Internet DRAFT - draft-iab-service-id-considerations


Internet Architecture Board                             M. St Johns, Ed.
Internet-Draft                                            G. Huston, Ed.
Expires: February 12, 2004                                           IAB
                                                          August 14, 2003

   Considerations on the use of a Service Identifier in Packet Headers

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

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    Copyright (C) The Internet Society (2003). All Rights Reserved.


    This memo describes some considerations relating to the use of IP
    protocol number fields and payload protocol (e.g.  TCP) port fields
    to identify particular services that may be associated with that port
    number or protocol number.

1. Introduction

    This memo describes some considerations relating to the use of IP
    protocol number fields and payload protocol (e.g.  TCP) port or
    service fields to identify particular services that may be associated
    with that port number or protocol number.  It is a general statement
    regarding appropriate processing and use of service identifiers by

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    intermediate systems.

    This memo points out that various measures by intermediate systems
    that are intended to filter or prevent the transmission of traffic
    based on the service identification within the traffic flow have
    limited effect, with a major side-effect of forcing the affected
    services to be redesigned using various forms of encapsulation or
    dynamic port negotiation in order to remove the fixed service
    identification from the IP packet headers. The IAB does not believe
    this serves the general interests of the Internet community related
    to the design of simple and reliable Internet applications. This memo
    suggests some thought be given to control mechanisms that do not rely
    on intermediary systems taking actions based on an assumed
    relationship between the service identifier in the packet and the
    actual service of which the packet is a part.

2. Service Identifiers

    Although not necessarily by design, certain conventions have evolved
    with respect to the IP protocol suite relative to the identification
    of services within an IP traffic flow:

    o  Within the IP protocol suite, end point identifiers (e.g.  TCP/
       UDP/SCTP port numbers, IP protocol numbers) are designed to
       identify services to end points.  In particular, TCP, UDP or SCTP
       (Stream Control Transmission Protocol) port numbers are intended
       to identify the source service location and the destination
       service entity to the destination end point.

    o  The IP [2] datagram header contains the source and destination
       address of the datagram as well as an indication of the upper-
       level protocol (ULP) carried within the datagram.  If the ULP is
       either TCP [3], UDP [1], or SCTP [8] the payload will contain both
       source and destination port numbers which allows differentiation
       between services (e.g.  TELNET, HTTP) and between multiple
       instances of the same service between the pair of hosts described
       by the source and destination address.

    o  By convention, for at least TCP and UDP, certain port numbers are
       used as rendezvous points and are considered "well known" on the
       source or destination side of the communication.  Such rendezvous
       points are maintained in an IANA registry currently located at
       [11].  Specific registries for protocol and port numbers are at
       [12] and  [13].

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    o  Notwithstanding the "well-knownness" of any given port, port
       numbers are only guaranteed to be meaningful to the end systems.
       An intermediate system should generally not impute specific
       meaning to any given port number, unless specifically indicated by
       an end system (e.g.  via the Resource Reservation Protocol
       (RSVP)[4] ) or agreed to by convention among the end systems and
       one or more specific intermediate systems (e.g.  firewall
       traversal for the IP Security Protocol (IPSEC)[5]).

    o  Some services make use of protocol interactions to dynamically
       allocate service identifiers (i.e.  port numbers) to specific
       communications.  One specific example of this is the Session
       Initiation Protocol (SIP)[9]. The implication of this is that
       intermediate systems cannot relate the service identifiers to the
       actual service unless they participate in the protocols which
       allocate the service identifiers, or are explicitly notified of
       the outcome of the allocation.

    o  Various products and service-related mechanisms deployed today
       take advantage of the fact that some service identifiers are
       relatively stable (and well known) to do various things (e.g.
       firewall filtering, QOS marking).

    o  Certain network operations, such as various forms of packet
       encapsulation (e.g.  tunnelling) and encryption, can occlude this
       port number (or service identifier) while an IP packet is in
       transit within the network.  For example, both the IPSEC
       Encapsulating Security Payload (ESP) [6] and Generic Routing
       Encapsulation (GRE) [7] both provide means for tunneling an IP
       datagram within another IP datagram.  The service information
       becomes obscured and, in some instances, encrypted.

    o  Cooperating end systems may elect to use arbitrarily selected port
       numbers for any service.  The port numbers used in such cases may
       be statically defined, through coordinated configuration of the
       cooperating end systems through use of a common application or
       operating system, or by dynamic selection as an outcome of a
       rendezvous protocol.

    Intermediate system imposed service-based controls may block
    legitimate uses by subscribers.  For example, some service providers
    are blocking port 25 (i.e.  notionally SMTP) traffic for the stated
    purpose of trying to prevent SPAM, but which can also block
    legitimate email to the end user.

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    Attempts by intermediate systems to impose service-based controls on
    communications against the perceived interests of the end parties to
    the communication are often circumvented[10].  Services may be
    tunneled within other services, proxied by a collaborating external
    host (e.g.  an anonymous redirector), or simply run over an alternate
    port (e.g.  port 8080 vs port 80 for HTTP). Another means of
    circumvention is alteration of the service behaviour to use a dynamic
    port negotiation phase, in order to avoid use of a constant port

    For the purposes of this memo a "party to a communication" is either
    the sender, receiver or an authorized agent of the sender or receiver
    in the path.

    If intermediate systems take actions on behalf of one or more parties
    to the communication or affecting the communication, a good rule of
    thumb is they should only take actions that are beneficial to or
    approved by one or more of the parties, within the operational
    parameters of the service-specific protocol, or otherwise unlikely to
    lead to widespread evasion by the user community.

3. Ramifications

    The IAB observes that having stable and globally meaningful service
    identifiers visible at points other than the end systems can be
    useful for the purposes of determining network behavior and network
    loading on a macro level.  The IAB also observes that application
    protocols that include dynamic port negotiation for both ends of a
    connection tend to add to the complexity of the applications.

    Dynamic port negotiation for a protocol may also limit or prohibit
    its use in situations where the service provider (e.g.  ISP or
    employer) has instituted some form of service filtering through port
    blocking mechanisms.

    From this perspective of network and application utility, it is
    preferable that no action or activity be undertaken by any agency,
    carrier, service provider or organization which would tend to cause
    end-users and protocol designers to generally obscure service
    identification information from the IP packet header.

    Nothing in this statement should be construed as opposing
    encapsulation, application security, end-to-end encryption, or other
    processes beneficial or specifically desired by the end-users.

4. Security Considerations

    This document is a general statement regarding appropriate processing

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    and use of service identifiers by intermediate systems.  If enough
    agencies, carriers, service providers and organizations ignore the
    concerns voiced here, the utility of port and protocol numbers,
    general network analysis, end-user beneficial filtering (e.g.
    preventing DDOS attacks), and other common uses of these service
    identifiers might be adversely affected.


    [1]   Postel, J., "User Datagram Protocol", STD 6, RFC 768, August

    [2]   Postel, J., "Internet Protocol", STD 5, RFC 791, September

    [3]   Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
          September 1981.

    [4]   Braden, B., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
          "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
          Specification", RFC 2205, September 1997.

    [5]   Kent, S. and R. Atkinson, "Security Architecture for the
          Internet Protocol", RFC 2401, November 1998.

    [6]   Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
          (ESP)", RFC 2406, November 1998.

    [7]   Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
          "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

    [8]   Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
          H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
          "Stream Control Transmission Protocol", RFC 2960, October 2000.

    [9]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
          Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
          Session Initiation Protocol", RFC 3261, June 2002.

          MEDIA FILES", 27th November 2002.

    [11]  IANA, "IANA Protocol Numbers and Assignment Services", May
          2003, <>.

    [12]  IANA, "IANA Protocol Number Registry", May 2003, <http://

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    [13]  IANA, "IANA Port Number Registry", May 2003, <http://

Authors' Addresses

    Mike St Johns
    Internet Architecture Board

    Geoff Huston
    Internet Architecture Board

Appendix A. IAB Members

    Internet Architecture Board Members at the time this document was
    completed were:

       Bernard Aboba
       Harald Alvestrand
       Rob Austein
       Leslie Daigle, Chair
       Patrik Faltstrom
       Sally Floyd
       Jun-ichiro Itojun Hagino
       Mark Handley
       Geoff Huston
       Charlie Kaufman
       James Kempf
       Eric Rescorla
       Michael StJohns

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