Internet DRAFT - draft-ietf-rreq-iprouters-require

draft-ietf-rreq-iprouters-require











                        INTERNET DRAFT

                 Requirements for IP Routers

          <draft-ietf-rreq-iprouters-require-00.txt>

                       21 December 1993
                    Document Revision 1.47
                   Revision Date: 12/21/93


                  Frank Kastenholz (Editor)
                      FTP Software, Inc
                        2 High Street
              North Andover, Mass 01845-2620 USA

                        kasten@ftp.com






Status of this Memo

This document is an Internet Draft.  Internet Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups.  Note that other
groups may also distribute working documents as Internet
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Draft.












Internet Draft    Requirements for IP Routers    December 1993


This is a working document only, it should neither be cited
nor quoted in any formal document.

This document will expire before 26 June 1994.

Distribution of this document is unlimited.

Please send comments to the editor.

If your comment pertains to a particular piece of text, please
remember to mention the section number, this document is very
large and locating the text solely by context might not be
possible.  Please also mention the date of this draft
(12/21/93) and the revision level (1.47).



































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1.  INTRODUCTION

The memo replaces for RFC-1009, "Requirements for Internet
Gateways" ([INTRO:1]).

This memo defines and discusses requirements for devices which
perform the network layer forwarding function of the Internet
protocol suite.  The Internet community usually refers to such
devices as "IP routers" or simply "routers"; The OSI community
refers to such devices as "intermediate systems".  Many older
Internet documents refer to these devices as "gateways", a
name which more recently has largely passed out of favor to
avoid confusion with application gateways.

An IP router can be distinguished from other sorts of packet
switching devices in that a router examines the IP protocol
header as part of the switching process.  It generally has to
modify the IP header and to strip off and replace the Link
Layer framing.

The authors of this memo recognize, as should its readers,
that many routers support multiple protocol suites, and that
support for multiple protocol suites will be required in
increasingly large parts of the Internet in the future.  This
memo, however, does not attempt to specify Internet
requirements for protocol suites other than TCP/IP.

This document enumerates standard protocols that a router
connected to the Internet must use, and it incorporates by
reference the RFCs and other documents describing the current
specifications for these protocols.  It corrects errors in the
referenced documents and adds additional discussion and
guidance for an implementor.

For each protocol, this memo also contains an explicit set of
requirements, recommendations, and options.  The reader must
understand that the list of requirements in this memo is
incomplete by itself; the complete set of requirements for an
Internet protocol router is primarily defined in the standard
protocol specification documents, with the corrections,
amendments, and supplements contained in this memo.

This memo should be read in conjunction with the Requirements
for Internet Hosts RFCs ([INTRO:2] and [INTRO:3]).  Internet





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hosts and routers must both be capable of originating IP
datagrams and receiving IP datagrams destined for them.  The
major distinction between Internet hosts and routers is that
routers are required to implement forwarding algorithms and
Internet hosts do not require forwarding capabilities.  Any
Internet host acting as a router must adhere to the
requirements contained in this memo.

The goal of "open system interconnection" dictates that
routers must function correctly as Internet hosts when
necessary.  To achieve this, this memo provides guidelines for
such instances.  For simplification and ease of document
updates, this memo tries to avoid overlapping discussions of
host requirements with [INTRO:2] and [INTRO:3] and
incorporates the relevant requirements of those documents by
reference.  In some cases the requirements stated in [INTRO:2]
and [INTRO:3] are superseded  by this document.

A good-faith implementation of the protocols produced after
careful reading of the RFCs, with some interaction with the
Internet technical community, and that follows good
communications software engineering practices, should differ
from the requirements of this memo in only minor ways.  Thus,
in many cases, the "requirements" in this document are already
stated or implied in the standard protocol documents, so that
their inclusion here is, in a sense, redundant.  However, they
were included because some past implementation has made the
wrong choice, causing problems of interoperability,
performance, and/or robustness.

This memo includes discussion and explanation of many of the
requirements and recommendations.  A simple list of
requirements would be dangerous, because:

+  Some required features are more important than others, and
   some features are optional.

+  Some features are critical in some applications of routers
   but irrelevant in others.

+  There may be valid reasons why particular vendor products
   that are designed for restricted contexts might choose to
   use different specifications.






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However, the specifications of this memo must be followed to
meet the general goal of arbitrary router interoperation
across the diversity and complexity of the Internet.  Although
most current implementations fail to meet these requirements
in various ways, some minor and some major, this specification
is the ideal towards which we need to move.

These requirements are based on the current level of Internet
architecture.  This memo will be updated as required to
provide additional clarifications or to include additional
information in those areas in which specifications are still
evolving.


1.1  Reading this Document



1.1.1  Organization

      This memo emulates the layered organization used by
      [INTRO:2] and [INTRO:3].  Thus, Chapter 2 describes the
      layers found in the Internet architecture.  Chapter 3
      covers the Link Layer.  Chapters 4 and 5 are concerned
      with the Internet Layer protocols and forwarding
      algorithms.  Chapter 6 covers the Transport Layer.
      Upper layer protocols are divided between Chapter 7,
      which discusses the protocols which routers use to
      exchange routing information with each other, Chapter 8,
      which discusses network management, and Chapter 9, which
      discusses other upper layer protocols.  The final
      chapter covers operations and maintenance features.
      This organization was chosen for simplicity, clarity,
      and consistency with the Host Requirements RFCs.
      Appendices to this memo include a bibliography, a
      glossary, and some conjectures about future directions
      of router standards.

      In describing the requirements, we assume that an
      implementation strictly mirrors the layering of the
      protocols.  However, strict layering is an imperfect
      model, both for the protocol suite and for recommended
      implementation approaches.  Protocols in different
      layers interact in complex and sometimes subtle ways,





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      and particular functions often involve multiple layers.
      There are many design choices in an implementation, many
      of which involve creative "breaking" of strict layering.
      Every implementor is urged to read [INTRO:4] and
      [INTRO:5].

      In general, each major section of this memo is organized
      into the following subsections:

      (1)  Introduction

      (2)  Protocol Walk-Through -- considers the protocol
           specification documents section-by-section,
           correcting errors, stating requirements that may be
           ambiguous or ill-defined, and providing further
           clarification or explanation.

      (3)  Specific Issues -- discusses protocol design and
           implementation issues that were not included in the
           walk-through.

      Under many of the individual topics in this memo, there
      is parenthetical material labeled "DISCUSSION" or
      "IMPLEMENTATION". This material is intended to give a
      justification, clarification or explanation to the
      preceding requirements text.  The implementation
      material contains suggested approaches that an
      implementor may want to consider.  The DISCUSSION and
      IMPLEMENTATION sections are not part of the standard.


1.1.2  Requirements

      In this memo, the words that are used to define the
      significance of each particular requirement are
      capitalized.  These words are:

      +  "MUST"
         This word means that the item is an absolute
         requirement of the specification.

      +  "MUST IMPLEMENT"
         This phrase means that this specification requires
         that the item be implemented, but does not require





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         that it be enabled by default.

      +  "MUST NOT"
         This phrase means that the item is an absolute
         prohibition of the specification.

      +  "SHOULD"
         This word means that there may exist valid reasons in
         particular circumstances to ignore this item, but the
         full implications should be understood and the case
         carefully weighed before choosing a different course.

      +  "SHOULD IMPLEMENT"
         This phrase is similar in meaning to SHOULD, but is
         used when we recommend that a particular feature be
         provided but does not necessarily recommend that it
         be enabled by default.

      +  "SHOULD NOT"
         This phrase means that there may exist valid reasons
         in particular circumstances when the described
         behavior is acceptable or even useful, but the full
         implications should be understood and the case
         carefully weighed before implementing any behavior
         described with this label.

      +  "MAY"
         This word means that this item is truly optional.
         One vendor may choose to include the item because a
         particular marketplace requires it or because it
         enhances the product, for example; another vendor may
         omit the same item.


1.1.3  Compliance

      Some requirements are applicable to all routers.  Other
      requirements are applicable only to those which
      implement particular features or protocols.  In the
      following paragraphs, "Relevant" refers to the union of
      the requirements applicable to all routers and the set
      of requirements applicable to a particular router
      because of the set of features and protocols it has
      implemented.





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      Note that not all Relevant requirements are stated
      directly in this memo.  Various parts of this memo
      incorporate by reference sections of the Host
      Requirements specification, [INTRO:2] and [INTRO:3].
      For purposes of determining compliance with this memo,
      it does not matter whether a Relevant requirement is
      stated directly in this memo or merely incorporated by
      reference from one of those documents.

      An implementation is said to be "conditionally
      compliant" if it satisfies all of the Relevant MUST,
      MUST IMPLEMENT, and MUST NOT requirements.  An
      implementation is said to be "unconditionally compliant"
      if it is conditionally compliant and also satisfies all
      of the Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT
      requirements.  An implementation is not compliant if it
      is not conditionally compliant (i.e., it fails to
      satisfy one or more of the Relevant MUST, MUST
      IMPLEMENT, or MUST NOT requirements).

      For any of the SHOULD and SHOULD NOT requirements, a
      router may provide a configuration option that will
      cause the router to act other than as specified by the
      requirement.  Having such a configuration option does
      not void a router's claim to unconditional compliance as
      long as the option has a default setting, and that
      leaving the option at its default setting causes the
      router to operate in a manner which conforms to the
      requirement.

      Likewise, routers may provide, except where explicitly
      prohibited by this memo, options which cause them to
      violate MUST or MUST NOT requirements.  A router which
      provides such options is compliant (either fully or
      conditionally) if and only if each such option has a
      default setting which causes the router to conform to
      the requirements of this memo.  Please note that the
      authors of this memo, although aware of market
      realities, strongly recommend against provision of such
      options.  Requirements are labeled MUST or MUST NOT
      because experts in the field have judged them to be
      particularly important to interoperability or proper
      functioning in the Internet.  Vendors should weigh
      carefully the customer support costs of providing





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      options which violate those rules.

      Of course, this memo is not a complete specification of
      an IP router, but rather is closer to what in the OSI
      world is called a profile.  For example, this memo
      requires that a number of protocols be implemented.
      Although most of the contents of their protocol
      specifications are not repeated in this memo,
      implementors are nonetheless required to implement the
      protocols according to those specifications.


1.2  Relationships to Other Standards

   There are several reference documents of interest in
   checking the current status of protocol specifications and
   standardization:

     +  INTERNET OFFICIAL PROTOCOL STANDARDS
        This document describes the Internet standards process
        and lists the standards status of the protocols.  As
        of this writing (December, 1993) the current version
        of this document is RFC 1540, [ARCH:7].  This document
        is periodically re-issued.  You should always consult
        an RFC repository and use the latest version of this
        document.

     +  Assigned Numbers
        This document lists the assigned values of the
        parameters used in the various protocols.  For
        example, IP protocol codes, TCP port numbers, Telnet
        Option Codes, ARP hardware types, and Terminal Type
        names.  As of this writing (December, 1993) the
        current version of this document is RFC 1340,
        [INTRO:7].  This document is periodically re-issued.
        You should always consult an RFC repository and use
        the latest version of this document.

     +  Host Requirements
        This pair of documents reviews the specifications that
        apply to hosts and supplies guidance and clarification
        for any ambiguities.  Note that these requirements
        also apply to routers, except where otherwise
        specified in this memo.  As of this writing (December,





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        1993) the current versions of these documents are RFC
        1122 and RFC 1123, [INTRO:2], and [INTRO:3]
        respectively.

     +  Router Requirements (formerly "Gateway Requirements")
        This memo.

     Note that these documents are revised and updated at
     different times; in case of differences between these
     documents, the most recent must prevail.

     These and other Internet protocol documents may be
     obtained from the:
                  DDN Network Information Center
                14200 Park Meadow Drive, Suite 200
                       Chantilly, VA  22021
                                USA

                        nic@ds.internic.net

                 (800) 365-3642 or (703) 802-4535


1.3  General Considerations

   There are several important lessons that vendors of
   Internet software have learned and which a new vendor
   should consider seriously.


1.3.1  Continuing Internet Evolution

      The enormous growth of the Internet has revealed
      problems of management and scaling in a large datagram-
      based packet communication system.  These problems are
      being addressed, and as a result there will be
      continuing evolution of the specifications described in
      this memo.  New routing protocols, algorithms, and
      architectures are constantly being developed.  New and
      additional internet-layer protocols are also constantly
      being devised.  Because routers play such a crucial role
      in the Internet, and because the number of routers
      deployed in the Internet is much smaller than the number
      of hosts, vendors should expect that router standards





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      will continue to evolve much more quickly than host
      standards.  These changes will be carefully planned and
      controlled since there is extensive participation in
      this planning by the vendors and by the organizations
      responsible for operation of the networks.

      Development, evolution, and revision are characteristic
      of computer network protocols today, and this situation
      will persist for some years.  A vendor who develops
      computer communications software for the Internet
      protocol suite (or any other protocol suite!) and then
      fails to maintain and update that software for changing
      specifications is going to leave a trail of unhappy
      customers.  The Internet is a large communication
      network, and the users are in constant contact through
      it.  Experience has shown that knowledge of deficiencies
      in vendor software propagates quickly through the
      Internet technical community.


1.3.2  Robustness Principle

      At every layer of the protocols, there is a general rule
      (from [TRANS:2] by Jon Postel) whose application can
      lead to enormous benefits in robustness and
      interoperability:

                  "Be conservative in what you do,
            be liberal in what you accept from others."

      Software should be written to deal with every
      conceivable error, no matter how unlikely; sooner or
      later a packet will come in with that particular
      combination of errors and attributes, and unless the
      software is prepared, chaos can ensue.  In general, it
      is best to assume that the network is filled with
      malevolent entities that will send packets designed to
      have the worst possible effect.  This assumption will
      lead to suitably protective design.  The most serious
      problems in the Internet have been caused by unforeseen
      mechanisms triggered by low probability events; mere
      human malice would never have taken so devious a course!

      Adaptability to change must be designed into all levels





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      of router software.  As a simple example, consider a
      protocol specification that contains an enumeration of
      values for a particular header field -- e.g., a type
      field, a port number, or an error code; this enumeration
      must be assumed to be incomplete.  If the protocol
      specification defines four possible error codes, the
      software must not break when a fifth code shows up.  An
      undefined code might be logged, but it must not cause a
      failure.

      The second part of the principle is almost as important:
      software on hosts or other routers may contain
      deficiencies that make it unwise to exploit legal but
      obscure protocol features.  It is unwise to stray far
      from the obvious and simple, lest untoward effects
      result elsewhere.  A corollary of this is "watch out for
      misbehaving hosts"; router software should be prepared
      to survive in the presence of misbehaving hosts.  An
      important function of routers in the Internet is to
      limit the amount of disruption such hosts can inflict on
      the shared communication facility.


1.3.3  Error Logging

      The Internet includes a great variety of systems, each
      implementing many protocols and protocol layers, and
      some of these contain bugs and misfeatures in their
      Internet protocol software.  As a result of complexity,
      diversity, and distribution of function, the diagnosis
      of problems is often very difficult.

      Problem diagnosis will be aided if routers include a
      carefully designed facility for logging erroneous or
      "strange" events.  It is important to include as much
      diagnostic information as possible when an error is
      logged.  In particular, it is often useful to record the
      header(s) of a packet that caused an error.  However,
      care must be taken to ensure that error logging does not
      consume prohibitive amounts of resources or otherwise
      interfere with the operation of the router.

      There is a tendency for abnormal but harmless protocol
      events to overflow error logging files; this can be





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      avoided by using a "circular" log, or by enabling
      logging only while diagnosing a known failure.  It may
      be useful to filter and count duplicate successive
      messages.  One strategy that seems to work well is to
      both:
      +  Always count abnormalities and make such counts
         accessible through the management protocol (see
         Chapter 8); and
      +  Allow the logging of a great variety of events to be
         selectively enabled.  For example, it might useful to
         be able to "log everything" or to "log everything for
         host X".

      This topic is further discussed in [MGT:5].


1.3.4  Configuration

      In an ideal world, routers would be easy to configure,
      and perhaps even entirely self-configuring.  However,
      practical experience in the real world suggests that
      this is an impossible goal, and that in fact many
      attempts by vendors to make configuration easy actually
      cause customers more grief than they prevent.  As an
      extreme example, a router designed to come up and start
      routing packets without requiring any configuration
      information at all would almost certainly choose some
      incorrect parameter, possibly causing serious problems
      on any networks unfortunate enough to be connected to
      it.

      Often this memo requires that a parameter be a
      configurable option.  There are several reasons for
      this.  In a few cases there currently is some
      uncertainty or disagreement about the best value and it
      may be necessary to update the recommended value in the
      future.  In other cases, the value really depends on
      external factors -- e.g., the distribution of its
      communication load, or the speeds and topology of nearby
      networks -- and self-tuning algorithms are unavailable
      and may be insufficient.  In some cases, configurability
      is needed because of administrative requirements.

      Finally, some configuration options are required to





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      communicate with obsolete or incorrect implementations
      of the protocols, distributed without sources, that
      persist in many parts of the Internet.  To make correct
      systems coexist with these faulty systems,
      administrators must occasionally misconfigure the
      correct systems.  This problem will correct itself
      gradually as the faulty systems are retired, but cannot
      be ignored by vendors.

      When we say that a parameter must be configurable, we do
      not intend to require that its value be explicitly read
      from a configuration file at every boot time.  For many
      parameters, there is one value that is appropriate for
      all but the most unusual situations.  In such cases, it
      is quite reasonable that the parameter default to that
      value if not explicitly set.

      This memo requires a particular value for such defaults
      in some cases.  The choice of default is a sensitive
      issue when the configuration item controls accommodation
      of existing, faulty, systems.  If the Internet is to
      converge successfully to complete interoperability, the
      default values built into implementations must implement
      the official protocol, not misconfigurations to
      accommodate faulty implementations.  Although marketing
      considerations have led some vendors to choose
      misconfiguration defaults, we urge vendors to choose
      defaults that will conform to the standard.

      Finally, we note that a vendor needs to provide adequate
      documentation on all configuration parameters, their
      limits and effects.


1.4  Algorithms

   In several places in this memo, specific algorithms that a
   router ought to follow are specified.  These algorithms are
   not, per se, required of the router.  A router need not
   implement each algorithm as it is written in this document.
   Rather, an implementation must present a behavior to the
   external world that is the same as a strict, literal,
   implementation of the specified algorithm.






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   Algorithms are described in a manner that differs from the
   way a good implementor would implement them.  For
   expository purposes, a style that emphasizes conciseness,
   clarity, and independence from implementation details has
   been chosen.  A good implementor will choose algorithms and
   implementation methods which produce the same results as
   these algorithms, but may be more efficient or less
   general.

   We note that the art of efficient router implementation is
   outside of the scope of this memo.






































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2.  INTERNET ARCHITECTURE

This chapter does not contain any requirements.  However, it
does contain useful background information on the general
architecture of the Internet and of routers.

General background and discussion on the Internet architecture
and supporting protocol suite can be found in the DDN Protocol
Handbook [ARCH:1]; for background see for example [ARCH:2],
[ARCH:3], and [ARCH:4].  The Internet architecture and
protocols are also covered in an ever-growing number of
textbooks, such as [ARCH:5] and [ARCH:6].


2.1  Introduction

   The Internet system consists of a number of interconnected
   packet networks supporting communication among host
   computers using the Internet protocols.  These protocols
   include the Internet Protocol (IP), the Internet Control
   Message Protocol (ICMP), the Internet Group Management
   Protocol (IGMP), and a variety transport and application
   protocols that depend upon them.  As was described in
   Section [1.2], the Internet Engineering Steering Group
   periodically releases an "Official Protocols" memo listing
   all of the Internet protocols.

   All Internet protocols use IP as the basic data transport
   mechanism.  IP is a datagram, or connectionless,
   internetwork service and includes provision for addressing,
   type-of-service specification, fragmentation and
   reassembly, and security.  ICMP and IGMP are considered
   integral parts of IP, although they are architecturally
   layered upon IP.  ICMP provides error reporting, flow
   control, first-hop router redirection, and other
   maintenance and control functions.  IGMP provides the
   mechanisms by which hosts and routers can join and leave IP
   multicast groups.

   Reliable data delivery is provided in the Internet protocol
   suite by Transport Layer protocols such as the Transmission
   Control Protocol (TCP), which provides end-end
   retransmission, resequencing and connection control.
   Transport Layer connectionless service is provided by the





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   User Datagram Protocol (UDP).


2.2  Elements of the Architecture



2.2.1  Protocol Layering

      To communicate using the Internet system, a host must
      implement the layered set of protocols comprising the
      Internet protocol suite.  A host typically must
      implement at least one protocol from each layer.

      The protocol layers used in the Internet architecture
      are as follows [ARCH:7]:

      +  Application Layer
         The Application Layer is the top layer of the
         Internet protocol suite.  The Internet suite does not
         further subdivide the Application Layer, although
         some application layer protocols do contain some
         internal sub-layering.  The application layer of the
         Internet suite essentially combines the functions of
         the top two layers -- Presentation and Application --
         of the OSI Reference Model [ARCH:8].  The Application
         Layer in the Internet protocol suite also includes
         some of the function relegated to the Session Layer
         in the OSI Reference Model.

         We distinguish two categories of application layer
         protocols:  user protocols that provide service
         directly to users, and support protocols that provide
         common system functions.  The most common Internet
         user protocols are:
         -- Telnet (remote login)
         -- FTP (file transfer)
         -- SMTP (electronic mail delivery)

         There are a number of other standardized user
         protocols and many private user protocols.

         Support protocols, used for host name mapping,
         booting, and management, include SNMP, BOOTP, TFTP,





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         the Domain Name System (DNS) protocol, and a variety
         of routing protocols.

         Application Layer protocols relevant to routers are
         discussed in chapters 7, 8, and 9 of this memo.

      +  Transport Layer
         The Transport Layer provides end-to-end communication
         services.  This layer is roughly equivalent to the
         "Transport Layer" in the OSI Reference Model, except
         that it also incorporates some of OSI's Session Layer
         establishment and destruction functions.

         There are two primary Transport Layer protocols at
         present:
         -- Transmission Control Protocol (TCP)
         -- User Datagram Protocol (UDP)

         TCP is a reliable connection-oriented transport
         service that provides end-to-end reliability,
         resequencing, and flow control.  UDP is a
         connectionless ("datagram") transport service.  Other
         transport protocols have been developed by the
         research community, and the set of official Internet
         transport protocols may be expanded in the future.

         Transport Layer protocols relevant to routers are
         discussed in Chapter 6.

      +  Internet Layer
         All Internet transport protocols use the Internet
         Protocol (IP) to carry data from source host to
         destination host.  IP is a connectionless or datagram
         internetwork service, providing no end-to-end
         delivery guarantees. IP datagrams may arrive at the
         destination host damaged, duplicated, out of order,
         or not at all.  The layers above IP are responsible
         for reliable delivery service when it is required.
         The IP protocol includes provision for addressing,
         type-of-service specification, fragmentation and
         reassembly, and security.

         The datagram or connectionless nature of IP is a
         fundamental and characteristic feature of the





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         Internet architecture.

         The Internet Control Message Protocol (ICMP) is a
         control protocol that is considered to be an integral
         part of IP, although it is architecturally layered
         upon IP, i.e., it uses IP to carry its data end-to-
         end.  ICMP provides error reporting, congestion
         reporting, and first-hop router redirection.

         The Internet Group Management Protocol (IGMP) is an
         Internet layer protocol used for establishing dynamic
         host groups for IP multicasting.

         The Internet layer protocols IP, ICMP, and IGMP are
         discussed in chapter 4.

      +  Link Layer
         To communicate on its directly-connected network, a
         host must implement the communication protocol used
         to interface to that network.  We call this a Link
         Layer layer protocol.

         Some older Internet documents refer to this layer as
         the "Network Layer", but it is not the same as the
         "Network Layer" in the OSI Reference Model.

         This layer contains everything "below" the Internet
         Layer.

         Protocols in this Layer are generally outside the
         scope of Internet standardization; the Internet
         (intentionally) uses existing standards whenever
         possible.  Thus, Internet Link Layer standards
         usually address only address resolution and rules for
         transmitting IP packets over specific Link Layer
         protocols.  Internet Link Layer standards are
         discussed in chapter 3.


2.2.2  Networks

      The constituent networks of the Internet system are
      required to provide only packet (connectionless)
      transport.  According to the IP service specification,





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      datagrams can be delivered out of order, be lost or
      duplicated, and/or contain errors.

      For reasonable performance of the protocols that use IP
      (e.g., TCP), the loss rate of the network should be very
      low.  In networks providing connection-oriented service,
      the extra reliability provided by virtual circuits
      enhances the end-end robustness of the system, but is
      not necessary for Internet operation.

      Constituent networks may generally be divided into two
      classes:

        +  Local-Area Networks (LANs)
           LANs may have a variety of designs.  In general, a
           LAN will cover a small geographical area (e.g., a
           single building or plant site) and provide high
           bandwidth with low delays.  LANs may be passive
           (similar to Ethernet) or they may be active (such
           as ATM).

        +  Wide-Area Networks (WANs)
           Geographically-dispersed hosts and LANs are
           interconnected by wide-area networks, also called
           long-haul networks.  These networks may have a
           complex internal structure of lines and packet-
           switches, or they may be as simple as point-to-
           point lines.


2.2.3  Routers

      In the Internet model, constituent networks are
      connected together by IP datagram forwarders which are
      called "routers" or "IP routers".  In this document,
      every use of the term "router" is equivalent to "IP
      router".  Many older Internet documents refer to routers
      as "gateways".

      Historically, routers have been realized with packet-
      switching software executing on a general-purpose CPU.
      However, as custom hardware development becomes cheaper
      and as higher throughput is required, but special-
      purpose hardware is becoming increasingly common.  This





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      specification applies to routers regardless of how they
      are implemented.

      A router is connected to two or more networks, appearing
      to each of these networks as a connected host.  Thus, it
      has (at least) one physical interface and (at least) one
      IP address on each of the connected networks (this
      ignores the concept of un-numbered links, which is
      discussed in section [2.2.7]).  Forwarding an IP
      datagram generally requires the router to choose the
      address of the next-hop router or (for the final hop)
      the destination host.  This choice, called "routing",
      depends upon a routing database within the router.  The
      routing database is also sometimes known as a routing
      table or forwarding table.

      The routing database should be maintained dynamically to
      reflect the current topology of the Internet system.  A
      router normally accomplishes this by participating in
      distributed routing and reachability algorithms with
      other routers.

      Routers provide datagram transport only, and they seek
      to minimize the state information necessary to sustain
      this service in the interest of routing flexibility and
      robustness.

      Packet switching devices may also operate at the Link
      Layer; such devices are usually called "bridges".
      Network segments which are connected by bridges share
      the same IP network number, i.e., they logically form a
      single IP network.  These other devices are outside of
      the scope of this document.

      Another variation on the simple model of networks
      connected with routers sometimes occurs: a set of
      routers may be interconnected with only serial lines, to
      form a network in which the packet switching is
      performed at the Internetwork (IP) Layer rather than the
      Link Layer.









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2.2.4  Autonomous Systems

      For technical, managerial, and sometimes political
      reasons, the routers of the Internet system are grouped
      into collections called "autonomous systems".  The
      routers included in a single autonomous system (AS) are
      expected to:

      +  Be under the control of a single operations and
         maintenance (O&M) organization;

      +  Employ common routing protocols among themselves, to
         dynamically maintain their routing databases.

      A number of different dynamic routing protocols have
      been developed (see Section [7.2]); the routing protocol
      within a single AS is generically called an interior
      gateway protocol or IGP.

      An IP datagram may have to traverse the routers of two
      or more ASs to reach its destination, and the ASs must
      provide each other with topology information to allow
      such forwarding.  An exterior gateway protocol
      (generally BGP or EGP) is used for this purpose.


2.2.5  Addresses and Subnets

      An IP datagram carries 32-bit source and destination
      addresses, each of which is partitioned into two parts
      -- a constituent network number and a host number on
      that network.  Symbolically:

         IP-address  ::=  { <Network-number>, <Host-number> }

      To finally deliver the datagram, the last router in its
      path must map the Host-number (or "rest") part of an IP
      address into the physical address of a host connection
      to the constituent network.

      This simple notion has been extended by the concept of
      "subnets", which were introduced in order to allow
      arbitrary complexity of interconnected LAN structures
      within an organization, while insulating the Internet





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      system against explosive growth in network numbers and
      routing complexity.  Subnets essentially provide a
      multi-level hierarchical routing structure for the
      Internet system.  The subnet extension, described in
      [INTERNET:2], is now a required part of the Internet
      architecture.  The basic idea is to partition the <Host-
      number> field into two parts: a subnet number, and a
      true host number on that subnet:

         IP-address  ::=
           { <Network-number>, <Subnet-number>, <Host-number>
           }

      The interconnected physical networks within an
      organization will be given the same network number but
      different subnet numbers.  The distinction between the
      subnets of such a subnetted network is normally not
      visible outside of that network.  Thus, routing in the
      rest of the Internet will be based only upon the
      <Network-number> part of the IP destination address;
      routers outside the network will combine <Subnet-number>
      and <Host-number> together to form an uninterpreted
      "rest" part of the 32-bit IP address.  Within the
      subnetted network, the routers must route on the basis
      of an extended network number:

         { <Network-number>, <Subnet-number> }

      Under certain circumstances, it may be desirable to
      support subnets of a particular network being
      interconnected only via a path which is not part of the
      subnetted network.  Even though many IGP's and no EGP's
      currently support this configuration effectively,
      routers need to be able to support this configuration of
      subnetting (see Section [4.2.3.4]).  In general, routers
      should not make assumptions about what are subnets and
      what are not, but simply ignore the concept of Class in
      networks, and treat each route as a { network, mask
      }-tuple.

      DISCUSSION:
         It is becoming clear that as the Internet grows
         larger and larger, the traditional uses of Class A,
         B, and C networks will be modified in order to





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         achieve better use of IP's 32-bit address space.
         Classless Interdomain Routing (CIDR) [INTERNET:15] is
         a method currently being deployed in the Internet
         backbones to achieve this added efficiency.  CIDR
         depends on the ability of assigning and routing to
         networks that are not based on Class A, B, or C
         networks.  Thus, routers should always treat a route
         as a network with a mask.

      Furthermore, for similar reasons, a subnetted network
      need not have a consistent subnet mask through all parts
      of the network.  For example, one subnet may use an 8
      bit subnet mask, another 10 bit, and another 6 bit.
      Routers need to be able to support this type of
      configuration (see Section [4.2.3.4]).

      The bit positions containing this extended network
      number are indicated by a 32-bit mask called the "subnet
      mask"; it is recommended but not required that the
      <Subnet-number> bits be contiguous and fall between the
      <Network-number> and the <Host-number> fields.  No
      subnet should be assigned the value zero or -1 (all one
      bits).

      Although the inventors of the subnet mechanism probably
      expected that each piece of an organization's network
      would have only a single subnet number, in practice it
      has often proven necessary or useful to have several
      subnets share a single physical cable.

      There are special considerations for the router when a
      connected network provides a broadcast or multicast
      capability; these will be discussed later.


2.2.6  IP Multicasting

      IP multicasting is an extension of Link Layer multicast
      to IP internets.  Using IP multicasts, a single datagram
      can be addressed to multiple hosts. This collection of
      hosts is called a multicast group.  Each multicast group
      is represented as a Class D IP address.  An IP datagram
      sent to the group is to be delivered to each group
      member with the same best-effort delivery as that





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      provided for unicast IP traffic.  The sender of the
      datagram does not itself need to be a member of the
      destination group.

      The semantics of IP multicast group membership are
      defined in [INTERNET:4].  That document describes how
      hosts and routers join and leave multicast groups.  It
      also defines a protocol, the Internet Group Management
      Protocol (IGMP), that monitors IP multicast group
      membership.

      Forwarding of IP multicast datagrams is accomplished
      either through static routing information or via a
      multicast routing protocol.  Devices that forward IP
      multicast datagrams are called multicast routers. They
      may or may not also forward IP unicasts. In general,
      multicast datagrams are forwarded on the basis of both
      their source and destination addresses.  Forwarding of
      IP multicast packets is described in more detail in
      Section [5.2.1]. Appendix D discusses multicast routing
      protocols.


2.2.7  Unnumbered Lines and Networks and Subnets

      Traditionally, each network interface on an IP host or
      router has its own IP address.  Over the years, people
      have observed that this can cause inefficient use of the
      scarce IP address space, since it forces allocation of
      an IP network number, or at least a subnet number, to
      every point-to-point link.

      To solve this problem, a number of people have proposed
      and implemented the concept of "unnumbered serial
      lines".  An unnumbered serial line does not have any IP
      network or subnet number associated with it.  As a
      consequence, the network interfaces connected to an
      unnumbered serial line do not have IP addresses.

      Because the IP architecture has traditionally assumed
      that all interfaces had IP addresses, these unnumbered
      interfaces cause some interesting dilemmas.  For
      example, some IP options (e.g.  Record Route) specify
      that a router must insert the interface address into the





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      option, but an unnumbered interface has no IP address.
      Even more fundamental (as we shall see in chapter 5) is
      that routes contain the IP address of the next hop
      router.  A router expects that that IP address will be
      on an IP (sub)net that the router is connected to.  That
      assumption is of course violated if the only connection
      is an unnumbered serial line.

      To get around these difficulties, two schemes have been
      invented.  The first scheme says that two routers
      connected by an unnumbered serial line aren't really two
      routers at all, but rather two "half-routers" which
      together make up a single (virtual) router.  The
      unnumbered serial line is essentially considered to be
      an internal bus in the virtual router.  The two halves
      of the virtual router must coordinate their activities
      in such a way that they act exactly like a single
      router.

      This scheme fits in well with the IP architecture, but
      suffers from two important drawbacks.  The first is
      that, although it handles the common case of a single
      unnumbered serial line, it is not readily extensible to
      handle the case of a mesh of routers and unnumbered
      serial lines.  The second drawback is that the
      interactions between the half routers are necessarily
      complex and are not standardized, effectively precluding
      the connection of equipment from different vendors using
      unnumbered serial lines.

      Because of these drawbacks, this memo has adopted an
      alternative scheme, which has been invented multiple
      times but which is probably originally attributable to
      Phil Karn.  In this scheme, a router which has
      unnumbered serial lines also has a special IP address,
      called a "router-id" in this memo.  The router-id is one
      of the router's IP addresses (a router is required to
      have at least one IP address).  This router-id is used
      as if it is the IP address of all unnumbered interfaces.










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2.2.8  Notable Oddities



2.2.8.1  Embedded Routers

         A router may be a stand-alone computer system,
         dedicated to its IP router functions.  Alternatively,
         it is possible to embed router functions within a
         host operating system which supports connections to
         two or more networks.  The best-known example of an
         operating system with embedded router code is the
         Berkeley BSD system.  The embedded router feature
         seems to make internetting easy, but it has a number
         of hidden pitfalls:

         (1)  If a host has only a single constituent-network
              interface, it should not act as a router.

              For example, hosts with embedded router code
              that gratuitously forward broadcast packets or
              datagrams on the same net often cause packet
              avalanches.

         (2)  If a (multihomed) host acts as a router, it must
              implement ALL the relevant router requirements
              contained in this document.

              For example, the routing protocol issues and the
              router control and monitoring problems are as
              hard and important for embedded routers as for
              stand-alone routers.

              Since Internet router requirements and
              specifications may change independently of
              operating system changes, an administration that
              operates an embedded router in the Internet is
              strongly advised to have the ability to maintain
              and update the router code (e.g., this might
              require router code source).

         (3)  Once a host runs embedded router code, it
              becomes part of the Internet system.  Thus,
              errors in software or configuration can hinder





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              communication between other hosts.  As a
              consequence, the host administrator must lose
              some autonomy.

              In many circumstances, a host administrator will
              need to disable router code embedded in the
              operating system, and any embedded router code
              must be organized so that it can be easily
              disabled.

         (4)  If a host running embedded router code is
              concurrently used for other services, the O&M
              (Operation and Maintenance) requirements for the
              two modes of use may be in serious conflict.

              For example, router O&M will in many cases be
              performed remotely by an operations center; this
              may require privileged system access which the
              host administrator would not normally want to
              distribute.


2.2.8.2  Transparent Routers

         There are two basic models for interconnecting local-
         area networks and wide-area (or long-haul) networks
         in the Internet.  In the first, the local-area
         network is assigned a network number and all routers
         in the Internet must know how to route to that
         network.  In the second, the local-area network
         shares (a small part of) the address space of the
         wide-area network.  Routers that support this second
         model are called "address sharing routers" or
         "transparent routers".  The focus of this memo is on
         routers that support the first model, but this is not
         intended to exclude the use of transparent routers.

         The basic idea of a transparent router is that the
         hosts on the local-area network behind such a router
         share the address space of the wide-area network in
         front of the router.  In certain situations this is a
         very useful approach and the limitations do not
         present significant drawbacks.






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         The words "in front" and "behind" indicate one of the
         limitations of this approach: this model of
         interconnection is suitable only for a geographically
         (and topologically) limited stub environment.  It
         requires that there be some form of logical
         addressing in the network level addressing of the
         wide-area network.  All of the IP addresses in the
         local environment map to a few (usually one) physical
         address in the wide-area network.  This mapping
         occurs in a way consistent with the { IP address <->
         network address } mapping used throughout the wide-
         area network.

         Multihoming is possible on one wide-area network, but
         may present routing problems if the interfaces are
         geographically or topologically separated.
         Multihoming on two (or more) wide-area networks is a
         problem due to the confusion of addresses.

         The behavior that hosts see from other hosts in what
         is apparently the same network may differ if the
         transparent router cannot fully emulate the normal
         wide-area network service.  For example, the ARPANET
         used a Link Layer protocol that provided a
         "Destination Dead" indication in response to an
         attempt to send to a host which was powered off.
         However, if there were a transparent router between
         the ARPANET and an Ethernet, a host on the ARPANET
         would not receive a Destination Dead indication if it
         sent a datagram to a host that was powered off and
         was connected to the ARPANET via the transparent
         router instead of directly.


2.3  Router Characteristics

   An Internet router performs the following functions:

   (1)  Conforms to specific Internet protocols specified in
        this document, including the Internet Protocol (IP),
        Internet Control Message Protocol (ICMP), and others
        as necessary.

   (2)  Interfaces to two or more packet networks.  For each





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        connected network the router must implement the
        functions required by that network.  These functions
        typically include:

        +  Encapsulating and decapsulating the IP datagrams
           with the connected network framing (e.g., an
           Ethernet header and checksum),

        +  Sending and receiving IP datagrams up to the
           maximum size supported by that network, this size
           is the network's "Maximum Transmission Unit" or
           "MTU",

        +  Translating the IP destination address into an
           appropriate network-level address for the connected
           network (e.g., an Ethernet hardware address), if
           needed, and

        +  Responding to the network flow control and error
           indication, if any.

        See chapter 3 (Link Layer).

   (3)  Receives and forwards Internet datagrams.  Important
        issues in this process are buffer management,
        congestion control, and fairness.

        +  Recognizes various error conditions and generates
           ICMP error and information messages as required.

        +  Drops datagrams whose time-to-live fields have
           reached zero.

        +  Fragments datagrams when necessary to fit into the
           MTU of the next network.

        See chapter 4 (Internet Layer -- Protocols) and
        chapter 5 (Internet Layer -- Forwarding) for more
        information.

   (4)  Chooses a next-hop destination for each IP datagram,
        based on the information in its routing database.  See
        chapter 5 (Internet Layer -- Forwarding) for more
        information.





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   (5)  (Usually) supports an interior gateway protocol (IGP)
        to carry out distributed routing and reachability
        algorithms with the other routers in the same
        autonomous system.  In addition, some routers will
        need to support an exterior gateway protocol (EGP)  to
        exchange topological information with other autonomous
        systems.  See chapter 7 (Application Layer -- Routing
        Protocols) for more information.

   (6)  Provides network management and system support
        facilities, including loading, debugging, status
        reporting, exception reporting and control.  See
        chapter 8 (Application Layer -- Network Management
        Protocols) and chapter 10 (Operation and Maintenance)
        for more information.

   A router vendor will have many choices on power,
   complexity, and features for a particular router product.
   It may be helpful to observe that the Internet system is
   neither homogeneous nor fully-connected.  For reasons of
   technology and geography it is growing into a global
   interconnect system plus a "fringe" of LANs around the
   "edge". More and more these fringe LANs are becoming richly
   interconnected, thus making them less out on the fringe and
   more demanding on router requirements.

   +  The global interconnect system is comprised of a number
      of wide-area networks to which are attached routers of
      several Autonomous Systems (AS); there are relatively
      few hosts connected directly to the system.

   +  Most hosts are connected to LANs.  Many organizations
      have clusters of LANs interconnected by local routers.
      Each such cluster is connected by routers at one or more
      points into the global interconnect system.  If it is
      connected at only one point, a LAN is known as a "stub"
      network.

   Routers in the global interconnect system generally
   require:

   +  Advanced Routing and Forwarding Algorithms

      These routers need routing algorithms which are highly





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      dynamic and also offer type-of-service routing.
      Congestion is still not a completely resolved issue (see
      Section [5.3.6]).  Improvements in these areas are
      expected, as the research community is actively working
      on these issues.

   +  High Availability

      These routers need to be highly reliable, providing 24
      hours a day, 7 days a week service.  Equipment and
      software faults can have a wide-spread (sometimes
      global) effect.  In case of failure, they must recover
      quickly.  In any environment, a router must be highly
      robust and able to operate, possibly in a degraded
      state, under conditions of extreme congestion or failure
      of network resources.

   +  Advanced O&M Features

      Internet routers normally operate in an unattended mode.
      They will typically be operated remotely from a
      centralized monitoring center.  They need to provide
      sophisticated means for monitoring and measuring traffic
      and other events and for diagnosing faults.

   +  High Performance

      Long-haul lines in the Internet today are most
      frequently 56 Kbps, DS1 (1.4Mbps), and DS3 (45Mbps)
      speeds.  LANs are typically Ethernet (10Mbps) and, to a
      lesser degree, FDDI (100Mbps).  However, network media
      technology is constantly advancing and even higher
      speeds are likely in the future.  Full-duplex operation
      is provided at all of these speeds.

   The requirements for routers used in the LAN fringe (e.g.,
   campus networks) depend greatly on the demands of the local
   networks.  These may be high or medium-performance devices,
   probably competitively procured from several different
   vendors and operated by an internal organization (e.g., a
   campus computing center).  The design of these routers
   should emphasize low average latency and good burst
   performance, together with delay and type-of-service
   sensitive resource management. In this environment there





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   may be less formal O&M but it will not be less important.
   The need for the routing mechanism to be highly dynamic
   will become more important as networks become more complex
   and interconnected.  Users will demand more out of their
   local connections because of the speed of the global
   interconnects.

   As networks have grown, and as more networks have become
   old enough that they are phasing out older equipment, it
   has become increasingly imperative that routers
   interoperate with routers from other vendors.

   Even though the Internet system is not fully
   interconnected, many parts of the system need to have
   redundant connectivity.  Rich connectivity allows reliable
   service despite failures of communication lines and
   routers, and it can also improve service by shortening
   Internet paths and by providing additional capacity.
   Unfortunately, this richer topology can make it much more
   difficult to choose the best path to a particular
   destination.


2.4  Architectural Assumptions

   The current Internet architecture is based on a set of
   assumptions about the communication system.  The
   assumptions most relevant to routers are as follows:

   +  The Internet is a network of networks.

      Each host is directly connected to some particular
      network(s); its connection to the Internet is only
      conceptual.  Two hosts on the same network communicate
      with each other using the same set of protocols that
      they would use to communicate with hosts on distant
      networks.

   +  Routers don't keep connection state information.

      To improve the robustness of the communication system,
      routers are designed to be stateless, forwarding each IP
      packet independently of other packets.  As a result,
      redundant paths can be exploited to provide robust





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      service in spite of failures of intervening routers and
      networks.

      All state information required for end-to-end flow
      control and reliability is implemented in the hosts, in
      the transport layer or in application programs.  All
      connection control information is thus co-located with
      the end points of the communication, so it will be lost
      only if an end point fails.  Routers effect flow control
      only indirectly, by dropping packets or increasing
      network delay.

      Note that future protocol developments may well end up
      putting some more state into routers.  This is
      especially likely for resource reservation and flows.

   +  Routing complexity should be in the routers.

      Routing is a complex and difficult problem, and ought to
      be performed by the routers, not the hosts.  An
      important objective is to insulate host software from
      changes caused by the inevitable evolution of the
      Internet routing architecture.

   +  The system must tolerate wide network variation.

      A basic objective of the Internet design is to tolerate
      a wide range of network characteristics -- e.g.,
      bandwidth, delay, packet loss, packet reordering, and
      maximum packet size.  Another objective is robustness
      against failure of individual networks, routers, and
      hosts, using whatever bandwidth is still available.
      Finally, the goal is full "open system interconnection":
      an Internet router must be able to interoperate robustly
      and effectively with any other router or Internet host,
      across diverse Internet paths.

      Sometimes implementors have designed for less ambitious
      goals.  For example, the LAN environment is typically
      much more benign than the Internet as a whole; LANs have
      low packet loss and delay and do not reorder packets.
      Some vendors have fielded implementations that are
      adequate for a simple LAN environment, but work badly
      for general interoperation.  The vendor justifies such a





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      product as being economical within the restricted LAN
      market.  However, isolated LANs seldom stay isolated for
      long; they are soon connected to each other, to
      organization-wide internets, and eventually to the
      global Internet system.  In the end, neither the
      customer nor the vendor is served by incomplete or
      substandard routers.

      The requirements spelled out in this document are
      designed for a full-function router.  It is intended
      that fully compliant routers will be usable in almost
      any part of the Internet.





































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3.  LINK LAYER

Although  [INTRO:1] covers Link Layer standards (IP over foo,
ARP, etc.), this document anticipates that Link-Layer material
will be covered in a separate Link Layer Requirements
document.  A Link-Layer requirements document would be
applicable to both hosts and routers.  Thus, this document
will not obsolete the parts of [INTRO:1] that deal with link-
layer issues.


3.1  INTRODUCTION

   Routers have essentially the same Link Layer protocol
   requirements as other sorts of Internet systems.  These
   requirements are given in chapter 3 of "Requirements for
   Internet Gateways" [INTRO:1].  A router MUST comply with
   its requirements and SHOULD comply with its
   recommendations.  Since some of the material in that
   document has become somewhat dated, some additional
   requirements and explanations are included below.

   DISCUSSION:
      It is expected that the Internet community will produce
      a "Requirements for Internet Link Layer" standard which
      will supersede both this chapter and chapter 3 of
      [INTRO:1].



3.2  LINK/INTERNET LAYER INTERFACE

   Although this document does not attempt to specify the
   interface between the Link Layer and the upper layers, it
   is worth noting here that other parts of this document,
   particularly chapter 5, require various sorts of
   information to be passed across this layer boundary.

   This section uses the following definitions:

   +  Source physical address

      The source physical address is the Link Layer address of
      the host or router from which the packet was received.





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   +  Destination physical address

      The destination physical address is the Link Layer
      address to which the packet was sent.

   The information that must pass from the Link Layer to the
   Internetwork Layer for each received packet is:

   (1)  The IP packet [5.2.2],

   (2)  The length of the data portion (i.e., not including
        the Link-Layer framing) of the Link Layer frame
        [5.2.2],

   (3)  The identity of the physical interface from which the
        IP packet was received [5.2.3], and

   (4)  The classification of the packet's destination
        physical address as a Link Layer unicast, broadcast,
        or multicast [4.3.2], [5.3.4].

   In addition, the Link Layer also should provide:

   (5)  The source physical address.

   The information that must pass from the Internetwork Layer
   to the Link Layer for each transmitted packet is:

   (1)  The IP packet [5.2.1]

   (2)  The length of the IP packet [5.2.1]

   (3)  The destination physical interface [5.2.1]

   (4)  The next hop IP address [5.2.1]

   In addition, the Internetwork Layer also should provide:

   (5)  The Link Layer priority value [5.3.3.2]

   The Link Layer must also notify the Internetwork Layer if
   the packet to be transmitted causes a Link Layer
   precedence-related error [5.3.3.3].






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3.3  SPECIFIC ISSUES



3.3.1  Trailer Encapsulation

      Routers which can connect to 10Mb Ethernets MAY be able
      to receive and forward Ethernet packets encapsulated
      using the trailer encapsulation described in [LINK:1].
      However, a router SHOULD NOT originate trailer
      encapsulated packets.  A router MUST NOT originate
      trailer encapsulated packets without first verifying,
      using the mechanism described in section 2.3.1 of
      [INTRO:2], that the immediate destination of the packet
      is willing and able to accept trailer-encapsulated
      packets.  A router SHOULD NOT agree (using these same
      mechanisms) to accept trailer-encapsulated packets.


3.3.2  Address Resolution Protocol -- ARP

      Routers which implement ARP MUST be compliant and SHOULD
      be unconditionally compliant with the requirements in
      section 2.3.2 of [INTRO:2].

      The link layer MUST NOT report a Destination Unreachable
      error to IP solely because there is no ARP cache entry
      for a destination.

      A router MUST not believe any ARP reply which claims
      that the Link Layer address of another host or router is
      a broadcast or multicast address.


3.3.3  Ethernet and 802.3 Coexistence

      Routers which can connect to 10Mb Ethernets MUST be
      compliant and SHOULD be unconditionally compliant with
      the requirements of Section [2.3.3] of [INTRO:2].










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3.3.4  Maximum Transmission Unit -- MTU

      The MTU of each logical interface MUST be configurable.

      Many Link Layer protocols define a maximum frame size
      that may be sent.  In such cases, a router MUST NOT
      allow an MTU to be set which would allow sending of
      frames larger than those allowed by the Link Layer
      protocol.  However, a router SHOULD be willing to
      receive a packet as large as the maximum frame size even
      if that is larger than the MTU.

      DISCUSSION:
         Note that this is a stricter requirement than imposed
         on hosts by [INTRO:2], which requires that the MTU of
         each physical interface be configurable.

         If a network is using an MTU smaller than the maximum
         frame size for the Link Layer, a router may receive
         packets larger than the MTU from hosts which are in
         the process of initializing themselves, or which have
         been misconfigured.

         In general, the Robustness Principle indicates that
         these packets should be successfully received, if at
         all possible.



3.3.5  Point-to-Point Protocol -- PPP

      Contrary to [INTRO:1], the Internet does have a standard
      serial line protocol: the Point-to-Point Protocol (PPP),
      defined in [LINK:2], [LINK:3], [LINK:4], and [LINK:5].

      A "serial line interface" is any interface which is
      designed to send data over a telephone, leased,
      dedicated or direct line (either 2 or 4 wire) using a
      standardized modem or bit serial interface (such as
      RS-232, RS-449 or V.35), using either synchronous or
      asynchronous clocking.

      A "general purpose serial interface" is a serial line
      interface which is not solely for use as an access line





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      to a network for which an alternative IP link layer
      specification exists (such as X.25 or Frame Relay).

      Routers which contain such general purpose serial
      interfaces MUST implement PPP.

      PPP MUST be supported on all general purpose serial
      interfaces on a router.  The router MAY allow the line
      to be configured to use serial line protocols other than
      PPP, all general purpose serial interfaces MUST default
      to using PPP.


3.3.5.1  Introduction

         This section provides guidelines to router
         implementors so that they can ensure interoperability
         with other routers using PPP over either synchronous
         or asynchronous links.

         It is critical that an implementor understand the
         semantics of the option negotiation mechanism.
         Options are a means for a local device to indicate to
         a remote peer what the local device will *accept*
         from the remote peer, not what it wishes to send.  It
         is up to the remote peer to decide what is most
         convenient to send within the confines of the set of
         options that the local device has stated that it can
         accept.  Therefore it is perfectly acceptable and
         normal for a remote peer to ACK all the options
         indicated in an LCP Configuration Request (CR) even
         if the remote peer does not support any of those
         options.  Again, the options are simply a mechanism
         for either device to indicate to its peer what it
         will accept, not necessarily what it will send.


3.3.5.2  Link Control Protocol (LCP) Options

         The PPP Link Control Protocol (LCP) offers a number
         of options that may be negotiated.  These options
         include (among others) address and control field
         compression, protocol field compression, asynchronous
         character map, Maximum Receive Unit (MRU), Link





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         Quality Monitoring (LQM), magic number (for loopback
         detection), Password Authentication Protocol (PAP),
         Challenge Handshake Authentication Protocol (CHAP),
         and the 32-bit Frame Check Sequence (FCS).

         A router MAY do address/control field compression on
         either synchronous or asynchronous links.  A router
         MAY do protocol field compression on either
         synchronous or asynchronous links.  A router MAY
         indicate that it can accept these compressions, but
         MUST be able to accept uncompressed PPP header
         information even if it has indicated a willingness to
         receive compressed PPP headers.

         DISCUSSION:
            These options control the appearance of the PPP
            header.  Normally the PPP header consists of the
            address field (one byte containing the value
            0xff), the control field (one byte containing the
            value 0x03), and the two-byte protocol field that
            identifies the contents of the data area of the
            frame.  If a system negotiates address and control
            field compression it indicates to its peer that it
            will accept PPP frames that have or do not have
            these fields at the front of the header.  It does
            not indicate that it will be sending frames with
            these fields removed.  The protocol field may also
            be compressed from two to one byte in most cases.


         IMPLEMENTATION:
            Some hardware does not deal well with variable
            length header information.  In those cases it
            makes most sense for the remote peer to send the
            full PPP header.  Implementations may ensure this
            by not sending the address/control field and
            protocol field compression options to the remote
            peer.  Even if the remote peer has indicated an
            ability to receive compressed headers there is no
            requirement for the local router to send
            compressed headers.

         A router MUST negotiate the Async Control Character
         Map (ACCM) for asynchronous PPP links, but SHOULD NOT





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         negotiate the ACCM for synchronous links.  If a
         router receives an attempt to negotiate the ACCM over
         a synchronous link, it MUST ACKnowledge the option
         and then ignore it.

         DISCUSSION:
            There are implementations that offer both sync and
            async modes of operation and may use the same code
            to implement the option negotiation.  In this
            situation it is possible that one end or the other
            may send the ACCM option on a synchronous link.

         A router SHOULD properly negotiate the maximum
         receive unit (MRU).  Even if a system negotiates an
         MRU smaller than 1,500 bytes, it MUST be able to
         receive a 1,500 byte frame.

         A router SHOULD negotiate and enable the link quality
         monitoring (LQM) option.

         DISCUSSION:
            This memo does not specify a policy for deciding
            whether the link's quality is adequate.  However,
            it is important (see Section [3.3.6]) that a
            router disable failed links.

         A router SHOULD implement and negotiate the magic
         number option for loopback detection.

         A router MAY support the authentication options (PAP
         - password authentication protocol, and/or CHAP -
         challenge handshake authentication protocol).

         A router MUST support 16-bit CRC frame check sequence
         (FCS) and MAY support the 32-bit CRC.


3.3.5.3  IP Control Protocol (ICP) Options

         A router MAY offer to perform IP address negotiation.
         A router MUST accept a refusal (REJect) to perform IP
         address negotiation from the peer.

         A router SHOULD NOT perform Van Jacobson header





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         compression of TCP/IP packets if the link speed is in
         excess of 64 Kbps.  Below that speed the router MAY
         perform Van Jacobson (VJ) header compression.  At
         link speeds of 19,200 bps or less the router SHOULD
         perform VJ header compression.


3.3.6  Interface Testing

      A router MUST have a mechanism to allow routing software
      to determine whether a physical interface is available
      to send packets or not.  A router SHOULD have a
      mechanism to allow routing software to judge the quality
      of a physical interface.  A router MUST have a mechanism
      for informing the routing software when a physical
      interface becomes available or unavailable to send
      packets because of administrative action.  A router MUST
      have a mechanism for informing the routing software when
      it detects a Link level interface has become available
      or unavailable, for any reason.

      DISCUSSION:
         It is crucial that routers have workable mechanisms
         for determining that their network connections are
         functioning properly, since failure to do so (or
         failure to take the proper actions when a problem is
         detected) can lead to black holes.

         The mechanisms available for detecting problems with
         network connections vary considerably, depending on
         the Link Layer protocols in use and also in some
         cases on the interface hardware chosen by the router
         manufacturer.  The intent is to maximize the
         capability to detect failures within the Link-Layer
         constraints.














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4.  INTERNET LAYER -- PROTOCOLS



4.1  INTRODUCTION

   This chapter and chapter 5 discuss the protocols used at
   the Internet Layer: IP, ICMP, and IGMP.  Since forwarding
   is obviously a crucial topic in a document discussing
   routers, chapter 5 limits itself to the aspects of the
   protocols which directly relate to forwarding.  The current
   chapter contains the remainder of the discussion of the
   Internet Layer protocols.


4.2  INTERNET PROTOCOL -- IP



4.2.1  INTRODUCTION

      Routers MUST implement the IP protocol, as defined by
      [INTERNET:1].  They MUST also implement its mandatory
      extensions: subnets (defined in [INTERNET:2]), and IP
      broadcast (defined in [INTERNET:3]).

      A router  MUST be compliant, and SHOULD be
      unconditionally compliant, with the requirements of
      sections 3.2.1 and 3.3 of [INTRO:2], except that:

      +  Section 3.2.1.1 may be ignored, since it duplicates
         requirements found in this memo.

      +  Section 3.2.1.2 may be ignored, since it duplicates
         requirements found in this memo.

      +  Section 3.2.1.3 should be ignored, since it is
         superseded by Section [4.2.2.11] of this memo.

      +  Section 3.2.1.4 may be ignored, since it duplicates
         requirements found in this memo.

      +  Section 3.2.1.6 should be ignored, since it is
         superseded by Section [4.2.2.4] of this memo.





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      +  Section 3.2.1.8 should be ignored, since it is
         superseded by Section [4.2.2.1] of this memo.

      In the following, the action specified in certain cases
      is to "silently discard" a received datagram.  This
      means that the datagram will be discarded without
      further processing and that the router will not send any
      ICMP error message (see Section [4.3]) as a result.
      However, for diagnosis of problems a router SHOULD
      provide the capability of logging the error (see Section
      [1.3.3]), including the contents of the silently-
      discarded datagram, and SHOULD record the event in a
      statistics counter.


4.2.2  PROTOCOL WALK-THROUGH

      RFC 791 is [INTERNET:1], the specification for the
      Internet Protocol.


4.2.2.1  Options: RFC-791 Section 3.2

         In datagrams received by the router itself, the IP
         layer MUST interpret those IP options that it
         understands and preserve the rest unchanged for use
         by higher layer protocols.

         Higher layer protocols may require the ability to set
         IP options in datagrams they send or examine IP
         options in datagrams they receive.  Later sections of
         this document discuss specific IP option support
         required by higher layer protocols.

         DISCUSSION:
            Neither this memo nor [INTRO:2] define the order
            in which a receiver must process multiple options
            in the same IP header.  Hosts and routers
            originating datagrams containing multiple options
            must be aware that this introduces an ambiguity in
            the meaning of certain options when combined with
            a source-route option.

         Here are the requirements for specific IP options:





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         (a)  Security Option

              Some environments require the Security option in
              every packet originated or received.  Routers
              SHOULD IMPLEMENT the revised security option
              described in [INTERNET:5].

              DISCUSSION:
                 Note that the security options described in
                 [INTERNET:1] and RFC 1038 ([INTERNET:16]) are
                 obsolete.

         (b)  Stream Identifier Option

              This option is obsolete; routers SHOULD NOT
              place this option in a datagram that the router
              originates.  This option MUST be ignored in
              datagrams received by the router.

         (c)  Source Route Options

              A router MUST be able to act as the final
              destination of a source route.  If a router
              receives a packet containing a completed source
              route (i.e., the pointer points beyond the last
              field and the destination address in the IP
              header addresses the router), the packet has
              reached its final destination; the option as
              received (the recorded route) MUST be passed up
              to the transport layer (or to ICMP message
              processing).

              In order to respond correctly to source-routed
              datagrams it receives, a router MUST provide a
              means whereby transport protocols and
              applications can reverse the source route in a
              received datagram and insert the reversed source
              route into datagrams they originate (see Section
              4 of [INTRO:2] for details).

              Some applications in the router MAY require that
              the user be able to enter a source route.

              A router MUST NOT originate a datagram





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              containing multiple source route options.  What
              a router should do if asked to forward a packet
              containing multiple source route options is
              described in Section [5.2.4.1].

              When a source route option is created, it MUST
              be correctly formed even if it is being created
              by reversing a recorded route that erroneously
              includes the source host (see case (B) in the
              discussion below).

              DISCUSSION:
                 Suppose a source routed datagram is to be
                 routed from source S to destination D via
                 routers G1, G2, ... Gn.  Source S constructs
                 a datagram with G1's IP address as its
                 destination address, and a source route
                 option to get the datagram the rest of the
                 way to its destination.  However, there is an
                 ambiguity in the specification over whether
                 the source route option in a datagram sent
                 out by S should be (A) or (B):

                 (A):  {>>G2, G3, ... Gn, D}     <--- CORRECT

                 (B):  {S, >>G2, G3, ... Gn, D}  <---- WRONG

                 (where >> represents the pointer).  If (A) is
                 sent, the datagram received at D will contain
                 the option: {G1, G2, ... Gn >>}, with S and D
                 as the IP source and destination addresses.
                 If (B) were sent, the datagram received at D
                 would again contain S and D as the same IP
                 source and destination addresses, but the
                 option would be: {S, G1, ...Gn >>}; i.e., the
                 originating host would be the first hop in
                 the route.

         (d)  Record Route Option

              Routers MAY support the Record Route option in
              datagrams originated by the router.

         (e)  Timestamp Option





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              Routers MAY support the timestamp option in
              datagrams originated by the router.  The
              following rules apply:

              +  When originating a datagram containing a
                 Timestamp Option, a router MUST record a
                 timestamp in the option if

                 -- Its Internet address fields are not pre-
                    specified or
                 -- Its first pre-specified address is the IP
                    address of the logical interface over
                    which the datagram is being sent (or the
                    router's router-id if the datagram is
                    being sent over an unnumbered interface).

              +  If the router itself receives a datagram
                 containing a Timestamp Option, the router
                 MUST insert the current timestamp into the
                 Timestamp Option (if there is space in the
                 option to do so) before passing the option to
                 the transport layer or to ICMP for
                 processing.

              +  A timestamp value MUST follow the rules given
                 in Section [3.2.2.8] of [INTRO:2].

              IMPLEMENTATION:
                 To maximize the utility of the timestamps
                 contained in the timestamp option, it is
                 suggested that the timestamp inserted be, as
                 nearly as practical, the time at which the
                 packet arrived at the router.  For datagrams
                 originated by the router, the timestamp
                 inserted should be, as nearly as practical,
                 the time at which the datagram was passed to
                 the Link Layer for transmission.



4.2.2.2  Addresses in Options: RFC-791 Section 3.1

         When a router inserts its address into a Record
         Route, Strict Source and Record Route, Loose Source





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         and Record Route, or Timestamp, it MUST use the IP
         address of the logical interface on which the packet
         is being sent.  Where this rule cannot be obeyed
         because the output interface has no IP address (i.e.,
         is an unnumbered interface), the router MUST instead
         insert its "router-id".  The router's router-id is
         one of the router's IP addresses.  Which of the
         router's addresses is used as the router-id MUST NOT
         change (even across reboots) unless changed by the
         network manager or unless the configuration of the
         router is changed such that the IP address used as
         the router-id ceases to be one of the router's IP
         addresses.  Routers with multiple unnumbered
         interfaces MAY have multiple router-id's.  Each
         unnumbered interface MUST be associated with a
         particular router-id.  This association MUST NOT
         change (even across reboots) without reconfiguration
         of the router.

         DISCUSSION:
            This specification does not allow for routers
            which do not have at least one IP address.  We do
            not view this as a serious limitation, since a
            router needs an IP address to meet the
            manageability requirements of Chapter [8] even if
            the router is connected only to point-to-point
            links.


         IMPLEMENTATION:
            One possible method of choosing the router-id that
            fulfills this requirement is to use the
            numerically smallest (or greatest) IP address
            (treating the address as a 32-bit integer) that is
            assigned to the router.



4.2.2.3  Unused IP Header Bits: RFC-791 Section 3.1

         The IP header contains two reserved bits: one in the
         Type of Service byte and the other in the Flags
         field.  A router MUST NOT set either of these bits to
         one in datagrams originated by the router.  A router





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         MUST NOT drop (refuse to receive or forward) a packet
         merely because one or more of these reserved bits has
         a non-zero value.

         DISCUSSION:
            Future revisions to the IP protocol may make use
            of these unused bits.  These rules are intended to
            ensure that these revisions can be deployed
            without having to simultaneously upgrade all
            routers in the Internet.



4.2.2.4  Type of Service: RFC-791 Section 3.1

         The "Type-of-Service" byte in the IP header is
         divided into three sections:  the Precedence field
         (high-order 3 bits), a field that is customarily
         called "Type of Service" or "TOS" (next 4 bits), and
         a reserved bit (the low order bit).

         Rules governing the reserved bit were described in
         Section [4.2.2.3].

         A more extensive discussion of the TOS field and its
         use can be found in [ROUTE:11].

         The description of the IP Precedence field is
         superseded by Section [5.3.3].  RFC-795, "Service
         Mappings", is obsolete and SHOULD NOT be implemented.


4.2.2.5  Header Checksum: RFC-791 Section 3.1

         As stated in Section [5.2.2], a router MUST verify
         the IP checksum of any packet which is received.  The
         router MUST NOT provide a means to disable this
         checksum verification.

         IMPLEMENTATION:
            A more extensive description of the IP checksum,
            including extensive implementation hints, can be
            found in [INTERNET:6] and [INTERNET:7].






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4.2.2.6  Unrecognized Header Options: RFC-791 Section 3.1

         A router MUST ignore IP options which it does not
         recognize.  A corollary of this requirement is that a
         router MUST implement the End of Option List option
         and the No Operation option, since neither contains
         an explicit length.

         DISCUSSION:
            All future IP options will include an explicit
            length.



4.2.2.7  Fragmentation: RFC-791 Section 3.2

         Fragmentation, as described in [INTERNET:1], MUST be
         supported by a router.

         When a router fragments an IP datagram, it SHOULD
         minimize the number of fragments.  When a router
         fragments an IP datagram, it MUST send the fragments
         in order.  A fragmentation method which may generate
         one IP fragment which is significantly smaller than
         the other MAY cause the first IP fragment to be the
         smaller one.

         DISCUSSION:
            There are several fragmentation techniques in
            common use in the Internet.  One involves
            splitting the IP datagram into IP fragments with
            the first being MTU sized, and the others being
            approximately the same size, smaller than the MTU.
            The reason for this is twofold.  The first IP
            fragment in the sequence will be the effective MTU
            of the current path between the hosts, and the
            following IP fragments are sized to hopefully
            minimize the further fragmentation of the IP
            datagram.  Another technique is to split the IP
            datagram into MTU sized IP fragments, with the
            last fragment being the only one smaller, as per
            page 26 of [INTERNET:1].

            A common trick used by some implementations of





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            TCP/IP is to fragment an IP datagram into IP
            fragments that are no larger than 576 bytes when
            the IP datagram is to travel through a router.  In
            general, this allows the resulting IP fragments to
            pass the rest of the path without further
            fragmentation.  This would, though, create more of
            a load on the destination host, since it would
            have a larger number of IP fragments to reassemble
            into one IP datagram.  It would also not be
            efficient on networks where the MTU only changes
            once, and stays much larger than 576 bytes (such
            as an 802.5 network with a MTU of 2048 or an
            Ethernet network with an MTU of 1536).

            One other fragmentation technique discussed was
            splitting the IP datagram into approximately equal
            sized IP fragments, with the size being smaller
            than the next hop network's MTU.  This is intended
            to minimize the number of fragments that would
            result from additional fragmentation further down
            the path.

            In most cases, routers should try and create
            situations that will generate the lowest number of
            IP fragments possible.

            Work with slow machines leads us to believe that
            if it is necessary to send small packets in a
            fragmentation scheme, sending the small IP
            fragment first maximizes the chance of a host with
            a slow interface of receiving all the fragments.



4.2.2.8  Reassembly: RFC-791 Section 3.2

         As specified in Section 3.3.2 of [INTRO:2], a router
         MUST support reassembly of datagrams which it
         delivers to itself.










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4.2.2.9  Time to Live: RFC-791 Section 3.2

         Time to Live (TTL) handling for packets originated or
         received by the router is governed by [INTRO:2].
         Note in particular that a router MUST NOT check the
         TTL of a packet except when forwarding it.


4.2.2.10  Multi-subnet Broadcasts: RFC-922

         All-subnets broadcasts (called "multi-subnet
         broadcasts" in [INTERNET:3]) have been deprecated.
         See Section [5.3.5.3].


4.2.2.11  Addressing: RFC-791 Section 3.2

         There are now five classes of IP addresses: Class A
         through Class E.  Class D addresses are used for IP
         multicasting [INTERNET:4], while Class E addresses
         are reserved for experimental use.

         A multicast (Class D) address is a 28-bit logical
         address that stands for a group of hosts, and may be
         either permanent or transient.  Permanent multicast
         addresses are allocated by the Internet Assigned
         Number Authority [INTRO:7], while transient addresses
         may be allocated dynamically to transient groups.
         Group membership is determined dynamically using IGMP
         [INTERNET:4].

         We now summarize the important special cases for
         Unicast (that is class A, B, and C) IP addresses,
         using the following notation for an IP address:

            { <Network-number>, <Host-number> }

         or

            { <Network-number>, <Subnet-number>, <Host-number> }

         and the notation "-1" for a field that contains all 1
         bits and the notation "0" for a field that contains
         all 0 bits.  This notation is not intended to imply





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         that the 1-bits in a subnet mask need be contiguous.

         (a)  { 0, 0 }

              This host on this network.  It MUST NOT be used
              as a source address by routers, except the
              router MAY use this as a source address as part
              of an initialization procedure (e.g., if the
              router is using BOOTP to load its configuration
              information).

              Incoming datagrams with a source address of { 0,
              0 } which are received for local delivery (see
              Section [5.2.3]), MUST be accepted if the router
              implements the associated protocol and that
              protocol clearly defines appropriate action to
              be taken.  Otherwise, a router MUST silently
              discard any locally-delivered datagram whose
              source address is { 0, 0 }.

              DISCUSSION:
                 Some protocols define specific actions to
                 take in response to a received datagram whose
                 source address is { 0, 0 }.  Two examples are
                 BOOTP and ICMP Mask Request.  The proper
                 operation of these protocols often depends on
                 the ability to receive datagrams whose source
                 address is { 0, 0 }.  For most protocols,
                 however, it is best to ignore datagrams
                 having a source address of { 0, 0 } since
                 they were probably generated by a
                 misconfigured host or router.  Thus, if a
                 router knows how to deal with a given
                 datagram having a { 0, 0 } source address,
                 the router MUST accept it.  Otherwise, the
                 router MUST discard it.

              See also Section [4.2.3.1] for a non-standard
              use of { 0, 0 }.

         (b)  { 0, <Host-number> }

              Specified host on this network.  It MUST NOT be
              sent by routers except that the router MAY uses





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              this as a source address as part of an
              initialization procedure by which the it learns
              its own IP address.

         (c)  { -1, -1 }

              Limited broadcast.  It MUST NOT be used as a
              source address.

              A datagram with this destination address will be
              received by every host and router on the
              connected physical network, but will not be
              forwarded outside that network.

         (d)  { <Network-number>, -1 }

              Network Directed Broadcast -- a broadcast
              directed to the specified network.  It MUST NOT
              be used as a source address.  A router MAY
              originate Network Directed Broadcast packets.  A
              router MUST receive Network Directed Broadcast
              packets; however a router MAY have a
              configuration option to prevent reception of
              these packets.  Such an option MUST default to
              allowing reception.

         (e)  { <Network-number>, <Subnet-number>, -1 }

              Subnetwork Directed Broadcast -- a broadcast
              sent to the specified subnet.  It MUST NOT be
              used as a source address.  A router MAY
              originate Network Directed Broadcast packets.  A
              router MUST receive Network Directed Broadcast
              packets; however a router MAY have a
              configuration option to prevent reception of
              these packets.  Such an option MUST default to
              allowing reception.

         (f)  { <Network-number>, -1, -1 }

              All Subnets Directed Broadcast -- a broadcast
              sent to all subnets of the specified subnetted
              network.  It MUST NOT be used as a source
              address.  A router MAY originate Network





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              Directed Broadcast packets.  A router MUST
              receive Network Directed Broadcast packets;
              however a router MAY have a configuration option
              to prevent reception of these packets.  Such an
              option MUST default to allowing reception.

         (g)  { 127, <any> }

              Internal host loopback address.  Addresses of
              this form MUST NOT appear outside a host.

         The <Network-number> is administratively assigned so
         that its value will be unique in the entire world.

         IP addresses are not permitted to have the value 0 or
         -1 for any of the <Host-number>, <Network-number>, or
         <Subnet-number> fields (except in the special cases
         listed above).  This implies that each of these
         fields will be at least two bits long.

         For further discussion of broadcast addresses, see
         Section [4.2.3.1].

         Since (as described in Section [4.2.1]) a router must
         support the subnet extensions to IP, there will be a
         subnet mask of the form: { -1, -1, 0 } associated
         with each of the host's local IP addresses; see
         Sections [4.3.3.9], [5.2.4.2], and [10.2.2].

         When a router originates any datagram, the IP source
         address MUST be one of its own IP addresses (but not
         a broadcast or multicast address).  The only
         exception is during initialization.

         For most purposes, a datagram addressed to a
         broadcast or multicast destination is processed as if
         it had been addressed to one of the router's IP
         addresses; that is to say:

         +  A router MUST receive and process normally any
            packets with a broadcast destination address.

         +  A router MUST receive and process normally any
            packets sent to a multicast destination address





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            which the router is interested in.

         The term "specific-destination address" means the
         equivalent local IP address of the host.  The
         specific-destination address is defined to be the
         destination address in the IP header unless the
         header contains a broadcast or multicast address, in
         which case the specific-destination is an IP address
         assigned to the physical interface on which the
         datagram arrived.

         A router MUST silently discard any received datagram
         containing an IP source address that is invalid by
         the rules of this section.  This validation could be
         done either by the IP layer or by each protocol in
         the transport layer.

         DISCUSSION:
            A misaddressed datagram might be caused by a Link
            Layer broadcast of a unicast datagram or by
            another router or host that is confused or
            misconfigured.



4.2.3  SPECIFIC ISSUES



4.2.3.1  IP Broadcast Addresses

         For historical reasons, there are a number of IP
         addresses (some standard and some not) which are used
         to indicate that an IP packet is an IP broadcast.  A
         router

         (1)  MUST treat as IP broadcasts packets addressed to
              255.255.255.255, { <Network-number>, -1 }, {
              <Network-number>, <Subnet-number>, -1 }, and {
              <Network-number>, -1, -1 }.

         (2)  SHOULD silently discard on receipt (i.e., don't
              even deliver to applications in the router) any
              packet addressed to 0.0.0.0, { <Network-number>,





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              0 }, { <Network-number>, <Subnet-number>, 0 },
              or { <Network-number>, 0, 0 }; if these packets
              are not silently discarded, they MUST be treated
              as IP broadcasts (see Section [5.3.5]).  There
              MAY be a configuration option to allow receipt
              of these packets.  This option SHOULD default to
              discarding them.

         (3)  SHOULD (by default) use the limited broadcast
              address (255.255.255.255) when originating an IP
              broadcast destined for a connected network or
              subnet (except when sending an ICMP Address Mask
              Reply, as discussed in Section [4.3.3.9]).  A
              router MUST receive limited broadcasts.

         (4)  SHOULD NOT originate datagrams addressed to
              0.0.0.0, { <Network-number>, 0 }, { <Network-
              number>, <Subnet-number>, 0 }, or { <Network-
              number>, 0, 0 }.  There MAY be a configuration
              option to allow generation of these packets
              (instead of using the relevant "1s" format
              broadcast).  This option SHOULD default to not
              generating them.

         DISCUSSION:
            In the second bullet, the router obviously cannot
            recognize addresses of the form { <Network-
            number>, <Subnet-number>, 0 } if the router does
            not know how the particular network is subnetted.
            In that case, the rules of the second bullet do
            not apply because, from the point of view of the
            router, the packet is not an IP broadcast packet.



4.2.3.2  IP Multicasting

         An IP router SHOULD satisfy the Host Requirements
         with respect to IP multicasting, as specified in
         Section 3.3.7 of [INTRO:2].  An IP router SHOULD
         support local IP multicasting on all connected
         networks for which a mapping from Class D IP
         addresses to link-layer addresses has been specified
         (see the various IP-over-xxx specifications), and on





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         all connected point-to-point links.  Support for
         local IP multicasting includes originating multicast
         datagrams, joining multicast groups and receiving
         multicast datagrams, and leaving multicast groups.
         This implies support for all of [INTERNET:4]
         including IGMP (see Section [4.4]).

         DISCUSSION:
            Although [INTERNET:4] is entitled Host Extensions
            for IP Multicasting, it applies to all IP systems,
            both hosts and routers.  In particular, since
            routers may join multicast groups, it is correct
            for them to perform the "host" part of IGMP,
            reporting their group memberships to any multicast
            routers that may be present on their attached
            networks (whether or not they themselves are
            multicast routers).

            Some router protocols may specifically require
            support for IP multicasting (e.g., OSPF
            [ROUTE:1]), or may recommend it (e.g., ICMP Router
            Discovery [INTERNET:13]).



4.2.3.3  Path MTU Discovery

         In order to eliminate fragmentation or minimize it,
         it is desirable to know what is the path MTU along
         the path from the source to destination.  The path
         MTU is the minimum of the MTUs of each hop in the
         path.  [INTERNET:14] describes a technique for
         dynamically discovering the maximum transmission unit
         (MTU) of an arbitrary internet path.  For a path that
         passes through a router that does not support
         [INTERNET:14], this technique might not discover the
         correct Path MTU, but it will always choose a Path
         MTU as accurate as, and in many cases more accurate
         than, the Path MTU that would be chosen by older
         techniques or the current practice.

         When a router is originating an IP datagram, it
         SHOULD use the scheme described in [INTERNET:14] to
         limit the datagram's size.  If the router's route to





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         the datagram's destination was learned from a routing
         protocol that provides Path MTU information, the
         scheme described in [INTERNET:14] is still used, but
         the Path MTU information from the routing protocol
         SHOULD be used as the initial guess as to the Path
         MTU and also as an upper bound on the Path MTU.


4.2.3.4  Subnetting

         Under certain circumstances, it may be desirable to
         support subnets of a particular network being
         interconnected only via a path which is not part of
         the subnetted network.  This is known as
         discontiguous subnetwork support.

         Routers MUST support discontiguous subnetworks.

         IMPLEMENTATION:
            In general, a router should not make assumptions
            about what are subnets and what are not, but
            simply ignore the concept of Class in networks,
            and treat each route as a { network, mask }-tuple.


         DISCUSSION:
            The Internet has been growing at a tremendous rate
            of late.  This has been placing severe strains on
            the IP addressing technology.  A major factor in
            this strain is the strict IP Address class
            boundaries.  These make it difficult to
            efficiently size network numbers to their networks
            and aggregate several network numbers into a
            single route advertisement.  By eliminating the
            strict class boundaries of the IP address and
            treating each route as a {network number,
            mask}-tuple these strains may be greatly reduced.

            The technology for currently doing this is
            Classless Interdomain Routing (CIDR)
            [INTERNET:15].

         Furthermore, for similar reasons, a subnetted network
         need not have a consistent subnet mask through all





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         parts of the network.  For example, one subnet may
         use an 8 bit subnet mask, another 10 bit, and another
         6 bit.  This is known as variable subnet-masks.

         Routers MUST support variable subnet-masks.


4.3  INTERNET CONTROL MESSAGE PROTOCOL -- ICMP



4.3.1  INTRODUCTION

      ICMP is an auxiliary protocol, which provides routing,
      diagnostic and and error functionality for IP. It is
      described in [INTERNET:8].  A router MUST support ICMP.

      ICMP messages are grouped in two classes which are
      discussed in the following sections:

      ICMP error messages:

      Destination Unreachable     Section 4.3.3.1
      Redirect                    Section 4.3.3.2
      Source Quench               Section 4.3.3.3
      Time Exceeded               Section 4.3.3.4
      Parameter Problem           Section 4.3.3.5

      ICMP query messages:
      Echo                        Section 4.3.3.6
      Information                 Section 4.3.3.7
      Timestamp                   Section 4.3.3.8
      Address Mask                Section 4.3.3.9
      Router Discovery            Section 4.3.3.10


      General ICMP requirements and discussion are in the next
      section.


4.3.2  GENERAL ISSUES








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4.3.2.1  Unknown Message Types

         If an ICMP message of unknown type is received, it
         MUST be passed to the ICMP user interface (if the
         router has one) or silently discarded (if the router
         doesn't have one).


4.3.2.2  ICMP Message TTL

         When originating an ICMP message, the router MUST
         initialize the TTL.  The TTL for ICMP responses must
         not be taken from the packet which triggered the
         response.


4.3.2.3  Original Message Header

         Every ICMP error message includes the Internet header
         and at least the first 8 data bytes of the datagram
         that triggered the error.  More than 8 bytes MAY be
         sent, but the resulting ICMP datagram SHOULD have a
         length of less than or equal to 576 bytes.  The
         returned IP header (and user data) MUST be identical
         to that which was received, except that the router is
         not required to undo any modifications to the IP
         header that are normally performed in forwarding that
         were performed before the error was detected (e.g.,
         decrementing the TTL, updating options).  Note that
         the requirements of Section [4.3.3.5] supersede this
         requirement in some cases (i.e., for a Parameter
         Problem message, if the problem  is in a modified
         field, the router must "undo" the modification).  See
         Section [4.3.3.5])


4.3.2.4  ICMP Message Source Address

         Except where this document specifies otherwise, the
         IP source address in an ICMP message originated by
         the router MUST be one of the IP addresses associated
         with the physical interface over which the ICMP
         message is transmitted.  If the interface has no IP
         addresses associated with it, the router's router-id





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         (see Section [5.2.5]) is used instead.


4.3.2.5  TOS and Precedence

         ICMP error messages SHOULD have their TOS bits set to
         the same value as the TOS bits in the packet which
         provoked the sending of the ICMP error message,
         unless setting them to that value would cause the
         ICMP error message to be immediately discarded
         because it could not be routed to its destination.
         Otherwise, ICMP error messages MUST be sent with a
         normal (i.e. zero) TOS.  An ICMP reply message SHOULD
         have its TOS bits set to the same value as the TOS
         bits in the ICMP request that provoked the reply.

         EDITOR'S COMMENTS:
            The following paragraph originally read:

               ICMP error messages MUST have their IP
               Precedence field set to the same value as the
               IP Precedence field in the packet which
               provoked the sending of the ICMP error message,
               except that the precedence value MUST be 6
               (INTERNETWORK CONTROL) or 7 (NETWORK CONTROL),
               SHOULD be 7, and MAY be settable for the
               following types of ICMP error messages:
               Unreachable, Redirect, Time Exceeded, and
               Parameter Problem.

            I believe that the following paragraph is
            equivalent and easier for humans to parse (Source
            Quench is the only other ICMP Error message).
            Other interpretations of the original are sought.

         ICMP Source Quench error messages MUST have their IP
         Precedence field set to the same value as the IP
         Precedence field in the packet which provoked the
         sending of the ICMP Source Quench message.  All other
         ICMP error messages (Destination Unreachable,
         Redirect, Time Exceeded, and Parameter Problem) MUST
         have their precedence value set to 6 (INTERNETWORK
         CONTROL) or 7 (NETWORK CONTROL), SHOULD be 7.  The IP
         Precedence value for these error messages MAY be





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         settable.

         An ICMP reply message MUST have its IP Precedence
         field set to the same value as the IP Precedence
         field in the ICMP request that provoked the reply.


4.3.2.6  Source Route

         If the packet which provokes the sending of an ICMP
         error message contains a source route option, the
         ICMP error message SHOULD also contain a source route
         option of the same type (strict or loose), created by
         reversing the portion before the pointer of the route
         recorded in the source route option of the original
         packet UNLESS the ICMP error message is an ICMP
         Parameter Problem complaining about a source route
         option in the original packet.

         DISCUSSION:
            In environments which use the U.S. Department of
            Defense security option (defined in [INTERNET:5]),
            ICMP messages may need to include a security
            option.  Detailed information on this topic should
            be available from the Defense Communications
            Agency.



4.3.2.7  When Not to Send ICMP Errors

         An ICMP error message MUST NOT be sent as the result
         of receiving:

         +  An ICMP error message, or

         +  A packet which fails the IP header validation
            tests described in Section [5.2.2] (except where
            that section specifically permits the sending of
            an ICMP error message), or

         +  A packet destined to an IP broadcast or IP
            multicast address, or






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         +  A packet sent as a Link Layer broadcast or
            multicast, or

         +  A packet whose source address has a network number
            of zero or is an invalid source address (as
            defined in Section [5.3.7]), or

         +  Any fragment of a datagram other then the first
            fragment (i.e., a packet for which the fragment
            offset in the IP header is nonzero).

         Furthermore, an ICMP error message MUST NOT be sent
         in any case where this memo states that a packet is
         to be "silently discarded".

         NOTE:  THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY
         REQUIREMENT ELSEWHERE IN THIS DOCUMENT FOR SENDING
         ICMP ERROR MESSAGES.

         DISCUSSION:
            These rules aim to prevent the "broadcast storms"
            that have resulted from routers or hosts returning
            ICMP error messages in response to broadcast
            packets.  For example, a broadcast UDP packet to a
            non-existent port could trigger a flood of ICMP
            Destination Unreachable datagrams from all devices
            that do not have a client for that destination
            port.  On a large Ethernet, the resulting
            collisions can render the network useless for a
            second or more.

            Every packet that is broadcast on the connected
            network should have a valid IP broadcast address
            as its IP destination (see Section [5.3.4] and
            [INTRO:2]).  However, some devices violate this
            rule.  To be certain to detect broadcast packets,
            therefore, routers are required to check for a
            link-layer broadcast as well as an IP-layer
            address.


         IMPLEMENTATION:
            This requires that the link layer inform the IP
            layer when a link-layer broadcast packet has been





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            received; see Section [3.1].



4.3.2.8  Rate Limiting

         A router which sends ICMP Source Quench messages MUST
         be able to limit the rate at which the messages can
         be generated.  A router SHOULD also be able to limit
         the rate at which it sends other sorts of ICMP error
         messages (Destination Unreachable, Redirect, Time
         Exceeded, Parameter Problem).  The rate limit
         parameters SHOULD be settable as part of the
         configuration of the router.  How the limits are
         applied (e.g., per router or per interface) is left
         to the implementor's discretion.

         DISCUSSION:
            Two problems for a router sending ICMP error
            message are:
            (1)  The consumption of bandwidth on the reverse
                 path, and
            (2)  The use of router resources (e.g., memory,
                 CPU time)

            To help solve these problems a router can limit
            the frequency with which it generates ICMP error
            messages.  For similar reasons, a router may limit
            the frequency at which some other sorts of
            messages, such as ICMP Echo Replies, are
            generated.


         IMPLEMENTATION:
            Various mechanisms have been used or proposed for
            limiting the rate at which ICMP messages are sent:

            (1)  Count-based -- for example, send an ICMP
                 error message for every N dropped packets
                 overall or per given source host.  This
                 mechanism might be appropriate for ICMP
                 Source Quench, but probably not for other
                 types of ICMP messages.






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            (2)  Timer-based -- for example, send an ICMP
                 error message to a given source host or
                 overall at most once per T milliseconds.

            (3)  Bandwidth-based -- for example, limit the
                 rate at which ICMP messages are sent over a
                 particular interface to some fraction of the
                 attached network's bandwidth.



4.3.3  SPECIFIC ISSUES



4.3.3.1  Destination Unreachable

         If a route can not forward a packet because it has no
         routes at all to the destination network specified in
         the packet then the router MUST generate a
         Destination Unreachable, Code 0 (Network Unreachable)
         ICMP message.  If the router does have routes to the
         destination network specified in the packet but the
         TOS specified for the routes is neither the default
         TOS (0000) nor the TOS of the packet that the router
         is attempting to route, then the router MUST generate
         a Destination Unreachable, Code 11 (Network
         Unreachable for TOS) ICMP message.

         If a packet is to be forwarded to a host on a network
         that is directly connected to the router (i.e., the
         router is the last-hop router) and the router has
         ascertained that there is no path to the destination
         host then the router MUST generate a Destination
         Unreachable, Code 1 (Host Unreachable) ICMP message.
         If a packet is to be forwarded to a host that is on a
         network that is directly connected to the router and
         the router cannot forward the packet because because
         no route to the destination has a TOS that is either
         equal to the TOS requested in the packet or is the
         default TOS (0000) then the router MUST generate a
         Destination Unreachable, Code 12 (Host Unreachable
         for TOS) ICMP message.






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         DISCUSSION:
            The intent is that a router generates the
            "generic" host/network unreachable if it has no
            path at all (including default routes) to the
            destination.  If the router has one or more paths
            to the destination, but none of those paths have
            an acceptable TOS, then the router generates the
            "unreachable for TOS" message.



4.3.3.2  Redirect

         The ICMP Redirect message is generated to inform a
         host on the same subnet that the router used by the
         host to route certain packets should be changed.

         Contrary to section 3.2.2.2 of [INTRO:2], a router
         MAY ignore ICMP Redirects when choosing a path for a
         packet originated by the router if the router is
         running a routing protocol or if forwarding is
         enabled on the router and on the interface over which
         the packet is being sent.


4.3.3.3  Source Quench

         A router SHOULD NOT originate ICMP Source Quench
         messages.  As specified in Section [4.3.2], a router
         which does originate Source Quench messages MUST be
         able to limit the rate at which they are generated.

         DISCUSSION:
            Research seems to suggest that Source Quench
            consumes network bandwidth but is an ineffective
            (and unfair) antidote to congestion.  See, for
            example, [INTERNET:9] and [INTERNET:10].  Section
            [5.3.6] discusses the current thinking on how
            routers ought to deal with overload and network
            congestion.

         A router MAY ignore any ICMP Source Quench messages
         it receives.






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         DISCUSSION:
            A router itself may receive a Source Quench as the
            result of originating a packet sent to another
            router or host.  Such datagrams might be, e.g., an
            EGP update sent to another router, or a telnet
            stream sent to a host.  A mechanism has been
            proposed ([INTERNET:11], [INTERNET:12]) to make
            the IP layer respond directly to Source Quench by
            controlling the rate at which packets are sent,
            however, this proposal is currently experimental
            and not currently recommended.



4.3.3.4  Time Exceeded

         When a router is forwarding a packet and the TTL
         field of the packet is reduced to 0, the requirements
         of section [5.2.3.8] apply.

         When the router is reassembling a packet that is
         destined for the router, it MUST fulfill requirements
         of [INTRO:2], section [3.3.2] apply.

         When the router receives (i.e., is destined for the
         router) a Time Exceeded message, it MUST comply with
         section 3.2.2.4 of [INTRO:2].


4.3.3.5  Parameter Problem

         A router MUST generate a Parameter Problem message
         for any error not specifically covered by another
         ICMP message.  The IP header field or IP option
         including the byte indicated by the pointer field
         MUST be included unchanged in the IP header returned
         with this ICMP message.  Section [4.3.2] defines an
         exception to this requirement.

         A new variant of the Parameter Problem message was
         defined in [INTRO:2]:
              Code 1 = required option is missing.

         DISCUSSION:





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            This variant is currently in use in the military
            community for a missing security option.



4.3.3.6  Echo Request/Reply

         A router MUST implement an ICMP Echo server function
         that receives Echo Requests and sends corresponding
         Echo Replies.  A router MUST be prepared to receive,
         reassemble and echo an ICMP Echo Request datagram at
         least as large as the maximum of 576 and the MTUs of
         all the connected networks.

         The Echo server function MAY choose not to respond to
         ICMP echo requests addressed to IP broadcast or IP
         multicast addresses.

         A router SHOULD have a configuration option which, if
         enabled, causes the router to silently ignore all
         ICMP echo requests; if provided, this option MUST
         default to allowing responses.

         DISCUSSION:
            The neutral provision about responding to
            broadcast and multicast Echo Requests results from
            the conclusions reached in section [3.2.2.6] of
            [INTRO:2].

         As stated in Section [10.3.3], a router MUST also
         implement an user/application-layer interface for
         sending an Echo Request and receiving an Echo Reply,
         for diagnostic purposes.  All ICMP Echo Reply
         messages MUST be passed to this interface.

         The IP source address in an ICMP Echo Reply MUST be
         the same as the specific-destination address of the
         corresponding ICMP Echo Request message.

         Data received in an ICMP Echo Request MUST be
         entirely included in the resulting Echo Reply.

         If a Record Route and/or Timestamp option is received
         in an ICMP Echo Request, this option (these options)





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         SHOULD be updated to include the current router and
         included in the IP header of the Echo Reply message,
         without "truncation".  Thus, the recorded route will
         be for the entire round trip.

         If a Source Route option is received in an ICMP Echo
         Request, the return route MUST be reversed and used
         as a Source Route option for the Echo Reply message.


4.3.3.7  Information Request/Reply

         A router SHOULD NOT originate or respond to these
         messages.

         DISCUSSION:
            The Information Request/Reply pair was intended to
            support self-configuring systems such as diskless
            workstations, to allow them to discover their IP
            network numbers at boot time.  However, these
            messages are now obsolete.  The RARP and BOOTP
            protocols provide better mechanisms for a host to
            discover its own IP address.



4.3.3.8  Timestamp and Timestamp Reply

         A router MAY implement Timestamp and Timestamp Reply.
         If they are implemented then:

         +  The ICMP Timestamp server function MUST return a
            Timestamp Reply to every Timestamp message that is
            received.  It SHOULD be designed for minimum
            variability in delay.

         +  An ICMP Timestamp Request message to an IP
            broadcast or IP multicast address MAY be silently
            discarded.

         +  The IP source address in an ICMP Timestamp Reply
            MUST be the same as the specific-destination
            address of the corresponding Timestamp Request
            message.





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         +  If a Source Route option is received in an ICMP
            Timestamp Request, the return route MUST be
            reversed and used as a Source Route option for the
            Timestamp Reply message.

         +  If a Record Route and/or Timestamp option is
            received in a Timestamp Request, this (these)
            option(s) SHOULD be updated to include the current
            router and included in the IP header of the
            Timestamp Reply message.

         +  If the router provides an application-layer
            interface for sending Timestamp Request messages
            then incoming Timestamp Reply messages MUST be
            passed up to the ICMP user interface.

         The preferred form for a timestamp value (the
         "standard value") is milliseconds since midnight,
         Universal Time.  However, it may be difficult to
         provide this value with millisecond resolution. For
         example, many systems use clocks that update only at
         line frequency, 50 or 60 times per second.
         Therefore, some latitude is allowed in a "standard
         value":

         (a)  A "standard value" MUST be updated at least 16
              times per second (i.e., at most the six low-
              order bits of the value may be undefined).

         (b)  The accuracy of a "standard value" MUST
              approximate that of operator-set CPU clocks,
              i.e., correct within a few minutes.

         IMPLEMENTATION:
            To meet the second condition, a router may need to
            query some time server when the router is booted
            or restarted. It is recommended that the UDP Time
            Server Protocol be used for this purpose. A more
            advanced implementation would use the Network Time
            Protocol (NTP) to achieve nearly millisecond clock
            synchronization; however, this is not required.








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4.3.3.9  Address Mask Request/Reply

         A router MUST implement support for receiving ICMP
         Address Mask Request messages and responding with
         ICMP Address Mask Reply messages.  These messages are
         defined in [INTERNET:2].

         A router SHOULD have a configuration option for each
         logical interface specifying whether the router is
         allowed to answer Address Mask Requests for that
         interface; this option MUST default to allowing
         responses.  A router MUST NOT respond to an Address
         Mask Request before the router knows the correct
         subnet mask.

         A router MUST NOT respond to an Address Mask Request
         which has a source address of 0.0.0.0 and which
         arrives on a physical interface which has associated
         with it multiple logical interfaces and the subnet
         masks for those interfaces are not all the same.

         A router SHOULD examine all ICMP Address Mask Replies
         which it receives to determine whether the
         information it contains matches the router's
         knowledge of the subnet mask.  If the ICMP Address
         Mask Reply appears to be in error, the router SHOULD
         log the subnet mask and the sender's IP address.  A
         router MUST NOT use the contents of an ICMP Address
         Mask Reply to determine the correct subnet mask.

         Because hosts may not be able to learn the subnet
         mask if a router is down when the host boots up, a
         router MAY broadcast a gratuitous ICMP Address Mask
         Reply on each of its logical interfaces after it has
         configured its own subnet masks.  However, this
         feature can be dangerous in environments which use
         variable length subnet masks.  Therefore, if this
         feature is implemented, gratuitous Address Mask
         Replies MUST NOT be broadcast over any logical
         interface(s) which either:

         +  Are not configured to send gratuitous Address Mask
            Replies.  Each logical interface MUST have a
            configuration parameter controlling this, and that





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            parameter MUST default to not sending the
            gratuitous Address Mask Replies.

         +  Share the same IP network number and physical
            interface but have different subnet masks.

         The { <Network-number>, -1, -1 } form (on subnetted
         networks) or the { <Network-number>, -1 } form (on
         non-subnetted networks) of the IP broadcast address
         MUST be used for broadcast Address Mask Replies.

         DISCUSSION:
            The ability to disable sending Address Mask
            Replies by routers is required at a few sites
            which intentionally lie to their hosts about the
            subnet mask.  The need for this is expected to go
            away as more and more hosts become compliant with
            the Host Requirements standards.

            The reason for both the second bullet above and
            the requirement about which IP broadcast address
            to use is to prevent problems when multiple IP
            networks or subnets are in use on the same
            physical network.



4.3.3.10  Router Advertisement and Solicitations

         An IP router MUST support the router part of the ICMP
         Router Discovery Protocol [INTERNET:13] on all
         connected networks on which the router supports
         either IP multicast or IP broadcast addressing.  The
         implementation MUST include all of the configuration
         variables specified for routers, with the specified
         defaults.

         DISCUSSION:
            Routers are not required to implement the host
            part of the ICMP Router Discovery Protocol, but
            might find it useful for operation while IP
            forwarding is disabled (i.e., when operating as a
            host).






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         DISCUSSION:
            We note that it is quite common for hosts to use
            RIP as the "router discovery" protocol.  Such
            hosts listen to RIP traffic and use and use
            information extracted from that traffic to
            discover routers and to make decisions as to which
            router to use as a first-hop router for a given
            destination.  While this behavior is discouraged,
            it is still common and implementors should be
            aware of it.



4.4  INTERNET GROUP MANAGEMENT PROTOCOL -- IGMP

   IGMP [INTERNET:4] is a protocol used between hosts and
   multicast routers on a single physical network to establish
   hosts' membership in particular multicast groups.
   Multicast routers use this information, in conjunction with
   a multicast routing protocol, to support IP multicast
   forwarding across the Internet.

   A router SHOULD implement the host part of IGMP.


























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5.  INTERNET LAYER -- FORWARDING



5.1  INTRODUCTION

   This section describes the process of forwarding packets.


5.2  FORWARDING WALK-THROUGH

   There is no separate specification of the forwarding
   function in IP.  Instead, forwarding is covered by the
   protocol specifications for the internet layer protocols
   ([INTERNET:1], [INTERNET:2], [INTERNET:3], [INTERNET:8],
   and [ROUTE:11]).


5.2.1  Forwarding Algorithm

      Since none of the primary protocol documents describe
      the forwarding algorithm in any detail, we present it
      here.  This is just a general outline, and omits
      important details, such as handling of congestion, that
      are dealt with in later sections.

      It is not required that an implementation follow exactly
      the algorithms given in sections [5.2.1.1], [5.2.1.2],
      and [5.2.1.3].  Much of the challenge of writing router
      software is to maximize the rate at which the router can
      forward packets while still achieving the same effect of
      the algorithm.  Details of how to do that are beyond the
      scope of this document, in part because they are heavily
      dependent on the architecture of the router.  Instead,
      we merely point out the order dependencies among the
      steps:

      (1)  A router MUST verify the IP header, as described in
           section [5.2.2], before performing any actions
           based on the contents of the header.  This allows
           the router to detect and discard bad packets before
           the expenditure of other resources.

      (2)  Processing of certain IP options requires that the





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           router insert its IP address into the option.  As
           noted in Section [5.2.4], the address inserted MUST
           be the address of the logical interface on which
           the packet is sent or the router's router-id if the
           packet is sent over an unnumbered interface.  Thus,
           processing of these options cannot be completed
           until after the output interface is chosen.

      (3)  The router cannot check and decrement the TTL
           before checking whether the packet should be
           delivered to the router itself, for reasons
           mentioned in Section [4.2.2.9].

      (4)  More generally, when a packet is delivered locally
           to the router, its IP header MUST NOT be modified
           in any way (except that a router may be required to
           insert a timestamp into any Timestamp options in
           the IP header).  Thus, before the router determines
           whether the packet is to be delivered locally to
           the router, it cannot update the IP header in any
           way that it is not prepared to undo.


5.2.1.1  General

         This section covers the general forwarding algorithm.
         This algorithm applies to all forms of packets to be
         forwarded: unicast, multicast, and broadcast.


         (1)  The router receives the IP packet (plus
              additional information about it, as described in
              Section [3.1]) from the Link Layer.

         (2)  The router validates the IP header, as described
              in Section [5.2.2].  Note that IP reassembly is
              not done, except on IP fragments to be queued
              for local delivery in step (4).

         (3)  The router performs most of the processing of
              any IP options.  As described in Section
              [5.2.4], some IP options require additional
              processing after the routing decision has been
              made.





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         (4)  The router examines the destination IP address
              of the IP datagram, as described in Section
              [5.2.3], to determine how it should continue to
              process the IP datagram.  There are three
              possibilities:

              +  The IP datagram is destined for the router,
                 and should be queued for local delivery,
                 doing reassembly if needed.

              +  The IP datagram is not destined for the
                 router, and should be queued for forwarding.

              +  The IP datagram should be queued for
                 forwarding, but (a copy) must also be queued
                 for local delivery.


5.2.1.2  Unicast

         Since the local delivery case is well-covered by
         [INTRO:2], the following assumes that the IP datagram
         was queued for forwarding.  If the destination is an
         IP unicast address:

         (5)  The forwarder determines the next hop IP address
              for the packet, usually by looking up the
              packet's destination in the router's routing
              table.  This procedure is described in more
              detail in Section [5.2.4].  This procedure also
              decides which network interface should be used
              to send the packet.

         (6)  The forwarder verifies that forwarding the
              packet is permitted.  The source and destination
              addresses should be valid, as described in
              Section [5.3.7] and Section [5.3.4] If the
              router supports administrative constraints on
              forwarding, such as those described in Section
              [5.3.9], those constraints must be satisfied.

         (7)  The forwarder decrements (by at least one) and
              checks the packet's TTL, as described in Section
              [5.3.1].





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         (8)  The forwarder performs any IP option processing
              that could not be completed in step 3.

         (9)  The forwarder performs any necessary IP
              fragmentation, as described in Section
              [4.2.2.7].  Since this step occurs after
              outbound interface selection (step 5), all
              fragments of the same datagram will be
              transmitted out the same interface.

         (10) The forwarder determines the Link Layer address
              of the packet's next hop.  The mechanisms for
              doing this are Link Layer-dependent (see chapter
              3).

         (11) The forwarder encapsulates the IP datagram (or
              each of the fragments thereof) in an appropriate
              Link Layer frame and queues it for output on the
              interface selected in step 5.

         (12) The forwarder sends an ICMP redirect if
              necessary, as described in Section [4.3.3.2].


5.2.1.3  Multicast

         If the destination is an IP multicast, the following
         steps are taken.

         Note that the main differences between the forwarding
         of IP unicasts and the forwarding of IP multicasts
         are

         +  IP multicasts are usually forwarded based on both
            the datagram's source and destination IP
            addresses,

         +  IP multicast uses an expanding ring search,

         +  IP multicasts are forwarded as Link Level
            multicasts, and

         +  ICMP errors are never sent in response to IP
            multicast datagrams.





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         Note that the forwarding of IP multicasts is still
         somewhat experimental. As a result, the algorithm
         presented below is not mandatory, and is provided as
         an example only.

         (5a) Based on the IP source and destination addresses
              found in the datagram header, the router
              determines whether the datagram has been
              received on the proper interface for forwarding.
              If not, the datagram is dropped silently.  The
              method for determining the proper receiving
              interface depends on the multicast routing
              algorithm(s) in use. In one of the simplest
              algorithms, reverse path forwarding (RPF), the
              proper interface is the one that would be used
              to forward unicasts back to the datagram source.

         (6a) Based on the IP source and destination addresses
              found in the datagram header, the router
              determines the datagram's outgoing interfaces.
              In order to implement IP multicast's expanding
              ring search (see [INTERNET:4]) a minimum TTL
              value is specified for each outgoing interface.
              A copy of the multicast datagram is forwarded
              out each outgoing interface whose minimum TTL
              value is less than or equal to the TTL value in
              the datagram header, by separately applying the
              remaining steps on each such interface.

         (7a) The router decrements the packet's TTL by one.

         (8a) The forwarder performs any IP option processing
              that could not be completed in step (3).

         (9a) The forwarder performs any necessary IP
              fragmentation, as described in Section
              [4.2.2.7].

         (10a) The forwarder determines the Link Layer address
              to use in the Link Level encapsulation. The
              mechanisms for doing this are Link Layer-
              dependent. On LANs a Link Level multicast or
              broadcast is selected, as an algorithmic
              translation of the datagrams' class D





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              destination address.  See the various IP-over-
              xxx specifications for more details.

         (11a) The forwarder encapsulates the packet (or each
              of the fragments thereof) in an appropriate Link
              Layer frame and queues it for output on the
              appropriate interface.


5.2.2  IP Header Validation

      Before a router can process any IP packet, it MUST
      perform a the following basic validity checks on the
      packet's IP header to ensure that the header is
      meaningful.  If the packet fails any of the following
      tests, it MUST be silently discarded, and the error
      SHOULD be logged.

      (1)  The packet length reported by the Link Layer must
           be large enough to hold the minimum length legal IP
           datagram (20 bytes).

      (2)  The IP checksum must be correct.

      (3)  The IP version number must be 4.  If the version
           number is not 4 then the packet may well be another
           version of IP, such as ST-II.

      (4)  The IP header length field must be at least 5.

      (5)  The IP total length field must be at least 4 * IP
           header length field.

      A router MUST NOT have a configuration option which
      allows disabling any of these tests.

      If the packet passes the second and third tests, the IP
      header length field is at least 4, and both the IP total
      length field and the packet length reported by the Link
      Layer are at least 16 then, despite the above rule, the
      router MAY respond with an ICMP Parameter Problem
      message, whose pointer points at the IP header length
      field (if it failed the fourth test) or the IP total
      length field (if it failed the fifth test).  However, it





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      still MUST discard the packet and still SHOULD log the
      error.

      These rules (and this entire document) apply only to
      version 4 of the Internet Protocol.  These rules should
      not be construed as prohibiting routers from supporting
      other versions of IP.  Furthermore, if a router can
      truly classify a packet as being some other version of
      IP then it ought not treat that packet as an error
      packet within the context of this memo.

      IMPLEMENTATION:
         It is desirable for purposes of error reporting,
         though not always entirely possible, to determine why
         a header was invalid.  There are four possible
         reasons:

         +  The Link Layer truncated the IP header

         +  The datagram is using a version of IP other than
            the standard one (version 4).

         +  The IP header has been corrupted in transit.

         +  The sender generated an illegal IP header.

         It is probably desirable to perform the checks in the
         order listed, since we believe that this ordering is
         most likely to correctly categorize the cause of the
         error.  For purposes of error reporting, it may also
         be desirable to check if a packet which fails these
         tests has an IP version number equal to 6.  If it
         does, the packet is probably an ST-II datagram and
         should be treated as such.  ST-II is described in
         [FORWARD:1].

      Additionally, the router SHOULD verify that the packet
      length reported by the Link Layer is at least as large
      as the IP total length recorded in the packet's IP
      header.  If it appears that the packet has been
      truncated, the packet MUST be discarded, the error
      SHOULD be logged, and the router SHOULD respond with an
      ICMP Parameter Problem message whose pointer points at
      the IP total length field.





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      DISCUSSION:
         Because any higher layer protocol which concerns
         itself with data corruption will detect truncation of
         the packet data when it reaches its final
         destination, it is not absolutely necessary for
         routers to perform the check suggested above in order
         to maintain protocol correctness.  However, by making
         this check a router can simplify considerably the
         task of determining which hop in the path is
         truncating the packets.  It will also reduce the
         expenditure of resources "down-stream" from the
         router in that down-stream systems will not need to
         deal with the packet.

      Finally, if the destination address in the IP header is
      not one of the addresses of the router, the router
      SHOULD verify that the packet does not contain a Strict
      Source and Record Route option.  If a packet fails this
      test, the router SHOULD log the error and SHOULD respond
      with an ICMP Parameter Problem error with the pointer
      pointing at the offending packet's IP destination
      address.

      DISCUSSION:
         Some people might suggest that the router should
         respond with a Bad Source Route message instead of a
         Parameter Problem message.  However, when a packet
         fails this test, it usually indicates a protocol
         error by the previous hop router, whereas Bad Source
         Route would suggest that the source host had
         requested a nonexistent or broken path through the
         network.



5.2.3  Local Delivery Decision

      When a router receives an IP packet, it must decide
      whether the packet is addressed to the router (and
      should be delivered locally) or the packet is addressed
      to another system (and should be handled by the
      forwarder).  There is also a hybrid case, where certain
      IP broadcasts and IP multicasts are both delivered
      locally and forwarded.  A router MUST determine which of





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      the these three cases applies using the following rules:

      +  An unexpired source route option is one whose pointer
         value does not point past the last entry in the
         source route.  If the packet contains an unexpired
         source route option, the pointer in the option is
         advanced until either the pointer does point past the
         last address in the option or else the next address
         is not one of the router's own addresses.  In the
         latter (normal) case, the  packet is forwarded (and
         not delivered locally) regardless of the rules below.

      +  The packet is delivered locally and not considered
         for forwarding in the following cases:

         -- The packet's destination address exactly matches
            one of the router's IP addresses,

         -- The packet's destination address is a limited
            broadcast address ({-1, -1}), and

         -- The packet's destination is an IP multicast
            address which is limited to a single subnet (such
            as 224.0.0.1 or 224.0.0.2) and (at least) one of
            the logical interfaces associated with the
            physical interface on which the packet arrived is
            a member of the destination multicast group.

      +  The packet is passed to the forwarder AND delivered
         locally in the following cases:

         -- The packet's destination address is an IP
            broadcast address that addresses at least one of
            the router's logical interfaces but does not
            address any of the logical interfaces associated
            with the physical interface on which the packet
            arrived

         -- The packet's destination is an IP multicast
            address which is not limited to a single
            subnetwork (such as 224.0.0.1 and 224.0.0.2 are)
            and (at least) one of the logical interfaces
            associated with the physical interface on which
            the packet arrived is a member of the destination





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            multicast group.

      +  The packet is delivered locally if the packet's
         destination address is an IP broadcast address (other
         than a limited broadcast address) that addresses at
         least one of the logical interfaces associated with
         the physical interface on which the packet arrived.
         The packet is ALSO passed to the forwarder unless the
         link on which the packet arrived uses an IP
         encapsulation that does not encapsulate broadcasts
         differently than unicasts (e.g. by using different
         Link Layer destination addresses).

      +  The packet is passed to the forwarder in all other
         cases.

      DISCUSSION:
         The purpose of the requirement in the last sentence
         of the fourth bullet is to deal with a directed
         broadcast to another net or subnet on the same
         physical cable.  Normally, this works as expected:
         the sender sends the broadcast to the router as a
         Link Layer unicast.  The router notes that it arrived
         as a unicast, and therefore must be destined for a
         different logical net (or subnet) than the sender
         sent it on.  Therefore, the router can safely send it
         as a Link Layer broadcast out the same (physical)
         interface over which it arrived.  However, if the
         router can't tell whether the packet was received as
         a Link Layer unicast, the sentence ensures that the
         router does the safe but wrong thing rather than the
         unsafe but right thing.


      IMPLEMENTATION:
         As described in Section [5.3.4], packets received as
         Link Layer broadcasts are generally not forwarded.
         It may be advantageous to avoid passing to the
         forwarder packets it would later discard because of
         the rules in that section.

         Some Link Layers (either because of the hardware or
         because of special code in the drivers) can deliver
         to the router copies of all Link Layer broadcasts and





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         multicasts it transmits.  Use of this feature can
         simplify the implementation of cases where a packet
         has to both be passed to the forwarder and delivered
         locally, since forwarding the packet will
         automatically cause the router to receive a copy of
         the packet that it can then deliver locally.  One
         must use care in these circumstances in order to
         prevent treating a received loop-back packet as a
         normal packet that was received (and then being
         subject to the rules of forwarding, etc etc).

         Even in the absence of such a Link Layer, it is of
         course hardly necessary to make a copy of an entire
         packet in order to queue it both for forwarding and
         for local delivery, though care must be taken with
         fragments, since reassembly is performed on locally
         delivered packets but not on forwarded packets.  One
         simple scheme is to associate a flag with each packet
         on the router's output queue which indicates whether
         it should be queued for local delivery after it has
         been sent.


5.2.4  Determining the Next Hop Address

      When a router is going to forward a packet, it must
      determine whether it can send it directly to its
      destination, or whether it needs to pass it through
      another router.  If the latter, it needs to determine
      which router to use.  This section explains how these
      determinations are made.

      This section makes use of the following definitions:

      +  "LSRR" -- IP Loose Source and Record Route option

      +  "SSRR" -- IP Strict Source and Record Route option

      +  "Source Route Option" -- an LSRR or an SSRR

      +  "Ultimate Destination Address" -- where the packet is
         being sent to: the last address in the source route
         of a source-routed packet, or the destination address
         in the IP header of a non-source-routed packet





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      +  "Adjacent" -- reachable without going through any IP
         routers

      +  "Next Hop Address" -- the IP address of the adjacent
         host or router to which the packet should be sent
         next

      +  "Immediate Destination Address" -- the ultimate
         destination address, except in source routed packets,
         where it is the next address specified in the source
         route

      +  Immediate Destination -- the node, system, router,
         end-system, or whatever that is addressed by the
         Immediate Destination Address.


5.2.4.1  Immediate Destination Address

         If the destination address in the IP header is one of
         the addresses of the router and the packet contains a
         Source Route Option, the Immediate Destination
         Address is the address pointed at by the pointer in
         that option if the pointer does not point past the
         end of the option.  Otherwise, the Immediate
         Destination Address is the same as the IP destination
         address in the IP header.

         A router MUST use the Immediate Destination Address,
         not the Ultimate Destination Address, when
         determining how to handle a packet.

         It is an error for more than one source route option
         to appear in a datagram.  If it receives one, it
         SHOULD discard the packet and reply with an ICMP
         Parameter Problem message whose pointer points at the
         beginning of the second source route option.


5.2.4.2  Local/Remote Decision

         After it has been determined that the IP packet needs
         to be forwarded in accordance with the rules
         specified in Section [5.2.3], the following algorithm





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         MUST be used to determine if the Immediate
         Destination is directly accessible (see
         [INTERNET:2]):

         (1)  For each network interface that has not been
              assigned any IP address (the "unnumbered lines"
              as described in Section [2.2.7]), compare the
              router-id of the other end of the line to the
              Immediate Destination Address.  If they are
              exactly equal, the packet can be transmitted
              through this interface.

              DISCUSSION:
                 In other words, the router or host at the
                 remote end of the line is the destination of
                 the packet or is the next step in the source
                 route of a source routed packet.

         (2)  If no network interface has been selected in the
              first step, for each IP address assigned to the
              router:
              (a)  Apply the subnet mask associated with the
                   address to this IP address.

                   IMPLEMENTATION:
                      The result of this operation will
                      usually have been computed and saved
                      during initialization.

              (b)  Apply the same subnet mask to the Immediate
                   Destination Address of the packet.
              (c)  Compare the resulting values. If they are
                   equal to each other, the packet can be
                   transmitted through the corresponding
                   network interface.

         (3)  If an interface has still not been selected, the
              Immediate Destination is accessible only through
              some other router.  The selection of the router
              and the "next hop" IP address is described in
              Section [5.2.4.3].








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5.2.4.3  Next Hop Address


         EDITOR'S COMMENTS:
            Note that this section has been extensively
            rewritten.  The original document indicated that
            Phil Almquist wished to revise this section to
            conform to his "Ruminations on the Next Hop"
            document.  I am under the assumption that the
            working group generally agreed with this goal;
            there was an editor's note from Phil that remained
            in this document to that effect, and the RoNH
            document contains a "mandatory RRWG algorithm".

            So, I have taken said algorithm from RoNH and
            moved it into here.

            Additional useful or interesting information from
            RoNH has been extracted and placed into an
            appendix to this note.

         The router applies the algorithm in the previous
         section to determine if the Immediate Destination
         Address is adjacent.  If so, the next hop address is
         the same as the Immediate Destination Address.
         Otherwise, the packet must be forwarded through
         another router to reach its Immediate Destination.
         The selection of this router is the topic of this
         section.

         If the packet contains an SSRR, the router MUST
         discard the packet and reply with an ICMP Bad Source
         Route error.  Otherwise, the router looks up the
         Immediate Destination Address in its routing table to
         determine an appropriate next hop address.

         DISCUSSION:
            Per the IP specification, a Strict Source Route
            must specify a sequence of nodes through which the
            packet must traverse; the packet must go from one
            node of the source route to the next, traversing
            intermediate networks only.  Thus, if the router
            is not adjacent to the next step of the source
            route, the source route can not be fulfilled.





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            Therefore, the ICMP Bad Source Route error.

         The goal of the next-hop selection process is to
         examine the entries in the router's Forwarding
         Information Base (FIB) and select the best route (if
         there is one) for the packet from those available in
         the FIB.

         Conceptually, any route lookup algorithm starts out
         with a set of candidate routes which consists of the
         entire contents of the FIB.  The algorithm consists
         of a series of steps which discard routes from the
         set.  These steps are referred to as Pruning Rules.
         Normally, when the algorithm terminates there is
         exactly one route remaining in the set.  If the set
         ever becomes empty, the packet is discarded because
         the destination is unreachable.  It is also possible
         for the algorithm to terminate when more than one
         route remains in the set.  In this case, the router
         may arbitrarily discard all but one of them, or may
         perform "load-splitting" by choosing whichever of the
         routes has been least recently used.

         With the exception of rule 3 (Weak TOS), a router
         MUST use the following Pruning Rules when selecting a
         next hop for a packet.  If a router does consider TOS
         when making next-hop decisions, the Rule 3 must be
         applied in the order indicated below.  These rules
         MUST be (conceptually) applied to the FIB in the
         order that they are presented.  (For some historical
         perspective, additional pruning rules, and other
         common algorithms in use, see Appendix E).

         DISCUSSION:
            Rule 3 is optional in that Section [5.3.2] says
            that a router only SHOULD consider TOS when making
            forwarding decisions.


         (1)  Basic Match
              This rule discards any routes to destinations
              other than the Immediate Destination Address of
              the packet.  For example, if a packet's
              Immediate Destination Address is 36.144.2.5,





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              this step would discard a route to net
              128.12.0.0 but would retain any routes to net
              36.0.0.0, any routes to subnet 36.144.0.0, and
              any default routes.

              More precisely, we assume that each route has a
              destination attribute, called route.dest, and a
              corresponding mask, called route.mask, to
              specify which bits of route.dest are
              significant.  The Immediate Destination Address
              of the packet being forwarded is ip.dest.  This
              rule discards all routes from the set of
              candidate routes except those for which
              (route.dest & route.mask) = (ip.dest &
              route.mask).

         (2)  Longest Match
              Longest Match is a refinement of Basic Match,
              described above.  After Basic Match pruning is
              performed, the remaining routes are examined to
              determine the maximum number of bits set in any
              of their route.mask attributes.  The step then
              discards from the set of candidate routes any
              routes which have fewer than that maximum number
              of bits set in their route.mask attributes.

              For example, if a packet's Immediate Destination
              Address is 36.144.2.5 and there are
              {route.dest, route.mask} pairs of {36.144.2.0,
              255.255.255.0}, {36.144.0.5, 255.255.0.255},
              {36.144.0.0, 255.255.0.0}, and {36.0.0.0,
              255.0.0.0}, then this rule would keep only the
              first two pairs; {36.144.2.0, 255.255.255.0} and
              {36.144.0.5, 255.255.0.255}.

         (3)  Weak TOS
              Each route has a type of service attribute,
              called route.tos, whose possible values are
              assumed to be identical to those used in the TOS
              field of the IP header.  Routing protocols which
              distribute TOS information fill in route.tos
              appropriately in routes they add to the FIB;
              routes from other routing protocols are treated
              as if they have the default TOS (0000).  The TOS





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              field in the IP header of the packet being
              routed is called ip.tos.

              The set of candidate routes is examined to
              determine if it contains any routes for which
              route.tos = ip.tos.  If so, all routes except
              those for which route.tos = ip.tos are
              discarded.  If not, all routes except those for
              which route.tos = 0000 are discarded from the
              set of candidate routes.

              Additional discussion of routing based on Weak
              TOS may be found in [ROUTE:11].

              DISCUSSION:
                 The effect of this rule is to select only
                 those routes which have a TOS that matches
                 the TOS requested in the packet.  If no such
                 routes exist then routes with the default TOS
                 are considered.  Routes with a non-default
                 TOS that is not the TOS requested in the
                 packet are never used, even if such routes
                 are the only available routes that go to the
                 packet's destination.

         (4)  Best Metric
              Each route has a metric attribute, called
              route.metric, and a routing domain identifier,
              called route.domain.  Each member of the set of
              candidate routes is compared with each other
              member of the set.  If route.domain is equal for
              the two routes and route.metric is strictly
              "inferior" for one when compared with the other,
              then the one with the "inferior" metric is
              discarded from the set.  The determination of
              "inferior" is usually by a simple arithmetic
              comparison, though some protocols may have
              structured metrics requiring more complex
              comparisons.

         (5)  Vendor Policy
              Vendor Policy is sort of a catch-all to make up
              for the fact that the previously listed rules
              are often inadequate to chose from among the





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              possible routes.  Vendor Policy pruning rules
              are extremely vendor-specific.  See section
              [5.2.4.4].

         This algorithm has two distinct disadvantages.
         Presumably, a router implementor might develop
         techniques to deal with these disadvantages and make
         them a part of the Vendor Policy pruning rule.

         (1)  IS-IS and OSPF route classes are not directly
              handled.

         (2)  Path properties other than type of service (e.g.
              MTU) are ignored.

         It is also worth noting a deficiency in the way that
         TOS is supported: routing protocols which support TOS
         are implicitly preferred when forwarding packets
         which have non-zero TOS values.

         The Basic Match and Longest Match pruning rules
         generalize the treatment of a number of particular
         types of routes.  These routes are selected in the
         following, decreasing, order of preference:

         (1)  Host Route: This is a route to a specific end
              system.

         (2)  Subnetwork Route: This is a route to a
              particular subnet of a network.

         (3)  Default Subnetwork Route: This is a route to all
              subnets of a particular net for which there are
              not (explicit) subnet routes.

         (4)  Network Route: This is a route to a particular
              network.

         (5)  Default Network Route (also known as the
              "default route"): This is a route to all
              networks for which there are no explicit routes
              to the net or any of its subnets.

         If, after application of the pruning rules, the set





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         of routes is empty (i.e., no routes were found), the
         packet MUST be discarded and an appropriate ICMP
         error generated (ICMP Bad Source Route if the
         Immediate Destination Address came from a source
         route option; otherwise, whichever of ICMP
         Destination Host Unreachable or Destination Network
         Unreachable is appropriate, as described in Section
         [4.3.3.1]).


5.2.4.4  Administrative Preference

         One suggested mechanism for the Vendor Policy Pruning
         Rule is to use administrative preference.

         Each route has associated with it a "preference
         value", based on various attributes of the route
         (specific mechanisms for assignment of preference
         values are suggested below).  This preference value
         is an integer in the range [0..255], with zero being
         the most preferred and 254 being the least preferred.
         255 is a special value that means that the route
         should never be used.  The first step in the Vendor
         Policy pruning rule discards all but the most
         preferable routes (and always discards routes whose
         preference value is 255).

         This policy is not "safe" in that it can easily be
         misused to create routing loops.  Since no protocol
         ensures that the preferences configured for a router
         are consistent with the preferences configured in its
         neighbors, network managers must exercise care in
         configuring preferences.

         +  Address Match
            It is useful to be able to assign a single
            preference value to all routes (learned from the
            same routing domain) to any of a specified set of
            destinations, where the set of destinations is all
            destinations that match a specified address/mask
            pair.

         +  Route Class
            For routing protocols which maintain the





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            distinction, it is useful to be able to assign a
            single preference value to all routes (learned
            from the same routing domain) which have a
            particular route class (intra-area, inter-area,
            external with internal metrics, or external with
            external metrics).

         +  Interface
            It is useful to be able to assign a single
            preference value to all routes (learned from a
            particular routing domain) that would cause
            packets to be routed out a particular logical
            interface on the router (logical interfaces
            generally map one-to-one onto the router's network
            interfaces, except that any network interface
            which has multiple IP addresses will have multiple
            logical interfaces associated with it).

         +  Source router
            It is useful to be able to assign a single
            preference value to all routes (learned from the
            same routing domain) which were learned from any
            of a set of routers, where the set of routers are
            those whose updates have a source address which
            match a specified address/mask pair.

         +  Originating AS
            For routing protocols which provide the
            information, it is useful to be able to assign a
            single preference value to all routes (learned
            from a particular routing domain) which originated
            in another particular routing domain.  For BGP
            routes, the originating AS is the first AS listed
            in the route's AS_PATH attribute.  For OSPF
            external routes, the originating AS may be
            considered to be the low order 16 bits of the
            route's external route tag if the tag's Automatic
            bit is set and the tag's PathLength is not equal
            to 3.

         +  External route tag
            It is useful to be able to assign a single
            preference value to all OSPF external routes
            (learned from the same routing domain) whose





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            external route tags match any of a list of
            specified values.  Because the external route tag
            may contain a structured value, it may be useful
            to provide the ability to match particular
            subfields of the tag.

         +  AS path
            It may be useful to be able to assign a single
            preference value to all BGP routes (learned from
            the same routing domain) whose AS path "matches"
            any of a set of specified values.  It is not yet
            clear exactly what kinds of matches are most
            useful.  A simple option would be to allow
            matching of all routes for which a particular AS
            number appears (or alternatively, does not appear)
            anywhere in the route's AS_PATH attribute.  A more
            general but somewhat more difficult alternative
            would be to allow matching all routes for which
            the AS path matches a specified regular
            expression.


5.2.4.6  Load Splitting

         At the end of the Next-hop selection process,
         multiple routes may still remain.  A router has
         several options when this occurs.  It may arbitrarily
         discard some of the routes.  It may reduce the number
         of candidate routes by comparing metrics of routes
         from routing domains which are not considered
         equivalent.  It may retain more than one route and
         employ a "load-splitting" mechanism to divide traffic
         among them.  Perhaps the only thing that can be said
         about the relative merits of the options is that
         load-splitting is useful in some situations but not
         in others, so a wise implementor who implements load-
         splitting will also provide a way for the network
         manager to disable it.


5.2.5  Unused IP Header Bits: RFC-791 Section 3.1

      The IP header contains several reserved bits, in the
      Type of Service field and in the Flags field.  Routers





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      MUST NOT drop packets merely because one or more of
      these reserved bits has a non-zero value.

      Routers MUST ignore and MUST pass through unchanged the
      values of these reserved bits.  If a router fragments a
      packet, it MUST copy these bits into each fragment.

      DISCUSSION:
         Future revisions to the IP protocol may make use of
         these unused bits.  These rules are intended to
         ensure that these revisions can be deployed without
         having to simultaneously upgrade all routers in the
         Internet.



5.2.6  Fragmentation and Reassembly: RFC-791 Section 3.2

      As was discussed in Section [4.2.2.7], a router MUST
      support IP fragmentation.

      A router MUST NOT reassemble any datagram before
      forwarding it.

      DISCUSSION:
         A few people have suggested that there might be some
         topologies where reassembly of transit datagrams by
         routers might improve performance.  In general,
         however, the fact that fragments may take different
         paths to the destination precludes safe use of such a
         feature.

         Nothing in this section should be construed to
         control or limit fragmentation or reassembly
         performed as a link layer function by the router.



5.2.7  Internet Control Message Protocol -- ICMP

      General requirements for ICMP were discussed in Section
      [4.3].  This section discusses ICMP messages which are
      sent only by routers.






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5.2.7.1  Destination Unreachable

         The ICMP Destination Unreachable message is sent by a
         router in response to a packet which it cannot
         forward because the destination (or next hop) is
         unreachable or a service is unavailable

         A router MUST be able to generate ICMP Destination
         Unreachable messages and SHOULD choose a response
         code that most closely matches the reason why the
         message is being generated.

         The following codes are defined in [INTERNET:8] and
         [INTRO:2]:

         0 =  Network Unreachable - generated by a router if a
              forwarding path (route) to the destination
              network is not available;

         1 =  Host Unreachable - generated by a router if a
              forwarding path (route) to the destination host
              on a directly connected network is not
              available;

         2 =  Protocol Unreachable - generated if the
              transport protocol designated in a datagram is
              not supported in the transport layer of the
              final destination;

         3 =  Port Unreachable -  generated if the designated
              transport protocol (e.g. UDP) is unable to
              demultiplex the datagram in the transport layer
              of the final destination but has no protocol
              mechanism to inform the sender;

         4 =  Fragmentation Needed and DF Set - generated if a
              router needs to fragment a datagram but cannot
              since the DF flag is set;

         5 =  Source Route Failed - generated if a router
              cannot forward a packet to the next hop in a
              source route option;

         6 =  Destination Network Unknown - This code SHOULD





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              NOT be generated since it would imply on the
              part of the router that the destination network
              does not exist (net unreachable code 0 SHOULD be
              used in place of code 6);

         7 =  Destination Host Unknown - generated only when a
              router can determine (from link layer advice)
              that the destination host does not exist;

         11 = Network Unreachable For Type Of Service -
              generated by a router if a forwarding path
              (route) to the destination network with the
              requested or default TOS is not available;

         12 = Host Unreachable For Type Of Service - generated
              if a router cannot forward a packet because its
              route(s) to the destination do not match either
              the TOS requested in the datagram or the default
              TOS (0).

         The following additional codes are hereby defined:

         13 = Communication Administratively Prohibited -
              generated if a router cannot forward a packet
              due to administrative filtering;

         14 = Host Precedence Violation.  Sent by the first
              hop router to a host to indicate that a
              requested precedence is not permitted for the
              particular combination of source/destination
              host or network, upper layer protocol, and
              source/destination port;

         15 = Precedence cutoff in effect.  The network
              operators have imposed a minimum level of
              precedence required for operation, the datagram
              was sent with a precedence below this level;

         NOTE: [INTRO:2] defined Code 8 for "source host
         isolated".  Routers SHOULD NOT generate Code 8;
         whichever of Codes 0 (Network Unreachable) and 1
         (Host Unreachable) is appropriate SHOULD be used
         instead.  [INTRO:2] also defined Code 9 for
         communication with destination network





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         administratively prohibited and Code 10 for
         communication with destination host administratively
         prohibited.  These codes were intended for use by
         end-to-end encryption devices used by U.S military
         agencies.  Routers SHOULD use the newly defined Code
         13 (Communication Administratively Prohibited) if
         they administratively filter packets.

         Routers MAY have a configuration option that causes
         Code 13 (Communication Administratively Prohibited)
         messages not to be generated.  When this option is
         enabled, no ICMP error message is sent in response to
         a packet which is dropped because its forwarding is
         administratively prohibited.

         Similarly, routers MAY have a configuration option
         that causes Code 14 (Host Precedence Violation) and
         Code 15 (Precedence Cutoff in Effect) messages not to
         be generated.  When this option is enabled, no ICMP
         error message is sent in response to a packet which
         is dropped  because of a precedence violation.

         Routers MUST use Host Unreachable or Destination Host
         Unknown codes whenever other hosts on the same
         destination network might be reachable; otherwise,
         the source host may erroneously conclude that all
         hosts on the network are unreachable, and that may
         not be the case.

         [INTERNET:14] describes a slight modification the
         form of Destination Unreachable messages containing
         Code 4 (Fragmentation needed and DF set).  A router
         MUST use this modified form when originating Code 4
         Destination Unreachable messages.


5.2.7.2  Redirect

         The ICMP Redirect message is generated to inform a
         host on the same subnet that the router used by the
         host to route certain packets should be changed.

         Routers MUST NOT generate the Redirect for Network or
         Redirect for Network and Type of Service messages





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         (Codes 0 and 2) specified in [INTERNET:8].  Routers
         MUST be able to generate the Redirect for Host
         message (Code 1) and SHOULD be able to generate the
         Redirect for Type of Service and Host message (Code
         3) specified in [INTERNET:8].

         DISCUSSION:
            If the directly-connected network is not
            subnetted, a router can normally generate a
            network Redirect which applies to all hosts on a
            specified remote network.  Using a network rather
            than a host Redirect may economize slightly on
            network traffic and on host routing table storage.
            However, the savings are not significant, and
            subnets create an ambiguity about the subnet mask
            to be used to interpret a network Redirect.  In a
            general subnet environment, it is difficult to
            specify precisely the cases in which network
            Redirects can be used.  Therefore, routers must
            send only host (or host and type of service)
            Redirects.

         A Code 3 (Redirect for Host and Type of Service)
         message is generated when the packet provoking the
         redirect has a destination for which the path chosen
         by the router would depend (in part) on the TOS
         requested.

         Routers which can generate Code 3 redirects (Host and
         Type of Service) MUST have a configuration option
         (which defaults to on) to enable Code 1 (Host)
         redirects to be substituted for Code 3 redirects.  A
         router MUST send a Code 1 Redirect in place of a Code
         3 Redirect if it has been configured to do so.

         If a router is not able to generate Code 3 Redirects
         then it MUST generate Code 1 Redirects in situations
         where a Code 3 Redirect is called for.

         Routers MUST NOT generate a Redirect Message unless
         all of the following conditions are met:

         +  The packet is being forwarded out the same
            physical interface that it was received from,





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         +  The IP source address in the packet is on the same
            Logical IP (sub)network as the next-hop IP
            address, and

         +  The packet does not contain an IP source route
            option.

         The source address used in the ICMP Redirect MUST
         belong to the same logical (sub)net as the
         destination address.

         A router using a routing protocol (other than static
         routes) MUST NOT consider paths learned from ICMP
         Redirects when forwarding a packet.  If a router is
         not using a routing protocol, a router MAY have a
         configuration which, if set, allows the router to
         consider routes learned via ICMP Redirects when
         forwarding packets.

         DISCUSSION:
            ICMP Redirect is a mechanism for routers to convey
            routing information to hosts.  Routers use other
            mechanisms to learn routing information, and
            therefore have no reason to obey redirects.
            Believing a redirect which contradicted the
            router's other information would likely create
            routing loops.

            On the other hand, when a router is not acting as
            a router, it MUST comply with the behavior
            required of a host.



5.2.7.3  Time Exceeded

         A router MUST generate a Time Exceeded message Code 0
         (In Transit) when it discards a packet due to an
         expired TTL field.  A router MAY have a per-interface
         option to disable origination of these messages on
         that interface, but that option MUST default to
         allowing the messages to be originated.







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5.2.8  INTERNET GROUP MANAGEMENT PROTOCOL -- IGMP

      IGMP [INTERNET:4] is a protocol used between hosts and
      multicast routers on a single physical network to
      establish hosts' membership in particular multicast
      groups.  Multicast routers use this information, in
      conjunction with a multicast routing protocol, to
      support IP multicast forwarding across the Internet.

      A router SHOULD implement the multicast router part of
      IGMP.


5.3  SPECIFIC ISSUES



5.3.1  Time to Live (TTL)

      The Time-to-Live (TTL) field of the IP header is defined
      to be a timer limiting the lifetime of a datagram.  It
      is an 8-bit field and the units are seconds.  Each
      router (or other module) that handles a packet MUST
      decrement the TTL by at least one, even if the elapsed
      time was much less than a second.  Since this is very
      often the case, the TTL is effectively a hop count limit
      on how far a datagram can propagate through the
      Internet.

      When a router forwards a packet, it MUST reduce the TTL
      by at least one.  If it holds a packet for more than one
      second, it MAY decrement the TTL by one for each second.

      If the TTL is reduced to zero (or less), the packet MUST
      be discarded, and if the destination is not a multicast
      address the router MUST send an ICMP Time Exceeded
      message, Code 0 (TTL Exceeded in Transit) message to the
      source.  Note that a router MUST NOT discard an IP
      unicast or broadcast packet with a non-zero TTL merely
      because it can predict that another router on the path
      to the packet's final destination will decrement the TTL
      to zero.  However, a router MAY do so for IP multicasts,
      in order to more efficiently implement IP multicast's
      expanding ring search algorithm (see [INTERNET:4]).





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      DISCUSSION:
         The IP TTL is used, somewhat schizophrenically, as
         both a hop count limit and a time limit.  Its hop
         count function is critical to ensuring that routing
         problems can't melt down the network by causing
         packets to loop infinitely in the network.  The time
         limit function is used by transport protocols such as
         TCP to ensure reliable data transfer.  Many current
         implementations treat TTL as a pure hop count, and in
         parts of the Internet community there is a strong
         sentiment that the time limit function should instead
         be performed by the transport protocols that need it.

         In this specification, we have reluctantly decided to
         follow the strong belief among the router vendors
         that the time limit function should be optional.
         They argued that implementation of the time limit
         function is difficult enough that it is currently not
         generally done.  They further pointed to the lack of
         documented cases where this shortcut has caused TCP
         to corrupt data (of course, we would expect the
         problems created to be rare and difficult to
         reproduce, so the lack of documented cases provides
         little reassurance that there haven't been a number
         of undocumented cases).

         IP multicast notions such as the expanding ring
         search may not work as expected unless the TTL is
         treated as a pure hop count.  The same thing is
         somewhat true of traceroute.

         ICMP Time Exceeded messages are required because the
         traceroute diagnostic tool depends on them.

         Thus, the tradeoff is between severely crippling, if
         not eliminating, two very useful tools vs. a very
         rare and transient data transport problem (which may
         not occur at all).











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5.3.2  Type of Service (TOS)

      The "Type-of-Service" byte in the IP header is divided
      into three sections:  the Precedence field (high-order 3
      bits), a field that is customarily called "Type of
      Service" or "TOS" (next 4 bits), and a reserved bit (the
      low order bit).  Rules governing the reserved bit were
      described in Section [4.2.2.3].  The Precedence field
      will be discussed in Section [5.3.3].  A more extensive
      discussion of the TOS field and its use can be found in
      [ROUTE:11].

      A router SHOULD consider the TOS field in a packet's IP
      header when deciding how to forward it.  The remainder
      of this section describes the rules that apply to
      routers that conform to this requirement.

      A router MUST maintain a TOS value for each route in its
      routing table.  Routes learned via a routing protocol
      which does not support TOS MUST be assigned a TOS of
      zero (the default TOS).

      To choose a route to a destination, a router MUST use an
      algorithm equivalent to the following:

      (1)  The router locates in its routing table all
           available routes to the destination (see Section
           [5.2.4]).

      (2)  If there are none, the router drops the packet
           because the destination is unreachable.  See
           section [5.2.4].

      (3)  If one or more of those routes have a TOS that
           exactly matches the TOS specified in the packet,
           the router chooses the route with the best metric.

      (4)  Otherwise, the router repeats the above step,
           except looking at routes whose TOS is zero.

      (5)  If no route was chosen above, the router drops the
           packet because the destination is unreachable.  The
           router returns an ICMP Destination Unreachable
           error specifying the appropriate code: either





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           Network Unreachable with Type of Service (code 11)
           or Host Unreachable with Type of Service (code 12).

      DISCUSSION:
         Although TOS has been little used in the past, its
         use by hosts is now mandated by the Requirements for
         Internet Hosts RFCs ([INTRO:2] and [INTRO:3]).
         Support for TOS in routers may become a MUST in the
         future, but is a SHOULD for now until we get more
         experience with it and can better judge both its
         benefits and its costs.

         Various people have proposed that TOS should affect
         other aspects of the forwarding function.  For
         example:

         (1)  A router could place packets which have the "Low
              Delay" bit set ahead of other packets in its
              output queues.

         (2)  a router is forced to discard packets, it could
              try to avoid discarding those which have the
              "High Reliability" bit set.

         These ideas have been explored in more detail in
         [INTERNET:17] but we don't yet have enough experience
         with such schemes to make requirements in this area.



5.3.3  IP Precedence

      This section specifies requirements and guidelines for
      appropriate processing of the IP Precedence field in
      routers.  Precedence is a scheme for allocating
      resources in the network based on the relative
      importance of different traffic flows.  The IP
      specification defines specific values to be used in this
      field for various types of traffic.

      The basic mechanisms for precedence processing in a
      router are preferential resource allocation, including
      both precedence-ordered queue service and precedence-
      based congestion control, and selection of Link Layer





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      priority features.  The router also selects the IP
      precedence for routing, management and control traffic
      it originates.  For a more extensive discussion of IP
      Precedence and its implementation see [FORWARD:6].

      Precedence-ordered queue service, as discussed in this
      section, includes but is not limited to the queue for
      the forwarding process and queues for outgoing links.
      It is intended that a router supporting precedence
      should also use the precedence indication at whatever
      points in its processing are concerned with allocation
      of finite resources, such as packet buffers or Link
      Layer connections.  The set of such points is
      implementation-dependent.

      DISCUSSION:
         Although the Precedence field was originally provided
         for use in DOD systems where large traffic surges or
         major damage to the network are viewed as inherent
         threats, it has useful applications for many non-
         military IP networks.  Although the traffic handling
         capacity of networks has grown greatly in recent
         years, the traffic generating ability of the users
         has also grown, and network overload conditions still
         occur at times.  Since IP-based routing and
         management protocols have become more critical to the
         successful operation of the Internet, overloads
         present two additional risks to the network:

         (1)  High delays may result in routing protocol
              packets being lost.  This may cause the routing
              protocol to falsely deduce a topology change and
              propagate this false information to other
              routers.  Not only can this cause routes to
              oscillate, but an extra processing burden may be
              placed on other routers.

         (2)  High delays may interfere with the use of
              network management tools to analyze and perhaps
              correct or relieve the problem in the network
              that caused the overload condition to occur.

         Implementation and appropriate use of the Precedence
         mechanism alleviates both of these problems.





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5.3.3.1  Precedence-Ordered Queue Service

         Routers SHOULD implement precedence-ordered queue
         service.  Precedence-ordered queue service means that
         when a packet is selected for output on a (logical)
         link, the packet of highest precedence that has been
         queued for that link is sent.  Routers that implement
         precedence-ordered queue service MUST also have a
         configuration option to suppress precedence-ordered
         queue service in the Internet Layer.

         Any router MAY implement other policy-based
         throughput management procedures that result in other
         than strict precedence ordering, but it MUST be
         configurable to suppress them (i.e., use strict
         ordering).

         As detailed in Section [5.3.6], routers that
         implement precedence-ordered queue service discard
         low precedence packets before discarding high
         precedence packets for congestion control purposes.

         Preemption (interruption of processing or
         transmission of a packet) is not envisioned as a
         function of the Internet Layer.  Some protocols at
         other layers may provide preemption features.


5.3.3.2  Lower Layer Precedence Mappings

         Routers that implement precedence-ordered queueing
         MUST IMPLEMENT, and other routers SHOULD IMPLEMENT,
         Lower Layer Precedence Mapping.

         A router which implements Lower Layer Precedence
         Mapping:

         +  MUST be able to map IP Precedence to Link Layer
            priority mechanisms for link layers that have such
            a feature defined.

         +  MUST have a configuration option to select the
            Link Layer's default priority treatment for all IP
            traffic





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         +  SHOULD be able to configure specific nonstandard
            mappings of IP precedence values to Link Layer
            priority values for each interface.

         DISCUSSION:
            Some research questions the workability of the
            priority features of some Link Layer protocols,
            and some networks may have faulty implementations
            of the link layer priority mechanism.  It seems
            prudent to provide an escape mechanism in case
            such problems show up in a network.

            On the other hand, there are proposals to use
            novel queueing strategies to implement special
            services such as low-delay service.  Special
            services and queueing strategies to support them
            need further research and experimentation before
            they are put into widespread use in the Internet.
            Since these requirements are intended to encourage
            (but not force) the use of precedence features in
            the hope of providing better Internet service to
            all users, routers supporting precedence-ordered
            queue service should default to maintaining strict
            precedence ordering regardless of the type of
            service requested.

            Implementors may wish to consider that correct
            link layer mapping of IP precedence is required by
            DOD policy for TCP/IP systems used on DOD
            networks.



5.3.3.3  Precedence Handling For All Routers

         A router (whether or not it employs precedence-
         ordered queue service):

         (1)  MUST accept and process incoming traffic of all
              precedence levels normally, unless it has been
              administratively configured to do otherwise.

         (2)  MAY implement a validation filter to
              administratively restrict the use of precedence





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              levels by particular traffic sources.  If
              provided, this filter MUST NOT filter out or cut
              off the following sorts of ICMP error messages:
              Destination Unreachable, Redirect, Time
              Exceeded, and Parameter Problem.  If this filter
              is provided, the procedures required for packet
              filtering by addresses are required for this
              filter also.

              DISCUSSION:
                 Precedence filtering should be applicable to
                 specific source/destination IP Address pairs,
                 specific protocols, specific ports, and so
                 on.

              An ICMP Destination Unreachable message with
              code 14 SHOULD be sent when a packet is dropped
              by the validation filter, unless this has been
              suppressed by configuration choice.

         (3)  MAY implement a cutoff function which allows the
              router to be set to refuse or drop traffic with
              precedence below a specified level.  This
              function may be activated by management actions
              or by some implementation dependent heuristics,
              but there MUST be a configuration option to
              disable any heuristic mechanism that operates
              without human intervention.  An ICMP Destination
              Unreachable message with code 15 SHOULD be sent
              when a packet is dropped by the cutoff function,
              unless this has been suppressed by configuration
              choice.

              A router MUST NOT refuse to forward datagrams
              with IP precedence of 6 (Internetwork Control)
              or 7 (Network Control) solely due to precedence
              cutoff.  However, other criteria may be used in
              conjunction with precedence cutoff to filter
              high precedence traffic.

              DISCUSSION:
                 Unrestricted precedence cutoff could result
                 in an unintentional cutoff of routing and
                 control traffic.  In general, host traffic





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                 should be restricted to a value of 5
                 (CRITIC/ECP) or below although this is not a
                 requirement and may not be valid in certain
                 systems.


         (4)  MUST NOT change precedence settings on packets
              it did not originate.

         (5)  SHOULD be able to configure distinct precedence
              values to be used for each routing or management
              protocol supported (except for those protocols,
              such as OSPF, which specify which precedence
              value must be used).

         (6)  MAY be able to configure routing or management
              traffic precedence values independently for each
              peer address.

         (7)  MUST respond appropriately to Link Layer
              precedence-related error indications where
              provided.  An ICMP Destination Unreachable
              message with code 15 SHOULD be sent when a
              packet is dropped because a link cannot accept
              it due to a precedence-related condition, unless
              this has been suppressed by configuration
              choice.

              DISCUSSION:
                 The precedence cutoff mechanism described in
                 (3) is somewhat controversial.  Depending on
                 the topological location of the area affected
                 by the cutoff, transit traffic may be
                 directed by routing protocols into the area
                 of the cutoff, where it will be dropped.
                 This is only a problem if another path which
                 is unaffected by the cutoff exists between
                 the communicating points.  Proposed ways of
                 avoiding this problem include providing some
                 minimum bandwidth to all precedence levels
                 even under overload conditions, or
                 propagating cutoff information in routing
                 protocols.  In the absence of a widely
                 accepted (and implemented) solution to this





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                 problem, great caution is recommended in
                 activating cutoff mechanisms in transit
                 networks.

                 A transport layer relay could legitimately
                 provide the function prohibited by (4) above.
                 Changing precedence levels may cause subtle
                 interactions with TCP and perhaps other
                 protocols; a correct design is a non-trivial
                 task.

                 The intent of (5) and (6) (and the discussion
                 of IP Precedence in ICMP messages in Section
                 [4.3.2]) is that the IP precedence bits
                 should be appropriately set, whether or not
                 this router acts upon those bits in any other
                 way.  We expect that in the future
                 specifications for routing protocols and
                 network management protocols will specify how
                 the IP Precedence should be set for messages
                 sent by those protocols.

                 The appropriate response for (7) depends on
                 the link layer protocol in use.  Typically,
                 the router should stop trying to send
                 "offensive" traffic to that destination for
                 some period of time, and should return an
                 ICMP Destination Unreachable message with
                 code 15 (service not available for precedence
                 requested) to the traffic source.  It also
                 should not try to reestablish a preempted
                 Link Layer connection for some period of
                 time.



5.3.4  Forwarding of Link Layer Broadcasts

      The encapsulation of IP packets in most Link Layer
      protocols (except PPP) allows a receiver to distinguish
      broadcasts and multicasts from unicasts simply by
      examining the Link Layer protocol headers (most
      commonly, the Link Layer destination address).  The
      rules in this section which refer to "Link Layer





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      broadcasts" apply only to Link Layer protocols which
      allow broadcasts to be distinguished; likewise, the
      rules which refer to "Link Layer multicasts" apply only
      to Link Layer protocols which allow multicasts to be
      distinguished.

      A router MUST NOT forward any packet which the router
      received as a Link Layer broadcast (even if the IP
      destination address is also some form of broadcast
      address) unless the packet is an all-subnets-directed
      broadcast being forwarded as specified in [INTERNET:3].

      DISCUSSION:
         As noted in Section [5.3.5.3], forwarding of all-
         subnets-directed broadcasts in accordance with
         [INTERNET:3] is optional and is not something that
         routers do by default.

      A router MUST NOT forward any packet which the router
      received as a Link Layer multicast unless the packet's
      destination address is an IP multicast address.

      A router SHOULD silently discard a packet that is
      received via a Link Layer broadcast but does not specify
      an IP multicast or IP broadcast destination address.

      When a router sends a packet as a Link Layer broadcast,
      the IP destination address MUST be a legal IP broadcast
      or IP multicast address.


5.3.5  Forwarding of Internet Layer Broadcasts

      There are two major types of IP broadcast addresses;
      limited broadcast and directed broadcast.  In addition,
      there are three subtypes of directed broadcast; a
      broadcast directed to a specified network, a broadcast
      directed to a specified subnetwork, and a broadcast
      directed to all subnets of a specified network.
      Classification by a router of a broadcast into one of
      these categories depends on the broadcast address and on
      the router's understanding (if any) of the subnet
      structure of the destination network.  The same
      broadcast will be classified differently by different





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      routers.

      A limited IP broadcast address is defined to be all-
      ones: { -1, -1 } or 255.255.255.255.

      A net-directed broadcast is composed of the network
      portion of the IP address with a local part of all-ones,
      { <Network-number>, -1 }.  For example, a Class A net
      broadcast address is net.255.255.255, a Class B net
      broadcast address is net.net.255.255 and a Class C net
      broadcast address is net.net.net.255 where "net" is a
      byte of the network address.

      An all-subnets-directed broadcast is composed of the
      network part of the IP address with a subnet and a host
      part of all-ones, { <Network-number>, -1, -1 }.  For
      example, an all-subnets broadcast on a subnetted class B
      network is net.net.255.255.  A network must be known to
      be subnetted and the subnet part must be all-ones before
      a broadcast can be classified as all-subnets-directed.

      A subnet-directed broadcast address is composed of the
      network and subnet part of the IP address with a host
      part of all-ones, { <Network-number>, <Subnet-number>,
      -1 }.  For example, a subnet-directed broadcast to
      subnet 2 of a class B network might be net.net.2.255 (if
      the subnet mask was 255.255.255.0) or net.net.1.127 (if
      the subnet mask was 255.255.255.128).  A network must be
      known to be subnetted and the net and subnet part must
      not be all-ones before an IP broadcast can be classified
      as subnet-directed.

      As was described in Section [4.2.3.1], a router may
      encounter certain non-standard IP broadcast addresses:

      +  0.0.0.0 is an obsolete form of the limited broadcast
         address

      +  { <Network-number>, 0 } is an obsolete form of a net-
         directed broadcast address.

      +  { <Network-number>, 0, 0 } is an obsolete form of a
         all-subnets-directed broadcast address.






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      +  { <Network-number>, <Subnet-number>, 0 } is an
         obsolete form of a subnet-directed broadcast address.

      As was described in that section, packets addressed to
      any of these addresses SHOULD be silently discarded, but
      if they are not, they MUST be treated in accordance with
      the same rules that apply to packets addressed to the
      non-obsolete forms of the broadcast addresses described
      above.  These rules are described in the next few
      sections.


5.3.5.1  Limited Broadcasts

         Limited broadcasts MUST NOT be forwarded.  Limited
         broadcasts MUST NOT be discarded.  Limited broadcasts
         MAY be sent and SHOULD be sent instead of directed
         broadcasts where limited broadcasts will suffice.

         DISCUSSION:
            Some routers contain UDP servers which function by
            resending the requests (as unicasts or directed
            broadcasts) to other servers.  This requirement
            should not be interpreted as prohibiting such
            servers.  Note, however, that such servers can
            easily cause packet looping if misconfigured.
            Thus, providers of such servers would probably be
            well-advised to document their setup carefully and
            to consider carefully the TTL on packets which are
            sent.



5.3.5.2  Net-directed Broadcasts

         A router MUST classify as net-directed broadcasts all
         valid, directed broadcasts destined for a remote
         network or an attached nonsubnetted network.  A
         router MUST forward net-directed broadcasts.  Net-
         directed broadcasts MAY be sent.

         A router MAY have an option to disable receiving net-
         directed broadcasts on an interface and MUST have an
         option to disable forwarding net-directed broadcasts.





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         These options MUST default to permit receiving and
         forwarding net-directed broadcasts.

         DISCUSSION:
            There has been some debate about forwarding or not
            forwarding directed broadcasts.  In this memo we
            have made the forwarding decision depend on the
            router's knowledge of the subnet mask for the
            destination network.  Forwarding decisions for
            subnetted networks should be made by routers with
            an understanding of the subnet structure.
            Therefore, in general, routers must forward
            directed broadcasts for networks they are not
            attached to and for which they do not understand
            the subnet structure.  One router may interpret
            and handle the same IP broadcast packet
            differently than another, depending on its own
            understanding of the structure of the destination
            (sub)network.



5.3.5.3  All-subnets-directed Broadcasts

         A router MUST classify as all-subnets-directed
         broadcasts all valid directed broadcasts destined for
         a directly attached subnetted network which have all-
         ones in the subnet part of the address.  If the
         destination network is not subnetted, the broadcast
         MUST be treated as a net-directed broadcast.

         A router MUST forward an all-subnets-directed
         broadcast as a link level broadcast out all physical
         interfaces connected to the IP network addressed by
         the broadcast, except that:

         +  A router MUST NOT forward an all-subnet-directed
            broadcast that was received by the router as a
            Link Layer broadcast, unless the router is
            forwarding the broadcast in accordance with
            [INTERNET:3] (see below).

         +  If a router receives an all-subnets-directed
            broadcast over a network which does not indicate





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            via Link Layer framing whether the frame is a
            broadcast or a unicast, the packet MUST NOT be
            forwarded to any network which likewise does not
            indicate whether a frame is a broadcast.

         +  A router MUST NOT forward an all-subnets-directed
            broadcast if the router is configured not to
            forward such broadcasts.  A router MUST have a
            configuration option to deny forwarding of all-
            subnets-directed broadcasts.  The configuration
            option MUST default to permit forwarding of all-
            subnets-directed broadcasts.

         EDITOR'S COMMENTS:
            The algorithm presented here is broken.  The
            working group explicitly desired this algorithm,
            knowing its failures.

            The second bullet, above, prevents All Subnets
            Directed Broadcasts from traversing more than one
            PPP (or other serial) link in a row.  Such a
            topology is easily conceived.  Suppose that some
            corporation builds its corporate backbone out of
            PPP links, connecting routers at geographically
            dispersed locations.  Suppose that this
            corporation has 3 sites (S1, S2, and S3) and there
            is a router at each site (R1, R2, and R3).  At
            each site there are also several LANs connected to
            the local router.  Let there be a PPP link
            connecting S1 to S2 and one connecting S2 to S3
            (i.e. the links are R1-R2 and R2-R3).  So, if a
            host on a LAN at S1 sends a All Subnets Directed
            Broadcast, R1 will forward the broadcast over the
            R1-R2 link to R2.  R2 will forward the broadcast
            to the LAN(s) connected to R2.  Since the PPP does
            not differentiate broadcast from non-broadcast
            frames, R2 will NOT forward the broadcast onto the
            R2-R3 link.  Therefore, the broadcast will not
            reach S3.

         [INTERNET:3] describes an alternative set of rules
         for forwarding of all-subnets-directed broadcasts
         (called multi-subnet-broadcasts in that document).  A
         router MAY IMPLEMENT that alternative set of rules,





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         but MUST use the set of rules described above unless
         explicitly configured to use the [INTERNET:3] rules.
         If routers will do [INTERNET:3]-style forwarding,
         then the router MUST have a configuration option
         which MUST default to doing the rules presented in
         this document.

         DISCUSSION:
            As far as we know, the rules for multi-subnet
            broadcasts described in [INTERNET:3] have never
            been implemented, suggesting that either they are
            too complex or the utility of multi-subnet
            broadcasts is low.  The rules described in this
            section match current practice.  In the future, we
            expect that IP multicast (see [INTERNET:4]) will
            be used to better solve the sorts of problems that
            multi-subnets broadcasts were intended to address.

            We were also concerned that hosts whose system
            managers neglected to configure with a subnet mask
            could unintentionally send multi-subnet
            broadcasts.

         A router SHOULD NOT originate all-subnets broadcasts,
         except as required by Section [4.3.3.9] when sending
         ICMP Address Mask Replies on subnetted networks.

         DISCUSSION:
            The current intention is to decree that (like
            0-filled IP broadcasts) the notion of the all-
            subnets broadcast is obsolete.  It should be
            treated as a directed broadcast to the first
            subnet of the net in question that it appears on.

            Routers may implement a switch (default off) which
            if turned on enables the [INTERNET:3] behavior for
            all-subnets broadcasts.

            If a router has a configuration option to allow
            for forwarding all-subnet broadcasts, it should
            use a spanning tree, RPF, or other multicast
            forwarding algorithm (which may be computed for
            other purposes such as bridging or OSPF) to
            distribute the all-subnets broadcast efficiently.





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            In general, it is better to use an IP multicast
            address rather than an all-subnets broadcast.



5.3.5.4  Subnet-directed Broadcasts

         A router MUST classify as subnet-directed broadcasts
         all valid directed broadcasts destined for a directly
         attached subnetted network in which the subnet part
         is not all-ones.  If the destination network is not
         subnetted, the broadcast MUST be treated as a net-
         directed broadcast.

         A router MUST forward subnet-directed broadcasts.

         A router MUST have a configuration option to prohibit
         forwarding of subnet-directed broadcasts.  Its
         default setting MUST permit forwarding of subnet-
         directed broadcasts.

         A router MAY have a configuration option to prohibit
         forwarding of subnet-directed broadcasts from a
         source on a network on which the router has an
         interface.  If such an option is provided, its
         default setting MUST permit forwarding of subnet-
         directed broadcasts.


5.3.6  Congestion Control

      Congestion in a network is loosely defined as a
      condition where demand for resources (usually bandwidth
      or CPU time) exceeds capacity.  Congestion avoidance
      tries to prevent demand from exceeding capacity, while
      congestion recovery tries to restore an operative state.
      It is possible for a router to contribute to both of
      these mechanisms.  A great deal of effort has been spent
      studying the problem.  The reader is encouraged to read
      [FORWARD:2] for a survey of the work.  Important papers
      on the subject include [FORWARD:3], [FORWARD:4],
      [FORWARD:5], and [INTERNET:10], among others.

      The amount of storage that router should have available





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      to handle peak instantaneous demand when hosts use
      reasonable congestion policies, such as described in
      [FORWARD:5], is a function of the product of the
      bandwidth of the link times the path delay of the flows
      using the link, and therefore storage should increase as
      this Bandwidth*Delay product increases.  The exact
      function relating storage capacity to probability of
      discard is not known.

      When a router receives a packet beyond its storage
      capacity it must (by definition, not by decree) discard
      it or some other packet or packets.  Which packet to
      discard is the subject of much study but, unfortunately,
      little agreement so far.

      A router MAY discard the packet it has just received;
      this is the simplest but not the best policy.  It is
      considered better policy to randomly pick some transit
      packet on the queue and discard it (see [FORWARD:2]).  A
      router MAY use this Random Drop algorithm to determine
      which packet to discard.

      If a router implements a discard policy (such as Random
      Drop) under which it chooses a packet to discard from
      among a pool of eligible packets:

      +  If precedence-ordered queue service (described in
         Section [5.3.3.1]) is implemented and enabled, the
         router MUST NOT discard a packet whose IP precedence
         is higher than that of a packet which is not
         discarded.

      +  A router MAY protect packets whose IP headers request
         the "maximize reliability" TOS, except where doing so
         would be in violation of the previous rule.

      +  A router MAY protect fragmented IP packets, on the
         theory that dropping a fragment of a datagram may
         increase congestion by causing all fragments of the
         datagram to be retransmitted by the source.

      +  To help prevent routing perturbations or disruption
         of management functions, the router MAY protect
         packets used for routing control, link control, or





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         network management from being discarded.  Dedicated
         routers (i.e.. routers which are not also general
         purpose hosts, terminal servers, etc.) can achieve an
         approximation of this rule by protecting packets
         whose source or destination is the router itself.

      Advanced methods of congestion control include a notion
      of fairness, so that the 'user' that is penalized by
      losing a packet is the one that contributed the most to
      the congestion.  No matter what mechanism is implemented
      to deal with bandwidth congestion control, it is
      important that the CPU effort expended be sufficiently
      small that the router is not driven into CPU congestion
      also.

      As described in Section [4.3.3.3], this document
      recommends that a router should not send a Source Quench
      to the sender of the packet that it is discarding.  ICMP
      Source Quench is a very weak mechanism, so it is not
      necessary for a router to send it, and host software
      should not use it exclusively as an indicator of
      congestion.


5.3.7  Martian Address Filtering

      An IP source address is invalid if it is an IP broadcast
      address or is not a class A, B, or C address.

      An IP destination address is invalid if it is not a
      class A, B, C, or D address.

      A router SHOULD NOT forward any packet which has an
      invalid IP source address or a source address on network
      0.  A router SHOULD NOT forward, except over a loopback
      interface, any packet which has a source address on
      network 127.  A router MAY have a switch which allows
      the network manager to disable these checks.  If such a
      switch is provided, it MUST default to performing the
      checks.

      A router SHOULD NOT forward any packet which has an
      invalid IP destination address or a destination address
      on network 0.  A router SHOULD NOT forward, except over





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      a loopback interface, any packet which has a destination
      address on network 127.  A router MAY have a switch
      which allows the network manager to disable these
      checks.  If such a switch is provided, it MUST default
      to performing the checks.

      If a router discards a packet because of these rules, it
      SHOULD log at least the IP source address, the IP
      destination address, and, if the problem was with the
      source address, the physical interface on which the
      packet was received and the Link Layer address of the
      host or router from which the packet was received.


5.3.8  Source Address Validation

      A router SHOULD IMPLEMENT the ability to filter traffic
      based on a comparison of the source address of a packet
      and the forwarding table for a logical interface on
      which the packet was received.  If this filtering is
      enabled, the router MUST silently discard a packet if
      the interface on which the packet was received is not
      the interface on which a packet would be forwarded to
      reach the address contained in the source address.  In
      simpler terms, if a router wouldn't route a packet
      containing this address through a particular interface,
      it shouldn't believe the address if it appears as a
      source address in a packet read from this interface.

      If this feature is implemented, it MUST be disabled by
      default.

      DISCUSSION:
         This feature can provide useful security improvements
         in some situations, but can erroneously discard valid
         packets in situations where paths are asymmetric.



5.3.9  Packet Filtering and Access Lists

      As a means of providing security and/or limiting traffic
      through portions of a network a router SHOULD provide
      the ability to selectively forward (or filter) packets.





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      If this capability is provided, filtering of packets
      MUST be configurable either to forward all packets or to
      selectively forward them based upon the source and
      destination addresses.  Each source and destination
      address SHOULD allow specification of an arbitrary mask.

      If supported, a router MUST be configurable to allow one
      of an

      +  Include list --  specification of a list of address
         pairs to be forwarded, or an

      +  Exclude list --  specification of a list of address
         pairs NOT to be forwarded.

      A router MAY provide a configuration switch which allows
      a choice between specifying an include or an exclude
      list.

      A value matching any address (e.g. a keyword "any" or an
      address with a mask of all 0's) MUST be allowed as a
      source and/or destination address.

      In addition to address pairs, the router MAY allow any
      combination of transport and/or application protocol and
      source and destination ports to be specified.

      The router MUST allow packets to be silently discarded
      (i.e..  discarded without an ICMP error message being
      sent).

      The router SHOULD allow an appropriate ICMP unreachable
      message to be sent when a packet is discarded. The ICMP
      message SHOULD specify Communication Administratively
      Prohibited (code 13) as the reason for the destination
      being unreachable.

      The router SHOULD allow the sending of ICMP destination
      unreachable messages (code 13) to be configured for each
      combination of address pairs, protocol types, and ports
      it allows to be specified.

      The router SHOULD count and SHOULD allow selective
      logging of packets not forwarded.





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5.3.10  Multicast Routing

      An IP router SHOULD support forwarding of IP multicast
      packets, based either on static multicast routes or on
      routes dynamically determined by a multicast routing
      protocol (e.g., DVMRP [ROUTE:9]).  A router that
      forwards IP multicast packets is called a multicast
      router.


5.3.11  Controls on Forwarding

      For each physical interface, a router SHOULD have a
      configuration option which specifies whether forwarding
      is enabled on that interface.  When forwarding on an
      interface is disabled, the router:

      +  MUST silently discard any packets which are received
         on that interface but are not addressed to the router

      +  MUST NOT send packets out that interface, except for
         datagrams originated by the router

      +  MUST NOT announce via any routing protocols the
         availability of paths through the interface

      DISCUSSION:
         This feature allows the network manager to
         essentially turn off an interface but leaves it
         accessible for network management.

         Ideally, this control would apply to logical rather
         than physical interfaces, but cannot because there is
         no known way for a router to determine which logical
         interface a packet arrived on when there is not a
         one-to-one correspondence between logical and
         physical interfaces.



5.3.12  State Changes

      During the course of router operation, interfaces may
      fail or be manually disabled, or may become available





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      for use by the router.  Similarly, forwarding may be
      disabled for a particular interface or for the entire
      router or may be (re)enabled.  While such transitions
      are (usually) uncommon, it is important that routers
      handle them correctly.


5.3.12.1  When a Router Ceases Forwarding

         When a router ceases forwarding it MUST stop
         advertising all routes, except for third party
         routes.  It MAY continue to receive and use routes
         from other routers in its routing domains.  If the
         forwarding database is retained, the router MUST NOT
         cease timing the routes in the forwarding database.
         If routes that have been received from other routers
         are remembered, the router MUST NOT cease timing the
         routes which it has remembered.  It MUST discard any
         routes whose timers expire while forwarding is
         disabled, just as it would do if forwarding were
         enabled.

         DISCUSSION:
            When a router ceases forwarding, it essentially
            ceases being a router.  It is still a host, and
            must follow all of the requirements of Host
            Requirements [INTRO: 2].  The router may still be
            a passive member of one or more routing domains,
            however.  As such, it is allowed to maintain its
            forwarding database by listening to other routers
            in its routing domain.  It may not, however,
            advertise any of the routes in its forwarding
            database, since it itself is doing no forwarding.
            The only exception to this rule is when the router
            is advertising a route which uses only some other
            router, but which this router has been asked to
            advertise.

         A router MAY send ICMP destination unreachable (host
         unreachable) messages to the senders of packets that
         it is unable to forward. It SHOULD NOT send ICMP
         redirect messages.

         DISCUSSION:





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            Note that sending an ICMP destination unreachable
            (host unreachable) is a router action.  This
            message should not be sent by hosts.   This
            exception to the rules for hosts is allowed so
            that packets may be rerouted in the shortest
            possible time, and so that "black holes" are
            avoided.



5.3.12.2  When a Router Starts Forwarding

         When a router begins forwarding, it SHOULD expedite
         the sending of new routing information to all routers
         with which it normally exchanges routing information.


5.3.12.3  When an Interface Fails or is Disabled

         If an interface fails or is disabled a router MUST
         remove and stop advertising all routes in its
         forwarding database which make use of that interface.
         It MUST disable all static routes which make use of
         that interface.  If other routes to the same
         destination and TOS are learned or remembered by the
         router, the router MUST choose the best alternate,
         and add it to its forwarding database.  The router
         SHOULD send ICMP destination unreachable or ICMP
         redirect messages, as appropriate, in reply to all
         packets which it is unable to forward due to the
         interface being unavailable.


5.3.12.4  When an Interface is Enabled

         If an interface which had not been available becomes
         available, a router MUST reenable any static routes
         which use that interface.  If routes which would use
         that interface are learned by the router,  then these
         routes MUST be evaluated along with all of the other
         learned routes, and the router MUST make a decision
         as to which routes should be placed in the forwarding
         database.  The implementor is referred to Chapter
         [7], "Application Layer -- Routing Protocols" for





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         further information on how this decision is made.

         A router SHOULD expedite the sending of new routing
         information to all routers with which it normally
         exchanges routing information.


5.3.13  IP Options

      Several options, such as Record Route and Timestamp,
      contain "slots" into which a router inserts its address
      when forwarding the packet.  However, each such option
      has a finite number of slots, and therefore a router may
      find that there is not free slot into which it can
      insert its address.  No requirement listed below should
      be construed as requiring a router to insert its address
      into an option that has no remaining slot to insert it
      into.  Section [5.2.5] discusses how a router must
      choose which of its addresses to insert into an option.


5.3.13.1  Unrecognized Options

         Unrecognized IP options in forwarded packets MUST be
         passed through unchanged.


5.3.13.2  Security Option

         Some environments require the Security option in
         every packet; such a requirement is outside the scope
         of this document and the IP standard specification.
         Note, however, that the security options described in
         [INTERNET:1] and [INTERNET:16] are obsolete.  Routers
         SHOULD IMPLEMENT the revised security option
         described in [INTERNET:5].


5.3.13.3  Stream Identifier Option

         This option is obsolete.  If the Stream Identifier
         option is present in a packet forwarded by the
         router, the option MUST be ignored and passed through
         unchanged.





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5.3.13.4  Source Route Options

         A router MUST implement support for source route
         options in forwarded packets.  A router MAY implement
         a configuration option which, when enabled, causes
         all source-routed packets to be discarded.  However,
         such an option MUST NOT be enabled by default.

         DISCUSSION:
            The ability to source route datagrams through the
            Internet is important to various network
            diagnostic tools.  However, in a few rare cases,
            source routing may be used to bypass
            administrative and security controls within a
            network.  Specifically, those cases where
            manipulation of routing tables is used to provide
            administrative separation in lieu of other methods
            such as packet filtering may be vulnerable through
            source routed packets.



5.3.13.5  Record Route Option

         Routers MUST support the Record Route option in
         forwarded packets.

         A router MAY provide a configuration option which, if
         enabled, will cause the router to ignore (i.e. pass
         through unchanged) Record Route options in forwarded
         packets.  If provided, such an option MUST default to
         enabling the record-route.  This option does not
         affect the processing of Record Route options in
         datagrams received by the router itself (in
         particular, Record Route options in ICMP echo
         requests will still be processed in accordance with
         Section [4.3.3.6]).

         DISCUSSION:
            There are some people who believe that Record
            Route is a security problem because it discloses
            information about the topology of the network.
            Thus, this document allows it to be disabled.






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5.3.13.6  Timestamp Option

         Routers MUST support the timestamp option in
         forwarded packets.  A timestamp value MUST follow the
         rules given in Section [3.2.2.8] of [INTRO:2].

         If the flags field = 3 (timestamp and prespecified
         address), the router MUST add its timestamp if the
         next prespecified address matches any of the router's
         IP addresses.  It is not necessary that the
         prespecified address be either the address of the
         interface on which the packet arrived or the address
         of the interface over which it will be sent.

         IMPLEMENTATION:
            To maximize the utility of the timestamps
            contained in the timestamp option, it is suggested
            that the timestamp inserted be, as nearly as
            practical, the time at which the packet arrived at
            the router.  For datagrams originated by the
            router, the timestamp inserted should be, as
            nearly as practical, the time at which the
            datagram was passed to the network layer for
            transmission.

         A router MAY provide a configuration option which, if
         enabled, will cause the router to ignore (i.e. pass
         through unchanged) Timestamp options in forwarded
         datagrams when the flag word is set to zero
         (timestamps only) or one (timestamp and registering
         IP address).  If provided, such an option MUST
         default to off (that is, the router does not ignore
         the timestamp).  This option does not affect the
         processing of Timestamp options in datagrams received
         by the router itself (in particular, a router will
         insert timestamps into Timestamp options in datagrams
         received by the router, and Timestamp options in ICMP
         echo requests will still be processed in accordance
         with Section [4.3.3.6]).

         DISCUSSION:
            Like the Record Route option, the Timestamp option
            can reveal information about a network's topology.
            Some people consider this to be a security





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            concern.
















































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6.  TRANSPORT LAYER

A router is not required to implement any Transport Layer
protocols except those required to support Application Layer
protocols supported by the router.  In practice, this means
that most routers implement both the Transmission Control
Protocol (TCP) and the User Datagram Protocol (UDP).


6.1  USER DATAGRAM PROTOCOL -- UDP

   The User Datagram Protocol (UDP) is specified in [TRANS:1].

   A router which implements UDP MUST be compliant, and SHOULD
   be unconditionally compliant, with the requirements of
   section 4.1.3 of [INTRO:2], except that:

   +  This specification does not specify the interfaces
      between the various protocol layers.  Thus, a router
      need not comply with sections 4.1.3.2, 4.1.3.3, and
      4.1.3.5 of [INTRO:2] (except of course where compliance
      is required for proper functioning of Application Layer
      protocols supported by the router).

   +  Contrary to section 4.1.3.4 of [INTRO:2], an application
      MUST NOT be able to disable to generation of UDP
      checksums.


   DISCUSSION:
      Although a particular application protocol may require
      that UDP datagrams it receives must contain a UDP
      checksum, there is no general requirement that received
      UDP datagrams contain UDP checksums.  Of course, if a
      UDP checksum is present in a received datagram, the
      checksum must be verified and the datagram discarded if
      the checksum is incorrect.



6.2  TRANSMISSION CONTROL PROTOCOL -- TCP

   The Transmission Control Protocol (TCP) is specified in
   [TRANS:2].





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   A router which implements TCP MUST be compliant, and SHOULD
   be unconditionally compliant, with the requirements of
   section 4.2 of [INTRO:2], except that:

   +  This specification does not specify the interfaces
      between the various protocol layers.  Thus, a router
      need not comply with the following requirements of
      [INTRO:2] (except of course where compliance is required
      for proper functioning of Application Layer protocols
      supported by the router):

      Section 4.2.2.2:
           "Passing a received PSH flag to the application
           layer is now OPTIONAL."

      Section 4.2.2.4:
           "A TCP MUST inform the application layer
           asynchronously whenever it receives an Urgent
           pointer and there was previously no pending urgent
           data, or whenever the Urgent pointer advances in
           the data stream.  There MUST be a way for the
           application to learn how much urgent data remains
           to be read from the connection, or at least to
           determine whether or not more urgent data remains
           to be read."

      Section 4.2.3.5:
           "An application MUST be able to set the value for
           R2 for a particular connection.  For example, an
           interactive application might set R2 to
           ``infinity,'' giving the user control over when to
           disconnect."

      Section 4.2.3.7:
           "If an application on a multihomed host does not
           specify the local IP address when actively opening
           a TCP connection, then the TCP MUST ask the IP
           layer to select a local IP address before sending
           the (first) SYN.  See the function GET_SRCADDR() in
           Section 3.4."

      Section 4.2.3.8:
           "An application MUST be able to specify a source
           route when it actively opens a TCP connection, and





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           this MUST take precedence over a source route
           received in a datagram."

   +  For similar reasons, a router need not comply with any
      of the requirements of section 4.2.4 of [INTRO:2].

   +  The requirements of section 4.2.2.6 of [INTRO:2] are
      amended as follows: a router which implements the host
      portion of MTU discovery (discussed in Section [4.2.3.3]
      of this memo) uses 536 as the default value of SendMSS
      only if the path MTU is unknown; if the path MTU is
      known, the default value for SendMSS is the path MTU -
      40.

   +  The requirements of section 4.2.2.6 of [INTRO:2] are
      amended as follows: ICMP Destination Unreachable codes
      11 and 12 are additional soft error conditions.
      Therefore, these message MUST NOT cause TCP to abort a
      connection.

   DISCUSSION:
      It should particularly be noted that a TCP
      implementation in a router must conform to the following
      requirements of [INTRO:2]:

      +  Providing a configurable TTL. [4.2.2.1]

      +  Providing an interface to configure keep-alive
         behavior, if keep-alives are used at all. [4.2.3.6]

      +  Providing an error reporting mechanism, and the
         ability to manage it.  [4.2.4.1]

      +  Specifying type of service. [4.2.4.2]

      The general paradigm applied is that if a particular
      interface is visible outside the router, then all
      requirements for the interface must be followed.  For
      example, if a router provides a telnet function, then it
      will be generating traffic, likely to be routed in the
      external networks.  Therefore, it must be able to set
      the type of service correctly or else the telnet traffic
      may not get through.






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7.  APPLICATION LAYER -- ROUTING PROTOCOLS



7.1  INTRODUCTION

   An Autonomous System (AS) is defined as a set of routers
   all belonging under the same authority and all subject to a
   consistent set of routing policies.  Interior gateway
   protocols (IGPs) are used to distribute routing information
   inside of an AS (i.e.  intra-AS routing). Exterior gateway
   protocols are used to exchange routing information between
   ASs (i.e. inter-AS routing).


7.1.1  Routing Security Considerations

      Routing is one of the few places where the Robustness
      Principle ("be liberal in what you accept") does not
      apply.  Routers should be relatively suspicious in
      accepting routing data from other routing systems.

      A router SHOULD provide the ability to rank routing
      information sources from "most trustworthy" to "least
      trustworthy" and to accept routing information about any
      particular destination from the most trustworthy sources
      first.  This was implicit in the original core/stub
      autonomous system routing model using EGP and various
      interior routing protocols.  It is even more important
      with the demise of a central, "trusted" core.

      A router SHOULD provide a mechanism to filter out
      "obviously invalid" routes (such as those for net 127).

      Routers MUST NOT by default redistribute routing data
      they do not themselves use, trust or otherwise consider
      invalid.  In rare cases, it may be necessary to
      redistribute suspicious information, but this should
      only happen under direct intercession by some human
      agency.

      In general, routers must be at least a little paranoid
      about accepting routing data from anyone, and must be
      especially careful when they distribute routing





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      information provided to them by another party.  See
      below for specific guidelines.

      Routers SHOULD IMPLEMENT peer-to-peer authentication for
      those routing protocols that support them.


7.1.2  Precedence

      Except where the specification for a particular routing
      protocol specifies otherwise, a router SHOULD set the IP
      Precedence value for IP datagrams carrying routing
      traffic it originates to 6 (INTERNETWORK CONTROL).

      DISCUSSION:
         Routing traffic with VERY FEW exceptions should be
         the highest precedence traffic on any network.  If a
         system's routing traffic can't get through, chances
         are nothing else will.



7.2  INTERIOR GATEWAY PROTOCOLS



7.2.1  INTRODUCTION

      An Interior Gateway Protocol (IGP) is used to distribute
      routing information between the various routers in a
      particular AS. Independent of the algorithm used to
      implement a particular IGP, it should perform the
      following functions:

      (1)  Respond quickly to changes in the internal topology
           of an AS

      (2)  Provide a mechanism such that circuit flapping does
           not cause continuous routing updates

      (3)  Provide quick convergence to loop-free routing

      (4)  Utilize minimal bandwidth






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      (5)  Provide "equal cost" routes to enable "load-
           splitting"

      (6)  Provide a means for authentication of routing
           updates

      Current IGPs used in the internet today are
      characterized as either being being based on a distance-
      vector or a link-state algorithm.

      Several IGPs are detailed in this section, including
      those most commonly used and some recently developed
      protocols which may be widely used in the future.
      Numerous other protocols intended for use in intra-AS
      routing exist in the Internet community.

      A router which implements any routing protocol (other
      than static routes) MUST IMPLEMENT OSPF (see Section
      [7.2.2]) and MUST IMPLEMENT RIP (see Section [7.2.4]).
      A router MAY implement additional IGPs.


7.2.2  OPEN SHORTEST PATH FIRST -- OSPF



7.2.2.1  Introduction

         Shortest Path First (SPF) based routing protocols are
         a class of link-state algorithms which are based on
         the shortest-path algorithm of Dijkstra.  Although
         SPF based algorithms have been around since the
         inception of the ARPANet, it is only recently that
         they have achieved popularity both inside both the IP
         and the OSI communities.  In an SPF based system,
         each router obtains an exact replica of the entire
         topology database via a process known as flooding.
         Flooding insures a reliable transfer of the
         information. Each individual router then runs the SPF
         algorithm on its database to build the IP routing
         table.  The OSPF routing protocol is an
         implementation of an SPF algorithm.  The current
         version, OSPF version 2, is specified in [ROUTE:1].
         Note that RFC-1131, which describes OSPF version 1,





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         is obsolete.

         Note that to comply with Section [8.3] of this memo,
         a router which implements OSPF MUST implement the
         OSPF MIB [MGT:14].


7.2.2.2  Specific Issues


         Virtual Links

              There is a minor error in the specification that
              can cause routing loops when all of the
              following conditions are simultaneously true:

              (1)  A virtual link is configured through a
                   transit area,

              (2)  Two separate paths exist, each having the
                   same endpoints, but one utilizing only non-
                   virtual backbone links, and the other using
                   links in the transit area, and

              (3)  The latter path is part of the (underlying
                   physical representation of the) configured
                   virtual link, routing loops may occur.

              To prevent this, an implementation of OSPF
              SHOULD invoke the calculation in Section 16.3 of
              [ROUTE:1] whenever any part of the path to the
              destination is a virtual link (the specification
              only says this is necessary when the first hop
              is a virtual link).


7.2.2.3  New Version of OSPF

         As of this writing (12/21/93) there is a new version
         of the OSPF specification that is winding its way
         through the Internet standardization process.  A
         prudent implementor will be aware of this and develop
         an implementation accordingly.






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         The new version fixes several errors in the current
         specification [ROUTE:1].  For this reason,
         implementors and vendors ought to expect to upgrade
         to the new version relatively soon.  In particular,
         the following problems exist in [ROUTE:1] that the
         new version fixes:

         +  In [ROUTE:1], certain configurations of virtual
            links can lead to incorrect routing and/or routing
            loops. A fix for this is specified in the new
            specification.

         +  In [ROUTE:1], OSPF external routes to "subnet 0"s
            cannot be expressed.  For example, a router cannot
            import into an OSPF domain external routes both
            for 192.2.0.0, 255.255.0.0 and 192.2.0.0,
            255.255.255.0.  Routes such as these may become
            common with the deployment of CIDR [INTERNET:15].
            This has been addressed in the new OSPF
            specification.

         +  In [ROUTE:1], OSPF Network-LSAs originated before
            a router changes its OSPF Router ID can confuse
            the Dijkstra calculation if the router again
            becomes Designated Router for the network. This
            has been fixed.


7.2.3  INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM -- DUAL
IS-IS

      The American National Standards Institute (ANSI) X3S3.3
      committee has defined an intra-domain routing protocol.
      This protocol is titled "Intermediate System to
      Intermediate System Routeing Exchange Protocol".

      Its application to an IP network has been defined in
      [ROUTE:2], and is referred to as Dual IS-IS (or
      sometimes as Integrated IS-IS).  IS-IS is based on a
      link-state (SPF) routing algorithm and shares all the
      advantages for this class of protocols.








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7.2.4  ROUTING INFORMATION PROTOCOL -- RIP



7.2.4.1  Introduction

         RIP is specified in [ROUTE:3].  Although RIP is still
         quite important in the Internet, it is being replaced
         in sophisticated applications by more modern IGPs
         such as the ones described above.

         Another common use for RIP is as a "router discovery"
         protocol.  Section [4.3.3.10] briefly touches upon
         this subject.


7.2.4.2  Protocol Walk-Through


         Dealing with changes in topology: [ROUTE:3], pp. 11

              An implementation of RIP MUST provide a means
              for timing out routes.  Since messages are
              occasionally lost, implementations MUST NOT
              invalidate a route based on a single missed
              update.

              Implementations MUST by default wait six times
              the update interval before invalidating a route.
              A router MAY have configuration options to alter
              this value.

              DISCUSSION:
                 It is important to routing stability that all
                 routers in a RIP autonomous system use
                 similar timeout value for invalidating
                 routes, and therefore it is important that an
                 implementation default to the timeout value
                 specified in the RIP specification.  However,
                 that timeout value is overly conservative in
                 environments where packet loss is reasonably
                 rare.  In such an environment, a network
                 manager may wish to be able to decrease the
                 timeout period in order to promote faster





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                 recovery from failures.


              IMPLEMENTATION:
                 There is a very simple mechanism which a
                 router may use to meet the requirement to
                 invalidate routes promptly after they time
                 out.  Whenever the router scans the routing
                 table to see if any routes have timed out, it
                 also notes the age of the least recently
                 updated route which has not yet timed out.
                 Subtracting this age from the timeout period
                 gives the amount of time until the router
                 again needs to scan the table for timed out
                 routes.


         Split Horizon: [ROUTE:3], pp. 14-15

              An implementation of RIP MUST implement "split
              horizon", a scheme used for avoiding problems
              caused by including routes in updates sent to
              the router from which they were learned.

              An implementation of RIP SHOULD implement "Split
              horizon with poisoned reverse", a variant of
              split horizon which includes routes learned from
              a router sent to that router, but sets their
              metric to infinity.  Because of the routing
              overhead which may be incurred by implementing
              split horizon with poisoned reverse,
              implementations MAY include an option to select
              whether poisoned reverse is in effect.  An
              implementation SHOULD limit the period of time
              in which it sends reverse routes at an infinite
              metric.

              IMPLEMENTATION:
                 Each of the following algorithms can be used
                 to limit the period of time for which
                 poisoned reverse is applied to a route.  The
                 first algorithm is more complex but does a
                 more complete job of limiting poisoned
                 reverse to only those cases where it is





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                 necessary.

                 The goal of both algorithms is to ensure that
                 poison reverse is done for any destination
                 whose route has changed in the last Route
                 Lifetime (typically 180 seconds), unless it
                 can be sure that the previous route used the
                 same output interface.  The Route Lifetime is
                 used because that is the amount of time RIP
                 will keep around an old route before
                 declaring it stale.

                 The time intervals (and derived variables)
                 used in the following algorithms are as
                 follows:

                 Tu   The Update Timer; the number of seconds
                      between RIP updates.  This typically
                      defaults to 30 seconds.

                 Rl   The Route Lifetime, in seconds.  This is
                      the amount of time that a route is
                      presumed to be good, without requiring
                      an update.  This typically defaults to
                      180 seconds.

                 Ul   The Update Loss; the number of
                      consecutive updates that have to be lost
                      or fail to mention a route before RIP
                      deletes the route.  Ul is calculated to
                      be (Rl/Tu)+1.  The "+1" is to account
                      for the fact that the first time the
                      ifcounter is decremented will be less
                      than Tu seconds after it is initialized.
                      Typically, Ul will be 7: (180/30)+1.


                 In   The value to set ifcounter to when a
                      destination is newly learned.  This
                      value is Ul-4, where the "4" is RIP's
                      garbage collection timer/30

                 The first algorithm is:






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                 -- Associated with each destination is a
                    counter, called the ifcounter below.
                    Poison reverse is done for any route whose
                    destination's ifcounter is greater than
                    zero.

                 -- After a regular (not triggered or in
                    response to a request) update is sent, all
                    of the non-zero ifcounters are decremented
                    by one.

                 -- When a route to a destination is created,
                    its ifcounter is set as follows:

                    -- If the new route is superseding a valid
                       route, and the old route used a
                       different (logical) output interface,
                       then the ifcounter is set to Ul.

                    -- If the new route is superseding a stale
                       route, and the old route used a
                       different (logical) output interface,
                       then the ifcounter is set to MAX(0, Ul
                       - INT(seconds that the route has been
                       stale/Ut).

                    -- If there was no previous route to the
                       destination, the ifcounter is set to
                       In.

                    -- Otherwise, the ifcounter is set to zero

                 -- RIP also maintains a timer, called the
                    resettimer below.  Poison reverse is done
                    on all routes whenever resettimer has not
                    expired (regardless of the ifcounter
                    values).

                 -- When RIP is started, restarted, reset, or
                    otherwise has its routing table cleared,
                    it sets the resettimer to go off in Rl
                    seconds.

                 The second algorithm is identical to the





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                 first except that:

                 -- The rules which set the ifcounter to non-
                    zero values are changed to always set it
                    to Rl/Tu, and

                 -- The resettimer is eliminated.

            Triggered updates: [ROUTE:3], pp. 15-16; pp. 29

                 Triggered updates (also called "flash
                 updates") are a mechanism for immediately
                 notifying a router's neighbors when the
                 router adds or deletes routes or changes
                 their metrics.  A router MUST send a
                 triggered update when routes are deleted or
                 their metrics are increased.  A router MAY
                 send a triggered update when routes are added
                 or their metrics decreased.

                 Since triggered updates can cause excessive
                 routing overhead, implementations MUST use
                 the following mechanism to limit the
                 frequency of triggered updates:

                 (1)  When a router sends a triggered update,
                      it sets a timer to a random time between
                      one and five seconds in the future.  The
                      router must not generate additional
                      triggered updates before this timer
                      expires.

                 (2)  If the router would generate a triggered
                      update during this interval it sets a
                      flag indicating that a triggered update
                      is desired.  The router also logs the
                      desired triggered update.

                 (3)  When the triggered update timer expires,
                      the router checks the triggered update
                      flag. If the flag is set then the router
                      sends a single triggered update which
                      includes all of the changes that were
                      logged.  The router then clears the flag





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                      and, since a triggered update was sent,
                      restarts this algorithm.

                 (4)  The flag is also cleared whenever a
                      regular update is sent.

                 Triggered updates SHOULD include all routes
                 that have changed since the most recent
                 regular (non-triggered) update.  Triggered
                 updates MUST NOT include routes that have not
                 changed since the most recent regular update.

                 DISCUSSION:
                    Sending all routes, whether they have
                    changed recently or not, is unacceptable
                    in triggered updates because the
                    tremendous size of many Internet routing
                    tables could otherwise result in
                    considerable bandwidth being wasted on
                    triggered updates.

            Use of UDP: [ROUTE:3], pp. 18-19.

                 RIP packets sent to an IP broadcast address
                 SHOULD have their initial TTL set to one.

                 Note that to comply with Section [6.1] of
                 this memo, a router MUST use UDP checksums in
                 RIP packets which it originates, MUST discard
                 RIP packets received with invalid UDP
                 checksums, but MUST not discard received RIP
                 packets simply because they do not contain
                 UDP checksums.

            Addressing Considerations: [ROUTE:3], pp. 22

                 A RIP implementation SHOULD support host
                 routes.  If it does not, it MUST (as
                 described on page 27 of [ROUTE:3]) ignore
                 host routes in received updates.  A router
                 MAY log ignored hosts routes.

                 The special address 0.0.0.0 is used to
                 describe a default route. A default route is





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                 used as the route of last resort (i.e. when a
                 route to the specific net does not exist in
                 the routing table). The router MUST be able
                 to create a RIP entry for the address
                 0.0.0.0.

            Input Processing - Response: [ROUTE:3], pp. 26

                 When processing an update, the following
                 validity checks MUST be performed:

                 +  The response MUST be from UDP port 520.

                 +  The source address MUST be on a directly
                    connected subnet (or on a directly
                    connected, non-subnetted network) to be
                    considered valid.

                 +  The source address MUST NOT be one of the
                    router's addresses.

                    DISCUSSION:
                       Some networks, media, and interfaces
                       allow a sending node to receive packets
                       that it broadcasts.  A router must not
                       accept its own packets as valid routing
                       updates and process them.  The last
                       requirement prevents a router from
                       accepting its own routing updates and
                       processing them (on the assumption that
                       they were sent by some other router on
                       the network).

                 An implementation MUST NOT replace an
                 existing route if the metric received is
                 equal to the existing metric except in
                 accordance with the following heuristic.

                 An implementation MAY choose to implement the
                 following heuristic to deal with the above
                 situation. Normally, it is useless to change
                 the route to a network from one router to
                 another if both are advertised at the same
                 metric. However, the route being advertised





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                 by one of the routers may be in the process
                 of timing out. Instead of waiting for the
                 route to timeout, the new route can be used
                 after a specified amount of time has elapsed.
                 If this heuristic is implemented, it MUST
                 wait at least halfway to the expiration point
                 before the new route is installed.


7.2.4.3  Specific Issues


         RIP Shutdown

              An implementation of RIP SHOULD provide for a
              graceful shutdown using the following steps:

              (1)  Input processing is terminated,

              (2)  Four updates are generated at random
                   intervals of between two and four seconds,
                   These updates contain all routes that were
                   previously announced, but with some metric
                   changes.  Routes that were being announced
                   at a metric of infinity should continue to
                   use this metric.  Routes that had been
                   announced with a non-infinite metric should
                   be announced with a metric of 15 (infinity
                   - 1).

                   DISCUSSION:
                      The metric used for the above really
                      ought to be 16 (infinity); setting it to
                      15 is a kludge to avoid breaking certain
                      old hosts which wiretap the RIP
                      protocol.  Such a host will
                      (erroneously) abort a TCP connection if
                      it tries to send a datagram on the
                      connection while the host has no route
                      to the destination (even if the period
                      when the host has no route lasts only a
                      few seconds while RIP chooses an
                      alternate path to the destination).






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         RIP Split Horizon and Static Routes

              Split horizon SHOULD be applied to static routes
              by default.  An implementation SHOULD provide a
              way to specify, per static route, that split
              horizon should not be applied to this route.


7.2.5  GATEWAY TO GATEWAY PROTOCOL -- GGP

      The Gateway to Gateway protocol is considered obsolete
      and SHOULD NOT be implemented.


7.3  EXTERIOR GATEWAY PROTOCOLS



7.3.1  INTRODUCTION

      Exterior Gateway Protocols are utilized for inter-
      Autonomous System routing to exchange reachability
      information for a set of networks internal to a
      particular autonomous system to a neighboring autonomous
      system.

      The area of inter-AS routing is a current topic of
      research inside the Internet Engineering Task Force.
      The Exterior Gateway Protocol (EGP) described in Section
      [7.3.3] has traditionally been the inter-AS protocol of
      choice.  The Border Gateway Protocol (BGP) eliminates
      many of the restrictions and limitations of EGP, and is
      therefore growing rapidly in popularity.  A router is
      not required to implement any inter-AS routing protocol.
      However, if a router does implement EGP it also MUST
      IMPLEMENT BGP.

      Although it was not designed as an exterior gateway
      protocol, RIP (described in Section [7.2.4]) is
      sometimes used for inter-AS routing.









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7.3.2  BORDER GATEWAY PROTOCOL -- BGP



7.3.2.1  Introduction

         The Border Gateway Protocol (BGP) is an inter-AS
         routing protocol which exchanges network reachability
         information with other BGP speakers. The information
         for a network includes the complete list of ASs that
         traffic must transit to reach that network. This
         information can then be used to insure loop-free
         paths.  This information is sufficient to construct a
         graph of AS connectivity from which routing loops may
         be pruned and some policy decisions at the AS level
         may be enforced.

         BGP is defined by [ROUTE:4].  [ROUTE:5] specifies the
         proper usage of BGP in the Internet, and provides
         some useful implementation hints and guidelines.
         [ROUTE:12] and [ROUTE:13] provide additional useful
         information.

         To comply with Section [8.3] of this memo, a router
         which implements BGP MUST also implement the BGP MIB
         [MGT:15].

         To characterize the set of policy decisions that can
         be enforced using BGP, one must focus on the rule
         that an AS advertises to its neighbor ASs only those
         routes that it itself uses.  This rule reflects the
         "hop-by-hop" routing paradigm generally used
         throughout the current Internet.  Note that some
         policies cannot be supported by the "hop-by-hop"
         routing paradigm and thus require techniques such as
         source routing to enforce.  For example, BGP does not
         enable one AS to send traffic to a neighbor AS
         intending that that traffic take a different route
         from that taken by traffic originating in the
         neighbor AS.  On the other hand, BGP can support any
         policy conforming to the "hop-by-hop" routing
         paradigm.

         Implementors of BGP are strongly encouraged to follow





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         the recommendations outlined in Section 6 of
         [ROUTE:5].


7.3.2.2  Protocol Walk-through

         While BGP provides support for quite complex routing
         policies (as an example see Section 4.2 in
         [ROUTE:5]), it is not required for all BGP
         implementors to support such policies.  At a minimum,
         however, a BGP implementation:

         (1)  SHOULD allow an AS to control announcements of
              the BGP learned routes to adjacent AS's.
              Implementations SHOULD support such control with
              at least the granularity of a single network.
              Implementations SHOULD also support such control
              with the granularity of an autonomous system,
              where the autonomous system may be either the
              autonomous system that originated the route, or
              the autonomous system that advertised the route
              to the local system (adjacent autonomous
              system).

         (2)  SHOULD allow an AS to prefer a particular path
              to a destination (when more than one path is
              available).  Such function SHOULD be implemented
              by allowing system administrator to assign
              "weights" to Autonomous Systems, and making
              route selection process to select a route with
              the lowest "weight" (where "weight" of a route
              is defined as a sum of "weights" of all AS's in
              the AS_PATH path attribute associated with that
              route).

         (3)  SHOULD allow an AS to ignore routes with certain
              AS's in the AS_PATH path attribute. Such
              function can be implemented by using technique
              outlined in (2), and by assigning "infinity" as
              "weights" for such AS's. The route selection
              process must ignore routes that have "weight"
              equal to "infinity".







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7.3.3  EXTERIOR GATEWAY PROTOCOL -- EGP



7.3.3.1  Introduction

         The Exterior Gateway Protocol (EGP) specifies an EGP
         which is used to exchange reachability information
         between routers of the same or differing autonomous
         systems. EGP is not considered a routing protocol
         since there is no standard interpretation (i.e.
         metric) for the distance fields in the EGP update
         message, so distances are comparable only among
         routers of the same AS.  It is however designed to
         provide high-quality reachability information, both
         about neighbor routers and about routes to non-
         neighbor routers.

         EGP is defined by [ROUTE:6].  An implementor almost
         certainly wants to read [ROUTE:7] and [ROUTE:8] as
         well, for they contain useful explanations and
         background material.

         DISCUSSION:
            The present EGP specification has serious
            limitations, most importantly a restriction which
            limits routers to advertising only those networks
            which are reachable from within the router's
            autonomous system.  This restriction against
            propagating "third party" EGP information is to
            prevent long-lived routing loops.  This
            effectively limits EGP to a two-level hierarchy.

            RFC-975 is not a part of the EGP specification,
            and should be ignored.



7.3.3.2  Protocol Walk-through


         Indirect Neighbors: RFC-888, pp. 26

            An implementation of EGP MUST include indirect





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            neighbor support.

         Polling Intervals: RFC-904, pp. 10

            The interval between Hello command retransmissions
            and the interval between Poll retransmissions
            SHOULD be configurable but there MUST be a minimum
            value defined.

            The interval at which an implementation will
            respond to Hello commands and Poll commands SHOULD
            be configurable but there MUST be a minimum value
            defined.

         Network Reachability: RFC-904, pp. 15

            An implementation MUST default to not providing
            the external list of routers in other autonomous
            systems; only the internal list of routers
            together with the nets which are reachable via
            those routers should be included in an Update
            Response/Indication packet.  However, an
            implementation MAY elect to provide a
            configuration option enabling the external list to
            be provided.  An implementation MUST NOT include
            in the external list routers which were learned
            via the external list provided by a router in
            another autonomous system. An implementation MUST
            NOT send a network back to the autonomous system
            from which it is learned, i.e. it MUST do split-
            horizon on an autonomous system level.

            If more than 255 internal or 255 external routers
            need to be specified in a Network Reachability
            update, the networks reachable from routers that
            can not be listed MUST be merged into the list for
            one of the listed routers.  Which of the listed
            routers is chosen for this purpose SHOULD be user
            configurable, but SHOULD default to the source
            address of the EGP update being generated.

            An EGP update contains a series of blocks of
            network numbers, where each block contains a list
            of network numbers reachable at a particular





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            distance via a particular router.  If more than
            255 networks are reachable at a particular
            distance via a particular router, they are split
            into multiple blocks (all of which have the same
            distance).  Similarly, if more than 255 blocks are
            required to list the networks reachable via a
            particular router, the router's address is listed
            as many times as necessary to include all of the
            blocks in the update.
         Unsolicited Updates: RFC-904, pp. 16

            If a network is shared with the peer, an
            implementation MUST send an unsolicited update
            upon entry to the Up state assuming that the
            source network is the shared network.

         Neighbor Reachability: RFC-904, pp. 6, 13-15

            The table on page 6 which describes the values of
            j and k (the neighbor up and down thresholds) is
            incorrect.  It is reproduced correctly here:

               Name    Active  Passive Description
               -----------------------------------------------
                j         3       1    neighbor-up threshold
                k         1       0    neighbor-down threshold

            The value for k in passive mode also specified
            incorrectly in RFC-904, pp. 14 The values in
            parenthesis should read:

               (j = 1, k = 0, and T3/T1 = 4)

            As an optimization, an implementation can refrain
            from sending a Hello command when a Poll is due.
            If an implementation does so, it SHOULD provide a
            user configurable option to disable this
            optimization.

         Abort timer: RFC-904, pp. 6, 12, 13

            An EGP implementation MUST include support for the
            abort timer (as documented in section 4.1.4 of
            RFC-904).  An implementation SHOULD use the abort





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            timer in the Idle state to automatically issue a
            Start event to restart the protocol machine.
            Recommended values are P4 for a critical error
            (Administratively prohibited, Protocol Violation
            and Parameter Problem) and P5 for all others.  The
            abort timer SHOULD NOT be started when a Stop
            event was manually initiated (such as via a
            network management protocol).

         Cease command received in Idle state: RFC-904, pp. 13

            When the EGP state machine is in the Idle state,
            it MUST reply to Cease commands with a Cease-ack
            response.

         Hello Polling Mode: RFC-904, pp. 11

            An EGP implementation MUST include support for
            both active and passive polling modes.

         Neighbor Acquisition Messages: RFC-904, pp. 18

            As noted the Hello and Poll Intervals should only
            be present in Request and Confirm messages.
            Therefore the length of an EGP Neighbor
            Acquisition Message is 14 bytes for a Request or
            Confirm message and 10 bytes for a Refuse, Cease
            or Cease-ack message.  Implementations MUST NOT
            send 14 bytes for Refuse, Cease or Cease-ack
            messages but MUST allow for implementations that
            send 14 bytes for these messages.

         Sequence Numbers: RFC-904, pp. 10

            Response or indication packets received with a
            sequence number not equal to S MUST be discarded.
            The send sequence number S MUST be incremented
            just before the time a Poll command is sent and at
            no other times.










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7.3.4  INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL

      It is possible to exchange routing information between
      two autonomous systems or routing domains without using
      a standard exterior routing protocol between two
      separate, standard interior routing protocols.  The most
      common way of doing this is to run both interior
      protocols independently in one of the border routers
      with an exchange of route information between the two
      processes.

      As with the exchange of information from an EGP to an
      IGP, without appropriate controls these exchanges of
      routing information between two IGPs in a single router
      are subject to creation of routing loops.


7.4  STATIC ROUTING

   Static routing provides a means of explicitly defining the
   next hop from a router for a particular destination.  A
   router SHOULD provide a means for defining a static route
   to a destination, where the destination is defined by an
   address and an address mask.  The mechanism SHOULD also
   allow for a metric to be specified for each static route.

   A router which supports a dynamic routing protocol MUST
   allow static routes to be defined with any metric valid for
   the routing protocol used.  The router MUST provide the
   ability for the user to specify a list of static routes
   which may or may not be propagated via the routing
   protocol.  In addition, a router SHOULD support the
   following additional information if it supports a routing
   protocol that could make use of the information. They are:

   +  TOS,

   +  Subnet mask, or

   +  A metric specific to a given routing protocol that can
      import the route.

   DISCUSSION:
      We intend that one needs to support only the things





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      useful to the given routing protocol.  The need for TOS
      should not require the vendor to implement the other
      parts if they are not used.

   Whether a router prefers a static route over a dynamic
   route (or vice versa) or whether the associated metrics are
   used to choose between conflicting static and dynamic
   routes SHOULD be configurable for each static route.

   A router MUST allow a metric to be assigned to a static
   route for each routing domain that it supports.  Each such
   metric MUST be explicitly assigned to a specific routing
   domain.  For example:

        route 36.0.0.0 255.0.0.0 via 192.19.200.3 rip metric 3

        route 36.21.0.0 255.255.0.0 via 192.19.200.4 ospf
        inter-area metric 27

        route 36.22.0.0 255.255.0.0 via 192.19.200.5 egp 123
        metric 99

        route 36.23.0.0 255.255.0.0 via 192.19.200.6 igrp 47
        metric 1 2 3 4 5

   DISCUSSION:
      It has been suggested that, ideally, static routes
      should have preference values rather than metrics (since
      metrics can only be compared with metrics of other
      routes in the same routing domain, the metric of a
      static route could only be compared with metrics of
      other static routes).  This is contrary to some current
      implementations, where static routes really do have
      metrics, and those metrics are used to determine whether
      a particular dynamic route overrides the static route to
      the same destination.  Thus, this document uses the term
      metric rather than preference.

      This technique essentially makes the static route into a
      RIP route, or an OSPF route (or whatever, depending on
      the domain of the metric).  Thus, the route lookup
      algorithm of that domain applies.  However, this is NOT
      route leaking, in that coercing a static route into a
      dynamic routing domain does not authorize the router to





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      redistribute the route into the dynamic routing domain.

      For static routes not put into a specific routing
      domain, the route lookup algorithm is:

      (1)  Basic match

      (2)  Longest match

      (3)  Weak TOS (if TOS supported)

      (4)  Best metric (where metric are implementation-
           defined)

      The last step may not be necessary, but it's useful in
      the case where you want to have a primary static route
      over one interface and a secondary static route over an
      alternate interface, with failover to the alternate path
      if the interface for the primary route fails.



7.5  FILTERING OF ROUTING INFORMATION

   Each router within a network makes forwarding decisions
   based upon information contained within its forwarding
   database.  In a simple network the contents of the database
   may be statically configured.  As the network grows more
   complex, the need for dynamic updating of the forwarding
   database becomes critical to the efficient operation of the
   network.

   If the data flow through a network is to be as efficient as
   possible, it is necessary to provide a mechanism for
   controlling the propagation of the information a router
   uses to build its forwarding database.  This control takes
   the form of choosing which sources of routing information
   should be trusted and selecting which pieces of the
   information to believe.  The resulting forwarding database
   is a filtered version of the available routing information.

   In addition to efficiency, controlling the propagation of
   routing information can reduce instability by preventing
   the spread of incorrect or bad routing information.





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   In some cases local policy may require that complete
   routing information not be widely propagated.

   These filtering requirements apply only to non-SPF-based
   protocols (and therefore not at all to routers which don't
   implement any distance vector protocols).


7.5.1  Route Validation

      A router SHOULD log as an error any routing update
      advertising a route to network zero, subnet zero, or
      subnet -1, unless the routing protocol from which the
      update was received uses those values to encode special
      routes (such as default routes).


7.5.2  Basic Route Filtering

      Filtering of routing information allows control of paths
      used by a router to forward packets it receives.  A
      router should be selective in which sources of routing
      information it listens to and what routes it believes.
      Therefore, a router MUST provide the ability to specify:

      +  On which logical interfaces routing information will
         be accepted and which routes will be accepted from
         each logical interface.

      +  Whether all routes or only a default route is
         advertised on a logical interface.

      Some routing protocols do not recognize logical
      interfaces as a source of routing information.  In such
      cases the router MUST provide the ability to specify

      +  from which other routers routing information will be
         accepted.

      For example, assume a router connecting one or more leaf
      networks to the main portion or backbone of a larger
      network.  Since each of the leaf networks has only one
      path in and out, the router can simply send a default
      route to them.  It advertises the leaf networks to the





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      main network.


7.5.3  Advanced Route Filtering

      As the topology of a network grows more complex, the
      need for more complex route filtering arises.
      Therefore, a router SHOULD provide the ability to
      specify independently for each routing protocol:

      +  Which logical interfaces or routers routing
         information (routes) will be accepted from and which
         routes will be believed from each other router or
         logical interface,

      +  Which routes will be sent via which logical
         interface(s), and

      +  Which routers routing information will be sent to, if
         this is supported by the routing protocol in use.

      In many situations it is desirable to assign a
      reliability ordering to routing information received
      from another router instead of the simple believe or
      don't believe choice listed in the first bullet above.
      A router MAY provide the ability to specify:

      +  A reliability or preference to be assigned to each
         route received.  A route with higher reliability will
         be chosen over one with lower reliability regardless
         of the routing metric associated with each route.

      If a router supports assignment of preferences, the
      router MUST NOT propagate any routes it does not prefer
      as first party information.  If the routing protocol
      being used to propagate the routes does not support
      distinguishing between first and third party
      information, the router MUST NOT propagate any routes it
      does not prefer.

      DISCUSSION:
         For example, assume a router receives a route to
         network C from router R and a route to the same
         network from router S.  If router R is considered





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         more reliable than router S traffic destined for
         network C will be forwarded to router R regardless of
         the route received from router S.

      Routing information for routes which the router does not
      use (router S in the above example) MUST NOT be passed
      to any other router.


7.6  INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE

   Routers MUST be able to exchange routing information
   between separate IP interior routing protocols, if
   independent IP routing processes can run in the same
   router.  Routers MUST provide some mechanism for avoiding
   routing loops when routers are configured for bi-
   directional exchange of routing information between two
   separate interior routing processes.  Routers MUST provide
   some priority mechanism for choosing routes from among
   independent routing processes.  Routers SHOULD provide
   administrative control of IGP-IGP exchange when used across
   administrative boundaries.

   Routers SHOULD provide some mechanism for translating or
   transforming metrics on a per network basis.  Routers (or
   routing protocols) MAY allow for global preference of
   exterior routes imported into an IGP.

   DISCUSSION:
      Different IGPs use different metrics, requiring some
      translation technique when introducing information from
      one protocol into another protocol with a different form
      of metric.  Some IGPs can run multiple instances within
      the same router or set of routers.  In this case metric
      information can be preserved exactly or translated.

      There are at least two techniques for translation
      between different routing processes.  The static (or
      reachability) approach uses the existence of a route
      advertisement in one IGP to generate a route
      advertisement in the other IGP with a given metric.  The
      translation or tabular approach uses the metric in one
      IGP to create a metric in the other IGP through use of
      either a function (such as adding a constant) or a table





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      lookup.

      Bi-directional exchange of routing information is
      dangerous without control mechanisms to limit feedback.
      This is the same problem that distance vector routing
      protocols must address with the split horizon technique
      and that EGP addresses with the third-party rule.
      Routing loops can be avoided explicitly through use of
      tables or lists of permitted/denied routes or implicitly
      through use of a split horizon rule, a no-third-party
      rule, or a route tagging mechanism.  Vendors are
      encouraged to use implicit techniques where possible to
      make administration easier for network operators.




































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8.  APPLICATION LAYER -- NETWORK MANAGEMENT PROTOCOLS

Note that this chapter supersedes any requirements stated in
section 6.3 of [INTRO:3].


8.1  The Simple Network Management Protocol -- SNMP



8.1.1  SNMP Protocol Elements

      Routers MUST be manageable by SNMP [MGT:3].  The SNMP
      MUST operate using UDP/IP as its transport and network
      protocols.  Others MAY be supported (e.g., see [MGT:25,
      MGT:26, MGT:27, and MGT:28]).  SNMP management
      operations MUST operate as if the SNMP was implemented
      on the router itself. Specifically, management
      operations MUST be effected by sending SNMP management
      requests to any of the IP addresses assigned to any of
      the router's interfaces. The actual management operation
      may be performed either by the router or by a proxy for
      the router.

      DISCUSSION:
         This wording is intended to allow management either
         by proxy, where the proxy device responds to SNMP
         packets which have one of the router's IP addresses
         in the packets destination address field, or the SNMP
         is implemented directly in the router itself and
         receives packets and responds to them in the proper
         manner.

         It is important that management operations can be
         sent to one of the router's IP Addresses.  In
         diagnosing network problems the only thing
         identifying the router that is available may be one
         of the router's IP address; obtained perhaps by
         looking through another router's routing table.

      All SNMP operations (get, get-next, get-response, set,
      and trap) MUST be implemented.

      Routers MUST provide a mechanism for rate-limiting the





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      generation of SNMP trap messages.  Routers MAY provide
      this mechanism via the algorithms for asynchronous alert
      management described in [MGT:5].

      DISCUSSION:
         Although there is general agreement about the need to
         rate-limit traps, there is not yet consensus on how
         this is best achieved.  The reference cited is
         considered experimental.



8.2  Community Table

   For the purposes of this specification, we assume that
   there is an abstract `community table' in the router.  This
   table contains several entries, each entry for a specific
   community and containing the parameters necessary to
   completely define the attributes of that community.  The
   actual implementation method of the abstract community
   table is, of course, implementation specific.

   A router's community table MUST allow for at least one
   entry and SHOULD allow for at least two entries.

   DISCUSSION:
      A community table with zero capacity is useless.  It
      means that the router will not recognize any communities
      and, therefore, all SNMP operations will be rejected.

      Therefore, one entry is the minimal useful size of the
      table.  Having two entries allows one entry to be
      limited to read-only access while the other would have
      write capabilities.

   Routers MUST allow the user to manually (i.e., without
   using SNMP) examine, add, delete and change entries in the
   SNMP community table. The user MUST be able to set the
   community name.  The user MUST be able to configure
   communities as read-only (i.e., they do not allow SETs) or
   read-write (i.e., they do allow SETs).

   The user MUST be able to define at least one IP address to
   which traps are sent for each community.  These addresses





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   MUST be definable on a per-community basis.  Traps MUST be
   enablable or disablable on a per-community basis.

   A router SHOULD provide the ability to specify a list of
   valid network managers for any particular community.  If
   enabled, a router MUST validate the source address of the
   SNMP datagram against the list and MUST discard the
   datagram if its address does not appear.  If the datagram
   is discarded the router MUST take all actions appropriate
   to an SNMP authentication failure.

   DISCUSSION:
      This is a rather limited authentication system, but
      coupled with various forms of packet filtering may
      provide some small measure of increased security.

   The community table MUST be saved in non-volatile storage.

   The initial state of the community table SHOULD contain one
   entry, with the community name string "public" and read-
   only access.  The default state of this entry MUST NOT send
   traps.  If it is implemented, then this entry MUST remain
   in the community table until the administrator changes it
   or deletes it.

   DISCUSSION:
      By default, traps are not sent to this community.  Trap
      PDUs are sent to unicast IP addresses. This address must
      be configured into the router in some manner. Before the
      configuration occurs, there is no such address, so to
      whom should the trap be sent? Therefore trap sending to
      the "public" community defaults to be disabled. This
      can, of course, be changed by an administrative
      operation once the router is operational.



8.3  Standard MIBS

   All MIBS relevant to a router's configuration are to be
   implemented.  To wit:

   +  The System, Interface, IP, ICMP, and UDP groups of MIB-
      II [MGT:2] MUST be implemented.





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   +  The Interface Extensions MIB [MGT:18] MUST be
      implemented.

   +  The IP Forwarding Table MIB [MGT:20] MUST be
      implemented.

   +  If the router implements TCP (e.g. for Telnet) then the
      TCP group of MIB-II [MGT:2] MUST be implemented.

   +  If the router implements EGP then the EGP group of MIB-
      II [MGT:2] MUST be implemented.

   +  If the router supports OSPF then the OSPF MIB [MGT:14]
      MUST be implemented.

   +  If the router supports BGP then the BGP MIB [MGT:15]
      MUST be implemented.

   +  If the router has Ethernet, 802.3, or StarLan interfaces
      then the Ethernet-Like MIB [MGT:6] MUST be implemented.

   +  If the router has 802.4 interfaces then the 802.4 MIB
      [MGT:7] MAY be implemented.

   +  If the router has 802.5 interfaces then the 802.5 MIB
      [MGT:8] MUST be implemented.

   +  If the router has FDDI interfaces that implement ANSI
      SMT 7.3 then the FDDI MIB [MGT:9] MUST be implemented.

   +  If the router has FDDI interfaces that implement ANSI
      SMT 6.2 then the FDDI MIB [MGT:29] MUST be implemented.

   +  If the router has RS-232 interfaces then the RS-232
      [MGT:10] MIB MUST be implemented.

   +  If the router has T1/DS1 interfaces then the T1/DS1 MIB
      [MGT:16] MUST be implemented.

   +  If the router has T3/DS3 interfaces then the T3/DS3 MIB
      [MGT:17] MUST be implemented.

   +  If the router has SMDS interfaces then the SMDS
      Interface Protocol MIB [MGT:19] MUST be implemented.





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   +  If the router supports PPP over any of its interfaces
      then the PPP MIBs [MGT:11], [MGT:12], and [MGT:13] MUST
      be implemented.

   +  If the router supports RIP Version 2 then the RIP
      Version 2 MIB [MGT:21] MUST be implemented.

   +  If the router supports X.25 over any of its interfaces
      then the X.25 MIBs [MGT:22, MGT:23 and MGT:24] MUST be
      implemented.


8.4  Vendor Specific MIBS

   The Internet Standard and Experimental MIBs do not cover
   the entire range of statistical, state, configuration and
   control information that may be available in a network
   element. This information is, never the less, extremely
   useful. Vendors of routers (and other network devices)
   generally have developed MIB extensions that cover this
   information. These MIB extensions are called Vendor
   Specific MIBs.

   The Vendor Specific MIB for the router MUST provide access
   to all statistical, state, configuration, and control
   information that is not available through the Standard and
   Experimental MIBs that have been implemented.  This
   information MUST be available for both monitoring and
   control operations.

   DISCUSSION:
      The intent of this requirement is to provide the ability
      to do anything on the router via SNMP that can be done
      via a console.  A certain minimal amount of
      configuration is necessary before SNMP can operate
      (e.g., the router must have an IP address). This initial
      configuration can not be done via SNMP. However, once
      the initial configuration is done, full capabilities
      ought to be available via network management.

   The vendor SHOULD make available the specifications for all
   Vendor Specific MIB variables. These specifications MUST
   conform to the SMI [MGT:1] and the descriptions MUST be in
   the form specified in [MGT:4].





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   DISCUSSION:
      Making the Vendor Specific MIB available to the user is
      necessary. Without this information the users would not
      be able to configure their network management systems to
      be able to access the Vendor Specific parameters.  These
      parameters would then be useless.

      The format of the MIB specification is also specified.
      Parsers which read MIB specifications and generate the
      needed tables for the network management station are
      available.  These parsers generally understand only the
      standard MIB specification format.



8.5  Saving Changes

   Parameters altered by SNMP MAY be saved to non-volatile
   storage.

   DISCUSSION:
      Reasons why this "requirement" is a MAY:

      +  The exact physical nature of non-volatile storage is
         not specified in this document.  Hence, parameters
         may be saved in NVRAM/EEPROM, local floppy or hard
         disk, or in some TFTP file server or BOOTP server,
         etc. Suppose that that this information is in a file
         that is retrieved via TFTP. In that case, a change
         made to a configuration parameter on the router would
         need to be propagated back to the file server holding
         the configuration file.  Alternatively, the SNMP
         operation would need to be directed to the file
         server, and then the change somehow propagated to the
         router.  The answer to this problem does not seem
         obvious.

         This also places more requirements on the host
         holding the configuration information than just
         having an available tftp server, so much more that
         its probably unsafe for a vendor to assume that any
         potential customer will have a suitable host
         available.






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      +  The timing of committing changed parameters to non-
         volatile storage is still an issue for debate. Some
         prefer to commit all changes immediately. Others
         prefer to commit changes to non-volatile storage only
         upon an explicit command.












































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9.  APPLICATION LAYER -- MISCELLANEOUS PROTOCOLS

For all additional application protocols that a router
implements, the router MUST be compliant and SHOULD be
unconditionally compliant with the relevant requirements of
[INTRO:3].


9.1  BOOTP



9.1.1  Introduction

      The Bootstrap Protocol (BOOTP) is a UDP/IP-based
      protocol which allows a booting host to configure itself
      dynamically and without user supervision.  BOOTP
      provides a means to notify a host of its assigned IP
      address, the IP address of a boot server host, and the
      name of a file to be loaded into memory and executed
      ([APPL:1]).  Other configuration information such as the
      local subnet mask, the local time offset, the addresses
      of default routers, and the addresses of various
      Internet servers can also be communicated to a host
      using BOOTP ([APPL:2]).


9.1.2  BOOTP Relay Agents

      In many cases, BOOTP clients and their associated BOOTP
      server(s) do not reside on the same IP network or
      subnet.  In such cases, a third-party agent is required
      to transfer BOOTP messages between clients and servers.
      Such an agent was originally referred to as a "BOOTP
      forwarding agent."  However, in order to avoid confusion
      with the IP forwarding function of a router, the name
      "BOOTP relay agent" has been adopted instead.

      DISCUSSION:
         A BOOTP relay agent performs a task which is distinct
         from a router's normal IP forwarding function.  While
         a router normally switches IP datagrams between
         networks more-or-less transparently, a BOOTP relay
         agent may more properly be thought to receive BOOTP





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         messages as a final destination and then generate new
         BOOTP messages as a result.  One should resist the
         notion of simply forwarding a BOOTP message "straight
         through like a regular packet."

      This relay-agent functionality is most conveniently
      located in the routers which interconnect the clients
      and servers (although it may alternatively be located in
      a host which is directly connected to the client
      subnet).

      A router MAY provide BOOTP relay-agent capability.  If
      it does, it MUST conform to the specifications in
      [APPL:3].

      Section [5.2.3] discussed the circumstances under which
      a packet is delivered locally (to the router).  All
      locally delivered UDP messages whose UDP destination
      port number is BOOTPS (67) are considered for special
      processing by the router's logical BOOTP relay agent.

      Sections [4.2.2.11] and [5.3.7] discussed invalid IP
      source addresses.  According to these rules, a router
      must not forward any received datagram whose IP source
      address is 0.0.0.0.  However, routers which support a
      BOOTP relay agent MUST accept for local delivery to the
      relay agent BOOTREQUEST messages whose IP source address
      is 0.0.0.0.





















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10.  OPERATIONS AND MAINTENANCE

This chapter supersedes any requirements stated in section 6.2
of [INTRO:3].

Facilities to support operation and maintenance (O&M)
activities form an essential part of any router
implementation.  Although these functions do not seem to
relate directly to interoperability, they are essential to the
network manager who must make the router interoperate and must
track down problems when it doesn't.  This chapter also
includes some discussion of router initialization and of
facilities to assist network managers in securing and
accounting for their networks.


10.1  Introduction

   The following kinds of activities are included under router
   O&M:

   +  Diagnosing hardware problems in the router's processor,
      in its network interfaces, or in its connected networks,
      modems, or communication lines.

   +  Installing new hardware

   +  Installing new software.

   +  Restarting or rebooting the router after a crash.

   +  Configuring (or reconfiguring) the router.

   +  Detecting and diagnosing Internet problems such as
      congestion, routing loops, bad IP addresses, black
      holes, packet avalanches, and misbehaved hosts.

   +  Changing network topology, either temporarily (e.g., to
      bypass a communication line problem) or permanently.

   +  Monitoring the status and performance of the routers and
      the connected networks.

   +  Collecting traffic statistics for use in (Inter-)network





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      planning.

   +  Coordinating the above activities with appropriate
      vendors and telecommunications specialists.

   Routers and their connected communication lines are often
   operated as a system by a centralized O&M organization.
   This organization may maintain a (Inter-)network operation
   center, or NOC, to carry out its O&M functions.  It is
   essential that routers support remote control and
   monitoring from such a NOC through an Internet path, since
   routers might not be connected to the same network as their
   NOC.  Since a network failure may temporarily preclude
   network access, many NOCs insist that routers be accessible
   for network management via an alternative means, often
   dialup modems attached to console ports on the routers.

   Since an IP packet traversing an internet will often use
   routers under the control of more than one NOC, Internet
   problem diagnosis will often involve cooperation of
   personnel of more than one NOC.  In some cases, the same
   router may need to be monitored by more than one NOC, but
   only if necessary, because excessive monitoring could
   impact a router's performance.

   The tools available for monitoring at a NOC may cover a
   wide range of sophistication. Current implementations
   include multi-window, dynamic displays of the entire router
   system. The use of AI techniques for automatic problem
   diagnosis is proposed for the future.

   Router O&M facilities discussed here are only a part of the
   large and difficult problem of Internet management.  These
   problems encompass not only multiple management
   organizations, but also multiple protocol layers.  For
   example, at the current stage of evolution of the Internet
   architecture, there is a strong coupling between host TCP
   implementations and eventual IP-level congestion in the
   router system [OPER:1].  Therefore, diagnosis of congestion
   problems will sometimes require the monitoring of TCP
   statistics in hosts.  There are currently a number of R&D
   efforts in progress in the area of Internet management and
   more specifically router O&M. These R&D efforts have
   already produced standards for router O&M. This is also an





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   area in which vendor creativity can make a significant
   contribution.


10.2  Router Initialization



10.2.1  Minimum Router Configuration

      There exists a minimum set of conditions that must be
      satisfied before a router may forward packets.  A router
      MUST NOT enable forwarding on any physical interface
      unless either:

      (1)  The router knows the IP address and associated
           subnet mask of at least one logical interface
           associated with that physical interface, or

      (2)  The router knows that the interface is an
           unnumbered interface and also knows its router-id.

      These parameters MUST be explicitly configured:

      +  A router MUST NOT use factory-configured default
         values for its IP addresses, subnet masks, or router-
         id, and

      +  A router MUST NOT assume that an unconfigured
         interface is an unnumbered interface.

      DISCUSSION:
         There have been instances in which routers have been
         shipped with vendor-installed default addresses for
         interfaces.  In a few cases, this has resulted in
         routers advertising these default addresses into
         active networks.



10.2.2  Address and Address Mask Initialization

      A router MUST allow its IP addresses and their subnet
      masks to be statically configured and saved in permanent





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      storage.

      A router MAY obtain its IP addresses and their
      corresponding subnet masks dynamically as a side effect
      of the system initialization process (see Section
      10.2.3]);

      If the dynamic method is provided, the choice of method
      to be used in a particular router MUST be configurable.

      As was described in Section [4.2.2.11], IP addresses are
      not permitted to have the value 0 or -1 for any of the
      <Host-number>, <Network-number>, or <Subnet-number>
      fields.  Therefore, a router SHOULD NOT allow an IP
      address or subnet mask to be set to a value which would
      make any of the the three fields above have the value
      zero or -1.

      DISCUSSION:
         It is possible using variable length subnet masks to
         create situations in which routing is ambiguous
         (i.e., two routes with different but equally-specific
         subnet masks match a particular destination address).
         We suspect that a router could, when setting a subnet
         mask, check whether the mask would cause routing to
         be ambiguous, and that implementors might be able to
         decrease their customer support costs by having
         routers prohibit or log such erroneous
         configurations.  However, at this time we do not
         require routers to make such checks because we know
         of no published method for accurately making this
         check.

      A router SHOULD make the following checks on any subnet
      mask it installs:

      +  The mask is not all 1-bits.

      +  The bits which correspond to the network number part
         of the address are all set to 1.


      DISCUSSION:
         The masks associated with routes are also sometimes





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         called "subnet masks", this test should not be
         applied to them.



10.2.3  Network Booting using BOOTP and TFTP

      There has been a lot of discussion on how routers can
      and should be booted from the network.  In general,
      these discussions have centered around BOOTP and TFTP.
      Currently, there are routers that boot with TFTP from
      the network.  There is no reason that BOOTP could not be
      used for locating the server that the boot image should
      be loaded from.

      In general, BOOTP is a protocol used to boot end
      systems, and requires some stretching to accommodate its
      use with routers.  If a router is using BOOTP to locate
      the current boot host, it should send a BOOTP Request
      with its hardware address for its first interface, or,
      if it has been previously configured otherwise, with
      either another interface's hardware address, or another
      number to put in the hardware address field of the BOOTP
      packet.  This is to allow routers without hardware
      addresses (like sync line only routers) to use BOOTP for
      bootload discovery.  TFTP can then be used to retrieve
      the image found in the BOOTP Reply.  If there are no
      configured interfaces or numbers to use, a router MAY
      cycle through the interface hardware addresses it has
      until a match is found by the BOOTP server.

      A router SHOULD IMPLEMENT the ability to store
      parameters learned via BOOTP into local stable storage.
      A router MAY implement the ability to store a system
      image loaded over the network into local stable storage.

      A router MAY have a facility to allow a remote user to
      request that the router get a new boot image.
      Differentiation should be made between getting the new
      boot image from one of three locations: the one included
      in the request, from the last boot image server, and
      using BOOTP to locate a server.







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10.3  Operation and Maintenance



10.3.1  Introduction

      There is a range of possible models for performing O&M
      functions on a router.  At one extreme is the local-only
      model, under which the O&M functions can only be
      executed locally (e.g., from a terminal plugged into the
      router machine).  At the other extreme, the fully-remote
      model allows only an absolute minimum of functions to be
      performed locally (e.g., forcing a boot), with most O&M
      being done remotely from the NOC.  There are
      intermediate models, such as one in which NOC personnel
      can log into the router as a host, using the Telnet
      protocol, to perform functions which can also be invoked
      locally.  The local-only model may be adequate in a few
      router installations, but in general remote operation
      from a NOC will be required, and therefore remote O&M
      provisions are required for most routers.

      Remote O&M functions may be exercised through a control
      agent (program).  In the direct approach, the router
      would support remote O&M functions directly from the NOC
      using standard Internet protocols (e.g., SNMP, UDP or
      TCP); in the indirect approach, the control agent would
      support these protocols and control the router itself
      using proprietary protocols.  The direct approach is
      preferred, although either approach is acceptable.  The
      use of specialized host hardware and/or software
      requiring significant additional investment is
      discouraged; nevertheless, some vendors may elect to
      provide the control agent as an integrated part of the
      network in which the routers are a part.  If this is the
      case, it is required that a means be available to
      operate the control agent from a remote site using
      Internet protocols and paths and with equivalent
      functionality with respect to a local agent terminal.

      It is desirable that a control agent and any other NOC
      software tools which a vendor provides operate as user
      programs in a standard operating system.  The use of the
      standard Internet protocols UDP and TCP for





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      communicating with the routers should facilitate this.

      Remote router monitoring and (especially) remote router
      control present important access control problems which
      must be addressed.  Care must also be taken to ensure
      control of the use of router resources for these
      functions.  It is not desirable to let router monitoring
      take more than some limited fraction of the router CPU
      time, for example.  On the other hand, O&M functions
      must receive priority so they can be exercised when the
      router is congested, since often that is when O&M is
      most needed.


10.3.2  Out Of Band Access

      Routers MUST support Out-Of-Band (OOB) access.  OOB
      access SHOULD provide the same functionality as in-band
      access.

      DISCUSSION:
         This Out-Of-Band access will allow the NOC a way to
         access isolated routers during times when network
         access is not available.

         Out-Of-Band access is an important management tool
         for the network administrator.  It allows the access
         of equipment independent of the network connections.
         There are many ways to achieve this access.
         Whichever one is used it is important that the access
         is independent of the network connections.  An
         example of Out-Of-Band access would be a serial port
         connected to a modem that provides dial up access to
         the router.

         It is important that the OOB access provides the same
         functionality as in-band access.  In-band access, or
         accessing equipment through the existing network
         connection, is limiting, because most of the time,
         administrators need to reach equipment to figure out
         why it is unreachable.  In band access is still very
         important for configuring a router, and for
         troubleshooting more subtle problems.






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10.3.2  Router O&M Functions



10.3.2.1  Maintenance -- Hardware Diagnosis

         Each router SHOULD operate as a stand-alone device
         for the purposes of local hardware maintenance.
         Means SHOULD be available to run diagnostic programs
         at the router site using only on-site tools.  A
         router SHOULD be able to run diagnostics in case of a
         fault.  For suggested hardware and software
         diagnostics see Section [10.3.3].


10.3.2.2  Control -- Dumping and Rebooting

         A router MUST include both in-band and out-of-band
         mechanisms to allow the network manager to reload,
         stop, and restart the router.  A router SHOULD also
         contain a mechanism (such as a watchdog timer) which
         will reboot the router automatically if it "hangs"
         due to a software or hardware fault.

         A router SHOULD IMPLEMENT a mechanism for dumping the
         contents of a router's memory (and/or other state
         useful for vendor debugging after a crash), and
         either saving them on a stable storage device local
         to the router or saving them on another host via an
         up-line dump mechanism such as TFTP (see [OPER:2],
         [INTRO:3]).


10.3.2.3  Control -- Configuring the Router

         Every router has configuration parameters which may
         need to be set.  It SHOULD be possible to update the
         parameters without rebooting the router; at worst, a
         restart MAY be required.  There may be cases when it
         is not possible to change parameters without
         rebooting the router (for instance, changing the IP
         address of an interface).  In these cases, care
         should be taken to minimize disruption to the router
         and the surrounding network.





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         There SHOULD be a way to configure the router over
         the network either manually or automatically.  A
         router SHOULD be able to upload or download its
         parameters from a host or another router, and these
         parameters SHOULD be convertible into some sort of
         text format for making changes and then back to the
         form the router can read.  A router SHOULD have some
         sort of stable storage for its configuration. A
         router SHOULD NOT believe protocols such as RARP,
         ICMP Address Mask Reply, and MAY not believe BOOTP.

         DISCUSSION:
            It is necessary to note here that in the future
            RARP, ICMP Address Mask Reply, BOOTP and other
            mechanisms may be needed to allow a router to
            auto-configure.  Although routers may in the
            future be able to configure automatically, the
            intent here is to discourage this practice in a
            production environment until such time as auto-
            configuration has been tested more thoroughly. The
            intent is NOT to discourage auto-configuration all
            together.  In cases where a router is expected to
            get its configuration automatically it may be wise
            to allow the router to believe these things as it
            comes up and then ignore them after it has gotten
            its configuration.



10.3.2.4  Netbooting of System Software

         A router SHOULD keep its system image in local non-
         volatile storage such as PROM, NVRAM, or disk. It MAY
         also be able to load its system software over the
         network from a host or another router.

         A router which can keep its system image in local
         non-volatile storage MAY be configurable to boot its
         system image over the network.  A router which offers
         this option SHOULD be configurable to boot the system
         image in its non-volatile local storage if it is
         unable to boot its system image over the network.

         DISCUSSION:





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            It is important that the router be able to come up
            and run on its own.  NVRAM may be a particular
            solution for routers used in large networks, since
            changing PROMs can be quite time consuming for a
            network manager responsible for numerous or
            geographically dispersed routers.  It is important
            to be able to netboot the system image because
            there should be an easy way for a router to get a
            bug fix or new feature more quickly than getting
            PROMS installed.  Also if the router has NVRAM
            instead of PROMs, it will netboot the image and
            then put it in NVRAM.

         A router MAY also be able to distinguish between
         different configurations based on which software it
         is running. If configuration commands change from one
         software version to another, it would be helpful if
         the router could use the configuration that was
         compatible with the software.


10.3.2.5  Detecting and responding to misconfiguration

         There MUST be mechanisms for detecting and responding
         to misconfigurations.  If a command is executed
         incorrectly, the router SHOULD give an error message.
         The router SHOULD NOT accept a poorly formed command
         as if it were correct.

         DISCUSSION:
            There are cases where it is not possible to detect
            errors: the command is correctly formed, but
            incorrect with respect to the network.  This may
            be detected by the router, but may not be
            possible.

         Another form of misconfiguration is misconfiguration
         of the network to which the router is attached.  A
         router MAY detect misconfigurations in the network.
         The router MAY log these findings to a file, either
         on the router or a host, so that the network manager
         will see that there are possible problems on the
         network.






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         DISCUSSION:
            Examples of such misconfigurations might be
            another router with the same address as the one in
            question or a router with the wrong subnet mask.
            If a router detects such problems it is probably
            not the best idea for the router to try to fix the
            situation.  That could cause more harm than good.



10.3.2.6  Minimizing Disruption

         Changing the configuration of a router SHOULD have
         minimal affect on the network.   Routing tables
         SHOULD NOT be unnecessarily flushed when a simple
         change is made to the router.  If a router is running
         several routing protocols, stopping one routing
         protocol SHOULD NOT disrupt other routing protocols,
         except in the case where one network is learned by
         more than one routing protocol.

         DISCUSSION:
            It is the goal of a network manager to run a
            network so that users of the network get the best
            connectivity possible.  Reloading a router for
            simple configuration changes can cause disruptions
            in routing and ultimately cause disruptions to the
            network and its users.  If routing tables are
            unnecessarily flushed, for instance, the default
            route will be lost as well as specific routes to
            sites within the network.  This sort of disruption
            will cause significant downtime for the users. It
            is the purpose of this section to point out that
            whenever possible, these disruptions should be
            avoided.



10.3.2.7  Control -- Troubleshooting Problems


         (1)  A router MUST provide in-band network access,
              but (except as required by Section [8.2]) for
              security considerations this access SHOULD be





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              disabled by default.  Vendors MUST document the
              default state of any in-band access.

              DISCUSSION:
                 In-band access primarily refers to access via
                 the normal network protocols which may or may
                 not affect the permanent operational state of
                 the router.  This includes, but is not
                 limited to Telnet/RLOGIN console access and
                 SNMP operations.

                 This was a point of contention between the
                 "operational out of the box" and "secure out
                 of the box" contingents.  Any "automagic"
                 access to the router may introduce
                 insecurities, but it may be more important
                 for the customer to have a router which is
                 accessible over the network as soon as it is
                 plugged in.  At least one vendor supplies
                 routers without any external console access
                 and depends on being able to access the
                 router via the network to complete its
                 configuration.

                 Basically, it is the vendors call whether or
                 not in-band access is enabled by default; but
                 it is also the vendors responsibility to make
                 its customers aware of possible insecurities.

         (2)  A router MUST provide the ability to initiate an
              ICMP echo.  The following options SHOULD be
              implemented:

              +  Choice of data patterns

              +  Choice of packet size

              +  Record route

              and the following additional options MAY be
              implemented:

              +  Loose source route






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              +  Strict source route

              +  Timestamps

         (3)  A router SHOULD provide the ability to initiate
              a traceroute.  If traceroute is provided, then
              the 3rd party traceroute SHOULD be implemented.

         Each of the above three facilities (if implemented)
         SHOULD have access restrictions placed on it to
         prevent its abuse by unauthorized persons.


10.4  Security Considerations



10.4.1  Auditing and Audit Trails

      Auditing and billing are the bane of the network
      operator, but are the two features most requested by
      those in charge of network security and those who are
      responsible for paying the bills.  In the context of
      security, auditing is desirable if it helps you keep
      your network working and protects your resources from
      abuse, without costing you more than those resources are
      worth.

      (1)  Configuration Changes

           Router SHOULD provide a method for auditing a
           configuration change of a router, even if it's
           something as simple as recording the operator's
           initials and time of change.

           DISCUSSION:
              Having the ability to track who made changes and
              when is highly desirable, especially if your
              packets suddenly start getting routed through
              Alaska on their way across town.

      (2)  Packet Accounting

           Vendors should strongly consider providing a system





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           for tracking traffic levels between pairs of hosts
           or networks.  A mechanism for limiting the
           collection of this information to specific pairs of
           hosts or networks is also strongly encouraged.

           DISCUSSION:
              A "host traffic matrix" as described above can
              give the network operator a glimpse of traffic
              trends not apparent from other statistics.  It
              can also identify hosts or networks which are
              "probing" the structure of the attached networks
              -- e.g., a single external host which tries to
              send packets to every IP address in the network
              address range for a connected network.

      (3)  Security Auditing

           Routers MUST provide a method for auditing security
           related failures or violations to include:

           +  Authorization Failures:  bad passwords, invalid
              SNMP communities, invalid authorization tokens,

           +  Violations of Policy Controls:  Prohibited
              Source Routes, Filtered Destinations, and

           +  Authorization Approvals:  good passwords --
              Telnet in-band access, console access.

           Routers MUST provide a method of limiting or
           disabling such auditing but auditing SHOULD be on
           by default.  Possible methods for auditing include
           listing violations to a console if present, logging
           or counting them internally, or logging them to a
           remote security server via the SNMP trap mechanism
           or the Unix logging mechanism as appropriate.  A
           router MUST implement at least one of these
           reporting mechanisms -- it MAY implement more than
           one.










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10.4.2  Configuration Control

      A vendor has a responsibility to use good configuration
      control practices in the creation of the
      software/firmware loads for their routers.  In
      particular, if a vendor makes updates and loads
      available for retrieval over the Internet, the vendor
      should also provide a way for the customer to confirm
      the load is a valid one, perhaps by the verification of
      a checksum over the load.

      DISCUSSION:
         Many vendors currently provide short notice updates
         of their software products via the Internet.  This a
         good trend and should be encouraged, but provides a
         point of vulnerability in the configuration control
         process.

      If a vendor provides the ability for the customer to
      change the configuration parameters of a router
      remotely, for example via a Telnet session, the ability
      to do so SHOULD be configurable and SHOULD default to
      off.  The router SHOULD require a password or other
      valid authentication before permitting remote
      reconfiguration.

      DISCUSSION:
         Allowing your properly identified network operator to
         twiddle with your routers is necessary; allowing
         anyone else to do so is foolhardy.

      A router MUST NOT have undocumented "back door" access
      and "master passwords".  A vendor MUST ensure any such
      access added for purposes of debugging or product
      development are deleted before the product is
      distributed to its customers.

      DISCUSSION:
         A vendor has a responsibility to its customers to
         ensure they are aware of the vulnerabilities present
         in its code by intention - e.g.  in-band access.
         "Trap doors", "back doors" and "master passwords"
         intentional or unintentional can turn a relatively
         secure router into a major problem on an operational





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         network.  The supposed operational benefits are not
         matched by the potential problems.















































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11.  REFERENCES

Implementors should be aware that Internet protocol standards
are occasionally updated.  These references are current as of
this writing, but a cautious implementor will always check a
recent version of the RFC index to ensure that an RFC has not
been updated or superseded by another, more recent RFC.
Reference [INTRO:6] explains various ways to obtain a current
RFC index.

APPL:1.
     B. Croft and J. Gilmore, "Bootstrap Protocol (BOOTP),
     Request For Comments (RFC) 951, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     September 1985.

APPL:2.
     S. Alexander and R. Droms, "DHCP Options and BOOTP Vendor
     Extensions", Request For Comments (RFC) 1533, October
     1993.

APPL:3.
     W. Wimer, "Clarifications and Extensions for the
     Bootstrap Protocol", Request For Comments (RFC) 1542,
     October 1993.

ARCH:1.
     "DDN Protocol Handbook, NIC-50004, NIC-50005, NIC-50006
     (three volumes), DDN Network Information Center, SRI
     International, Menlo Park, California, USA, December
     1985.

ARCH:2.
     V. Cerf and R. Kahn, "A Protocol for Packet Network
     Intercommunication," IEEE Transactions on Communication,
     May 1974.  Also included in [ARCH:1].

ARCH:3.
     J. Postel, C. Sunshine, and D. Cohen, "The ARPA Internet
     Protocol," Computer Networks, vol. 5, no. 4, July 1981.
     Also included in [ARCH:1].

ARCH:4.
     B. Leiner, J. Postel, R. Cole, and D. Mills, "The DARPA





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     Internet Protocol Suite," Proceedings of INFOCOM '85,
     IEEE, Washington, DC, March 1985.  Also in: IEEE
     Communications Magazine, March 1985.  Also available from
     the Information Sciences Institute, University of
     Southern California as Technical Report ISI-RS-85-153.

ARCH:5.
     D. Comer, "Internetworking With TCP/IP Volume 1:
     Principles, Protocols, and Architecture", Prentice Hall,
     Englewood Cliffs, NJ, 1991.

ARCH:6.
     W. Stallings, "Handbook of Computer-Communications
     Standards Volume 3: The TCP/IP Protocol Suite",
     Macmillan, New York, NY, 1990.

ARCH:7.
     J. Postel, "Internet Official Protocol Standards",
     Request For Comments (RFC) 1540, October 1993.

ARCH:8.
     "Information processing systems -- Open Systems
     Interconnection -- Basic Reference Model", ISO 7489,
     International Standards Organization, 1984.

FORWARD:1.
     IETF CIP Working Group (C. Topolcic, Editor),
     "Experimental Internet Stream Protocol, Version 2 (ST-
     II)", Request For Comments (RFC) 1190, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, October 1990.

FORWARD:2.
     A. Mankin and K. Ramakrishnan, Editors, "Gateway
     Congestion Control Survey", Request For Comments (RFC)
     1254, DDN Network Information Center, SRI International,
     Menlo Park, California, USA, August 1991.

FORWARD:3.
     J. Nagle, "On Packet Switches with Infinite Storage,"
     IEEE Transactions on Communications, vol. COM-35, no. 4,
     April 1987.

FORWARD:4.





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     R. Jain, K. Ramakrishnan, and D. Chiu, "Congestion
     Avoidance in Computer Networks With a Connectionless
     Network Layer", Technical Report DEC-TR-506, Digital
     Equipment Corporation.

FORWARD:5.
     V. Jacobson, "Congestion Avoidance and Control,"
     Proceedings of SIGCOMM '88, Association for Computing
     Machinery, August 1988.

FORWARD:6.
     W. Barns, "Precedence and Priority Access Implementation
     for Department of Defense Data Networks", Technical
     Report MTR-91W00029, The Mitre Corporation, McLean,
     Virginia, USA, July 1991.

INTERNET:1.
     J. Postel, "Internet Protocol", Request For Comments
     (RFC) 791, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, September
     1981.

INTERNET:2.
     J. Mogul and J. Postel, "Internet Standard Subnetting
     Procedure", Request For Comments (RFC) 950, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, August 1985.

INTERNET:3.
     J. Mogul, "Broadcasting Internet Datagrams in the
     Presence of Subnets", Request For Comments (RFC) 922, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, October 1984.

INTERNET:4.
     S. Deering, "Host Extensions for IP Multicasting",
     Request For Comments (RFC) 1112, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     August 1989.

INTERNET:5.
     S. Kent, "U.S. Department of Defense Security Options for
     the Internet Protocol", Request for Comments (RFC) 1108,
     November 1991.





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INTERNET:6.
     R. Braden, D. Borman, and C. Partridge, "Computing the
     Internet Checksum", Request For Comments (RFC) 1071, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, September 1988.

INTERNET:7.
     T. Mallory and A. Kullberg, "Incremental Updating of the
     Internet Checksum", Request For Comments (RFC) 1141, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, January 1990.

INTERNET:8.
     J. Postel, "Internet Control Message Protocol", Request
     For Comments (RFC) 792, DDN Network Information Center,
     SRI International, Menlo Park, California, USA, September
     1981.

INTERNET:9.
     A. Mankin, G. Hollingsworth, G. Reichlen, K. Thompson, R.
     Wilder, and R. Zahavi, "Evaluation of Internet
     Performance -- FY89", Technical Report MTR-89W00216,
     MITRE Corporation, February, 1990.

INTERNET:10.
     G. Finn, "A Connectionless Congestion Control Algorithm,"
     Computer Communications Review, vol. 19, no. 5,
     Association for Computing Machinery, October 1989.

INTERNET:11.
     W. Prue, "The Source Quench Introduced Delay (SQuID)",
     Request For Comments (RFC) 1016, DDN Network Information
     Center, SRI International, J. Postel, August 1987.

INTERNET:12.
     A. McKenzie, "Some comments on SQuID", Request For
     Comments (RFC) 1018, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, August 1987.

INTERNET:13.
     S. Deering, "ICMP Router Discovery Messages", Request For
     Comments (RFC) 1256, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, September
     1991.





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INTERNET:14.
     J. Mogul and S. Deering, "Path MTU Discovery", Request
     For Comments (RFC) 1191, DDN Network Information Center,
     SRI International, Menlo Park, California, USA, November
     1990.

INTERNET:15
     V. Fuller, T. Li, J. Yi, and K. Varadhan, "Classless
     Inter-Domain Routing (CIDR): an Address Assignment and
     Aggregation Strategy" Request For Comments (RFC) 1519,
     DDN Network Information Center, SRI International Menlo
     Park, California, USA September 1993.

INTERNET:16
     M. St. Johns, "Draft Revised IP Security Option", Request
     for Comments 1038, January 1988.

INTERNET:17
     W. Prue and J. Postel, "Queuing Algorithm to Provide
     Type-of-service For IP Links", Request for Comments 1046,
     February 1988.

INTRO:1.
     R. Braden and J. Postel, "Requirements for Internet
     Gateways", Request For Comments (RFC) 1009, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, June 1987.

INTRO:2.
     Internet Engineering Task Force (R. Braden, Editor),
     "Requirements for Internet Hosts -- Communication
     Layers", Request For Comments (RFC) 1122, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, October 1989.

INTRO:3.
     Internet Engineering Task Force (R. Braden, Editor),
     "Requirements for Internet Hosts -- Application and
     Support", Request For Comments (RFC) 1123, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, October 1989.

INTRO:4.
     D. Clark, "Modularity and Efficiency in Protocol





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     Implementations", Request For Comments (RFC) 817, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, July 1982.

INTRO:5.
     D. Clark, "The Structuring of Systems Using Upcalls,"
     Proceedings of 10th ACM SOSP, December 1985.

INTRO:6.
     O. Jacobsen and J. Postel, "Protocol Document Order
     Information", Request For Comments (RFC) 980, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, March 1986.

INTRO:7.
     J. Reynolds and J. Postel, "Assigned Numbers", Request
     For Comments (RFC) 1340, July 1992.  This document is
     periodically updated and reissued with a new number.  It
     is wise to verify occasionally that the version you have
     is still current.

INTRO:8.
     "DoD Trusted Computer System Evaluation Criteria", DoD
     publication 5200.28-STD, U.S. Department of Defense,
     December 1985.

INTRO:9
     G. Malkin and T. LaQuey Parker, "Internet Users'
     Glossary", Request for Comments (RFC) 1392 (also FYI
     0018), Network Information Center, January 1993.

LINK:1.
     S. Leffler and M. Karels, "Trailer Encapsulations",
     Request For Comments (RFC) 893, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     April 1984.

LINK:2
     W. Simpson, "The Point-to-Point Protocol (PPP) for the
     Transmission of Multi-protocol Datagrams over Point-to-
     Point Links", Request For Comments (RFC) 1331, May 1992.

LINK:3
     G. McGregor, "The PPP Internet Protocol Control Protocol





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     (IPCP)", Request For Comments (RFC) 1332, May 1992.

LINK:4
     B. Lloyd, W. Simpson, "PPP Authentication Protocols",
     Request For Comments (RFC) 1334, May 1992.

LINK:5
     W. Simpson "PPP Link Quality Monitoring", Request For
     Comments (RFC) 1333, May 1992.

MGT:1.
     M. Rose and K. McCloghrie, "Structure and Identification
     of Management Information of TCP/IP-based Internets",
     Request For Comments (RFC) 1155, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     May 1990.

MGT:2.
     K. McCloghrie and M. Rose (Editors), "Management
     Information Base of TCP/IP-Based Internets: MIB-II",
     Request For Comments (RFC) 1213, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     March 1991.

MGT:3.
     J. Case, M. Fedor, M. Schoffstall, and J. Davin, "Simple
     Network Management Protocol", Request For Comments (RFC)
     1157, DDN Network Information Center, SRI International,
     Menlo Park, California, USA, May 1990.

MGT:4.
     M. Rose and K. McCloghrie (Editors), "Towards Concise MIB
     Definitions", Request For Comments (RFC) 1212, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, March 1991.

MGT:5.
     L. Steinberg, "Techniques for Managing Asynchronously
     Generated Alerts", Request for Comments (RFC) 1224, May
     1991.

MGT:6.
     F. Kastenholz, "Definitions of Managed Objects for the
     Ethernet-like Interface Types", Request for Comments





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     (RFC) 1398, January 1993.

MGT:7.
     R. Fox and K. McCloghrie, "IEEE 802.4 Token Bus MIB",
     Request for Comments (RFC) 1230, May 1991.

MGT:8.
     E. Decker, R. Fox and K. McCloghrie, "IEEE 802.5 Token
     Ring MIB", Request for Comments (RFC) 1231, February
     1993.

MGT:9.
     J. Case and A. Rijsinghani, "FDDI Management Information
     Base", Request for Comments (RFC) 1512, September 1993.

MGT:10.
     B. Stewart, "Definitions of Managed Objects for
     RS-232-like Hardware Devices", Request for Comments (RFC)
     1317, April 1992.

MGT:11.
     F. Kastenholz, " Definitions of Managed Objects for the
     Link Control Protocol of the Point-to-Point Protocol",
     Request For Comments (RFC) 1471 June 1992.

MGT:12.
     F. Kastenholz, "The Definitions of Managed Objects for
     the Security Protocols of the Point-to-Point Protocol",
     Request For Comments (RFC) 1472 June 1992.

MGT:13.
     F. Kastenholz, "The Definitions of Managed Objects for
     the IP Network Control Protocol of the Point-to-Point
     Protocol", Request For Comments (RFC) 1473 June 1992.

MGT:14.
     F. Baker and R. Coltun, "OSPF Version 2 Management
     Information Base", Request For Comments (RFC) 1253,
     August 1991.

MGT:15.
     S. Willis and J. Burruss, "Definitions of Managed Objects
     for the Border Gateway Protocol (Version 3)", Request For
     Comments (RFC) 1269, October 1991.





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MGT:16.
     F. Baker, J. Watt, "Definitions of Managed Objects for
     the DS1 and E1 Interface Types", Request For Comments
     (RFC) 1406, January 1993.

MGT:17.
     T. Cox and K. Tesink, "Definitions of Managed Objects for
     the DS3/E3 Interface Types", Request For Comments (RFC)
     1407, January 1993.

MGT:18.
     K. McCloghrie, "Extensions to the Generic-Interface MIB",
     Request For Comments (RFC) 1229, August 1992.

MGT:19.
     T. Cox and K. Tesink, "Definitions of Managed Objects for
     the SIP Interface Type", Request For Comments (RFC) 1304,
     February 1992.

MGT:20
     F. Baker, "IP Forwarding Table MIB", Request For Comments
     (RFC) 1354, July 1992.

MGT:21.
     G. Malkin and F. Baker, "RIP Version 2 MIB Extension",
     Request For Comments (RFC) 1389, January 1993.

MGT:22.
     D. Throop, "SNMP MIB Extension for the X.25 Packet
     Layer", Request For Comments (RFC) 1382, November 1992.

MGT:23.
     D. Throop and F. Baker, "SNMP MIB Extension for X.25
     LAPB", Request For Comments (RFC) 1381, November 1992.

MGT:24.
     D. Throop and F. Baker, "SNMP MIB Extension for
     MultiProtocol Interconnect over X.25", Request For
     Comments (RFC) 1461, May 1993.

MGT:25.
     M. Rose, "SNMP over OSI", Request For Comments (RFC)
     1418, March 1993.






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MGT:26.
     G. Minshall and M. Ritter, "SNMP over AppleTalk", Request
     For Comments (RFC) 1419, March 1993.

MGT:27.
     S. Bostock, "SNMP over IPX", Request For Comments (RFC)
     1420, March 1993.

MGT:28.
     M. Schoffstall, C. Davin, M. Fedor, J. Case, "SNMP over
     Ethernet", Request For Comments (RFC) 1089, February
     1989.

MGT:29.
     J. Case, "FDDI Management Information Base", Request For
     Comments (RFC) 1285, January 1992.

OPER:1.
     J. Nagle, "Congestion Control in IP/TCP Internetworks",
     Request For Comments (RFC) 896, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     January 1984.

OPER:2.
     K.R. Sollins, "TFTP Protocol (revision 2)", Request For
     Comments (RFC) 1350, July 1992.

ROUTE:1.
     J. Moy, "OSPF Version 2", Request For Comments (RFC)
     1247, DDN Network Information Center, SRI International,
     Menlo Park, California, USA, July 1991.

ROUTE:2.
     R. Callon, "Use of OSI IS-IS for Routing in TCP/IP and
     Dual Environments", Request For Comments (RFC) 1195, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, December 1990.

ROUTE:3.
     C. L. Hedrick, "Routing Information Protocol", Request
     For Comments (RFC) 1058, DDN Network Information Center,
     SRI International, Menlo Park, California, USA, June
     1988.






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ROUTE:4.
     K. Lougheed and Y. Rekhter, "A Border Gateway Protocol 3
     (BGP-3)", Request For Comments (RFC) 1267, October 1991.

ROUTE:5.
     P. Gross and Y. Rekhter, "Application of the Border
     Gateway Protocol in the Internet", Request For Comments
     (RFC) 1268, October 1991.

ROUTE:6.
     D. Mills, "Exterior Gateway Protocol Formal
     Specification", Request For Comments (RFC) 904, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, April 1984.

ROUTE:7.
     E. Rosen, "Exterior Gateway Protocol (EGP)", Request For
     Comments (RFC) 827, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, October 1982.

ROUTE:8.
     L. Seamonson and E. Rosen, ""STUB" Exterior Gateway
     Protocol", Request For Comments (RFC) 888, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, January 1984.

ROUTE:9.
     D. Waitzman, C. Partridge, and S. Deering, "Distance
     Vector Multicast Routing Protocol", Request For Comments
     (RFC) 1075, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, November
     1988.

ROUTE:10.
     S. Deering, "Multicast Routing in Internetworks and
     Extended LANs," Proceedings of SIGCOMM '88, Association
     for Computing Machinery, August 1988.

ROUTE:11.
     P. Almquist, "Type of Service in the Internet Protocol
     Suite", Request for Comments (RFC) 1349, July 1992.

ROUTE:12.
     Y. Rekhter, "Experience with the BGP Protocol", Request





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     For Comments (RFC) 1266, October 1991.

ROUTE:13.
     Y. Rekhter, "BGP Protocol Analysis", Request For Comments
     (RFC) 1265, October 1991.

TRANS:1.
     J. Postel, "User Datagram Protocol", Request For Comments
     (RFC) 768, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, August 1980.

TRANS:2.
     J. Postel, "Transmission Control Protocol", Request For
     Comments (RFC) 793, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, September
     1981.

































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APPENDIX  A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS

Subject to restrictions given below, a host MAY be able to act
as an intermediate hop in a source route, forwarding a source-
routed datagram to the next specified hop.

However, in performing this router-like function, the host
MUST obey all the relevant rules for a router forwarding
source-routed datagrams [INTRO:2].  This includes the
following specific provisions:

(A)  TTL
     The TTL field MUST be decremented and the datagram
     perhaps discarded as specified for a router in [INTRO:2].

(B)  ICMP Destination Unreachable
     A host MUST be able to generate Destination Unreachable
     messages with the following codes:
     4 (Fragmentation Required but DF Set) when a source-
       routed datagram cannot be fragmented to fit into the
       target network;
     5 (Source Route Failed) when a source-routed datagram
       cannot be forwarded, e.g., because of a routing problem
       or because the next hop of a strict source route is not
       on a connected network.

(C)  IP Source Address
     A source-routed datagram being forwarded MAY (and
     normally will) have a source address that is not one of
     the IP addresses of the forwarding host.

(D)  Record Route Option
     A host that is forwarding a source-routed datagram
     containing a Record Route option MUST update that option,
     if it has room.

(E)  Timestamp Option
     A host that is forwarding a source-routed datagram
     containing a Timestamp Option MUST add the current
     timestamp to that option, according to the rules for this
     option.

To define the rules restricting host forwarding of source-
routed datagrams, we use the term "local source-routing" if





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the next hop will be through the same physical interface
through which the datagram arrived; otherwise, it is "non-
local source-routing".

A host is permitted to perform local source-routing without
restriction.

A host that supports non-local source-routing MUST have a
configurable switch to disable forwarding, and this switch
MUST default to disabled.

The host MUST satisfy all router requirements for configurable
policy filters [INTRO:2] restricting non-local forwarding.

If a host receives a datagram with an incomplete source route
but does not forward it for some reason, the host SHOULD
return an ICMP Destination Unreachable (code 5, Source Route
Failed) message, unless the datagram was itself an ICMP error
message.






























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APPENDIX  B. GLOSSARY


This Appendix defines specific terms used in this memo.  It
also defines some general purpose terms that may be of
interest.  See also [INTRO:9] for a more general set of
definitions.

AS
     Autonomous System A collection of routers under a single
     administrative authority using a common Interior Gateway
     Protocol for routing packets.

Connected Network
     A network to which a router is interfaced is often known
     as the "local network" or the "subnetwork" relative to
     that router.  However, these terms can cause confusion,
     and therefore we use the term "Connected Network" in this
     memo.

Connected (Sub)Network
     A Connected (Sub)Network is an IP subnetwork to which a
     router is interfaced, or a connected network if the
     connected network is not subnetted.  See also Connected
     Network.

Datagram
     The unit transmitted between a pair of internet modules.
     data, called datagrams, from sources to destinations.
     The Internet Protocol does not provide a reliable
     communication facility.  There are no acknowledgments
     either end-to-end or hop-by-hop.  There is no error no
     retransmissions.  There is no flow control.  See IP.

Default Route
     A routing table entry which is used to direct any data
     addressed to any network numbers not explicitly listed in
     the routing table.

EGP
     Exterior Gateway Protocol A protocol which distributes
     routing information to the gateways (routers) which
     connect autonomous systems.  See IGP.






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EGP-2
     Exterior Gateway Protocol version 2 This is an EGP
     routing protocol developed to handle traffic between AS's
     in the Internet.

Forwarder
     The logical entity within a router that is responsible
     for switching packets among the router's interfaces.  The
     Forwarder also makes the decisions to queue a packet for
     local delivery, to queue a packet for transmission out
     another interface, or both.

Forwarding
     Forwarding is the process a router goes through for each
     packet received by the router.  The packet may be
     consumed by the router, it may be output on one or more
     interfaces of the router, or both.  Forwarding includes
     the process of deciding what to do with the packet as
     well as queuing it up for (possible) output or internal
     consumption.

Fragment
     An IP datagram which represents a portion of a higher
     layer's packet which was too large to be sent in its
     entirety over the output network.

IGP
     Interior Gateway Protocol A protocol which distributes
     routing information with an Autonomous System (AS).  See
     EGP.

Interface IP Address
     The IP Address and subnet mask that is assigned to a
     specific interface of a router.

Internet Address
     An assigned number which identifies a host in an
     internet.  It has two or three parts: network number,
     optional subnet number, and host number.

IP
     Internet Protocol The network layer protocol for the
     Internet.  It is a packet switching, datagram protocol
     defined in RFC 791.  IP does not provide a reliable





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     communications facility; that is, there are no end-to-end
     of hop-by-hop acknowledgments.

IP Datagram
     An IP Datagram is the unit of end-to-end transmission in
     the Internet Protocol.  An IP Datagram consists of an IP
     header followed by all of higher-layer data (such as TCP,
     UDP, ICMP, and the like).  An IP Datagram is an IP header
     followed by a message.

     An IP Datagram is a complete IP end-to-end transmission
     unit.  An IP Datagram is composed of one or more IP
     Fragments.

     In this memo, the unqualified term "Datagram" should be
     understood to refer to an IP Datagram.

IP Fragment
     An IP Fragment is a component of an IP Datagram.  An IP
     Fragment consists of an IP header followed by all or part
     of the higher-layer of the original IP Datagram.

     One or more IP Fragments comprises a single IP Datagram.

     In this memo, the unqualified term "Fragment" should be
     understood to refer to an IP Fragment.

IP Packet
     An IP Datagram or an IP Fragment.

     In this memo, the unqualified term "Packet" should
     generally be understood to refer to an IP Packet.

Logical [network] interface
     We define a logical [network] interface to be a logical
     path, distinguished by a unique IP address, to a
     connected network.

Martian Filtering
     A packet which contains an invalid source or destination
     address is considered to be "martian" and discarded.

MTU (Maximum Transmission Unit)
     The size of the largest packet that can be transmitted or





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     received through a logical interface.  This size includes
     the IP header but does not include the size of any Link
     Layer headers or framing.

Multicast
     A packet which is destined for multiple hosts.  See
     "broadcast".

Multicast Address
     A special type of address which is recognized by multiple
     hosts.

     A Multicast Address is sometimes known as a Functional
     Address or a Group Address.

Originate
     Packets can be transmitted by a router for one of two
     reasons: 1) the packet was received and is being
     forwarded or 2) the router itself created the packet for
     transmission (such as route advertisements).  Packets
     that the router creates for transmission are said to
     originate at the router.

Packet
     A packet is the unit of data passed across the interface
     between the Internet Layer and the Link Layer.  It
     includes an IP header and data.  A packet may be a
     complete IP datagram or a fragment of an IP datagram.

Path
     The sequence of routers and (sub-)networks which a packet
     traverses from a particular router to a particular
     destination host.  Note that a path is uni-directional;
     it is not unusual to have different paths in the two
     directions between a given host pair.

Physical Network
     A Physical Network is a network (or a piece of an
     internet) which is contiguous at the Link Layer.  Its
     internal structure (if any) is transparent to the
     Internet Layer.

     In this memo, several media components that are connected
     together via devices such as bridges or repeaters are





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     considered to be a single Physical Network since such
     devices are transparent to the IP.

Physical Network Interface
     This is a physical interface to a Connected Network and
     has a (possibly unique) Link-Layer address.  Multiple
     Physical Network Interfaces on a single router may share
     the same Link-Layer address, but the address must be
     unique for different routers on the same Physical
     Network.

router
     A special-purpose dedicated computer that attaches
     several networks together.  Routers switch packets
     between these networks in a process known as forwarding.
     This process may be repeated several times on a single
     packet by multiple routers until the packet can be
     delivered to the final destination -- switching the
     packet from router to router to router... until the
     packet gets to its destination.

RPF
     Reverse Path Forwarding A method used to deduce the next
     hops for broadcast and multicast packets.

serial line
     A physical medium which we cannot define, but we
     recognize one when we see one.  See the U.S. Supreme
     Court's definitions on pornography.

Silently Discard
     This memo specifies several cases where a router is to
     "Silently Discard" a received packet (or datagram).  This
     means that the router should discard the packet without
     further processing, and that the router will not send any
     ICMP error message (see Section [4.3.2]) as a result.
     However, for diagnosis of problems, the router should
     provide the capability of logging the error (see Section
     [1.3.3]), including the contents of the silently-
     discarded packet, and should record the event in a
     statistics counter.

Silently Ignore
     A router is said to "Silently Ignore" an error or





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     condition if it takes no action other than possibly
     generating an error report in an error log or via some
     network management protocol, and discarding, or ignoring,
     the source of the error.  In particular, the router does
     NOT generate an ICMP error message.

Specific-destination address
     This is defined to be the destination address in the IP
     header unless the header contains an IP broadcast or IP
     multicast address, in which case the specific-destination
     is an IP address assigned to the physical interface on
     which the packet arrived.

subnet
     A portion of a network, which may be a physically
     independent network, which shares a network address with
     other portions of the network and is distinguished by a
     subnet number.  A subnet is to a network what a network
     is to an internet.

subnet number
     A part of the internet address which designates a subnet.
     It is ignored for the purposes internet routing, but is
     used for intranet routing.

TOS
     Type Of Service A field in the IP header which represents
     the degree of reliability expected from the network layer
     by the transport layer or application.

TTL
     Time To Live A field in the IP header which represents
     how long a packet is considered valid.  It is a
     combination "hop count" and "timer value".















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APPENDIX  C. FUTURE DIRECTIONS

This appendix lists work that future revisions of this
document may wish to address.

In the preparation of Router Requirements, we stumbled across
several other architectural issues.  Each of these is dealt
with somewhat in the document, but still ought to be
classified as an open issue in the IP architecture.

Most of the he topics presented here generally indicate areas
where the technology is still relatively new and it is not
appropriate to develop specific requirements since the
community is still gaining operational experience.

Other topics represent areas of ongoing research and indicate
areas that the prudent developer would closely monitor.

(1)  SNMP Version 2

(2)  Additional SNMP MIBs

(3)  IDPR

(4)  CIPSO

(5)  IP Next Generation research

(6)  More detailed requirements for next-hop selection

(7)  More detailed requirements for leaking routes between
     routing protocols

(8)  Router system security

(9)  Routing protocol security

(10) Internetwork Protocol layer security.  There has been
     extensive work refining the security of IP since the
     original work writing this document.  This security work
     should be included in here.

(11) Route caching






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(12) Load Splitting

(13) Sending fragments along different paths

(14) Variable width subnet masks (i.e., not all subnets of a
     particular net use the same subnet mask).  Routers are
     required (MUST) support them, but are not required to
     detect ambiguous configurations.

(15) Multiple logical (sub)nets on the same wire.  Router
     Requirements does not require support for this.  We made
     some attempt to identify pieces of the architecture (e.g.
     forwarding of directed broadcasts and issuing of
     Redirects) where the wording of the rules has to be done
     carefully to make "the right thing" happen, and tried to
     clearly distinguish logical interfaces from physical
     interfaces.  However, we did not study this issue in
     detail, and we are not at all confident that all of the
     rules in the document are correct in the presence of
     multiple logical (sub)nets on the same wire.

(15) Congestion control and resource management.  On the
     advice of the IETF's experts (Mankin and Ramakrishnan) we
     deprecated (SHOULD NOT) Source Quench and said little
     else concrete (Section 5.3.6).

(16) Developing a Link-Layer requirements document that would
     be common for both routers and hosts.

(17) Developing a common PPP LQM algorithm.

(18) Investigate of other information (above and beyond
     section [3.2]) that passes between the layers, such as
     physical network MTU, mappings of IP precedence to Link
     Layer priority values, etc.

(19) Should the Link Layer notify IP if address resolution
     failed (just like it notifies IP when there is a Link
     Layer priority value problem)?

(20) Should all routers be required to implement a DNS
     resolver?

(21) Should a human user be able to use a host name anywhere





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     you can use an IP address when configuring the router?
     Even in ping and traceroute?

(22) Almquist's draft ruminations on the next hop and
     ruminations on route leaking need to be reviewed, brought
     up to date, and published.

(23) Investigation is needed to determine if a redirect
     message for precedence is needed or not. If not, are the
     type-of-service redirects acceptable?

(24) RIPv2 and RIP+CIDR and variable length subnet masks.

(25) BGP-4 CIDR is going to be important, and everyone is
     betting on BGP-4. We can't avoid mentioning it.  Probably
     need to describe the differences between BGP-3 and BGP-4,
     and explore upgrade issues...
































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APPENDIX D.  Multicast Routing Protocols

Multicasting is a relatively new technology within the
Internet Protocol family.  It is not widely deployed or
commonly in use yet.  Its importance, however, is expected to
grow over the coming years.

This Appendix describes some of the technologies being
investigated for routing multicasts through the Internet.

A diligent implementor will keep abreast of developments in
this area in order to properly develop multicast facilities.

This Appendix does not specify any standards or requirements.


D.1  Introduction

   Multicast routing protocols enable the forwarding of IP
   multicast datagrams throughout a TCP/IP internet. Generally
   these algorithms forward the datagram based on its source
   and destination addresses.  Additionally, the datagram may
   need to be forwarded to several multicast group members, at
   times requiring the datagram to be replicated and sent out
   multiple interfaces.

   The state of multicast routing protocols is less developed
   than the protocols available for the forwarding of IP
   unicasts.  Two multicast routing protocols have been
   documented for TCP/IP; both are currently considered to be
   experimental.  Both also use the IGMP protocol (discussed
   in Section [4.4]) to monitor multicast group membership.


D.2  Distance Vector Multicast Routing Protocol -- DVMRP

   DVMRP, documented in [ROUTE:9], is based on Distance Vector
   or Bellman-Ford technology. It routes multicast datagrams
   only, and does so within a single Autonomous System. DVMRP
   is an implementation of the Truncated Reverse Path
   Broadcasting algorithm described in [ROUTE:10].  In
   addition, it specifies the tunneling of IP multicasts
   through non-multicast-routing-capable IP domains.






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D.3  Multicast Extensions to OSPF -- MOSPF

   MOSPF, currently under development, is a backward-
   compatible addition to OSPF that allows the forwarding of
   both IP multicasts and unicasts within an Autonomous
   System. MOSPF routers can be mixed with OSPF routers within
   a routing domain, and they will interoperate in the
   forwarding of unicasts. OSPF is a link-state or SPF-based
   protocol. By adding link state advertisements that pinpoint
   group membership, MOSPF routers can calculate the path of a
   multicast datagram as a tree rooted at the datagram source.
   Those branches that do not contain group members can then
   be discarded, eliminating unnecessary datagram forwarding
   hops.



































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APPENDIX E  Additional Next-Hop Selection Algorithms

Section [5.2.4.3] specifies an algorithm that routers ought to
use when selecting a next-hop for a packet.

This appendix provides historical perspective for the next-hop
selection problem.  It also presents several additional
pruning rules and next-hop selection algorithms that might be
found in the Internet.

This appendix presents material drawn from an earlier,
unpublished, work by Philip Almquist; "Ruminations on the Next
Hop".

This Appendix does not specify any standards or requirements.


E.1. Some Historical Perspective

   It is useful to briefly review the history of the topic,
   beginning with what is sometimes called the "classic model"
   of how a router makes routing decisions.  This model
   predates IP.  In this model, a router speaks some single
   routing protocol such as RIP.  The protocol completely
   determines the contents of the router's FIB.  The route
   lookup algorithm is trivial: the router looks in the FIB
   for a route whose destination attribute exactly matches the
   network number portion of the destination address in the
   packet.  If one is found, it is used; if none is found, the
   destination is unreachable.  Because the routing protocol
   keeps at most one route to each destination, the problem of
   what to do when there are multiple routes which match the
   same destination cannot arise.

   Over the years, this classic model has been augmented in
   small ways.  With the advent of default routes, subnets,
   and host routes, it became possible to have more than one
   routing table entry which in some sense matched the
   destination.  This was easily resolved by a consensus that
   there was a hierarchy of routes: host routes should be
   preferred over subnet routes, subnet routes over net
   routes, and net routes over default routes.

   With the advent of variable length subnet masks, the





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   general approach remained the same although its description
   became a little more complicated. We now say that each
   route has a bit mask associated with it.  If a particular
   bit in a route's bit mask is set, the corresponding bit in
   the route's destination attribute is significant. A route
   cannot be used to route a packet unless each significant
   bit in the route's destination attribute matches the
   corresponding bit in the packet's destination address, and
   routes with more bits set in their masks are preferred over
   routes which have fewer bits set in their masks. This is
   simply a generalization of the hierarchy of routes
   described above, and will be referred to for the rest of
   this memo as choosing a route by preferring longest match.

   Another way the classic model has been augmented is through
   a small amount of relaxation of the notion that a routing
   protocol has complete control over the contents of the
   routing table.  First, static routes were introduced.  For
   the first time, it was possible to simultaneously have two
   routes (one dynamic and one static) to the same
   destination.  When this happened, a router had to have a
   policy (in some cases configurable, and in other cases
   chosen by the author of the router's software) which
   determined whether the static route or the dynamic route
   was preferred. However, this policy was only used as a tie-
   breaker when longest match didn't uniquely determine which
   route to use. Thus, for example, a static default route
   would never be preferred over a dynamic net route even if
   the policy preferred static routes over dynamic routes.

   The classic model had to be further augmented when inter-
   domain routing protocols were invented. Traditional routing
   protocols came to be called "interior gateway protocols"
   (IGPs), and at each Internet site there was a strange new
   beast called an "exterior gateway", a router which spoke
   EGP to several "BBN Core Gateways" (the routers which made
   up the Internet backbone at the time) at the same time as
   it spoke its IGP to the other routers at its site. Both
   protocols wanted to determine the contents of the router's
   routing table. Theoretically, this could result in a router
   having three routes (EGP, IGP, and static) to the same
   destination.  Because of the Internet topology at the time,
   it was resolved with little debate that routers would be
   best served by a policy of preferring IGP routes over EGP





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   routes.  However, the sanctity of longest match remained
   unquestioned: a default route learned from the IGP would
   never be preferred over a net route from learned EGP.

   Although the Internet topology, and consequently routing in
   the Internet, have evolved considerably since then, this
   slightly augmented version of the classic model has
   survived pretty much intact to this day in the Internet
   (except that BGP has replaced EGP).  Conceptually (and
   often in implementation) each router has a routing table
   and one or more routing protocol processes.  Each of these
   processes can add any entry that it pleases, and can delete
   or modify any entry that it has created. When routing a
   packet, the router picks the best route using longest
   match, augmented with a policy mechanism to break ties.
   Although this augmented classic model has served us well,
   it has a number of shortcomings:

   +  It ignores (although it could be augmented to consider)
      path characteristics such as quality of service and MTU.

   +  It doesn't support routing protocols (such as OSPF and
      Integrated IS-IS) that require route lookup algorithms
      different than pure longest match.

   +  There has not been a firm consensus on what the tie-
      breaking mechanism ought to be. Tie-breaking mechanisms
      have often been found to be difficult if not impossible
      to configure in such a way that the router will always
      pick what the network manger considers to be the
      "correct" route.


E.2. Additional Pruning Rules

   Section [5.2.4.3] defined several pruning rules to use to
   select routes from the FIB.  There are other rules that
   could also be used.

   +  OSPF Route Class
      Routing protocols which have areas or make a distinction
      between internal and external routes divide their routes
      into classes, where classes are rank-ordered in terms of
      preference. A route is always chosen from the most





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      preferred class unless none is available, in which case
      one is chosen from the second most preferred class, and
      so on. In OSPF, the classes (in order from most
      preferred to least preferred) are intra-area, inter-
      area, type 1 external (external routes with internal
      metrics), and type 2 external. As an additional wrinkle,
      a router is configured to know what addresses ought to
      be accessible via intra-area routes, and will not use
      inter- area or external routes to reach these
      destinations even when no intra-area route is available.

      More precisely, we assume that each route has a class
      attribute, called route.class, which is assigned by the
      routing protocol.  The set of candidate routes is
      examined to determine if it contains any for which
      route.class = intra-area.  If so, all routes except
      those for which route.class = intra-area are discarded.
      Otherwise, router checks whether the packet's
      destination falls within the address ranges configured
      for the local area.  If so, the entire set of candidate
      routes is deleted.  Otherwise, the set of candidate
      routes is examined to determine if it contains any for
      which route.class = inter-area.  If so, all routes
      except those for which route.class = inter-area are
      discarded.  Otherwise, the set of candidate routes is
      examined to determine if it contains any for which
      route.class = type 1 external.  If so, all routes except
      those for which route.class = type 1 external are
      discarded.

   +  IS-IS Route Class
      IS-IS route classes work identically to OSPF's. However,
      the set of classes defined by Integrated IS-IS is
      different, such that there isn't a one-to-one mapping
      between IS-IS route classes and OSPF route classes. The
      route classes used by Integrated IS-IS are (in order
      from most preferred to least preferred) intra-area,
      inter-area, and external.

      The Integrated IS-IS internal class is equivalent to the
      OSPF internal class. Likewise, the Integrated IS-IS
      external class is equivalent to OSPF's type 2 external
      class. However, Integrated IS-IS does not make a
      distinction between inter-area routes and external





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      routes with internal metrics -- both are considered to
      be inter-area routes. Thus, OSPF prefers true inter-area
      routes over external routes with internal metrics,
      whereas Integrated IS-IS gives the two types of routes
      equal preference.

   +  IDPR Policy
      A specific case of Policy. The IETF's Inter-domain
      Policy Routing Working Group is devising a routing
      protocol called Inter-Domain Policy Routing (IDPR) to
      support true policy-based routing in the Internet.
      Packets with certain combinations of header attributes
      (such as specific combinations of source and destination
      addresses or special IDPR source route options) are
      required to use routes provided by the IDPR protocol.
      Thus, unlike other Policy pruning rules, IDPR Policy
      would have to be applied before any other pruning rules
      except Basic Match.

      Specifically, IDPR Policy examines the packet being
      forwarded to ascertain if its attributes require that it
      be forwarded using policy-based routes. If so, IDPR
      Policy deletes all routes not provided by the IDPR
      protocol.


E.3  Some Route Lookup Algorithms

   This section examines several route lookup algorithms that
   are in use or have been proposed.  Each is described by
   giving the sequence of pruning rules it uses.  The
   strengths and weaknesses of each algorithm are presented


E.3.1 The Revised Classic Algorithm

      The Revised Classic Algorithm is the form of the
      traditional algorithm which was discussed in Section
      [E.1].  The steps of this algorithm are:
      1.  Basic match
      2.  Longest match
      3.  Best metric
      4.  Policy






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      Some implementations omit the Policy step, since it is
      needed only when routes may have metrics that are not
      comparable (because they were learned from different
      routing domains).

      The advantages of this algorithm are:

      (1)  It is widely implemented.

      (2)  Except for the Policy step (which an implementor
           can choose to make arbitrarily complex) the
           algorithm is simple both to understand and to
           implement.

      Its disadvantages are:

      (1)  It does not handle IS-IS or OSPF route classes, and
           therefore cannot be used for Integrated IS-IS or
           OSPF.

      (2)  It does not handle TOS or other path attributes.

      (3)  The policy mechanisms are not standardized in any
           way, and are therefore are often implementation-
           specific.  This causes extra work for implementors
           (who must invent appropriate policy mechanisms) and
           for users (who must learn how to use the
           mechanisms.  This lack of a standardized mechanism
           also makes it difficult to build consistent
           configurations for routers from different vendors.
           This presents a significant practical deterrent to
           multi-vendor interoperability.

      (4)  The proprietary policy mechanisms currently
           provided by vendors are often inadequate in complex
           parts of the Internet.

      (5)  The algorithm has not been written down in any
           generally available document or standard.  It is,
           in effect, a part of the Internet Folklore.









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E.3.2 The Variant Router Requirements Algorithm

      Some Router Requirements Working Group members have
      proposed a slight variant of the algorithm described in
      the Section [5.2.4.3].  In this variant, matching the
      type of service requested is considered to be more
      important, rather than less important, than matching as
      much of the destination address as possible.  For
      example, this algorithm would prefer a default route
      which had the correct type of service over a network
      route which had the default type of service, whereas the
      algorithm in [5.2.4.3] would make the opposite choice.

      The steps of the algorithm are:
      1.  Basic match
      2.  Weak TOS
      3.  Longest match
      4.  Best metric
      5.  Policy

      Debate between the proponents of this algorithm and the
      regular Router Requirements Algorithm suggests that each
      side can show cases where its algorithm leads to
      simpler, more intuitive routing than the other's
      algorithm does.  In general, this variant has the same
      set of advantages and disadvantages that the algorithm
      specified in [5.2.4.3] does, except that pruning on Weak
      TOS before pruning on Longest Match makes this algorithm
      less compatible with OSPF and Integrated IS-IS than the
      standard Router Requirements Algorithm.


E.3.3 The OSPF Algorithm

      OSPF uses an algorithm which is virtually identical to
      the Router Requirements Algorithm except for one crucial
      difference: OSPF considers OSPF route classes.

      The algorithm is:
      1.  Basic match
      2.  OSPF route class
      3.  Longest match
      4.  Weak TOS
      5.  Best metric





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      6.  Policy

      Type of service support is not always present.  If it is
      not present then, of course, the fourth step would be
      omitted

      This algorithm has some advantages over the Revised
      Classic Algorithm:

      (1)  It supports type of service routing.

      (2)  Its rules are written down, rather than merely
           being a part of the Internet folklore.

      (3)  It (obviously) works with OSPF.

      However, this algorithm also retains some of the
      disadvantages of the Revised Classic Algorithm:

      (1)  Path properties other than type of service (e.g.
           MTU) are ignored.

      (2)  As in the Revised Classic Algorithm, the details
           (or even the existence) of the Policy step are left
           to the discretion of the implementor.

      The OSPF Algorithm also has a further disadvantage
      (which is not shared by the Revised Classic Algorithm).
      OSPF internal (intra-area or inter-area) routes are
      always considered to be superior to routes learned from
      other routing protocols, even in cases where the OSPF
      route matches fewer bits of the destination address.
      This is a policy decision that is inappropriate in some
      networks.

      Finally, it is worth noting that the OSPF Algorithm's
      TOS support suffers from a deficiency in that routing
      protocols which support TOS are implicitly preferred
      when forwarding packets which have non-zero TOS values.
      This may not be appropriate in some cases.









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E.3.4 The Integrated IS-IS Algorithm

      Integrated IS-IS uses an algorithm which is similar to
      but not quite identical to the OSPF Algorithm.
      Integrated IS-IS uses a different set of route classes,
      and also differs slightly in its handling of type of
      service.  The algorithm is:
      1. Basic Match
      2. IS-IS Route Classes
      3. Longest Match
      4. Weak TOS
      5. Best Metric
      6. Policy

      Although Integrated IS-IS uses Weak TOS, the protocol is
      only capable of carrying routes for a small specific
      subset of the possible values for the TOS field in the
      IP header.  Packets containing other values in the TOS
      field are routed using the default TOS.

      Type of service support is optional; if disabled, the
      fourth step would be omitted.  As in OSPF, the
      specification does not include the Policy step.

      This algorithm has some advantages over the Revised
      Classic Algorithm:
      (1)  It supports type of service routing.
      (2)  Its rules are written down, rather than merely
           being a part of the Internet folklore.
      (3)  It (obviously) works with Integrated IS-IS.

      However, this algorithm also retains some of the
      disadvantages of the Revised Classic Algorithm:
      (1)  Path properties other than type of service (e.g.
           MTU) are ignored.
      (2)  As in the Revised Classic Algorithm, the details
           (or even the existence) of the Policy step are left
           to the discretion of the implementor.
      (3)  It doesn't work with OSPF because of the
           differences between IS-IS route classes and OSPF
           route classes.  Also, because IS-IS supports only a
           subset of the possible TOS values, some obvious
           implementations of the Integrated IS-IS algorithm
           would not support OSPF's interpretation of TOS.





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      The Integrated IS-IS Algorithm also has a further
      disadvantage (which is not shared by the Revised Classic
      Algorithm): IS-IS internal (intra-area or inter-area)
      routes are always considered to be superior to routes
      learned from other routing protocols, even in cases
      where the IS-IS route matches fewer bits of the
      destination address and doesn't provide the requested
      type of service.  This is a policy decision that may not
      be appropriate in all cases.

      Finally, it is worth noting that the Integrated IS-IS
      Algorithm's TOS support suffers from the same deficiency
      noted for the OSPF Algorithm.




































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Security Considerations

Although the focus of this document is interoperability rather
than security, there are obviously many sections of this
document which have some ramifications on network security.

"Security" means different things to different people.
Security from a router's point of view is anything that helps
to keep its own networks operational and in addition helps to
keep the Internet as a whole healthy.  For the purposes of
this document, the security services we are concerned with are
"denial of service", "integrity", and "authentication" as it
applies to the first two.  "Privacy" as a security service is
important, but only peripherally a concern of a router -- at
least as of the date of this document.

In several places in this document there are sections entitled
"... Security Considerations".  These sections discuss
specific considerations that apply to the general topic under
discussion.

Rarely does this document say "do this and your router/network
will be secure".  More likely, it says "this is a good idea
and if you do it, it *may* improve the security of the
Internet and your local system in general."

Unfortunately, this is the state-of-the-art AT THIS TIME.  Few
if any of the network protocols a router is concerned with
have reasonable, built-in security features.  Industry and the
protocol designers have been and are continuing to struggle
with these issues.  There is progress, but only small baby
steps such as the peer-to-peer authentication available in the
BGP and OSPF routing protocols.

In particular, this document notes the current research into
developing and enhancing network security.  Specific areas of
research, development, and engineering that are underway as of
this writing (December 1993) are in IP Security, SNMP
Security, and common authentication technologies.

Notwithstanding all of the above, there are things both
vendors and users can do to improve the security of their
router.  Vendors should get a copy of "Trusted Computer System
Interpretation" [INTRO:8].  Even if a vendor decides not to





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submit their device for formal verification under these
guidelines, the publication provides excellent guidance on
general security design and practices for computing devices.














































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Acknowledgments


O that we now had here
But one ten thousand of those men in England
That do no work to-day!

What's he that wishes so?
My cousin Westmoreland? No, my fair cousin:
If we are mark'd to die, we are enow
To do our country loss; and if to live,
The fewer men, the greater share of honour.
God's will! I pray thee, wish not one man more.
By Jove, I am not covetous for gold,
Nor care I who doth feed upon my cost;
It yearns me not if men my garments wear;
Such outward things dwell not in my desires:
But if it be a sin to covet honour,
I am the most offending soul alive.
No, faith, my coz, wish not a man from England:
God's peace! I would not lose so great an honour
As one man more, methinks, would share from me
For the best hope I have. O, do not wish one more!
Rather proclaim it, Westmoreland, through my host,
That he which hath no stomach to this fight,
Let him depart; his passport shall be made
And crowns for convoy put into his purse:
We would not die in that man's company
That fears his fellowship to die with us.
This day is called the feast of Crispian:
He that outlives this day, and comes safe home,
Will stand a tip-toe when the day is named,
And rouse him at the name of Crispian.
He that shall live this day, and see old age,
Will yearly on the vigil feast his neighbours,
And say 'To-morrow is Saint Crispian:'
Then will he strip his sleeve and show his scars.
And say 'These wounds I had on Crispin's day.'
Old men forget: yet all shall be forgot,
But he'll remember with advantages
What feats he did that day: then shall our names.
Familiar in his mouth as household words
Harry the king, Bedford and Exeter,
Warwick and Talbot, Salisbury and Gloucester,





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Be in their flowing cups freshly remember'd.
This story shall the good man teach his son;
And Crispin Crispian shall ne'er go by,
From this day to the ending of the world,
But we in it shall be remember'd;
We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother; be he ne'er so vile,
This day shall gentle his condition:
And gentlemen in England now a-bed
Shall think themselves accursed they were not here,
And hold their manhoods cheap whiles any speaks
That fought with us upon Saint Crispin's day.

This memo is a product of the IETF's Router Requirements
Working Group.  A memo such as this one is of necessity the
work of many more people than could be listed here.  A wide
variety of vendors, network managers, and other experts from
the Internet community graciously contributed their time and
wisdom to improve the quality of this memo.  The editor wishes
to extend sincere thanks to all of them.

The current editor also wishes to single out and extend his
heartfelt gratitude and appreciation to the original editor of
this document; Philip Almquist.  Without Philip's work, both
as the original editor and as the Chair of the working group,
this document would not have been produced.

Philip Almquist, Jeffrey Burgan, Frank Kastenholz, and Cathy
Wittbrodt each wrote major chapters of this memo.  Others who
made major contributions to the document included Bill Barns,
Steve Deering, Kent England, Jim Forster, Martin Gross, Jeff
Honig, Steve Knowles, Yoni Malachi, Michael Reilly, and Walt
Wimer.

Additional text came from Art Berggreen, John Cavanaugh, Ross
Callon, John Lekashman, Brian Lloyd, Gary Malkin, Milo Medin,
John Moy, Craig Partridge, Stephanie Price, Yakov Rekhter,
Steve Senum, Richard Smith, Frank Solensky, Rich Woundy, and
others who have been inadvertently overlooked.

Some of the text in this memo has been (shamelessly)
plagiarized from earlier documents, most notably RFC-1122 by
Bob Braden and the Host Requirements Working Group, and





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RFC-1009 by Bob Braden and Jon Postel.  The work of these
earlier authors is gratefully acknowledged.

Jim Forster was a co-chair of the Router Requirements Working
Group during its early meetings, and was instrumental in
getting the group off to a good start.  Jon Postel, Bob
Braden, and Walt Prue also contributed to the success by
providing a wealth of good advice prior to the group's first
meeting.  Later on, Phill Gross, Vint Cerf, and Noel Chiappa
all provided valuable advice and support.

Mike St. Johns coordinated the Working Group's interactions
with the security community, and Frank Kastenholz coordinated
the Working Group's interactions with the network management
area.  Allison Mankin and K.K. Ramakrishnan provided expertise
on the issues of congestion control and resource allocation.

Many more people than could possibly be listed or credited
here participated in the deliberations of the Router
Requirements Working Group, either through electronic mail or
by attending meetings.  However, the efforts of Ross Callon
and Vince Fuller in sorting out the difficult issues of route
choice and route leaking are especially acknowledged.

The previous editor, Philip Almquist, wishes to extend his
thanks and appreciation to his former employers, Stanford
University and BARRNet, for allowing him to spend a large
fraction (probably far more than they ever imagined when he
started on this) of his time working on this project.

The current editor wishes to thank his employer, FTP Software,
for allowing him to spend the time necessary to finish this
document.
















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Editor's Address

The address of the current editor of this document is
   Frank J. Kastenholz
   FTP Software
   2 High Street
   North Andover, MA, 01845-2620
   USA

   Phone: +1 508-685-4000

   EMail: kasten@ftp.com





































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Table of Contents


 Status of this Memo ....................................    i
 1.  INTRODUCTION .......................................    1
 1.1  Reading this Document .............................    3
 1.1.1  Organization ....................................    3
 1.1.2  Requirements ....................................    4
 1.1.3  Compliance ......................................    5
 1.2  Relationships to Other Standards ..................    7
 1.3  General Considerations ............................    8
 1.3.1  Continuing Internet Evolution ...................    8
 1.3.2  Robustness Principle ............................    9
 1.3.3  Error Logging ...................................   10
 1.3.4  Configuration ...................................   11
 1.4  Algorithms ........................................   12
 2.  INTERNET ARCHITECTURE ..............................   14
 2.1  Introduction ......................................   14
 2.2  Elements of the Architecture ......................   15
 2.2.1  Protocol Layering ...............................   15
 2.2.2  Networks ........................................   17
 2.2.3  Routers .........................................   18
 2.2.4  Autonomous Systems ..............................   20
 2.2.5  Addresses and Subnets ...........................   20
 2.2.6  IP Multicasting .................................   22
 2.2.7  Unnumbered Lines and Networks and Subnets .......   23
 2.2.8  Notable Oddities ................................   25
 2.2.8.1  Embedded Routers ..............................   25
 2.2.8.2  Transparent Routers ...........................   26
 2.3  Router Characteristics ............................   27
 2.4  Architectural Assumptions .........................   31
 3.  LINK LAYER .........................................   34
 3.1  INTRODUCTION ......................................   34
 3.2  LINK/INTERNET LAYER INTERFACE .....................   34
 3.3  SPECIFIC ISSUES ...................................   36
 3.3.1  Trailer Encapsulation ...........................   36
 3.3.2  Address Resolution Protocol -- ARP ..............   36
 3.3.3  Ethernet and 802.3 Coexistence ..................   36
 3.3.4  Maximum Transmission Unit -- MTU ................   37
 3.3.5  Point-to-Point Protocol -- PPP ..................   37
 3.3.5.1  Introduction ..................................   38
 3.3.5.2  Link Control Protocol (LCP) Options ...........   38
 3.3.5.3  IP Control Protocol (ICP) Options .............   40
 3.3.6  Interface Testing ...............................   41





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 4.  INTERNET LAYER -- PROTOCOLS ........................   42
 4.1  INTRODUCTION ......................................   42
 4.2  INTERNET PROTOCOL -- IP ...........................   42
 4.2.1  INTRODUCTION ....................................   42
 4.2.2  PROTOCOL WALK-THROUGH ...........................   43
 4.2.2.1  Options: RFC-791 Section 3.2 ..................   43
 4.2.2.2  Addresses in Options: RFC-791 Section 3.1 .....   46
 4.2.2.3   Unused  IP Header Bits: RFC-791 Section 3.1
     ....................................................   47
 4.2.2.4  Type of Service: RFC-791 Section 3.1 ..........   48
 4.2.2.5  Header Checksum: RFC-791 Section 3.1 ..........   48
 4.2.2.6  Unrecognized Header Options: RFC-791 Section
     3.1 ................................................   49
 4.2.2.7  Fragmentation: RFC-791 Section 3.2 ............   49
 4.2.2.8  Reassembly: RFC-791 Section 3.2 ...............   50
 4.2.2.9  Time to Live: RFC-791 Section 3.2 .............   51
 4.2.2.10  Multi-subnet Broadcasts: RFC-922 .............   51
 4.2.2.11  Addressing: RFC-791 Section 3.2 ..............   51
 4.2.3  SPECIFIC ISSUES .................................   55
 4.2.3.1  IP Broadcast Addresses ........................   55
 4.2.3.2  IP Multicasting ...............................   56
 4.2.3.3  Path MTU Discovery ............................   57
 4.2.3.4  Subnetting ....................................   58
 4.3  INTERNET CONTROL MESSAGE PROTOCOL -- ICMP .........   59
 4.3.1  INTRODUCTION ....................................   59
 4.3.2  GENERAL ISSUES ..................................   59
 4.3.2.1  Unknown Message Types .........................   60
 4.3.2.2  ICMP Message TTL ..............................   60
 4.3.2.3  Original Message Header .......................   60
 4.3.2.4  ICMP Message Source Address ...................   60
 4.3.2.5  TOS and Precedence ............................   61
 4.3.2.6  Source Route ..................................   62
 4.3.2.7  When Not to Send ICMP Errors ..................   62
 4.3.2.8  Rate Limiting .................................   64
 4.3.3  SPECIFIC ISSUES .................................   65
 4.3.3.1  Destination Unreachable .......................   65
 4.3.3.2  Redirect ......................................   66
 4.3.3.3  Source Quench .................................   66
 4.3.3.4  Time Exceeded .................................   67
 4.3.3.5  Parameter Problem .............................   67
 4.3.3.6  Echo Request/Reply ............................   68
 4.3.3.7  Information Request/Reply .....................   69
 4.3.3.8  Timestamp and Timestamp Reply .................   69
 4.3.3.9  Address Mask Request/Reply ....................   71





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 4.3.3.10  Router Advertisement and Solicitations .......   72
 4.4  INTERNET GROUP MANAGEMENT PROTOCOL -- IGMP ........   73
 5.  INTERNET LAYER -- FORWARDING .......................   74
 5.1  INTRODUCTION ......................................   74
 5.2  FORWARDING WALK-THROUGH ...........................   74
 5.2.1  Forwarding Algorithm ............................   74
 5.2.1.1  General .......................................   75
 5.2.1.2  Unicast .......................................   76
 5.2.1.3  Multicast .....................................   77
 5.2.2  IP Header Validation ............................   79
 5.2.3  Local Delivery Decision .........................   81
 5.2.4  Determining the Next Hop Address ................   84
 5.2.4.1  Immediate Destination Address .................   85
 5.2.4.2  Local/Remote Decision .........................   85
 5.2.4.3  Next Hop Address ..............................   87
 5.2.4.4  Administrative Preference .....................   92
 5.2.4.6  Load Splitting ................................   94
 5.2.5  Unused IP Header Bits: RFC-791 Section 3.1 ......   94
 5.2.6   Fragmentation and Reassembly: RFC-791 Section
     3.2 ................................................   95
 5.2.7  Internet Control Message Protocol -- ICMP .......   95
 5.2.7.1  Destination Unreachable .......................   96
 5.2.7.2  Redirect ......................................   98
 5.2.7.3  Time Exceeded .................................  100
 5.2.8  INTERNET GROUP MANAGEMENT PROTOCOL -- IGMP ......  101
 5.3  SPECIFIC ISSUES ...................................  101
 5.3.1  Time to Live (TTL) ..............................  101
 5.3.2  Type of Service (TOS) ...........................  103
 5.3.3  IP Precedence ...................................  104
 5.3.3.1  Precedence-Ordered Queue Service ..............  106
 5.3.3.2  Lower Layer Precedence Mappings ...............  106
 5.3.3.3  Precedence Handling For All Routers ...........  107
 5.3.4  Forwarding of Link Layer Broadcasts .............  110
 5.3.5  Forwarding of Internet Layer Broadcasts .........  111
 5.3.5.1  Limited Broadcasts ............................  113
 5.3.5.2  Net-directed Broadcasts .......................  113
 5.3.5.3  All-subnets-directed Broadcasts ...............  114
 5.3.5.4  Subnet-directed Broadcasts ....................  117
 5.3.6  Congestion Control ..............................  117
 5.3.7  Martian Address Filtering .......................  119
 5.3.8  Source Address Validation .......................  120
 5.3.9  Packet Filtering and Access Lists ...............  120
 5.3.10  Multicast Routing ..............................  122
 5.3.11  Controls on Forwarding .........................  122





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 5.3.12  State Changes ..................................  122
 5.3.12.1  When a Router Ceases Forwarding ..............  123
 5.3.12.2  When a Router Starts Forwarding ..............  124
 5.3.12.3  When an Interface Fails or is Disabled .......  124
 5.3.12.4  When an Interface is Enabled .................  124
 5.3.13  IP Options .....................................  125
 5.3.13.1  Unrecognized Options .........................  125
 5.3.13.2  Security Option ..............................  125
 5.3.13.3  Stream Identifier Option .....................  125
 5.3.13.4  Source Route Options .........................  126
 5.3.13.5  Record Route Option ..........................  126
 5.3.13.6  Timestamp Option .............................  127
 6.  TRANSPORT LAYER ....................................  129
 6.1  USER DATAGRAM PROTOCOL -- UDP .....................  129
 6.2  TRANSMISSION CONTROL PROTOCOL -- TCP ..............  129
 7.  APPLICATION LAYER -- ROUTING PROTOCOLS .............  132
 7.1  INTRODUCTION ......................................  132
 7.1.1  Routing Security Considerations .................  132
 7.1.2  Precedence ......................................  133
 7.2  INTERIOR GATEWAY PROTOCOLS ........................  133
 7.2.1  INTRODUCTION ....................................  133
 7.2.2  OPEN SHORTEST PATH FIRST -- OSPF ................  134
 7.2.2.1  Introduction ..................................  134
 7.2.2.2  Specific Issues ...............................  135
 7.2.2.3  New Version of OSPF ...........................  135
 7.2.3   INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM --
     DUAL IS-IS .........................................  136
 7.2.4  ROUTING INFORMATION PROTOCOL -- RIP .............  137
 7.2.4.1  Introduction ..................................  137
 7.2.4.2  Protocol Walk-Through .........................  137
 7.2.4.3  Specific Issues ...............................  144
 7.2.5  GATEWAY TO GATEWAY PROTOCOL -- GGP ..............  145
 7.3  EXTERIOR GATEWAY PROTOCOLS ........................  145
 7.3.1  INTRODUCTION ....................................  145
 7.3.2  BORDER GATEWAY PROTOCOL -- BGP ..................  146
 7.3.2.1  Introduction ..................................  146
 7.3.2.2  Protocol Walk-through .........................  147
 7.3.3  EXTERIOR GATEWAY PROTOCOL -- EGP ................  148
 7.3.3.1  Introduction ..................................  148
 7.3.3.2  Protocol Walk-through .........................  148
 7.3.4  INTER-AS ROUTING WITHOUT AN EXTERIOR  PROTOCOL
     ....................................................  152
 7.4  STATIC ROUTING ....................................  152
 7.5  FILTERING OF ROUTING INFORMATION ..................  154





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 7.5.1  Route Validation ................................  155
 7.5.2  Basic Route Filtering ...........................  155
 7.5.3  Advanced Route Filtering ........................  156
 7.6  INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE .......  157
 8.  APPLICATION LAYER -- NETWORK MANAGEMENT PROTOCOLS
     ....................................................  159
 8.1  The Simple Network Management Protocol  --  SNMP
     ....................................................  159
 8.1.1  SNMP Protocol Elements ..........................  159
 8.2  Community Table ...................................  160
 8.3  Standard MIBS .....................................  161
 8.4  Vendor Specific MIBS ..............................  163
 8.5  Saving Changes ....................................  164
 9.  APPLICATION LAYER -- MISCELLANEOUS PROTOCOLS .......  166
 9.1  BOOTP .............................................  166
 9.1.1  Introduction ....................................  166
 9.1.2  BOOTP Relay Agents ..............................  166
 10.  OPERATIONS AND MAINTENANCE ........................  168
 10.1  Introduction .....................................  168
 10.2  Router Initialization ............................  170
 10.2.1  Minimum Router Configuration ...................  170
 10.2.2  Address and Address Mask Initialization ........  170
 10.2.3  Network Booting using BOOTP and TFTP ...........  172
 10.3  Operation and Maintenance ........................  173
 10.3.1  Introduction ...................................  173
 10.3.2  Out Of Band Access .............................  174
 10.3.2  Router O&M Functions ...........................  175
 10.3.2.1  Maintenance -- Hardware Diagnosis ............  175
 10.3.2.2  Control -- Dumping and Rebooting .............  175
 10.3.2.3  Control -- Configuring the Router ............  175
 10.3.2.4  Netbooting of System Software ................  176
 10.3.2.5   Detecting  and responding to misconfigura-
     tion ...............................................  177
 10.3.2.6  Minimizing Disruption ........................  178
 10.3.2.7  Control -- Troubleshooting Problems ..........  178
 10.4  Security Considerations ..........................  180
 10.4.1  Auditing and Audit Trails ......................  180
 10.4.2  Configuration Control ..........................  182
 11.  REFERENCES ........................................  184
 APPENDIX  A. REQUIREMENTS  FOR  SOURCE-ROUTING  HOSTS
     ....................................................  196
 APPENDIX  B. GLOSSARY ..................................  198
 APPENDIX  C. FUTURE DIRECTIONS .........................  204
 APPENDIX D.  Multicast Routing Protocols ...............  207





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 D.1  Introduction ......................................  207
 D.2   Distance  Vector  Multicast Routing Protocol --
     DVMRP ..............................................  207
 D.3  Multicast Extensions to OSPF -- MOSPF .............  208
 APPENDIX E  Additional Next-Hop Selection  Algorithms
     ....................................................  209
 E.1. Some Historical Perspective .......................  209
 E.2. Additional Pruning Rules ..........................  211
 E.3  Some Route Lookup Algorithms ......................  213
 E.3.1 The Revised Classic Algorithm ....................  213
 E.3.2 The Variant Router Requirements Algorithm ........  215
 E.3.3 The OSPF Algorithm ...............................  215
 E.3.4 The Integrated IS-IS Algorithm ...................  217
 Security Considerations ................................  219
 Acknowledgments ........................................  221
 Editor's Address .......................................  224

































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