Internet Engineering Task Force                            O. Troan, Ed.
Internet-Draft                                                     Cisco
Intended status: Informational                                  D. Miles
Expires: March 3, 2012                                    Alcatel-Lucent
                                                           S. Matsushima
                                                        Softbank Telecom
                                                              T. Okimoto
                                                                NTT West
                                                                 D. Wing
                                                                   Cisco
                                                         August 31, 2011


          IPv6 Multihoming without Network Address Translation
          draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-01

Abstract

   Network Address and Port Translation (NAPT) works well for conserving
   global addresses and addressing multihoming requirements, because an
   IPv4 NAPT router implements three functions: source address
   selection, next-hop resolution and optionally DNS resolution.  For
   IPv6 hosts one approach could be the use of IPv6 NAT.  However, NAT
   should be avoided, if at all possible, to permit transparent end-to-
   end connectivity.  In this document, we analyze the use cases of
   multihoming.  We also describe functional requirements and possible
   solutions for multihoming without the use of NAT in IPv6 for hosts
   and small IPv6 networks that would otherwise be unable to meet
   minimum IPv6 allocation criteria.  Nevertheless, we mention that the
   possible needs for IPv6 NAT in the transition phase to the fully
   deployment of the proposed solutions.

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 March 3, 2012.



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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
   Provisions Relating to IETF Documents
   (http://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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  IPv6 multihomed network scenarios  . . . . . . . . . . . . . .  5
     3.1.  Classification of network scenarios for multihomed host  .  5
     3.2.  Multihomed network environment . . . . . . . . . . . . . .  8
     3.3.  Problem Statement  . . . . . . . . . . . . . . . . . . . .  9
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  End-to-End transparency  . . . . . . . . . . . . . . . . . 10
     4.2.  Policy distribution  . . . . . . . . . . . . . . . . . . . 11
     4.3.  Scalability  . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Problem statement and analysis . . . . . . . . . . . . . . . . 11
     5.1.  Source address selection . . . . . . . . . . . . . . . . . 12
     5.2.  Next-hop selection . . . . . . . . . . . . . . . . . . . . 12
     5.3.  DNS recursive name server selection  . . . . . . . . . . . 13
   6.  Implementation approach  . . . . . . . . . . . . . . . . . . . 14
     6.1.  Source address selection . . . . . . . . . . . . . . . . . 14
     6.2.  Next-hop selection . . . . . . . . . . . . . . . . . . . . 14
     6.3.  DNS recursive name server selection  . . . . . . . . . . . 15
   7.  Considerations for host without multi-prefix support . . . . . 16
     7.1.  IPv6 NAT . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.2.  Co-existence consideration . . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     11.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21





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1.  Introduction

   IPv6 provides enough globally unique addresses to permit every
   conceivable host on the Internet to be uniquely addressed without the
   requirement for Network Address Port Translation (NAPT [RFC3022]),
   offering a renaissance in end-to-end transparent connectivity.

   Unfortunately, this may not be possible due to the necessity of NAT
   even in IPv6, because of multihoming.  Though there are some
   mechanisms to implement multihoming, such as BGP multihoming
   [RFC4116] in network level, and SCTP based multihoming [RFC4960] in
   application transport level, there is no mechanism in IPv6 that
   serves as a replacement for NAT based multihoming in IPv4.  In IPv4,
   for a host or a small network, NAT based multihoming is easily
   deployable and an already deployed technique.  Some of the same
   reasons for IPv4 NATs may be applicable to IPv6.

   Whenever a host or small network (which does not meet minimum IPv6
   allocation criteria) is connected to multiple upstream networks, an
   IPv6 address is assigned by each respective service provider
   resulting in hosts with multiple global scope IPv6 addresses with
   different prefixes.  As each service provider is allocated a
   different address space from its Internet Registry, it in-turn
   assigns a different address space to the end-user network or host.
   For example, a remote access user's host or router may use a VPN to
   simultaneously connect to a remote network and retain a default route
   to the Internet for other purposes.

   In IPv4 a common solution to the multihoming problem is to employ
   NAPT on a border router and use private address space for individual
   host addressing.  The use of NAPT allows hosts to have exactly one IP
   address visible on the public network and the combination of NAPT
   with provider-specific outside addresses (one for each uplink) and
   destination-based routing insulates a host from the impacts of
   multiple upstream networks.  The border router may also implement a
   DNS cache or DNS policy to resolve address queries from hosts.

   It is our goal to avoid the IPv6 equivalent of NAT.  So, the goals
   for IPv6 multihoming defined in [RFC3582] do not match the goals of
   this document.  Also regardless of what the IPv6 NAT's specification
   is, we are trying to avoid any form of network address translation
   technique that may not be visible for either of the end hosts.  To
   reach this goal, mechanisms are needed for end-user hosts to have
   multiple address assignments and resolve issues such as which address
   to use for sourcing traffic to which destination:






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   o  If multiple routers exist on a single link the host must select
      the appropriate next-hop for each connected network.  Each router
      is in turn connected to a different service provider network,
      which provides independent address assignment.  Routing protocols
      that would normally be employed for router-to-router network
      advertisement seem inappropriate for use by individual hosts.

   o  Source address selection also becomes difficult whenever a host
      has more than one address within the same address scope.  Current
      address selection criteria may result in hosts using an arbitrary
      or random address when sourcing upstream traffic.  Unfortunately,
      for the host, the appropriate source address is a function of the
      upstream network for which the packet is bound for.  If an
      upstream service provider uses IP anti-spoofing or ingress
      filtering, it is conceivable that the packets that have an
      inappropriate source address for the upstream network would never
      reach their destination.

   o  In a multihomed environment, different DNS scopes or partitions
      may exist in each independent upstream network.  A DNS query sent
      to an arbitrary upstream DNS recursive name servier may result in
      incorrect or poisoned responses.

   In short, while IPv6 facilitates hosts having more than one address
   in the same address scope, the application of this causes significant
   issues for a host from routing, source address selection and DNS
   resolution perspectives.  A possible consequence of assigning a host
   multiple identically-scoped addresses is severely impaired IP
   connectivity.

   If a host connects to a network behind an IPv4 NAPT, the host has one
   private address in the local network.  There is no confusion.  The
   NAT becomes the gateway of the host and forwards the packet to an
   appropriate network when it is multihomed.  It also operates a DNS
   cache server, which receives all DNS inquires, and gives a correct
   answer to the host.

   In this document, we analyze the use cases of multihoming.  We also
   describe functional requirements and possible solutions for
   multihoming without the use of NAT in IPv6 for hosts and small IPv6
   networks that would otherwise be unable to meet minimum IPv6
   allocation criteria.  Nevertheless, we mention that the possible
   needs for IPv6 NAT in the transition phase to the fully deployment of
   the proposed solutions.







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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   IPv6 NAT              The terms "NAT66" and "IPv6 NAT" refer to NPTv6
                         [RFC6296].

   NAPT                  Network Address Port Translation as described
                         in [RFC3022].  In other contexts, NAPT is often
                         pronounced "NAT" or written as "NAT".

   Multihomed with multi-prefix (MHMP)  A host implementation which
                         supports the mechanisms described in this
                         document.  Namely source address selection
                         policy, next-hop selection and DNS selection
                         policy.


3.  IPv6 multihomed network scenarios

   In this section, we classify three scenarios of the multihoming
   environment.

3.1.  Classification of network scenarios for multihomed host

   Scenario 1:

   In this scenario, two or more routers are present on a single link
   shared with the host(s).  Each router is in turn connected to a
   different service provider network, which provides independent
   address assignment and DNS recursive name servers.  A host in this
   environment would be offered multiple prefixes and DNS recursive name
   servers advertised from the two different routers.
















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                                +------+       ___________
                                |      |      /           \
                            +---| rtr1 |=====/   network   \
                            |   |      |     \      1      /
               +------+     |   +------+      \___________/
               |      |     |
               | hosts|-----+
               |      |     |
               +------+     |   +------+       ___________
                            |   |      |      /           \
                            +---| rtr2 |=====/   network   \
                                |      |     \      2      /
                                +------+      \___________/

        Figure 1: single uplink, multiple next-hop, multiple prefix
                               (Scenario 1)

   Figure 1 illustrates the host connecting to rtr1 and rtr2 via a
   shared link.  Networks 1 and 2 are reachable via rtr1 and rtr2
   respectively.  When the host sends packets to network 1, the next-hop
   to network 1 is rtr1.  Similarly, rtr2 is the next-hop to network 2.

   - e.g., broadband service (Internet, VoIP, IPTV, etc.)

   Scenario 2:

   In this scenario, a single gateway router connects the host to two or
   more upstream service provider networks.  This gateway router would
   receive prefix delegations and a different set of DNS recursive name
   servers from each independent service provider network.  The gateway
   in turn advertises the provider prefixes to the host, and for DNS,
   may either act as a lightweight DNS cache server or may advertise the
   complete set of service provider DNS recursive name servers to the
   hosts.

















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                                     +------+       ___________
                       +-----+       |      |      /           \
                       |     |=======| rtr1 |=====/   network   \
                       |     |port1  |      |     \      1      /
          +------+     |     |       +------+      \___________/
          |      |     |     |
          | hosts|-----| GW  |
          |      |     | rtr |
          +------+     |     |       +------+       ___________
                       |     |port2  |      |      /           \
                       |     |=======| rtr2 |=====/   network   \
                       +-----+       |      |     \      2      /
                                     +------+      \___________/

         Figure 2: single uplink, single next-hop, multiple prefix
                               (Scenario 2)

   Figure 2 illustrates the host connected to GW rtr.  GW rtr connects
   to networks 1 and 2 via port1 and 2 respectively.  When the host
   sends packets to either network 1 or 2, the next-hop is GW rtr.  When
   the packets are sent to network 1 (network 2), GW rtr forwards the
   packets to port1 (port2).

   - e.g, Internet + VPN/Application Service Provider (ASP)

   Scenario 3:

   In this scenario, a host has more than one active interface that
   connects to different routers and service provider networks.  Each
   router provides the host with a different address prefix and set of
   DNS recursive name servers, resulting in a host with a unique address
   per link/interface.



















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                 +------+     +------+       ___________
                 |      |     |      |      /           \
                 |      |-----| rtr1 |=====/   network   \
                 |      |     |      |     \      1      /
                 |      |     +------+      \___________/
                 |      |
                 | host |
                 |      |
                 |      |     +------+       ___________
                 |      |     |      |      /           \
                 |      |=====| rtr2 |=====/   network   \
                 |      |     |      |     \      2      /
                 +------+     +------+      \___________/

       Figure 3: Multiple uplink, multiple next-hop, multiple prefix
                               (Scenario 3)

   Figure 3 illustrates the host connecting to rtr1 and rtr2 via a
   direct connection or a virtual link.  When the host sends packets
   network 1, the next-hop to network 1 is rtr1.  Similarly, rtr2 is the
   next-hop to network 2.

   - e.g., Mobile Wifi + 3G, ISP A + ISP B

3.2.  Multihomed network environment

   In an IPv6 multihomed network, a host is assigned two or more IPv6
   addresses and DNS recursive name servers from independent service
   provider networks.  When this multihomed host attempts to connect
   with other hosts, it may incorrectly resolve the next-hop router, use
   an inappropriate source address, or use a DNS response from an
   incorrect service provider that may result in impaired IP
   connectivity.

   Multihomed networks in IPv4 have been commonly implemented through
   the use of a gateway router with NAPT function (scenario 2 with
   NAPT).  An analysis of the current IPv4 NAPT and DNS functions within
   the gateway router should provide a baseline set of requirements for
   IPv6 multihomed environments.  A destination prefix/route is often
   used on the gateway router to separate traffic between the networks.











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                                     +------+       ___________
                                     |      |      /           \
                                 +---| rtr1 |=====/   network   \
                                 |   |      |     \      1      /
          +------+     +-----+   |   +------+      \___________/
          | IPv4 |     |     |   |
          | host |-----| GW  |---+
          |      |     | rtr |   |
          +------+     +-----+   |   +------+       ___________
                      (NAPT&DNS) |   |      |      /           \
          (private               +---| rtr2 |=====/   network   \
              address                |      |     \      2      /
                 space)              +------+      \___________/

   Figure 4: IPv4 Multihomed environment with Gateway Router performing
                                   NAPT

3.3.  Problem Statement

   A multihomed IPv6 host has one or more assigned IPv6 addresses and
   DNS recursive name servers from each upstream service provider,
   resulting in the host having multiple valid IPv6 addresses and DNS
   recursive name servers.  The host must be able to resolve the
   appropriate next-hop, the correct source address and DNS recursive
   name server to use based on the destination prefix.  To prevent IP
   spoofing, operators will often implement ingress filtering to discard
   traffic with an inappropriate source address, making it essential for
   the host to correctly resolve these three items before sourcing the
   first packet.

   IPv6 has mechanisms for the provision of multiple routers on a single
   link and multiple address assignments to a single host.  However,
   when these mechanisms are applied to the three scenarios in
   Section 3.1 a number of connectivity issues are identified:

   Scenario 1:

   The host has been assigned an address from each router and recognizes
   both rtr1 and rtr2 as valid default routers (in the default routers
   list).

   o  The source address selection policy on the host does not
      deterministically resolve a source address.  Ingress filtering or
      filter policies will discard traffic with source addresses that
      the operator did not assign.

   o  The host will select one of the two routers as the active default
      router.  No traffic is sent to the other router.



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   Scenario 2:

   The host has been assigned two different addresses from the single
   gateway router.  The gateway router is the only default router on the
   link.

   o  The source address selection policy on the host does not
      deterministically resolve a source address.  Ingress filtering or
      filter policies will discard traffic with source addresses that
      the operator did not assign.

   o  The gateway router does not have an autonomous mechanism for
      determining which traffic should be sent to which network.  If the
      gateway router is implementing host functions (i.e., processing
      Router Advertisement) then two valid default routers may be
      recognized.

   Scenario 3:

   A host has two separate interfaces and on each interface a different
   address is assigned.  Each link has its own router.

   o  The host does not have enough information for determining which
      traffic should be sent to which upstream routers.  The host will
      select one of the two routers as the active default router, and no
      traffic is sent to the other router.  The default address
      selection rules select the address assigned to the outgoing
      interface as the source address.  So, if a host has an appropriate
      routing table, an appropriate source address will be selected.

   All scenarios:

   o  In network deployments utilizing local namespaces, the host may
      choose to communicate with a "wrong" DNS recursive server unable
      to serve a local namespace.


4.  Requirements

   This section describes requirements that any solution multi-address
   and multi-uplink architectures need to meet.

4.1.  End-to-End transparency

   One of the major design goals for IPv6 is to restore the end-to-end
   transparency of the Internet.  If NAT mechanism is applied to IP
   communication between hosts, it is required to apply complex NAT
   traversal mechanism to establish bi-directional IP communication.



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   Essentially, extra NAT traversal meachanism does not need to be
   implemented on application, on an environment with end-to-end
   transparency.  Therefore, The IPv6 multihoming solution SHOULD
   guarantee end-to-end transparency by avoiding IPv6 NAT.

4.2.  Policy distribution

   The solution SHOULD have a function to provide a policy on sites/
   nodes.  In particular, a network service provider has to control his
   or her user nodes such as CPE devices.  All nodes are not necessarily
   controlled evenly with policy providing.  It is required to identify
   a nodes and provide indepenent policy by each node.

   The providing mechanisms should have:

   o  a function to distribute policies to nodes dynamically to update
      their behavior.  When the network environment changes and the
      nodes' behavior has to be changed, a network administrator can
      modify the behavior of the nodes.

   o  a function to control every node centrally.  A site administrator
      or a service provider could determine or could have an effect on
      the behavior at their users' hosts.

   o  a function to control node-specific behavior.  Even when multiple
      nodes are on the same subnet, the mechanism should be able to
      provide a method for the network administrator to make nodes
      behave differently.  For example, each node may have a different
      set of assigned prefixes.  In such a case, the appropriate
      behavior may be different.

4.3.  Scalability

   The solution will have to be able to manage a large number of sites/
   nodes.  In services for residential users, provider edge devices have
   to manage thousands of sites.  In such environments, sending packets
   periodically to each site may affect edge system performance.


5.  Problem statement and analysis

   The problems described in Section 3 can be classified into these
   three types:

   o  Wrong source address selection

   o  Wrong next-hop selection




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   o  Wrong DNS server selection

   This section reviews the problem statements presented above and the
   proposed functional requirements to resolve the issues.

5.1.  Source address selection

   A multihomed IPv6 host will typically have different addresses
   assigned from each service provider either on the same link
   (scenarios 1 & 2) or different links (scenario 3).  When the host
   wishes to send a packet to any given destination, the current source
   address selection rules [RFC3484] may not deterministically resolve
   the correct source address when the host addressing was via Router
   Advertisement (RA) or DHCPv6.
   [I-D.ietf-6man-addr-select-considerations] describes the use of the
   policy table [RFC3484] to resolve this problem, but there is no
   mechanism defined to disseminate the policy table information to a
   host.  A proposal is in [I-D.ietf-6man-addr-select-opt] to provide a
   DHCPv6 mechanism for host policy table management.

   Again, by employing DHCPv6, the server could restrict address
   assignment (of additional prefixes) only to hosts that support policy
   table management.

   Scenario 1: "Host" needs to support the solution for this problem.

   Scenario 2: "Host" needs to support the solution for this problem.

   Scenario 3: If "Host" support the next-hop selection solution, there
   is no need to support the address selection functionality on the
   host.

   It is noted that the service providers (i.e., ISP and enterprise/VPN)
   must also support [I-D.ietf-6man-addr-select-opt].

5.2.  Next-hop selection

   A multihomed IPv6 host or gateway may have multiple uplinks to
   different service providers.  Here each router would use Router
   Advertisements [RFC4861] for distributing default route/next-hop
   information to the host or gateway router.

   In this case, the host or gateway router may select any valid default
   router from the default routers list, resulting in traffic being sent
   to the wrong router and discarded by the upstream service provider.
   Using the above scenarios as an example, whenever the host wishes to
   reach a destination in network 2 and there is no connectivity between
   networks 1 and 2 (as is the case for a walled-garden or closed



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   service), the host or gateway router does not know whether to forward
   traffic to rtr1 or rtr2 to reach a destination in network 2.  The
   host or gateway router may choose rtr1 as the default router, and
   traffic fails to reach the destination server.  The host or gateway
   router requires route information for each upstream service provider,
   but the use of a routing protocol between the gateway and the two
   routers causes both configuration and scaling issues.  For IPv4
   hosts, the gateway router is often pre-configured with static route
   information or uses of Classless Static Route Options [RFC3442] for
   DHCPv4.  Extensions to Router Advertisements through Default Router
   Preference and More-Specific Routes [RFC4191] provides for link-
   specific preferences but does not address per-host configuration in a
   multi-access topology because of its reliance on Router
   Advertisements.  A DHCPv6 option, such as that in
   [I-D.ietf-mif-dhcpv6-route-option], is preferred for host-specific
   configuration.  By employing a DHCPv6 solution, a DHCPv6 server could
   restrict address assignment (of additional prefixes) only to hosts
   that support more advanced next-hop and address selection
   requirements.

   Scenario 1: "Host" needs to support the solution for this problem.

   Scenario 2: "GW rtr" needs to support the solution for this problem.

   Scenario 3: "Host" needs to support the solution for this problem.

   It is noted that the service providers (i.e., ISP and enterprise/VPN)
   must also support [I-D.ietf-mif-dhcpv6-route-option].

5.3.  DNS recursive name server selection

   A multihomed IPv6 host or gateway router may be provided multiple DNS
   recursive name servers through DHCPv6 [RFC3646] or RA [RFC6106].
   When the host or gateway router sends a DNS query, it would normally
   choose one of the available DNS recursive name servers for the query.

   In the IPv6 gateway router scenario, the Broadband Forum [TR124]
   required that the query be sent to all DNS recursive name servers,
   and the gateway waits for the first reply.  In IPv6, given our use of
   specific destination-based policy for both routing and source address
   selection, it is desirable to extend a policy-based concept to DNS
   recursive name server selection.  Doing so can minimize DNS recursive
   name server load and avoid issues where DNS recursive name servers in
   different networks have connectivity issues, or the DNS recursive
   name server are not publicly accessible.  In the worst case, a DNS
   query for a name from a local namespace may not be resolved correctly
   if sent towards a DNS server not aware of said local namespace,
   resulting in a lack of connectivity.



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   It is not issue of Domain Name System model itself, but an IPv6
   multihomed host or gateway router should have the ability to select
   appropriate DNS recursive name servers for each service based on the
   domain space for the destination, and each service should provide
   rules specific to that network.  [I-D.ietf-mif-dns-server-selection]
   proposes a solution for distributing DNS server selection policy
   using a DHCPv6 option.

   Scenario 1: "Host" needs to support the solution for this problem.

   Scenario 2: "GW rtr" needs to support the solution for this problem.

   Scenario 3: "Host" needs to support the solution for this problem.

   It is noted that the service providers (i.e., ISP and enterprise/VPN)
   must also support [I-D.ietf-mif-dns-server-selection].


6.  Implementation approach

   As mentioned in Section 5, in the multi-prefix environment, we have
   three problems in source address selection, next-hop selection, and
   DNS recursive name server selection.  In this section, possible
   solution mechanisms for each problem are introduced and evaluated
   against the requirements in Section 4.

6.1.  Source address selection

   Possible solutions and their evaluation are summarized in
   [I-D.ietf-6man-addr-select-considerations].  When those solutions are
   examined against the requirements in Section 4, the proactive
   approaches, such as the policy table distribution mechanism and the
   routing hints mechanism, are more appropriate in that they can
   propagate the network administrator's policy directly.  The policy
   distribution mechanism has an advantage with regard to the host's
   protocol stack impact and the static nature of the assumed target
   network environment.

6.2.  Next-hop selection

   As for the source address selection problem, both a policy-based
   approach and a non policy-based approach are possible with regard to
   the next-hop selection problem.  Because of the same requirements,
   the policy propagation-based solution mechanism, whatever the policy,
   should be more appropriate.

   Routing information is a typical example of policy related to next-
   hop selection.  If we assume source address-based routing at hosts or



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   intermediate routers, the pairs of source prefixes and next-hops can
   be another example of next-hop selection policy.

   The routing information-based approach has a clear advantage in
   implementation and is already commonly used.

   The existing proposed or standardized routing information
   distribution mechanisms are routing protocols, such as RIPng and
   OSPFv3, the RA extension option defined in [RFC4191], the DHCPv6
   route information option proposed in
   [I-D.ietf-mif-dhcpv6-route-option], and the [TR069] standardized at
   BBF.

   The RA-based mechanism has difficulty in per-host routing information
   distribution.  The dynamic routing protocols such as RIPng are not
   usually used between the residential users and ISP networks because
   of their scalability implications.  The DHCPv6 mechanism does not
   have these difficulties and has the advantage of its relaying
   functionality.  It is commonly used and is thus easy to deploy.

   [TR069], mentioned above, is a possible solution mechanism for
   routing information distribution to customer-premises equipment
   (CPE).  It assumes, however, IP reachability to the Auto
   Configuration Server (ACS) is established.  Therefore, if the CPE
   requires routing information to reach the ACS, [TR069] cannot be used
   to distribute this information.

6.3.  DNS recursive name server selection

   As in the above two problems, a policy-based approach and non policy-
   based approach are possible.  In a non policy-based approach, a host
   or a home gateway router is assumed to send DNS queries to several
   DNS recursive name servers at once or to select one of the available
   servers.

   In the non policy-based approach, by making a query to a DNS
   recursive name server in a different service provider to that which
   hosts the service, a user could be directed to unexpected IP address
   or receive an invalid response, and thus cannot connect to the
   service provider's private and legitimate service.  For example, some
   DNS recursive name servers reply with different answers depending on
   the source address of the DNS query, which is sometimes called split-
   horizon.  When the host mistakenly makes a query to a different
   provider's DNS recursive name server to resolve a FQDN of another
   provider's private service, and the DNS recursive name server adopts
   the split-horizon configuration, the queried server returns an IP
   address of the non-private side of the service.  Another problem with
   this approach is that it causes unnecessary DNS traffic to the DNS



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   recursive name servers that are visible to the users.

   The alternative of a policy-based approach is documented in
   [I-D.ietf-mif-dns-server-selection], where several pairs of DNS
   recursive name server addresses and DNS domain suffixes are defined
   as part of a policy and conveyed to hosts in a new DHCP option.  In
   an environment where there is a home gateway router, that router can
   act as a DNS recursive name server, interpret this option and
   distribute DNS queries to the appropriate DNS servers according to
   the policy.


7.  Considerations for host without multi-prefix support

   This section presents an alternative approach to mitigate the problem
   in a multihomed network.  This approach will help IPv6 hosts that are
   not capable of the enhancements for the source address selection
   policy, next-hop selection policy, and DNS selection policy described
   in Section 6.

7.1.  IPv6 NAT

   In a typical IPv4 multihomed network deployment, IPv4 NAPT is
   practically used and it can eventually avoid assigning multiple
   addresses to the hosts and solve the next-hop selection problem.  In
   a similar fashion, IPv6 NAT can be used as a last resort for IPv6
   multihomed network deployments where one needs to assign a single
   IPv6 address to a host.


                                                       __________
                                                      /          \
                                                 +---/  Internet  \
                             gateway router      |   \            /
           +------+     +---------------------+  |    \__________/
           |      |     |   |        |  WAN1  +--+
           | host |-----|LAN| Router |--------|
           |      |     |   |        |NAT|WAN2+--+
           +------+     +---------------------+  |     __________
                                                 |    /          \
                                                 +---/    ASP     \
                                                     \            /
                                                      \__________/

                           Figure 5: Legacy Host

   The gateway router also has to support the two features, next-hop
   selection and DNS server selection, shown in Section 6.



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   The implementation and issues of IPv6 NAT are out of the scope of
   this document.  They may be covered by another document under
   discussion [RFC6296].

7.2.  Co-existence consideration

   To allow the co-existence of non-MHMP hosts and MHMP hosts (i.e.
   hosts supporting multi-prefix with the enhancements for the source
   address selection), GW-rtr may need to treat those hosts separately.

   An idea to achieve this is that GW-rtr identifies the hosts, and then
   assigns a single prefix to non-MHMP hosts and assigns multiple
   prefixes to MHMP hosts.  In this case, GW-rtr can perform IPv6 NAT
   only for the traffic from non-MHMP hosts if its source address is not
   appropriate.

   Another idea is that GW-rtr assigns multiple prefixes to the both
   hosts, and it performs IPv6 NAT for the traffic from non-MHMP hosts
   if its source address is not appropriate.

   In scenario 1 and 3, the non-MHMP hosts can be placed behind the NAT
   box.  In this case, the non-MHMP host can access the service through
   the NAT box.

   The implementation of identifying non-MHMP hosts and NAT policy is
   outside the scope of this document.


8.  Security Considerations

   This document requires that the solutions for MHMP should have policy
   providing functions.  New security threats can be introduced
   depending on what kind and what form of the policy.  The threats can
   be categorized in two parts: the policy receiver side and the policy
   distributor side.

   A policy receiver may receive an evil policy from a policy
   distributor.  A policy distributor should expect some hosts in its
   network do not follow the distributed policy.  The security threats
   related to IPv6 multihoming are described in [RFC4218].  Those
   threats that are specific to MHMP solutions are enumerated below.

   Threats related to the policy distributor side:

         Policy collision can happen.  When multiple policy distributor
         exists, a policy receiver may not follow one or each of the
         received policy.  Especially when a policy conflicts with
         another policy, a policy receiver cannot implement each of the



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         policy.  To solve or mitigate this issue, policy prioritization
         rule should be defined, such as preference for the policy
         received on a trusted interface.  Another solution is to
         preclude the functionality of multiple policy acceptance at the
         receiver side.  In this case, a policy distributor should
         cooperate with other policy distributors, and a single
         representative provider should distribute a merged policy.

         Apart from the policy collision, a network service provider
         should expect the existence of hosts that will not obey the
         received policy.  A possible solutions is to ingress-filter
         those packets that do not match the distributed policy and drop
         them.  About the route selection, packet forwarding or
         redirection can be another possible solution.  About the source
         address selection, IPv6 NAT can be another possible solution.

   Threats related to the policy receiver side:

         A policy receiver are exposed to the threats of unauthorized
         policy, which can lead to session hijack, DoS, wiretapping and
         phishing.  Unauthorized policy here means a policy distributed
         from an entity that does not have rights to do so.  Usually,
         only a site administrator and a network service provider have
         rights to distribute these policies just as well as IP address
         assignment and DNS server address notification.  Regarding
         source address selection, unauthorized policy can expose an IP
         address that will not usually be exposed to an external server,
         which can be a privacy problem.  To solve or mitigate this
         problem of unauthorized policy, one approach is limiting on use
         of these policy distribution mechanisms, as described in the
         section 4.4 of [I-D.ietf-mif-dns-server-selection].  For
         example, a policy should be preferred or accepted when the
         policy is delivered across a secure, trusted channel such as 3G
         connection in cellular services.  The proposed solutions are
         baed on DHCP, so the limitation of local site communication,
         which is often used in WiFi access services, should be another
         solution or mitigation for this problem.  About DNS server
         selection issue, DNSSEC can be another solution.  About source
         address selection, the ingress filter at the network service
         provider router can be a solution.

         Another threat is the leakage of the policy and privacy issues
         resulting from that.  Especially when each client is
         distributed its own policy from the network service provider,
         the policy can give a hint of which service the client
         subscribes.  Encryption of communication channel, separation of
         communication channel per host can be solutions for this
         problem.



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9.  IANA Considerations

   This document has no IANA actions.


10.  Contributors

   The following people contributed to this document: Akiko Hattori,
   Arifumi Matsumoto, Frank Brockners, Fred Baker, Tomohiro Fujisaki,
   Jun-ya Kato, Shigeru Akiyama, Seiichi Morikawa, Mark Townsley,
   Wojciech Dec, Yasuo Kashimura, Yuji Yamazaki.  This document has
   greatly benefited from inputs by Randy Bush, Brian Carpenter, and
   Teemu Savolainen.


11.  References

11.1.  Normative References

   [I-D.ietf-6man-addr-select-considerations]
              Chown, T., "Considerations for IPv6 Address Selection
              Policy Changes",
              draft-ietf-6man-addr-select-considerations-03 (work in
              progress), March 2011.

   [I-D.ietf-6man-addr-select-opt]
              Matsumoto, A., Fujisaki, T., Kato, J., and T. Chown,
              "Distributing Address Selection Policy using DHCPv6",
              draft-ietf-6man-addr-select-opt-01 (work in progress),
              June 2011.

   [I-D.ietf-mif-dhcpv6-route-option]
              Dec, W., Mrugalski, T., Sun, T., and B. Sarikaya, "DHCPv6
              Route Options", draft-ietf-mif-dhcpv6-route-option-02
              (work in progress), July 2011.

   [I-D.ietf-mif-dns-server-selection]
              Savolainen, T., Kato, J., and T. Lemon, "Improved DNS
              Server Selection for Multi-Homed Nodes",
              draft-ietf-mif-dns-server-selection-03 (work in progress),
              June 2011.

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

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




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   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, June 2011.

11.2.  Informative References

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC3442]  Lemon, T., Cheshire, S., and B. Volz, "The Classless
              Static Route Option for Dynamic Host Configuration
              Protocol (DHCP) version 4", RFC 3442, December 2002.

   [RFC3582]  Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
              Multihoming Architectures", RFC 3582, August 2003.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC4116]  Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
              Gill, "IPv4 Multihoming Practices and Limitations",
              RFC 4116, July 2005.

   [RFC4218]  Nordmark, E. and T. Li, "Threats Relating to IPv6
              Multihoming Solutions", RFC 4218, October 2005.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, November 2010.

   [TR069]    The BroadBand Forum, "TR-069, CPE WAN Management Protocol
              v1.1, Version: Issue 1 Amendment 2", December 2007.

   [TR124]    The BroadBand Forum, "TR-124i2, Functional Requirements
              for Broadband Residential Gateway Devices (work in
              progress)", May 2010.




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Authors' Addresses

   Ole Troan (editor)
   Cisco
   Bergen
   Norway

   Email: ot@cisco.com


   David Miles
   Alcatel-Lucent
   Melbourne
   Australia

   Email: david.miles@alcatel-lucent.com


   Satoru Matsushima
   Softbank Telecom
   Tokyo
   Japan

   Email: satoru.matsushima@tm.softbank.co.jp


   Tadahisa Okimoto
   NTT West
   Osaka
   Japan

   Email: t.okimoto@rdc.west.ntt.co.jp


   Dan Wing
   Cisco
   170 West Tasman Drive
   San Jose
   USA

   Email: dwing@cisco.com










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