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Internet Engineering Task Force (IETF)                        M. EubanksRequest for Comments: 6935                            AmericaFree.TV LLCUpdates:2460                                                P. ChimentoCategory: Standards Track               Johns Hopkins University AppliedISSN: 2070-1721                                       Physics Laboratory                                                           M. Westerlund                                                                Ericsson                                                              April 2013IPv6 and UDP Checksums for Tunneled PacketsAbstract   This document updates the IPv6 specification (RFC 2460) to improve   performance when a tunnel protocol uses UDP with IPv6 to tunnel   packets.  The performance improvement is obtained by relaxing the   IPv6 UDP checksum requirement for tunnel protocols whose header   information is protected on the "inner" packet being carried.   Relaxing this requirement removes the overhead associated with the   computation of UDP checksums on IPv6 packets that carry the tunnel   protocol packets.  This specification describes how the IPv6 UDP   checksum requirement can be relaxed when the encapsulated packet   itself contains a checksum.  It also describes the limitations and   risks of this approach and discusses the restrictions on the use of   this method.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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6935.Eubanks, et al.              Standards Track                    [Page 1]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .42.1.  Requirements Language  . . . . . . . . . . . . . . . . . .43.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .44.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .44.1.  Analysis of Corruption in Tunnel Context . . . . . . . . .54.2.  Limitation to Tunnel Protocols . . . . . . . . . . . . . .74.3.  Middleboxes  . . . . . . . . . . . . . . . . . . . . . . .85.  The Zero UDP Checksum Update . . . . . . . . . . . . . . . . .96.  Additional Observations  . . . . . . . . . . . . . . . . . . .107.  Security Considerations  . . . . . . . . . . . . . . . . . . .108.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .119.  References . . . . . . . . . . . . . . . . . . . . . . . . . .119.1.  Normative References . . . . . . . . . . . . . . . . . . .119.2.  Informative References . . . . . . . . . . . . . . . . . .11Eubanks, et al.              Standards Track                    [Page 2]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 20131.  Introduction   This document constitutes an update of the IPv6 specification   [RFC2460] for cases 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 new protocol on a critical mass of   routers, middleboxes, and hosts on the global Internet 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.  The result is 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 the   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 tunnel protocol,   especially if the inner packets are already protected by a checksum.   IPv4 has a checksum over the IP packet header, and the checksum on   the outer UDP packet may be set to zero.  However, IPv6 has no   checksum in the IP header, andRFC 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 updatesRFC 2460 to allow   endpoints to use a zero UDP checksum under constrained situations   (primarily for IPv6 tunnel transports that carry checksum-protected   packets), following the applicability statements and constraints in   [RFC6936].   When reading this document, the advice in "Unicast UDP Usage   Guidelines for Application Designers" [RFC5405] is applicable.  It   discusses both UDP tunnels (Section 3.1.3) and the usage of checksums   (Section 3.4).   While the origin of this specification is the problem raised by the   draft titled "Automatic Multicast Tunnels", also known as "AMT"   [AMT], we expect it to have wide applicability.  Since the first   draft of this RFC was written, the need for an efficient UDP   tunneling mechanism has increased.  Other IETF Working Groups,   notably LISP [RFC6830] and Softwires [RFC5619], have expressed a needEubanks, et al.              Standards Track                    [Page 3]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 2013   to update the UDP checksum processing inRFC 2460.  We therefore   expect this update to be applicable in the future to other tunnel   protocols specified by these and other IETF Working Groups.2.  Terminology   This document discusses only IPv6, because the problem being   addressed does not exist for IPv4.  Therefore, all references to "IP"   should be understood as references 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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 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, the cost of the   computation may outweigh the benefit of the checksum.  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" [RFC6936] describes issues related to allowing UDP over   IPv6 to have a valid zero UDP checksum and is the starting point for   this discussion.  Sections4 and5 of [RFC6936], respectively,   identify node implementation and usage requirements for datagrams   sent and received with a zero UDP checksum.  These sections introduce   constraints on the usage of a zero checksum for UDP over IPv6.  The   remainder of this section analyzes the use of general tunnels and   explains the motivations for why tunnel protocols are being permitted   to use the method described in this update.  It also discusses issues   with middleboxes.Eubanks, et al.              Standards Track                    [Page 4]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 20134.1.  Analysis of Corruption in Tunnel Context   This section analyzes the impact of the different corruption modes in   the context of a tunnel protocol.  It specifies what needs to be   considered by the designer and user of a tunnel protocol for the   protocol to be robust.  It also summarizes why use of a zero UDP   checksum is thought to be 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 using other protocols that provide      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.   o  Keepalive 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 keepalives.  To enable the tunnel endpoints to discover and      react to this behavior in a timely way, the keepalive traffic      should include datagrams with a non-zero checksum and datagrams      with a zero checksum.   o  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 check.      *  If the datagram header matches the 5-tuple and the node has         enabled the zero checksum 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         node has enabled zero checksum for the port, the datagram         payload is matched to the wrong context and may be processed by         the wrong tunnel protocol, provided that it also passes the         verification of that protocol.Eubanks, et al.              Standards Track                    [Page 5]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 2013      *  If a corrupted datagram matches a 5-tuple and node has not         enabled the zero checksum for this port, the datagram will be         discarded.      When only the source information is corrupted, the datagram could      arrive at the intended applications or 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 also decreases      the likelihood of matching a context.  However, the exception is      when errors replace one packet header with another, so both      packets could be tunneled, and therefore the corrupted packet      could match a previously defined context.   o  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, 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      (although [RFC6936] 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  Compared to other applications, tunnel protocols using UDP have      some advantages that reduce the risk for a corrupted tunnel packet      reaching a destination that will receive it.  These advantages      result from processing by the network of the inner (tunneled)      packet after it is forwarded from the tunnel egress using a wrong      context:      *  A tunneled packet may be forwarded to the wrong address domain,         for example, to a private address domain where the inner         packet's address is not routable, or it 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 be         reachable at all from the delivered domain.  An example is an         Ethernet frame where the destination MAC address is not present         on the LAN segment that was reached.Eubanks, et al.              Standards Track                    [Page 6]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 2013      *  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.   Each of these checks significantly reduces 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 risks 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 ones that are corrupted in tunnels or corrupted by other      usage of the zero checksum.  This will require an integrity      mechanism.  Using a standard UDP checksum reduces the      computational load in the receiver that is necessary 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 no functions other than a transport      from tunnel ingress to tunnel egress.  A corruption will commonly      affect only the single tunneled packet, not the establishedEubanks, et al.              Standards Track                    [Page 7]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 2013      protocol state.  One common effect is that the inner packet flow      will see only a corruption and a misdelivery of the outer packet      as a lost packet.   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 it is      necessary 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; without      this, the server may spend significant resources processing the      packet, including sending replies or error messages.   Tunnel protocols that encapsulate IP will generally be safe for   deployment, because 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, further analysis is necessary   to understand the implications of misdelivery of corrupted packets   for 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 side   effects.   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 in the face   of misdelivery.4.3.  Middleboxes   "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero   Checksums" [RFC6936] specifies requirements for middleboxes and   tunnels that need to traverse middleboxes.  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.  This is   especially important as middleboxes that conform toRFC 2460 are   likely to discard datagrams with a zero UDP checksum.Eubanks, et al.              Standards Track                    [Page 8]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 20135.  The Zero UDP 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" [RFC6936].   The following text in[RFC2460], Section 8.1, fourth bullet should be   deleted:      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 used UDP 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,      the checksum MUST be changed to hex FFFF for placement in the UDP      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 zero-checksum mode for a specific port      (or set of ports) for sending and/or receiving.  Any node      implementing zero-checksum mode MUST follow the node requirements      specified inSection 4 of "Applicability Statement for the use of      IPv6 UDP Datagrams with Zero Checksums" [RFC6936].      Any protocol that enables zero-checksum mode for a specific port      or ports MUST follow the usage requirements specified inSection 5      of "Applicability Statement for the Use of IPv6 UDP Datagrams with      Zero Checksums" [RFC6936].      Middleboxes supporting IPv6 MUST follow requirements 9, 10, and 11      of the usage requirements specified inSection 5 of "Applicability      Statement for the Use of IPv6 UDP Datagrams with Zero Checksums"      [RFC6936].Eubanks, et al.              Standards Track                    [Page 9]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 20136.  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 2013.  We strongly suggest that a new empirical study be      performed, along with extensive analysis of the corruption      probabilities of the IPv6 header.  This could 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 [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.  UDP-Lite 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.  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.   In general, any user of zero UDP checksums should apply the checks   and context verification that are possible to minimize the risk of   unintended traffic to reach a particular context.  This will,   however, not protect against an intentional attack that creates   packets 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 those that   calculate a checksum.  This processing overhead may need to be   considered when deciding whether to enable a tunnel and to determineEubanks, et al.              Standards Track                   [Page 10]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 2013   an acceptable rate for transmission.  This processing overhead 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 forwarding zero checksum UDP packets and cause overload   in the processing of the non-zero checksums, e.g., if this happens in   a router's slow path.  Therefore, protection mechanisms should 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.8.  Acknowledgments   We would like to thank Brian Haberman, Dan Wing, Joel 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 helping to ensure alignment between this document   and [RFC6936].9.  References9.1.  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.   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement              for the Use of IPv6 UDP Datagrams with Zero Checksums",RFC 6936, April 2013.9.2.  Informative References   [AMT]      Bumgardner, G.,"Automatic Multicast Tunneling", 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.Eubanks, et al.              Standards Track                   [Page 11]

RFC 6935         IPv6/UDP Checksums for Tunneled Packets      April 2013   [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   USA   Phone: +1-703-501-4376   EMail: marshall.eubanks@gmail.com   P.F. Chimento   Johns Hopkins University Applied Physics Laboratory   11100 Johns Hopkins Road   Laurel, Maryland  20723   USA   Phone: +1-443-778-1743   EMail: Philip.Chimento@jhuapl.edu   Magnus Westerlund   Ericsson   Farogatan 6   SE-164 80 Kista   Sweden   Phone: +46 10 719 00 00   EMail: magnus.westerlund@ericsson.comEubanks, et al.              Standards Track                   [Page 12]