IPv6 Prefix Length Recommendation for Forwarding
draft-boucadair-6man-prefix-routing-reco-03

Abstract

The length of IP prefixes is an information used by forwarding and routing processes is policy-based. As such, no maximum length must be assumed by design.

Discussions on the 64-bit boundary in IPv6 addressing revealed a need for a clear recommendation on which bits must be used by forwarding decision-making processes. This document sketches a recommendation to be followed by forwarding and routing designs with regards to the prefix length. The aim is to avoid hard-coded routing and forwarding designs that exclude some IP prefix lengths.

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/.

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 March 28, 2015.

Copyright Notice

Copyright (c) 2014 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

Recent discussions on the 64-bit boundary in IPv6 addressing ([I-D.ietf-6man-why64]) revealed a need for a clear recommendation on which bits must be used by forwarding decision-making processes.

A detailed analysis of the 64-bit boundary in IPv6 addressing, and the implication for end-site prefix assignment, is documented in [I-D.ietf-6man-why64]. No recommendation is included in [I-D.ietf-6man-why64].

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 made by routers primarily rely upon a longest prefix-match algorithm. Like in IPv4, the IPv6 prefix-match algorithms involve one critical operation which is the comparison of a destination address with a prefix present in a routing table (e.g., compare the 2001:db8::1 address with the 2001:db8::/64 prefix). The recommendation of this document is to be followed by that critical operation.

It is important that the compare operation be a bit-wise comparison, and not a byte-wise comparison.

2. Recommendation

Forwarding decision-making processes MUST NOT restrict by design the length of IPv6 prefixes. 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 involved when IPv6 stateless address autoconfiguration (SLAAC, [RFC4862]) is used on links such as Ethernet [RFC2464].

Some lookup algorithm implementations (find the prefix matching a given destination address) may be affected by this recommendation, even more so for IPv6 than IPv4. The performance of some implementations may be degraded when prefix lengths are longer than /64.

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 and Christian Jacquenet for their comments.

Special thanks to Randy Bush and Brian Carpenter for their support.

6. References

6.1. Normative References

6.2. Informative References

Authors' Addresses