Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Informational                             M. Westerlund
Expires: April 27, 2012                                         Ericsson
                                                        October 25, 2011

                    IPv6 UDP Checksum Considerations


   This document examines the role of the UDP 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 as an
   indication that no checksum is present.  This method is compared with
   some other possibilities.  The document also describes the issues and
   design principles that need to be considered when UDP is used with
   IPv6 to support tunnel encapsulations.  It concludes that UDP with a
   zero checksum in IPv6 can safely be used for this purpose, provided
   that this usage is governed by a set of constraints.

Status of this Memo

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   This Internet-Draft will expire on April 27, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Document Structure . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Background . . . . . . . . . . . . . . . . . . . . . . . .  5
       1.2.1.  The Role of a Transport Endpoint . . . . . . . . . . .  5
       1.2.2.  The UDP Checksum . . . . . . . . . . . . . . . . . . .  5
       1.2.3.  Differences between IPv6 and IPv4  . . . . . . . . . .  7
     1.3.  Use of UDP Tunnels . . . . . . . . . . . . . . . . . . . .  7
       1.3.1.  Motivation for new approaches  . . . . . . . . . . . .  8
       1.3.2.  Reducing forwarding cost . . . . . . . . . . . . . . .  8
       1.3.3.  Need to inspect the entire packet  . . . . . . . . . .  9
       1.3.4.  Interactions with middleboxes  . . . . . . . . . . . .  9
       1.3.5.  Support for load balancing . . . . . . . . . . . . . . 10
   2.  Standards-Track Transports . . . . . . . . . . . . . . . . . . 10
     2.1.  UDP with Standard Checksum . . . . . . . . . . . . . . . . 10
     2.2.  UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . . 11
       2.2.1.  Using UDP-Lite as a Tunnel Encapsulation . . . . . . . 11
     2.3.  General Tunnel Encapsulations  . . . . . . . . . . . . . . 11
   3.  Issues Requiring Consideration . . . . . . . . . . . . . . . . 12
     3.1.  Effect of packet modification in the network . . . . . . . 13
       3.1.1.  Corruption of the destination IP address . . . . . . . 14
       3.1.2.  Corruption of the source IP address  . . . . . . . . . 14
       3.1.3.  Corruption of Port Information . . . . . . . . . . . . 15
       3.1.4.  Delivery to an unexpected port . . . . . . . . . . . . 15
       3.1.5.  Corruption of Fragmentation Information  . . . . . . . 16
     3.2.  Validating the network path  . . . . . . . . . . . . . . . 18
     3.3.  Applicability of method  . . . . . . . . . . . . . . . . . 19
     3.4.  Impact on non-supporting devices or applications . . . . . 20
   4.  Evaluation of proposal to update RFC 2460 to support zero
       checksum . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     4.1.  Alternatives to the Standard Checksum  . . . . . . . . . . 20
     4.2.  Comparison . . . . . . . . . . . . . . . . . . . . . . . . 22
       4.2.1.  Middlebox Traversal  . . . . . . . . . . . . . . . . . 22
       4.2.2.  Load Balancing . . . . . . . . . . . . . . . . . . . . 23
       4.2.3.  Ingress and Egress Performance Implications  . . . . . 23
       4.2.4.  Deployability  . . . . . . . . . . . . . . . . . . . . 23
       4.2.5.  Corruption Detection Strength  . . . . . . . . . . . . 24
       4.2.6.  Comparison Summary . . . . . . . . . . . . . . . . . . 24
   5.  Requirements on the specification of transported protocols . . 26

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     5.1.  Constraints required on usage of a zero checksum . . . . . 26
   6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     10.2. Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  Document Change History . . . . . . . . . . . . . . . 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33

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1.  Introduction

   The User Datagram Protocol (UDP) [RFC0768] transport is defined for
   the Internet Protocol (IPv4) [RFC0791] and is defined in Internet
   Protocol, Version 6 (IPv6) [RFC2460] for IPv6 hosts and routers.  The
   UDP transport protocol has a minimal set of features.  This limited
   set has enabled a wide range of applications to use UDP, but these
   application do need to provide many important transport functions on
   top of UDP.  The UDP Usage Guidelines [RFC5405] provides overall
   guidance for application designers, including the use of UDP to
   support tunneling.  The key difference between UDP usage with IPv4
   and IPv6 is that IPv6 mandates use of the UDP checksum, i.e. a non-
   zero value, due to the lack of an IPv6 header checksum.

   The lack of a possibility to use UDP with a zero-checksum in IPv6 has
   been observed as a real problem for certain classes of application,
   primarily tunnel applications.  This class of application has been
   deployed with a zero checksum using IPv4.  The design of IPv6 raises
   different issues when considering the safety of using a zero checksum
   for UDP with IPv6.  These issues can significantly affect
   applications, both when an endpoint is the intended user and when an
   innocent bystander (received by a different endpoint to that
   intended).  The document examines these issues and compares the
   strengths and weaknesses of a number of proposed solutions.  This
   analysis presents a set of issues that must be considered and
   mitigated to be able to safely deploy UDP with a zero checksum over
   IPv6.  The provided comparison of methods is expected to also be
   useful when considering applications that have different goals from
   the ones that initiated the writing of this document, especially the
   use of already standardized methods.

   The analysis concludes that using UDP with a zero checksum is the
   best method of the proposed alternatives to meet the goals for
   certain tunnel applications.  Unfortunately, this usage is expected
   to have some deployment issues related to middleboxes, limiting the
   usability more than desired in the currently deployed internet.
   However, this limitation will be largest initially and will reduce as
   updates for support of UDP zero checksum for IPv6 are provided to
   middleboxes.  The document therefore derives a set of constraints
   required to ensure safe deployment of zero checksum in UDP.  It also
   identifies some issues that require future consideration and possibly
   additional research.

1.1.  Document Structure

   Section 1 provides a background to key issues, and introduces the use
   of UDP as a tunnel transport protocol.

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

   Section 3 discusses issues with a zero checksum in UDP for IPv6.  It
   considers the impact of corruption, the need for validation of the
   path and when it is suitable to use a zero checksum.

   Section 4 evaluates a set of proposals to update the UDP transport
   behaviour and other alternatives intended to improve support for
   tunnel protocols.  It focuses on a proposal to allow a zero checksum
   for this use-case with IPv6 and assess the trade-offs that would

   Section 5.1 lists the constraints perceived for safe deployment of

   Section 6 provides the recommendations for standardization of zero-
   checksum with a summary of the findings and notes remaining issues
   needing future work.

1.2.  Background

   This section provides a background on topics relevant to the
   following discussion.

1.2.1.  The Role of a Transport Endpoint

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

   o  Protection of the endpoint transport state from unnecessary extra
      state (e.g.  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-filtering of incorrectly addressed destination packets, before
      responding to a source address.

1.2.2.  The UDP Checksum

   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

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   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
   datagram, because the transport pseudo header covers the IP
   addresses, port numbers, transport payload length, and Next Header/
   Protocol value corresponding to the UDP transport protocol [RFC1071].
   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

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

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

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

   o  Upper layer payload type (always included in the pseudo header of
      the checksum)

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

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

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

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

   o  Transport payload size (always included in the checksum)

   Transport endpoints also need to verify the correctness of reassembly
   of any fragmented datagram.  For UDP, this is normally provided as a
   part of the integrity check.  Disabling the IPv4 checksum prevents
   this check.  A lack of the UDP header and checksum in fragments can
   lead to issues in a translator or middlebox.  For example, many IPv4
   Network Address Translators, NATs, rely on port numbers to find the
   mappings, packet fragments do not carry port numbers, so fragments
   get dropped.  IP/ICMP Translation Algorithm [RFC6145] provides some

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   guidance on the processing of fragmented IPv4 UDP datagrams that do
   not carry a UDP checksum.

   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 also Networking Interface Cards (NICs) that automatically
   calculate TCP [RFC0793] and UDP checksums on transmission when a
   checksum of zero is sent to the NIC, using a method known as checksum

1.2.3.  Differences between IPv6 and IPv4

   IPv6 does not provide a network-layer integrity check.  The removal
   of the header checksum from the IPv6 specification released routers
   from a need to update a network-layer checksum for each router hop as
   the IPv6 Hop Count is changed (in contrast to the checksum update
   needed when an IPv4 router modifies the Time-To-Live (TTL)).

   The IP header checksum calculation was seen as redundant for most
   traffic (with UDP or TCP checksums enabled), and people wanted to
   avoid this extra processing.  However, there was concern that the
   removal of the IP header checksum in IPv6 combined with a UDP
   checksum set to zero 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.  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 therefore required
   [RFC2460] when endpoint applications transmit UDP datagrams over

1.3.  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 the 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.

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1.3.1.  Motivation for new approaches

   A number of tunnel encapsulations deployed over IPv4 have used the
   UDP transport with a zero checksum.  Users of these protocols expect
   a similar solution for IPv6.

   A number of tunnel protocols are also currently being defined (e.g.
   Automated Multicast Tunnels, AMT [I-D.ietf-mboned-auto-multicast],
   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 encapsulations (often using the IPv4, UDP, UDP-Lite,
      or TCP checksums).

   o  Eliminating a need to access the entire packet when forwarding the
      packet by a tunnel endpoint.

   o  Enhancing ability to traverse middleboxes, especially Network
      Address Translators, NATs.

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

1.3.2.  Reducing forwarding cost

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

   Automatic IP Multicast Without Explicit Tunnels, known as AMT
   [I-D.ietf-mboned-auto-multicast] currently specifies UDP as the
   transport protocol for 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 of
   [I-D.ietf-mboned-auto-multicast]).  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 encapsulation.

   The argument concerning redundant processing costs is valid regarding
   the integrity of a tunneled packet.  In some architectures (e.g.  PC-

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   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 receivers to reject corrupted packets
   without further processing.  However, there are certain classes of
   tunnel end-points where this off-loading is not available and
   unlikely to become available in the near future.

1.3.3.  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 a router that is unable to access the entire packet and does
   not provide checksum-offloading.  Thus enabling checksum calculation
   over the complete packet can impact router design, performance
   improvement, energy consumption and/or cost.

1.3.4.  Interactions with middleboxes

   In IPv4, UDP-encapsulation may be desirable for NAT traversal, since
   UDP support is commonly provided.  It is also necessary due to the
   almost ubiquitous deployment of IPv4 NATs.  There has also been
   discussion of NAT for IPv6, although not for the same reason as in
   IPv4.  If IPv6 NAT becomes a reality they hopefully do not present
   the same protocol issues as for IPv4.  If NAT is defined for IPv6, it
   should take UDP zero checksum into consideration.

   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.  If an zero-checksum
   for UDP were to be allowed for IPv6, this would need firewalls to be
   updated before full utility of the change is available.

   It can be expected that UDP with zero-checksum will initially not
   have the same middlebox traversal characteristics as regular UDP.
   However, if standardized we can expect an improvement over time of
   the traversal capabilities.  We also note that deployment of IPv6-
   capable middleboxes is still in its initial phases.  Thus, it might
   be that the number of non-updated boxes quickly become a very small
   percentage of the deployed middleboxes.

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1.3.5.  Support for load balancing

   The UDP port number fields have been used as a basis to design load-
   balancing solutions for IPv4.  This approach has also been leveraged
   for IPv6.  An alternate method would be to utilise the IPv6 Flow
   Label as basis for entropy for the load balancing.  This would have
   the desirable effect of releasing IPv6 load-balancing devices from
   the need to assume semantics for the use of the transport port field
   and also works for all type of transport protocols.  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.  Equal-Cost Multi-Path routing, ECMP [ECMP]).

   Router vendors could be encouraged to start using the IPv6 Flow Label
   as a part of the flow hash, providing support for ECMP without
   requiring use of UDP.  However, the method for populating the outer
   IPv6 header with a value for the flow label is not trivial: If the
   inner packet uses IPv6, then the flow label value could be copied to
   the outer packet header.  However, many current end-points set the
   flow label to a zero value (thus no entropy).  The ingress of a
   tunnel seeking to provide good entropy in the flow label field would
   therefore need to create a random flow label value and keep
   corresponding state, so that all packets that were associated with a
   flow would be consistently given the same flow label.  Although
   possible, this complexity may not be desirable in a tunnel ingress.

   The end-to-end use of flow labels for load balancing is a long-term
   solution.  Even if the usage of the flow label is clarified, there
   would be a transition time before a significant proportion of end-
   points start to assign a good quality flow label to the flows that
   they originate, with continued use of load balancing using the
   transport header fields until any widespread deployment is finally

2.  Standards-Track Transports

   The IETF has defined a set of transport protocols that may be
   applicable for tunnels with IPv6.  There are also a set of network
   layer encapsulation tunnels such as IP-in-IP and GRE.  These already
   standardized solutions are discussed here prior to the issues, as
   background for the issue description and some comparison of where the
   issue may already occur.

2.1.  UDP with Standard Checksum

   UDP [RFC0768] with standard checksum behaviour is defined in RFC 2460
   has already been discussed.  UDP usage guidelines are provided in

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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].  There is at least one open
   source implementation as a part of the Linux kernel since version

   UDP-Lite provides a checksum with 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).  When the checksum covers the entire packet, UDP-Lite is
   fully equivalent with UDP.  Errors/corruption in the insensitive part
   will not cause the datagram to be discarded by the transport layer at
   the receiving endpoint.  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. radio access bearers).  Most link-layers will
   cover the insensitive part with the same strong layer 2 frame CRC
   that covers the sensitive part.

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 [RFC5415]), since UDP-
   Lite provides a transport-layer checksum, including an IP pseudo
   header checksum, in IPv6, without the need for a router/middelbox to
   traverse the entire packet payload.  This provides most of the
   delivery verifications and still keep the complexity of the
   checksumming operation low.  UDP-Lite may set the length of checksum
   coverage on a per packet basis.  This feature could be used if a
   tunnel protocol is designed to only verify delivery of the tunneled
   payload and uses full checksumming for control information.

   There is currently poor support for middlebox traversal using UDP-
   Lite, because UDP-Lite uses a different IPv6 network-layer Next
   Header value to that of UDP, and few middleboxes are able to
   interpret UDP-Lite and take appropriate actions when forwarding the
   packet.  This makes UDP-Lite less suited to protocols needing general
   Internet support, until such time that UDP-Lite has achieved better
   support in middleboxes and end-points.

2.3.  General Tunnel Encapsulations

   The IETF has defined a set of tunneling protocols or network layer
   encapsulations, like IP-in-IP and GRE.  These either do not include a

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   checksum or use a checksum that is optional, since tunnel
   encapsulations are typically layered directly over the Internet layer
   (identified by the upper layer type in the IPv6 Next Header field)
   and are also not used as endpoint transport protocols.  There is
   little chance of confusing a tunnel-encapsulated packet with other
   application data that could result in corruption of application state
   or data.

   From the end-to-end perspective, the principal difference is that the
   network-layer 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 applications
   and therefore UDP demultiplexing relies solely on the port numbers.

3.  Issues Requiring Consideration

   This section evaluates issues around the proposal to update IPv6
   [RFC2460], to provide the option of using a UDP transport checksum
   set to zero.  Some of the identified issues are shared with other
   protocols already in use.

   The decision by IPv6 to omit an integrity check at the network level
   has meant that the transport check was overloaded with many
   functions, including validating:

   o  the endpoint address was not corrupted within a router - i.e.  A
      packet was intended to be received by this destination and a wrong
      header has not been spliced to a different payload;

   o  that extension header processing is correctly delimited - i.e.
      The start of data has not been corrupted.  In this case, reception
      of a valid next header value provides some protection;

   o  reassembly processing, when used;

   o  the length of the payload;

   o  the port values - i.e.  The correct application receives the
      payload (applications should also check the expected use of source

   o  the payload integrity.

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

   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
   [RFC4443] 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.  Effect of packet modification in the network

   IP packets may be corrupted as they traverse 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 [RFC3819].  There is no current evidence that such cases
   are rare in the modern Internet, nor that they may not be applicable
   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
      wrong transport entity within an endpoint.  Such a datagram needs
      to be discarded;

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

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

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

   The following sections examine the impact of modifying each of these
   header fields.

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3.1.1.  Corruption of the destination IP address

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

   There are two possible outcomes:

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

   o  Delivery to a different destination 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 endpoint 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

3.1.2.  Corruption of the source IP address

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

   Corruption of an IPv6 source address does not result in the IP packet
   being delivered to a different endpoint protocol or destination
   address.  If only the source address is corrupted, the datagram 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 are:

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

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

   o  Some datagram applications build state using the information from
      packet headers.  A previously unused source address would result

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      in receiver processing and the creation of unnecessary transport-
      layer state at the receiver.  For example, Real Time Protocol
      (RTP) [RFC3550] sessions commonly employ a source independent
      receiver port.  State is created for each received flow.
      Reception of a datagram with a corrupted source address will
      therefore 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 tunnel application is expected to correctly handle a
   corrupted source address.

   The impact of source address modification is more difficult to
   quantify when the receiving application is not that originally
   intended and several fields have been modified in transit.

3.1.3.  Corruption of Port Information

   This section describes 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 carried in the transport header of an
   IPv6 packet 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

3.1.4.  Delivery to an unexpected port

   If one combines the corruption effects, such as destination address
   and ports, there is a number of potential outcomes when traffic
   arrives at an unexpected port.  This section discusses these
   possibilities and their outcomes for a packet that does not use the
   UDP checksum validation:

   o  Delivery to a port that is not in use.  The packet 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,

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      and may be misinterpreted, generating side-effects or accumulated

   The probability of each outcome depends on the statistical
   probability that the address or the port information for the source
   or destination becomes corrupt in the datagram such that they match
   those of an existing flow or server port.  Unfortunately, such a
   match may be more likely for UDP than for connection-oriented
   transports, because:

   1.  There is no handshake prior to communication and no sequence
       numbers (as in TCP, DCCP, or SCTP).  Together, this makes it hard
       to verify that an application is given only the data associated
       with a transport session.

   2.  Applications writers often bind to wild-card values in endpoint
       identifiers and do not always validate correctness of datagrams
       they receive (guidance on this topic is provided in [RFC5405]).

   While these rules could, in principle, be revised to declare naive
   applications as "Historic".  This remedy is not realistic: the
   transport owes it to the stack to do its best to reject bogus

   If checksum coverage is suppressed, the application therefore needs
   to provide a method to detect and discard the unwanted data.  A
   tunnel protocol would need to perform its own integrity checks on any
   control information if transported in UDP with zero-checksum.  If the
   tunnel payload is another IP packet, the packets requiring checksums
   can be assumed to have their own checksums provided that the rate of
   corrupted packets is not significantly larger due to the tunnel
   encapsulation.  If a tunnel transports other inner payloads that do
   not use IP, the assumptions of corruption detection for that
   particular protocol must be fulfilled, this may require an additional
   checksum/CRC and/or integrity protection of the payload and tunnel

   A protocol using UDP zero-checksum can never assume that it is the
   only protocol using a zero checksum.  Therefore, it needs to
   gracefully handle misdelivery.  It must be robust to reception of
   malformed packets received on a listening port and expect that these
   packets may contain corrupted data or data associated with a
   completely different protocol.

3.1.5.  Corruption of Fragmentation Information

   The fragmentation information in IPv6 employs a 32-bit identity
   field, compared to only a 16-bit filed in IPv4, a 13-bit fragment

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   offset and a 1-bit flag, indicating if there are more fragments.
   Corruption of any of these field may result in one of two outcomes:

   Reassembly failure:   An error in the "More Fragments" field for the
      last fragment will for example result in the packet never being
      considered complete and will eventually be timed out and
      discarded.  A corruption in the ID field will result in the
      fragment not being delivered to the intended context thus leaving
      the rest incomplete, unless that packet has been duplicated prior
      to corruption.  The incomplete packet will eventually be timed out
      and discarded.

   Erroneous reassembly:  The re-assemblied packet did not match the
      original packet.  This can occur when the ID field of a fragment
      is corrupted, resulting in a fragment becoming associated with
      another packet and taking the place of another fragment.
      Corruption in the offset information can cause the fragment to be
      misaligned in the reassembly buffer, resulting in incorrect
      reassembly.  Corruption can cause the packet to become shorter or
      longer, however completion of reassembly is much less probable,
      since this would requires consistent corruption of the IPv6
      headers payload length field and the offset field.  The
      possibility of mis-assembly requires the reassembling stack to
      provide strong checks that detect overlap or missing data, note
      however that this is not guaranteed and has recently been
      clarified in "Handling of Overlapping IPv6 Fragments" [RFC5722].

   The erroneous reassembly of packets is a general concern and such
   packets should be discarded instead of being passed to higher layer
   processes.  The primary detector of packet length changes is the IP
   payload length field, with a secondary check by the transport
   checksum.  The Upper-Layer Packet length field included in the pseudo
   header assists in verifying correct reassembly, since the Internet
   checksum has a low probability of detecting insertion of data or
   overlap errors (due to misplacement of data).  The checksum is also
   incapable of detecting insertion or removal of all zero-data that
   occurs in a multiple of a 16-bit chunk.

   The most significant risk of corruption results following mis-
   association of a fragment with a different packet.  This risk can be
   significant, since the size of fragments is often the same (e.g.
   fragments resulting when the path MTU results in fragmentation of a
   larger packet, common when addition of a tunnel encapsulation header
   expands the size of a packet).  Detection of this type of error
   requires a checksum or other integrity check of the headers and the
   payload.  Such protection is anyway desirable for tunnel
   encapsulations using IPv4, since the small fragmentation ID can
   easily result in wrap-around [RFC4963], this is especially the case

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   for tunnels that perform flow aggregation [I-D.ietf-intarea-tunnels].

   Tunnel fragmentation behavior matters.  There can be outer or inner
   fragmentation "Tunnels in the Internet Architecture"
   [I-D.ietf-intarea-tunnels].  If there is inner fragmentation by the
   tunnel, the outer headers will never be fragmented and thus a zero-
   checksum in the outer header will not affect the reassembly process.
   When a tunnel performs outer header fragmentation, the tunnel egress
   needs to perform reassembly of the outer fragments into an inner
   packet.  The inner packet is either a complete packet or a fragment.
   If it is a fragment, the destination endpoint of the fragment will
   perform reassembly of the received fragments.  The complete packet or
   the reassembled fragments will then be processed according to the
   packet next header field.  The receiver may only detect reassembly
   anomalies when it uses a protocol with a checksum.  The larger the
   number of reassembly processes to which a packet has been subjected,
   the greater the probability of an error.

   o  An IP-in-IP tunnel that performs inner fragmentation has similar
      properties to a UDP tunnel with a zero-checksum that also performs
      inner fragmentation.

   o  An IP-in-IP tunnel that performs outer fragmentation has similar
      properties to a UDP tunnel with a zero checksum that performs
      outer fragmentation.

   o  A tunnel that performs outer fragmentation can result in a higher
      level of corruption due to both inner and outer fragmentation,
      enabling more chances for reassembly errors to occur.

   o  Recursive tunneling can result in fragmentation at more than one
      header level, even for inner fragmentation unless it goes to the
      inner most IP header.

   o  Unless there is verification at each reassembly the probability
      for undetected error will increase with the number of times
      fragmentation is recursively applied.  Making IP-in-IP and UDP
      with zero checksum equal subject to this effect.

   In conclusion fragmentation of packets with a zero-checksum does not
   worsen the situation compared to some other commonly used tunnel
   encapsulations.  However, caution is needed for recursive tunneling
   without any additional verification at the different tunnel layers.

3.2.  Validating the network path

   IP transports designed for use in the general Internet should not
   assume specific path characteristics.  Network protocols may reroute

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   packets that change the set of routers and middleboxes along a path.
   Therefore transports such as TCP, SCTP and DCCP have been designed to
   negotiate protocol parameters, adapt to different network path
   characteristics, and receive feedback to verify that the current path
   is suited to the intended application.  Applications using UDP and
   UDP-Lite need to provide their own mechanisms to confirm the validity
   of the current network path.

   The zero-checksum in UDP is explicitly disallowed in RFC2460.  Thus
   it may be expected that any device on the path that has a reason to
   look beyond the IP header will consider such a packet as erroneous or
   illegal and may likely discard it, unless the device is updated to
   support a new behavior.  A pair of end-points intending to use a new
   behavior will therefore not only need to ensure support at each end-
   point, but also that the path between them will deliver packets with
   the new behavior.  This may require negotiation or an explicit
   mandate to use the new behavior by all nodes intended to use a new

   Support along the path between end points may be guaranteed in
   limited deployments by appropriate configuration.  In general, it can
   be expected to take time for deployment of any updated behaviour to
   become ubiquitous.  A sender will need to probe the path to verify
   the expected behavior.  Path characteristics may change, and usage
   therefore should be robust and able to detect a failure of the path
   under normal usage and re-negotiate.  This will require periodic
   validation of the path, adding complexity to any solution using the
   new behavior.

3.3.  Applicability of method

   The expectation of the present proposal defined in
   [I-D.ietf-6man-udpchecksums] is that this change would only apply to
   IPv6 router nodes that implement specific protocols that permit
   omission of UDP checksums.  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 also 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 the mechanism can (and can not) be used.
   Enabling this, and ensuring correct interactions with the stack,
   implies much more than simply disabling the checksum algorithm for
   specific packets at the transport interface.

   The IETF should carefully consider constraints on sanctioning the use
   of any new transport mode.  If this is specified and widely

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   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, 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 unless they accept that

3.4.  Impact on non-supporting devices or applications

   It is important to consider what potential impact the zero-checksum
   behavior may have on end-points, devices or applications that are not
   modified to support the new behavior or by default or preference, use
   the regular behavior.  These applications must not be significantly
   impacted by the changes.

   To illustrate a potential issue, consider the implications of a node
   that were to enable use of a zero-checksum at the interface level:
   This would result in all applications that listen to a UDP socket
   receiving datagram where the checksum was not verified.  This could
   have a significant impact on an application that was not designed
   with the additional robustness needed to handle received packets with
   corruption, creating state or destroying existing state in the

   In contrast, the use of a zero-checksum could be enabled only for
   individual ports using an explicit request by the application.  In
   this case, applications using other ports would maintain the current
   IPv6 behavior, discarding incoming UDP datagrams with a zero-
   checksum.  These other applications would not be effected by this
   changed behavior.  An application that allows the changed behavior
   should be aware of the risk for corruption and the increased level of
   misdirected traffic, and can be designed robustly to handle this

4.  Evaluation of proposal to update RFC 2460 to support zero checksum

   This section evaluates the proposal to update IPv6 [RFC2460], to
   provide the option that some nodes may suppress generation and
   checking of the UDP transport checksum.  It also compares the
   proposal with other alternatives.

4.1.  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

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   decreasing order of complexity):

   o  Delta computation of the checksum from an encapsulated checksum
      field.  Since the checksum is a cumulative sum [RFC1624], 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, in a manner resembling incremental checksum update
      [RFC1141].  This would not require access to the whole packet, but
      does require 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

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

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

   o  There has been a proposal to simply ignore the UDP checksum value
      on reception at the tunnel egress, allowing a tunnel ingress to
      insert any value correct or false.  For tunnel usage, a non
      standard checksum value may be used, forcing an RFC 2460 receiver
      to drop the packet.  The main downside is that it would be
      impossible to identify a UDP packet (in the network or an
      endpoint) that is treated in this way compared to a packet that
      has actually been corrupted.

   o  A method has been proposed that uses a new (to be defined) IPv6
      Destination Options Header to provide an end-to-end validation
      check at the network layer.  This would allow an endpoint to
      verify delivery to an appropriate end point, but would also
      require IPv6 nodes to correctly handle the additional header, and
      would require changes to middlebox behavior (e.g. when used with a
      NAT that always adjusts the checksum value).

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   o  UDP modified to disable checksum processing
      [I-D.ietf-6man-udpchecksums].  This requires no checksum
      calculation, but would require constraints on appropriate usage
      and updates to end-points and middleboxes.

   o  IP-in-IP tunneling.  As this method completely dispenses with a
      transport protocol in the outer-layer it has reduced overhead and
      complexity, but also reduced functionality.  There is no outer
      checksum over the packet and also no ports to perform
      demultiplexing between different tunnel types.  This reduces the
      information available upon which a load balancer may act.

   These options are compared and discussed further in the following

4.2.  Comparison

   This section compares the above listed methods to support datagram
   tunneling.  It includes proposals for updating the behaviour of UDP.

4.2.1.  Middlebox Traversal

   Regular UDP with a standard checksum or the delta encoded
   optimization for creating correct checksums have the best
   possibilities for successful traversal of a middlebox.  No new
   support is required.

   A method that ignores the UDP checksum on reception is expected to
   have a good probability of traversal, because most middleboxes
   perform an incremental checksum update.  UDPTT may also traverse a
   middlebox with this behaviour.  However, a middlebox on the path that
   attempts to verify a standard checksum will not forward packets using
   either of these methods, preventing traversal.  The methods that
   ignores the checksum has an additional downside in that middlebox
   traversal can not be improved, because there is no way to identify
   which packets use the modified checksum behaviour.

   IP-in-IP or GRE tunnels offer good traversal of middleboxes that have
   not been designed for security, e.g. firewalls.  However, firewalls
   may be expected to be configured to block general tunnels as they
   present a large attack surface.

   A new IPv6 Destination Options header will suffer traversal issues
   with middleboxes, especially Firewalls and NATs, and will likely
   require them to be updated before the extension header is passed.

   Packets using UDP with a zero checksum will not be passed by any
   middlebox that validates the checksum using RFC 2460 or updates the

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   checksum field, such as NAT or firewalls.  This would require an
   update to correctly handle the zero checksum packets.

   UDP-Lite will require an update of almost all type of middleboxes,
   because it requires support for a separate network-layer protocol
   number.  Once enabled, the method to support incremental checksum
   update would be identical to that for UDP, but different for checksum

4.2.2.  Load Balancing

   The usefulness of solutions for load balancers depends on the
   difference in entropy in the headers for different flows that can be
   included in a hash function.  All the proposals that use the UDP
   protocol number have equal behavior.  UDP-Lite has the potential for
   equally good behavior as for UDP.  However, UDP-Lite is currently
   likely to not be supported by deployed hashing mechanisms, which may
   cause a load balancer to not use the transport header in the computed
   hash.  A load balancer that only uses the IP header will have low
   entropy, but could be improved by including the IPv6 the flow label,
   providing that the tunnel ingress ensures that different flow labels
   are assigned to different flows.  However, a transition to the common
   use of good quality flow labels is likely to take time to deploy.

4.2.3.  Ingress and Egress Performance Implications

   IP-in-IP tunnels are often considered efficient, because they
   introduce very little processing and low data overhead.  The other
   proposals introduce a UDP-like header incurring associated data
   overhead.  Processing is minimised for the zero-checksum method,
   ignoring the checksum on reception, and only slightly higher for
   UDPTT, the extension header and UDP-Lite.  The delta-calculation
   scheme operates on a few more fields, but also introduces serious
   failure modes that can result in a need to calculate a checksum over
   the complete packet.  Regular UDP is clearly the most costly to
   process, always requiring checksum calculation over the entire

   It is important to note that the zero-checksum method, ignoring
   checksum on reception, the Option Header, UDPTT and UDP-Lite will
   likely incur additional complexities in the application to
   incorporate a negotiation and validation mechanism.

4.2.4.  Deployability

   The major factors influencing deployability of these solutions are a
   need to update both end-points, a need for negotiation and the need
   to update middleboxes.  These are summarised below:

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   o  The solution with the best deployability is regular UDP.  This
      requires no changes and has good middlebox traversal

   o  The next easiest to deploy is the delta checksum solution.  This
      does not modify the protocol on the wire and only needs changes in
      tunnel ingress.

   o  IP-in-IP tunnels should not require changes to the end-points, but
      raise issues when traversing firewalls and other security-type
      devices, which are expected to require updates.

   o  Ignoring the checksum on reception will require changes at both
      end-points.  The never ceasing risk of path failure requires
      additional checks to ensure this solution is robust and will
      require changes or additions to the tunneling control protocol to
      negotiate support and validate the path.

   o  The remaining solutions offer similar deployability.  UDP-Lite
      requires support at both end-points and in middleboxes.  UDPTT and
      Zero-checksum with or without an Extension header require support
      at both end-points and in middleboxes.  UDP-Lite, UDPTT, and Zero-
      checksum and Extension header may additionally require changes or
      additions to the tunneling control protocol to negotiate support
      and path validation.

4.2.5.  Corruption Detection Strength

   The standard UDP checksum and the delta checksum can both provide
   some verification at the tunnel egress.  This can significantly
   reduce the probability that a corrupted inner packet is forwarded.
   UDP-Lite, UDPTT and the extension header all provide some
   verification against corruption, but do not verify the inner packet.
   They only provide a strong indication that the delivered packet was
   intended for the tunnel egress and was correctly delimited.  The
   Zero-checksum, ignoring the checksum on reception and IP-and-IP
   encapsulation provide no verification that a received packet was
   intended to be processed by a specific tunnel egress or that the
   inner packet was correct.

4.2.6.  Comparison Summary

   The comparisons above may be summarised as "there is no silver bullet
   that will slay all the issues".  One has to select which down side(s)
   can best be lived with.  Focusing on the existing solutions, this can
   be summarized as:

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   Regular UDP:  Good middlebox traversal and load balancing and
      multiplexing, requiring a checksum in the outer headers covering
      the whole packet.

   IP in IP:  A low complexity encapsulation, with limited middlebox
      traversal, no multiplexing support, and currently poor load
      balancing support that could improve over time.

   UDP-Lite:  A medium complexity encapsulation, with good multiplexing
      support, limited middlebox traversal, but possible to improve over
      time, currently poor load balancing support that could improve
      over time, in most cases requiring application level negotiation
      and validation.

   The delta-checksum is an optimization in the processing of UDP, as
   such it exhibits some of the drawbacks of using regular UDP.

   The remaining proposals may be described in similar terms:

   Zero-Checksum:  A low complexity encapsulation, with good
      multiplexing support, limited middlebox traversal that could
      improve over time, good load balancing support, in most cases
      requiring application level negotiation and validation.

   UDPTT:  A medium complexity encapsulation, with good multiplexing
      support, limited middlebox traversal, but possible to improve over
      time, good load balancing support, in most cases requiring
      application level negotiation and validation.

   IPv6 Destination Option IP in IP tunneling:  A medium complexity,
      with no multiplexing support, limited middlebox traversal,
      currently poor load balancing support that could improve over
      time, in most cases requiring application level negotiation and

   IPv6 Destination Option combined with UDP Zero-checksuming:  A medium
      complexity encapsulation, with good multiplexing support, limited
      load balancing support that could improve over time, in most cases
      requiring application level negotiation and validation.

   Ignore the checksum on reception:  A low complexity encapsulation,
      with good multiplexing support, medium middlebox traversal that
      never can improve, good load balancing support, in most cases
      requiring application level negotiation and validation.

   There is no clear single optimum solution.  If the most important
   need is to traverse middleboxes, then the best choice is to stay with
   regular UDP and consider the optimizations that may be required to

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   perform the checksumming.  If one can live with limited middlebox
   traversal, low complexity is necessary and one does not require load
   balancing, then IP-in-IP tunneling is the simplest.  If one wants
   strengthened error detection, but with currently limited middlebox
   traversal and load-balancing.  UDP-Lite is appropriate.  UDP Zero-
   checksum addresses another set of constraints, low complexity and a
   need for load balancing from the current Internet, providing it can
   live with currently limited middlebox traversal.

   Techniques for load balancing and middlebox traversal do continue to
   evolve.  Over a long time, developments in load balancing have good
   potential to improve.  This time horizon is long since it requires
   both load balancer and end-point updates to get full benefit.  The
   challenges of middlebox traversal are also expected to change with
   time, as device capabilities evolve.  Middleboxes are very prolific
   with a larger proportion of end-user ownership, and therefore may be
   expected to take long time cycles to evolve.  One potential advantage
   is that the deployment of IPv6 capable middleboxes are still in its
   initial phase and the quicker zero-checksum becomes standardized the
   fewer boxes will be non-compliant.

   Thus, the question of whether to allow UDP with a zero-checksum for
   IPv6 under reasonable constraints, is therefore best viewed as a
   trade-off between a number of more subjective questions:

   o  Is there sufficient interest in zero-checksum with the given
      constraints (summarised below)?

   o  Are there other avenues of change that will resolve the issue in a
      better way and sufficiently quickly ?

   o  Do we accept the complexity cost of having one more solution in
      the future?

   The authors do think the answer to the above questions are such that
   zero-checksum should be standardized for use by tunnel

5.  Requirements on the specification of transported protocols

5.1.  Constraints required on usage of a zero checksum

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

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   1.  IPv6 protocol stack implementations should not by default allow
       the new method.  The default node receiver behaviour must discard
       all IPv6 packets carrying UDP packets with a zero checksum.

   2.  Implementations must provide a way to signal the set of ports
       that will be enabled to receive UDP datagrams with a zero
       checksum.  An IPv6 node that enables reception of UDP packets
       with a zero-checksum, must enable this only for a specific port
       or port-range.  This may be implemented via a socket API call, or
       similar mechanism.

   3.  RFC 2460 specifies that IPv6 nodes should log UDP datagrams with
       a zero-checksum.  This should remain the case for any datagram
       received on a port that does not explicitly enable zero-checksum
       processing.  A port for which zero-checksum has been enabled must
       not log the datagram.

   4.  A stack may separately identify UDP datagrams that are discarded
       with a zero checksum.  It should not add these to the standard
       log, since the endpoint has not been verified.

   5.  Tunnels that encapsulate IP may rely on the inner packet
       integrity checks provided that the tunnel will not significantly
       increase the rate of corruption of the inner IP packet.  If a
       significantly increased corruption rate can occur, then the
       tunnel must provide an additional integrity verification
       mechanism.  An integrity mechanisms is always recommended at the
       tunnel layer to ensure that corruption rates of the inner most
       packet are not increased.

   6.  Tunnels that encapsulate Non-IP packets must have a CRC or other
       mechanism for checking packet integrity, unless the Non-IP packet
       specifically is designed for transmission over lower layers that
       do not provide any packet integrity guarantee.  In particular,
       the application must be designed so that corruption of this
       information does not result in accumulated state or incorrect
       processing of a tunneled payload.

   7.  UDP applications that support use of a zero-checksum, should not
       rely upon correct reception of the IP and UDP protocol
       information (including the length of the packet) when decoding
       and processing the packet payload.  In particular, the
       application must be designed so that corruption of this
       information does not result in accumulated state or incorrect
       processing of a tunneled payload.

   8.  If a method proposes recursive tunnels, it needs to provide
       guidance that is appropriate for all use-cases.  Restrictions may

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       be needed to the use of a tunnel encapsulations and the use of
       recursive tunnels (e.g.  Necessary when the endpoint is not

   9.  IPv6 nodes that receive ICMPv6 messages that refer to packets
       with a zero UDP checksum must provide appropriate checks
       concerning the consistency of the reported packet to verify that
       the reported packet actually originated from the node, before
       acting upon the information (e.g. validating the address and port
       numbers in the ICMPv6 message body).

   Deployment of the new method needs to remain restricted to endpoints
   that explicitly enable this mode and adopt the above procedures.  Any
   middlebox that examines or interact with the UDP header over IPv6
   should support the new method.

6.  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.  In most cases tunnels encapsulating IP packets can
   rely on the inner packets own integrity protection.  When correctly
   implemented, such a tunnel endpoint will not be negatively impacted
   by omission of the transport-layer checksum.  Recursive tunneling and
   fragmentation is a potential issues that can raise corruption rates
   significantly, and requires careful consideration.

   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 checksum.  It is particularly 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 with a zero UDP
   checksum, thus this is not an issue.  For nodes that implement
   support for zero-checksum it is important to ensure that only UDP
   applications that desire zero-checksum can receive and originate
   zero-checksum packets.  Thus, the enabling of zero-checksum needs to
   be at a port level, not for the entire host or for all use of an

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   The implications on firewalls, NATs and other middleboxes need to be
   considered.  It is not expected that IPv6 NATs handle IPv6 UDP
   datagrams in the same way that they handle IPv4 UDP datagrams.  This
   possibly reduces the need to update the checksum.  Firewalls are
   intended to be configured, and therefore may need to be explicitly
   updated to allow new services or protocols.  IPv6 middlebox
   deployment is not yet as prolific as it is in IPv4.  Thus, relatively
   few current middleboxes may actually block IPv6 UDP with a zero

   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.

   The use of UDP with no checksum has merits for some applications,
   such as tunnel encapsulation, and is widely used in IPv4.  However,
   there are dangers for IPv6: There is a bigger risk of corruption and
   miss-delivery when using zero-checksum in IPv6 compared to IPv4 due
   to the removed IP header checksum.  Thus, applications needs to make
   a new evaluation of the risks of enabling a zero-checksum.  Some
   applications will need to re-consider their usage of zero-checksum,
   and possibly consider a solution that at least provides the same
   delivery protection as for IPv4, for example by utilizing UDP-Lite,
   or by enabling the UDP checksum.  Tunnel applications using UDP for
   encapsulation can in many case use zero-checksum without significant
   impact on the corruption rate.  In some cases, the use of checksum
   off-loading may help alleviate the checksum processing cost.

   Recursive tunneling and fragmentation is a difficult issue relating
   to tunnels in general.  There is an increased risk of an error in the
   inner-most packet when fragmentation when several layers of tunneling
   and several different reassembly processes are run without any
   verification of correctness.  This issue requires future thought and

   The conclusion is that UDP zero checksum in IPv6 should be
   standardized, as it satisfies usage requirements that are currently
   difficult to address.  We do note that a safe deployment of zero-
   checksum will need to follow a set of constraints listed in
   Section 5.1.

7.  Acknowledgements

   Brian Haberman, Brian Carpenter, Magaret Wasserman, Lars Eggert,

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   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

8.  IANA Considerations

   This document does not require any actions by IANA.

9.  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.

10.  References

10.1.  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.

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

10.2.  Informative References

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

              Eubanks, M., "UDP Checksums for Tunneled Packets",
              draft-ietf-6man-udpchecksums-00 (work in progress),
              March 2011.


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              Touch, J. and M. Townsley, "Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-00 (work in
              progress), March 2010.

              Thaler, D., Talwar, M., Aggarwal, A., Vicisano, L.,
              Pusateri, T., and T. Morin, "Automatic IP Multicast
              Tunneling", draft-ietf-mboned-auto-multicast-11 (work in
              progress), July 2011.

   [LISP]     D. Farinacci et al, "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.

   [RFC1624]  Rijsinghani, A., "Computation of the Internet Checksum via
              Incremental Update", RFC 1624, May 1994.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [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.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.

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

   [RFC5415]  Calhoun, P., Montemurro, M., and D. Stanley, "Control And
              Provisioning of Wireless Access Points (CAPWAP) Protocol

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              Specification", RFC 5415, March 2009.

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, December 2009.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

              Jonathan Stone and Craig Partridge , "When the CRC and TCP
              Checksum Disagree", 2000.

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

Appendix A.  Document Change History

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

   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.

   Working Group Draft 00

      *  Working Group Version -00 corrects some typos and removes much
         of rationale for UDPTT.  It also adds some discussion of IPv6
         extension header.

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   Working Group Draft 01

      *  Working Group Version -01 updates the rules and incorporates
         off-list feedback.  This version is intended for wider review
         within the 6man working group.

   Working Group Draft 02

      *  This version is the result of a major rewrite and re-ordering
         of the document.

      *  A new section comparing the results have been added.

      *  The constraints list has been significantly altered by removing
         some and rewording other constraints.

      *  This contains other significant language updates to clarify the
         intent of this draft.

   Working Group Draft 03

      *  Editorial updates

   Working Group Draft 04

      *  Resubmission only updating the AMT and RFC2765 references.

Authors' Addresses

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


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   Magnus Westerlund
   Farogatan 6
   Stockholm,   SE-164 80

   Phone: +46 8 719 0000

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