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Homenet working Group                                           O. Troan
Internet-Draft                                             Cisco Systems
Intended status: Informational                                L. Colitti
Expires: March 08, 2014                                           Google
                                                      September 04, 2013

     IPv6 Multihoming with Source Address Dependent Routing (SADR)


   A multihomed network using provider aggregatable addresses must send
   the packet out the right path to avoid violating the provider's
   ingress filtering.  This memo suggests a mechanism called Source
   Address Dependent Routing to solve that problem.

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
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   This Internet-Draft will expire on March 08, 2014.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2

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   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   4.  Using SADR for multihoming . . . . . . . . . . . . . . . . . .  3
   5.  A Conceptual Forwarding Algorithm  . . . . . . . . . . . . . .  3
   6.  Routing considerations . . . . . . . . . . . . . . . . . . . .  4
     6.1.  Routing Protocol extensions  . . . . . . . . . . . . . . .  5
     6.2.  Simplified SADR in home networks . . . . . . . . . . . . .  5
   7.  Interaction between routers and hosts  . . . . . . . . . . . .  5
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     11.1.  Normative References  . . . . . . . . . . . . . . . . . .  6
     11.2.  Informative References  . . . . . . . . . . . . . . . . .  7
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  7

1.  Introduction

   IPv6 is designed to support multiple addresses on an interface, and
   the intention was to use this feature to support multihoming with
   provider aggregatable addresses.

   One difficulty of multihoming with provider-aggregatable space is
   that providers typically employ BCP38 [RFC2827] filtering.  If a
   network sends traffic to its upstream provider using a source address
   that was not assigned by that provider, the traffic will be dropped.
   Thus, if a network is multihomed to multiple providers, it must
   ensure that traffic is sent out the correct exit for the packet's
   source address.

   As long as upstream traffic is sent to the correct provider, hosts
   inside the network are free to use source addresses assigned by any
   of the network's upstream providers.  In such a scenario, each host
   has multiple addresses, one or more from each provider the network is
   connected to.  The network ensures that packets are sent to the
   correct upstream by forwarding packets based on the destination
   address and the source address.  This we call source address
   dependent routing (SADR).  This memo shows how SADR can be used to
   implement multihoming.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Terminology

   Service Provider        An entity that provides the network with
                           external connectivity, e.g.  to the Internet.

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   WAN Interface           An interface connected to a Service
                           Providers.  WAN interfaces may either be
                           physical links or virtual interfaces such as
                           tunnels.  WAN interfaces are used to send
                           ingress traffic from the Internet to the End-
                           User, and egress traffic from the End-User
                           network to the Internet.  Ingress traffic may
                           be received on any active interface at any
                           time.  Egress traffic follows a set of rules
                           within the router in order to choose the
                           proper WAN interface.

   Border Router           A border router has one or more external
                           interfaces connecting it to one or more
                           Service Providers.  The border router
                           receives one or more delegated prefixes, each
                           associated with one or more WAN interfaces.

   External Route          A route that is learned from a Service
                           Provider.  Each External Route has an
                           Acceptable Source Prefix which determines
                           which source addresses may use that route.

   Internal Router         A router that is not a Border Router.

   Internal Route          A route to a destination inside the network.

4.  Using SADR for multihoming

   SADR is similar to policy based routing.  This memo proposes a simple
   extension to the destination based longest match algorithm to
   constrain it to source address.

   In order to support ingress filtering by upstream networks, the
   network MUST treat external routes specially.  Ingress filtering MAY
   also be used internally, by installing (S,D) routes for locally
   assigned prefixes, where the source prefix would be the aggregatable
   prefix.  If no ingress filtering is performed inside the network,
   then normal non-source constrained forwarding is used.

5.  A Conceptual Forwarding Algorithm

   This section describes a conceptual forwarding algorithm.  An
   implementation might implement this differently, e.g.  with multiple
   tables, as long as the external behaviour is as described.

   First a longest match lookup is done in the routing table for the
   destination address, then for the resulting set a longest matching
   lookup is done for the source address.

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   In a destination based routing table, an entry in the routing table
   can be shown as "D -> NH".  That is, to get to a destination D, use
   next-hop NH.  For a source constrained routing table we propose the
   following notation.  (Source Network, Destination network) -> Next-
   hop.  (S, D) -> NH.  A route that is not source constrained can be
   represented as (*, D) -> NH.

   For convenience this document shows the routing table as a single
   destination based routing table, with source address constrained
   paths.  This does not preclude other implementations, as long as the
   external behaviour is the same.

   A router forwarding a packet does a longest match look-up on the
   destination address.  If this is a (*, D) entry, it forwards the
   packet out the best next-hop as before (doing equal cost multi path
   load balancing etc).  If the look-up results in a (S, D) entry, the
   look-up function does a longest match on the source address among the
   set of (S, D) paths.  If there is a match the packet is forwarded out
   the given next-hop, if not an ICMP destination address unreachable
   message, code 5 is returned [RFC4443].  A routing entry may have both
   (S, D) paths and (*, D) paths.  The longest match wins.

   The following example show the routing table of a network connected
   to two ISPs, ISP A and ISP B. Both ISPs offer default connectivity
   and ISP B also offers a more specific route to a walled garden

    (2001:db8::/56, ::/0) -> ISP_A          # Default route to ISP A
    (2001:db9::/56, ::/0) -> ISP_B          # Default route to ISP B
    (*, 2001:db8::/64) -> R1                # Internal network, prefix from A
    (*, 2001:db8:1::/64) -> R2              # Internal network, prefix from A
    (*, 2001:db9::/64) -> R1                # Internal network, prefix from B
    (*, 2001:db9:1::/64) -> R2              # Internal network, prefix from B
    (*, fd00::/64) -> R3                    # Internal network ULA
    (2001:db9::/56, 2001:420::/32) -> ISP_B # Walled garden route from ISP B

                    Figure 1: Example Routing Table

   A packet with the SA, DA of 2001:db8::1, 2001:dead:beef::1 would be
   forwarded to ISP A, likewise a packet with SA, DA 2001:db9::1,
   2001:dead:beef::1 would be forward to ISP B. An packet with SA,DA
   2001:db8::1, 2001:db9::1 would be forwarded using normal destination
   based routing.  A packet to the walled garden SA,DA 2001:db9::1,
   2001:420::1 would be sent to ISP B. A packet with SA,DA
   2001:db8::1,2001:420::1 would be dropped with an ICMP unreachable
   message being sent back.

6.  Routing considerations

   Now that we have described the function of the source constrained
   routing table.  How does the table get populated?

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6.1.  Routing Protocol extensions

   The generic answer is that the routing protocol used in the network
   has to be extended to support (S, D) routes.  Specifically, the
   routing protocol should distribute, for each External Route, the
   Acceptable Source Prefix(es) for that route.  This may be done, for
   example, using [I-D.baker-ipv6-ospf-dst-src-routing] or [I-D.baker-
   ipv6-isis-dst-src-routing].  In the case of OSPFv3, for example,
   external routes are advertised in an AS-External-prefix LSA,

6.2.  Simplified SADR in home networks

   In a home network using a dynamic prefix assignment mechanism such as
   [I-D.arkko-homenet-prefix-assignment] it may be known that a
   particular Border Router is announcing both an External Route and an
   Aggregated Prefix (for example, if the same router ID is announcing
   both).  In this case, interior routers may assume that the Acceptable
   Source Prefix of the External Route announced by that Border Router
   is in fact the Aggregated Prefix announced by that Border Router.

   An internal router when receiving a AS-External LSA route will
   install that in the routing table as normal.  When the internal
   router receives a aggregated prefix as part of prefix assignment, the
   router shall add source constrained entries to all the AS-External
   routes received from the same border router (matching router-ID).

   Routes that are not associated with a border router or are not AS-
   External do not have source constrained paths.

   The routing protocol requirements for simplified SADR in the home
   network are:

   1.  Routing protocol must flood all information to all routers in the
       home network.  (Single area).

   2.  Prefix assignment and unicast routing must be done in the same

   3.  A router must be uniquely identified (router-id) so that router
       advertisements and prefix assignment can be tied together

7.  Interaction between routers and hosts

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   Generally, hosts need not be aware that SADR is in use in the
   network.  Hosts simply choose source addresses and the network will
   deliver the traffic to the appropriate upstream.  One exception is
   when an Acceptable Source Prefix becomes invalid (e.g., if the Border
   Router which announced it crashes, or its WAN link goes down).  In
   this case, current hosts will continue to use source addresses in
   that Acceptable Source Prefix without knowing that all communication
   outside the network is likely to fail.  In this case, interior
   routers can improve responsiveness by deprecating the addresses in
   that Acceptable Source Prefix.

   ICMP [RFC4443] includes a Destination unreachable code 5 - "Source
   address failed ingress/egress policy".  Hosts MUST adhered to this
   message, and based on the unreachable message try another source

8.  IANA Considerations

   This specification does not require any IANA actions.

9.  Security Considerations

10.  Acknowledgements

   The authors would like to thank Jari Arkko and Andrew Yourtchenko for
   their ideas and review.

11.  References

11.1.  Normative References

              Arkko, J., Lindem, A., and B. Paterson, "Prefix Assignment
              in a Home Network", draft-arkko-homenet-prefix-
              assignment-03 (work in progress), October 2012.

              Lindem, A. and J. Arkko, "OSPFv3 Auto-Configuration",
              draft-ietf-ospf-ospfv3-autoconfig-00 (work in progress),
              October 2012.

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

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

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   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

11.2.  Informative References

              Baker, F., "IPv6 Source/Destination Routing using IS-IS",
              draft-baker-ipv6-isis-dst-src-routing-00 (work in
              progress), February 2013.

              Baker, F., "IPv6 Source/Destination Routing using OSPFv3",
              draft-baker-ipv6-ospf-dst-src-routing-00 (work in
              progress), February 2013.

              Chown, T., Arkko, J., Brandt, A., Troan, O., and J. Weil,
              "Home Networking Architecture for IPv6", draft-ietf-
              homenet-arch-06 (work in progress), October 2012.

Authors' Addresses

   Ole Troan
   Cisco Systems
   Philip Pedersens vei 1
   Lysaker  1366

   Email: ot@cisco.com

   Lorenzo Colitti
   Roppongi Hills Mori Tower PO box 22
   Minato, Tokyo  106-6126

   Email: lorenzo@google.com

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