Source-Address-Dependent Routing and Source Address Selection for IPv6 Hosts: Overview of the Problem Space
RFC 8043

Independent Submission                                       B. Sarikaya
Request for Comments: 8043                                    Huawei USA
Category: Informational                                     M. Boucadair
ISSN: 2070-1721                                                   Orange
                                                            January 2017

     Source-Address-Dependent Routing and Source Address Selection
             for IPv6 Hosts: Overview of the Problem Space

Abstract

   This document presents the source-address-dependent routing (SADR)
   problem space from the host's perspective.  Both multihomed hosts and
   hosts with multiple interfaces are considered.  Several network
   architectures are presented to illustrate why source address
   selection and next-hop resolution are needed in view of
   source-address-dependent routing.

   The document is scoped on identifying a set of scenarios for
   source-address-dependent routing from the host's perspective and
   analyzing a set of solutions to mitigate encountered issues.  The
   document does not make any solution recommendations.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 7841.

   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/rfc8043.

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Copyright Notice

   Copyright (c) 2017 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overall Context . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Source-Address-Dependent Routing (SADR) Scenarios . . . . . .   4
     2.1.  Multi-Prefix Multihoming  . . . . . . . . . . . . . . . .   5
     2.2.  Multi-Prefix Multi-Interface  . . . . . . . . . . . . . .   5
     2.3.  Home Network (Homenet)  . . . . . . . . . . . . . . . . .   7
     2.4.  Service-Specific Egress Routing . . . . . . . . . . . . .   7
   3.  Analysis of Source-Address-Dependent Routing  . . . . . . . .   8
     3.1.  Scenarios Analysis  . . . . . . . . . . . . . . . . . . .   8
     3.2.  Provisioning Domains and SADR . . . . . . . . . . . . . .  10
   4.  Discussion of Alternate Solutions . . . . . . . . . . . . . .  11
     4.1.  Router Advertisement Option . . . . . . . . . . . . . . .  11
     4.2.  Router Advertisement Option Set . . . . . . . . . . . . .  12
     4.3.  Rule 5.5 for Source Address Selection . . . . . . . . . .  12
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

1.1.  Overall Context

   BCP 38 recommends ingress traffic filtering to prohibit Denial-of-
   Service (DoS) attacks.  As such, datagrams with source addresses that
   do not match with the network where the host is attached are
   discarded [RFC2827].  Preventing packets from being dropped due to
   ingress filtering is difficult, especially in multihomed networks
   where the host receives more than one prefix from the networks it is
   connected to, and consequently may have more than one source address.
   Based on BCP 38, BCP 84 introduced recommendations on the routing
   system for multihomed networks [RFC3704].

   Recommendations on the routing system for ingress filtering such as
   in BCP 84 inevitably involve source address checks.  This leads to
   source-address-dependent-routing (SADR).  Source-address-dependent
   routing can be problematic, especially when the host is connected to
   a multihomed network and is communicating with another host in
   another multihomed network.  In such a case, the communication can be
   broken in both directions if Network Providers apply ingress
   filtering and the datagrams contain the wrong source addresses (see
   [INGRESS_FIL] for more details).

   Hosts with simultaneously active interfaces receive multiple prefixes
   and have multiple source addresses.  Datagrams originating from such
   hosts are likely to be filtered due to ingress filtering policies.
   The source address selection algorithm needs to carefully avoid
   ingress filtering on the next-hop router [RFC6724].

   Many use cases have been reported for source/destination routing --
   see [SD_RTG].  These use cases clearly indicate that the multihomed
   host or Customer Premises Equipment (CPE) router needs to be
   configured with the correct source prefixes/addresses so that it can
   forward packets upstream correctly to prevent the ingress filtering
   applied by an upstream Network Provider from dropping the packets.

   In multihomed networks, there is a need to enforce source-address-
   based routing if some providers are performing ingress filtering.
   This requires that the routers consider the source addresses as well
   as the destination addresses in determining the packet's next hop.

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1.2.  Scope

   Based on the use cases defined in [SD_RTG], the routers may be
   informed about the source addresses to use for forwarding using
   extensions to the routing protocols like IS-IS [ISO.10589.1992]
   [SD_RTG_ISIS], OSPF [RFC5340] [SD_RTG_OSPF].

   In this document, we describe the scenarios for source-address-
   dependent routing from the host's perspective.  Two flavors can be
   considered:

   1.  A host may have a single interface with multiple addresses (from
       different prefixes or /64s).  Each prefix is delegated from
       different exit routers, and this case can be called "multihomed
       with multi-prefix" (MHMP).  In such case, source address
       selection is performed by the host while source-dependent routing
       is enforced by an upstream router.

   2.  A host may have simultaneously connected multiple interfaces
       where each interface is connected to a different exit router, and
       this case can be called "multi-prefix multiple interface" (MPMI).
       For this case, the host is required to support both source
       address selection and source-dependent routing to avoid the need
       for an upstream router to rewrite the IPv6 prefix.

   Several limitations arise in multihoming contexts based on NAT and
   IPv6-to-IPv6 Network Prefix Translation (NPTv6) [RFC6296]; see, for
   example, [RFC4116].  NPTv6 is out of scope for this document.

   This document was initially written to inform the community about the
   SADR problem space.  It was updated to record the various sets of
   alternate solutions to address that problem space.  The 6man WG
   consensus is documented in [RFC8028].

2.  Source-Address-Dependent Routing (SADR) Scenarios

   This section describes a set of scenarios to illustrate the SADR
   problem.  Scenarios are listed in order of increasing complexity.

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2.1.  Multi-Prefix Multihoming

   The scenario shown in Figure 1 is a multi-prefix multihoming use
   case.  "rtr" is a CPE router that is connected to two Network
   Providers, each advertising its own prefixes.  In this case, the host
   may have a single interface, but it receives multiple prefixes from
   the upstream Network Providers.  Assuming that providers apply the
   ingress filtering policy, the packets for any external communication
   from the host should follow source-address-dependent routing in order
   to avoid getting dropped.

   In this scenario, the host does not need to perform source-dependent
   routing; it only needs to perform source address selection.

      +------+                  |
      |      |                  |        (Network)
      |      |                  |=====|(Provider 1)|=====
      |      |     +------+     |
      |      |     |      |     |
      |      |=====| rtr  |=====|
      | host |     |      |     |
      |      |     +------+     |
      |      |                  |
      |      |                  |        (Network)
      |      |                  |=====|(Provider 2)|=====
      |      |                  |
      +------+                  |

            Figure 1: Multihomed Host with Multiple CPE Routers

2.2.  Multi-Prefix Multi-Interface

   The scenario shown in Figure 2 is multi-prefix multi-interface, where
   "rtr1" and "rtr2" represent CPE routers and there are exit routers in
   both "network 1" and "network 2".  If the packets from the host
   communicating with a remote destination are routed to the wrong exit
   router, i.e., they carry the wrong source address, they will get
   dropped due to ingress filtering.

   In order to avoid complications when sending packets and to avoid the
   need to rewrite the source IPv6 prefix, the host is required to
   perform both source address selection and source-dependent routing so
   that the appropriate next hop is selected while taking into account
   the source address.

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

          Figure 2: Multiple Interfaced Host with Two CPE Routers

   There is a variant of Figure 2 that is often referred to as a
   corporate VPN, i.e., a secure tunnel from the host to a router
   attached to a corporate network.  In this case, "rtr2" provides
   access directly to the corporate network, and the link from the host
   to "rtr2" is a secure tunnel (for example, an IPsec tunnel).
   Therefore, the interface is a virtual interface with its own IP
   address/prefix assigned by the corporate network.

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

                            Figure 3: VPN Case

   There are at least two sub-cases:

   a.  Dedicated forwarding entries are created in the host such that
       only traffic directed to the corporate network is sent to "rtr2";
       everything else is sent to "rtr1".

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   b.  All traffic is sent to "rtr2" and then routed to the Internet if
       necessary.  This case doesn't need host routes but leads to
       unnecessary traffic and latency because of the path stretch via
       "rtr2".

2.3.  Home Network (Homenet)

   In the homenet scenario depicted in Figure 4, representing a simple
   home network, there is a host connected to a local network that is
   serviced with two CPEs that are connected to Providers 1 and 2,
   respectively.  Each network delegates a different prefix.  Also, each
   router provides a different prefix to the host.  The issue in this
   scenario is that ingress filtering is used by each provider.  This
   scenario can be considered as a variation of the scenario described
   in Section 2.2.

      +------+
      |      |     +------+
      |      |     |      |      (Network)
      |      |==+==| rtr1 |====|(Provider 1)|=====
      |      |  |  |      |
      |      |  |  +------+
      | host |  |
      |      |  |
      |      |  |  +------+
      |      |  |  |      |      (Network)
      |      |  +==| rtr2 |====|(Provider 2)|=====
      |      |     |      |
      +------+     +------+

            Figure 4: Simple Home Network with Two CPE Routers

   The host has to select the source address from the prefixes of
   Providers 1 or 2 when communicating with other hosts in Provider 1 or
   2.  The next issue is to select the correct next-hop router, "rtr1"
   or "rtr2" that can reach the correct provider, Network Provider 1 or
   2.

2.4.  Service-Specific Egress Routing

   A variation of the scenario in Section 2.1 is specialized egress
   routing.  Upstream networks offer different services with specific
   requirements, e.g., Voice over IP (VoIP) or IPTV.  The hosts using
   this service need to use the service's source and destination
   addresses.  No other service will accept this source address, i.e.,
   those packets will be dropped [SD_RTG].

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   Both source address selection and source-dependent routing are
   required to be performed by the host.

    ___________                +------+
   /           \   +------+    |      |
  /   network   \  |      |    |      |
  \      1      /--| rtr1 |----|      |
   \___________/   |      |    |      |     +------+       ___________
                   +------+    | host |     |      |      /           \
                               |      |=====| rtr3 |=====/   network   \
    ___________                |      |     |      |     \      3      /
   /           \   +------+    |      |     +------+      \___________/
  /   network   \  |      |    |      |
  \      2      /--| rtr2 |----|      |
   \___________/   |      |    |      |
                   +------+    |      |
                               +------+

          Figure 5: Multi-Interfaced Host with Three CPE Routers

   The scenario shown in Figure 5 is a variation of a multi-prefix
   multi-interface scenario (Section 2.2).  "rtr1", "rtr2", and "rtr3"
   are CPE routers.  The networks apply ingress routing.  Source-
   address-dependent routing should be used to avoid dropping any
   external communications.

3.  Analysis of Source-Address-Dependent Routing

   SADR can be facilitated at the host with proper source address and
   next-hop selection.  For this, each router connected to different
   interfaces of the host uses Router Advertisements (RAs) [RFC4861] to
   distribute a default route, the next hop, and the source address/
   prefix information to the host.  As a reminder, while the Prefix
   Information Option (PIO) is defined in [RFC4861], the Route
   Information Option (RIO) is defined in [RFC4191].

   Section 3.1 presents an analysis of the scenarios in Section 2, and
   Section 3.2 discusses the relevance of SADR to the provisioning
   domains.

3.1.  Scenarios Analysis

   As in [RFC7157], we assume that the routers in Section 2 use RAs to
   distribute default route and source address prefixes supported in
   each next hop to the hosts or that the gateway/CPE router relays this
   information to the hosts.

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   Referring to Section 2.1, source address selection is undertaken by
   the host while source-dependent routing must be followed by "rtr" to
   avoid packets being dropped.  No particular modification is required
   for next-hop selection at the host.

   Referring to the scenario in Figure 2, source-address-dependent
   routing can present a solution to the problem of when the host wishes
   to reach a destination in network 2 and the host chooses "rtr1" as
   the default router.  The solution assumes that the host is correctly
   configured.  The host should be configured with the prefixes
   supported in these next hops.  This way the host, having received
   many prefixes, will have the correct information for selecting the
   right source address and next hop when sending packets to remote
   destinations.

   Note that similar considerations apply to the scenario in Figure 5.

   In the configuration of the scenario (Figure 1), it is also useful to
   configure the host with the prefixes and source address prefixes they
   support.  This will enable the host to select the right prefix when
   sending packets to the right next hop and avoid any issues with
   ingress filtering.

   Let us analyze the scenario in Section 2.3.  If a source-address-
   dependent routing protocol is used, the two routers ("rtr1" and
   "rtr2") are both able to route traffic correctly, no matter which
   next-hop router and source address the host selects.  In case the
   host chooses the wrong next-hop router, e.g., "rtr1" is selected for
   Provider 2, "rtr1" will forward the traffic to "rtr2" to be sent to
   Network Provider 2 and no ingress filtering will happen.

   Note that home networks are expected to comply with requirements for
   source-address-dependent routing and that the routers will be
   configured accordingly no matter which routing protocol is used
   [RFC7788].

   This would work, but with some issues.  The host traffic to Provider
   2 will have to go over two links instead of one, i.e., the link
   bandwidth will be halved.  Another possibility is that "rtr1" can
   send an ICMPv6 Redirect message to the host to direct the traffic to
   "rtr2".  The host would then redirect Provider 2 traffic to "rtr2".

   The problem with redirects is that the ICMPv6 Redirect message can
   only convey two addresses, i.e., in this case the router address, or
   "rtr2" address and the destination address, or the destination host
   in Provider 2.  That means that the source address will not be
   communicated.  As a result, the host would send packets to the same
   destination using both source addresses, which causes "rtr2" to send

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   a redirect message to "rtr1", resulting in ping-pong redirects sent
   by "rtr1" and "rtr2".

   A solution to these issues is to configure the host with the source
   address prefixes that the next hop supports.  In a homenet context,
   each interface of the host can be configured by its next-hop router,
   so that all that is needed is to add the information about source
   address prefixes.  This results in the hosts selecting the right
   router, no matter what.

   Source-address-dependent routing in the use case of specialized
   egress routing (Section 2.4) may work as follows.  The specialized
   service router advertises one or more specific prefixes with
   appropriate source prefixes, e.g., to the CPE router, "rtr" in
   Figure 1.  The CPE router in turn advertises the specific service's
   prefixes and source prefixes to the host.  This will allow proper
   configuration at the host so that the host can use the service by
   sending the packets with the correct source and destination
   addresses.

3.2.  Provisioning Domains and SADR

   A consistent set of network configuration information is called a
   provisioning domain (PvD).  In the case of multihomed with multi-
   prefix (MHMP), more than one provisioning domain is present on a
   single link.  In the case of multi-prefix multiple interface (MPMI)
   environments, elements of the same domain may be present on multiple
   links.  PvD-aware nodes support association of configuration
   information into PvDs and use these PvDs to serve requests for
   network connections, e.g., choosing the right source address for the
   packets.  PvDs can be constructed from one of more DHCP or Router
   Advertisement (RA) options carrying such information as PvD identity
   and PvD container [MPvD_NDP] [MPvD_DHCP].  PvDs constructed based on
   such information are called explicit PvDs [RFC7556].

   Apart from PvD identity, PvD content may be encapsulated in separate
   RA or DHCP options called the PvD Container Option.  These options
   are placed in the container options of an explicit PvD.

   Explicit PvDs may be received from different interfaces.  A single
   PvD may be accessible over one interface or simultaneously accessible
   over multiple interfaces.  Explicit PvDs may be scoped to a
   configuration related to a particular interface; however, in general,
   this does not apply.  What matters is that the PvD identity is
   authenticated by the node even in cases where the node has a single
   connected interface.  The authentication of the PvD ID should meet
   the level required by the node policy.  Single PvD information may be
   received over multiple interfaces as long as the PvD ID is the same.

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   This applies to the Router Advertisements (RAs) in which case a
   multihomed host (that is, with multiple interfaces) should trust a
   message from a router on one interface to install a route to a
   different router on another interface.

4.  Discussion of Alternate Solutions

   We presented many topologies in which a host with multiple interfaces
   or a multihomed host is connected to various networks or Network
   Providers, which in turn may apply ingress routing.  The scenario
   analysis in Section 3.1 shows that in order to prevent packets from
   being dropped due to ingress routing, source-address-dependent
   routing is needed.  Also, source-address-dependent routing should be
   supported by routers throughout a site that has multiple egress
   points.

   In this section, we provide some alternate solutions vis-a-vis the
   scenarios presented in Section 2.  We start with Rule 5.5 in
   [RFC6724] for source address selection and the scenarios it solves,
   and then continue with solutions that state exactly what information
   hosts need in terms of new Router Advertisement options for correct
   source address selection in those scenarios.  No recommendation is
   made in this section.

4.1.  Router Advertisement Option

   There is a need to configure the host not only with the prefixes, but
   also with the source prefixes that the next-hop routers support.
   Such a configuration may prevent the host from getting ingress/egress
   policy error messages such as ICMP source address failure messages.

   If host configuration is done using Router Advertisement messages,
   then there is a need to define new Router Advertisement options for
   source-address-dependent routing.  These options include the Route
   Prefix with Source Address/Prefix Option.  Other options such as the
   Next-Hop Address with the Route Prefix Option and the Next-Hop
   Address with the Source Address and Route Prefix Option will be
   considered in Section 4.2.

   As discussed in Section 3.1, the scenario in Figure 4 can be solved
   by defining a new Router Advertisement option.

   If host configuration is done using DHCP, then there is a need to
   define new DHCP options for Route Prefix with Source Address/Prefix.
   As mentioned above, DHCP server configuration is interface specific.
   New DHCP options for source-address-dependent routing such as route
   prefix and source prefix need to be configured separately for each
   interface.

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   The scenario in Figure 4 can be solved by defining a new DHCP option.

4.2.  Router Advertisement Option Set

   Rule 5.5 for source address selection may be a solution for selecting
   the right source addresses for each next hop, but there are cases
   where the next-hop routers on each interface of the host are not
   known by the host initially.  Such use cases are out of scope.
   Guidelines for use cases that require the Router Advertisement option
   set involving third-party next-hop addresses are also out of scope.

4.3.  Rule 5.5 for Source Address Selection

   One possible solution is Rule 5.5 in [RFC6724], the default rule for
   source address selection, which recommends selecting the source
   addresses advertised by the next hop.  Considering the above
   scenarios, we can state that this rule can solve the problem in
   Figures 1, 2, and 5.

   Source address selection rules can be distributed by the DHCP server
   using the DHCP option OPTION_ADDRSEL_TABLE defined in [RFC7078].

   In case of DHCP-based host configuration, the DHCP server can
   configure only the interface of the host to which it is directly
   connected.  In order for Rule 5.5 to apply on other interfaces, the
   option should be sent on those interfaces as well using the DHCPv6
   address selection policy option defined in [RFC7078].

   Rule 5.5, the default rule for source address selection, solves that
   problem when an application sends a packet with an unspecified source
   address.  In the presence of two default routes, one route will be
   chosen, and Rule 5.5 will make sure that the right source address is
   used.

   When the application selects a source address, i.e., the source
   address is chosen before next-hop selection, even though the source
   address is a way for the application to select the exit point, in
   this case, that purpose will not be served.  In the presence of
   multiple default routes, one will be picked, ignoring the source
   address that was selected by the application because it is known that
   IPv6 implementations are not required to remember which next hops
   advertised which prefixes.  Therefore, the next-hop router may not be
   the correct one, and the packets may be filtered.

   This implies that the hosts should register which next-hop router
   announced each prefix.  It is required that RAs be sent by the
   routers and that they contain PIO on all links.  It is also required
   that the hosts remember the source addresses of the routers that sent

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   PIOs together with the prefixes advertised.  This can be achieved by
   updating redirect rules specified in [RFC4861].  [RFC8028] further
   elaborates this to specify to which router a host should present its
   transmission.

   The source-address-dependent routing solution is not complete without
   support from the edge routers.  All routers in edge networks need to
   be required to support routing based on not only the destination
   address but also the source address.  All edge routers need to be
   required to satisfy filters as defined in BCP 38.

5.  Security Considerations

   This document describes some use cases, and thus brings no additional
   security risks.  Solution documents should further elaborate on
   specific security considerations.

6.  References

6.1.  Normative References

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <http://www.rfc-editor.org/info/rfc3704>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <http://www.rfc-editor.org/info/rfc5340>.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
              <http://www.rfc-editor.org/info/rfc6296>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <http://www.rfc-editor.org/info/rfc6724>.

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   [RFC7078]  Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
              Address Selection Policy Using DHCPv6", RFC 7078,
              DOI 10.17487/RFC7078, January 2014,
              <http://www.rfc-editor.org/info/rfc7078>.

   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <http://www.rfc-editor.org/info/rfc8028>.

6.2.  Informative References

   [INGRESS_FIL]
              Huitema, C., Draves, R., and M. Bagnulo, "Ingress
              filtering compatibility for IPv6 multihomed sites", Work
              in Progress, draft-huitema-multi6-ingress-filtering-00,
              October 2004.

   [ISO.10589.1992]
              International Organization for Standardization,
              "Intermediate system to intermediate system intra-domain-
              routing routine information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473), ISO
              Standard 10589", ISO ISO.10589.1992, 1992.

   [MPvD_DHCP]
              Krishnan, S., Korhonen, J., and S. Bhandari, "Support for
              multiple provisioning domains in DHCPv6", Work in
              Progress, draft-ietf-mif-mpvd-dhcp-support-02, October
              2015.

   [MPvD_NDP] Korhonen, J., Krishnan, S., and S. Gundavelli, "Support
              for multiple provisioning domains in IPv6 Neighbor
              Discovery Protocol", Work in Progress, draft-ietf-mif-
              mpvd-ndp-support-03, February 2016.

   [RFC4116]  Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
              Gill, "IPv4 Multihoming Practices and Limitations",
              RFC 4116, DOI 10.17487/RFC4116, July 2005,
              <http://www.rfc-editor.org/info/rfc4116>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <http://www.rfc-editor.org/info/rfc4191>.

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   [RFC7157]  Troan, O., Ed., Miles, D., Matsushima, S., Okimoto, T.,
              and D. Wing, "IPv6 Multihoming without Network Address
              Translation", RFC 7157, DOI 10.17487/RFC7157, March 2014,
              <http://www.rfc-editor.org/info/rfc7157>.

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
              <http://www.rfc-editor.org/info/rfc7556>.

   [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
              2016, <http://www.rfc-editor.org/info/rfc7788>.

   [SD_RTG]   Baker, F., Xu, M., Yang, S., and J. Wu, "Requirements and
              Use Cases for Source/Destination Routing", Work in
              Progress, draft-baker-rtgwg-src-dst-routing-use-cases-02,
              April 2016.

   [SD_RTG_ISIS]
              Baker, F. and D. Lamparter, "IPv6 Source/Destination
              Routing using IS-IS", Work in Progress, draft-baker-ipv6-
              isis-dst-src-routing-06, October 2016.

   [SD_RTG_OSPF]
              Baker, F., "IPv6 Source/Destination Routing using OSPFv3",
              Work in Progress, draft-baker-ipv6-ospf-dst-src-routing-
              03, August 2013.

Acknowledgements

   In writing this document, we benefited from the ideas expressed by
   the electronic mail discussion participants on 6man Working Group:
   Brian Carpenter, Ole Troan, Pierre Pfister, Alex Petrescu, Ray
   Hunter, Lorenzo Colitti, and others.

   Pierre Pfister proposed the scenario in Figure 4 as well as some text
   for Rule 5.5.

   The text on corporate VPN in Section 2 was provided by Brian
   Carpenter.

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Authors' Addresses

   Behcet Sarikaya
   Huawei USA
   5340 Legacy Dr. Building 175
   Plano, TX  75024
   United States of America

   Email: sarikaya@ieee.org

   Mohamed Boucadair
   Orange
   Rennes 35000
   France

   Email: mohamed.boucadair@orange.com

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