v6ops Working Group                                         M. Boucadair
Internet-Draft                                            France Telecom
Intended status: Best Current Practice                       A. Petrescu
Expires: July 22, August 17, 2015                                       CEA, LIST
                                                                F. Baker
                                                           Cisco Systems
                                                        January 18,
                                                       February 13, 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
   algorithms 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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   3   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   4   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 (mis)interpretation
   is mainly induced by the 64-bit boundary in IPv6 addressing.

   A detailed analysis

   As discussed in [RFC7421], "the notion of the 64-bit 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
   together Addressing architecture, resulting in [RFC4291], and followed
   with the implication addition of /127 prefixes for point-to-point links, clearly
   demonstrates the intent for end-site prefix assignment are
   documented in [RFC7421], but no recommendation is included in that
   document. future IPv6 developments to have the
   flexibility to change this part of the architecture when justified.

   It is fundamental to not 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, barring the configuration of some
   overriding policy.

   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 reminder 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].

   Forwarding decision-making processes 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.

   Obviously, 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.

   This recommendation does not conflict with the 64-bit boundary for
   some IPv6 stateless address autoconfiguration (SLAAC, [RFC4862])
   based schemes such as [RFC2464].

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].

   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, and Ross Chandler Chandler, and David Farmer
   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-v6-05 (work in progress), October 2014.

   [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

   [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

   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

   Email: fred@cisco.com