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6MAN Working Group                                           A. Petrescu
Internet-Draft                                                 CEA, LIST
Intended status: Standards Track                       December 14, 2018
Expires: June 17, 2019


The length of the prefix of an IPv6 link-local address ranges from 10 to
                                  127
                  draft-petrescu-6man-ll-prefix-len-05

Abstract

   A rejected Errata to RFC4291 "IPv6 Addr Archi" on the topic of link-
   local addresses 'needs' a draft.  This is an answer to that need.

   The length of the prefix of an IPv6 link-local address is variable.
   The minimal value is 10 decimal.  The maximum value is 127 decimal.

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
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   This Internet-Draft will expire on June 17, 2019.

Copyright Notice

   Copyright (c) 2018 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
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   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Definitions and Statements  . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Example of use of LL Prefix Length 32 . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  ChangeLog  . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Definitions and Statements

   The prefix of an IP address is formed by the n leftmost bits of the
   address.  (in a left-to-right writing system).

   The prefix of an IP address is used for goals such as: identify the
   type of an IPv6 address (link-local, global, others), identify the
   belonging of an IP address to a particular subnetwork, assist the
   forwarding (or not forwarding) decisions, and others.

   The minimal length of the prefix of an IPv6 link-local address (the
   value of n) is equal to 10 decimal.  The maximum is 127.

   The prefix of an IPv6 link-local address is represented textually as
   "fe80::/n", where n MAY be any value between 10 and 127.

   Regardless of the prefix length, the leftmost 10 bits of an IPv6
   link-local address MUST be set to binary 1111111010 (hexadecimal
   fe80).

   The illustration of an IPv6 link-local address is:


     | leftmost |         Subnet ID and Interface ID
     | 10 bits  |                 118 bits                             |
     +----------+------------------------------------------------------+
     |1111111010+          Bits that MAY be either 0 or 1              |
     +----------+------------------------------------------------------+


                   Figure 1: The IPv6 link-local address



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   Examples: fe80::1/10, fe80:1::1/32 and fe80::1:1/64 are all IPv6
   link-local addresses; their prefix lengths are 10, 32 and 64
   respectively.  Each such IPv6 address has the leftmost 10 bits equal
   to binary 1111111010.

   The Difficulty: the number binary 1111111010 can not be written in
   hexadecimal without specifying the number of significant bits
   (fe80::/10); yet that does not make it a 'prefix'.  Converting
   1111111010 to hexadecimal leads to 3FA (because in a left-to-right
   writing system the leading 0s before comma are irrelevant); yet '3FA'
   is not commonly known to be the leading bits of an IPv6 link-local
   address, fe80::/10 is.

2.  Terminology

   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].

   prefix: a contiguous string of bits valid for forwarding operations
   and for subnet formation.

   textual representation of a prefix: e.g. fe80::/64.

   n leading bits: the first n bits in a string of bits read from left
   to right in a writing system that is read left-to-right.  E.g. the 10
   leading bits of the fe80::/64 textual representation of the IPv6
   link-local prefix are 1111111010.

3.  Context

   The RFC "IPv6 Address Archi" illustrates the format of the link-local
   addresses.  From the illustration it MAY be understood that the
   length of the link-local prefix is 10 bits of value 1111111010 and 54
   0 bits.

   IANA lists the "IPv6 prefix", and "Address Block", to be "fe80::/10"
   on its website.  It is possible that in the future the IETF could
   decide to use the bits 11-53.

   The RFC 2464 "IPv6-over-Ethernet" states that the prefix for link-
   local addresses is "fe80::/64".

   RFC 6874, "Representing IPv6 Zone Identifiers in Address Literals and
   Uniform Resource Identifiers" specifies the link-local addresses to
   be under prefix "fe80::/10".





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   Several knowledgeable interpretations state that, generally speaking,
   the prefix length of link-local addresses is 10, but it is 64 in the
   particular case of Stateless Address-Autoconfiguration (SLAAC).  In
   this latter case, the prefix is named a "subnet prefix", or "prefix
   on a link", and it is "fe80::/64".

   Implementations of an IPv6 stack in a particular operating system
   allow for the manual configuration of both prefix lengths 64 and 10
   for link-local addresses.  In another operating system the prefix
   length for link-local addresses can not be explicitely specified by
   the end user, but may be indirectly derived from two distinct textual
   formats by using an unspecified rule.

   Misconfigurations and lack of interoperability MAY arise between
   computers that use mixed prefix lengths for link-local addresses.

   A memo describes the use of IPv6 link-local addresses in
   applications.  The filename of the Internet Draft is draft-smith-
   ipv6-link-locals-apps-00.

   Historical note: earlier, the link-local prefix fe80::/10 and site-
   local prefix fec0::/10 were grouped into a common fe80::/9.  If bits
   10-64 were 0 then the prefix was a link-local, otherwise a site-
   local.  The site-local addresses were later deprecated by RFC 3879.

4.  Example of use of LL Prefix Length 32

   This figure shows two routers each with two interfaces; one such
   interface is connected to the other router; there are two interfaces
   that point elsewhere.


      i1 ------- i2      i3-------i4
      --|Router1|---------|Router2|---
         -------           -------

i2 address is fe80:12::1:1/32 ('12' means subnet between R1 and R2,
                                '1' is R1, 2nd '1' is 'front' interface)
i3 address is fe80:12::2:2/32


                             Figure 2: Figure

   One router's interface (connected to the other router) uses address
   fe80:12::1:1/32 and the other router's corresponding interface uses
   address fe80:12::2:2/32.





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5.  Security Considerations

   The clarification of the definition of the prefix length of the IPv6
   link-local prefix at IANA is: call it 'leading bits' and not
   'prefix', or state that the IPv6 prefix length of link-local
   addresses is 10 decimal.  This clarification has beneficial impact in
   the algorithm implementation for calculation of the opaque and stable
   Interface Identifiers for IPv6 link-local addresses.  It also
   positively impacts some implementations of IPv6 forwarding.

6.  IANA Considerations

   IANA is requested to change the name of the column head in the table
   that depicts the "Internet Protocol Version 6 Address Space".  The
   name should be "The n leading bits of an address" instead of "IPv6
   Prefix".

   The desired effect of this change is that the IPv6 link-local prefix
   be "fe80::/n" and that the 10 leading bits of this prefix be
   1111111010.  A second effect is that the textual representation
   "fe80::/10" as an IPv6 link-local prefix should disappear from that
   IANA page, because it is wrong.

7.  Contributors

   Listed from 6man WG discussion.

8.  Acknowledgements

   The following persons are acknowledged for the discussion that is
   reflected in this draft.  Not all points are reflected.  Some points
   are copied almost entirely.

   Ole Troan, Scott Timothy Morizot, Brian Carpenter, Fred Baker, Mark
   Smith, Peter Occil, Philip Homburg, Albert Manfredi, _–3/4
   ’BAE (TATUYA Jinmei), Fernando Gont, Christian Huitema,
   Simon Hobson, Matthew Petach, Yucel Guven, Sander Steffann, Dennis
   Ferguson, Musa Stephen Honlue, Philip Homburg.

   Peter Paluch submitted the Errata suggestion to RFC 4291 about link-
   local addresses, and Brian Haberman rejected it, by requiring a
   draft.  Igor Lubashev pointed to that Errata.

9.  Normative References







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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

Appendix A.  ChangeLog

   The changes are listed in reverse chronological order, most recent
   changes appearing at the top of the list.

   -04: added an example advantage of using prefix length 32.

   -03:

   -02: corrected a typo in "fe80::/1" and added a 7-bit encoding for
   one persons name (in addition to the japanese-shift-jis encoding
   which is not understood by xml2rfc.)

Author's Address

   Alexandre Petrescu
   CEA, LIST
   CEA Saclay
   Gif-sur-Yvette , Ile-de-France   91190
   France

   Phone: +33169089223
   Email: Alexandre.Petrescu@cea.fr























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