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IPv6 Prefix Length Recommendation for Forwarding

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7608.
Authors Mohamed Boucadair , Alexandre Petrescu , Fred Baker
Last updated 2015-10-14 (Latest revision 2015-05-26)
Replaces draft-boucadair-6man-prefix-routing-reco
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Best Current Practice
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Lee Howard
Shepherd write-up Show Last changed 2015-02-17
IESG IESG state Became RFC 7608 (Best Current Practice)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Joel Jaeggli
Send notices to (None)
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IANA Actions
v6ops Working Group                                         M. Boucadair
Internet-Draft                                            France Telecom
Intended status: Best Current Practice                       A. Petrescu
Expires: November 27, 2015                                     CEA, LIST
                                                                F. Baker
                                                           Cisco Systems
                                                            May 26, 2015

            IPv6 Prefix Length Recommendation for Forwarding


   IPv6 prefix length, as in IPv4, is a parameter conveyed and used in
   IPv6 routing and forwarding processes in accordance with the
   Classless Inter-domain Routing (CIDR) architecture.  The length of an
   IPv6 prefix may be any number from zero to 128, although subnets
   using stateless address autoconfiguration (SLAAC) for address
   allocation conventionally use a /64 prefix.  Hardware and software
   implementations of routing and forwarding should therefore impose no
   rules on prefix length, but implement longest-match-first on prefixes
   of any valid length.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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

   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 November 27, 2015.

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Internet-Draft                                                  May 2015

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Discussions on the 64-bit boundary in IPv6 addressing ([RFC7421])
   revealed a need for a clear recommendation on which bits must be used
   by forwarding decision-making processes.  However, such a
   recommendation was out of scope for that document.

   Although Section 2.5 of [RFC4291] states "IPv6 unicast addresses are
   aggregatable with prefixes of arbitrary bit-length, similar to IPv4
   addresses under Classless Inter-Domain Routing" (CIDR, [RFC4632]),
   there is still a misinterpretation that IPv6 prefixes can be either
   /127 ([RFC6164]) or any length up to /64.  This misinterpretation is
   mainly induced by the 64-bit boundary in IPv6 addressing.

   As discussed in [RFC7421], "the notion of a /64 boundary in the
   address was introduced after the initial design of IPv6, following a
   period when it was expected to be at /80".  This evolution of the
   IPv6 Addressing architecture, resulting in [RFC4291], and followed
   with the addition of /127 prefixes for point-to-point links, clearly
   demonstrates the intent for future IPv6 developments to have the
   flexibility to change this part of the architecture when justified.

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Internet-Draft                                                  May 2015

   It is fundamental not to link routing and forwarding to the IPv6
   prefix/address semantics [RFC4291].  This document includes a
   recommendation for that aim.

   Forwarding decisions rely on the longest-match-first algorithm, which
   stipulates that, given a choice between two prefixes in the
   Forwarding Information Base (FIB) of different length that match the
   destination address in each bit up to their respective lengths, the
   longer prefix is used.  This document's recommendation (Section 2) is
   that IPv6 forwarding must follow the longest-match-first rule,
   regardless of prefix length, unless some overriding policy is

   This recommendation does not conflict with the 64-bit boundary for
   some IPv6 stateless address autoconfiguration (SLAAC, [RFC4862])
   based schemes such as [RFC2464].  Indeed, [RFC7421] clarifies this is
   only a parameter in the SLAAC process and other longer prefix lengths
   are in operational use (e.g., either manually configured or based
   upon DHCPv6 [RFC3315]).

   A historical background of CIDR is documented in [RFC1380] and
   Section 2 of [RFC4632].

2.  Recommendation

   IPv6 implementations MUST conform to the rules specified in
   Section 5.1 of [RFC4632].

   Decision-making processes for forwarding MUST NOT restrict the length
   of IPv6 prefixes by design.  In particular, forwarding processes MUST
   be designed to process prefixes of any length up to /128, by
   increments of 1.

   Policies can be enforced to restrict the length of IP prefixes
   advertised within a given domain or in a given interconnection link.
   These policies are deployment-specific and/or driven by
   administrative (interconnection) considerations.

3.  IANA Considerations

   This document does not require any action from IANA.

4.  Security Considerations

   This document does not introduce security issues in addition to what
   is discussed in [RFC4291].

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Internet-Draft                                                  May 2015

   IPv6 security issues, including operational ones, are discussed in
   [RFC4942] and [I-D.ietf-opsec-v6].

5.  Acknowledgements

   Thanks to Eric Vyncke, Christian Jacquenet, Brian Carpenter, Fernando
   Gont, Tatuya Jinmei, Lorenzo Colitti, Ross Chandler, David Farmer,
   David Black, and Barry Leiba for their contributions and comments.

   Special thanks to Randy Bush for his support.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, August 2006.

6.2.  Informative References

              Chittimaneni, K., Kaeo, M., and E. Vyncke, "Operational
              Security Considerations for IPv6 Networks", draft-ietf-
              opsec-v6-06 (work in progress), March 2015.

   [RFC1380]  Gross, P. and P. Almquist, "IESG Deliberations on Routing
              and Addressing", RFC 1380, November 1992.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, December 1998.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942, September

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Internet-Draft                                                  May 2015

   [RFC6164]  Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
              L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
              Router Links", RFC 6164, April 2011.

   [RFC7421]  Carpenter, B., Chown, T., Gont, F., Jiang, S., Petrescu,
              A., and A. Yourtchenko, "Analysis of the 64-bit Boundary
              in IPv6 Addressing", RFC 7421, January 2015.

Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes  35000


   Alexandre Petrescu
   CEA Saclay
   Gif-sur-Yvette, Ile-de-France  91190

   Phone: +33169089223

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117


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