Internet Engineering Task Force (IETF)                      M. Boucadair
Request for Comments: 7608                                France Telecom
BCP: 198                                                     A. Petrescu
Category: Best Current Practice                                CEA, LIST
ISSN: 2070-1721                                                 F. Baker
                                                           Cisco Systems
                                                               July 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.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 7608                                                       July 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
   (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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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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RFC 7608                                                       July 2015

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

   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 schemes that based on IPv6 stateless address autoconfiguration
   (SLAAC) [RFC4862], 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].

1.1.  Requirements Language

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

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.

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RFC 7608                                                       July 2015

3.  Security Considerations

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

   IPv6 security issues, including operational ones, are discussed in
   [RFC4942] and [OPSEC-v6].

4.  References

4.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
              2006, <http://www.rfc-editor.org/info/rfc4632>.

4.2.  Informative References

   [OPSEC-v6] Chittimaneni, K., Kaeo, M., and E. Vyncke, "Operational
              Security Considerations for IPv6 Networks", Work in
              Progress, draft-ietf-opsec-v6-06, March 2015.

   [RFC1380]  Gross, P. and P. Almquist, "IESG Deliberations on Routing
              and Addressing", RFC 1380, DOI 10.17487/RFC1380, November
              1992, <http://www.rfc-editor.org/info/rfc1380>.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

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RFC 7608                                                       July 2015

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942,
              DOI 10.17487/RFC4942, September 2007,

   [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, DOI 10.17487/RFC6164, April 2011,

   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,

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RFC 7608                                                       July 2015


   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.

Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes  35000

   Email: mohamed.boucadair@orange.com

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

   Phone: +33169089223
   Email: alexandre.petrescu@cea.fr

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117
   United States

   Email: fred@cisco.com

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