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Versions: (draft-fairhurst-tsvwg-6man-udpzero) 00 01 02 03 04 05 06 07 08 09 10 12 RFC 6936

Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Standards Track                           M. Westerlund
Expires: August 29, 2013                                        Ericsson
                                                       February 25, 2013


  Applicability Statement for the use of IPv6 UDP Datagrams with Zero
                               Checksums
                       draft-ietf-6man-udpzero-12

Abstract

   This document provides an applicability statement for the use of UDP
   transport checksums with IPv6.  It defines recommendations and
   requirements for the use of IPv6 UDP datagrams with a zero UDP
   checksum.  It describes the issues and design principles that need to
   be considered when UDP is used with IPv6 to support tunnel
   encapsulations and examines the role of the IPv6 UDP transport
   checksum.  The document also identifies issues and constraints for
   deployment on network paths that include middleboxes.  An appendix
   presents a summary of the trade-offs that were considered in
   evaluating the safety of the update to RFC 2460 that updates use of
   the UDP checksum with IPv6.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 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



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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Document Structure . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Use of UDP Tunnels . . . . . . . . . . . . . . . . . . . .  5
       1.3.1.  Motivation for new approaches  . . . . . . . . . . . .  6
       1.3.2.  Reducing forwarding cost . . . . . . . . . . . . . . .  6
       1.3.3.  Need to inspect the entire packet  . . . . . . . . . .  7
       1.3.4.  Interactions with middleboxes  . . . . . . . . . . . .  7
       1.3.5.  Support for load balancing . . . . . . . . . . . . . .  8
   2.  Standards-Track Transports . . . . . . . . . . . . . . . . . .  9
     2.1.  UDP with Standard Checksum . . . . . . . . . . . . . . . .  9
     2.2.  UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.2.1.  Using UDP-Lite as a Tunnel Encapsulation . . . . . . . 10
     2.3.  General Tunnel Encapsulations  . . . . . . . . . . . . . . 10
     2.4.  Relation to UDP-Lite and UDP with checksum . . . . . . . . 10
   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  . . . . . . . . . 15
       3.1.3.  Corruption of Port Information . . . . . . . . . . . . 16
       3.1.4.  Delivery to an unexpected port . . . . . . . . . . . . 16
       3.1.5.  Corruption of Fragmentation Information  . . . . . . . 17
     3.2.  Where Packet Corruption Occurs . . . . . . . . . . . . . . 19
     3.3.  Validating the network path  . . . . . . . . . . . . . . . 20
     3.4.  Applicability of method  . . . . . . . . . . . . . . . . . 21
     3.5.  Impact on non-supporting devices or applications . . . . . 21
   4.  Constraints on implementation of IPv6 nodes supporting
       zero checksum  . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.  Requirements on usage of the zero UDP checksum . . . . . . . . 24
   6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     10.2. Informative References . . . . . . . . . . . . . . . . . . 29



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   Appendix A.  Evaluation of proposal to update RFC 2460 to
                support zero checksum . . . . . . . . . . . . . . . . 31
     A.1.  Alternatives to the Standard Checksum  . . . . . . . . . . 31
     A.2.  Comparison . . . . . . . . . . . . . . . . . . . . . . . . 33
       A.2.1.  Middlebox Traversal  . . . . . . . . . . . . . . . . . 33
       A.2.2.  Load Balancing . . . . . . . . . . . . . . . . . . . . 34
       A.2.3.  Ingress and Egress Performance Implications  . . . . . 34
       A.2.4.  Deployability  . . . . . . . . . . . . . . . . . . . . 34
       A.2.5.  Corruption Detection Strength  . . . . . . . . . . . . 35
       A.2.6.  Comparison Summary . . . . . . . . . . . . . . . . . . 35
   Appendix B.  Document Change History . . . . . . . . . . . . . . . 38
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41







































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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 RFC 2460 mandates use of a calculated UDP checksum,
   i.e. a non-zero value, due to the lack of an IPv6 header checksum.
   The inclusion of the pseudo header in the checksum computation
   provides a statistical check that datagrams have been delivered to
   the intended IPv6 destination node.  Algorithms for checksum
   computation are described in [RFC1071].

   The lack of a possibility to use an IPv6 datagram with a zero UDP
   checksum 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 UDP checksum using IPv4.
   The design of IPv6 raises different issues when considering the
   safety of using a UDP checksum with IPv6.  These issues can
   significantly affect applications, both when an endpoint is the
   intended user and when an innocent bystander (when a packet is
   received by a different endpoint to that intended).

   This document examines the issues and an appendix compares the
   strengths and weaknesses of a number of proposed solutions.  This
   identifies a set of issues that must be considered and mitigated to
   be able to safely deploy IPv6 applications that use a zero UDP
   checksum.  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 a zero UDP checksum is the best method of the proposed
   alternatives to meet the goals for certain tunnel applications.

   This document defines recommendations and requirements for use of
   IPv6 datagrams with a zero UDP checksum.  This usage is expected to
   have initial 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 are provided in middleboxes that support the zero UDP
   checksum for IPv6.  The document therefore derives a set of
   constraints required to ensure safe deployment of a zero UDP
   checksum.



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   Finally, the document 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.

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

   Section 4 is an applicability statement that defines requirements and
   recommendations on the implementation of IPv6 nodes that support the
   use of a zero UDP checksum.

   Section 5 provides an applicability statement that defines
   requirements and recommendations for protocols and tunnel
   encapsulations that are transported over an IPv6 transport that does
   not perform a UDP checksum calculation to verify the integrity at the
   transport endpoints.

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

   Appendix A evaluates the set of proposals to update the UDP transport
   behaviour and other alternatives intended to improve support for
   tunnel protocols.  It concludes by assessing the trade-offs of the
   various methods, identifying advantages and disadvantages for each
   method.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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,



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   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.

1.3.1.  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 motivated an update to IPv6 UDP checksum processing to
   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 and function with middleboxes.

   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.  The processing cost per tunnel includes both
   state (memory requirements) and per-packet processing at the tunnel
   ingress and egress.

   Automatic IP Multicast Tunneling, 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 states that the UDP
   checksum in the outer packet header should be zero (see Section 6.6
   of [I-D.ietf-mboned-auto-multicast]).  This argues that the
   computation of an additional checksum is an unwarranted burden on
   nodes implementing lightweight tunneling protocols when an inner
   packet is already adequately protected, .  The AMT protocol needs to



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   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-
   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.

   When this design is used to support a tunnel ingress and egress, it
   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

   Many paths in the Internet include one or more middleboxes of various
   types.  There exist large classes of middleboxes that will handle
   zero UDP checksum packets, which would not support UDP-Lite or the
   other investigated proposals.  These middleboxes includes load
   balancers (see Section 1.3.5) including Equal Cost Multipath Routing,
   traffic classifiers and other functions that reads some fields in the
   UDP headers but does not validate the UDP checksum.

   There are also middleboxes that either validates or modify the UDP
   checksum.  The two most common classes are Firewalls and NATs.  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



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   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 into consideration the use of a zero UDP checksum.

   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
   datagrams with a zero UDP checksum as valid.  Use of a zero UDP
   checksum with IPv6 requires firewalls to be updated before the full
   utility of the change is available.

   It can be expected that datagrams with zero UDP checksum will
   initially not have the same middlebox traversal characteristics as
   regular UDP (RFC 2460).  However when implementations follow the
   requirements specified in this document, we expect the traversal
   capabilities to improve over time.  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.

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 [RFC6437] as a basis for entropy for 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 for load balancing is consistent with the
   intended use, although further clarity was needed to ensure the field
   can be consistently used for this purpose, therefore an updated IPv6
   Flow Label [RFC6437] and Equal-Cost Multi-Path routing usage, (ECMP)
   [RFC6438] was produced.  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



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   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
   achieved.


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, as defined in RFC
   2460, has already been discussed.  UDP usage 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 [RFC5097] and
   unicast usage guidelines in [RFC5405].  There is at least one open
   source implementation as a part of the Linux kernel since version
   2.6.20.

   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, with the exception that it uses a
   different value in the Next Header field in the IPv6 header.  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.




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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/middlebox to
   traverse the entire packet payload.  This provides most of the
   verification required for delivery and still keeps a low complexity
   for the checksumming operation.  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 a calculated checksum 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, e.g., IP-in-IP and GRE.  These either do not include
   a 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.

2.4.  Relation to UDP-Lite and UDP with checksum

   The operation of IPv6 with UDP with a zero-checksum is not the same
   as IPv4 with UDP with a zero-checksum.  Protocol designers should not



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   be fooled into thinking the two are the same.  The requirements below
   list a set of additional considerations.

   Where possible, existing general tunnel encapsulations, such as GRE,
   IP-in-IP, should be used.  This section assumes that such existing
   tunnel encapsulations do not offer the functionally required to
   satisfy the protocol designer's goals.  The section considers the
   standardized alternative solutions, rather than the full set of ideas
   evaluated in Appendix A.  The alternatives to UDP with a zero
   checksum are UDP with a (calculated) checksum, and UDP-Lite.

   UDP with a checksum has the advantage of close to universal support
   in both endpoints and middleboxes.  It also provides statistical
   verification of delivery to the intended destination (address and
   port).  However, some classes of device have limited support for
   calculation of a checksum that covers a full datagram.  For these
   devices, this can incur significant processing cost (e.g. requiring
   processing in the router slow-path) and can hence reduce capacity or
   fail to function.

   UDP-Lite has the advantage of using a checksum that is calculated
   only over the pseudo header and the UDP header.  This provides a
   statistical verification of delivery to the intended destination
   (address and port).  The checksum can be calculated without access to
   the datagram payload, only requiring access to the part to be
   protected.  A drawback is that UDP-Lite has currently limited support
   in both end-points (i.e. is not supported on all operating system
   platforms) and middleboxes (that require support for the UDP-Lite
   header type).  A path verification method is therefore recommended.

   IPv6 and UDP with a zero-checksum can also be used by nodes that do
   not permit calculation of a payload checksum.  Many existing classes
   of middleboxes do not verify or change the transport checksum.  For
   these middleboxes, IPv6 with a zero UDP checksum is expected to
   function where UDP-Lite would not.  However, support for the zero UDP
   checksum in middleboxes that do change or verify the checksum is
   currently limited, and this may result in datagrams with a zero UDP
   checksum being discarded, therefore a path verification method is
   recommended.

   There are sets of constrains for which no solution exist: A protocol
   designer that needs to originate or receive datagrams on a device
   that can not efficiently calculate a checksum over a full datagram
   and also needs these packets to pass through a middlebox that
   verifies or changes a UDP checksum, but does not support a zero UDP
   checksum, can not use the zero UDP checksum method.  Similarly, one
   that originates datagrams on a device with UDP-Lite support, but
   needs the packets to pass through a middlebox that does not support



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   UDP-Lite, can not use UDP-Lite.  For such cases, there is no optimal
   solution and the current recommendation is to use or fall-back to
   using UDP with full checksum coverage.


3.  Issues Requiring Consideration

   This informative section evaluates issues around the proposal to
   update IPv6 [RFC2460], to enable the UDP transport checksum to be set
   to zero.  Some of the identified issues are shared with other
   protocols already in use.  The section also provides background to
   the requirements and recommendations that follow.

   The decision in RFC 2460 to omit an integrity check at the network
   level meant that the IPv6 transport checksum 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
      validate that the packet does not consist of a wrong header
      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
      ports/addresses);

   o  the payload integrity.

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

   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.






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3.1.  Effect of packet modification in the network

   IP packets may be corrupted as they traverse an Internet path.  Older
   evidence in "When the CRC and TCP Checksum Disagree" [Sigcomm2000]
   show that this was once an issue in year 2000 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].  During
   the development of this document in 2009, individuals provided
   reports of observed rates for received UDP datagrams using IPv4 where
   the UDP checksum had been detected as corrupt.  These rates where as
   high as 1.39E-4 for some paths, but also close to zero for some other
   paths.

   There is extensive experience of deployment using tunnel protocols in
   well-managed networks (e.g. corporate networks or service provider
   core networks).  This has shown the robustness of methods such as PWE
   and MPLS that do not employ a transport protocol checksum and have
   not specified mechanisms to protect from corruption of the
   unprotected headers (such as the VPN Identifier in MPLS).  Reasons
   for the robustness may include:

   o  A reduced probability of corruption on paths through well-managed
      networks.

   o  IP form the majority of the inner traffic carried by these tunnel.
      Hence from a transport perspective, endpoint verification is
      already being performed when processing a received IPv4 packet or
      by the transport pseudo-header for an IPv6 packet.  This update to
      UDP does not change this behaviour.

   o  In certain cases, a combination of additional filtering (e.g.
      filter of a MAC destination address in a L2 tunnel) significantly
      reduces the probability of final mis-delivery to the IP stack.

   o  The tunnel protocols did not use a UDP transport header, any
      corruption is therefore unlikely to result in misdelivery to
      another UDP-based application.  This concern is specific to the
      use of UDP with IPv6.

   While this experience can guide the present recommendations, any
   update to UDP must preserve operation in the general Internet.  This
   is heterogeneous and can include links and systems of very varying
   characteristics.  Transport protocols used by hosts need to be
   designed with this in mind, especially when there is need to traverse
   edge networks, where middlebox deployments are common.

   For the general Internet, there is no current evidence that



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   corruption is rare, nor that this may not be applicable to IPv6.  It
   therefore seems prudent not to relax checks on misdelivery .  The
   emergence of low-end IPv6 routers and the proposed use of NAT with
   IPv6 further motivate the need to protect from misdelivery.

   Corruption in the network may result in:

   o  A datagram being misdelivered 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.

3.1.1.  Corruption of the destination IP address

   An IPv6 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.
   Detection of this corruption by a UDP receiver relies on the IPv6
   pseudo header incorporated in the transport checksum.

   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 a computed 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



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      port processing).

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.  Detection of this corruption by a UDP
   receiver relies on the IPv6 pseudo header incorporated in the
   transport checksum.

   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.  When using Unicast Reverse Path Forwarding
   [RFC2827], a change in address may result in the router discarding
   the packet when the route to the modified source address is different
   to that of the source address of the original packet.

   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
      activated).

   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
      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).

   o  ICMP messages relating to a corrupted packet can be misdirected to
      the wrong source node.



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   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 is
   used with a zero UDP checksum, but not when UDP checksums are
   calculated or when UDP-Lite is used.  If the ports carried in the
   transport header of an IPv6 packet were corrupted in transit, packets
   may be delivered to the wrong application process (on the intended
   machine) and/or responses or errors sent to the wrong application
   process (on the intended machine).

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,
      and may be misinterpreted, generating side-effects or accumulated
      state.

   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:




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   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 process is given only the
       application data associated with a specific 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
   datagrams.

   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 datagrams with a zero UDP
   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 headers.

   A protocol that uses a zero UDP checksum can not assume that it is
   the only protocol using a zero UDP 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 field in IPv4, a 13-bit fragment
   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



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      and discarded.

   Erroneous reassembly:  The re-assembled 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 require 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 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
   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
   UDP 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



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   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 UDP 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 UDP 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 UDP checksum both vulnerable to undetected errors.

   In conclusion, fragmentation of datagrams with a zero UDP checksum
   does not worsen the performance 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.  Where Packet Corruption Occurs

   Corruption of IP packets can occur at any point along a network path,
   during packet generation, during transmission over the link, in the
   process of routing and switching, etc.  Some transmission steps
   include a checksum or Cyclic Redundancy Check (CRC) that reduces the
   probability for corrupted packets being forwarded, but there still
   exists a probability that errors may propagate undetected.

   Unfortunately the community lacks reliable information to identify
   the most common functions or equipment that result in packet
   corruption.  However, there are indications that the place where
   corruption occurs can vary significantly from one path to another.



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   There is therefore a risk in applying evidence from one domain of
   usage to infer characteristics for another.  Methods intended for
   general Internet usage must therefore assume that corruption can
   occur and deploy mechanisms to mitigate the effect of corruption
   and/or resulting misdelivery.

3.3.  Validating the network path

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

   A zero value in the UDP checksum field 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, for example to validate
   the UDP checksum, will consider such a packet as erroneous or illegal
   and may discard it, unless the device is updated to support the new
   behavior.  Any middlebox that modifies the UDP checksum, for example
   a NAT that changes the values of the IP and UDP header in such a way
   that the checksum over the pseudo header changes value, will need to
   be updated to support this behavior.  Until then, a zero UDP checksum
   packet is likely to be discarded either directly in the middlebox or
   at the destination, when a zero UDP checksum has been modified to a
   non-zero by an incremental update.

   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 using negotiation or an explicit mandate to use the new
   behavior by all nodes that support the new protocol.

   Enabling the use of a zero checksum places new requirements on
   equipment deployed within the network, such as middleboxes.  A
   middlebox (e.g.  Firewalls, Network Address Translators) may enable
   zero checksum usage for a particular range of ports.  Note that
   checksum off-loading and operating system design may result in all
   IPv6 UDP traffic being sent with a calculated checksum.  This
   requires middleboxes that are configured to enable a zero UDP
   checksum to continue to work with bidirectional UDP flows that use a
   zero UDP checksum in only one direction, and therefore they must not
   maintain separate state for a UDP flow based on its checksum usage.




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   Support along the path between end points can 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.  Note that a bidirectional path does not necessarily
   support the same checksum usage in both the forward and return
   directions: Receipt of a datagram with a zero UDP checksum, does not
   imply that the remote endpoint can also receive a datagram with a
   zero UDP checksum.  This will require periodic validation of the
   path, adding complexity to any solution using the new behavior.

3.4.  Applicability of method

   The update to the IPv6 specification defined in
   [I-D.ietf-6man-udpchecksums] only modifies IPv6 nodes that implement
   specific protocols designed to permit omission of a UDP checksum.
   This document therefore provides an applicability statement for the
   updated method 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.

   When the method is 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, needs to minimise the possibility that application
   processes could confuse a corrupted or wrongly delivered UDP datagram
   with that of data addressed to the application running on their
   endpoint.

   The protocol or application that uses the zero checksum method must
   ensure that the lack of checksum does not affect the protocol
   operation.  This includes being robust to receiving a unintended
   packet from another protocol or context following corruption of a
   destination or source address and/or port value.  It also includes
   considering the need for additional implicit protection mechanisms
   required when using the payload of a UDP packet received with a zero
   checksum.

3.5.  Impact on non-supporting devices or applications

   It is important to consider the potential impact of using a zero UDP
   checksum on end-point devices or applications that are not modified
   to support the new behavior or by default or preference, use the



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   regular behavior.  These applications must not be significantly
   impacted by the update.

   To illustrate why this necessary, consider the implications of a node
   that enables use of a zero UDP checksum at the interface level: This
   would result in all applications that listen to a UDP socket
   receiving datagrams 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
   application.

   A zero UDP checksum therefore needs to 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 datagrams with a zero UDP checksum.
   These other applications would not be affected by this changed
   behavior.  An application that allows the changed behavior should be
   aware of the risk of corruption and the increased level of
   misdirected traffic, and can be designed robustly to handle this
   risk.


4.  Constraints on implementation of IPv6 nodes supporting zero checksum

   This section is an applicability statement that defines requirements
   and recommendations on the implementation of IPv6 nodes that support
   use of a zero value in the checksum field of a UDP datagram.

   All implementations that support this zero UDP checksum method MUST
   conform to the requirements defined below.

   1.   An IPv6 sending node MAY use a calculated RFC 2460 checksum for
        all datagrams that it sends.  This explicitly permits an
        interface that supports checksum offloading to insert an updated
        UDP checksum value in all UDP datagrams that it forwards,
        however note that sending a calculated checksum requires the
        receiver to also perform the checksum calculation.  Checksum
        offloading can normally be switched off for a particular
        interface to ensure that datagrams are sent with a zero UDP
        checksum.

   2.   IPv6 nodes SHOULD by default NOT allow the zero UDP checksum
        method for transmission.

   3.   IPv6 nodes MUST provide a way for the application/protocol to
        indicate the set of ports that will be enabled to send datagrams
        with a zero UDP checksum.  This may be implemented by enabling a



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        transport mode using a socket API call when the socket is
        established, or a similar mechanism.  It may also be implemented
        by enabling the method for a pre-assigned static port used by a
        specific tunnel protocol.

   4.   IPv6 nodes MUST provide a method to allow an application/
        protocol to indicate that a particular UDP datagram is required
        to be sent with a UDP checksum.  This needs to be allowed by the
        operating system at any time (e.g. to send keep-alive
        datagrams), not just when a socket is established in the zero
        checksum mode.

   5.   The default IPv6 node receiver behaviour MUST discard all IPv6
        packets carrying datagrams with a zero UDP checksum.

   6.   IPv6 nodes MUST provide a way for the application/protocol to
        indicate the set of ports that will be enabled to receive
        datagrams with a zero UDP checksum.  This may be implemented via
        a socket API call, or similar mechanism.  It may also be
        implemented by enabling the method for a pre-assigned static
        port used by a specific tunnel protocol.

   7.   IPv6 nodes supporting usage of zero UDP checksums MUST also
        allow reception using a calculated UDP checksum on all ports
        configured to allow zero UDP checksum usage.  (The sending
        endpoint, e.g. encapsulating ingress, may choose to compute the
        UDP checksum, or may calculate this by default.)  The receiving
        endpoint MUST use the reception method specified in RFC2460 when
        the checksum field is not zero.

   8.   RFC 2460 specifies that IPv6 nodes SHOULD log received datagrams
        with a zero UDP checksum.  This remains the case for any
        datagram received on a port that does not explicitly enable
        processing of a zero UDP checksum.  A port for which the zero
        UDP checksum has been enabled MUST NOT log the datagram solely
        because the checksum value is zero.

   9.   IPv6 nodes MAY separately identify received UDP datagrams that
        are discarded with a zero UDP checksum.  It SHOULD NOT add these
        to the standard log, since the endpoint has not been verified.
        This may be used to support other functions (such as a security
        policy).

   10.  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



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        port numbers in the ICMPv6 message body).


5.  Requirements on usage of the zero UDP checksum

   This section is an applicability statement that identifies
   requirements and recommendations for protocols and tunnel
   encapsulations that are transported over an IPv6 transport flow (e.g.
   tunnel) that does not perform a UDP checksum calculation to verify
   the integrity at the transport endpoints.  Before deciding to use the
   zero UDP checksum and loose the integrity verification provided, a
   protocol developer should seriously consider if they can use
   checksummed UDP packets or UDP-Lite [RFC3828], because IPv6 with a
   zero UDP checksum is not equivalent in behavior to IPv4 with zero UDP
   checksum.

   The requirements and recommendations for protocols and tunnel
   encapsulations using an IPv6 transport flow that does not perform a
   UDP checksum calculation to verify the integrity at the transport
   endpoints are:

   1.   Transported protocols that enable the use of zero UDP checksum
        MUST only enable this for a specific port or port-range.  This
        needs to be enabled at the sending and receiving endpoints for a
        UDP flow.

   2.   An integrity mechanism is always RECOMMENDED at the transported
        protocol layer to ensure that corruption rates of the delivered
        payload is not increased (e.g. the inner-most packet of a UDP
        tunnel).  A mechanism that isolates the causes of corruption
        (e.g. identifying misdelivery, IPv6 header corruption, tunnel
        header corruption) is expected to also provide additional
        information about the status of the tunnel (e.g. to suggest a
        security attack).

   3.   A transported protocol that encapsulates Internet Protocol (IPv4
        or IPv6) packets MAY rely on the inner packet integrity checks,
        provided that the tunnel protocol will not significantly
        increase the rate of corruption of the inner IP packet.  If a
        significantly increased corruption rate can occur, then the
        tunnel protocol MUST provide an additional integrity
        verification mechanism.  Early detection is desirable to avoid
        wasting unnecessary computation, transmission capacity or
        storage for packets that will subsequently be discarded.

   4.   A transported protocol that supports use of a zero UDP checksum,
        MUST be designed so that corruption of this information does not
        result in accumulated state for the protocol.



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   5.   A transported protocol with a non-tunnel payload or one that
        encapsulates non-IP packets MUST have a CRC or other mechanism
        for checking packet integrity, unless the non-IP packet is
        specifically designed for transmission over a lower layer that
        does not provide a packet integrity guarantee.

   6.   A transported protocol with control feedback SHOULD be robust to
        changes in the network path, since the set of middleboxes on a
        path may vary during the life of an association.  The UDP
        endpoints need to discover paths with middleboxes that drop
        packets with a zero UDP checksum.  Therefore, transported
        protocols SHOULD send keep-alive messages with a zero UDP
        checksum.  An endpoint that discovers an appreciable loss rate
        for keep-alive packets MAY terminate the UDP flow (e.g. tunnel).
        Section 3.1.3 of RFC 5405 describes requirements for congestion
        control when using a UDP-based transport.

   7.   A protocol with control feedback that can fall-back to using UDP
        with a calculated RFC 2460 checksum is expected to be more
        robust to changes in the network path.  Therefore, keep-alive
        messages SHOULD include both UDP datagrams with a checksum and
        datagrams with a zero UDP checksum.  This will enable the remote
        endpoint to distinguish between a path failure and dropping of
        datagrams with a zero UDP checksum.

   8.   A middlebox implementation MUST allow forwarding of an IPv6 UDP
        datagram with both a zero and standard UDP checksum using the
        same UDP port.

   9.   A middlebox MAY configure a restricted set of specific port
        ranges that forward UDP datagrams with a zero UDP checksum.  The
        middlebox MAY drop IPv6 datagrams with a zero UDP checksum that
        are outside a configured range.

   10.  When a middlebox forwards an IPv6 UDP flow containing datagrams
        with both a zero and standard UDP checksum, the middlebox MUST
        NOT maintain separate state for flows depending on the value of
        their UDP checksum field.  (This requirement is necessary to
        enable a sender that always calculates a checksum to communicate
        via a middlebox with a remote endpoint that uses a zero UDP
        checksum.)

   Special considerations are required when designing a UDP tunnel
   protocol, where the tunnel ingress or egress may be a router that may
   not have access to the packet payload.  When the node is acting as a
   host (i.e., sending or receiving a packet addressed to itself), the
   checksum processing is similar to other hosts.  However, when the
   node (e.g. a router) is acting as a tunnel ingress or egress that



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   forwards a packet to or from a UDP tunnel, there may be restricted
   access to the packet payload.  This prevents calculating (or
   verifying) a UDP checksum.  In this case, the tunnel protocol may use
   a zero UDP checksum and must:

   o  Ensure that tunnel ingress and tunnel egress router are both
      configured to use a zero UDP checksum.  For example, this may
      include ensuring that hardware checksum offloading is disabled.

   o  The tunnel operator must ensure that middleboxes on the network
      path are updated to support use of a zero UDP checksum.

   o  A tunnel egress should implement appropriate security techniques
      to protect from overload, including source address filtering to
      prevent traffic injection by an attacker, and rate-limiting of any
      packets that incur additional processing, such as UDP datagrams
      used for control functions that require verification of a
      calculated checksum to verify the network path.  Usage of common
      control traffic for multiple tunnels between a pair of nodes can
      assist in reducing the number of packets to be processed.


6.  Summary

   This document provides an applicability statement for the use of UDP
   transport checksums with IPv6.

   It examines the role of the UDP transport checksum when used with
   IPv6 and presents a summary of the trade-offs in evaluating the
   safety of updating RFC 2460 to permit an IPv6 endpoint to use a zero
   UDP checksum field to indicate that no checksum is present.

   Application designers should first examine whether their transport
   goals may be met using standard UDP (with a calculated checksum) or
   by using UDP-Lite.  The use of UDP with a zero UDP checksum has
   merits for some applications, such as tunnel encapsulation, and is
   widely used in IPv4.  However, there are different dangers for IPv6:
   There is an increased risk of corruption and misdelivery when using
   zero UDP checksum in IPv6 compared to using IPv4 due to the lack of
   an IPv6 header checksum.  Thus, applications need to evaluate the
   risks of enabling use of a zero UDP checksum and 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.  The
   use of checksum off-loading may help alleviate the cost of checksum
   processing and permit use of a checksum using method defined in RFC
   2460.

   Tunnel applications using UDP for encapsulation can in many cases use



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   a zero UDP checksum without significant impact on the corruption
   rate.  A well-designed tunnel application should include consistency
   checks to validate the header information encapsulated with a
   received packet.  In most cases, tunnels encapsulating IP packets can
   rely on the integrity protection provided by the transported protocol
   (or tunneled inner packet).  When correctly implemented, such an
   endpoint will not be negatively impacted by omission of the
   transport-layer checksum.  Recursive tunneling and fragmentation is a
   potential issue that can raise corruption rates significantly, and
   requires careful consideration.

   Other UDP applications at the intended destination node or another
   node can be impacted if they are allowed to receive datagrams that
   have a zero UDP checksum.  It is important that already deployed
   applications are not impacted by a change at the transport layer.  If
   these applications execute on nodes that implement RFC 2460, they
   will discard (and log) all datagrams with a zero UDP checksum.  This
   is not an issue.

   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 data stream of
   the application 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 transport of recursive tunneling and the use of fragmentation
   pose difficult issues that need to be considered in the design of
   tunnel protocols.  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
   verification of correctness.  This requires extra thought and careful
   consideration in the design of transported tunnels.

   Any use of the updated method must consider the implications on
   firewalls, NATs and other middleboxes.  It is not expected that IPv6
   NATs handle IPv6 UDP datagrams in the same way that they handle IPv4
   UDP datagrams.  In many deployed cases this will require an update to
   support an IPv6 zero UDP 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, and therefore new devices are expected to
   follow the methods specified in this document.

   Each application should consider the implications of choosing an IPv6
   transport that uses a zero UDP checksum, and consider whether other
   standard methods may be more appropriate, and may simplify
   application design.



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7.  Acknowledgements

   Brian Haberman, Brian Carpenter, Margaret Wasserman, Lars Eggert,
   others in the TSV directorate.  Barry Leiba, Ronald Bonica, Pete
   Resnick, and Stewart Bryant are thanked for resulting in a document
   with much greater applicability.  Thanks to P.F. Chimento for careful
   review and editorial corrections.

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


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.

   Depending on the hardware design, the processing requirements may
   differ for tunnels that have a zero UDP checksum and those that
   calculate a checksum.  This processing overhead may need to be
   considered when deciding whether to enable a tunnel and to determine
   an acceptable rate for transmission.  This can become a security risk
   for designs that can handle a significantly larger number of packets
   with zero UDP checksums compared to datagrams with a non-zero
   checksum, such as tunnel egress.  An attacker could attempt to inject
   non-zero checksummed UDP packets into a tunnel forwarding zero
   checksum UDP packets and cause overload in the processing of the non-
   zero checksums, e.g. if this happens in a routers slow path.
   Protection mechanisms should therefore be employed when this threat
   exists.  Protection may include source address filtering to prevent
   an attacker injecting traffic, as well as throttling the amount of
   non-zero checksum traffic.  The latter may impact the function of the
   tunnel protocol.

   Transmission of IPv6 packets with a zero UDP checksum could reveal
   additional information to an on-path attacker to identify the
   operating system or configuration of a sending node.  There is a need
   to probe the network path to determine whether the current path
   supports using IPv6 packets with a zero UDP checksum.  The details of
   the probing mechanism may differ for different tunnel encapsulations



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   and if visible in the network (e.g. if not using IPsec in encryption
   mode) could reveal additional information to an on-path attacker to
   identify the type of tunnel being used.

   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.  This applicability statement
   therefore permits this method to be enabled only for specific ranges
   of ports.

   When the zero UDP checksum mode is enabled for a range of ports,
   nodes and middleboxes must forward received UDP datagrams that have
   either a calculated checksum or a zero checksum.


10.  References

10.1.  Normative References

   [I-D.ietf-6man-udpchecksums]
              Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets",
              draft-ietf-6man-udpchecksums-08 (work in progress),
              February 2013.

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

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

   [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.

10.2.  Informative References

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

   [I-D.ietf-mboned-auto-multicast]
              Bumgardner, G., "Automatic Multicast Tunneling",
              draft-ietf-mboned-auto-multicast-14 (work in progress),



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              June 2012.

   [LISP]     D. Farinacci et al, "Locator/ID Separation Protocol
              (LISP)", November 2012.

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

   [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.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [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.

   [RFC5097]  Renker, G. and G. Fairhurst, "MIB for the UDP-Lite
              protocol", RFC 5097, January 2008.

   [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.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437, November 2011.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, November 2011.

   [Sigcomm2000]
              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.  Evaluation of proposal to update RFC 2460 to support zero
             checksum

   This informative appendix documents the evaluation of 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, and notes that
   for a particular application some standard methods may be more
   appropriate than using IPv6 with a zero UDP checksum.

A.1.  Alternatives to the Standard Checksum

   There are several alternatives to the normal method for calculating
   the UDP Checksum [RFC1071] that do not require a tunnel endpoint to
   inspect the entire packet when computing a checksum.  These include
   (in 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 used).



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   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] 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 datagram (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).

   o  UDP modified to disable checksum processing
      [I-D.ietf-6man-udpchecksums].  This eliminates the need for a
      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
   sections.





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A.2.  Comparison

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

   While this comparison focuses on applications that are expected to
   execute on routers, 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 the receiving node from a
   received packet.

A.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 would also have been
   able to 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.  A method that ignores the checksum has an additional
   downside in that it prevents improvement of middlebox traversal,
   because there is no way to identify UDP datagrams that 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.

   Datagrams with a zero UDP checksum will not be passed by any
   middlebox that validates the checksum using RFC 2460 or updates the
   checksum field, such as NAT or firewalls.  This would require an
   update to correctly handle a datagram with a zero UDP checksum.

   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



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   validation.

A.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
   unlikely to be supported by deployed hashing mechanisms, which could
   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.

A.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 method that uses a zero
   UDP checksum, ignoring the UDP 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 datagram.  Regular UDP is
   clearly the most costly to process, always requiring checksum
   calculation over the entire datagram.

   It is important to note that the zero UDP 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.

A.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:

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

   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



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      tunnel ingress.

   o  IP-in-IP tunnels should not require changes to the end-points, but
      raise issues when traversing firewalls and other security 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 tunnel control protocol to
      negotiate support and validate the path.

   o  The remaining solutions (including the zero checksum method) offer
      similar deployability.  UDP-Lite requires support at both end-
      points and in middleboxes.  UDPTT and the zero UDP checksum method
      with or without an extension header require support at both end-
      points and in middleboxes.  UDP-Lite, UDPTT, and the zero UDP
      checksum method and use of extension headers may additionally
      require changes or additions to the tunnel control protocol to
      negotiate support and path validation.

A.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 methods using a zero UDP checksum, ignoring the UDP checksum on
   reception and IP-and-IP encapsulation all provide no verification
   that a received datagram was intended to be processed by a specific
   tunnel egress or that the inner encapsulated packet was correct.
   Section 3.1 discusses experience using specific protocols in well-
   managed networks.

A.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:  The method defined in RFC 2460 has 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
      to select the protocol and validation to confirm the path forwards
      UDP-Lite.

   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 to confirm
      the path forwards a zero UDP checksum.

   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 to select the transport and
      validation to confirm the path forwards UDPTT datagrams.

   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 negotiation to confirm the option is
      supported and validation to confirm the path forwards the option.

   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 negotiation to confirm the option is supported and
      validation to confirm the path forwards the option.

   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 negotiation to confirm the option is supported by the



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      remote endpoint and validation to confirm the path forwards a zero
      UDP checksum.

   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
   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.  Zero UDP
   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 a new
   method becomes standardized, the fewer boxes will be non-compliant.

   Thus, the question of whether to permit use of datagrams with a zero
   UDP 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 using a zero UDP 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 analysis concludes that the IETF should carefully consider
   constraints on sanctioning the use of any new transport mode.  The
   6man working group of the IETF has determined that the answer to the
   above questions are sufficient to update IPv6 to standardise use of a
   zero UDP checksum for use by tunnel encapsulations for specific
   applications.




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   Each application should consider the implications of choosing an IPv6
   transport that uses a zero UDP checksum.  In many cases, standard
   methods may be more appropriate, and may simplify application design.
   The use of checksum off-loading may help alleviate the checksum
   processing cost and permit use of a checksum using method defined in
   RFC 2460.


Appendix B.  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.

   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.








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   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.

   Working Group Draft 05

      *  Resubmission to correct editorial NiTs - thanks to Bill Atwood
         for noting these.Group Draft 05.

   Working Group Draft 06

      *  Resubmission to keep draft alive (spelling updated from 05).

   Working Group Draft 07

      *  Interim Version

      *  Submission after IESG Feedback Added

      *  Updates to enable the document to become a PS Applicability
         Statement

   Working Group Draft 08

      *  First Version written as a PS Applicability Statement

      *  Changes to reflect decision to update RFC 2460, rather than
         recommend decision

      *  Updates to requirements for middleboxes




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      *  Inclusion of requirements for security, API, and tunnel

      *  Move of the rationale for the update to an Annex (former
         section 4)

   Working Group Draft 09

      *  Submission after second WGLC (note mistake corrected in -09).

      *  Clarified role of API for supporting full checksum.

      *  Clarified that full checksum is required in security
         considerations, and therefore noting that full checksum should
         not be treated as an attack - consistent with remainder of
         document.

      *  Added mention that API can set a mode in transport stack - to
         link to similar statement in RFC 2460 update.

      *  Fixed typos.

   Working Group Draft 10

      *  Submission to correct unwanted removal of text from section 5
         bullets 5-7 by GF.

      *  Replaced section 5 text with the text from 08, and reapplied
         the editorial correction.

      *  Note to reviewers: Please compare this revision with -08 used
         in the IETF LC).

   Working Group Draft 11

      *  Added REF for 5097 (Noted by S.Turner)

      *  Added text in response to P. Resnick on place where checksum is
         calculated.

      *  Added text to note experience with MPLS/PWE; Appendix updated
         to refer to this (S. Bryant)

      *  Added text in response to P.Resnick's 2nd comments.

      *  Request to make UDP-Lite more clearly recommended (J Touch,
         P.Resnick)





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      *  Added considerations around usage of zero checksum in routers.

      *  Added text in response to Stewart Bryant's comments on router
         requirements.


Authors' Addresses

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

   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk/users/gorry


   Magnus Westerlund
   Ericsson
   Farogatan 6
   Stockholm,  SE-164 80
   Sweden

   Phone: +46 8 719 0000
   Email: magnus.westerlund@ericsson.com

























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