v6ops Working Group M. Boucadair
Internet-Draft France Telecom
Intended status: Best Current Practice A. Petrescu
Expires: November 27, 2015 CEA, LIST
May 26, 2015
IPv6 Prefix Length Recommendation for Forwarding
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.
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 27, 2015.
Boucadair, et al. Expires November 27, 2015 [Page 1]
Internet-Draft May 2015
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
2. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4
6.2. Informative References . . . . . . . . . . . . . . . . . 4
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
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. Expires November 27, 2015 [Page 2]
Internet-Draft May 2015
It is fundamental not to link routing and forwarding to the IPv6
prefix/address semantics [RFC4291]. This document includes a
recommendation for that aim.
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
This recommendation does not conflict with the 64-bit boundary for
some IPv6 stateless address autoconfiguration (SLAAC, [RFC4862])
based schemes 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].
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.
3. IANA Considerations
This document does not require any action from IANA.
4. Security Considerations
This document does not introduce security issues in addition to what
is discussed in [RFC4291].
Boucadair, et al. Expires November 27, 2015 [Page 3]
Internet-Draft May 2015
IPv6 security issues, including operational ones, are discussed in
[RFC4942] and [I-D.ietf-opsec-v6].
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.
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
6.2. Informative References
Chittimaneni, K., Kaeo, M., and E. Vyncke, "Operational
Security Considerations for IPv6 Networks", draft-ietf-
opsec-v6-06 (work in progress), March 2015.
[RFC1380] Gross, P. and P. Almquist, "IESG Deliberations on Routing
and Addressing", RFC 1380, November 1992.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
Co-existence Security Considerations", RFC 4942, September
Boucadair, et al. Expires November 27, 2015 [Page 4]
Internet-Draft May 2015
[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, April 2011.
[RFC7421] Carpenter, B., Chown, T., Gont, F., Jiang, S., Petrescu,
A., and A. Yourtchenko, "Analysis of the 64-bit Boundary
in IPv6 Addressing", RFC 7421, January 2015.
Gif-sur-Yvette, Ile-de-France 91190
Santa Barbara, California 93117
Boucadair, et al. Expires November 27, 2015 [Page 5]