





Internet Engineering Task Force (IETF)                      G. Fairhurst
Request for Comments: 6936                        University of Aberdeen
Category: Standards Track                                  M. Westerlund
ISSN: 2070-1721                                                 Ericsson
                                                              April 2013


       Applicability Statement for the Use of IPv6 UDP Datagrams
                          with Zero Checksums

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 it 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 changes the use
   of the UDP checksum with IPv6.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6936.














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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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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Document Structure . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Use of UDP Tunnels . . . . . . . . . . . . . . . . . . . .  6
       1.3.1.  Motivation for New Approaches  . . . . . . . . . . . .  6
       1.3.2.  Reducing Forwarding Costs  . . . . . . . . . . . . . .  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.  Relationship of Zero UDP Checksum to UDP-Lite and UDP
           with Checksum  . . . . . . . . . . . . . . . . . . . . . . 11
   3.  Issues Requiring Consideration . . . . . . . . . . . . . . . . 12
     3.1.  Effect of Packet Modification in the Network . . . . . . . 13
       3.1.1.  Corruption of the Destination IP Address Field . . . . 14
       3.1.2.  Corruption of the Source IP Address Field  . . . . . . 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  . . . . . . . 18
     3.2.  Where Packet Corruption Occurs . . . . . . . . . . . . . . 20
     3.3.  Validating the Network Path  . . . . . . . . . . . . . . . 20
     3.4.  Applicability of the Zero UDP Checksum Method  . . . . . . 21
     3.5.  Impact on Non-Supporting Devices or Applications . . . . . 22
   4.  Constraints on Implementation of IPv6 Nodes Supporting
       Zero Checksum  . . . . . . . . . . . . . . . . . . . . . . . . 23
   5.  Requirements on Usage of the Zero UDP Checksum . . . . . . . . 24
   6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 30
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  Evaluation of Proposal to Update RFC 2460 to
                Support Zero Checksum . . . . . . . . . . . . . . . . 33
     A.1.  Alternatives to the Standard Checksum  . . . . . . . . . . 33
     A.2.  Comparison of Alternative Methods  . . . . . . . . . . . . 34
       A.2.1.  Middlebox Traversal  . . . . . . . . . . . . . . . . . 34
       A.2.2.  Load Balancing . . . . . . . . . . . . . . . . . . . . 35
       A.2.3.  Ingress and Egress Performance Implications  . . . . . 36
       A.2.4.  Deployability  . . . . . . . . . . . . . . . . . . . . 36
       A.2.5.  Corruption Detection Strength  . . . . . . . . . . . . 37
       A.2.6.  Comparison Summary . . . . . . . . . . . . . . . . . . 37



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1.  Introduction

   The User Datagram Protocol (UDP) [RFC0768] transport is defined for
   IPv4 [RFC0791], and it 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 applications do
   need to provide many important transport functions on top of UDP.
   The UDP usage guidelines [RFC5405] provide 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 inability to use an IPv6 datagram with a zero UDP checksum has
   been found to be 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, whether an endpoint is the intended user or an innocent
   bystander (i.e., when a packet is received by a different endpoint to
   that intended).

   This document identifies a set of issues that must be considered and
   mitigated to enable safe deployment of IPv6 applications that use a
   zero UDP checksum.  The appendix compares the strengths and
   weaknesses of a number of proposed solutions.  The comparison of
   methods provided in this document is also expected to be useful when
   considering applications that have different goals from the ones
   whose needs led to the writing of this document, especially
   applications that can use existing standardized transport protocols.
   The analysis concludes that using a zero UDP checksum is the best
   method of the proposed alternatives to meet the goals of 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 decrease
   as updates are provided in middleboxes that support the zero UDP





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   checksum for IPv6.  Therefore, in this document, we derive a set of
   constraints required to ensure safe deployment of a zero UDP
   checksum.

   Finally, the document 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 the remaining
   issues that need future work.

   Appendix A evaluates the set of proposals to update the UDP transport
   behavior and other alternatives intended to improve support for
   tunnel protocols.  It concludes by assessing the trade-offs of the
   various methods and by 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].







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1.3.  Use of UDP Tunnels

   One increasingly popular use of UDP is as a tunneling protocol, where
   a tunnel endpoint encapsulates the packets of another protocol inside
   UDP datagrams and transmits them to another tunnel endpoint.  Using
   UDP as a tunneling protocol is attractive when the payload protocol
   is not supported by the middleboxes that may exist along the path,
   because many middleboxes support transmission using UDP.  In this
   use, the receiving endpoint decapsulates the UDP datagrams and
   forwards the original packets contained in the payload [RFC5405].
   Tunnels establish virtual links that appear to directly connect
   locations that are distant in the physical Internet topology, and
   they 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] and Locator/Identifier Separation
   Protocol (LISP) [RFC6830]).  These protocols provided several
   motivations to update IPv6 UDP checksum processing so that it would
   benefit from simpler checksum processing, including:

   o  Reducing forwarding costs, motivated by redundancy present in the
      encapsulated packet header, because in tunnel encapsulations,
      payload integrity and length verification may be provided by
      higher-layer encapsulations (often using the IPv4, UDP, UDP-Lite
      [RFC3828], or TCP checksums [RFC0793]).

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

   o  Enhancing the 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 Costs

   It is a common requirement to terminate a large number of tunnels on
   a single router or host.  The processing cost per tunnel includes
   both state (memory requirements) and per-packet processing at the
   tunnel ingress and egress.






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   Automatic IP Multicast Tunneling, known as AMT [AMT], 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 [AMT]).  That section 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 replicate a
   multicast packet to each gateway tunnel.  In this case, the outer IP
   addresses are different for each tunnel; therefore, a different
   pseudo-header must be built to form the header for each tunnel egress
   that receives replicated multicast packets.

   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.  For example, there are implementations
   that have optimized 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, for certain classes of tunnel
   endpoints, this off-loading is not available and is 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 provides only 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 for use within a router that is unable to access the
   entire packet and does not provide checksum off-loading.  Thus,
   enabling checksum calculation over the complete packet can impact
   router design, performance, energy consumption, and cost.

1.3.4.  Interactions with Middleboxes

   Many paths in the Internet include one or more middleboxes of various
   types.  Large classes of middleboxes will handle zero UDP checksum
   packets, but do not support UDP-Lite or the other investigated
   proposals.  These middleboxes include load balancers (see
   Section 1.3.5) including equal-cost multipath (ECMP) routing, traffic
   classifiers, and other functions that reads some fields in the UDP
   headers but does not validate the UDP checksum.



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   There are also middleboxes that either validate or modify the UDP
   checksum.  The two most common classes are firewalls and NATs.  In
   IPv4, UDP encapsulation may be desirable for NAT traversal, because
   UDP support is commonly provided.  It is also necessary due to the
   almost ubiquitous deployment of IPv4 NATs.  There has also been
   discussion of NAT for IPv6, although not for the same reason as in
   IPv4.  If IPv6 NAT becomes a reality, it hopefully will 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 becomes 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 becomes 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 utilize the IPv6 flow
   label [RFC6437] as a basis for entropy for load balancing.  This
   would have the desirable effect of freeing IPv6 load-balancing
   devices from the need to assume semantics for the use of the
   transport port field, and also, it works for all types 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 ECMP routing [RFC6438] usage were
   specified.  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,
   the flow label value could be copied to the outer packet header.



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   However, many current endpoints set the flow label to a zero value
   (thus, no entropy).  The ingress of a tunnel seeking to provide good
   entropy in the flow label field would therefore need to create a
   random flow label value and keep corresponding state so that all
   packets that were associated with a flow would be consistently given
   the same flow label.  Although possible, this complexity may not be
   desirable in a tunnel ingress.

   The end-to-end use of flow labels for load balancing is a long-term
   solution.  Even if the usage of the flow label has been clarified,
   there will be a transition time before a significant proportion of
   endpoints start to assign a good quality flow label to the flows that
   they originate.  The use of load balancing using the transport header
   fields would continue 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 is also a set of network-
   layer encapsulation tunnels, such as IP-in-IP and Generic Routing
   Encapsulation (GRE).  These solutions, which are already
   standardized, are discussed first, before discussing the issues,
   because they provide background for the description of the issues and
   allow some comparison with existing issues.

2.1.  UDP with Standard Checksum

   UDP [RFC0768] with standard checksum behavior, 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 and is
   specified as a proposed standard, RFC 3828.  A MIB is defined in
   [RFC5097], and unicast usage guidelines are defined in [RFC5405].
   There has been at least one open-source implementation of UDP-Lite as
   a part of the Linux kernel since version 2.6.20.

   UDP-Lite provides a checksum with an option for 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
   or corruption in the insensitive part will not cause the datagram to
   be discarded by the transport layer at the receiving endpoint.  A



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   minor side effect of using UDP-Lite is that it 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 Cyclic Redundancy Check (CRC) that
   covers the sensitive part.

2.2.1.  Using UDP-Lite as a Tunnel Encapsulation

   Tunnel encapsulations, such as Control And Provisioning of Wireless
   Access Points (CAPWAP) [RFC5415], can use UDP-Lite, because it
   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 verify only delivery of the
   tunneled payload and uses a calculated checksum for control
   information.

   Currently, support for middlebox traversal using UDP-Lite is poor,
   because UDP-Lite uses a different IPv6 network-layer Next Header
   value than that used for UDP; therefore, 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 as UDP-Lite has achieved better
   support in middleboxes and endpoints.

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

   From an end-to-end perspective, the principal difference between an
   endpoint transport and a tunnel encapsulation is the value of the
   network-layer Next Header field.  In the former, it identifies a
   transport protocol that supports endpoint applications.  In the
   latter, it identifies a tunnel protocol egress.  This separation of
   function reduces the probability that corruption of a tunneled packet
   could result in the packet being erroneously delivered to an



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   application.  Specifically, packets are delivered only 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.  Relationship of Zero UDP Checksum 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
   be fooled into thinking that the two are the same.  The requirements
   below list a set of additional considerations for IPv6.

   Where possible, existing general tunnel encapsulations, such as GRE
   and 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.  This 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 limited support can incur significant processing costs
   (e.g., requiring processing in the router's slow path) and hence can
   reduce capacity or fail to function.

   UDP-Lite has the advantage of using a checksum that can be 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, requiring access only to the part that is to be
   protected.  A drawback is that UDP-Lite currently has limited support
   in both endpoints (i.e., is not supported on all operating system
   platforms) and middleboxes (which must support the UDP-Lite header
   type).  Therefore, using a path verification method is 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



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   currently limited, and this may result in datagrams with a zero UDP
   checksum being discarded.  Therefore, using a path verification
   method is recommended.

   For some sets of constraints, no solution exists.  For example, a
   protocol designer who needs to originate or receive datagrams on a
   device that cannot 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 that does not support a
   zero UDP checksum, cannot use the zero UDP checksum method.
   Similarly, a protocol designer who needs to originate datagrams on a
   device with UDP-Lite support, but needs the packets to pass through a
   middlebox that does not support UDP-Lite, cannot use UDP-Lite.  For
   such cases, there is no optimal solution.  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 about the proposal to
   update IPv6 [RFC2460] to enable the UDP transport checksum to be set
   to zero.  Some of the identified issues are common to other protocols
   already in use.  This section also provides background to help in
   understanding 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  That the endpoint address was not corrupted within a router, i.e.,
      a packet was intended to be received by this destination, and 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.





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   In IPv4, the first four of these 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.

3.1.  Effect of Packet Modification in the Network

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

   There is extensive experience with deployments using tunnel protocols
   in well-managed networks (e.g., corporate networks and service
   provider core networks).  This has shown the robustness of methods
   such as Pseudowire Emulation Edge-to-Edge (PWE3) and MPLS that do not
   employ a transport protocol checksum and that 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 forms the majority of the inner traffic carried by these
      tunnels.  Hence, from a transport perspective, endpoint
      verification is already being performed when a received IPv4
      packet is processed or by the transport pseudo-header for an IPv6
      packet.  This update to UDP does not change this behavior.

   o  In certain cases, a combination of additional filtering (e.g.,
      filtering a MAC destination address in a Layer 2 tunnel)
      significantly reduces the probability of final misdelivery to the
      IP stack.





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

   While this experience can guide the present recommendations, any
   update to UDP must preserve operation in the general Internet, which
   is heterogeneous and can include links and systems of widely 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.

   Currently, for the general Internet, there is no evidence that
   corruption is rare, nor is there evidence that corruption in IPv6 is
   rare.  Therefore, it seems prudent not to relax checks on
   misdelivery.  The emergence of low-end IPv6 routers and the proposed
   use of NAT with IPv6 provide further motivation 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.

   Using a checksum 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 effect of modifications to the
   destination and source IP address fields, the port fields, and the
   fragmentation information.

3.1.1.  Corruption of the Destination IP Address Field

   An IPv6 endpoint destination address could be modified in the
   network; for example, it could be 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.  When using
   IPv6, however, such modification in the network cannot be detected at



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   the network layer.  Detection of this corruption by a UDP receiver
   relies on the IPv6 pseudo-header that is 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 an error report could be generated.

   o  Delivery to a different destination address.  This modification
      will normally be detected by the transport checksum, resulting in
      a 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 Section 3.1.4 on port
      processing.)

3.1.2.  Corruption of the Source IP Address Field

   This section examines what happens when the source IP 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 that is 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
   from 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 pre-established context may
      disregard the datagram as invalid or could map it to another
      context (if a context for the modified source address were 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.



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   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.
      Therefore, reception of a datagram with a corrupted source address
      will result in the 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.

   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 the one 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 when 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 are corrupted in transit, packets
   may be delivered to the wrong application process (on the intended
   machine), responses or errors may be sent to the wrong application
   process (on the intended machine), or both may occur.

3.1.4.  Delivery to an Unexpected Port

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

   o  The packet could be delivered to a port that is not in use.  The
      packet is discarded, but could generate an ICMPv6 message (e.g.,
      port unreachable).




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   o  The packet could be delivered to a different node that implements
      the same application, so the packet may be accepted, but side
      effects could occur or accumulated state could be generated.

   o  The packet could be delivered to an application that does not
      implement the tunnel protocol, so the packet may be incorrectly
      parsed and may be misinterpreted, causing side effects or
      generating 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 corrupted in the datagram such that they match
   those of an existing flow or server port.  Unfortunately, such a
   match may be more likely for UDP than for connection-oriented
   transports, because:

   1.  There is no handshake prior to communication and no sequence
       numbers (as in TCP, Datagram Congestion Control Protocol (DCCP),
       and Stream Control Transmission Protocol (SCTP)).  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 wildcard values in endpoint
       identifiers and do not always validate the 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 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 it is 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 cannot assume that it is the
   only protocol using a zero UDP checksum.  Therefore, it needs to
   handle misdelivery gracefully.  It must be robust when malformed



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   packets are received on a listening port, and it must 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 whether there are more fragments.
   Corruption of any of these fields may result in one of two outcomes:

   o  Reassembly failure: An error in the "More Fragments" field for the
      last fragment will, for example, result in the packet never being
      considered complete, so it 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 of the packet incomplete, unless that packet has been
      duplicated before the corruption.  The incomplete packet will
      eventually be timed out and discarded.

   o  Erroneous reassembly: The reassembled 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, completing the reassembly is much less probable,
      because this would require consistent corruption of the IPv6
      header's payload length and offset fields.  To prevent erroneous
      assembly, the reassembling stack must provide strong checks that
      detect overlap and 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 provided by the
   transport checksum.  The Upper-Layer Packet length field included in
   the pseudo-header assists in verifying correct reassembly, because
   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 data that is
   all-zero in a chunk that is a multiple of 16 bits.






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   The most significant risk of corruption results following mis-
   association of a fragment with a different packet.  This risk can be
   significant, because the size of fragments is often the same (e.g.,
   fragments that form when the path MTU results in fragmentation of a
   larger packet, which is common when addition of a tunnel
   encapsulation header increases the size of a packet).  Detection of
   this type of error requires a checksum or other integrity check of
   the headers and the payload.  While such protection is desirable for
   tunnel encapsulations using IPv4, because the small fragmentation ID
   can easily result in wraparound [RFC4963], this is especially
   desirable for tunnels that perform flow aggregation [TUNNELS].

   Tunnel fragmentation behavior matters.  There can be outer or inner
   fragmentation tunnels in the Internet Architecture [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 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 detect reassembly anomalies only
   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.  The following list describes
   some tunnel fragmentation behaviors:

   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 fragmentation of the encapsulated packet,
      unless the fragmentation is performed on the innermost IP header.







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   o  Unless there is verification at each reassembly, the probability
      of undetected errors will increase with the number of times
      fragmentation is recursively applied, making both IP-in-IP and UDP
      with zero UDP checksum 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 that offers no 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 CRC that reduces the probability for corrupted
   packets being forwarded, but there still exists a probability that
   errors may propagate undetected.

   Unfortunately, the Internet community lacks reliable information to
   identify the most common functions or equipment that results in
   packet corruption.  However, there are indications that the place
   where corruption occurs can vary significantly from one path to
   another.  However, there is a risk in taking evidence from one usage
   domain and using it to infer characteristics for another.  Methods
   intended for general Internet usage must therefore assume that
   corruption can occur, and mechanisms must be deployed to mitigate the
   effects of corruption and any 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, thus changing 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
   RFC 2460.  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,



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   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 be
   non-zero by an incremental update.

   A pair of endpoints intending to use the new behavior will therefore
   need not only to ensure support at each endpoint, but also to ensure
   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., a firewall or NAT) 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.

   Support along the path between endpoints can be guaranteed in limited
   deployments by appropriate configuration.  In general, it can be
   expected to take time for deployment of any updated behavior 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
   should be able to renegotiate.  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 behavior will require
   periodic validation of the path, adding complexity to any solution
   using the new behavior.

3.4.  Applicability of the Zero UDP Checksum Method

   The update to the IPv6 specification defined in [RFC6935] modifies
   only IPv6 nodes that implement specific protocols designed to permit
   omission of a UDP checksum.  This document provides an applicability
   statement for the updated method, indicating when the mechanism can
   (and cannot) be used.  Enabling a zero UDP checksum, and ensuring




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   correct interactions with the stack, implies much more than simply
   disabling the checksum algorithm for specific packets at the
   transport interface.

   When the zero UDP checksum method is widely available, we expect that
   it will be used by applications that perceive to gain benefit from
   it.  Any solution that uses an end-to-end transport protocol rather
   than an IP-in-IP encapsulation needs to minimize 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.

   A protocol or application that uses the zero UDP checksum method must
   ensure that the lack of checksum does not affect the protocol
   operation.  This includes being robust to receiving an 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 endpoint devices and applications that are not modified
   to support the new behavior or, by default or preference, do not use
   the 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.

   Therefore, a zero UDP checksum 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.




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4.  Constraints on Implementation of IPv6 Nodes Supporting Zero Checksum

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

   All implementations that support the 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 off-loading to insert an
        updated UDP checksum value in all UDP datagrams that it
        forwards.  Note, however, that sending a calculated checksum
        requires the receiver to also perform the checksum calculation.
        Checksum off-loading 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
        transport mode using a socket API call when the socket is
        established, or by 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 keepalive
        datagrams), not just when a socket is established in zero
        checksum mode.

   5.   The default IPv6 node receiver behavior MUST be to 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 by a similar mechanism.  It may also be
        implemented by enabling the method for a pre-assigned static
        port used by a specific tunnel protocol.





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   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., the encapsulating ingress, may choose to compute
        the UDP checksum or may calculate it 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.  They SHOULD NOT add
        these to the standard log, because 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
        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., a 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 lose the integrity verification
   provided by non-zero checksumming, 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:






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   1.   Transported protocols that enable the use of zero UDP checksum
        MUST enable this only 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 are not increased (e.g., at the innermost packet of a
        UDP tunnel).  A mechanism that isolates the causes of corruption
        (e.g., identifying misdelivery, IPv6 header corruption, or
        tunnel header corruption) is also expected to 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, 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 the use of a zero UDP
        checksum MUST be designed so that corruption of any header
        information does not result in accumulation of incorrect state
        for the protocol.

   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, because 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 keepalive messages with a zero UDP
        checksum.  An endpoint that discovers an appreciable loss rate
        for keepalive packets MAY terminate the UDP flow (e.g., a
        tunnel).  Section 3.1.3 of RFC 5405 describes requirements for
        congestion control when using a UDP-based transport.






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   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, keepalive
        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 the 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 a 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 a 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
   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 off-loading 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



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      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
   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, application designers 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 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 the omission of the
   transport-layer checksum.  Recursive tunneling and fragmentation are
   potential issues that can raise corruption rates significantly, and
   they require careful consideration.

   Other UDP applications at the intended destination node or another
   node can be impacted if the nodes 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.



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   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, to protect both 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
   by 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
   innermost packet when fragmentation occurs across 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 for
   firewalls, NATs, and other middleboxes.  It is not expected that IPv6
   NATs will handle IPv6 UDP datagrams in the same way that they handle
   IPv4 UDP datagrams.  In many deployed cases, an update to support an
   IPv6 zero UDP checksum will be required.  Firewalls are intended to
   be configured, and therefore, they may need to be explicitly updated
   to allow new services or protocols.  Deployment of IPv6 middleboxes
   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 should consider whether
   other standard methods may be more appropriate and may simplify
   application design.

7.  Security Considerations

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

   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 a tunnel egress.  An attacker could attempt to
   inject non-zero checksummed UDP packets into a tunnel that is
   forwarding zero checksum UDP packets and cause overload in the



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   processing of the non-zero checksums, e.g., if it happens in a
   router's slow path.  Protection mechanisms should therefore be
   employed when this threat exists.  Protection may include source-
   address filtering to prevent an attacker from injecting traffic, as
   well as throttling the amount of non-zero checksum traffic.  The
   latter may impact the functioning of the tunnel protocol.

   Transmission of IPv6 packets with a zero UDP checksum could reveal
   additional information to help an on-path attacker 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 the use of IPv6 packets with a zero UDP checksum.  The
   details of the probing mechanism may differ for different tunnel
   encapsulations, and if they are visible in the network (e.g., if not
   using IPsec in encryption mode), they could reveal additional
   information to help an on-path attacker 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, because
   they present a large attack surface.  This applicability statement
   therefore permits this method to be enabled only for specific port
   ranges.

   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.

8.  Acknowledgments

   We would like to thank Brian Haberman, Brian Carpenter, Margaret
   Wasserman, Lars Eggert, and others in the TSV directorate.  Barry
   Leiba, Ronald Bonica, Pete Resnick, and Stewart Bryant helped to make
   this document one with 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.











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9.  References

9.1.  Normative References

   [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.

   [RFC6935]     Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
                 UDP Checksums for Tunneled Packets", RFC 6935,
                 April 2013.

9.2.  Informative References

   [AMT]         Bumgardner, G., "Automatic Multicast Tunneling", Work
                 in Progress, June 2012.

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

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

   [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.






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   [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 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.

   [RFC6830]     Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
                 "The Locator/ID Separation Protocol (LISP)", RFC 6830,
                 January 2013.

   [Sigcomm2000] Stone, J. and C. Partridge, "When the CRC and TCP
                 Checksum Disagree", 2000,
                 <http://conferences.sigcomm.org/sigcomm/2000/conf/
                 abstract/9-1.htm>.




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   [TUNNELS]     Touch, J. and M. Townsley, "Tunnels in the Internet
                 Architecture", Work in Progress, March 2010.

   [UDPTT]       Fairhurst, G., "The UDP Tunnel Transport mode", Work in
                 Progress, February 2010.














































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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] such that it provides the option that some
   nodes may suppress generation and checking of the UDP transport
   checksum.  It also compares this 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:

   o  IP-in-IP tunneling.  Because 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 there are no ports to perform
      demultiplexing among different tunnel types.  This reduces the
      available information upon which a load balancer may act.

   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 minimize 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  Delta computation of the checksum from an encapsulated checksum
      field.  Because 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 to be performed on each packet.  The method
      would work only 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 has been modified to disable checksum processing (Zero UDP
      Checksum) [RFC6935].  This eliminates the need for a checksum
      calculation, but would require constraints on appropriate usage
      and updates to endpoints and middleboxes.

   o  The proposed UDP Tunnel Transport [UDPTT] protocol 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 minimize per-packet processing.

   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 endpoint, 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  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.

   These options are compared and discussed further in the following
   sections.

A.2.  Comparison of Alternative Methods

   This section compares the methods listed above to support datagram
   tunneling.  It includes proposals for updating the behavior 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.
   From a received packet, there is no way to identify the role of the
   receiving node.

A.2.1.  Middlebox Traversal

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



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   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 be able to
   traverse a middlebox with this behavior.  However, a middlebox on the
   path that attempts to verify a standard checksum will not forward
   packets using either of these methods, thus preventing traversal.  A
   method that ignores the checksum has the additional downside that it
   prevents improvement of middlebox traversal, because there is no way
   to identify UDP datagrams that use the modified checksum behavior.

   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, because
   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 types of middleboxes,
   because it requires support for a separate network-layer protocol
   number.  Once enabled, the method to support incremental checksum
   updates would be identical to that for UDP, but different for
   checksum 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
   behavior that is equally as good as UDP.  However, UDP-Lite is
   currently unlikely to be supported by deployed hashing mechanisms,
   which could cause a load balancer not to use the transport header in
   the computed hash.  A load balancer that uses only the IP header will
   have low entropy, but this could be improved by including the IPv6
   the flow label, provided 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.






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A.2.3.  Ingress and Egress Performance Implications

   IP-in-IP tunnels are often considered efficient, because they
   introduce very little processing and have low data overhead.  The
   other proposals introduce a UDP-like header, which incurs an
   associated data overhead.  Processing is minimized for the method
   that uses a zero UDP checksum and for the method that ignores the UDP
   checksum on reception, and processing is 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 endpoints, a need for negotiation, and the need
   to update middleboxes.  These are summarized 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 needs changes only in
      the tunnel ingress.

   o  IP-in-IP tunnels should not require changes to the endpoints, but
      they raise issues regarding the traversal of firewalls and other
      security devices, which are expected to require updates.

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

   o  The remaining solutions (including the zero UDP checksum method)
      offer similar deployability.  UDP-Lite requires support at both
      endpoints and in middleboxes.  UDPTT and the zero UDP checksum
      method, with or without an extension header, require support at



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      both endpoints and in middleboxes.  UDP-Lite, UDPTT, and the zero
      UDP checksum method and the use of extension headers may also
      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 they do not verify the inner
   packet.  They provide only 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 summarized as, "there is no silver
   bullet that will slay all the issues".  One has to select which
   downsides can best be lived with.  Focusing on the existing
   solutions, they can be summarized as:

   Regular UDP:  The method defined in RFC 2460 has good middlebox
      traversal and load balancing and multiplexing, and requires a
      checksum in the outer headers to cover the whole packet.

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

   UDP-Lite:  A medium-complexity encapsulation that has good
      multiplexing support, limited middlebox traversal that may
      possibly improve over time, and poor load-balancing support that
      could improve over time, and that, in most cases, requires
      application-level negotiation to select the protocol and
      validation to confirm that the path forwards UDP-Lite.

   Delta computation of a tunnel checksum:  The delta checksum is an
      optimization in the processing of UDP, and, as such, it exhibits
      some of the drawbacks of using regular UDP.



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   The remaining proposals may be described in similar terms:

   Zero Checksum:  A low-complexity encapsulation that has good
      multiplexing support, limited middlebox traversal that could
      improve over time, and good load-balancing support, and that, in
      most cases, requires application-level negotiation and validation
      to confirm that the path forwards a zero UDP checksum.

   UDPTT:  A medium-complexity encapsulation that has good multiplexing
      support, limited middlebox traversal that may possibly improve
      over time, and good load-balancing support, and that, in most
      cases, requires 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
      encapsulation that has no multiplexing support, limited middlebox
      traversal, and poor load-balancing support that could improve over
      time, and that, in most cases, requires negotiation to confirm
      that the option is supported and validation to confirm the path
      forwards the option.

   IPv6 Destination Option Combined with Zero UDP Checksum:  A medium-
      complexity encapsulation that has good multiplexing support,
      limited load-balancing support that could improve over time, and
      that, in most cases, requires 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
      that has good multiplexing support, medium middlebox traversal
      that can never improve, and good load-balancing support, and that,
      in most cases, requires negotiation to confirm that the option is
      supported by the 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, 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, if low complexity is necessary, and one does not require
   load balancing, IP-in-IP tunneling is the simplest.  If one wants
   strengthened error detection, but with the 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,
   provided that the usage can accept the currently limited support for
   middlebox traversal.




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RFC 6936      Applicability of Zero UDP Checksum with IPv6    April 2013


   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, because it requires
   both load balancer and endpoint 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 a long time to evolve.

   However, we note that the deployment of IPv6-capable middleboxes is
   still in its initial phase, and if a new method becomes standardized
   quickly, 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 best viewed as
   a trade-off among a number of more subjective questions:

   o  Is there sufficient interest in using a zero UDP checksum with the
      given constraints (summarized 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 answers to the
   above questions are sufficient to update IPv6 to standardize use of a
   zero UDP checksum for use by tunnel encapsulations for specific
   applications.

   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 the method defined
   in RFC 2460.












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RFC 6936      Applicability of Zero UDP Checksum with IPv6    April 2013


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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