IPv6 Prefix Length Recommendation for Forwarding
draft-ietf-v6ops-cidr-prefix-00

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 algorithms should therefore impose no rules on prefix length, but implement longest-match-first on prefixes of any valid length.

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 [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/.

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This Internet-Draft will expire on July 24, 2015.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Recommendation
• 3. IANA Considerations
• 4. Security Considerations
• 5. Acknowledgements
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Authors' Addresses

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.

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 or any length up to /64. This (mis)interpretation is mainly induced by the 64-bit boundary in IPv6 addressing.

A detailed analysis of the 64-bit boundary in IPv6 addressing together with the implication for end-site prefix assignment are documented in [RFC7421], but no recommendation is included in that document.

It is fundamental to not 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 is that IPv6 forwarding must follow the longest-match-first rule, regardless of prefix length, barring the configuration of some overriding policy.

A historical reminder of CIDR is documented in [RFC1380] and Section 2 of [RFC4632].

2. Recommendation

IPv6 MUST conform to the rules specified in Section 5.1 of [RFC4632].

Forwarding decision-making processes 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.

Obviously, 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.

This recommendation does not conflict with the 64-bit boundary for some IPv6 stateless address autoconfiguration (SLAAC, [RFC4862]) based schemes such as [RFC2464].

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].

5. Acknowledgements

Thanks to Eric Vyncke, Christian Jacquenet, Brian Carpenter, Fernando Gont, Tatuya Jinmei, Lorenzo Colitti, and Ross Chandler for their comments.

Special thanks to Randy Bush for his support.

6. References

6.1. Normative References

6.2. Informative References

Authors' Addresses