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Network Working Group                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Informational                             June 20, 2018
Expires: December 22, 2018


              The Longest Acceptable Prefix for IPv6 Links
                      draft-carpenter-6man-lap-01

Abstract

   This document introduces the concepts of a Longest Acceptable Prefix
   (LAP) and a Shortest Acceptable Identifier Length (SAIL) for an IPv6
   link.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on December 22, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definition of Longest Acceptable Prefix . . . . . . . . . . .   2
   3.  Definition of Shortest Acceptable Identifier Length . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   3
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   3
   Appendix A.  Change log [RFC Editor: Please remove] . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   The IPv6 addressing architecture [RFC4291] clearly separates an
   address into a routing prefix of length n bits and an interface
   identifier of length 128-n bits.  IPv6 routers are required by BCP
   198 [RFC7608] to support any length of routing prefix.  For
   operational reasons, routing prefixes up to 127 bits have been
   recommended [RFC6164].

   Stateless Address Autoconfiguration (SLAAC) [RFC4862] requires a
   fixed prefix length for each Layer 2 medium, and for largely
   historical reasons [RFC7136] this has been fixed for all media as 64
   bits by the addressing architecture.

   Efforts to update the addressing architecture
   [I-D.ietf-6man-rfc4291bis] have shown that there are contradictory
   opinions about retaining this fixed length for all purposes, not just
   for SLAAC.  See for example [I-D.bourbaki-6man-classless-ipv6].

   This document does not aim to rehash those opinions and the arguments
   behind them.  Its only purpose is to propose simple terminology to
   make the discussion easier.  Both the terms introduced include the
   word "Acceptable" to make it clear that they are human operational
   choices.

2.  Definition of Longest Acceptable Prefix

   As noted above, any prefix length up to /128 is treated identically
   by routing protocols.  However, for a given network, end site, or
   link, there always exists a Longest Acceptable Prefix (LAP), whose
   length is locally determined.  Currently, a site or link that uses
   SLAAC has a LAP of /64, and will not work with a longer one.  A
   point-to-point link may have a LAP of /127, according to [RFC6164].
   Situations in which other LAPs might be used should be defined in
   other documents.




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3.  Definition of Shortest Acceptable Identifier Length

   The interface identifier is used to identify a given interface on a
   given link, and is therefore only of local significance, even though
   it is globally visible as part of an address.  For a given link,
   there always exists a Shortest Acceptable Identifier Length (SAIL).
   By definition,

                  LAP + SAIL <= 128

   Currently, a site or link that uses SLAAC has a SAIL of 64.
   Situations in which other SAILs might be used should be defined in
   other documents, with particular attention to security and privacy
   issues.

4.  Security Considerations

   As noted in the Security Considerations of
   [I-D.ietf-6man-rfc4291bis], the length of a SAIL, and therefore the
   length of a LAP, have important implications for privacy.  Proposals
   for adopting LAPs longer than /64 must take this into account.

   Additionally, the length of a SAIL has important implications for the
   feasability of network reconnaissance by scanning attacks [RFC7707].

5.  IANA Considerations

   This document makes no request of the IANA.

6.  Acknowledgements

   The term SAIL is directly based on a suggestion by Mark Smith.

7.  Informative References

   [I-D.bourbaki-6man-classless-ipv6]
              Bourbaki, N., "IPv6 is Classless", draft-bourbaki-6man-
              classless-ipv6-03 (work in progress), March 2018.

   [I-D.ietf-6man-rfc4291bis]
              Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", draft-ietf-6man-rfc4291bis-09 (work in
              progress), July 2017.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.




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   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC6164]  Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
              L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
              Router Links", RFC 6164, DOI 10.17487/RFC6164, April 2011,
              <https://www.rfc-editor.org/info/rfc6164>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <https://www.rfc-editor.org/info/rfc7136>.

   [RFC7608]  Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
              Length Recommendation for Forwarding", BCP 198, RFC 7608,
              DOI 10.17487/RFC7608, July 2015,
              <https://www.rfc-editor.org/info/rfc7608>.

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <https://www.rfc-editor.org/info/rfc7707>.

Appendix A.  Change log [RFC Editor: Please remove]

   draft-carpenter-6man-lap-00, 2018-06-13:

   Initial version

   draft-carpenter-6man-lap-01, 2018-06-20:

   Added SAIL, minor clarifications

Author's Address

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland  1142
   New Zealand

   Email: brian.e.carpenter@gmail.com








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