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: July 21, August 25, 2013                              Physics Laboratory
                                                           M. Westerlund
                                                        January 17,
                                                       February 21, 2013

              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 where a tunnel protocol uses UDP with IPv6 to tunnel packets.
   The performance improvement is obtained by relaxing the IPv6 UDP
   checksum requirement for any suitable tunnel 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.

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   This Internet-Draft will expire on July 21, August 25, 2013.

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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 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     10.2. Informative References . . . . . . . . . . . . . . . . . . 11 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

1.  Introduction

   This work constitutes an update of the Internet Protocol Version 6
   (IPv6) Specification [RFC2460], in the use case where a tunnel
   protocol uses UDP with IPv6 to tunnel packets.  With the rapid growth
   of the Internet, tunnel protocols have become increasingly important
   to enable the deployment of new protocols.  Tunnel 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 tunnel
   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

   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 tunnel 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 Multicast 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] [RFC6830] and Softwires [RFC5619] have
   expressed a need to update the UDP checksum processing in RFC 2460.
   We therefore expect this update to be applicable in the future to
   other tunnel protocols specified by these and other IETF Working

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, reducing the
   forwarding cost is important, e.g., for nodes that perform the
   checksum in software 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 checksum.

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.  Sections 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
   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
   to be safe for deployment.

   1.  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 may affect much more
       than the single packet that was corrupted.

   2.  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 a non-zero checksum
       and datagrams with a zero checksum.

   3.  Receivers should attempt to detect corruption of the address
       information in an encapsulating packet.  A robust tunnel protocol
       should track tunnel context based on the 5-tuple (tunneled
       protocol number, IPv6 source address, IPv6 destination address,
       UDP source port, UDP destination port).  A corrupted datagram
       that arrives at a destination may be filtered based on this

       *  If the datagram header matches the 5-tuple and the node has
          the zero checksum enabled for this port, 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 and the
          zero checksum was enabled for the port, the datagram payload
          is matched to the wrong context, and may be processed by the
          wrong tunnel protocol, if it also passes the verification of
          that protocol.

       *  If a corrupted datagram matches a 5-tuple and the zero
          checksum has not been enabled for this port, the datagram will
          be discarded.

       When only the source information is corrupted, the datagram could
       arrive at the intended applications/protocol, which will process
       the datagram and try to match it against an existing tunnel
       context.  The likelihood that a corrupted packet enters a valid
       context is reduced when the protocol restricts processing to only
       the source addresses with established contexts.  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 packets are tunnelled and
       therefore the corrupted packet could match a previously defined

   4.  Receivers should attempt to detect corruption of source-
       fragmented encapsulating packets.  A tunnel 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).

   5.  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
          frame where the destination MAC address is not present on the
          LAN segment that was reached.

       *  The type of the tunneled packet may prevent delivery.  For
          example, an attempt to interpret an IP packet payload as an
          Ethernet frame, would 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 checks each 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, which
      may make 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 network processing of the inner packet
      headers as discussed above, and verification of the inner packet
      checksums.  Non-IP inner packets are likely to be subject to
      similar effects that may reduce the likelihood of a misdelivered
      packet being delivered to a protocol listener that can be affected
      by the packet.

   o  Protocols that directly consume the payload must have sufficient
      robustness against misdelivered 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  The design for stateful protocols or protocols where corruption
      causes cascade effects requires extra care.  In tunnel usage, each
      encapsulating packet provides only a transport mechanism from
      tunnel ingress to tunnel egress.  A corruption will commonly only
      affect the single tunneled packet, not the established protocol
      state.  One common effect is that the inner packet flow will only
      see a corruption and misdelivery of the outer packet as a lost

   o  Some non-tunnel protocols operate with general servers that do not
      know the source from which they will receive a packet.  In such
      applications, a zero UDP checksum is unsuitable because there is a
      need to provide the first level of verification that the packet
      was intended for the receiving server.  A verification prevents
      the server from processing the datagram payload and without this
      it may spend significant resources processing the packet,
      including sending replies or error messages.

   Tunnel protocols that encapsulate IP will generally be safe for
   deployment, since all IPv4 and IPv6 packets include at least one
   checksum at either the network or transport layer.  The network
   delivery of the inner packet will then further reduce the effects of
   corruption.  Tunnel protocols carrying non-IP packets may offer
   equivalent protection when the non-IP networks reduce the risk of
   misdelivery to applications.  However, there is a need for further
   analysis to understand the implications of misdelievery 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 misdelivery or negative

   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 misdelivered.  This requires all tunnel
   protocols using this method to be designed to be robust to

4.3.  Middleboxes

   "Applicability Statement for the use of IPv6 UDP Datagrams with Zero
   Checksums" [I-D.ietf-6man-udpzero] notes that middleboxes that
   conform to RFC 2460 will discard datagrams with a zero UDP checksum
   and should log this as an error.  Tunnel protocols intending to use a
   zero UDP checksum need 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 tunnel protocol.  UDP endpoints
   that implement this update MUST follow the node requirements in
   "Applicability Statement for the use of IPv6 UDP Datagrams with Zero
   Checksums" [I-D.ietf-6man-udpzero].

   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:

      An IPv6 node associates a mode with each active used UDP port. port (for
      sending and/or receiving packets).

      Whenever originating a UDP packet for a port 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. header as
      specified in [RFC2460].  IPv6 receivers MUST by default discard
      UDP packets containing a zero checksum, and SHOULD log the error.

      As an alternative, certain protocols that use UDP as a tunnel
      encapsulation, MAY enable the zero-checksum mode for a specific
      port (or set of ports). ports) for sending and/or receiving.  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"

      Any protocol that enables the zero-checksum mode for a specific
      port or ports MUST follow 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].

      Middleboxes supporting IPv6 MUST follow 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"

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
      corruption (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
      IPv6 header.  This can potentially allow revising the
      recommendations in this document.

   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], [RFC6830], 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], UDP-Lite 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 misdelivered 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 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

   In general any user of zero UDP checksums should check apply the validity of checks and
   context verification that are possible to minimize the inner risk of
   unintended traffic to reach a particular context.  This will however
   not protect against an intended attack that create packet with the
   correct information.  Source address validation can help prevent
   injection of traffic into contexts by an attacker.

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

9.  Acknowledgements

   We would like to thank Brian Haberman, Dan Wing, Joel Halpern Halpern, David
   Waltermire, J.W. Atwood, Peter Yee, Joe Touch 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

10.  References

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-10 (work in progress),
              December 2012.
              January 2013.

   [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.

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

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

   Magnus Westerlund
   Farogatan 6
   SE-164 80 Kista

   Phone: +46 10 714 82 87