6man Working Group                                           S. Krishnan
Internet-Draft                                                  Ericsson
Intended status: Standards Track                           j h. woodyatt
Expires: May 3, 2012                                               Apple
                                                                E. Kline
                                                             J. Hoagland
                                                               M. Bhatia
                                                        October 31, 2011

              An uniform format for IPv6 extension headers


   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 alternative
   extension mechanisms in IPv6.  It also provides a format for defining
   new IPv6 extension headers that would allow implementations to
   process past unknown extension headers.

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 May 3, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Conventions used in this document . . . . . . . . . . . . . . . 3
   3.  Applicability . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Proposed IPv6 Extension Header format . . . . . . . . . . . . . 5
   5.  Backward Compatibility  . . . . . . . . . . . . . . . . . . . . 5
   6.  Future work . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   10. Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  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,
   some intermediate nodes such as firewalls, may need to look at the
   transport layer header fields in order to make a decision to allow or
   deny the packet.  If new extension headers are defined and the
   intermediate node is not aware of them, the intermediate node cannot
   proceed further in the header chain since it does not know where the
   unknown header ends and the next header begins.  The main issue is
   that the extension header format is not standardized and hence it is
   not possible to skip past the unknown header.  This document intends
   to define a standard format for IPv6 extension headers.

   Also, Several existing deployed IPv6 routers and several existing
   deployed IPv6 firewalls 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 ASICs for
   packet processing).  Hence, one must also consider that any new IPv6
   Extension Header will break IPv6 deployments that use these existing

   Any IPv6 header or option that has hop-by-hop behaviour and is
   intended for general use in the public IPv6 Internet could be
   subverted to create an attack on IPv6 routers processing 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-hop
   behaviour or to drop packets containing headers with hop-by-hop

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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

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   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 an
   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 behaviour can
   be problematic in the global public Internet.  So new IPv6 Extension
   Header(s) having hop-by-hop behaviour MUST NOT be created or
   specified.  Also, new options for the existing Hop-by-Hop Header
   SHOULD NOT be created or specified unless no alternative 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
   behaviour is absolutely essential in the Internet-Draft proposing the
   new option with hop-by-hop behaviour.

   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.

   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 Internet-Draft proposing the new IPv6 Extension Header.

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4.  Proposed IPv6 Extension Header format

   If any IPv6 Extension Headers are defined in future, keeping in mind
   the restrictions specified in Section 3 and also the restrictions
   specified in [RFC2460], they MUST use the consistent format defined
   in Figure 1.  This enables future IPv6 implementations to skip over
   unknown IPv6 Extension Headers and continue to further process the
   IPv6 header chain.

    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
   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 layout

5.  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, the
   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

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   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.  IANA Considerations

   This document does not require any IANA actions.

8.  Security Considerations

   This document proposes a standard format for the IPv6 extension
   headers so that intermediate nodes that do not understand the
   contents of these headers can look past them.  Intermediate nodes,
   such as firewalls, skipping 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.

9.  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 and Steven Blake for their reviews and
   suggestions that made this document better.

10.  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.

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Authors' Addresses

   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC

   Phone: +1 514 345 7900 x42871
   Email: suresh.krishnan@ericsson.com

   james woodyatt
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014

   Email: jhw@apple.com

   Erik Kline
   Mori Tower 26F
   Roppongi 6-10-1
   Minato ku
   Tokyo  106-6126

   Phone: +81 3-6384-9635
   Email: ek@google.com

   James Hoagland
   Symantec Corporation
   350 Ellis St.
   Mountain View, CA  94043

   Email: Jim_Hoagland@symantec.com
   URI:   http://symantec.com/

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   Manav Bhatia

   Email: manav.bhatia@alcatel-lucent.com

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