IPv6 Maintenance                                                F. Baker
Internet-Draft                                             Cisco Systems
Updates: 4861 (if approved)                                 B. Carpenter
Intended status: Standards Track                       Univ. of Auckland
Expires: May 6, 2016                                    November 3, 2015

          Routing packets from hosts in a multi-prefix network


   This document describes expected IPv6 host behavior in a network that
   has more than one prefix, each allocated by an upstream network that
   implements BCP 38 ingress filtering, when the host has multiple
   routers to choose from.  It also applies to other scenarios such as
   the usage of stateful firewalls that effectively act as address-based

   This host behavior may interact with source address selection in a
   given implementation, but logically follows it.  Given that the
   network or host is, or appears to be, multihomed with multiple
   provider-allocated addresses, that the host has elected to use a
   source address in a given prefix, and that some but not all
   neighboring routers are advertising that prefix in their Router
   Advertisement Prefix Information Options, this document specifies to
   which router a host should present its transmission.  It updates RFC

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 6, 2016.

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Copyright Notice

   Copyright (c) 2015 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
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   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 and Applicability  . . . . . . . . . . . . . . .   2
     1.1.  Host Model  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Sending context expected by the host  . . . . . . . . . . . .   5
     2.1.  Expectations the host has of the network  . . . . . . . .   5
     2.2.  Expectations of multihomed networks . . . . . . . . . . .   7
   3.  Reasonable expectations of the host . . . . . . . . . . . . .   7
     3.1.  Interpreting Router Advertisements  . . . . . . . . . . .   7
     3.2.  Default Router Selection  . . . . . . . . . . . . . . . .   8
     3.3.  Source Address Selection  . . . . . . . . . . . . . . . .   8
     3.4.  Redirects . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.5.  History . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Residual issues . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Change Log (RFC Editor: please delete) . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction and Applicability

   This document describes the expected behavior of an IPv6 [RFC2460]
   host in a network that has more than one prefix, each allocated by an
   upstream network that implements BCP 38 [RFC2827] ingress filtering,
   and in which the host is presented with a choice of routers.  It
   expects that the network will implement some form of egress routing,
   so that packets sent to a host outside the local network from a given
   ISP's prefix will go to that ISP.  If the packet is sent to the wrong

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   egress, it is liable to be discarded by the BCP 38 filter.  However,
   the mechanics of egress routing once the packet leaves the host are
   out of scope.  The question here is how the host interacts with that

   Various aspects of this issue, and possible solution approaches, are
   discussed in the document IPv6 Multihoming without Network Address
   Translation [RFC7157].

   BCP 38 filtering by ISPs is not the only scenario where such behavior
   is valuable.  Implementations that combine existing recommendations,
   such as [RFC6092] [RFC7084] can also result in such filtering.
   Another case is when the connections to the upstream networks include
   stateful firewalls, such that return packets in a stream will be
   discarded if they do not return via the firewall that created state
   for the outgoing packets.  A similar cause of such discards is
   unicast reverse path forwarding (uRPF) [RFC3704].

   In this document, the term "filter" is used for simplicity to cover
   all such cases.  In any case, one cannot assume the host to be aware
   whether an ingress filter, a stateful firewall, or any other type of
   filter is in place.  Therefore, the only consistent solution is to
   implement the features defined in this document.

   Note that, apart from ensuring that a message with a given source
   address is given to a first-hop router that appears to know about the
   prefix in question, this specification is consistent with [RFC4861].
   Nevertheless, implementers of Sections 5.2, 6.2.3, 6.3.4 and 8 of RFC
   4861 will need to extend their implementations accordingly.  This
   specification is fully consistent with [RFC6724] and implementers
   will need to add support for its Rule 5.5.  Hosts that do not support
   these features may fail to communicate in the presence of filters as
   described above.

1.1.  Host Model

   It could be argued that the proposal of this document, which is to
   send messages using a source address in a given prefix to the router
   that advertised the prefix in its RA, is a form of [RFC1122]'s Strong
   ES (e.g.  Host) Model, discussed in section of that document.
   In short, [RFC1122] identifies two basic models, in which the "strong
   host" model models the host as a set of hosts in one chassis, each of
   which uses a single address on a single interface, and always both
   sends and receives on that interface, and the "weak host" model
   treats the host as one system with zero or more addresses on every
   interface, and capable of using any interface for any communication.
   As noted there, neither model is completely satisfactory.  For
   example, a host with an addressless interface and a default route

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   pointing to the interface will necessarily send packets using any
   address using that interface, and therefore be a de facto weak host.
   If the router upstream from such a host implements BCP 38 Ingress
   Filtering [RFC2827], such as by implementing uRPF on each interface,
   the router will prevent communication by weak hosts.

              |                 |
              |     MIF Router  +---/--- other interfaces
              |                 |
                  |         | Two interfaces sharing a prefix
                --+-+--   --+-+--
                    |         |
                 |   MIF Host    |

                Figure 1: Hypothetical MIF interconnection

   The proposal also differs slightly from [RFC1122]'s language of the
   Strong Host Model.  The statement is that the packet will go to the
   router that advertised a given prefix, but doesn't state what
   interface that might happen on.  Hence, if the router is a MIF router
   and is using the same prefix on two or more LANs shared by the host
   (as in Figure 1), the host might use each of those LANs and meet the
   requirement.  The Strong Host Model is not stated in those terms, but
   in terms of the interface used, and would find a MIF router quite

      (A) A host [MUST] silently discard an incoming datagram whose
      destination address does not correspond to the physical interface
      through which it is received.

      (B) A host [MUST] restrict itself to sending (non-source- routed)
      IP datagrams only through the physical interface that corresponds
      to the IP source address of the datagrams.

   However, comparing the presumptive route lookup mechanisms in each
   model, this proposal is indeed most similar to the Strong Host Model,
   as is any source/destination routing paradigm.

   Strong:  route(src IP addr, dest IP addr, TOS) -> gateway

   Weak:  route(dest IP addr, TOS) -> gateway, interface

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   In the hypothetical MIF model suggested in Figure 1, the address
   fails to identify a single interface, but it does identify a single

1.2.  Requirements Language

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

2.  Sending context expected by the host

2.1.  Expectations the host has of the network

   A host receives prefixes in a Router Advertisement [RFC4861], which
   goes on to identify whether they are usable by SLAAC [RFC4862]
   [RFC4941] [RFC7217].  When no prefixes are usable for SLAAC, the
   Router Advertisement would normally signal the availability of DHCPv6
   [RFC3315] and the host would use it to configure its addresses.  In
   the latter case (or if both SLAAC and DHCPv6 are used on the same
   link for some reason) it will be generally the case that the
   configured addresses match one of the prefixes advertised in a Router
   Advertisement that are supposed to be on-link for that link.

   The simplest multihomed network implementation in which a host makes
   choices among routers might be a LAN with one or more hosts on it and
   two or more routers, one for each upstream network, or a host that is
   served by disjoint networks on separate interfaces.  In such a
   network, especially the latter, there is not necessarily a routing
   protocol, and the two routers may not even know that the other is a
   router as opposed to a host, or may be configured to ignore its
   presence.  One might expect that the routers may or may not receive
   each other's RAs and form an address in the other router's prefix
   (which is not per [RFC4862], but is implemented by some stub router
   implementations).  However, all hosts in such a network might be
   expected to create an address in each prefix so advertised.

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          +---------+   +---------+    +---------+    +---------+
          |   ISP   |   |   ISP   |    |   ISP   |    |   ISP   |
          +----+----+   +----+----+    +----+----+    +----+----+
               |             |              |              |
               |             |              |              |
          +----+----+   +----+----+    +----+----+    +----+----+
          |  Router |   |  Router |    |  Router |    |  Router |
          +----+----+   +----+----+    +----+----+    +----+----+
               |             |              |              |
               +------+------+              |  +--------+  |
                      |                     +--+  Host  +--+
                 +----+----+                   +--------+
                 |  Host   |
               Common LAN Case            Disjoint LAN Case
            (Multihomed Network)          (Multihomed Host)

                       Figure 2: Two simple networks

   If there is no routing protocol among those routers, there is no
   mechanism by which packets can be deterministically forwarded between
   the routers (as described in BCP 84 [RFC3704]) in order to avoid
   filters.  Even if there was routing, it would result in an indirect
   route, rather than a direct route originating with the host; this is
   not "wrong", but can be inefficient.  Therefore the host would do
   well to select the appropriate router itself.

   Since the host derives fundamental default routing information from
   the Router Advertisement, this implies that, in any network with
   hosts using multiple prefixes, each prefix SHOULD be advertised via a
   Prefix Information Option (PIO) [RFC4861] by one of the attached
   routers, even if addresses are being assigned using DHCPv6.  A router
   that advertises a prefix indicates that it is able to appropriately
   route packets with source addresses within that prefix, regardless of
   the setting of the L and A flags in the PIO.

   In some circumstances both L and A might be zero.  If SLAAC is not
   wanted (A=0) and there is no reason to announce an on-link prefix
   (L=0), a PIO SHOULD be sent to inform hosts that the prefix is
   source-routed by the router in question.  Although this does not
   violate the existing standard [RFC4861], such a PIO has not
   previously been common, and it is possible that existing host
   implementations simply ignore such a PIO or that a router
   implementation rejects such a PIO as a configuration error.  Newer
   implementations that support this mechanism will need to be updated
   accordingly: a host SHOULD NOT ignore a PIO simply because both L and
   A flags are cleared; a router SHOULD be able to send such a PIO.

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2.2.  Expectations of multihomed networks

   The direct implication of Section 2.1 is that, if the network uses a
   routing protocol, the routing protocols used in multihomed networks
   SHOULD implement source-prefix based egress routing.  Network designs
   exist that can usefully limit themselves to static routing (such as a
   simple tree network), or may internally use no routers at all, such
   as a single LAN with two CE routers, each of which leads to a
   different upstream network.

3.  Reasonable expectations of the host

3.1.  Interpreting Router Advertisements

   As described in [RFC4191] and [RFC4861], a Router Advertisement may
   contain zero or more Prefix information Options (PIOs), zero or more
   Route Information Options (RIOs), or a Default Router List.  In their
   original intent, these indicate general information to a host: "these
   are your default routers", "you might create or be configured with an
   address in this prefix", or "this router would be a good place to
   send traffic directed to a given destination prefix".  In a multi-
   homed network implementing source/destination routing, the
   interpretation of a default router list or an RIO has to be modified
   with the words "if the source address is in one of the prefixes I
   advertise in a PIO".  Additionally, the PIO must be reinterpreted to
   also imply that the advertising router would be a reasonable first
   hop for any packet using a source address in any advertised prefix.

                                                 +---------+  |
                                     ( ISP A ) - +  Bob-A  +--+  +-----+
+---------+                        /             +---------+  +--+     |
|         |                       /                           |  |     |
|  Alice  +---/---( The Internet )                               | Bob |
|         |                       \                           |  |     |
+---------+                        \             +---------+  +--+     |
                                     ( ISP B ) - +  Bob-B  +--+  +-----+
                                                 +---------+  |

                 Figure 3: PIOs, RIOs, and Default Routes

   The implications bear consideration.  Imagine, Figure 3, that hosts
   Alice and Bob are in communication, Bob's network is multihomed, and
   Bob's first hop routers are Bob-A (to upstream ISP A advertising
   prefix A') and Bob-B (to upstream network B and advertising prefix
   B').  If Bob is responding to a message from Alice, his choice of
   source address is forced to be the address Alice used as a

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   destination (which we may presume to have been in prefix A').  Hence,
   Bob created or was assigned an address in A', and can only reasonably
   send traffic using it to Bob-A as a first hop router.  If there were
   several instances of Bob-A and one had advertised itself as a default
   router or as having a route to Alice, that is the router Bob should
   choose.  If none of Bob-A have advertised that but Bob-B has, it is
   irrelevant; Bob is using the address allocated in A' and courts a BCP
   38 discard if he doesn't send the packet to Bob-A.

   In the special case that Bob is initiating the conversation, an RIO
   might, however, influence source address choice.  Bob could
   presumably use any address allocated to him, in this case his address
   in A' or B'.  If Bob-B has advertised an RIO for Alice's prefix and
   Bob-A has not, Bob MAY take that fact into account in address
   selection - choosing an address that would allow him to make use of
   the RIO.

3.2.  Default Router Selection

   Default Router Selection is modified as follows: A host SHOULD select
   default routers for each prefix it is assigned an address in.
   Routers that have advertised the prefix in its Router Advertisement
   message SHOULD be preferred over routers that do not advertise the

   As a result of doing so, when a host sends a packet using a source
   address in one of those prefixes and has no history directing it
   otherwise, it SHOULD send it to the indicated default router.  In the
   "simplest" network described in Section 2.1, that would get it to the
   only router that is directly capable of getting it to the right ISP.
   This will also apply in more complex networks, even when more than
   one physical or virtual interface is involved.

   In more complex cases, wherein routers advertise RAs for multiple
   prefixes whether or not they have direct or isolated upstream
   connectivity, the host is dependent on the routing system already.
   If the host gives the packet to a router advertising its source
   prefix, it should be able to depend on the router to do the right

3.3.  Source Address Selection

   There is an interaction with Default Address Selection [RFC6724].  A
   host following the recommendation in the previous section will store
   information about which next-hops advertised which prefixes.  Rule
   5.5 of RFC 6724 states that the source address used to send to a
   given destination address should if possible be chosen from a prefix
   known to be advertised by the next-hop router for that destination.

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   This selection rule would therefore be applicable in a host following
   the recommendation in the previous section.

3.4.  Redirects

   There is potential for adverse interaction with any off-link Redirect
   (Redirect for a GUA destination that is not on-link) message sent by
   a router in accordance with Section 8 of [RFC4861].  Hosts SHOULD
   apply off-link redirects only for the specific pair of source and
   destination addresses concerned, so the host's Destination Cache may
   need to contain appropriate source-specific entries.

3.5.  History

   Some modern hosts maintain history, in terms of what has previously
   worked or not worked for a given address or prefix and in some cases
   the effective window and MSS values for TCP or other protocols.  This
   might include a next hop address for use when a packet is sent to the
   indicated address.

   When such a host makes a successful exchange with a remote
   destination using a particular address pair, and the host has
   previously received a PIO that matches the source address, then the
   host SHOULD include the prefix in such history, whatever the setting
   of the L and A flags in the PIO.  On subsequent attempts to
   communicate with that destination, if it has an address in that
   prefix at that time, a host MAY use an address in the remembered
   prefix for the session.

4.  Residual issues

   Consider a network where routers on a link run a routing protocol and
   are configured with the same information.  Thus, on each link all
   routers advertise all prefixes on the link.  The assumption that
   packets will be forwarded to the appropriate egress by the local
   routing system might cause at least one extra hop in the local
   network (from the host to the wrong router, and from there to another
   router on the same link).

   In a slightly more complex situation such as the disjoint LAN case of
   Figure 2, which happens to be one of the authors' home plus corporate
   home-office configuration, the two upstream routers might be on
   different LANs and therefore different subnets (e.g., the host is
   itself multi-homed).  In that case, there is no way for the "wrong"
   router to detect the existence of the "right" router, or to route to

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   In such a case it is particularly important that hosts take the
   responsibility to memorize and select the best first-hop as described
   in Section 3.

5.  IANA Considerations

   This memo asks the IANA for no new parameters.

6.  Security Considerations

   This document does not create any new security or privacy exposures.
   It is intended to avoid connectivity issues in the presence of BCP 38
   ingress filters or stateful firewalls combined with multihoming.

   There might be a small privacy improvement, however: with the current
   practice, a multihomed host that sends packets with the wrong address
   to an upstream router or network discloses the prefix of one upstream
   to the other upstream network.  This practice reduces the probability
   of that occurrence.

7.  Acknowledgements

   Comments were received from Jinmei Tatuya and Ole Troan, who have
   suggested important text, plus Mikael Abrahamsson, Steven Barth,
   Juliusz Chroboczek, Toerless Eckert, David Farmer, Dusan Mudric,
   Tadahisa Okimoto, Pierre Pfister, Mark Smith, and James Woodyatt.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [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>.

   [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,

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   [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,

8.2.  Informative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [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>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [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>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,

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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,

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,

Appendix A.  Change Log (RFC Editor: please delete)

   Initial Version:  2015-08-05

   Version 01:  Update text on PIOs, added text on Redirects, and
      clarified the concept of a "simple" network, 2015-08-13.

   Version 02:  Clarifications after WG discussions, 2015-08-19.

   Version 03:  More clarifications after more WG discussions,
      especially adding stateful firewalls, uRPF, and more precise
      discussion of RFC 4861, 2015-09-03.

   Version 04:  Responds to various comments including

      *  Questions regarding RFC 1122's strong and weak host models.
         This model is, strictly speaking, neither, but is most similar
         to the strong host model.

      *  Some wording errors.

      *  Requests for discussion of the handling of the RIO, PIO, and
         Default Router List in an RA.

   WG Versions 00-02:  More clarifications after more WG discussions,

Authors' Addresses

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117

   Email: fred@cisco.com

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   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland  1142
   New Zealand

   Email: brian.e.carpenter@gmail.com

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