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