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Internet Engineering Task Force (IETF)                       S. KrishnanRequest for Comments: 6564                                      EricssonUpdates:2460                                                J. WoodyattCategory: Standards Track                                          AppleISSN: 2070-1721                                                 E. Kline                                                                  Google                                                             J. Hoagland                                                                Symantec                                                               M. Bhatia                                                          Alcatel-Lucent                                                              April 2012A Uniform Format for IPv6 Extension HeadersAbstract   In IPv6, optional internet-layer information is encoded in separate   headers that may be placed between the IPv6 header and the transport-   layer header.  There are a small number of such extension headers   currently defined.  This document describes the issues that can arise   when defining new extension headers and discusses the alternate   extension mechanisms in IPv6.  It also provides a common format for   defining any new IPv6 extension headers, if they are needed.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/rfc6564.Krishnan, et al.             Standards Track                    [Page 1]

RFC 6564            Format for IPv6 Extension Headers         April 2012Copyright Notice   Copyright (c) 2012 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. Conventions Used in This Document ...............................33. Applicability ...................................................34. Proposed IPv6 Extension Header Format ...........................45. Backward Compatibility ..........................................46. Future Work .....................................................57. Security Considerations .........................................58. Acknowledgements ................................................59. Normative References ............................................51.  Introduction   The base IPv6 standard [RFC2460] defines extension headers as an   expansion mechanism to carry optional internet-layer information.   Extension headers, with the exception of the Hop-by-Hop Options   header, are not usually processed on intermediate nodes.  However,   several existing deployed IPv6 routers and several existing deployed   IPv6 firewalls, in contradiction to [RFC2460], are capable of parsing   past or ignoring all currently defined IPv6 extension headers (e.g.,   to examine transport-layer header fields) at wire speed (e.g., by   using custom Application-specific Integrated Circuits (ASICs) for   packet processing).  Hence, one must also consider that any new IPv6   extension header will break IPv6 deployments that use these existing   capabilities.   Any IPv6 header or option that has hop-by-hop behavior, and is   intended for general use in the public IPv6 Internet, could be   subverted to create an attack on IPv6 routers that process packets   containing such a header or option.  Reports from the field indicate   that some IP routers deployed within the global Internet are   configured either to ignore the presence of headers with hop-by-hopKrishnan, et al.             Standards Track                    [Page 2]

RFC 6564            Format for IPv6 Extension Headers         April 2012   behavior or to drop packets containing headers with hop-by-hop   behavior.2.  Conventions Used in This Document   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.  Applicability   The base IPv6 standard [RFC2460] allows the use of both extension   headers and destination options in order to encode optional   destination information in an IPv6 packet.  The use of destination   options to encode this information provides more flexible handling   characteristics and better backward compatibility than using   extension headers.  Because of this, implementations SHOULD use   destination options as the preferred mechanism for encoding optional   destination information, and use a new extension header only if   destination options do not satisfy their needs.  The request for   creation of a new IPv6 extension header MUST be accompanied by a   specific explanation of why destination options could not be used to   convey this information.   The base IPv6 standard [RFC2460] defines 3 extension headers (i.e.,   Routing header, Destination Options header, Hop-by-Hop Options   header) to be used for any new IPv6 options.  The same standard only   allows the creation of new extension headers in limited circumstances   ([RFC2460], Section 4.6).   As noted above, the use of any option with hop-by-hop behavior can be   problematic in the global public Internet.  New IPv6 extension   header(s) having hop-by-hop behavior MUST NOT be created or   specified.  New options for the existing Hop-by-Hop Header SHOULD NOT   be created or specified unless no alternative solution is feasible.   Any proposal to create a new option for the existing Hop-by-Hop   Header MUST include a detailed explanation of why the hop-by-hop   behavior is absolutely essential in the document proposing the new   option with hop-by-hop behavior.   The use of IPv6 Destination Options to encode information provides   more flexible handling characteristics and better backward   compatibility than using a new extension header.  Because of this,   new optional information to be sent SHOULD be encoded in a new option   for the existing IPv6 Destination Options header.Krishnan, et al.             Standards Track                    [Page 3]

RFC 6564            Format for IPv6 Extension Headers         April 2012   Mindful of the need for compatibility with existing IPv6 deployments,   new IPv6 extension headers MUST NOT be created or specified, unless   no existing IPv6 extension header can be used by specifying a new   option for that existing IPv6 extension header.  Any proposal to   create or specify a new IPv6 extension header MUST include a detailed   technical explanation of why no existing IPv6 extension header can be   used in the document proposing the new IPv6 extension header.4.  Proposed IPv6 Extension Header Format   Any IPv6 extension headers defined in the future, keeping in mind the   restrictions specified inSection 3 and also the restrictions   specified in [RFC2460], MUST use the consistent format defined in   Figure 1.  This minimizes breakage in intermediate nodes that examine   these extension headers.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Next Header  |  Hdr Ext Len  |                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +   |                                                               |   .                                                               .   .                  Header Specific Data                         .   .                                                               .   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Next Header          8-bit selector.  Identifies the type of header                        immediately following the extension header.                        Uses the same values as the IPv4 Protocol field                        [IANA_IP_PARAM].   Hdr Ext Len          8-bit unsigned integer.  Length of the extension                        header in 8-octet units, not including the first                        8 octets.   Header Specific      Variable length.  Fields specific to the   Data                 extension header.                     Figure 1: Extension Header Layout5.  Backward Compatibility   The scheme proposed in this document is not intended to be backward   compatible with all the currently defined IPv6 extension headers.  It   applies only to newly defined extension headers.  Specifically, theKrishnan, et al.             Standards Track                    [Page 4]

RFC 6564            Format for IPv6 Extension Headers         April 2012   fragment header predates this document and does not follow the format   proposed in this document.6.  Future Work   This document proposes one step in easing the inspection of extension   headers by middleboxes.  There is further work required in this area.   Some issues that are left unresolved beyond this document include:   o  There can be an arbitrary number of extension headers.   o  Extension headers must be processed in the order they appear.   o  Extension headers may alter the processing of the payload itself,      and hence the packet may not be processed properly without      knowledge of said header.7.  Security Considerations   This document proposes a standard format for the IPv6 extension   headers that minimizes breakage at intermediate nodes that inspect   but do not understand the contents of these headers.  Intermediate   nodes, such as firewalls, that skip over unknown headers might end up   allowing the setup of a covert channel from the outside of the   firewall to the inside using the data field(s) of the unknown   extension headers.8.  Acknowledgements   The authors would like to thank Albert Manfredi, Bob Hinden, Brian   Carpenter, Erik Nordmark, Hemant Singh, Lars Westberg, Markku Savela,   Tatuya Jinmei, Thomas Narten, Vishwas Manral, Alfred Hoenes, Joel   Halpern, Ran Atkinson, Steven Blake, Jari Arkko, Kathleen Moriarty,   Stephen Farrell, Ralph Droms, Sean Turner, and Adrian Farrel for   their reviews and suggestions that made this document better.9.  Normative References   [IANA_IP_PARAM] IANA, "IP Parameters",                   <http://www.iana.org/assignments/ip-parameters>.   [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.Krishnan, et al.             Standards Track                    [Page 5]

RFC 6564            Format for IPv6 Extension Headers         April 2012Authors' Addresses   Suresh Krishnan   Ericsson   8400 Decarie Blvd.   Town of Mount Royal, QC   Canada   Phone: +1 514 345 7900 x42871   EMail: suresh.krishnan@ericsson.com   James Woodyatt   Apple Inc.   1 Infinite Loop   Cupertino, CA 95014   US   EMail: jhw@apple.com   Erik Kline   Google   Mori Tower 26F   Roppongi 6-10-1   Minato ku   Tokyo 106-6126   Japan   Phone: +81 3-6384-9635   EMail: ek@google.com   James Hoagland   Symantec Corporation   350 Ellis St.   Mountain View, CA 94043   US   EMail: Jim_Hoagland@symantec.com   URI:http://symantec.com/   Manav Bhatia   Alcatel-Lucent   Bangalore   India   EMail: manav.bhatia@alcatel-lucent.comKrishnan, et al.             Standards Track                    [Page 6]