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Using 127-Bit IPv6 Prefixes on Inter-Router Links
RFC 6164

Internet Engineering Task Force (IETF)                          M. Kohno
Request for Comments: 6164             Juniper Networks, Keio University
Category: Standards Track                                      B. Nitzan
ISSN: 2070-1721                                         Juniper Networks
                                                                 R. Bush
                                                            Y. Matsuzaki
                                               Internet Initiative Japan
                                                              L. Colitti
                                                                  Google
                                                               T. Narten
                                                         IBM Corporation
                                                              April 2011

           Using 127-Bit IPv6 Prefixes on Inter-Router Links

Abstract

   On inter-router point-to-point links, it is useful, for security and
   other reasons, to use 127-bit IPv6 prefixes.  Such a practice
   parallels the use of 31-bit prefixes in IPv4.  This document
   specifies the motivation for, and usages of, 127-bit IPv6 prefix
   lengths on inter-router point-to-point links.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6164.

Kohno, et al.                Standards Track                    [Page 1]
RFC 6164                   IPv6 prefixlen p2p                 April 2011

Copyright Notice

   Copyright (c) 2011 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. Scope of This Memo ..............................................3
   3. Conventions Used in This Document ...............................3
   4. Problems Identified with 127-Bit Prefix Lengths in the Past .....3
   5. Reasons for Using Longer Prefixes ...............................4
      5.1. Ping-Pong Issue ............................................4
      5.2. Neighbor Cache Exhaustion Issue ............................4
      5.3. Other Reasons ..............................................5
   6. Recommendations .................................................5
   7. Security Considerations .........................................6
   8. Contributors ....................................................6
   9. Acknowledgments .................................................6
   10. References .....................................................6
      10.1. Normative References ......................................6
      10.2. Informative References ....................................7

1.  Introduction

   [RFC4291] specifies that interface IDs for all unicast addresses,
   except those that start with the binary value 000, are required to be
   64 bits long and to be constructed in Modified EUI-64 format.  In
   addition, it defines the Subnet-Router anycast address, which is
   intended to be used for applications where a node needs to
   communicate with any one of the set of routers on a link.

   Some operators have been using 127-bit prefixes, but this has been
   discouraged due to conflicts with Subnet-Router anycast [RFC3627].
   However, using 64-bit prefixes creates security issues that are
   particularly problematic on inter-router links, and there are other
   valid reasons to use prefixes longer than 64 bits, in particular /127
   (see Section 5).

Kohno, et al.                Standards Track                    [Page 2]
RFC 6164                   IPv6 prefixlen p2p                 April 2011

   This document provides a rationale for using 127-bit prefix lengths,

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