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rfc:rfc7157

Internet Engineering Task Force (IETF) O. Troan, Ed. Request for Comments: 7157 Cisco Category: Informational D. Miles ISSN: 2070-1721 Google Fiber

                                                         S. Matsushima
                                                      Softbank Telecom
                                                            T. Okimoto
                                                              NTT West
                                                               D. Wing
                                                                 Cisco
                                                            March 2014
        IPv6 Multihoming without Network Address Translation

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-to-IPv6 Network
 Prefix Translation (NPTv6).  However, NAT and NPTv6 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.  We conclude that DHCPv6-based
 solutions are suitable to solve the multihoming issues described in
 this document, but NPTv6 may be required as an intermediate solution.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7157.

Troan, et al. Informational [Page 1] RFC 7157 IPv6 Multihoming without NAT March 2014

Copyright Notice

 Copyright (c) 2014 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 . . . . . . . . . . . . . .   6
   3.1.  Classification of Network Scenarios for Multihomed Host .   6
   3.2.  Multihomed Network Environment  . . . . . . . . . . . . .   8
   3.3.  Problem Statement . . . . . . . . . . . . . . . . . . . .   9
 4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.1.  End-to-End Transparency . . . . . . . . . . . . . . . . .  11
   4.2.  Scalability . . . . . . . . . . . . . . . . . . . . . . .  11
 5.  Problem Analysis  . . . . . . . . . . . . . . . . . . . . . .  11
   5.1.  Source Address Selection  . . . . . . . . . . . . . . . .  11
   5.2.  Next Hop Selection  . . . . . . . . . . . . . . . . . . .  12
   5.3.  DNS Recursive Name Server Selection . . . . . . . . . . .  13
 6.  Implementation Approach . . . . . . . . . . . . . . . . . . .  13
   6.1.  Source Address Selection  . . . . . . . . . . . . . . . .  14
   6.2.  Next Hop Selection  . . . . . . . . . . . . . . . . . . .  14
   6.3.  DNS Recursive Name Server Selection . . . . . . . . . . .  15
   6.4.  Other Algorithms Available in RFCs  . . . . . . . . . . .  16
 7.  Considerations for MHMP Deployment  . . . . . . . . . . . . .  16
   7.1.  Non-MHMP Host Consideration . . . . . . . . . . . . . . .  16
   7.2.  Coexistence Considerations  . . . . . . . . . . . . . . .  17
   7.3.  Policy Collision Consideration  . . . . . . . . . . . . .  17
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
 9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
   10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
   10.2.  Informative References . . . . . . . . . . . . . . . . .  20

Troan, et al. Informational [Page 2] RFC 7157 IPv6 Multihoming without NAT March 2014

1. Introduction

 In this document, we analyze the use cases of multihoming, describe
 functional requirements, and describe the problems with IPv6
 multihoming.  There are two ways to avoid the problems of IPv6
 multihoming:
 1.  using IPv6-to-IPv6 network prefix translation (NPTv6) [RFC6296],
     or;
 2.  refining IPv6 specifications to resolve the problems with IPv6
     multihoming.
 This document concerns itself with the latter and explores the
 solution space.  We hope this will encourage the development of
 solutions to the problem so that, in the long run, NPTv6 can be
 avoided.
 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 in every case, due to the
 possible necessity of NAT even in IPv6, because of multihoming.
 Though there are mechanisms to implement multihoming, such as BGP
 multihoming [RFC4116] at the network level and multihoming based on
 the Stream Control Transmission Protocol (SCTP) [RFC4960] in the
 transport layer, 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 is
 an already-deployed technique.
 Whenever a host or small network (that 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.

Troan, et al. Informational [Page 3] RFC 7157 IPv6 Multihoming without NAT March 2014

 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 NPTv6 specification is,
 we are trying to avoid any form of network address translation
 technique that may not be visible to either of the end hosts.  To
 reach this goal, several 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:
 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 becomes difficult whenever a host has
    more than one address of 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.  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 server 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.

Troan, et al. Informational [Page 4] RFC 7157 IPv6 Multihoming without NAT March 2014

 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 or DNS proxy, which receives all DNS inquires, and gives
 a correct answer to the host.

2. Terminology

 NPTv6       IPv6-to-IPv6 Network Prefix Translation as described in
             [RFC6296].
 NAPT        Network Address Port Translation as described in
             [RFC3022].  In other contexts, NAPT is often pronounced
             "NAT" or written as "NAT".
 MHMP        Multihomed with multi-prefix.  A host implementation that
             supports the mechanisms described in this document;
             namely, source address selection policy, next hop
             selection, and DNS selection policy.

Troan, et al. Informational [Page 5] RFC 7157 IPv6 Multihoming without NAT March 2014

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.
                              +------+       ___________
                              |      |      /           \
                          +---| 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.
 Example: multiple broadband service providers (Internet, VoIP, IPTV,
 etc.)

Troan, et al. Informational [Page 6] RFC 7157 IPv6 Multihoming without NAT March 2014

 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 advertise the
 complete set of service provider DNS recursive name servers to the
 hosts.
                                   +------+       ___________
                     +-----+       |      |      /           \
                     |     |=======| 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.  As the figure
 shows a logical topology of the scenario, port1 could be a pseudo-
 interface for tunneling, which connects to network 1 through network
 2 and vice versa.  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).
 Example: Internet + VPN / Application Service Provider (ASP)

Troan, et al. Informational [Page 7] RFC 7157 IPv6 Multihoming without NAT March 2014

 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.
 +------+     +------+       ___________
 |      |     |      |      /           \
 |      |-----| 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 to
 network 1, the next hop to network 1 is rtr1.  Similarly, rtr2 is the
 next hop to network 2.
 Example: 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.
 In many cases, multihomed networks in IPv4 have been 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

Troan, et al. Informational [Page 8] RFC 7157 IPv6 Multihoming without NAT March 2014

 requirements for IPv6 multihomed environments.  A destination prefix/
 route is often used on the gateway router to separate traffic between
 the networks.
                                   +------+       ___________
                                   |      |      /           \
                               +---| rtr1 |=====/   network   \
                               |   |      |     \      1      /
        +------+     +-----+   |   +------+      \___________/
        | IPv4 |     |     |   |
        | hosts|-----| 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 the correct 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 described 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).

Troan, et al. Informational [Page 9] RFC 7157 IPv6 Multihoming without NAT March 2014

 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.
 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 (RA)), then two valid default routers may be
    recognized.
 Scenario 3:
 A host has two separate interfaces, and each interface has a
 different address assigned.  Each link has its own router.
 o  The host does not have enough information to determine 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.

Troan, et al. Informational [Page 10] RFC 7157 IPv6 Multihoming without NAT March 2014

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 is applied to IP communication
 between hosts, NAT traversal mechanisms are required to establish
 bidirectional IP communication.  In an environment with end-to-end
 transparency, a NAT traversal mechanism does not need to be
 implemented in an application (e.g., ICE [RFC5245]).  Therefore, the
 IPv6 multihoming solution should strive to avoid NPTv6 to achieve
 end-to-end transparency.

4.2. 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 Analysis

 The problems described in Section 3 can be classified into these
 three types:
 o  Wrong source address selection
 o  Wrong next hop selection
 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 on either the same link
 (scenarios 1 and 2) or different links (scenario 3).  When the host
 wishes to send a packet to any given destination, the current source
 address selection rules [RFC6724] may not deterministically select
 the correct source address.  [RFC7078] describes the use of the
 policy table (as discussed in [RFC6724]) to resolve this problem,
 using a DHCPv6 mechanism for host policy table management.

Troan, et al. Informational [Page 11] RFC 7157 IPv6 Multihoming without NAT March 2014

 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 supports the next hop selection solution, there
 is no need to support the address selection functionality on the
 host.
 It is noted that the network's DHCP server and DHCP-forwarding
 routers must also support the Address Selection option [RFC7078].

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] to distribute 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
 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, but
 traffic will fail 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.  In
 IPv4, gateway routers are often pre-configured with static routes or
 use the Classless Static Route Options [RFC3442] for DHCPv4.  An
 extension 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.
 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.

Troan, et al. Informational [Page 12] RFC 7157 IPv6 Multihoming without NAT March 2014

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 (BBF)
 [TR-124] requires that the query be sent to all DNS recursive name
 servers and that the gateway wait 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 servers 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.
 It is not an issue of the 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.  [RFC6731] 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 network's DHCP server and DHCP-forwarding
 routers must also support the Address Selection option [RFC6731].

6. Implementation Approach

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

Troan, et al. Informational [Page 13] RFC 7157 IPv6 Multihoming without NAT March 2014

6.1. Source Address Selection

 The problems of address selection in multi-prefix environments are
 summarized in [RFC5220].  When 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
 intermediate routers, 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 Routing
 Information Protocol Next Generation (RIPng) and OSPFv3), the RA
 extension option defined in [RFC4191], and the CPE WAN Management
 Protocol (CWMP) [TR069] standardized at BBF.
 The RA-based mechanism doesn't handle distribution of per-host
 routing information easily.  Dynamic routing protocols are not
 typically used between residential users and ISPs, because of their
 scalability and security implications.  The DHCPv6 mechanism does not
 have these problems and has the advantage of relay functionality.  It
 is commonly used and is thus easy to deploy.
 [TR069], mentioned above, defines a possible solution mechanism for
 routing information distribution to customer premises equipment
 (CPE).  It assumes, however, that IP reachability to the Auto
 Configuration Server (ACS) has been established.  Therefore, if the
 CPE requires routing information to reach the ACS, CWMP [TR069]
 cannot be used to distribute this information.

Troan, et al. Informational [Page 14] RFC 7157 IPv6 Multihoming without NAT March 2014

6.3. DNS Recursive Name Server Selection

    Note: Split-horizon DNS is discussed in this section.  Split-
    horizon DNS is known to cause problems with applications to allow
    information leakage.  The discussion of split-horizon DNS is not
    condoning its use, but rather acknowledging that split-horizon DNS
    is used and that its use is another justification for network
    address translation.  The goal of this document is to encourage
    building solutions that do not need network address translation.
    Two solutions appear possible: improve the function of split-
    horizon DNS (which is discussed below) or meet network
    administrators' requirements without split-horizon DNS (which is
    out of scope of this document).
 As in the above two problems, a policy-based approach and a 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 an unexpected IP
 address or receive an invalid response, and thus it could not 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 Fully
 Qualified Domain Name (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 recursive name
 servers that are visible to the users.
 The alternative to a policy-based approach is documented in
 [RFC6731], 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.

Troan, et al. Informational [Page 15] RFC 7157 IPv6 Multihoming without NAT March 2014

6.4. Other Algorithms Available in RFCs

 The authors of this document are aware of a variety of other
 algorithms and architectures, such as Shim6 [RFC5533] and HIP
 [RFC5206], that may be useful in this environment.  At the time of
 this writing, there is not enough operational experience on which to
 base a recommendation.  Should such operational experience become
 available, this document may be updated in the future.

7. Considerations for MHMP Deployment

 This section describes considerations to mitigate possible problems
 in a network that implements MHMP (described in Section 6).

7.1. Non-MHMP Host Consideration

 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, NPTv6 can be used as a last resort for IPv6
 multihomed network deployments where one needs to assign a single
 IPv6 address to a non-MHMP 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.
 The implementation and issues of NPTv6 are out of the scope of this
 document, but are discussed in Section 5 of [RFC6296].

Troan, et al. Informational [Page 16] RFC 7157 IPv6 Multihoming without NAT March 2014

7.2. Coexistence Considerations

 To allow the coexistence 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 for how to achieve this would be for GW rtr to identify the
 hosts, and then assign a single prefix to non-MHMP hosts and assign
 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 could assign multiple prefixes to both
 hosts and perform IPv6 NAT for traffic from non-MHMP hosts if its
 source address is not appropriate.
 In scenarios 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.

7.3. Policy Collision Consideration

 When multiple policy distributors exist, a policy receiver may not
 follow each of the received policies.  In particular, when a policy
 conflicts with another policy, a policy receiver cannot implement
 both of the policies.  To solve or mitigate this issue, a
 prioritization rule is required to align the policies with the
 preferences of a trusted interface.  Another solution is to preclude
 the functionality of the acceptance of multiple policies 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.
 This document does not presume specific recommendations for resolving
 policy collision.  It is expected that the implementation will decide
 how to resolve the conflicts.  If they are not resolved consistently
 by different implementations, that could affect interoperability and
 security trust boundaries.  Future work is expected to address the
 need for consistent policy resolution to avoid interoperability and
 security trust boundary issues.

Troan, et al. Informational [Page 17] RFC 7157 IPv6 Multihoming without NAT March 2014

8. Security Considerations

 In today's multihomed IPv4 networks, it is difficult to resolve or
 coordinate conflicts between the two upstream networks.  This problem
 persists with IPv6, no matter if the hosts use IPv6 provider-
 dependent or provider-independent addresses.
 This document requires that MHMP solutions have functions that
 provide policy controls.  New security threats can be introduced
 depending on the kind and 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 that some hosts in
 its network will not follow the distributed policy.  At the time of
 this writing, there are no known methods to resolve conflicts between
 the host's own policy (policy receiver) and the policies of upstream
 providers (policy provider).  As this document is analyzing the
 problem space, rather than proposing a solution, we note the
 following problems:
 Threats related to the policy distributor side:
       The service provider should expect the existence of hosts that
       will not obey the received policy.  A possible solution is to
       ingress-filter those packets that do not match the distributed
       policy and drop them.  For route selection, packet forwarding
       or redirection can be another possible solution.  For source
       address selection, IPv6 NAT can be another possible solution.
       In a multihomed multiple-provider network, nodes in the network
       may be administered by different organizations.  Administrators
       might need to control policies (and a node's behavior)
       independently of other administrators.  Access control policies
       need to be in place to restrict the administrator's access to
       only the nodes it is authorized to control.
 Threats related to the policy receiver side:
       For the policy receiver side, who should be trusted to accept
       policies is a fundamental issue.  How is the trust established?
       How can the network element be assured that it can establish
       that trust before the network is fully configured?  If a policy
       receiver trusts an untrusted network, it will cause the
       distributing of the unwanted and unauthorized policy that is
       described below.

Troan, et al. Informational [Page 18] RFC 7157 IPv6 Multihoming without NAT March 2014

       A policy receiver is exposed to the threats of unauthorized
       policy, which can lead to session hijack, falsification, 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 in
       addition to 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 the problem of unauthorized policy, one
       approach is to limit the use of these policy distribution
       mechanisms, as described in the Section 4.4 of [RFC6731].  For
       example, a policy should be preferred or accepted if delivered
       over a secure, trusted channel such as a cellular data
       connection.  The proposed solutions are based 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.  For the DNS server selection issue, DNS
       Security (DNSSEC) can be another solution.  For 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 clients receive different
       policies from the network service provider, that difference
       provides hints about the host itself and can be useful to
       uniquely identify the host.  Encryption of the communication
       channel and separation of the communication channel per host
       can be solutions for this problem.
 The security threats related to IPv6 multihoming are described in
 [RFC4218].

9. 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, and Yuji Yamazaki.  This document has
 greatly benefited from inputs by Randy Bush, Brian Carpenter, and
 Teemu Savolainen.

Troan, et al. Informational [Page 19] RFC 7157 IPv6 Multihoming without NAT March 2014

10. References

10.1. Normative References

 [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.
 [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
            "Default Address Selection for Internet Protocol Version 6
            (IPv6)", RFC 6724, September 2012.
 [RFC6731]  Savolainen, T., Kato, J., and T. Lemon, "Improved
            Recursive DNS Server Selection for Multi-Interfaced
            Nodes", RFC 6731, December 2012.
 [RFC7078]  Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
            Address Selection Policy Using DHCPv6", RFC 7078, January
            2014.

10.2. 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.

Troan, et al. Informational [Page 20] RFC 7157 IPv6 Multihoming without NAT March 2014

 [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.
 [RFC5206]  Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End-
            Host Mobility and Multihoming with the Host Identity
            Protocol", RFC 5206, April 2008.
 [RFC5220]  Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
            "Problem Statement for Default Address Selection in Multi-
            Prefix Environments: Operational Issues of RFC 3484
            Default Rules", RFC 5220, July 2008.
 [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
            (ICE): A Protocol for Network Address Translator (NAT)
            Traversal for Offer/Answer Protocols", RFC 5245, April
            2010.
 [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
            Shim Protocol for IPv6", RFC 5533, June 2009.
 [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
            "IPv6 Router Advertisement Options for DNS Configuration",
            RFC 6106, November 2010.
 [TR-124]   The Broadband Forum, "TR-124, Functional Requirements for
            Broadband Residential Gateway Devices", Issue: 2, May
            2010, <http://www.broadband-forum.org/technical/download/
            TR-124_Issue-2.pdf>.
 [TR069]    The Broadband Forum, "TR-069, CPE WAN Management Protocol
            v1.1", Version: Issue 1 Amendment 2, December 2007,
            <http://www.broadband-forum.org/technical/download/
            TR-069_Amendment-2.pdf>.

Troan, et al. Informational [Page 21] RFC 7157 IPv6 Multihoming without NAT March 2014

Authors' Addresses

 Ole Troan (editor)
 Cisco
 Oslo
 Norway
 EMail: ot@cisco.com
 David Miles
 Google Fiber
 Mountain View, CA
 USA
 EMail: davidmiles@google.com
 Satoru Matsushima
 Softbank Telecom
 Tokyo
 Japan
 EMail: satoru.matsushima@g.softbank.co.jp
 Tadahisa Okimoto
 NTT West
 Osaka
 Japan
 EMail: t.okimoto@west.ntt.co.jp
 Dan Wing
 Cisco
 170 West Tasman Drive
 San Jose
 USA
 EMail: dwing@cisco.com

Troan, et al. Informational [Page 22]

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