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Versions: 00 01 02 03 04 05 06 07 08 rfc6935             Standards Track
Network Working Group                                         M. Eubanks
Internet-Draft                                        AmericaFree.TV LLC
Updates: 2460 (if approved)                                  P. Chimento
Intended status: Standards Track        Johns Hopkins University Applied
Expires: June 14, 2013                                Physics Laboratory
                                                           M. Westerlund
                                                       December 11, 2012

              IPv6 and UDP Checksums for Tunneled Packets


   This document provides an update of the Internet Protocol version 6
   (IPv6) specification (RFC2460) to improve the performance in the use
   case when a tunnel protocol uses UDP with IPv6 to tunnel packets.
   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 IPv6 packets used to carry tunnel protocols.  The
   specification describes how the IPv6 UDP checksum requirement can be
   relaxed for the situation where the encapsulated packet itself
   contains a checksum.  The limitations and risks of this approach are
   described, and restrictions specified on the use of the method.

Status of this Memo

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

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

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

   This Internet-Draft will expire on June 14, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Some Terminology . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Analysis of Corruption in Tunnel Context . . . . . . . . .  5
     4.2.  Limitation to Tunnel Protocols . . . . . . . . . . . . . .  7
     4.3.  Middleboxes  . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  The Zero-Checksum Update . . . . . . . . . . . . . . . . . . .  8
   6.  Additional Observations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     10.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   This work constitutes an update of the Internet Protocol Version 6
   (IPv6) Specification [RFC2460], in the use case when a tunnel
   protocol uses UDP with IPv6 to tunnel packets.  With the rapid growth
   of the Internet, tunneling protocols have become increasingly
   important to enable the deployment of new protocols.  Tunneled
   protocols can be deployed rapidly, while the time to upgrade and
   deploy a critical mass of routers, middleboxes and hosts on the
   global Internet for a new protocol is now measured in decades.  At
   the same time, the increasing use of firewalls and other security-
   related middleboxes 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 middleboxes that perform filtering that is a fact
   of life on the current Internet.

   Tunnel endpoints may be routers or middleboxes aggregating traffic
   from a number of tunnel users, therefore the computation of an
   additional checksum on the outer UDP packet, may be seen as an
   unwarranted burden on nodes that implement a tunneling protocol,
   especially if the inner packet(s) are already protected by a
   checksum.  In IPv4, there is a checksum over the IP packet header,
   and the checksum on the outer UDP packet may be set to zero.  However
   in IPv6 there is no checksum in the IP header and RFC 2460 [RFC2460]
   explicitly states that IPv6 receivers MUST discard UDP packets with a
   zero checksum.  So, while sending a UDP datagram with a zero checksum
   is permitted in IPv4 packets, it is explicitly forbidden in IPv6
   packets.  To improve support for IPv6 UDP tunnels, this document
   updates RFC 2460 to allow endpoints to use a zero UDP checksum under
   constrained situations (primarily IPv6 tunnel transports that carry
   checksum-protected packets), following the applicability statements
   and constraints in [I-D.ietf-6man-udpzero].

   Unicast UDP Usage Guidelines for Application Designers [RFC5405]
   should be consulted when reading this specification.  It discusses
   both UDP tunnels (Section 3.1.3) and the usage of checksums (Section

   While the origin of this specification 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.  Since the first version of this document, the
   need for an efficient UDP tunneling mechanism has increased.  Other
   IETF Working Groups, notably LISP [I-D.ietf-lisp] and Softwires

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   [RFC5619] have expressed a need to update the UDP checksum processing
   in RFC 2460.  We therefore expect this update to be applicable in
   future to other tunneling protocols specified by these and other IETF
   Working Groups.

2.  Some Terminology

   This document discusses only IPv6, since this problem does not exist
   for IPv4.  Therefore all reference to 'IP' should be understood as a
   reference to IPv6.

   The document uses 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.

2.1.  Requirements Language

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

3.  Problem Statement

   When using tunnel protocols based on UDP, there can be both a benefit
   and a cost to computing and checking the UDP checksum of the outer
   (encapsulating) UDP transport header.  In certain cases, where
   reducing the forwarding cost is important, such as for nodes that
   perform the checksum in software, where the cost may outweigh the
   benefit.  This document provides an update for usage of the UDP
   checksum with IPv6.  The update is specified for use by a tunnel
   protocol that transports packets that are themselves protected by a

4.  Discussion

   Applicability Statement for the use of IPv6 UDP Datagrams with Zero
   Checksums [I-D.ietf-6man-udpzero] describes issues related to
   allowing UDP over IPv6 to have a valid zero UDP checksum and is the
   starting point for this discussion.  Section 4 and 5 of
   [I-D.ietf-6man-udpzero], respectively identify node implementation
   and usage requirements for datagrams sent and received with a zero
   UDP checksum.  These introduce constraints on the usage of a zero
   checksum for UDP over IPv6.  The remainder of this section analyses

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   the use of general tunnels and motivates why tunnel protocols are
   being permitted to use the method described in this update.  Issues
   with middleboxes are also discussed.

4.1.  Analysis of Corruption in Tunnel Context

   This section analyzes the impact of the different corruption modes in
   the context of a tunnel protocol.  It indicates what needs to be
   considered by the designer and user of a tunnel protocol to be
   robust.  It also summarizes why use of a zero UDP checksum is thought
   safe for deployment.

   o  Context (i.e. tunneling state) should be established by exchanging
      application Protocol Data Units (PDUs) carried in checksummed UDP
      datagrams or by other protocols with integrity protection against
      corruption.  These control packets should also carry any
      negotiation required to enable the tunnel endpoint to accept UDP
      datagrams with a zero checksum and identify the set of ports that
      are used.  It is important that the control traffic is robust
      against corruption because undetected errors can lead to long-
      lived and significant failures that affect not only the single
      packet that was corrupted.

   o  Keep-alive datagrams with a zero UDP checksum should be sent to
      validate the network path, because the path between tunnel
      endpoints can change and therefore the set of middleboxes along
      the path may change during the life of an association.  Paths with
      middleboxes that drop datagrams with a zero UDP checksum will drop
      these keep-alives.  To enable the tunnel endpoints to discover and
      react to this behavior in a timely way, the keep-alive traffic
      should include datagrams with both a non-zero checksum and ones
      with a zero checksum.

   o  Corruption of the address information in an encapsulating packets,
      i.e.  IPv6 source address, destination address and/or the UDP
      source port, and destination port fields.  A robust tunnel
      protocol should track tunnel context based on the 5-tuple, i.e.
      the protocol and both the address and port for both the source and
      destination.  A corrupted datagram that arrives at a destination
      may be filtered based on this check.

      *  If the datagram header matches the 5-tuple with a zero checksum
         enabled, the payload is matched to the wrong context.  The
         tunneled packet will then be decapsulated and forwarded by the
         tunnel egress.

      *  If a corrupted datagram matches a different 5-tuple with a zero
         checksum enabled, the payload is matched to the wrong context,

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         and may be processed by the wrong tunneling protocol, if it
         passes the verification of that protocol.

      *  If a corrupted datagram matches a 5-tuple that does not have a
         zero checksum enabled, it will be discarded.

      When only the source information is corrupted, the datagram could
      arrive at the intended applications/protocol which will process it
      and try to match it against an existing tunnel context.  If the
      protocol restricts processing to only the source addresses with
      established contexts the likelihood that a corrupted packet enters
      a valid context is reduced.  When both source and destination
      fields are corrupted, this increases the likelihood of failing to
      match a context, with the exception of errors replacing one packet
      header with another one.  In this case it is possible that both
      are tunnels and thus the corrupted packet can match a previously
      defined context.

   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).

   o  Tunnel protocols using UDP have some advantages that reduce the
      risk for a corrupted tunnel packet reaching a destination that
      will receive it, compared to other applications.  This results
      from processing by the network of the inner (tunneled) packet
      after being forwarded from the tunnel egress using a wrong

      *  A tunneled packet may be forwarded to the wrong address domain,
         for example a private address domain where the inner packet's
         address is not routable, or may fail a source address check,
         such as Unicast Reverse Path Forwarding [RFC2827], resulting in
         the packet being dropped.

      *  The destination address of a tunneled packet may not at all be
         reachable from the delivered domain.  For example an Ethernet
         packet where the destination MAC address is not present on the
         LAN segment that was reached.

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      *  The type of the tunneled packet may prevent delivery for
         example if an IP packet payload was attempted to be interpreted
         as an Ethernet packet.  This is likely to result in the packet
         being dropped as invalid.

      *  The tunneled packet checksum or integrity mechanism may detect
         corruption of the inner packet caused at the same time as
         corruption to the outer packet header.  The resulting packet
         would likely be dropped as invalid.

   These different examples each help to significantly reduce the
   likelihood that a corrupted inner tunneled packet is finally
   delivered to a protocol listener that can be affected by the packet.
   While the methods do not guarantee correctness, they can reduce the
   risk of relaxing the UDP checksum requirement for a tunnel
   application using IPv6.

4.2.  Limitation to Tunnel Protocols

   This document describes the applicability of using a zero UDP
   checksum to support tunnel protocols.  There are good motivations
   behind this and the arguments are provided here.

   o  Tunnels carry inner packets that have their own semantics that
      makes any corruption less likely to reach the indicated
      destination and be accepted as a valid packet.  This is true for
      IP packets with the addition of verification that can be made by
      the tunnel protocol, the networks' processing of the inner packet
      headers as discussed above, and verification of the inner packet
      checksums.  Also non-IP inner packets are likely to be subject to
      similar effects that reduce the likelihood that an mis-delivered
      packet are delivered.

   o  Protocols that directly consume the payload must have sufficient
      robustness against mis-delivered packets from any context,
      including the ones that are corrupted in tunnels and any other
      usage of the zero checksum.  This will require an integrity
      mechanism.  Using a standard UDP checksum reduces the
      computational load in the receiver to verify this mechanism.

   o  Stateful protocols or protocols where corruption causes cascade
      effects need to be extra careful.  In tunnel usage each
      encapsulating packet provides only a transport mechanism from
      tunnel ingress to tunnel egress.  A corruption will commonly only
      effect the single packet, not established protocol state.  One
      common effect is that the inner packet flow will only see a
      corruption and mis-delivery of the outer packet as a lost packet.

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   o  Some non-tunnel protocols operate with general servers that do not
      know from where they will receive a packet.  In such applications,
      the usage of a zero UDP checksum is especially unsuitable because
      there is a need to provide the first level of verification that
      the packet was intended for the server.  This verification
      prevents the server from processing the datagram payload and spend
      any significant amount of resources on it, including sending
      replies or error messages.

   Tunnel protocols encapsulating IP this will generally be safe, since
   all IPv4 and IPv6 packets include at least one checksum at either the
   network or transport layer and the network delivery of the inner
   packet will further reduce the effects of corruption.  Tunnel
   protocols carrying non-IP packets may provide equivalent protection
   due to the non-IP networks reducing the risk of delivery to
   applications.  However, there is need for further analysis to
   understand the implications of mis-delievery of corrupted packets for
   that each non-IP protocol.  The analysis above suggests that non-
   tunnel protocols can be expected to have significantly more cases
   where a zero checksum would result in mis-delivery or negative side-

   One unfortunate side-effect of increased use of a zero-checksum is
   that it also increases the likelihood of acceptance when a datagram
   with a zero UDP checksum is mis-delivered.  This requires all tunnel
   protocols using this method to be designed to be robust to mis-

4.3.  Middleboxes

   Applicability Statement for the use of IPv6 UDP Datagrams with Zero
   Checksums [I-D.ietf-6man-udpzero] notes that middlebox devices that
   conform to RFC 2460 will discard datagrams with a zero UDP checksum
   and should log this as an error.  Thus tunnel protocols intending to
   use a zero UDP checksum needs to ensure that they have defined a
   method for handling cases when a middlebox prevents the path between
   the tunnel ingress and egress from supporting transmission of
   datagrams with a zero UDP checksum.

5.  The Zero-Checksum Update

   This specification updates IPv6 to allow a zero UDP checksum in the
   outer encapsulating datagram of a tunneling protocol.  UDP endpoints
   that implement this update MUST follow the node requirements
   "Applicability Statement for the use of IPv6 UDP Datagrams with Zero
   Checksums" [I-D.ietf-6man-udpzero].

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   The following text in [RFC2460] Section 8.1, 4th bullet should be

   "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 text should be replaced by:

      Whenever originating a UDP packet in the default mode, 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 by default discard UDP packets containing a zero
      checksum, and SHOULD log the error.  As an alternative usage for
      some protocols, such as protocols that use UDP as a tunnel
      encapsulation, MAY enable the zero-checksum mode for specific sets
      of ports.  Any node implementing the zero-checksum mode MUST
      follow the node requirements specified in Section 4 of
      Applicability Statement for the use of IPv6 UDP Datagrams with
      Zero Checksums [I-D.ietf-6man-udpzero].

      Any protocol using the zero-checksum mode MUST follow the usage
      requirements specified in Section 5 of Applicability Statement for
      the use of IPv6 UDP Datagrams with Zero Checksums

      Middleboxes supporting IPv6 MUST follow the requirements 9, 10 and
      11 of the usage requirements specified in Section 5 of
      Applicability Statement for the use of IPv6 UDP Datagrams with
      Zero Checksums [I-D.ietf-6man-udpzero].

6.  Additional Observations

   This update was motivated by the existence of a number of protocols
   being developed in the IETF that are expected to benefit from the
   change.  The following observations are made:

   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.  At the time of publication, it
      is now 2012.  We strongly suggest a new empirical study, along
      with an extensive analysis of the corruption probabilities of the

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      IPv6 header.

   o  A key motivation for the increase in use of UDP in tunneling is a
      lack of protocol support in middleboxes.  Specifically, new
      protocols, such as LISP [I-D.ietf-lisp], may prefer to use UDP
      tunnels to traverse an end-to-end path successfully and avoid
      having their packets dropped by middleboxes.  If middleboxes were
      updated to support UDP-Lite [RFC3828], this would provide better
      protection than offered by this update.  This may be suited to a
      variety of applications and would be expected to be preferred over
      this method for many tunnel protocols.

   o  Another issue is that the UDP checksum is overloaded with the task
      of protecting the IPv6 header for UDP flows (as is 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

   Less work is required required to generate an attack using a zero UDP
   checksum than one using a standard full UDP checksum.  However, this
   does not lead to significant new vulnerabilities because checksums
   are not a security measure and can be easily generated by any
   attacker.  Properly configured tunnels should check the validity of
   the inner packet and perform security checks.

9.  Acknowledgements

   We would like to thank Brian Haberman, Dan Wing, Joel Halpern and the
   IESG of 2012 for discussions and reviews.  Gorry Fairhurst has been
   very diligent in reviewing and help ensuring alignment between this
   document and [I-D.ietf-6man-udpzero].

10.  References

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10.1.  Normative References

              Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the use of IPv6 UDP Datagrams with Zero Checksums",
              draft-ietf-6man-udpzero-07 (work in progress),
              October 2012.

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

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

10.2.  Informative References

              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-24 (work in progress), November 2012.

              Bumgardner, G., "Automatic Multicast Tunneling",
              draft-ietf-mboned-auto-multicast-14 (work in progress),
              June 2012.

   [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.

   [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.

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

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

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Authors' Addresses

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

   Phone: +1-703-501-4376
   Email: marshall.eubanks@gmail.com

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

   Phone: +1-443-778-1743
   Email: Philip.Chimento@jhuapl.edu

   Magnus Westerlund
   Farogatan 6
   SE-164 80 Kista

   Phone: +46 10 714 82 87
   Email: magnus.westerlund@ericsson.com

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