Network Working Group                                         M. Eubanks
Internet-Draft                                        AmericaFree.TV LLC
Intended status: Standards Track                             P. Chimento
Expires: September 13, 2012             Johns Hopkins University Applied
                                                      Physics Laboratory
                                                          March 12, 2012

                   UDP Checksums for Tunneled Packets


   This document provides an update of RFC 2460[RFC2460] in order to
   improve the performance of IPv6 in an increasingly important use
   case, the use of tunneling to carry new transport protocols.  The
   performance improvement is obtained by relaxing the IPv6 UDP checksum
   requirement for suitable tunneling protocol where header information
   is protected on the "inner" packet being carried.  This relaxation
   removes the overhead associated with the computation of UDP checksums
   on tunneled IPv6 packets and thereby improves the efficiency of the
   traversal of firewalls and other network middleware by such new
   protocols.  We describe how the IPv6 UDP checksum requirement can be
   relaxed in the situation where the encapsulated packet itself
   contains a checksum, the limitations and risks of this approach, and
   provides restrictions on the use of this relaxation to mitigate these

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on September 13, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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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
   2.  Some Terminology . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  The Zero-Checksum Solution . . . . . . . . . . . . . . . . . .  7
   6.  Additional Observations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 11
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   This work constitutes the first upgrade of RFC 2460[RFC2460], in
   order to improve the performance of IPv6 with transport layer
   protocols carried encapsulated in tunnels.  With the rapid growth of
   the Internet, tunneling protocols have become increasingly important
   to enable the deployment of new transport layer protocols.  Tunneled
   protocols can be deployed rapidly, while the time to upgrade and
   deploy a critical mass of routers, switches and end hosts on the
   global Internet for a new transport protocol is now measured in
   decades.  At the same time, the increasing use of firewalls and other
   security related middleware means that truly new tunnel protocols,
   with new protocol numbers, are also unlikely to be deployable in a
   reasonable time frame, which has resulted in an increasing interest
   in and use of UDP-based tunneling protocols.  In such protocols,
   there is an encapsulated "inner" packet, and the "outer" packet
   carrying the tunneled inner packet is a UDP packet, which can pass
   through firewalls and other middleware filtering that is a fact of
   life on the current Internet.

   As tunnel endpoints may be routers or middleware aggregating traffic
   from large numbers of tunnel users, the computation of an additional
   checksum on the outer UDP packet, when protected, is seen to be an
   unwarranted burden on the nodes implementing lightweight tunneling
   protocols, especially if the inner packet(s) are already protected by
   a checksum.  In IPv4, there is a checksum on the IP packet itself,
   and the checksum on the outer UDP packet can be set to zero.  However
   in IPv6 there is not a checksum on the IP packet and RFC 2460
   [RFC2460] explicitly states that IPv6 receivers MUST discard UDP
   packets with a 0 checksum.  So, while sending a UDP packet with a 0
   checksum is permitted in IPv4 packets, it is explicitly forbidden in
   IPv6 packets.  In order to meet the needs of the deployers of IPv6
   UDP tunnels, this document modifies RFC 2460 to allow for the
   ignoring of UDP checksums under constrained situations (IPv6
   tunneling where the inner packet exists and has a checksum), based on
   the considerations set forth in [I-D.ietf-6man-udpzero].

   While the origin of this I-D is the problem raised by the draft
   titled "Automatic IP Multicast Without Explicit Tunnels", also known
   as "AMT," [I-D.ietf-mboned-auto-multicast] we expect it to have wide
   applicability, immediately to AMT and LISP [I-D.ietf-lisp], and in
   the future to other tunneling protocols to come out of Softwires and
   other IETF Working Groups.

   Since the first version of this document, the need for an efficient,
   lightweight UDP tunneling mechanism has increased.  Indeed, other
   workgroups, notably LISP [I-D.ietf-lisp] and Softwires [RFC5619] have
   also expressed a need to have exceptions to the RFC 2460 prohibition.

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   Other users of UDP as a tunneling protocol, for example, L2TP and
   Softwires may benefit from a relaxation of the RFC 2460 restriction.

   The third version of this document benefited from a close read by
   Magnus Westerlund and Gorry Fairhurst.

2.  Some Terminology

   For the remainder of this document, we discuss only IPv6, since this
   problem does not exist for IPv4.  So any reference to 'IP' should be
   understood as a reference to IPv6.

   Although we will try to avoid them when possible, we may use the
   terms "tunneling" and "tunneled" as adjectives when describing
   packets.  When we refer to 'tunneling packets' we refer to the outer
   packet header that provides the tunneling function.  When we refer to
   'tunneled packets' we refer to the inner packet, i.e. the packet
   being carried in the tunnel.

3.  Problem Statement

   The argument is that since in the case of AMT multicast packets
   already have a UDP header with a checksum, there is no additional
   benefit and indeed some cost to nodes to both compute and check the
   UDP checksum of the outer (encapsulating) header.  Consequently, IPv6
   should make an exception to the rule that the UDP checksum MUST not
   be 0, and allow tunneling protocols to set the checksum field of the
   outer header only to 0 and skip both the sender and receiver

4.  Discussion

   [I-D.ietf-6man-udpzero] describes the issues related to allowing UDP
   over IPv6 to have a valid checksum of zero and is not repeated here.

   In Section 5.1 of [I-D.ietf-6man-udpzero], the authors propose nine
   (9) constraints on the usage of a zero checksum for UDP over IPv6.
   We agree with the restrictions proposed, and in fact proposed some of
   those restrictions ourselves in the previous version of the current
   draft.  These restrictions are incorporated into the proposed changes

   As has been pointed out in [I-D.ietf-6man-udpzero] and in various
   mailing list discussions, there is still the possibility of deep-
   inspection firewall devices or other middleboxes actually checking

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   the UDP checksum field of the outer packet and thereby discarding the
   tunneling packets.  This is would be an issue also for legacy systems
   which have not implemented the change in the IPv6 specification.  So
   in any case, there may be packet loss of lightweight tunneling
   packets because of mixed new-rule and old-rule nodes.

   As an example, we discuss how can errors be detected and handled in a
   lightweight UDP tunneling protocol when the checksum protection is
   disabled.  Note that other (non-tunneling) protocols may have
   different approaches.  We suggest that the following could be an
   approach to this problem:

   o  Context (i.e. tunneling state) should be established via
      application PDUs that are carried in checksummed UDP packets.
      That is, any control packets flowing between the tunnel endpoints
      should be protected by UDP checksums.  The control packets can
      also contain any negotiation that is necessary to set up the
      endpoint/adapters to accept UDP packets with a zero checksum.

   o  Only UDP packets containing tunneled packets should have a UDP
      checksum equal to zero.

   o  UDP keep-alive packets with checksum zero can be sent to validate
      paths, given that paths between tunnel endpoints can change and so
      middleboxes in the path may vary during the life of the
      association.  Paths with middleboxes that are intolerant of a UDP
      checksum of zero will drop the keep-alives and the endpoints will
      discover that.  Note that this need only be done per tunnel
      endpoint pair, not per tunnel context.  Keep-alive traffic SHOULD
      include both packets with tunnel checksums and packets with
      checksums equal to zero to enable the remote end to distinguish
      between path failures and the blockage of packets with checksum
      equal to zero.

   o  Corruption of the encapsulating IPv6 source address, destination
      address and/or the UDP source port, destination port fields : If
      the 9 restrictions in [I-D.ietf-6man-udpzero] are followed, the
      inner packets (tunneled packets) should be protected and run the
      usual (presumably small) risk of having undetected corruption(s).
      If lightweight tunneling protocol contexts contain (at a minimum)
      source and destination IP addresses and source and destination
      ports, there are 16 possible corruption outcomes.  We note that
      these outcomes not equally likely, as most require multiple bit
      errors with errored bits in separate fields.  The possible
      corruption outcomes fall out this way:

      *  Half of the 16 possible corruption combinations have a
         corrupted destination address.  If the incorrect destination is

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         reached and the node doesn't have an application for the
         destination port, the packet will be dropped.  If the
         application at the incorrect destination is the same
         lightweight tunneling protocol and if it has a matching context
         (which can be assumed to be a very low probability event) the
         inner packet will be decapsulated and forwarded.  If it is some
         other application, with very high probability, the application
         will not recognize the contents of the packet.

      *  Half of the 8 possible corruption combinations with a correct
         destination address have a corrupted source address.  If the
         tunnel contexts contain all elements of the address-port
         4-tuple, then the likelihood is that this corruption will be

      *  Of the remaining 4 possibilities, with valid source and
         destination IPv6 addresses, 1 has all 4 fields valid, the other
         three have one or both ports corrupted.  Again, if the
         tunneling endpoint context contains sufficient information,
         these error should be detected with high probability.

   o  Corruption of source-fragmented encapsulating packets: In this
      case, a tunneling protocol may reassemble fragments associated
      with the wrong context at the right tunnel endpoint, or it may
      reassemble fragments associated with a context at the wrong tunnel
      endpoint, or corrupted fragments may be reassembled at the right
      context at the right tunnel endpoint.  In each of these cases, the
      IPv6 length of the encapsulating header may be checked (though
      [I-D.ietf-6man-udpzero] points out the weakness in this check).
      In addition, if the encapsulated packet is protected by a
      transport (or other) checksum, these errors can be detected (with
      some probability).

   While this is not a perfect solution, it can reduce the risks of
   relaxing the UDP checksum requirement for IPv6.

5.  The Zero-Checksum Solution

   The solution to the overhead associated with UDP packets carrying
   encapsulated tunnel traffic is to allow a UDP checksum of zero on the
   outer encapsulating packet of a lightweight tunneling protocol.  UDP
   endpoints that implement this solution MUST change their behavior and
   not discard UDP packets received with a 0 checksum on the outer
   packet of tunneling protocols.  If this is done constraints in
   Section 5.1 of [I-D.ietf-6man-udpzero] also MUST be adopted.

   Specifically, the text in [RFC2460] Section 8.1, 4th bullet is

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   amended.  We refer to the following text:

   "Unlike IPv4, when UDP packets are originated by an IPv6 node, the
   UDP checksum is not optional.  That is, whenever originating a UDP
   packet, an IPv6 node must compute a UDP checksum over the packet and
   the pseudo-header, and, if that computation yields a result of zero,
   it must be changed to hex FFFF for placement in the UDP header.  IPv6
   receivers must discard UDP packets containing a zero checksum, and
   should log the error."

   This item should be taken out of the bullet list and should be
   modified as follows:

      Whenever originating a UDP packet, an IPv6 node SHOULD compute a
      UDP checksum over the packet and the pseudo-header, and, if that
      computation yields a result of zero, it must be changed to hex
      FFFF for placement in the UDP header.  IPv6 receivers SHOULD
      discard UDP packets containing a zero checksum, and SHOULD log the
      error.  However, some protocols, such as lightweight tunneling
      protocols that use UDP as a tunnel encapsulation, MAY omit
      computing the UDP checksum of the encapsulating UDP header and set
      it to zero, subject to the constraints described in
      [I-D.ietf-6man-udpzero].  In cases where the encapsulating
      protocol uses a zero checksum for UDP, the receiver of packets
      sent to a port enabled to receive zero-checksum packets MUST NOT
      discard packets solely for having a UDP checksum of zero.  Note
      that these constraints apply only to encapsulating protocols that
      omit calculating the UDP checksum and set it to zero.  An
      encapsulating protocol can always choose to compute the UDP
      checksum, in which case, its behavior should be as specified

      1.  IPv6 protocol stack implementations SHOULD NOT by default
          allow the new method.  The default node receiver behavior MUST
          discard all IPv6 packets carrying UDP packets with a zero

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

      3.  RFC 2460 specifies that IPv6 nodes should log UDP datagrams
          with a zero-checksum.  A port for which zero-checksum has been

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          enabled MUST NOT log zero-checksum datagrams for that reason
          (of course, there might be other reasons to log such packets).

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

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

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

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

      8.  If a method proposes recursive tunnels, it MUST provide
          guidance that is appropriate for all use-cases.  Restrictions
          may be needed to the use of a tunnel encapsulations and the
          use of recursive tunnels (e.g.  Necessary when the endpoint is
          not verified).

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

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      Middleboxes MUST allow IPv6 packets with UDP checksum equal to
      zero to pass.  Implementations of middleboxes MAY allow
      configuration of specific port ranges for which a zero UDP
      checksum is valid and may drop IPv6 UDP packets outside those

6.  Additional Observations

   The persistence of this issue among a significant number of protocols
   being developed in the IETF requires a definitive policy.  The
   authors would like to make the following observations:

   o  An empirically-based analysis of the probabilities of packet
      corruptions (with or without checksums) has not (to our knowledge)
      been conducted since about 2000.  It is now 2011.  We strongly
      suggest that an empirical study is in order, along with an
      extensive analysis of IPv6 header corruption probabilities.

   o  A key cause of this issue generally is the lack of protocol
      support in middleboxes.  Specifically, new protocols, such as LISP
      [I-D.ietf-lisp], are being forced to use UDP tunnels just to
      traverse an end-to-end path successfully and avoid having their
      packets dropped by middleboxes.  If this were not the case, the
      use of UDP-lite [RFC3828] might become more viable for some (but
      not necessarily all) lightweight tunneling protocols.

   o  Another cause of this issue is that the UDP checksum is overloaded
      with the task of protecting the IPv6 header for UDP flows (as it
      the TCP checksum for TCP flows).  Protocols that do not use a
      pseudo-header approach to computing a checksum or CRC have
      essentially no protection from mis-delivered packets.

7.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

8.  Security Considerations

   It is of course less work to generate zero-checksum attack packets
   than ones with full UDP checksums.  However, this does not lead to
   any significant new vulnerabilities as checksums are not a security
   measure and can be easily generated by any attacker, as properly

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   configured tunnels should check the validity of the inner packet and
   perform any needed security checks, regardless of the checksum
   status, and finally as most attacks are generated from compromised
   hosts which automatically create checksummed packets (in other words,
   it would generally be more, not less, effort for most attackers to
   generate zero UDP checksums on the host).

9.  Acknowledgements

   We would like to thank Brian Haberman, Magnus Westerlund and Gorry
   Fairhurst for discussions and reviews.

10.  References

10.1.  Normative References

   [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.

   [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.

   [RFC5619]  Yamamoto, S., Williams, C., Yokota, H., and F. Parent,
              "Softwire Security Analysis and Requirements", RFC 5619,
              August 2009.

10.2.  Informative References

              Fairhurst, G. and M. Westerlund, "IPv6 UDP Checksum
              Considerations", draft-ietf-6man-udpzero-05 (work in
              progress), December 2011.

              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-22 (work in progress), February 2012.

              Bumgardner, G. and T. Morin, "Automatic Multicast
              Tunneling", draft-ietf-mboned-auto-multicast-12 (work in
              progress), February 2012.

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Authors' Addresses

   Marshall Eubanks
   AmericaFree.TV LLC
   P.O. Box 141
   Clifton, Virginia  20124

   Phone: +1-703-501-4376

   P.F. Chimento
   Johns Hopkins University Applied Physics Laboratory
   11100 Johns Hopkins Road
   Laurel, MD  20723

   Phone: +1-443-778-1743

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