IPv6 Prefix Length Recommendation for Routing Protocols
draft-boucadair-6man-prefix-routing-reco-01

Abstract

The length of IP prefixes to be manipulated by forwarding and routing processes is policy-based; no maximum length must be assumed by design. 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/.

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This Internet-Draft will expire on December 28, 2014.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Recommendation
• 3. IANA Considerations
• 4. Security Considerations
• 5. References
• 5.1. Normative References
• 5.2. 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 routing protocols (including route 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 semantic [RFC4291]. This document includes a recommendation for that aim.

Forwarding decisions performed by routers rely on implementations of a longest prefix-match algorithm. As in IPv4, the IPv6 prefix-match algorithms involve one critical operation which is the comparison of a destination address to a prefix present in a routing table (e.g. compare the address 2001:db8::1 to the prefix 2001:db8::/64). The recommendation on this draft apply to that critical operation.

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

A prefix length of value 0 in the routing table should not exist, unless the prefix value is also 0 - the default route.

A prefix length of value 128 in the routing table means a 'host-based route'.

2. Recommendation

Forwarding and routing protocols MUST NOT restrict by design the length of IPv6 prefixes. In particular, forwarding and routing processes MUST be designed to accept 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 followed for end-site prefix assignments.

Certain lookup algorithm implementations (find the prefix matching a given destination address) may be affected by this recommentation, and more so in IPv6 than in IPv4. Some implementations may prove less well performing when prefix lengths are above 64 (e.g. see discussion by F. Baker and 6MAN WG members at http://www.ietf.org/mail-archive/web/ipv6/current/msg20633.html)

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. References

5.1. Normative References

5.2. Informative References

Authors' Addresses