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Versions: (draft-arifumi-v6ops-addr-select-ps) 00 01 02 03 04 05 06 07 08 09 RFC 5220

IPv6 Operations Working Group                               A. Matsumoto
Internet-Draft                                               T. Fujisaki
Intended status: Informational                                       NTT
Expires: August 28, 2008                                       R. Hiromi
                                                             K. Kanayama
                                                           Intec Netcore
                                                       February 25, 2008


     Problem Statement of Default Address Selection in Multi-prefix
        Environment: Operational Issues of RFC3484 Default Rules
                 draft-ietf-v6ops-addr-select-ps-04.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on August 28, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   One physical link can carry multiple subnets.  Moreover, we can use
   multiple physical networks at the same time in a host.  In that
   environment, end hosts might have multiple IP addresses and be
   required to use them selectively.  Without an appropriate source/



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   destination address selection mechanism, the host will experience
   some trouble in communication.  RFC 3484 defines default source and
   destination address selection algorithms, but the multi-prefix
   environment considered here needs additional rules beyond those of
   the default operation.  This document describes the possible problems
   that end hosts could encounter in an environment with multiple
   subnets.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Scope of this document . . . . . . . . . . . . . . . . . .  3
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Source Address Selection . . . . . . . . . . . . . . . . .  3
       2.1.1.  Multiple Routers on Single Interface . . . . . . . . .  4
       2.1.2.  Ingress Filtering Problem  . . . . . . . . . . . . . .  5
       2.1.3.  Half-Closed Network Problem  . . . . . . . . . . . . .  5
       2.1.4.  Combined Use of Global and ULA . . . . . . . . . . . .  7
       2.1.5.  Site Renumbering . . . . . . . . . . . . . . . . . . .  8
       2.1.6.  Multicast Source Address Selection . . . . . . . . . .  8
       2.1.7.  Temporary Address Selection  . . . . . . . . . . . . .  8
     2.2.  Destination Address Selection  . . . . . . . . . . . . . .  9
       2.2.1.  IPv4 or IPv6 prioritization  . . . . . . . . . . . . .  9
       2.2.2.  ULA and IPv4 dual-stack environment  . . . . . . . . . 10
       2.2.3.  ULA or Global Prioritization . . . . . . . . . . . . . 10
   3.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Appendix. Revision History  . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 15
















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1.  Introduction

   One physical link can carry multiple subnets.  In that case, an end-
   host has multiple IP addresses.  In the IPv4-IPv6 dual stack
   environment or in a site connected to both a ULA [RFC4193] and Global
   scope networks, an end-host has multiple IP addresses.  These are
   examples of networks that we focus on in this document.  In such an
   environment, an end-host will encounter some communication troubles.

   Inappropriate source address selection at the end-host causes
   unexpected asymmetric routing, filtering by a router or discarding of
   packets bacause there is no route to the host.

   Considering a multi-prefix environment, destination address selection
   is also important for correct or better communication establishment.

   RFC 3484 [RFC3484] defines default source and destination address
   selection algorithms.  In most cases, the host will be able to
   communicate with the targeted host using the algorithms.  However,
   there are still problematic cases.  This document describes such
   possibilities of incorrect address selection, which leads to dropping
   packets and communication failure.

1.1.  Scope of this document

   There has been a lot of discussion about "multiple addresses/
   prefixes".  As other mechanisms already exists, the multi-homing
   techniques for achieving redundancy are out of our scope.

   We focus on an end-site network environment.  The scope of this
   document is to sort out problematic cases related to address
   selection.  It includes problems that cannot always be solved by
   changing the host's address selection algorithm, such as an address
   selection mechanism that depends on the IPv6 address types.  For
   example, a global address isn't always globally routable and ULA's
   routable domain is dependent on the network policy.  This document
   includes these kind of network policy related address selection
   problems, as long as these problems are serious enough and worth
   solving.


2.  Problem Statement

2.1.  Source Address Selection







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2.1.1.  Multiple Routers on Single Interface

                          ==================
                          |    Internet    |
                          ==================
                             |          |
          2001:db8:1000::/36 |          | 2001:db8:8000::/36
                        +----+-+      +-+----+
                        | ISP1 |      | ISP2 |
                        +----+-+      +-+----+
                             |          |
         2001:db8:1000:::/48 |          | 2001:db8:8000::/48
                       +-----+---+ +----+----+
                       | Router1 | | Router2 |
                       +-------+-+ +-+-------+
                               |     |
          2001:db8:1000:1::/64 |     | 2001:db8:8000:1::/64
                               |     |
                        -----+-+-----+------
                             |
                           +-+----+ 2001:db8:1000:1::100
                           | Host | 2001:db8:8000:1::100
                           +------+

                                [Fig. 1]

   Generally speaking, there is no interaction between next-hop
   determination and address selection.  In this example, when a host
   sends a packet via Router1, the host does not necessarily choose
   address 2001:db8:1000:1::100 given by Router1 as the source address.
   This causes the same problem as described in the next section
   'Ingress Filtering Problem'.



















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2.1.2.  Ingress Filtering Problem

                        ==================
                        |    Internet    |
                        ==================
                             |       |
          2001:db8:1000::/36 |       | 2001:db8:8000::/36
                        +----+-+   +-+----+
                        | ISP1 |   | ISP2 |
                        +----+-+   +-+----+
                             |       |
         2001:db8:1000:::/48 |       | 2001:db8:8000::/48
                            ++-------++
                            | Router  |
                            +----+----+
                                 |  2001:db8:1000:1::/64
                                 |  2001:db8:8000:1::/64
                       ------+---+----------
                             |
                           +-+----+ 2001:db8:1000:1::100
                           | Host | 2001:db8:8000:1::100
                           +------+

                                [Fig. 2]

   When a relatively small site, which we call a "customer network", is
   attached to two upstream ISPs, each ISP delegates a network address
   block, which is usually /48, and a host has multiple IPv6 addresses.

   When the source address of an outgoing packet is not the one that is
   delegated by an upstream ISP, there is a possibility that the packet
   will be dropped at the ISP by its Ingress Filter.  Ingress
   filtering(uRPF: unicast Reverse Path Forwarding) is becoming more
   popular among ISPs to mitigate the damage of DoS attacks.

   In this example, when the Router chooses the default route to ISP2
   and the Host chooses 2001:db8:1000:1::100 as the source address for
   packets sent to a host (2001:db8:2000::1) somewhere on the Internet,
   the packets may be dropped at ISP2 because of Ingress Filtering.

2.1.3.  Half-Closed Network Problem

   You can see a second typical source address selection problem in a
   multihome site with global-closed mixed connectivity like in the
   figure below.  In this case, Host-A is in a multihomed network and
   has two IPv6 addresses, one delegated from each of the upstream ISPs.
   Note that ISP2 is a closed network and does not have connectivity to
   the Internet.



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                           +--------+
                           | Host-C | 2001:db8:a000::1
                           +-----+--+
                                 |
                        ==============  +--------+
                        |  Internet  |  | Host-B | 2001:db8:8000::1
                        ==============  +--------+
                             |           |
           2001:db8:1000:/36 |           | 2001:db8:8000::/36
                        +----+-+   +-+---++
                        | ISP1 |   | ISP2 | (Closed Network/VPN tunnel)
                        +----+-+   +-+----+
                             |       |
           2001:db8:1000:/48 |       | 2001:db8:8000::/48
                            ++-------++
                            | Router  |
                            +----+----+
                                 |  2001:db8:1000:1::/64
                                 |  2001:db8:8000:1::/64
                       ------+---+----------
                             |
                          +--+-----+ 2001:db8:1000:1::100
                          | Host-A | 2001:db8:8000:1::100
                          +--------+

                                [Fig. 3]

   You do not need two physical network connections here.  The
   connection from the Router to ISP2 can be a logical link over ISP1
   and the Internet.

   When Host-A starts the connection to Host-B in ISP2, the source
   address of a packet that has been sent will be the one delegated from
   ISP2, that is 2001:db8:8000:1::100, because of rule 8 (longest
   matching prefix) in RFC 3484.

   Host-C is located somewhere on the Internet and has IPv6 address
   2001:db8:a000::1.  When Host-A sends a packet to Host-C, the longest
   matching algorithm chooses 2001:db8:8000:1::100 for the source
   address.  In this case, the packet goes through ISP1 and may be
   filtered by ISP1's ingress filter.  Even if the packet is not
   filtered by ISP1, a return packet from Host-C cannot possibly be
   delivered to Host-A because the return packet is destined for 2001:
   db8:8000:1::100, which is closed from the Internet.

   The important point is that each host chooses a correct source
   address for a given destination address as long as NAT does not exist
   in the IPv6 world.  To solve this kind of network policy based



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   address selection problems, it is likely that delivering addtional
   information to a node fits better than algorithmic solutions that are
   local to the node.

2.1.4.  Combined Use of Global and ULA

                        ============
                        | Internet |
                        ============
                              |
                              |
                         +----+----+
                         |   ISP   |
                         +----+----+
                              |
              2001:db8:a::/48 |
                         +----+----+
                         | Router  |
                         +-+-----+-+
                           |     | 2001:db8:a:100::/64
          fd01:2:3:200:/64 |     | fd01:2:3:100:/64
                   -----+--+-   -+--+----
                        |           |
      fd01:2:3:200::101 |           |      2001:db8:a:100::100
                   +----+----+    +-+----+ fd01:2:3:100::100
                   | Printer |    | Host |
                   +---------+    +------+

                                [Fig. 4]

   As NAP [I-D.ietf-v6ops-nap] describes, using a ULA may be beneficial
   in some scenarios.  If the ULA is used for internal communication,
   packets with ULA need to be filtered at the Router.

   There is no serious problem related to address selection in this
   case, because of the dissimilarity between the ULA and the Global
   Unicast Address.  The longest matching rule of RFC 3484 chooses the
   correct address for both intra-site and extra-site communication.

   In the future, however, there is a possibility that the longest
   matching rule will not be able to choose the correct address anymore.
   That is the moment when the assignment of those Global Unicast
   Addresses starts, where the first bit is 1.  In RFC 4291 [RFC4291],
   almost all address spaces of IPv6, including those whose first bit is
   1, are assigned as Global Unicast Addresses.

   Namely, when we start to assign a part of the address block 8000::/1
   as the global unicast address and that part is used somewhere in the



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   Internet, the longest matching rule ceases to function properly for
   the people trying to connect to the servers with those addresses.

2.1.5.  Site Renumbering

   RFC 4192 [RFC4192] describes a recommended procedure for renumbering
   a network from one prefix to another.  An autoconfigured address has
   a lifetime, so by stopping advertisement of the old prefix, the
   autoconfigured address is eventually invalidated.

   However, invalidating the old prefix takes a long time.  You cannot
   stop routing to the old prefix as long as the old prefix is not
   removed from the host.  This can be a tough issue for ISP network
   administrators.

   There is a technique of advertising the prefix with the preferred
   lifetime zero, however, RFC 4862 [RFC4862] 5.5.4 allows the use of a
   deprecated address for a new outgoing connection.  So, this technique
   isn't always perfect.

                              +-----+---+
                              | Router  |
                              +----+----+
                                   |  2001:db8:b::/64  (new)
                                   |  2001:db8:a::/64 (old)
                         ------+---+----------
                               |
                            +--+-----+ 2001:db8:b::100  (new)
                            | Host-A | 2001:db8:a::100 (old)
                            +--------+

                                [Fig. 5]

2.1.6.  Multicast Source Address Selection

   This case is an example of Site-local or Global prioritization.  When
   you send a multicast packet across site-borders, the source address
   of the multicast packet should be a globally routable address.  The
   longest matching algorithm, however, selects a ULA if the sending
   host has both a ULA and a global address.

2.1.7.  Temporary Address Selection

   RFC 3041 [RFC3041] defines a Temporary Address.  The usage of a
   Temporary Address has both pros and cons.  That is good for viewing
   web pages or communicating with the general public, but that is bad
   for a service that uses address-based authentication and for logging
   purposes.



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   If you could turn the temporary address on and off, that would be
   better.  If you could switch its usage per service (destination
   address), that would also be better.  The same situation can be found
   when using HA (home address) and CoA (care-of address)in a Mobile
   IPv6 [RFC3775] network.

2.2.  Destination Address Selection

2.2.1.  IPv4 or IPv6 prioritization

   The default policy table gives IPv6 addresses higher precedence than
   IPv4 addresses.  There seem to be many cases, however, where network
   administrators want to control the address selection policy of end-
   hosts the other way around.

                            +---------+
                            | Tunnel  |
                            | Service |
                            +--+---++-+
                               |   ||
                               |   ||
                        ===========||==
                        | Internet || |
                        ===========||==
                             |     ||
                192.0.2.0/24 |     ||
                        +----+-+   ||
                        | ISP  |   ||
                        +----+-+   ||
                             |     ||
               IPv4 (Native) |     || IPv6 (Tunnel)
                192.0.2.0/26 |     ||
                            ++-----++-+
                            | Router  |
                            +----+----+
                                 |  2001:db8:a:1::/64
                                 |  192.0.2.0/28
                                 |
                       ------+---+----------
                             |
                           +-+----+ 2001:db8:a:1::100
                           | Host | 192.0.2.2
                           +------+

                                [Fig. 6]

   In the figure above, a site has native IPv4 and tunneled-IPv6
   connectivity.  Therefore, the administrator may want to set a higher



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   priority for using IPv4 than using IPv6 because the quality of the
   tunnel network seems to be worse than that of the native transport.

2.2.2.  ULA and IPv4 dual-stack environment

   This is a special form of IPv4 and IPv6 prioritization.  When an
   enterprise has IPv4 Internet connectivity but does not yet have IPv6
   Internet connectivity, and the enterprise wants to provide site-local
   IPv6 connectivity, a ULA is the best choice for site-local IPv6
   connectivity.  Each employee host will have both an IPv4 global or
   private address and a ULA.  Here, when this host tries to connect to
   Host-C that has registered both A and AAAA records in the DNS, the
   host will choose AAAA as the destination address and the ULA for the
   source address.  This will clearly result in a connection failure.

                           +--------+
                           | Host-C | AAAA = 2001:db8::80
                           +-----+--+ A    = 192.0.2.1
                                 |
                        ============
                        | Internet |
                        ============
                             |  no IPv6 connectivity
                        +----+----+
                        | Router  |
                        +----+----+
                             |
                             | fd01:2:3::/48 (ULA)
                             | 192.0.2.128/25
                            ++--------+
                            | Router  |
                            +----+----+
                                 |  fd01:2:3:4::/64 (ULA)
                                 |  192.0.2.240/28
                       ------+---+----------
                             |
                           +-+----+ fd01:2:3:4::100 (ULA)
                           | Host | 192.0.2.245
                           +------+

                                [Fig. 7]

2.2.3.  ULA or Global Prioritization

   Differentiating services by the client's source address is very
   common.  IP-address-based authentication is an typical example of
   this.  Another typical example is a web service that has pages for
   the public and internal pages for employees or involved parties.  Yet



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   another example is DNS zone splitting.

   However, a ULA and IPv6 global address both have global scope, and
   RFC3484 default rules do not specify which address should be given
   priority.  This point makes IPv6 implementation of address-based
   service differentiation a bit harder.

                            +------+
                            | Host |
                            +-+--|-+
                              |  |
                      ===========|==
                      | Internet | |
                      ===========|==
                            |    |
                            |    |
                       +----+-+  +-->+------+
                       | ISP  +------+  DNS | 2001:db8:a::80
                       +----+-+  +-->+------+ fc12:3456:789a::80
                            |    |
            2001:db8:a::/48 |    |
        fc12:3456:789a::/48 |    |
                       +----+----|+
                       | Router  ||
                       +---+-----|+
                           |     |    2001:db8:a:100::/64
                           |     |    fc12:3456:789a:100::/64
                         --+-+---|-----
                             |   |
                           +-+---|+ 2001:db8:a:100::100
                           | Host | fc12:3456:789a:100::100
                           +------+

                                [Fig. 7]


3.  Conclusion

   We have covered problems related to destination or source address
   selection.  These problems have their roots in the situation where
   end-hosts have multiple IP addresses.  In this situation, every end-
   host must choose an appropriate destination and source address, which
   cannot be achieved only by routers.

   It should be noted that end-hosts must be informed about routing
   policies of their upstream networks for appropriate address
   selection.  A site administrator must consider every possible address
   false-selection problem and take countermeasures beforehand.



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4.  Security Considerations

   When an intermediate router performs policy routing (e.g. source
   address based routing), inappropriate address selection causes
   unexpected routing.  For example, in the network described in 2.1.3,
   when Host-A uses a default address selection policy and chooses an
   inappropriate address, a packet sent to VPN can be delivered to a
   location via the Internet.  This issue can lead to packet
   eavesdropping or session hijack.  However, sending the packet back to
   the correct path from the attacker to the node is not easy, so these
   two risk are not serious.

   As documented in the security consideration section in RFC 3484,
   address selection algorithms expose a potential privacy concern.
   When a malicious host can make a target host perform address
   selection, for example by sending a anycast or a multicast packet,
   the malicious host can know multiple addresses attached to the target
   host.  In a case like 2.1.4, if an attacker can make Host to send a
   multicast packet and Host performs the default address selection
   algorithm, the attacker may be able to determine the ULAs attached to
   the Host.

   These security risks have roots in inappropriate address selection.
   Therefore, if a countermeasure is taken, and hosts always select an
   appropriate address that is suitable to a site's network structure
   and routing, these risks can be avoided.


5.  IANA Considerations

   This document has no actions for IANA.


6.  References

6.1.  Normative References

   [I-D.ietf-v6ops-nap]
              Velde, G., "Local Network Protection for IPv6",
              draft-ietf-v6ops-nap-06 (work in progress), January 2007.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              January 2001.

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




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   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [RFC4192]  Baker, F., Lear, E., and R. Droms, "Procedures for
              Renumbering an IPv6 Network without a Flag Day", RFC 4192,
              September 2005.

   [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.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

6.2.  Informative References


Appendix A.  Appendix. Revision History

   01:
      IP addresse notations changed to docmentation address.
      Descriptoin of solutions deleted.
   02:
      Security considerations section rewritten according to comments
      from SECDIR.
   03:
      Intended status changed to Informational.
   04:
      This version reflects comments from IESG members.


Authors' Addresses

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

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








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

   Phone: +81 422 59 7351
   Email: fujisaki@nttv6.net


   Ruri Hiromi
   Intec Netcore, Inc.
   Shinsuna 1-3-3
   Koto-ku, Tokyo  136-0075
   Japan

   Phone: +81 3 5665 5069
   Email: hiromi@inetcore.com


   Ken-ichi Kanayama
   Intec Netcore, Inc.
   Shinsuna 1-3-3
   Koto-ku, Tokyo  136-0075
   Japan

   Phone: +81 3 5665 5069
   Email: kanayama@inetcore.com























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Full Copyright Statement

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   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





Matsumoto, et al.        Expires August 28, 2008               [Page 15]


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