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

Abstract

  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
  http://www.rfc-editor.org/info/rfc7608.















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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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  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
  configured.

  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",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  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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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,
             <http://www.rfc-editor.org/info/rfc2119>.

  [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,
             <http://www.rfc-editor.org/info/rfc2464>.

  [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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  [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
             Address Autoconfiguration", RFC 4862,
             DOI 10.17487/RFC4862, September 2007,
             <http://www.rfc-editor.org/info/rfc4862>.

  [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
             Co-existence Security Considerations", RFC 4942,
             DOI 10.17487/RFC4942, September 2007,
             <http://www.rfc-editor.org/info/rfc4942>.

  [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,
             <http://www.rfc-editor.org/info/rfc6164>.

  [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,
             <http://www.rfc-editor.org/info/rfc7421>.































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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.

Authors' Addresses

  Mohamed Boucadair
  France Telecom
  Rennes  35000
  France

  Email: [email protected]


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

  Phone: +33169089223
  Email: [email protected]


  Fred Baker
  Cisco Systems
  Santa Barbara, California  93117
  United States

  Email: [email protected]

















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