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IETF RFC 7608



Last modified on Thursday, July 23rd, 2015

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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/RFC 7608.















Boucadair, et al.         Best Current Practice              PAGE 1 top


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 ([RFC 7421]) 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 [RFC 4291] states "IPv6 unicast addresses are aggregatable with prefixes of arbitrary bit-length, similar to IPv4 addresses under Classless Inter-Domain Routing" (CIDR, [RFC 4632]), there is still a misinterpretation that IPv6 prefixes can be either /127 ([RFC 6164]) or any length up to /64. This misinterpretation is mainly induced by the 64-bit boundary in IPv6 addressing. As discussed in [RFC 7421], "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 [RFC 4291], 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 top

RFC 7608 July 2015 It is fundamental not to link routing and forwarding to the IPv6 prefix/address semantics [RFC 4291]. 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) [RFC 4862], such as [RFC 2464]. Indeed, [RFC 7421] 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 [RFC 3315]). A historical background of CIDR is documented in [RFC 1380] and Section 2 of [RFC 4632]. 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 [RFC 2119]. 2. Recommendation IPv6 implementations MUST conform to the rules specified in Section 5.1 of [RFC 4632]. 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 top

RFC 7608 July 2015 3. Security Considerations This document does not introduce security issues in addition to what is discussed in [RFC 4291]. IPv6 security issues, including operational ones, are discussed in [RFC 4942] and [OPSEC-v6]. 4. References 4.1. Normative References [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC 2119, March 1997, <http://www.rfc-editor.org/info/RFC 2119>. [RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC 4291, February 2006, <http://www.rfc-editor.org/info/RFC 4291>. [RFC 4632] Fuller, V. and T. Li, "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", BCP 122, RFC 4632, DOI 10.17487/RFC 4632, August 2006, <http://www.rfc-editor.org/info/RFC 4632>. 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. [RFC 1380] Gross, P. and P. Almquist, "IESG Deliberations on Routing and Addressing", RFC 1380, DOI 10.17487/RFC 1380, November 1992, <http://www.rfc-editor.org/info/RFC 1380>. [RFC 2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, DOI 10.17487/RFC 2464, December 1998, <http://www.rfc-editor.org/info/RFC 2464>. [RFC 3315] 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/RFC 3315, July 2003, <http://www.rfc-editor.org/info/RFC 3315>. Boucadair, et al. Best Current Practice PAGE 4 top

RFC 7608 July 2015 [RFC 4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC 4862, September 2007, <http://www.rfc-editor.org/info/RFC 4862>. [RFC 4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/ Co-existence Security Considerations", RFC 4942, DOI 10.17487/RFC 4942, September 2007, <http://www.rfc-editor.org/info/RFC 4942>. [RFC 6164] 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/RFC 6164, April 2011, <http://www.rfc-editor.org/info/RFC 6164>. [RFC 7421] 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/RFC 7421, January 2015, <http://www.rfc-editor.org/info/RFC 7421>. Boucadair, et al. Best Current Practice PAGE 5 top

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 top

RFC TOTAL SIZE: 10818 bytes PUBLICATION DATE: Thursday, July 23rd, 2015 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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