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6MAN Working Group                                           A. Petrescu
Internet-Draft                                                 CEA, LIST
Updates: RFC4291, RFC4007 (if approved)                    L. Velvindron
Intended status: Standards Track                           Cyberstorm.mu
Expires: October 31, 2019                                  N. Kottapalli
                                                           Benu Networks
                                                               G. Mishra
                                                  Verizon Communications
                                                          April 29, 2019

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


   A rejected Erratum to RFC4291 "IPv6 Addr Archi" on the topic of link-
   local addresses 'would need' a draft.  This draft 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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 31, 2019.

Copyright Notice

   Copyright (c) 2019 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

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   (https://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
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   described in the Simplified BSD License.

Table of Contents

   1.  Definitions and Statements  . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Kinds of Solutions  . . . . . . . . . . . . . . . . . . . . .   4
   5.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Example of use of LL Prefix Length 32 . . . . . . . . . . . .   7
   7.  Use-Case  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  ChangeLog  . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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:

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     | 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

   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.  Problem Statement

   IPv6 link-local addresses are typically self-configured according to
   4 RFCs and relying on the fe80::/10 IANA allocation, RFC4291 54 0
   bits, and RFC2462 MAC-based 64bit Interface IDs.  In some cases, it
   is advantageous to manually configure link-local addresses.  This is

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   useful for easy remembering by humans, and for parameter resilience
   during network interface replacement.

   Manual configuration of an LL address may use short IID and Subnet
   ID.  The Subnet ID presence in the link-local address is useful in
   some wireless settings where the subnet structure parameters depend
   on the link locality.  Other settings may also benefit.

   When manually setting the link-local address it is necessary to know
   the length of the prefix of the subnet on which this link-local
   address is present.  This length is necessary for on-link

   The problem is: setting a variable prefix length to link-local
   addresses may provoke glitches.

4.  Kinds of Solutions

   Some solutions to the problem are: use an address of the form
   fe80:1::2/32, or use an address of the form fe80::1:2/64, where 1 is
   the Subnet ID and 2 is the Interface ID.  Other solutions involve a
   hidden 'scope_id' and the use of special syntax ('%') to denote an
   interface.  Each of these solutions have other problems of their own:
   set some of the 54 mandatorily reset bits of RFC4291, not
   implementable on some OS, invade the IID with a Subnet ID, and
   potentially others.

   Other solutions involve the use of an 'fe80' prefix in the RA such
   that to configure link-local prefixes by a similar means than GUAs/
   ULAs.  This also has advantages and drawbacks.

   Other solutions may involve the use of other than LL addresses, i.e.
   use GUAs or ULAs; this has some advantages and some inconvenients
   (cant put LL in src of RA).

   Other solutions recommend the use of name-to-address mapping, instead
   of easy to remember LLs; DNS is such tool; can be used in order to
   facilitate the remembering by humans.  However, this has some
   advantages and some inconvenients (e.g. needs DNS-SD, mDNS and IP
   multicast routing for multi-subnets; chicken-and-egg between
   formation of LLs needing these DNS tools to work in the first place).

5.  Context

   draft-bourbaki-6man-classless-ipv6-05 describes the motivation of
   considering IPv6 to be classless.  It gives a little bit of history
   of why it is how it is.  It proposes the rigid 64 IID length to be
   probably the last remnant of the boundary.

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

   draft-farmer-6man-routing-64-02 describes the relationship between
   routing and the 64-bit boundary; mainly GUA, no LL; t is ambiguous in
   its recommendation.

   draft-farmer-6man-exceptions-64-09 describes the exceptions to the
   standard subnet boundary in IPv6 addressing; mainly about GUA, not
   about LL; the exceptions are: GUA with the first 3 bits 0, manually
   config'ed addresses, DHCPv6 assigned addresses, ND on-link
   determination, IPv6-over-foo.

   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".

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

   RFC 4007 "IPv6 Scoped Address Architecture" discusses Zone ID.  A
   Zone ID may be used - internally - in the 54bits of a link-local
   address, even though these 54bits appear to be reset.  The document
   mentions at a point that fe80::1 could be used in two separate
   physical links (not virtual, like the loopback).

   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.

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   RFC4862 "SLAAC" defines how GUAs and LLs self-configure.  Whereas the
   GUA gets its prefix length from the RA (not from an RFC), the LL gets
   it from RFC4291 (not from RA).  They are independent choices based on
   distinct sources.

   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".

   The term "link-local prefix" is sometimes used to mean the prefix for
   on-link determination, and is sometimes used to mean the reserved
   address space for link-local addresses (including all current and
   future use).  The latter is fe80::/10.  Of which the address
   architecture spec only gives the addresses that match fe80::/64 the
   standard format (by specifying intermediate 54 bits are all 0).  As a
   result the former is (currently) only fe80::/64.

   For people in the RIR world it's a common thing: you get a prefix
   from the RIR and then make assignments from it for specific purposes.
   You can route the aggregate allocation, but you're not allowed to use
   the unassigned parts (until you make an assignment).  In this case
   fe80::/10 is the allocation and fe80::/64 is the assignment.

   Independent testing shows that 'ifconfig add fe80:1::1' works on
   linux but fails on openbsd.  The same command works on a Cisco

   Interpretations of the situation of linux working ok with fe80:1::1
   call it a violation of standards.

   The address fe80::1/128 is present on the loopback interface of BSD.

   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 (FreeBSD) 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 source or destination address
   matches fe80::/10 and contains a non-0 value in bits 15-31 (like
   fe80:1::1 does).  The URL of the C code in OpenBSD that leads to make

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   that packet drop is

   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.

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

   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.

6.  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.

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7.  Use-Case

   The topology in a linear convoy of cars, in V2V manner is like this:

        car1                       car2                        car3
      ---------                   ---------                  ---------
     | IP-OBU1 | ---subnet1 ---- | IP-OBU2 | --- subnet2--- | IP-OBU3 |
      ---------                   ---------                  ---------
         |in-car                     |                          |
         |subnets: Ethernet, WiFi, CAN, BT, etc

   (subnet1 is on 5880 MHz, subnet2 is on 5890 MHz)

   (in the triangular convoy the figure is different)

                             Figure 3: Figure

   Details about the restrictions with the current LL definition in the
   above topology:

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In the above topology the restrictions with RFC4291 definitions, and
the FreeBSD implementations are the following:

- RFC4291 needs 64bit MAC-based IIDs on the LLs on subnet1 and subnet2.
  The inconvenients of these restrictions are the following:
  - 64bit IIDs are too long to remember and type; easy to remember
    addresses are good for network debugging.
  - MAC-based IIDs may have some privacy risk; attackers on the road
    may listen to these IIDs (they are sent outside the car) and make
    associations that may help tracking users, like web cookies do.
  - RFC 4291 54 0 bits make it impossible to assign subnet-specific
    link-local addresses to subnet1 and subnet2.
    A RFC4291-compliant link-local address, like fe80::IID/64, assigned
    an interface on subnet2, and replying from a ping from subnet1, does
    not give ensurance that subnets (on 5880 MHz or on 5890 MHz) are set
    up wrongly.  It may be that the channels are set wrong (subnets are
    set up wrongly) as much as it may be that that fe80::IID/64 is in
    same subnet as the pinger and the channels are right.

    On another hand, if the LL addresses were like
    fe80:1::X on subnet1 and fe80:2::Y on subnet 2, then a ping issued
    from subnet1 to fe80:2::X and replying OK means clearly that the
    channels are set wrongly.

    RFC4291 54 0 bits prevent this use of subnet-specific LL addresses.

- FreeBSD OS:
  - forbids the manual assignment of LL addresses on interfaces (it is
    impossible to ifconfig add fe80::2 on an interface).
  - FreeBSD OS does not implement OCB mode.  OCB mode is an
    essential kind of link in vehicular communications.

                          Figure 4: Restrictions

   Expected improvements:

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   - human users type short LL addresses, like fe80:1::1 instead of long
   to type addresses like fe80::IID64bit

   - use fe80:1::1 and fe80:2::1 in two distinct subnets; if a ping from
   fe80:1::1 to fe80:1::2 that does not reply means the channels are
   wrong; otherwise (with fe80::IID) it is impossible to say whether the
   channels are wrong or that wrong address was used to ping (all
   fe80::IID64bit) look the same to a human - they are 'random').

   - BSD allowing manual configuration of LL addresses may have other
   benefits outside the OCB context

                          Figure 5: Improvements

8.  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.

9.  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 would be that the textual representation
   "fe80::/10" as an IPv6 link-local prefix would disappear from that
   IANA page.

10.  Contributors

   Listed from 6man WG discussion.

11.  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.

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   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, Gyan Mishra, Yu
   Tianpeng, Dusan Mudric.

   Peter Paluch submitted the Erratum suggestion to RFC 4291 about link-
   local addresses, and Brian Haberman rejected it, by noting 'would
   need' a draft.  Igor Lubashev pointed to that Erratum.

12.  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.

   -15: added references to draft-farmer-6man-exceptions-64-09 and
   draft-farmer-6man-routing-64-02, and interpreted them; added
   explanations of the solutions mentioned in WG discussion; added a
   use-case of car convoy with details about current restrictions of LL
   addressing and how a variable len plen for LL can improve the

   -14: updated authorship.

   -13: added a Problem Statement section; added the name of the
   Organisation of one co-author; distinguished between 'need' and
   'would need' a draft.

   -12: the '64' in GUA vs '64' in LL issued by distinct sources: RA vs
   RFC4291 respectively; the address fe80::1/128 is present on the
   loopback interface of BSD; detailed, again, the distinction for 'on-
   link' determination; detailed, again, the distinction between
   'assignment' and 'allocation'; added the fact that Cisco supports
   manual assignment of fe80:1::1.

   -11: trying the attribute updates=RFC4291,RFC4007 in the rfc tag.

   -10: syntax error corrected; more explanation about how FreeBSD C
   code blocks fe80:1::1; clarification in IANA section, but doubtful.

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   -09: added a reference to RFC 4007 about Zone ID in LL; added a
   reference to draft-bourbaki about IPv6 being classless; added the
   result of independent testing showing ifconfig add fe80:1::1 works on
   linux but fails on BSD; added URL to C code in BSD flavor that may be
   in charge of dropping packets whose src/dst is an LL like fe80:1::1;
   added two co-authors.

   -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
   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.)

Authors' Addresses

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

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

   Loganaden Velvindron
   city , region   code

   Phone: +phonenumber
   Email: loganaden@gmail.com

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   Naveen Kottapalli
   Benu Networks
   City , Region   code

   Phone: +phonenumber
   Email: naveen.sarma@gmail.com

   Gyan Mishra
   Verizon Communications
   City , Region   code

   Phone: cell 202 734-1000
   Email: hayabusagsm@gmail.com

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