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rfc:bcp:bcp198

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.

Boucadair, et al. Best Current Practice [Page 1] 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.

Boucadair, et al. Best Current Practice [Page 2] 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
 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.

Boucadair, et al. Best Current Practice [Page 3] 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,
            <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>.

Boucadair, et al. Best Current Practice [Page 4] RFC 7608 July 2015

 [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>.

Boucadair, et al. Best Current Practice [Page 5] RFC 7608 July 2015

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: mohamed.boucadair@orange.com
 Alexandre Petrescu
 CEA, LIST
 CEA Saclay
 Gif-sur-Yvette, Ile-de-France  91190
 France
 Phone: +33169089223
 Email: alexandre.petrescu@cea.fr
 Fred Baker
 Cisco Systems
 Santa Barbara, California  93117
 United States
 Email: fred@cisco.com

Boucadair, et al. Best Current Practice [Page 6]

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