IPv6 Prefix Length Recommendation for Forwarding
RFC 7608

Document Type RFC - Best Current Practice (July 2015; No errata)
Also known as BCP 198
Last updated 2015-10-14
Replaces draft-boucadair-6man-prefix-routing-reco
Stream IETF
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Reviews
Stream WG state Submitted to IESG for Publication
Document shepherd Lee Howard
Shepherd write-up Show (last changed 2015-02-17)
IESG IESG state RFC 7608 (Best Current Practice)
Consensus Boilerplate Yes
Telechat date
Responsible AD Joel Jaeggli
Send notices to (None)
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IC
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
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