Internet DRAFT - draft-fairhurst-tsvwg-6man-udpzero


Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                             M. Westerlund
Expires: September 23, 2010                            Ericsson Research
                                                          March 22, 2010

                    IPv6 UDP Checksum Considerations


   This document examines the role of the transport checksum when used
   with IPv6, as defined in RFC2460.  It presents a summary of the
   trade-offs for evaluating the safety of updating RFC 2460 to permit
   an IPv6 UDP endpoint to use a zero value in the checksum field to
   indicate that no checksum is present.  The document describes issues
   and design principles that need to be considered and provides

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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Use of UDP Tunnels . . . . . . . . . . . . . . . . . . . .  5
       1.2.1.  Motivation for new approaches  . . . . . . . . . . . .  5
       1.2.2.  Reducing forwarding cost . . . . . . . . . . . . . . .  6
       1.2.3.  Need to inspect the entire packet  . . . . . . . . . .  6
       1.2.4.  Interactions with middleboxes  . . . . . . . . . . . .  7
       1.2.5.  Support for load balancing . . . . . . . . . . . . . .  7
   2.  Standards-Track Transports . . . . . . . . . . . . . . . . . .  7
     2.1.  UDP with Standard Checksum . . . . . . . . . . . . . . . .  8
     2.2.  UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.2.1.  Using UDP-Lite as a Tunnel Encapsulation . . . . . . .  8
     2.3.  IP in IPv6 Tunnel Encapsulations . . . . . . . . . . . . .  9
   3.  Evaluation of proposal to update to RFC 2460 to support
       zero checksum  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Alternatives to the Standard Checksum  . . . . . . . . . . 10
     3.2.  Applicability of method  . . . . . . . . . . . . . . . . . 11
     3.3.  Effect of packet modification in the network . . . . . . . 11
       3.3.1.  Corruption of the destination IP address . . . . . . . 12
       3.3.2.  Corruption of the source IP address  . . . . . . . . . 12
       3.3.3.  Delivery to an unexpected port . . . . . . . . . . . . 13
       3.3.4.  Validating the network path  . . . . . . . . . . . . . 14
     3.4.  Comparision  . . . . . . . . . . . . . . . . . . . . . . . 15
   4.  Requirements on the specification of transported protocols . . 15
   5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Document Change History . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

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1.  Introduction

   The User Datagram Protocol (UDP) transport was defined by RFC768
   [RFC0768] for IPv4 RFC791 [RFC0791] and is defined in RFC2460
   [RFC2460] for IPv6 hosts and routers.  A UDP transport endpoint may
   be either a host or a router.  The UDP Usage Guidelines [RFC5405]
   provides overall guidance for application designers, including the
   use of UDP to support tunneling.  These guidelines are applicable to
   this discussion.

   This section provides a background to key issues, and introduces the
   use of UDP as a tunnel transport protocol.

   Section 2 describes a set of standards-track datagram transport
   protocols that may be used to support tunnels.

   Section 3 evaluates proposals to update the UDP transport behaviour
   to allow for better support of tunnel protocols.  It focuses on a
   proposal to eliminate the checksum for this use-case with IPv6 and
   assess the trade-offs that would arise.

   Section 4 reviews the trade offs and provides recommendations.

1.1.  Background

   An Internet transport endpoint should concern itself with the
   following issues:

   o  Protection of the endpoint transport state from unnecessary extra
      state (i.e.  Invalid state from rogue packets).

   o  Protection of the endpoint transport state from corruption of
      internal state.

   o  Pre-filtering by the endpoint of erroneous data, to protect the
      transport from unnecessary processing and from corruption that it
      can not itself reject.

   o  Pre-filter of incorrectly addressed destination packets, before
      responding to a source address.

   UDP, as defined in [RFC0768], supports two checksum behaviours when
   used with IPv4.  The normal behaviour is for the sender to calculate
   a checksum over a block of data that includes a pseudo header and the
   UDP datagram payload.  The UDP header includes a 16-bit one's
   complement checksum that provides a statistical guarantee that the
   payload was not corrupted in transit.  This also allows a receiver to
   verify that the endpoint was the intended destination of the

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   datagram, because the pseudo header covers the IP addresses, port
   numbers, transport payload length, and Next Header/Protocol value
   corresponding to the UDP transport protocol.  The length field
   verifies that the datagram is not truncated or padded.  The checksum
   therefore protects an application against receiving corrupted payload
   data in place of, or in addition to, the data that was sent.
   Although the IPv4 UDP [RFC0768] checksum may be disabled,
   applications are recommended to enable UDP checksums [RFC5405].

   IPv4 UDP checksum control is often a kernel-wide configuration
   control (e.g.  In Linux and BSD), rather than a per socket call.
   There are Networking Interface Cards (NICs) that automatically
   calculate TCP/UDP checksums on transmission if a checksum of zero is
   sent to the NIC, using a method known as checksum offloading.

   The network-layer fields that are validated by a transport checksum

   o  Endpoint IP source address (always included in pseudo header of

   o  Endpoint IP destination address (always included in pseudo header
      of checksum)

   o  Upper Layer Payload type (always included in pseudo header of

   o  IP length of payload (always included in pseudo header of

   o  Length of the network layer extension headers (i.e.  By correct
      position of checksum bytes)

   The transport-layer fields that are validated by a transport checksum

   o  Transport demultiplexing, i.e. ports (always included in checksum)

   o  Transport payload size (always included in checksum)

   Transport endpoints also need to verify correctness of reassembly of
   any fragmented packets (unless the application use of the payload is
   corruption tolerant as indicated by UDP-Lite's checksum coverage
   field).  For UDP, this is normally provided as a part of the
   integrity check.  Disabling the IPv4 checksum prevents this check.  A
   lack of checksum can lead to issues in a translator or middlebox
   (e.g.  Many IPv4 Network Address Translators, NATs, rely on port
   numbers to find the mappings, packet fragments do not carry port

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   numbers, so fragments get dropped).  RFC2765 [RFC2765] provides some
   guidance on the processing of fragmented IPv4 UDP datagrams that do
   not carry a UDP checksum.

   IPv6 does not provide a network-layer integrity check.  The removal
   of the IPv6 header checksum released routers from a need to update a
   network-layer checksum on a hop-by-hop basis when they changed the
   IPv4 Time-To-Live (TTL) or IPv6 Hop Count.  The IP header checksum
   calculation was seen as redundant for most traffic (TCP and UDP with
   checksums enabled), and people wanted to avoid this extra processing.
   However, there was concern that the removal of the IP header checksum
   in IPv6 would lessen the protection of the source/destination IP
   addresses and result in a significant (a multiplier of ~32,000)
   increase in the number of times that a UDP packet was accidentally
   delivered to the wrong destination address and/or apparently sourced
   from the wrong source address when UDP checksums were set to zero.
   This would have had implications on the detectability of mis-delivery
   of a packet to an incorrect endpoint/socket, and the robustness of
   the Internet infrastructure.  The use of the UDP checksum is
   required[RFC2460] when applications transmit UDP over IPv6.

1.2.  Use of UDP Tunnels

   One increasingly popular use of UDP is as a tunneling protocol, where
   a tunnel endpoint encapsulates the packets of another protocol inside
   UDP datagrams and transmits them to another tunnel endpoint.  Using
   UDP as a tunneling protocol is attractive when the payload protocol
   is not supported by middleboxes that may exist along the path,
   because many middleboxes support transmission using UDP.  In this
   use, the receiving endpoint decapsulates the UDP datagrams and
   forwards the original packets contained in the payload [RFC5405].
   Tunnels establish virtual links that appear to directly connect
   locations that are distant in the physical Internet topology and can
   be used to create virtual (private) networks.

1.2.1.  Motivation for new approaches

   A number of tunnel protocols are currently being defined (e.g..
   Automated Multicast Tunnels, AMT [AMT], and the Locator/Identifier
   Separation Protocol, LISP [LISP]).  These protocols have proposed an
   update to IPv6 UDP checksum processing.  These tunnel protocols could
   benefit from simpler checksum processing for various reasons:

   o  Reducing forwarding costs, motivated by redundancy present in the
      encapsulated packet header, since in tunnel encapsulations,
      payload integrity and length verification may be provided by
      higher layer tunnel encapsulations (often using the IPv4, UDP,
      UDP-Lite, or TCP checksums).

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   o  Eliminating a need to access the entire packet when forwarding the

   o  Enhancing ability to traverse middleboxes, especially NATs.

   o  A desire to use the port number space to enable load-sharing.

1.2.2.  Reducing forwarding cost

   It is a common requirement to terminate a large number of tunnels on
   a single router/host.  Processing costs per tunnel concern both state
   (memory requirements) and processing costs.

   Automatic IP Multicast Without Explicit Tunnels, known as AMT [AMT]
   currently specifies UDP as the transport protocol for tunneled
   packets carrying tunneled IP multicast packets.  The current
   specification for AMT requires that the UDP checksum in the outer
   packet header SHOULD be 0 (see Section 6.6).  It argues that the
   computation of an additional checksum, when an inner packet is
   already adequately protected, is an unwarranted burden on nodes
   implementing lightweight tunneling protocols.  The AMT protocol needs
   to replicate a multicast packet to each gateway tunnel.  In this case
   the outer IP addresses are different for each tunnel and therefore
   require a different pseudo header to be built for each UDP replicated

   The argument concerning redundant processing costs is valid regarding
   the integrity of a tunneled packet.  In some architectures (e.g.  PC-
   based routers), other mechanisms may also significantly reduce
   checksum processing costs: There are implementations that have
   optimised checksum processing algorithms, including the use of
   checksum-offloading.  This processing is readily available for IPv4
   packets at high line rates.  Such processing may be anticipated for
   IPv6 endpoints, allowing them to reject corrupted packets without
   further processing.  Relaxing RFC 2460 to minimise the processing
   impact for existing hardware is a transition policy decision, which
   seems undesirable if at the same time it yields a solution that may
   reduce stability and functionality in future network scenarios.

1.2.3.  Need to inspect the entire packet

   The currently-deployed hardware in many routers uses a fast-path
   processing that only provides the first n bytes of a packet to the
   forwarding engine, where typically n < 128.  This prevents fast
   processing of a transport checksum over an entire (large) packet.
   Hence the currently defined IPv6 UDP checksum is poorly suited to use
   within routers that are unable to access the entire packet and do not
   provide checksum-offloading.

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1.2.4.  Interactions with middleboxes

   In IPv4, UDP-encapsulation may be desirable for NAT traversal, since
   UDP support is commonly provided.

   IPv6 NAT traversal does not necessarily present the same protocol
   issues as for IPv4.  It is not clear that NATs will work the same way
   for IPv6.  Any change to RFC 2460 is going to require rewriting IPv6
   (or defining it) NAT behaviour to achieve consistent widescale

   The requirements for IPv6 firewall traversal are likely be to be
   similar to those for IPv4.  In addition, it can be reasonably
   expected that a firewall conforming to RFC 2460 will not regard UDP
   datagrams with a zero checksum as valid packets, and if such a mode
   were to be defined for IPv6 these may also need to be updated.

   Key questions in this space include:

   o  What types of middleboxes does the protocol need to cross
      (routers, NAT boxes, firewalls, etc.), and how will those
      middleboxes deal with these packets?

   o  What do IPv6 routers do today with zero-checksum UDP packets?

   o  What other IPv6 middleboxes exist today, and what would they do?

1.2.5.  Support for load balancing

   The UDP port number fields have been used as a basis to design load-
   balancing solutions for IPv4.  This approach could also be leveraged
   for IPv6.  However, support for extension headers would increase the
   complexity of providing standards-compliant solutions for IPv6.

   An alternate method could utilise the IPv6 Flow Label to perform load
   balancing.  This would release IPv6 load-balancing devices from the
   need to assume semantics for the use of the transport port field.
   This use of the flow-label is consistent with the intended use,
   although further clarity may be needed to ensure the field can be
   consistently used for this purpose, (e.g.  ECMP [ECMP]).  Router
   vendors could be encouraged to start using the IPv6 Flow Label as a
   part of the flow hash.

2.  Standards-Track Transports

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2.1.  UDP with Standard Checksum

   UDP with standard checksum behaviour is defined in RFC 2460, and
   should be the default choice.  Guidelines are provided in [RFC5405].

2.2.  UDP-Lite

   UDP-Lite [RFC3828] offers an alternate transport to UDP, specified as
   a proposed standard, RFC 3828.  A MIB is defined in RFC 5097 and
   unicast usage guidelines in [RFC5405].  UDP-Lite has been
   implemented, e.g. as a part of the Linux kernel since version 2.6.20.

   UDP-Lite provides a checksum with an optional partial coverage.  When
   using this option, a datagram is divided into a sensitive part
   (covered by the checksum) and an insensitive part (not covered by the
   checksum).  Errors/corruption in the insensitive part will not cause
   the datagram to be discarded by the transport layer at the receiving
   host.  A minor side-effect of using UDP-Lite is that this was
   specified for damage-tolerant payloads, and some link-layers may
   employ different link encapsulations when forwarding UDP-Lite
   segments (e.g.  Over radio access bearers).  When the checksum covers
   the entire packet, which should be the default, UDP-Lite is
   semantically identical to UDP and is specified for use with IPv4 and
   IPv6.  It uses an IP protocol type (or IPv6 next header) with a value
   of 136 decimal.  This value is different to that used by UDP.

2.2.1.  Using UDP-Lite as a Tunnel Encapsulation

   Tunnel encapsulations can use UDP-Lite (e.g.  Control And
   Provisioning of Wireless Access Points, CAPWAP), since UDP-Lite
   provides a transport-layer checksum, including an IP pseudo header
   checksum, in IPv6, without the need to traverse the entire packet.

   In the LISP case, the bytes that would need to be "checksummed" for
   UDP-Lite would be the set of bytes that are added to the packet by
   the LISP encapsulating router.  When an IPv4/UDP header is per-pended
   by a LISP router, the LISP ETR needs to calculate the IP header
   checksum over 20 bytes (the IP header).  If an IPv6/UDP-Lite header
   were per-pended by a LISP router, the ETR would need to calculate an
   IP header checksum over 48 bytes (the IP pseudo header and the UDP
   header).  This results in an increase in the number of bytes to be
   the checksummed for IPv6 (48 bytes rather than 20), but this is not
   thought to be a major processing overhead for a well-optimized
   implementation where the pre-pended header bytes are already in

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2.3.  IP in IPv6 Tunnel Encapsulations

   The IETF has defined a set of tunneling protocols.  These do not
   include a checksum, since tunnel encapsulations are typically layered
   directly over the Internet layer (identified by the upper layer type
   field) and are also not used as endpoint transport protocols.  That
   is, there is little chance of confusing a tunnel-encapsulated packet
   with other application data that would result in corruption of
   application state or data.

   From the end-to-end perspective, the principal difference is that the
   Next Header field identifies a separate transport, which reduces the
   probability that corruption could result in the packet being
   delivered to the wrong endpoint or application.  Specifically,
   packets are only delivered to protocol modules that process a
   specific next header value.  The next header field therefore provides
   a first-level check of correct demultiplexing.  In contrast, the UDP
   port space is shared by many diverse application and therefore UDP de
   multiplexing relies solely on the port numbers.

3.  Evaluation of proposal to update to RFC 2460 to support zero

   This section evaluates a proposal to update IPv6 [RFC2460], to
   provide the option that some nodes may suppress generation and
   checking of the UDP transport checksum.  The decision to omit an
   integrity check at the IPv6 level means that the transport check is
   overloaded with many functions including validating:

   o  the endpoint address was not corrupted within a router - this
      packet was meant for this destination and a wrong header has not
      been spliced to a different payload.

   o  the extension header processing is correctly delimited - the start
      of data has not been corrupted.  The protocol type field also
      provides some protection.

   o  reassembly processing, when used.

   o  the length of the payload.

   o  the port values - i.e.  The correct application gets the payload
      (applications should also check source ports/address).

   o  the payload integrity.

   In IPv4, the first 4 checks are performed using the IPv4 header

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   In IPv6, these checks occur within the endpoint stack using the UDP
   checksum information.  An IPv6 node also relies on the header
   information to determine whether to send an ICMPv6 error message and
   to determine the node to which this is sent.  Corrupted information
   may lead to misdelivery to an unintended application socket on an
   unexpected host.

3.1.  Alternatives to the Standard Checksum

   There are several alternatives to the normal method for calculating
   the UDP Checksum that do not require a tunnel endpoint to inspect the
   entire packet when computing a checksum.  These include (in
   decreasing complexity):

   o  Delta computation of the checksum from an encapsulated checksum
      field.  Since the checksum is a cumulative sum (RFC 1624), an
      encapsulating header checksum can be derived from the new pseudo
      header, the inner checksum and the sum of the other network-layer
      fields not included in the pseudo header of the encapsulated
      packet.  This would not require access to the whole packet, but
      does require header fields to be collected across the header, and
      arithmetic operations on each packet.  The method would only work
      for packets that contain a 2's complement transport checksum (i.e.
      it would not be appropriate for SCTP or when IP fragmentation is
      used).  The process may be easier for IPv4 over IPv6
      encapsulation, where the encapsulated IPv4 header checksum could
      be used as a basis.

   o  UDP-Lite.  Where the checksum coverage may be set to only the
      header portion of a packet.  This requires a pseudo header
      checksum calculation only on the encapsulating packet header,
      which includes extracting the UDP payload length for the pseudo
      header, however this is expected to be also known when performing
      packet forwarding.  The value may be cached per flow/destination,
      and subsequently combined only with the Length field to minimise
      per-packet processing.

   o  The UDP Tunnel Transport, UDPTT [UDPTT](if progressed), where UDP
      is modified to derive the checksum only from the encapsulating
      packet protocol header.  This value does not change between
      packets in a flow.  The value may be cached per flow/destination
      to minimise per-packet processing.

   o  UDP modified to disable checksum processing [UDPZ](if progressed).
      This requires no checksum calculation.

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   These options are discussed further in later sections.

3.2.  Applicability of method

   The expectation of the present proposal to permit omission of UDP
   checksums [UDPZ] is that this would apply only to IPv6 router nodes
   that implement specific protocols.  However, the distinction between
   a router and a host is not always clear, especially at the transport
   level.  Systems (such as unix-based operating systems) routinely
   provide both functions.  There is no way to identify the role of a
   receiver from a received packet.

   Any new method would therefore need a specific applicability
   statement indicating when this mechanism can (and can not).  There
   are additional requirements, e.g. that fragmentation is not
   performed, since correct reassembly can not be verified at the
   receiver without a checksum.  This would also open the receiver to a
   wide range of mis-behaviours.  This implies disabling host-based
   fragmentation.  Policing this and ensuring correct interactions with
   the stack implies much more than simply disabling the checksum
   algorithm for specific packets at the transport interface.  There are
   also proposals to simply ignore a specific received UDP checksum
   value, however this also can result in problems (e.g. when used with
   a NAT that always adjusts the checksum value).

   The IETF should carefully consider constraints on sanctioning the use
   of this mode.  If this is specified and widely available, it may be
   expected to be used by applications that are perceived to gain
   benefit.  Any solution that uses an end-to-end transport protocol
   (rather than an IP in IP encapsulation) also needs to minimise the
   possibility that end-hosts could confuse a corrupted or wrongly
   delivered packet with that of data addressed to an application
   running on their endpoint.

3.3.  Effect of packet modification in the network

   When a checksum is used with UDP/IPv6, this significantly reduces the
   impact of such errors, reducing the probability of undetected
   corruption of state (and data) on both the host stack and the
   applications using the transport service.

   P packets may be corrupted as they travers an Internet path.
   Evidence has been presented [Sigcomm2000] to show that this was once
   an issue with IPv4 routers, and occasional corruption could result
   from bad internal router processing in routers or hosts.  These
   errors are not detected by the strong frame checksums employed at the
   link-layer (RFC 3819).  There is no current evidence that such cases
   are rare in the modern Internet, nor that they may not be applicable

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   to IPv6.  It therefore seems prudent not to relax this constraint.
   The emergence of low-end IPv6 routers and the proposed use of NAT
   with IPv6 further motivate the need to protect from this type of

   Corruption in the network may result in:

   o  a datagram being mis-delivered to the wrong host/router or the
      wring transport entity within a host/router.  Such a datagram
      needs to be discarded.

   o  a datagram payload being corrupted and delivered to the intended
      host/router transport entity.  Such a datagram needs to be either
      discarded or correctly processed by an application that has its
      own integrity checks.

   o  a datagram payload being truncated by corruption of the length
      field.  Such a datagram needs to be discarded.

3.3.1.  Corruption of the destination IP address

   An IP endpoint destination address could be modified in the network
   (corrupted by errors).  This modification can not be detected in the
   network when using IPv6.  This is not a concern in IPv4, because the
   IP header checksum will result in this packet being discarded by the
   receiving IP stack.

   There are two possible outcomes:

   o  Delivery to an address that is not in use (the packet will not be
      delivered, but could result in an error report).

   o  Delivery to a different address.  This modification will normally
      be detected by the transport checksum, resulting in silent
      discard.  Without this checksum, the packet would be passed to the
      port demultiplexing function.  If an application is bound to the
      associated ports, the packet payload will be passed to the
      application (see the subsequent section on port processing).

3.3.2.  Corruption of the source IP address

   This section examines what happens when the source IPv6 address is
   corrupted in transit.  (This is not a concern in IPv4, because the IP
   header checksum will result in this packet being discarded by the
   receiving IP stack).

   Corruption of an IPv6 packet's source address does not result in the
   IP packet being delivered to a different endpoint protocol or

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   destination address.  If only the source address is corrupted, the
   packet will likely be processed in the intended context, although
   with erroneous origin information.  The result will depend on the
   application or protocol that processes the packet.  Some examples

   o  An application that requires pre-established context may disregard
      the packet as invalid, or could map this to another context (if a
      context for the modified source address was already activated).

   o  A stateless application will process the packet outside of any
      context, a simple example is the ECHO server, which will respond
      with a packet to the modified source address.  This would create
      unwanted additional processing load, and generate traffic to the
      modified endpoint address.

   o  Some applications build state using the information from packet
      headers.  A previously unused source address would result in
      receiver processing and the creation of unnecessary transport-
      layer state at the receiver.  For example, RTP flows commonly
      employ a source independent receiver port.  State is created for
      each received flow.  Reception of a packet with a corrupted source
      address would result in accumulation of unnecessary state in the
      RTP state machine, including collision detection and response
      (since the same Synchronization source, SSRC, value will appear to
      arrive from multiple source IP addresses).

   In general, the effect of corrupting the source address will depend
   upon the protocol that processes the packet and its robustness to
   this error.  For the case where the packet is received by a tunnel
   endpoint, the application is expected to correctly handle a corrupted
   source address.

   This effect is more difficult to quantify when several fields have
   been modified in transit, and the receiving application is not that
   originally intended.

3.3.3.  Delivery to an unexpected port

   This section considers what happens if one or both of the UDP port
   values are corrupted in transit.  (This can also happen with IPv4 in
   the zero checksum case, but not when UDP checksums are enabled or
   with UDP-Lite).  If the ports were corrupted in transit, packets may
   be delivered to the wrong process (on the intended machine) and/or
   responses or errors sent to the wrong application process (on the
   intended machine).

   There are several possible outcomes for a packet that passes and does

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   not use the UDP checksum validation:

   o  Delivery to a port that is not in use.  This is discarded, but
      could generate an ICMPv6 message (e.g. port unreachable).

   o  It could be delivered to a different node that implements the same
      application, where the packet may be accepted, generating side-
      effects or accumulated state.

   o  It could be delivered to an application that does not implement
      the tunnel protocol, where the packet may be incorrectly parsed,
      and misinterpreted, generating side-effects or accumulated state.

   The probability of this happening depends on the statistical
   probability that the source address and the destination port of the
   datagram (the source port is not always used in UDP) match those of
   an existing connection.

   Unfortunately, this may be more likely for UDP than for connection-
   oriented transports: (a) There is no handshake prior to communication
   and no sequence numbers (as in TCP, DCCP, SCTP).  Together this makes
   it hard to verify that an application is given only the data
   associated with a session. (b) Applications writers often bind to
   wild-card values in endpoint identifiers and do not always validate
   correctness of datagrams they receive.  While we could revise these
   rules and declare naive applications as Historic, this is not
   realistic - the transport owes it to the stack to do its best to
   reject bogus datagrams.

   If checksum coverage is suppressed, the application needs to provide
   a method to detect and discard the unwanted data.  The encapsulated
   tunnel protocol would need to perform its own integrity checks on any
   control information and ensure an integrity check is applied to the
   tunneled packet.  It is not reasonable to assume that it is safe for
   one application to use a zero checksum value and that other
   applications will not.  It is important to consider the possibility
   that a packet will be received by a different node to that for which
   it was intended, or that it will arrive at the correct tunnel
   destination with the wrong source address in the external header.

3.3.4.  Validating the network path

   IP transports designed for use in the general Internet should not
   assume specific characteristics.  Network protocols may reroute
   packets and change the set of routers and middleboxes along a path.
   Therefore transports such as TCP, SCTP and DCCP are designed to
   negotiate protocol parameters, adapt to different characteristics,
   and receive feedback that the current path is suited to the intended

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   application.  Applications using UDP and UDP-Lite need to provide
   their own mechanisms to confirm the validity of the current network

   Any application/tunnel that seeks to make use of zero checksum must
   include functionality to both negotiate and verify that the zero
   checksum support is provided by the path and validate that this
   continues to work (e.g., in the case of re-routing events) between
   the intended parties.  This increases the complexity of using such a

3.4.  Comparision

   This section compares different methods.  This includes two proposals
   for updating the behaviour of UDP.  These are provided as examples,
   and do not seek to endorse any specific method or suggest that these
   proposals are ready to be standardised.

   Comparison of functions for selected methods
                               UDP UDPv4 UDPL IP   IP  UDPv6 UDPv6 UDPTT
                                    zero      in   in         zero
                                              IPv4 IPv6

   Incremental cksum update?    X    -     X  N/A   N/A  X     -    X
   Verification of IP length?   X    X     X  X     X    X     X    X
   Detect dest addr corruption? X    X     X  X     -    X     -    X
   Detect NH addr corruption?   -    -     -  X     -    -     -    -
   Flow demux fields present?   X    X     X  -     X    X     X    X
   Detect port corruption?      X    -     X  N/A   N/A  X     -    X
   Detect illegal pay length?   X    X     -  N/A   N/A  X     X    -
   Detect pay corruption?       X    -     ?  N/A   N/A  X     -    -
   Static cksum per flow?       -    X     -  N/A   N/A  -     X    X
   Partial/full midbox support? X    *     ?  ?     ?    X     ?    ?
   Restricted tunnel behaviour  X    *     X  X     ?    X     -    X

   X   = Provided/supported
   -   = Not provided/supported
   N/A = Not applicable
   ?   = Partial support
   *   = Supports a subset of functions (i.e. not all combinations)
   Table 1

4.  Requirements on the specification of transported protocols

   If the IETF were to revise the standard for UDP using IPv6 for
   specific use-cases there are a set of questions that would need to be

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   answered.  These include:

   Is there a reason why IP in IP is not a reasonable choice for

   o  Examples of arguments for requiring an encapsulation beyond
      IP-in-IP include the need for NAT traversal and/or firewall
      traversal.  However, the use of any non-standard transport
      protocol or variant would also require specific support in

   o  Another example is a need to perform port-demultiplexing (e.g. for
      load balancing).  This need could be met using UDP, UDP-Lite, or
      other transports, or by utilising the IPv6 flow label.

   Is there a reason why UDP-Lite is not a reasonable choice for

   o  One argument against using UDP-Lite is that this transport is that
      this transport is not implemented on all endpoints.  However,
      there is at least one open source implementation.

   o  Another argument against using UDP-Lite is that it uses a
      different IPv6 Next Header, which is currently not widely
      supported in middleboxes (see previous).

   o  It has also been argued that UDP-Lite requires a checksum
      computation.  The UDP-Lite checksum, for instance includes the
      length field, but need not include the IP payload, and therefore
      would not require access to the full datagram payload by the
      tunnel endpoints.

   If the IETF needs to revise the rationale for UDP checksums in RFC
   2460, should we remove the checksum or replace it with one closer to
   UDP-Lite (e.g.  UDPTT)?

   Topics to be considered in making this decision:

   o  The role of a router and host are not fixed, and a consistent
      method must be specified that can be used on all nodes.  It can
      not be assumed that a particular protocol (or transport mode) will
      only be used on a specific type of network node (e.g. permitting
      the UDP checksum to be disabled only on a router).  In IPv6, a
      node selects the role of a router or host on a per interface
      basis.  It is important to note that protocol changes intended for
      one specific use are often re-used for different applications.

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   o  Behaviour of NAT/Middleboxes needs to be updated for UDPTT and for
      UDP cksum==0.

   o  Load balancing may not be enabled for all transport protocols.

   o  Implications on host acting as routers and transport end points.

   o  Appropriate mechanisms to negotiate and validate the properties of
      the network path, including consideration of the impact of

   o  Whether this requires restrictions on recursive tunnels (e.g.
      Necessary when the endpoint is not verified).

   If a zero checksum approach were to be adopted by the IETF, the
   specification should consider adding the following constraints on

   1.  The method must be specified to verify the integrity of the inner
       (tunneled) packet.

   2.  The tunneling protocol must not allow fragmentation of the inner
       packets being carried.  We would suggest the following
       elaborations of the above restrictions, if a change in the IPv6
       specification moves forward: That is, an inner IPv4 packet with a
       UDP checksum equal to 0 must not be tunneled

   3.  If a method proposes selective ignoring of the checksum on
       reception, it needs to provide guidance that is appropriate for
       all use-cases, including defining how currently standardised
       nodes handle any new use.

   4.  Other tunneling protcocols that use the UDP checksum equal to 0
       must not be tunneled themselves, even if more deeply encapsulated
       packets have checksums or other integrity checking mechanisms.

   5.  Non-IP inner (tunneled) packets must have a CRC or other
       mechanism for checking packet integrity.

   6.  The specification needs to consider whether to prevent recursive
       tunnels (e.g. necessary when the endpoint is not verified).

   7.  It is recommended that general protocol stack implementations do
       not by default allow the new method.  The new method should
       remain restricted to devices serving as endpoints of the
       lightweight tunneling protocol adopting the change.

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5.  Summary

   This document examines the role of the transport checksum when used
   with IPv6, as defined in RFC2460.

   It presents a summary of the trade-offs for evaluating the safety of
   updating RFC 2460 to permit an IPv6 UDP endpoint to use a zero value
   in the checksum field to indicate that no checksum is present.  A
   decision not to include a UDP checksum in received IPv6 datagrams
   could impact a tunnel application that receives these packets.
   However, a well-designed tunnel application should include
   consistency checks to validate any header information encapsulated
   with a packet and ensure that a an integrity check is included for
   each tunneled packet.  When correctly implemented, such a tunnel
   endpoint will not be negatively impacted by omission of the
   transport-layer checksum.  However, other applications at the
   intended destination node or another IPv6 node can be impacted if
   they are allowed to receive datagrams without a transport-layer

   In particular, it is important that already deployed applications are
   not impacted by any change at the transport layer.  If these
   applications execute on nodes that implement RFC 2460, they will
   reject all datagrams without a UDP checksum.

   The implications on firewalls, NATs and other middleboxes need to be
   considered.  It should not be expected that NATs handle IPv6 UDP
   datagrams in the same way as they handle IPv4 UDP datagrams.
   Firewalls are intended to be configured, and therefore may need to be
   explicitly updated to allow new services or protocols.

   If the use of UDP transport without a checksum were to become
   prevalent for IPv6 (e.g. tunnel protocols using this are widely
   deployed), there would also be a significant danger of the Internet
   carrying an increased volume of packets without a transport checksum
   for other applications, potentially including applications that have
   traditionally used IPv4 UDP transport without a checksum.  This
   result is highly undesirable.  In general, UDP-based applications
   need to employ a mechanism that allows a large percentage of the
   corrupted packets to be removed before they reach an application,
   both to protect the applications data stream and the control plane of
   higher layer protocols.  These checks are currently performed by the
   UDP checksum for IPv6, or the reduced checksum for UDP-Lite when used
   with IPv6.

   Although the use of UDP over IPv6 with no checksum may have merits
   for use as a tunnel encapsulation and is widely used in IPv4, it is
   considered dangerous for all IPv6 nodes (hosts and routers).  Other

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   solutions need to be found.  This requires rthat the IPv4 and IPv6
   solutions to differ, since there are different deployed

6.  Acknowledgements

   Brian Haberman, Brian Carpenter, Magaret Wasserman, Lars Eggert,
   Magnus Westerlund, others in the TSV directorate.

   Thanks also to: Remi Denis-Courmont, Pekka Savola and many others who
   contributed comments and ideas via the 6man, behave, lisp and mboned

7.  IANA Considerations

   This document does not require IANA considerations.

8.  Security Considerations

   Transport checksums provide the first stage of protection for the
   stack, although they can not be considered authentication mechanisms.
   These checks are also desirable to ensure packet counters correctly
   log actual activity, and can be used to detect unusual behaviours.

9.  References

9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC1071]  Braden, R., Borman, D., Partridge, C., and W. Plummer,
              "Computing the Internet checksum", RFC 1071,
              September 1988.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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9.2.  Informative References

   [AMT]      Internet draft, draft-ietf-mboned-auto-multicast-09,
              "Automatic IP Multicast Without Explicit Tunnels (AMT)",
              June 2008.

   [ECMP]     "Using the IPv6 flow label for equal cost multipath
              routing in tunnels (draft-carpenter-flow-ecmp)".

   [LISP]     Internet draft, draft-farinacci-lisp-12.txt, "Locator/ID
              Separation Protocol (LISP)", March 2009.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC1141]  Mallory, T. and A. Kullberg, "Incremental updating of the
              Internet checksum", RFC 1141, January 1990.

   [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
              (SIIT)", RFC 2765, February 2000.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405,
              November 2008.

              9-1.htm, "When the CRC and TCP Checksum Disagree", 2000.

   [UDPTT]    "The UDP Tunnel Transport mode", Feb 2010.

   [UDPZ]     "UDP Checksums for Tunneled Packets", (Oct 2009.

Appendix A.  Document Change History

   {RFC EDITOR NOTE: This section must be deleted prior to publication}

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   Individual Draft 00   This is the first DRAFT of this document - It
      contains a compilation of various discussions and contributions
      from a variety of IETF WGs, including: mboned, tsv, 6man, lisp,
      and behave.  This includes contributions from Magnus with text on
      RTP, and various updates.

   Individual Draft 01

      *  This version corrects some typos and editorial NiTs and adds
         discussion of the need to negotiate and verify operation of a
         new mechanism (3.3.4).

   Individual Draft 02

      *  Version -02 corrects some typos and editorial NiTs.

      *  Added reference to ECMP for tunnels.

      *  Clarifies the recommendations at the end of the document.

Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Aberdeen, AB24 3UE,
   Scotland, UK


   Magnus Westerlund
   Ericsson Research
   Torshamgatan 23
   Stockholm,   SE-164 80


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