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rfc:rfc5220

Network Working Group A. Matsumoto Request for Comments: 5220 T. Fujisaki Category: Informational NTT

                                                             R. Hiromi
                                                         Intec Netcore
                                                           K. Kanayama
                                                         INTEC Systems
                                                             July 2008
  Problem Statement for Default Address Selection in Multi-Prefix
     Environments: Operational Issues of RFC 3484 Default Rules

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Abstract

 A single physical link can have multiple prefixes assigned to it.  In
 that environment, end hosts might have multiple IP addresses and be
 required to use them selectively.  RFC 3484 defines default source
 and destination address selection rules and is implemented in a
 variety of OSs.  But, it has been too difficult to use operationally
 for several reasons.  In some environments where multiple prefixes
 are assigned on a single physical link, the host using the default
 address selection rules will experience some trouble in
 communication.  This document describes the possible problems that
 end hosts could encounter in an environment with multiple prefixes.

Matsumoto, et al. Informational [Page 1] RFC 5220 Address Selection PS July 2008

Table of Contents

 1. Introduction ....................................................2
    1.1. Scope of This Document .....................................3
 2. Problem Statement ...............................................4
    2.1. Source Address Selection ...................................4
         2.1.1. Multiple Routers on a Single Interface ..............4
         2.1.2. Ingress Filtering Problem ...........................5
         2.1.3. Half-Closed Network Problem .........................6
         2.1.4. Combined Use of Global and ULA ......................7
         2.1.5. Site Renumbering ....................................8
         2.1.6. Multicast Source Address Selection ..................9
         2.1.7. Temporary Address Selection .........................9
    2.2. Destination Address Selection .............................10
         2.2.1. IPv4 or IPv6 Prioritization ........................10
         2.2.2. ULA and IPv4 Dual-Stack Environment ................11
         2.2.3. ULA or Global Prioritization .......................12
 3. Conclusion .....................................................13
 4. Security Considerations ........................................14
 5. Normative References ...........................................14

1. Introduction

 In IPv6, a single physical link can have multiple prefixes assigned
 to it.  In such cases, an end host may have multiple IP addresses
 assigned to an interface on that link.  In the IPv4-IPv6 dual-stack
 environment or in a site connected to both a Unique Local Address
 (ULA) [RFC4193] and globally routable networks, an end host typically
 has multiple IP addresses.  These are examples of the networks that
 we focus on in this document.  In such an environment, an end host
 may encounter some communication troubles.
 Inappropriate source address selection at the end host causes
 unexpected asymmetric routing, filtering by a router, or discarding
 of packets because 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 and is implemented in a variety of OSs.  But, it
 has been too difficult to use operationally for several reasons, such
 as lack of an autoconfiguration method.  There are some problematic
 cases where the hosts using the default address selection rules
 encounter communication troubles.
 This document describes the possibilities of incorrect address
 selection that lead to dropping packets and communication failure.

Matsumoto, et al. Informational [Page 2] RFC 5220 Address Selection PS July 2008

1.1. Scope of This Document

 As other mechanisms already exist, the multi-homing techniques for
 achieving redundancy are basically out of our scope.
 We focus on an end-site network environment and unmanaged hosts in
 such an environment.  This is because address selection behavior at
 these kinds of hosts is difficult to manipulate, owing to the users'
 lack of knowledge, hosts' location, or massiveness of the hosts.
 The scope of this document is to sort out problematic cases related
 to address selection.  It includes problems that can be solved in the
 framework of RFC 3484 and problems that cannot.  For the latter, RFC
 3484 might be modified to meet their needs, or another address
 selection solution might be necessary.  For the former, an additional
 mechanism that mitigates the operational difficulty might be
 necessary.
 This document also includes simple solution analysis for each
 problematic case.  This analysis basically just focuses on whether or
 not the case can be solved in the framework of RFC 3484.  If not,
 some possible solutions are described.  Even if a case can be solved
 in the framework of RFC 3484, as mentioned above, it does not
 necessarily mean that there is no operational difficulty.  For
 example, in the environment stated above, it is not a feasible
 solution to configure each host's policy table by hand.  So, for such
 a solution, the difficulty of configuration is yet another common
 problem.

Matsumoto, et al. Informational [Page 3] RFC 5220 Address Selection PS July 2008

2. Problem Statement

2.1. Source Address Selection

2.1.1. Multiple Routers on a 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
                         +------+
                                  Figure 1
 Generally speaking, there is no interaction between next-hop
 determination and address selection.  In this example, when a host
 starts a new connection and 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".
 Solution analysis:
    As this case depends on next-hop selection, controlling the
    address selection behavior at the Host alone doesn't solve the
    entire problem.  One possible solution for this case is adopting
    source-address-based routing at Router1 and Router2.  Another
    solution may be using static routing at Router1, Router2, and the
    Host, and using the corresponding static address selection policy
    at the Host.

Matsumoto, et al. Informational [Page 4] RFC 5220 Address Selection PS July 2008

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
                         +------+
                                  Figure 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
 is becoming more popular among ISPs to mitigate the damage of
 denial-of-service (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.
 Solution analysis:
    One possible solution for this case is adopting source-address-
    based routing at the Router.  Another solution may be using static
    routing at the Router, and using the corresponding static address
    selection policy at the Host.

Matsumoto, et al. Informational [Page 5] RFC 5220 Address Selection PS July 2008

2.1.3. Half-Closed Network Problem

 You can see a second typical source address selection problem in a
 multi-homed site with global half-closed connectivity, as shown in
 the figure below.  In this case, Host-A is in a multi-homed 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.
                         +--------+
                         | 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
                        +--------+
                                   Figure 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

Matsumoto, et al. Informational [Page 6] RFC 5220 Address Selection PS July 2008

 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.  To solve this kind of
 network-policy-based address selection problem, it is likely that
 delivering additional information to a node provides a better
 solution than using algorithms that are local to the node.
 Solution analysis:
    This problem can be solved in the RFC 3484 framework.  For
    example, configuring some address selection policies into Host-A's
    RFC 3484 policy table can solve this problem.

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 |
                 +---------+    +------+
                              Figure 4
 As RFC 4864 [RFC4864] describes, using a ULA may be beneficial in
 some scenarios.  If the ULA is used for internal communication,
 packets with the ULA need to be filtered at the Router.

Matsumoto, et al. Informational [Page 7] RFC 5220 Address Selection PS July 2008

 This case does not presently create an address selection problem
 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
 Internet, the longest matching rule ceases to function properly for
 the people trying to connect to the servers with those addresses.
 For example, when the destination host has an IPv6 address 8000::1,
 and the originating host has 2001:db8:a:100::100 and
 fd01:2:3:100::100, the source address will be fd01:2:3:100::100,
 because the longest matching bit length is 0 for 2001:db8:a:100::100
 and 1 for fd01:2:3:100::100, respectively.
 Solution analysis:
    This problem can be solved in the RFC 3484 framework.  For
    example, configuring some address selection policies into the
    Host's RFC 3484 policy table can solve this problem.  Another
    solution is to modify RFC 3484 and define ULA's scope smaller than
    the global scope.

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], Section 5.5.4, does not
 absolutely prohibit the use of a deprecated address for a new
 outgoing connection due to limitations on the capabilities of
 applications.

Matsumoto, et al. Informational [Page 8] RFC 5220 Address Selection PS July 2008

                            +-----+---+
                            | Router  |
                            +----+----+
                                 |  2001:db8:b::/64  (new)
                                 |  2001:db8:a::/64 (old)
                       ------+---+----------
                             |
                          +--+---+ 2001:db8:b::100  (new)
                          | Host | 2001:db8:a::100 (old)
                          +------+
                              Figure 5
 Solution analysis:
    This problem can be mitigated in the RFC 3484 framework.  For
    example, configuring some address selection policies into the
    Host's RFC 3484 policy table can solve this problem.

2.1.6. Multicast Source Address Selection

 This case is an example of site-local or global unicast
 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 Unicast
 Address.
 Solution analysis:
    This problem can be solved in the RFC 3484 framework.  For
    example, configuring some address selection policies into the
    sending host's RFC 3484 policy table can solve this problem.

2.1.7. Temporary Address Selection

 RFC 3041 [RFC3041] defines a Temporary Address.  The usage of a
 Temporary Address has both pros and cons.  It is good for viewing web
 pages or communicating with the general public, but it is bad for a
 service that uses address-based authentication and for logging
 purposes.
 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 an HA (home address) and a CoA (care-of address) in a
 Mobile IPv6 [RFC3775] network.

Matsumoto, et al. Informational [Page 9] RFC 5220 Address Selection PS July 2008

 Section 6 ("Future Work") of RFC 3041 discusses that an API extension
 might be necessary to achieve a better address selection mechanism
 with finer granularity.
 Solution analysis:
    This problem cannot be solved in the RFC 3484 framework.  A
    possible solution is to make applications to select desirable
    addresses by using the IPv6 Socket API for Source Address
    Selection defined in RFC 5014 [RFC5014].

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 so that it is the other way around.

Matsumoto, et al. Informational [Page 10] RFC 5220 Address Selection PS July 2008

                          +---------+
                          | 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
                         +------+
                              Figure 6
 In the figure above, a site has native IPv4 and tunneled IPv6
 connectivity.  Therefore, the administrator may want to set a higher
 priority for using IPv4 than for using IPv6 because the quality of
 the tunnel network seems to be worse than that of the native
 transport.
 Solution analysis:
    This problem can be solved in the RFC 3484 framework.  For
    example, configuring some address selection policies into the
    Host's RFC 3484 policy table can solve this problem.

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

Matsumoto, et al. Informational [Page 11] RFC 5220 Address Selection PS July 2008

 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-B 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-B | 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-A | 192.0.2.245
                         +--------+
                              Figure 7
 Solution analysis:
    This problem can be solved in the RFC 3484 framework.  For
    example, configuring some address selection policies into Host-A's
    RFC 3484 policy table can solve this problem.

2.2.3. ULA or Global Prioritization

 Differentiating services by the client's source address is very
 common.  IP-address-based authentication is a 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
 another example is DNS zone splitting.

Matsumoto, et al. Informational [Page 12] RFC 5220 Address Selection PS July 2008

 However, a ULA and an IPv6 global address both have global scope, and
 RFC 3484 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-B |
                          +-+--|---+
                            |  |
                    ===========|==
                    | 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-A | fc12:3456:789a:100::100
                         +--------+
                              Figure 8
 Solution analysis:
    This problem can be solved in the RFC 3484 framework.  For
    example, configuring some address selection policies into Host-A's
    RFC 3484 policy table can solve this problem.

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; this
 choice cannot be achieved only by routers.

Matsumoto, et al. Informational [Page 13] RFC 5220 Address Selection PS July 2008

 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.

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 Section
 2.1.3, when Host-A uses a default address selection policy and
 chooses an inappropriate address, a packet sent to a 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 risks are not serious.
 As documented in the Security Considerations section of 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 an anycast or multicast packet),
 the malicious host can get knowledge of multiple addresses attached
 to the target host.  In a case like Section 2.1.4, if an attacker can
 make the Host to send a multicast packet and the 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. Normative References

 [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.
 [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.

Matsumoto, et al. Informational [Page 14] RFC 5220 Address Selection PS July 2008

 [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.
 [RFC4864]  Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
            E. Klein, "Local Network Protection for IPv6", RFC 4864,
            May 2007.
 [RFC5014]  Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
            Socket API for Source Address Selection", RFC 5014,
            September 2007.

Matsumoto, et al. Informational [Page 15] RFC 5220 Address Selection PS July 2008

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
 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 Systems Institute, Inc.
 Shimoshin-machi 5-33
 Toyama-shi, Toyama  930-0804
 Japan
 Phone: +81 76 444 8088
 EMail: kanayama_kenichi@intec-si.co.jp

Matsumoto, et al. Informational [Page 16] RFC 5220 Address Selection PS July 2008

Full Copyright Statement

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 contained in BCP 78, and except as set forth therein, the authors
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Matsumoto, et al. Informational [Page 17]

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