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Internet Engineering Task Force (IETF)                           F. GontRequest for Comments: 7112                           Huawei TechnologiesUpdates:2460                                                  V. ManralCategory: Standards Track                                 Ionos NetworksISSN: 2070-1721                                                R. Bonica                                                        Juniper Networks                                                            January 2014Implications of Oversized IPv6 Header ChainsAbstract   The IPv6 specification allows IPv6 Header Chains of an arbitrary   size.  The specification also allows options that can, in turn,   extend each of the headers.  In those scenarios in which the IPv6   Header Chain or options are unusually long and packets are   fragmented, or scenarios in which the fragment size is very small,   the First Fragment of a packet may fail to include the entire IPv6   Header Chain.  This document discusses the interoperability and   security problems of such traffic, and updatesRFC 2460 such that the   First Fragment of a packet is required to contain the entire IPv6   Header Chain.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/rfc7112.Gont, et al.                 Standards Track                    [Page 1]

RFC 7112         Implications of Oversized Header Chains    January 2014Copyright Notice   Copyright (c) 2014 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 ....................................................22. Requirements Language ...........................................33. Terminology .....................................................34. Motivation ......................................................45. Updates toRFC 2460 .............................................56. IANA Considerations .............................................57. Security Considerations .........................................68. Acknowledgements ................................................69. References ......................................................79.1. Normative References .......................................79.2. Informative References .....................................71.  Introduction   With IPv6, optional internet-layer information is carried in one or   more IPv6 Extension Headers [RFC2460].  Extension Headers are placed   between the IPv6 header and the Upper-Layer Header in a packet.  The   term "Header Chain" refers collectively to the IPv6 header, Extension   Headers, and Upper-Layer Header occurring in a packet.  In those   scenarios in which the IPv6 Header Chain is unusually long and   packets are fragmented, or scenarios in which the fragment size is   very small, the Header Chain may span multiple fragments.   While IPv4 had a fixed maximum length for the set of all IPv4 options   present in a single IPv4 packet, IPv6 does not have any equivalent   maximum limit at present.  This document updates the set of IPv6   specifications to create an overall limit on the size of the   combination of IPv6 options and IPv6 Extension Headers that is   allowed in a single IPv6 packet.  Namely, it updatesRFC 2460 such   that the First Fragment of a fragmented datagram is required to   contain the entire IPv6 Header Chain.Gont, et al.                 Standards Track                    [Page 2]

RFC 7112         Implications of Oversized Header Chains    January 2014   It should be noted that this requirement does not preclude the use of   large payloads but, instead, merely requires that all headers,   starting from the IPv6 base header and continuing up to the Upper-   Layer Header (e.g., TCP or the like) be present in the First   Fragment.2.  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 in [RFC2119].3.  Terminology   For the purposes of this document, the terms Extension Header, IPv6   Header Chain, First Fragment, and Upper-Layer Header are used as   follows:   Extension Header:      Extension Headers are defined inSection 4 of [RFC2460].  As a      result of [RFC7045], [IANA-PROTO] provides a list of assigned      Internet Protocol Numbers and designates which of those protocol      numbers also represent Extension Headers.   First Fragment:      An IPv6 fragment with Fragment Offset equal to 0.   IPv6 Header Chain:      The IPv6 Header Chain contains an initial IPv6 header, zero or      more IPv6 Extension Headers, and optionally, a single Upper-Layer      Header.  If an Upper-Layer Header is present, it terminates the      header chain; otherwise, the "No Next Header" value (Next Header =      59) terminates it.      The first member of the IPv6 Header Chain is always an IPv6      header.  For a subsequent header to qualify as a member of the      header chain, it must be referenced by the "Next Header" field of      the previous member of the header chain.  However, if a second      IPv6 header appears in the header chain, as is the case when IPv6      is tunneled over IPv6, the second IPv6 header is considered to be      an Upper-Layer Header and terminates the header chain.  Likewise,      if an Encapsulating Security Payload (ESP) header appears in the      header chain, it is considered to be an Upper-Layer Header, and it      terminates the header chain.Gont, et al.                 Standards Track                    [Page 3]

RFC 7112         Implications of Oversized Header Chains    January 2014   Upper-Layer Header:      In the general case, the Upper-Layer Header is the first member of      the header chain that is neither an IPv6 header nor an IPv6      Extension Header.  However, if either an ESP header, or a second      IPv6 header occur in the header chain, they are considered to be      Upper-Layer Headers, and they terminate the header chain.      Neither the upper-layer payload, nor any protocol data following      the upper-layer payload, is considered to be part of the IPv6      Header Chain.  In a simple example, if the Upper-Layer Header is a      TCP header, the TCP payload is not part of the IPv6 Header Chain.      In a more complex example, if the Upper-Layer Header is an ESP      header, neither the payload data, nor any of the fields that      follow the payload data in the ESP header are part of the IPv6      Header Chain.4.  Motivation   Many forwarding devices implement stateless firewalls.  A stateless   firewall enforces a forwarding policy on a packet-by-packet basis.   In order to enforce its forwarding policy, the stateless firewall may   need to glean information from both the IPv6 and upper-layer headers.   For example, assume that a stateless firewall discards all traffic   received from an interface unless it is destined for a particular TCP   port on a particular IPv6 address.  When this firewall is presented   with a fragmented packet that is destined for a different TCP port,   and the entire header chain is contained within the First Fragment,   the firewall discards the First Fragment and allows subsequent   fragments to pass.  Because the First Fragment was discarded, the   packet cannot be reassembled at the destination.  Insomuch as the   packet cannot be reassembled, the forwarding policy is enforced.   However, when the firewall is presented with a fragmented packet and   the header chain spans multiple fragments, the First Fragment does   not contain enough information for the firewall to enforce its   forwarding policy.  Lacking sufficient information, the stateless   firewall either forwards or discards that fragment.  Regardless of   the action that it takes, it may fail to enforce its forwarding   policy.Gont, et al.                 Standards Track                    [Page 4]

RFC 7112         Implications of Oversized Header Chains    January 20145.  Updates toRFC 2460   When a host fragments an IPv6 datagram, it MUST include the entire   IPv6 Header Chain in the First Fragment.   A host that receives a First Fragment that does not satisfy the   above-stated requirement SHOULD discard the packet and SHOULD send an   ICMPv6 error message to the source address of the offending packet   (subject to the rules for ICMPv6 errors specified in [RFC4443]).   However, for backwards compatibility, implementations MAY include a   configuration option that allows such fragments to be accepted.   Likewise, an intermediate system (e.g., router or firewall) that   receives an IPv6 First Fragment that does not satisfy the above-   stated requirement MAY discard that packet, and it MAY send an ICMPv6   error message to the source address of the offending packet (subject   to the rules for ICMPv6 error messages specified in [RFC4443]).   Intermediate systems having this capability SHOULD support   configuration (e.g., enable/disable) of whether or not such packets   are dropped by the intermediate system.   If a host or intermediate system discards a First Fragment because it   does not satisfy the above-stated requirement and sends an ICMPv6   error message due to the discard, then the ICMPv6 error message MUST   be Type 4 ("Parameter Problem") and MUST use Code 3 ("First Fragment   has incomplete IPv6 Header Chain").  The Pointer field contained by   the ICMPv6 Parameter Problem message MUST be set to zero.  The format   for the ICMPv6 error message is the same regardless of whether a host   or intermediate system originates it.   As a result of the above-mentioned requirement, a packet's header   chain length cannot exceed the Path MTU associated with its   destination.  Hosts discover the Path MTU using procedures such as   those defined in [RFC1981] and [RFC4821].  Hosts that do not discover   the Path MTU MUST limit the IPv6 Header Chain length to 1280 bytes.   Limiting the IPv6 Header Chain length to 1280 bytes ensures that the   header chain length does not exceed the IPv6 minimum MTU [RFC2460].6.  IANA Considerations   IANA has added the following "Type 4 - Parameter Problem" message to   the "Internet Control Message Protocol version 6 (ICMPv6) Parameters"   registry:      CODE     NAME/DESCRIPTION       3       IPv6 First Fragment has incomplete IPv6 Header ChainGont, et al.                 Standards Track                    [Page 5]

RFC 7112         Implications of Oversized Header Chains    January 20147.  Security Considerations   No new security exposures or issues are raised by this document.   This document describes how undesirably fragmented packets can be   leveraged to evade stateless packet filtering.  Having made that   observation, this document updates [RFC2460] so that undesirably   fragmented packets are forbidden.  Therefore, a security   vulnerability is removed.   This specification allows nodes that drop the aforementioned packets   to signal such packet drops with ICMPv6 "Parameter Problem, IPv6   First Fragment has incomplete IPv6 header chain" (Type 4, Code 3)   error messages.   As with all ICMPv6 error/diagnostic messages, deploying Source   Address Forgery Prevention filters helps reduce the chances of an   attacker successfully performing a reflection attack by sending   forged illegal packets with the victim's/target's IPv6 address as the   IPv6 source address of the illegal packet [RFC2827] [RFC3704].   A firewall that performs stateless deep packet inspection (i.e.,   examines application payload content) might still be unable to   correctly process fragmented packets, even if the IPv6 Header Chain   is not fragmented.8.  Acknowledgements   The authors would like to thank Ran Atkinson for contributing text   and ideas that were incorporated into this document.   The authors would like to thank (in alphabetical order) Ran Atkinson,   Fred Baker, Stewart Bryant, Brian Carpenter, Benoit Claise, Dominik   Elsbroek, Wes George, Mike Heard, Bill Jouris, Suresh Krishnan, Dave   Thaler, Ole Troan, Eric Vyncke, and Peter Yee, for providing valuable   comments on earlier versions of this document.Gont, et al.                 Standards Track                    [Page 6]

RFC 7112         Implications of Oversized Header Chains    January 20149.  References9.1.  Normative References   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery              for IP version 6",RFC 1981, August 1996.   [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.   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control              Message Protocol (ICMPv6) for the Internet Protocol              Version 6 (IPv6) Specification",RFC 4443, March 2006.   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU              Discovery",RFC 4821, March 2007.   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing              of IPv6 Extension Headers",RFC 7045, December 2013.9.2.  Informative References   [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.   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed              Networks",BCP 84,RFC 3704, March 2004.   [IANA-PROTO]              Internet Assigned Numbers Authority, "Protocol Numbers",              <http://www.iana.org/assignments/protocol-numbers>.Gont, et al.                 Standards Track                    [Page 7]

RFC 7112         Implications of Oversized Header Chains    January 2014Authors' Addresses   Fernando Gont   Huawei Technologies   Evaristo Carriego 2644   Haedo, Provincia de Buenos Aires  1706   Argentina   Phone: +54 11 4650 8472   EMail: fgont@si6networks.com   Vishwas Manral   Ionos Networks   Sunnyvale, CA  94089   US   Phone: 408-447-1497   EMail: vishwas@ionosnetworks.com   Ronald P. Bonica   Juniper Networks   2251 Corporate Park Drive   Herndon, VA  20171   US   Phone: 571 250 5819   EMail: rbonica@juniper.netGont, et al.                 Standards Track                    [Page 8]