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Internet Engineering Task Force (IETF)                      M. BoucadairRequest for Comments: 7608                                France TelecomBCP: 198                                                     A. PetrescuCategory: Best Current Practice                                CEA, LISTISSN: 2070-1721                                                 F. Baker                                                           Cisco Systems                                                               July 2015IPv6 Prefix Length Recommendation for ForwardingAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7608.Boucadair, et al.         Best Current Practice                 [Page 1]

RFC 7608                                                       July 2015Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .32.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . .33.  Security Considerations . . . . . . . . . . . . . . . . . . .44.  References  . . . . . . . . . . . . . . . . . . . . . . . . .44.1.  Normative References  . . . . . . . . . . . . . . . . . .44.2.  Informative References  . . . . . . . . . . . . . . . . .4   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .61.  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.   AlthoughSection 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] andSection 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 inRFC 2119 [RFC2119].2.  Recommendation   IPv6 implementations MUST conform to the rules specified inSection 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 20153.  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.  References4.1.  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 2015Acknowledgements   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.comBoucadair, et al.         Best Current Practice                 [Page 6]