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Versions: (draft-arifumi-6man-rfc3484-revise) 00 01 02 03 04 05 draft-ietf-6man-rfc3484bis

Network Working Group                                       A. Matsumoto
Internet-Draft                                                   J. Kato
Intended status: Standards Track                             T. Fujisaki
Expires: September 15, 2011                                          NTT
                                                          March 14, 2011


         Update to RFC 3484 Default Address Selection for IPv6
                 draft-ietf-6man-rfc3484-revise-02.txt

Abstract

   RFC 3484 describes algorithms for source address selection and for
   destination address selection.  The algorithms specify default
   behavior for all Internet Protocol version 6 (IPv6) implementations.
   This document specifies a set of updates that modify the algorithms
   and provide fixes for the identified issues.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 September 15, 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
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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Specification  . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Changes related to the default policy table  . . . . . . .  3
       2.1.1.  ULAs in the policy table . . . . . . . . . . . . . . .  4
       2.1.2.  Teredo in the policy table . . . . . . . . . . . . . .  4
       2.1.3.  Deprecated addresses in the policy table . . . . . . .  4
       2.1.4.  Renewed default policy table . . . . . . . . . . . . .  4
     2.2.  The longest matching rule  . . . . . . . . . . . . . . . .  5
     2.3.  Utilize next-hop for source address selection  . . . . . .  5
     2.4.  Private IPv4 address scope . . . . . . . . . . . . . . . .  6
     2.5.  Deprecation of site-local unicast address  . . . . . . . .  6
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     5.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  8
   Appendix B.  Discussion  . . . . . . . . . . . . . . . . . . . . .  8
     B.1.  Centrally assigned ULA . . . . . . . . . . . . . . . . . .  8
     B.2.  6to4, Teredo, and IPv4 prioritization  . . . . . . . . . .  9
     B.3.  Deprecated address . . . . . . . . . . . . . . . . . . . .  9
     B.4.  The longest match rule . . . . . . . . . . . . . . . . . .  9
   Appendix C.  Revision History  . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11










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1.  Introduction

   The IPv6 addressing architecture [RFC4291] allows multiple unicast
   addresses to be assigned to interfaces.  Because of this IPv6
   implementations need to handle multiple possible source and
   destination addresses when initiating communication.  RFC 3484
   [RFC3484] specifies the default algorithms, common across all
   implementations, for selecting source and destination addresses so
   that it is easier to predict the address selection behavior.

   Since RFC 3484 was published, some issues have been identified with
   the algorithm specified there.  The issues are related to the longest
   match algorithm used in Rule 9 of Destination address selection
   breaking DNS round-robin techniques, and prioritization of poor IPv6
   connectivity using transition mechanisms over native IPv4
   connectivity.

   There have also been some significant changes to the IPv6 addressing
   architecture that require changes in the RFC 3484 policy table.  Such
   changes include the deprecation of site-local unicast addresses
   [RFC3879] and the IPv4-compatible IPv6 addresses, the introduction of
   Unique Local Addresses [RFC4193] etc.

   This document specifies a set of updates that modify the algorithms
   and provide fixes for the identified issues.


2.  Specification

2.1.  Changes related to the default policy table

   The default policy table is defined in RFC 3484 Section 2.1 as
   follows:

         Prefix        Precedence Label
         ::1/128               50     0
         ::/0                  40     1
         2002::/16             30     2
         ::/96                 20     3
         ::ffff:0:0/96         10     4

   The changes that should be included into the default policy table are
   those rules that are universally useful and do no harm in every
   reasonable network environment.  The changes we should consider for
   the default policy table are listed in this sub-section.

   The policy table is defined to be configurable.  If the local site
   policy needs to be different changes can be put into the policy table



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   manually or by using the auto-configuration mechanism proposed as a
   DHCP option [I-D.ietf-6man-addr-select-opt].

2.1.1.  ULAs in the policy table

   RFC 5220 [RFC5220] Section 2.1.4, 2.2.2, and 2.2.3 describes address
   selection problems related to ULAs [RFC4193].  These problems can be
   solved by either changing the scope of ULAs to site-local, or by
   adding an entry to the default policy table entry that has its own
   label for ULAs.

   ULAs has been specified with a global scope because the reachability
   of the ULAs was intended to be restricted by the routing system.
   Since a ULA will not be exposed outside of its reachability domain,
   if a ULA is available as a candidate destination address, it can be
   expected to be reachable.  In fact, such ULA to ULA communication is
   often desired (in particular in sites where ULAs are intended to
   provide stable addresses when the global prefix may be changing) and
   thus needs to be prioritized.

   Therefore, the scope of ULA should be kept global, and prioritization
   of ULA to ULA communication should be implemented in the policy
   table, by assigning a specific label for ULAs using fc00::/7.

2.1.2.  Teredo in the policy table

   Teredo [RFC4380] is defined and has been assigned 2001::/32.  This
   address block should be assigned its own label in the policy table.
   Teredo's priority should be less than or equal to 6to4, considering
   its characteristic of being a transitional tunnel mechanism.  Windows
   already implements this.

2.1.3.  Deprecated addresses in the policy table

   IPv4-compatible IPv6 addresses are deprecated [RFC4291].  IPv6 site-
   local unicast addresses are deprecated [RFC3879].  Moreover, the
   6bone testing address has also been phased out[RFC3701].  The issue
   is how we treat these outdated addresses.

2.1.4.  Renewed default policy table

   After applying these updates, the default policy table becomes:









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         Prefix        Precedence Label
         ::1/128               60     0
         fc00::/7              50     1
         ::/0                  40     2
         ::ffff:0:0/96         30     3
         2002::/16             20     4
         2001::/32             10     5
         ::/96                  1    10
         fec::/16               1    11
         3ffe::/16              1    12

2.2.  The longest matching rule

   This issue is related to a problem with the longest matching rule, as
   reported by Dave Thaler.  It is a malfunction of the DNS round-robin
   technique.  It is common for both IPv4 and IPv6.

   When a destination address DA, DB, and the source address of DA
   Source(DA) are on the same subnet and Source(DA) == Source(DB), DNS
   round robin load-balancing cannot function.  By considering prefix
   lengths that are longer than the subnet prefix, this rule establishes
   preference between addresses that have no substantive differences
   between them.  The rule functions as an arbitrary tie-breaker between
   the hosts in a round robin, causing a given host to always prefer a
   given member of the round robin.

   By limiting the calculation of common prefixes to a maximum length
   equal to the length of the subnet prefix of the source address, rule
   9 can continue to favor hosts that are nearby in the network
   hierarchy without arbitrarily sorting addresses within a given
   network.  This modification could be written as follows:

   Rule 9: Use longest matching prefix.

   When DA and DB belong to the same address family (both are IPv6 or
   both are IPv4): If CommonPrefixLen(DA & Netmask(Source(DA)),
   Source(DA)) > CommonPrefixLen(DB & Netmask(Source(DB)), Source(DB)),
   then prefer DA.  Similarly, if CommonPrefixLen(DA &
   Netmask(Source(DA)), Source(DA)) < CommonPrefixLen(DB &
   Netmask(Source(DB)), Source(DB)), then prefer DB.

2.3.  Utilize next-hop for source address selection

   RFC 3484 source address selection rule 5 states that the address that
   is attached to the outgoing interface should be preferred as the
   source address.  This rule is reasonable considering the prevalence
   of Ingress Filtering described in BCP 38 [RFC2827].  This is because
   an upstream network provider usually assumes it receives those



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   packets from customers that will use the delegated addresses as their
   source addresses.

   This rule, however, is not effective in an environment such as
   described in RFC 5220 Section 2.1.1, where a host has multiple
   upstream routers on the same link and has addresses delegated from
   each upstream on single interface.

   So, a new rule 5.1 that utilizes next-hop information for source
   address selection is inserted just after the rule 5.

   Rule 5.1: Use an address assigned by the selected next-hop.

   If SA is assigned by the selected next-hop that will be used to send
   to D and SB is assigned by a different next-hop, then prefer SA.
   Similarly, if SB is assigned by the next-hop that will be used to
   send to D and SA is assigned by a different next-hop, then prefer SB.

2.4.  Private IPv4 address scope

   When a packet goes through a NAT, its source or destination address
   can get replaced with another address with a different scope.  It
   follows that the result of the source address selection algorithm may
   be different when the original address is replaced with the NATed
   address.

   The algorithm currently specified in RFC 3484 is based on the
   assumption that a source address with a small scope cannot reach a
   destination address with a larger scope.  This assumption does not
   hold if private IPv4 addresses and a NAT are used to reach public
   IPv4 addresses.

   Due to this assumption, in the presence of both NATed private IPv4
   address and transitional addresses (like 6to4 and Teredo), the host
   will choose the transitional IPv6 address to access dual-stack peers
   [I-D.denis-v6ops-nat-addrsel].  Choosing transitional IPv6
   connectivity over native IPv4 connectivity is not desirable.

   This issue can be fixed by changing the address scope of private IPv4
   addresses to global.  Such a change has already been implemented in
   some OSes.

2.5.  Deprecation of site-local unicast address

   RFC 3484 contains a few "site-local unicast" and "fec::"
   descriptions.  It's better to remove examples related to site-local
   unicast address, or change examples to use ULAs.  Points that need to
   be re-written are:



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      - the 2nd paragraph in RFC 3484 Section 3.1 describing the scope
      comparison mechanism.
      - RFC 3484 Section 10 containing examples for site-local address.


3.  Security Considerations

   No security risk is found that degrades RFC 3484.


4.  IANA Considerations

   An address type number for the policy table may have to be assigned
   by IANA.


5.  References

5.1.  Normative References

   [RFC1794]  Brisco, T., "DNS Support for Load Balancing", RFC 1794,
              April 1995.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC3701]  Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
              Allocation) Phaseout", RFC 3701, March 2004.

   [RFC3879]  Huitema, C. and B. Carpenter, "Deprecating Site Local
              Addresses", RFC 3879, September 2004.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC5220]  Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
              "Problem Statement for Default Address Selection in Multi-



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              Prefix Environments: Operational Issues of RFC 3484
              Default Rules", RFC 5220, July 2008.

5.2.  Informative References

   [I-D.denis-v6ops-nat-addrsel]
              Denis-Courmont, R., "Problems with IPv6 source address
              selection and IPv4 NATs", draft-denis-v6ops-nat-addrsel-00
              (work in progress), February 2009.

   [I-D.ietf-6man-addr-select-considerations]
              Chown, T., "Considerations for IPv6 Address Selection
              Policy Changes",
              draft-ietf-6man-addr-select-considerations-02 (work in
              progress), July 2010.

   [I-D.ietf-6man-addr-select-opt]
              Matsumoto, A., Fujisaki, T., and J. Kato, "Distributing
              Address Selection Policy using DHCPv6",
              draft-ietf-6man-addr-select-opt-00 (work in progress),
              December 2010.

   [I-D.ietf-ipv6-ula-central]
              Hinden, R., "Centrally Assigned Unique Local IPv6 Unicast
              Addresses", draft-ietf-ipv6-ula-central-02 (work in
              progress), June 2007.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.


Appendix A.  Acknowledgements

   The authors would like to thank to Dave Thaler, Pekka Savola, Remi
   Denis-Courmont and the members of 6man's address selection design
   team for their invaluable contributions to this document.


Appendix B.  Discussion

B.1.  Centrally assigned ULA

   Discussion:  Centrally assigned ULA [I-D.ietf-ipv6-ula-central] is
      proposed, and assigned fc00::/8.  Using the different labels for
      fc00::/8 and fd00::/8 makes sense if we can assume the same kind
      of address block is assigned in the same or adjacent network.




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      However, the way of assignment and network adjancency may not have
      any relationships.

B.2.  6to4, Teredo, and IPv4 prioritization

   Discussion:  Regarding the prioritization between IPv4 and these
      transitional mechanisms, their connectivity quality is recently
      known to be worse than IPv4.  These mechiansms are said to be the
      last resort access to IPv6 resources.  The 6to4 should have higher
      precedence over Teredo, in that 6to4 host to 6to4 host
      communication runs over IPv4, which can result in a more optimal
      path, and 6to4 does not need NAT traversal.

B.3.  Deprecated address

   Discussion:  These addresses were removed from the current
      specification.  So, they should not be treated differently,
      especially if we think about future re-use of these address
      blocks.

      Considering the inappropriate use of these address blocks,
      especially in outdated implementations, and bad effects caused by
      them, however, they should be labeled differently from the
      legitimate address blocks.
      Or should we keep this entry for the sake of backward
      compatibility?

B.4.  The longest match rule

   RFC 3484 defines that the destination address selection rule 9 should
   be applied to both IPv4 and IPv6, which spoils the DNS based load
   balancing technique that is widely used in the IPv4 Internet today.

   When two or more destination addresses are acquired from one FQDN,
   rule 9 states that the longest matching destination and source
   address pair should be chosen.  As stated in RFC 1794, the DNS based
   load balancing technique is achieved by not re-ordering the
   destination addresses returned from the DNS server.  Rule 9 defines a
   deterministic rule for re-ordering at hosts, hence the technique of
   RFC 1794 is not available anymore.

   Regarding this problem, there was discussion in the IETF and other
   places that led to some different options being suggested, as listed
   below.

   Discussion: The possible changes to RFC 3484 are as follows:





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   1.  To delete Rule 9 completely.
   2.  To apply Rule 9 only for IPv6 and not for IPv4.  In IPv6,
       hierarchical address assignment is a general principle, hence the
       longest matching rule is beneficial in many cases.  In IPv4, as
       stated above, the DNS based load balancing technique is widely
       used.
   3.  To apply Rule 9 for IPv6 conditionally and not for IPv4.  When
       the length of matching bits of the destination address and the
       source address is longer than N, rule 9 is applied.  Otherwise,
       the order of the destination addresses do not change.  The N
       should be configurable and it should be 32 by default.  This is
       simply because the two sites whose matching bit length is longer
       than 32 are probably adjacent.

   Now that IPv6 PI addressing is being assigned by some RIRs,
   hierachical address assignment is not fully maintained anymore.  It
   seems that the longest matching algorithm may not be worth the
   adverse effect of disalbing the DNS based load balance technique.


Appendix C.  Revision History

   02:
      Suresh Krishnan's comments were incorporated.
      A new source address selection rule that utilizes the next-hop
      information is included in Section 2.3

   01:
      Restructured to contain only the actual changes to RFC 3484.

   00:
      Published as a 6man working group item.

   03:
      Added acknowledgements.
      Added longest matching algorithm malfunction regarding local DNS
      round robin.
      The proposed changes section was restructured.
      The issue of 6to4/Teredo and IPv4 prioritization was included.
      The issue of deprecated addresses was added.
      The renewed default policy table was changed accordingly.

   02:
      Added the reference to address selection design team's proposal.

   01:





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      The issue of private IPv4 address scope was added.
      The issue of ULA address scope was added.
      Discussion of longest matching rule was expanded.


Authors' Addresses

   Arifumi Matsumoto
   NTT SI Lab
   Midori-Cho 3-9-11
   Musashino-shi, Tokyo  180-8585
   Japan

   Phone: +81 422 59 3334
   Email: arifumi@nttv6.net


   Jun-ya Kato
   NTT SI Lab
   Midori-Cho 3-9-11
   Musashino-shi, Tokyo  180-8585
   Japan

   Phone: +81 422 59 2939
   Email: kato@syce.net


   Tomohiro Fujisaki
   NTT PF Lab
   Midori-Cho 3-9-11
   Musashino-shi, Tokyo  180-8585
   Japan

   Phone: +81 422 59 7351
   Email: fujisaki@syce.net
















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