6MAN Working Group                                           A. Petrescu
Internet-Draft                                                 CEA, LIST
Intended status: Standards Track                          April 17, 2019
Expires: October 19, 2019

The length of the prefix of an IPv6 link-local address ranges from 10 to


   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.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

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

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

   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",
   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.  A prefix MUST have an integer length value
   from 1 to 127 (except when the prefix length is for default route, in
   which case the value is 0) and a prefix length must be indicated in
   its textual representation (e.g. 2001:db8::/32 is the prefix and 32
   is the prefix length).

   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".

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

   RFC 8415 "DHCPv6" considers link-local addresses are indicated by the
   prefix fe80::/10.

   RFC4291 requires that an IPv6 link-local address be assigned on each
   interface.  Yet, it does not require the link-local prefix to be
   associated to an interface.

   RFC4861 requires that the link local prefix be present in the Prefix
   List associated with an interface, although it does not specify the
   length of the link local prefix.

   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
   (linux) 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.

   In yet another operating system (BSD) an end user can not use a link-
   local address whose value is fe80:1::1; because in that OS the hosts
   drop incoming packets whose or destination address matches fe80::/10
   and contains a non-0 value in bits 15-31 (like fe80:1::1 does).

   In a particular operating system (openbsd), it is possible to run
   SLAAC with Interface IDentifiers of length different than 64, e.g.
   100; this implements RFC7217.  In that same operating system it is
   not possible to use an Interface Identifier of length 100.  At the
   same time, in another operating system (linux) it is possible to use
   Interface IDentifiers of length 100, yet SLAAC does not work with IID
   that is not 64.  In an ideal linux-bsd operating system any length of
   IID would be possible.

   The loopback interface is required to have a link-local address too
   (RFC4291), although some OSs dont (linux).  The RFC4007 clarifies
   that, somehow.

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   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-

   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
                            -------           -------

    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.

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.

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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

   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, Fred Templin.

   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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

Appendix A.  ChangeLog

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

   -08: added explanation of which RFC requires the LL address to be
   present, and which requires the LL prefix to be present; named the
   OSs, instead of staying generic; explained that the lack of

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   requirement of ll address on lo in RFC4291 is covered by another
   RFC4007; explained that openbsd allows variable len IID for GUAs but
   not for LLs, yet linux allows the reverse, and concluded on an
   obvious ideal.

   -07: added the fact that DHCPv6 spec considers the link-local
   addresses to be fe80::/10; added a valuable explanation of ll
   behaviour of a particularly important OS.

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


   -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 Saclay
   Gif-sur-Yvette , Ile-de-France   91190

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

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