draft-ietf-v6ops-cidr-prefix-02.txt   draft-ietf-v6ops-cidr-prefix-03.txt 
v6ops Working Group M. Boucadair v6ops Working Group M. Boucadair
Internet-Draft France Telecom Internet-Draft France Telecom
Intended status: Best Current Practice A. Petrescu Intended status: Best Current Practice A. Petrescu
Expires: October 22, 2015 CEA, LIST Expires: November 27, 2015 CEA, LIST
F. Baker F. Baker
Cisco Systems Cisco Systems
April 20, 2015 May 26, 2015
IPv6 Prefix Length Recommendation for Forwarding IPv6 Prefix Length Recommendation for Forwarding
draft-ietf-v6ops-cidr-prefix-02 draft-ietf-v6ops-cidr-prefix-03
Abstract Abstract
IPv6 prefix length, as in IPv4, is a parameter conveyed and used in IPv6 prefix length, as in IPv4, is a parameter conveyed and used in
IPv6 routing and forwarding processes in accordance with the IPv6 routing and forwarding processes in accordance with the
Classless Inter-domain Routing (CIDR) architecture. The length of an Classless Inter-domain Routing (CIDR) architecture. The length of an
IPv6 prefix may be any number from zero to 128, although subnets IPv6 prefix may be any number from zero to 128, although subnets
using stateless address autoconfiguration (SLAAC) for address using stateless address autoconfiguration (SLAAC) for address
allocation conventionally use a /64 prefix. Hardware and software allocation conventionally use a /64 prefix. Hardware and software
implementations of routing and forwarding should therefore impose no implementations of routing and forwarding should therefore impose no
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 22, 2015. This Internet-Draft will expire on November 27, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Discussions on the 64-bit boundary in IPv6 addressing ([RFC7421]) Discussions on the 64-bit boundary in IPv6 addressing ([RFC7421])
revealed a need for a clear recommendation on which bits must be used revealed a need for a clear recommendation on which bits must be used
by forwarding decision-making processes. However, such a by forwarding decision-making processes. However, such a
recommendation was out of scope for that document. recommendation was out of scope for that document.
Although Section 2.5 of [RFC4291] states "IPv6 unicast addresses are Although Section 2.5 of [RFC4291] states "IPv6 unicast addresses are
aggregatable with prefixes of arbitrary bit-length, similar to IPv4 aggregatable with prefixes of arbitrary bit-length, similar to IPv4
addresses under Classless Inter-Domain Routing" (CIDR, [RFC4632]), addresses under Classless Inter-Domain Routing" (CIDR, [RFC4632]),
there is still a misinterpretation that IPv6 prefixes can be either there is still a misinterpretation that IPv6 prefixes can be either
/127 ([RFC6164]) or any length up to /64. This (mis)interpretation /127 ([RFC6164]) or any length up to /64. This misinterpretation is
is mainly induced by the 64-bit boundary in IPv6 addressing. mainly induced by the 64-bit boundary in IPv6 addressing.
As discussed in [RFC7421], "the notion of a /64 boundary in the As discussed in [RFC7421], "the notion of a /64 boundary in the
address was introduced after the initial design of IPv6, following a address was introduced after the initial design of IPv6, following a
period when it was expected to be at /80". This evolution of the period when it was expected to be at /80". This evolution of the
IPv6 Addressing architecture, resulting in [RFC4291], and followed IPv6 Addressing architecture, resulting in [RFC4291], and followed
with the addition of /127 prefixes for point-to-point links, clearly with the addition of /127 prefixes for point-to-point links, clearly
demonstrates the intent for future IPv6 developments to have the demonstrates the intent for future IPv6 developments to have the
flexibility to change this part of the architecture when justified. flexibility to change this part of the architecture when justified.
It is fundamental to not link routing and forwarding to the IPv6 It is fundamental not to link routing and forwarding to the IPv6
prefix/address semantics [RFC4291]. This document includes a prefix/address semantics [RFC4291]. This document includes a
recommendation for that aim. recommendation for that aim.
Forwarding decisions rely on the longest-match-first algorithm, which Forwarding decisions rely on the longest-match-first algorithm, which
stipulates that, given a choice between two prefixes in the stipulates that, given a choice between two prefixes in the
Forwarding Information Base (FIB) of different length that match the Forwarding Information Base (FIB) of different length that match the
destination address in each bit up to their respective lengths, the destination address in each bit up to their respective lengths, the
longer prefix is used. This document's recommendation (Section 2) is longer prefix is used. This document's recommendation (Section 2) is
that IPv6 forwarding must follow the longest-match-first rule, that IPv6 forwarding must follow the longest-match-first rule,
regardless of prefix length, barring the configuration of some regardless of prefix length, unless some overriding policy is
overriding policy. configured.
This recommendation does not conflict with the 64-bit boundary for This recommendation does not conflict with the 64-bit boundary for
some IPv6 stateless address autoconfiguration (SLAAC, [RFC4862]) some IPv6 stateless address autoconfiguration (SLAAC, [RFC4862])
based schemes such as [RFC2464]. Indeed, [RFC7421] clarifies this is based schemes such as [RFC2464]. Indeed, [RFC7421] clarifies this is
only a parameter in the SLAAC process and other longer prefix lengths only a parameter in the SLAAC process and other longer prefix lengths
are in operational use (e.g., either manually configured or based are in operational use (e.g., either manually configured or based
upon DHCPv6 [RFC3315]). upon DHCPv6 [RFC3315]).
A historical reminder of CIDR is documented in [RFC1380] and A historical background of CIDR is documented in [RFC1380] and
Section 2 of [RFC4632]. Section 2 of [RFC4632].
2. Recommendation 2. Recommendation
IPv6 implementations MUST conform to the rules specified in IPv6 implementations MUST conform to the rules specified in
Section 5.1 of [RFC4632]. Section 5.1 of [RFC4632].
Forwarding decision-making processes MUST NOT restrict the length of Decision-making processes for forwarding MUST NOT restrict the length
IPv6 prefixes by design. In particular, forwarding processes MUST be of IPv6 prefixes by design. In particular, forwarding processes MUST
designed to process prefixes of any length up to /128, by increments be designed to process prefixes of any length up to /128, by
of 1. increments of 1.
Obviously, policies can be enforced to restrict the length of IP Policies can be enforced to restrict the length of IP prefixes
prefixes advertised within a given domain or in a given advertised within a given domain or in a given interconnection link.
interconnection link. These policies are deployment-specific and/or These policies are deployment-specific and/or driven by
driven by administrative (interconnection) considerations. administrative (interconnection) considerations.
3. IANA Considerations 3. IANA Considerations
This document does not require any action from IANA. This document does not require any action from IANA.
4. Security Considerations 4. Security Considerations
This document does not introduce security issues in addition to what This document does not introduce security issues in addition to what
is discussed in [RFC4291]. is discussed in [RFC4291].
IPv6 security issues, including operational ones, are discussed in IPv6 security issues, including operational ones, are discussed in
[RFC4942] and [I-D.ietf-opsec-v6]. [RFC4942] and [I-D.ietf-opsec-v6].
5. Acknowledgements 5. Acknowledgements
Thanks to Eric Vyncke, Christian Jacquenet, Brian Carpenter, Fernando Thanks to Eric Vyncke, Christian Jacquenet, Brian Carpenter, Fernando
Gont, Tatuya Jinmei, Lorenzo Colitti, Ross Chandler, David Farmer, Gont, Tatuya Jinmei, Lorenzo Colitti, Ross Chandler, David Farmer,
and David Black for their contributions and comments. David Black, and Barry Leiba for their contributions and comments.
Special thanks to Randy Bush for his support. Special thanks to Randy Bush for his support.
6. References 6. References
6.1. Normative References 6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
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