INTERNET-DRAFT                                             Erik Nordmark
Jan 10, 2005                                            Sun Microsystems
                                                         Marcelo Bagnulo

                       Multihoming L3 Shim Approach


   Status of this Memo

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   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet Draft expires July 10, 2005.


   This document specifies a particular approach to IPv6 multihoming.
   The approach is based on using a multi6 shim placed between the IP
   endpoint sublayer and the IP routing sublayer, and, at least
   initially, using routable IP locators as the identifiers visible
   above the shim layer.  The approach does not introduce a "stack name"
   type of identifier, instead it ensures that all upper layer protocols
   can operate unmodified in a multihomed setting while still seeing a
   stable IPv6 address.

   This document does not specify the mechanism for authenticating and
   authorizing the "rehoming" - this is specified in the HBA document.

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   Nor does it specify the messages used to establish multihoming state.

   The document does not even specify the packet format used for the
   data packets.  Instead it discusses the issue of receive side
   demultiplexing and the different tradeoffs.  The resolution of this
   issue will effect the packet format for the data packets.

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      1.  Introduction.............................................    4
         1.1.  Non-Goals...........................................    4
         1.2.  Assumptions.........................................    5

      2.  Terminology..............................................    5
         2.1.  Notational Conventions..............................    6

      3.  Overview.................................................    6

      4.  Locators as Upper-layer Identifiers......................    7
         4.1.  IP Multicast........................................    8
         4.2.  Renumbering Implications............................    8

      5.  Placement of the multi6 shim.............................    9
         5.1.  Shim Implications on Flow Label Usage...............   11
         5.2.  Shim Implications on ICMP errors....................   11
         5.3.  Other Shim Protocol Implications....................   12
         5.4.  MTU Implications....................................   12

      6.  Deferred Context Establishment...........................   13

      7.  Assumptions about the DNS................................   13
         7.1.  DNS and Centrally Assigned Unique-local Addresses...   13

      8.  Protocol Walkthrough.....................................   14
         8.1.  Initial Context Establishment.......................   14
         8.2.  Locator Change......................................   15
         8.3.  Concurrent Context Establishment....................   15
         8.4.  Handling Initial Locator Failures...................   16

      9.  Demultiplexing of data packets in multi6 communications..   16
         9.1.  Approaches preventing the existence of ambiguities..   17
            9.1.1.  Pre-agreed identifiers.........................   18
            9.1.2.  N-square addresses.............................   18
         9.2.  Providing additional information to the receiver....   19
            9.2.1.  Flow-label.....................................   19
            9.2.2.  Extension Header...............................   21
         9.3.  Host-Pair Context...................................   21

      10.  IPSEC INTERACTIONS......................................   22

      11.  OPEN ISSUES.............................................   22

      12.  ACKNOWLEDGMENTS.........................................   23

      13.  REFERENCES..............................................   23

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         13.1.  Normative References...............................   23
         13.2.  Informative References.............................   23

      14.  CHANGE LOG..............................................   24

      AUTHORS' ADDRESSES...........................................   25

1.  Introduction

   The goal of the IPv6 multihoming work is to allow a site to take
   advantage of multiple attachments to the global Internet without
   having a specific entry for the site visible in the global routing
   table.  Specifically, a solution should allow users to use multiple
   attachments in parallel, or to switch between these attachment points
   dynamically in the case of failures, without an impact on the upper
   layer protocols.

   The goals for this approach is to:

    o Have no impact on upper layer protocols in general and on
      transport protocols in particular.

    o Address the security threats in [M6THREATS] through a separate
      document [HBA]

    o No extra roundtrip for setup; deferred setup.

    o Take advantage of multiple locators/addresses for load spreading
      so that different sets of communication to a host (e.g., different
      connections) might use different locators of the host.

1.1.  Non-Goals

   The assumption is that the problem we are trying to solve is site
   multihoming, with the ability to have the set of site locator
   prefixes change over time due to site renumbering.  Further, we
   assume that such changes to the set of locator prefixes can be
   relatively slow and managed; slow enough to allow updates to the DNS
   to propagate.  This proposal does not attempt to solve, perhaps
   related, problems such as host multihoming or host mobility.

   This proposal also does not try to provide an IP identifier.  Even
   though such a concept would be useful to ULPs and applications,
   especially if the management burden for such a name space was zero
   and there was an efficient yet secure mechanism to map from

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   identifiers to locators, such a name space isn't necessary (and
   furthermore doesn't seem to help) to solve the multihoming problem.

1.2.  Assumptions

   This approach assumes that packets with arbitrary combinations of
   source and destination locators will make it from end to end unless
   there is some form of failure.  Due to the interaction between
   ingress filtering [RFC2827] and source address selection, this
   assumption might not be true in IPv6 today.  As a result there is a
   need to work out a solution that doesn't make the ingress filtering
   in ISPs drop more packets than needed.  Some solutions to this have
   been proposed in [INGRESS].

2.  Terminology

      upper layer protocol (ULP)
                  - a protocol layer immediately above IP.  Examples are
                    transport protocols such as TCP and UDP, control
                    protocols such as ICMP, routing protocols such as
                    OSPF, and internet or lower-layer protocols being
                    "tunneled" over (i.e., encapsulated in) IP such as
                    IPX, AppleTalk, or IP itself.

      interface   - a node's attachment to a link.

      address     - an IP layer name that contains both topological
                    significance and acts as a unique identifier for an
                    interface.  128 bits.

      locator     - an IP layer topological name for an interface or a
                    set of interfaces.  128 bits.  The locators are
                    carried in the IP address fields as the packets
                    traverse the network.

      identifier  - an IP layer identifier for an IP layer endpoint
                    (stack name in [NSRG]).  The transport endpoint is a
                    function of the transport protocol and would
                    typically include the IP identifier plus a port
                    number.  NOTE: This proposal does not contain any IP
                    layer identifiers.

      upper-layer identifier (ULID)
                  - an IP locator which has been selected for
                    communication with a peer to be used by the upper
                    layer protocol.  128 bits.  This is used for
                    pseudo-header checksum computation and connection

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                    identification in the ULP.  Different sets of
                    communication to a host (e.g., different
                    connections) might use different ULIDs in order to
                    enable load spreading.

      address field
                  - the source and destination address fields in the
                    IPv6 header.  As IPv6 is currently specified this
                    fields carry "addresses".  If identifiers and
                    locators are separated these fields will contain

      FQDN        - Fully Qualified Domain Name

      Host-pair context
                  - the state that the multi6 shim maintains for a
                    particular peer.  The peer is identified by one or
                    more ULIDs.

2.1.  Notational Conventions

   A, B, and C are hosts.  X is a potentially malicious host.

   FQDN(A) is the domain name for A.

   Ls(A) is the locator set for A, which consists of the locators L1(A),
   L2(A), ... Ln(A).

   ULID(A) is an upper-layer ID for A.  In this proposal, ULID(A) is
   always one member of A's locator set.

3.  Overview

   This document specifies certain aspects of the approach, yet leaves
   other aspects open.

   The main points are about using locators as the ULIDs, and the exact
   placement of the multi6 shim in the protocol stack.

   The draft also discusses issues about receive side demultiplexing,
   which affects the packet format for data packets.

   The approach assumes that there are mechanisms (specified in other
   drafts) which:

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    - can prevent redirection attacks [HBA]

    - can prevent 3rd party DoS attacks [HBA, M6FUNC]

    - can detect whether or not a peer supports the multi6 protocol
      [M6FUNC, M6DET]

    - can explore all the locator pairs to find a working pair when the
      initial pair does not work [M6DET]

4.  Locators as Upper-layer Identifiers

   Central to this approach is to not introduce a new identifier name
   space but instead use one of the locators as the upper-layer ID,
   while allowing the locators used in the address fields to change over
   time in response to failures of using the original locator.

   This implies that the ULID selection is performed as today's default
   address selection as specified in [RFC 3484].  Underneath, and
   transparently, the multi6 shim selects working locator pairs with the
   initial locator pair being the ULID pair.  When communication fails
   the shim can test and select alternate locators.  A subsequent
   section discusses the issues when the selected ULID is not initially
   working hence there is a need to switch locators up front.

   Using one of the locators as the ULID has certain benefits for
   applications which have long-lived session state, or performs
   callbacks or referrals, because both the FQDN and the 128-bit ULID
   work as handles for the applications.  However, using a single 128-
   bit ULID doesn't provide seamless communication when that locator is
   unreachable.  See [M6REFER] for further discussion of the application

   There has been some discussion of using non-routable locators, such
   as unique-local addresses [ULA], as ULIDs in a multihoming solution.
   While this document doesn't specify all aspects of this, it is
   believed that the approach can be extended to handle such a case.
   For example, the protocol already needs to handle ULIDs that are not
   initially reachable.  Thus the same mechanism can handle ULIDs that
   are permanently unreachable from outside their site.  The issue
   becomes how to make the protocol perform well when the ULID is not
   reachable, for instance, avoiding any timeout and retries in this
   case.  In addition one would need to understand how the ULAs would be
   entered in the DNS to avoid a performance impact on existing, non-
   multi6 aware, IPv6 hosts potentially trying to communicate to the

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   (unreachable) ULA.
4.1.  IP Multicast

   IP Multicast requires that the IP source address field contain a
   topologically correct locator for interface that is used to send the
   packet, since IP multicast routing uses both the source address and
   the destination group to determine where to forward the packet.
   (This isn't much different than the situation with widely implemented
   ingress filtering [RFC2827] for unicast.)

   While in theory it would be possible to apply the shim re-mapping of
   the IP address fields between ULIDs and locators, the fact that all
   the multicast receivers would need to know the mapping to perform,
   makes such an approach difficult in practice.  Thus it makes sense to
   have multicast ULPs operate directly on locators and not use the
   shim.  This is quite a natural fit for protocols which use RTP
   [RFC3550], since RTP already has an explicit identifier in the form
   of the SSRC field in the RTP headers.  Thus the actual IP address
   fields are not important to the application.
4.2.  Renumbering Implications

   As stated above, this approach does not target to not make
   communication survive renumbering.  However, the fact that a ULID
   might be used with a different locator over time open up the
   possibility that communication between two ULIDs might continue to
   work after one or both of those ULIDs are no longer reachable as
   locators, for example due to a renumbering event.  This opens up the
   possibility that the ULID (or at least the prefix on which it is
   based) is reassigned to another site while it is still being used
   (with another locator) for existing communication.

   Worst case we could end up with two separate hosts using the same
   ULID while both of them are communicating with the same host.

   This potential source for confusion can be avoided if we require that
   any communication using a ULID must be terminated when the ULID
   becomes invalid (due to the underlying prefix becoming invalid).

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5.  Placement of the multi6 shim

                            | Transport Protocols |

             ------ ------- -------------- -------------     IP endpoint
             | AH | | ESP | | Frag/reass | | Dest opts |     sub-layer
             ------ ------- -------------- -------------

                         | multi6 shim layer |

                                ------                      IP routing
                                | IP |                      sub-layer

   Figure 1: Protocol stack

   The proposal uses an multi6 shim layer between IP and the ULPs as
   shown in figure 1, in order to provide ULP independence.
   Conceptually the multi6 shim layer behaves as if it is associated
   with an extension header, which would be ordered immediately after
   any hop-by-hop options in the packet.  However, the amount of data
   that needs to be carried in an actual multi6 extension header is
   close to zero, thus it might not be necessary to add bytes to each
   packet.  See section 9.

   We refer to packets that at least conceptually have this extension
   header, i.e., packets that should be processed by the multi6 shim on
   the receiver, as "multi6 packets" (analogous to "ESP packets" or "TCP

   Layering AH and ESP above the multi6 shim means that IPsec can be
   made to be unaware of locator changes the same way that transport
   protocols can be unaware.  Thus the IPsec security associations
   remain stable even though the locators are changing.  Layering the
   fragmentation header above the multi6 shim makes reassembly robust in
   the case that there is broken multi-path routing which results in
   using different paths, hence potentially different source locators,
   for different fragments.  Thus, effectively the multi6 shim layer is
   placed between the IP endpoint sublayer, which handles fragmentation,
   reassembly, and IPsec, and the IP routing sublayer, which on a host
   selects which default router to use etc.

   Applications and upper layer protocols use ULIDs which the multi6

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   layer will map to/from different locators.  The multi6 layer
   maintains state, called host-pair context, per ULID pairs (that is,
   applies to all ULP connections between the ULID pair) in order to
   perform this mapping.  The mapping is performed consistently at the
   sender and the receiver, thus from the perspective of the upper layer
   protocols, packets appear to be sent using ULIDs from end to end,
   even though the packets travel through the network containing
   locators in the IP address fields, and even though those locators
   might be changed by the transmitting multi6 shim layer.

   The context state in this approach is maintained per remote ULID i.e.
   approximately per peer host, and not at any finer granularity.  In
   particular, it is independent of the ULPs and any ULP connections.
   When two hosts are communicating with each other using multiple
   different ULID pairs, there is an option to "merge" the context state
   for the two (or more) ULID pairs.  Doing so would mean that the
   protocol to test and select working locators after a failure can be
   shared across the multiple ULID pairs between the two hosts.
   However, if it will be uncommon that two hosts communicate using
   multiple ULID pairs, the added complexity of merging the state might
   not be worth while.  Thus this is for further study.

      ----------------------------          ----------------------------
      | Sender A                 |          | Receiver B               |
      |                          |          |                          |
      |     ULP                  |          |     ULP                  |
      |      | src ULID(A)=L1(A) |          |      ^                   |
      |      | dst ULID(B)=L1(B) |          |      | src ULID(A)=L1(A) |
      |      v                   |          |      | dst ULID(B)=L1(B) |
      |   multi6 shim            |          |   multi6 shim            |
      |      | src L2(A)         |          |      ^                   |
      |      | dst L3(B)         |          |      | src L2(A)         |
      |      v                   |          |      | dst L3(B)         |
      |      IP                  |          |      IP                  |
      ----------------------------          ----------------------------
             |                                     ^
             ------- cloud with some routers -------

   Figure 2: Mapping with changed locators.

   The result of this consistent mapping is that there is no impact on
   the ULPs.  In particular, there is no impact on pseudo-header
   checksums and connection identification.

   Conceptually one could view this approach as if both ULIDs and
   locators are being present in every packet, but with a header
   compression mechanism applied that removes the need for the ULIDs
   once the state has been established.  In order for the receiver to

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   recreate a packet with the correct ULIDs there might be a need to
   include some "compression tag" in the data packets.  This would serve
   to indicate the correct context to use for decompression when the
   locator pair in the packet is insufficient to uniquely identify the

5.1.  Shim Implications on Flow Label Usage

   The use of a shim has implications on a class of protocols which have
   host as well as router functions.  This section discusses the
   implications for the flow label field, which might be used for QoS
   handling where the hosts set the flow label and initiate some flow
   signaling protocols, and the routers participate in the flow
   signaling protocol and as a result perform different action on
   packets based on the flow (which is identified by the IP address
   fields and the flow label field).

   The shim will leave the flow label unmodified.  This means that the
   {Flow Label, Source ULID, Dest ULID} that the upper-layer protocol
   sends will appear on the wire as the set of {flow label, source
   location, destination locator) for the different locators.

   This does have implications for protocols which do explicit signaling
   to create flow state; such protocols would somehow need to be made
   multi6 aware so that they can perform the signaling for all the
   tuples that are used on the wire.  Note that this need to modify such
   signaling protocols would apply even if the flows were identified
   without the use of the flow label field, due to the different
   locators which might be used.

5.2.  Shim Implications on ICMP errors

   Another protocol which requires consistency between the upper layer
   protocols on the hosts and the routers are the ICMP errors.  The
   routers (and in some cases the peer host) will generate ICMP errors
   based on the locators contained in the IP address fields, but when
   the host implementation delivers a notification to the ULP that an
   ICMP error was received, the ULP instance needs to be discovered
   based on the ULIDs.  This means that for ICMP error reception the
   host needs to be able to take the initial part of a multi6 packet
   (the part returned in an ICMP error) and do the inverse translation
   of the IP address fields that it did when sending the packet, and
   then deliver the notification to the ULP.

   Being able to demultiplex based on the information returned in an
   ICMP error presumably has some implications on the packet format and

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   mechanism for receive-side demultiplexing.

5.3.  Other Shim Protocol Implications

   As stated above, there might be other protocols in addition to flow
   management and ICMP errors that assume that the ULPs at the hosts and
   the IP address fields seen by the routers are the same.

   Any such protocol is potentially impacted by the introduction of the
   multihoming shim.

5.4.  MTU Implications

   Depending on how demultiplexing is handled at the receiver (see
   subsequent sections) there may or may not be a need for the shim to
   add an extension header to the packets.  In some cases such an
   extension header would only need to be added to some and not all data

   This has implications on the MTU that is available to the upper layer
   protocols hence will require some extra handling in the host
   implementation.  At some level this is similar to the MTU
   implementations of IPsec, in that the IP layer would add bytes to
   some ULP packets and not others, but in the case of IPsec one would
   expect all or no packets in a particular ULP connection to be
   affected, whereas in multi6 one some packets, such as those sent
   after a locator failure, would be subject to a reduction in the
   available MTU.

   While the effects of these are local to the host implementation they
   are likely to be a bit complicated.  There needs to be a mechanism so
   that the ULP can be notified when the available MTU changes due to an
   extension header either being added, or no longer being added.  Also,
   ICMPv6 packet to big errors need to result in a notification to the
   ULP which takes into account whether or not an extension header is
   being added.  Finally, certain ULPs such as UDP might need to rely on
   IP fragmentation down to the available MTU, while other ULPs such as
   TCP will adapt their segment size to the available MTU.

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6.  Deferred Context Establishment

   The protocol will use some context establishment exchange in order to
   setup multi6 state at the two endpoints.  Similar to MAST [MAST] this
   initial exchange can be performed asynchronously with data packets
   flowing between the two hosts; until context state has been
   established at both ends the packets would flow just as for
   unmodified IPv6 hosts i.e., without the ability for the hosts to
   switch locators.  This approach allows the hosts to have some local
   policy on when to attempt to establish multi6 state with a peer;
   perhaps based on the transport protocols and port numbers, or perhaps
   based on the number of packets that have flowed to/from the peer.

   Once the initial exchange has completed there is host-pair context
   state at both hosts, and both ends know a set of locators for the
   peer that are acceptable as the source in received packets.  This
   will trigger some verification of the set of locators, which is the
   subject of the security scheme.

7.  Assumptions about the DNS

   This approach assumes that hosts in multihomed sites have multiple
   AAAA records under a single name, in order to allow initial
   communication to try all the locators.  For multi6 capable hosts, the
   content of those records are the locators (which also serve as

   However, the approach does not assume that all the AAAA records for a
   given name refer to the same host.  Instead the context establishment
   allows each host to pass its locators to the peer.  This set could be
   either smaller or larger (or neither) than the AAAA record set.

   The approach makes no assumption about the reverse tree since the
   approach does not use it.  However, applications might rely on the
   reverse tree whether multi6 is used or not.

7.1.  DNS and Centrally Assigned Unique-local Addresses

   Earlier we've mentioned that the protocol might provide the basic
   mechanism to use Unique-local addresses as ULIDs.

   In the cases where hosts have been assigned centrally assigned ULAs
   [ULA-CENTRAL], one can potentially take advantage of this to provide
   better support for applications.  With centrally assigned ULAs it is
   possible to register them in the reverse DNS tree.  As a result, one
   could use the DNS not only for applications which care about reverse

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   and forward tree being consistent, but also to find the full set of
   locators from the ULID.

8.  Protocol Walkthrough
8.1.  Initial Context Establishment

   Here is the sequence of events when A starts talking to B:

    1.  A looks up FQDN(B) in the DNS which returns a locator set which
        includes some locators for B.  (The set could include locators
        for other hosts since e.g., might include AAAA
        records for multiple hosts.)  The application would typically
        try to connect using the first locator in the set i.e., ULID(B)
        = L1(B).  The application is prepared to try the other locators
        should the first one fail.

    2.  The ULP creates "connection" state between ULID(A)=L1(A) and
        ULID(B) and sends the first packet down to the IP/multi6 shim
        layer on A.  L1(A) was picked using regular source address
        selection mechanisms.

    3.  The packet passes through the multi6 layer, which has no state
        for ULID(B).  A local policy will be used to determine when, if
        at all, to attempt to setup multi6 state with the peer.  Until
        this state triggers packets pass back and forth between A and B
        as they do in unmodified IPv6 today.

        When the policy is triggered, which could be on either A or B,
        an initial context establishment takes place.  This exchange
        might fail should the peer not support the multi6 protocol.  If
        it succeeds it results in both ends receiving the locator sets
        from their respective peer, and the security mechanism provides
        some way to verify these sets.

        At this point in time it is possible for the hosts to change to
        a different locator in the set.  But until they have exchanged
        the locator sets, and probably until they rehome the context to
        use different locators, they continue sending and receiving IPv6
        packets as before.

        As long as both hosts have been informed of the state at the
        peer i.e., know the locators of the peer and know that the peer
        has received its locators, each host can make an independent
        decision when it sees a need to change either the source or
        destination locator in the packets it is sending.  Thus the

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        approach does not require coordinating the actual locator
        changes between the peers.

8.2.  Locator Change

   When a host detects that communication is no longer working it can
   try to switch to a different locator pair.  A host might suspect that
   communication isn't working due to

    - lack of positive advise from the ULP (akin to the NUD advise in

    - negative advise from the ULP

    - failure of some explicit multi6 "heartbeat" messages

    - local indications such as the local locator becoming invalid
      [RFC2462] or the interface being disabled

   Given that each host knows the locator set for its peer, the host can
   just switch to using a different locator pair.  It might make sense
   for the host to test the locator pair before using it for ULP
   traffic, both to verify that the locator pair is working and to
   verify that it is indeed the peer that is present at the other end;
   the latter to prevent 3rd party DoS attacks.  Such testing needs to
   complete before using the locator as a destination in order to
   prevent 3rd party DoS attacks [M6THREATS].

8.3.  Concurrent Context Establishment

   Should both A and B attempt to contact each other at about the same
   time using the same ULIDs for each other, the context establishment
   should create a single host-pair context.  The NOID draft [NOID]
   contains a proof-of-concept that a 4-way context establishment
   exchange can ensure that a single context is created in this case.

   However, if different ULIDs are used this would result in two
   completely independent contexts between the two hosts following the
   basic content establishment above; the context is per ULID pair.  As
   noted above, in this case it might be desirable to "merge" i.e.,
   share certain information, such as the reachability of different
   locator pairs, across the different ULID pairs that are between the
   same pair of hosts.

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8.4.  Handling Initial Locator Failures

   Should not all locators be working when the communication is
   initiated some extra complexity arises, because the ULP has already
   been told which ULIDs to use.  If the locators that were selected to
   be ULIDs are not working and the multi6 shim does not know of
   alternate locators, it has no other choice than to have the
   application try a different ULID.

   Thus the simplest approach is to always punt initial locator failures
   up the stack to the application.  However, this might imply
   significant delays while transport protocol times out.

   It is possible to optimize this case when the multi6 shim already has
   alternate locators for the peer.  This might be the case when the two
   hosts have already communicated, and it might be possible to have the
   DNS resolver library provide alternate locators to the shim in the
   speculation that they might be useful.  Such an optimization must not
   assume that the AAAA records refer to the same host, since it isn't
   uncommon that a FQDN have multiple AAAA records for the same
   *service* but for different hosts.  For instance, the protocol would
   need to verify with the peer that the ULID in question is in fact
   assigned to the peer in this case.  Potentially the trust level for
   the different locators retrieved from the DNS in this case, as
   opposed to retrieving the ULIDs from the DNS and the locators from
   the peer itself using the multihoming protocol, might be different.
   Note however, that this is an optimization and is not required for
   the protocol to work.

   Should the multi6 shim know alternate locators for the peer, it needs
   to perform the multi6 protocol before upper layer protocol packets
   are exchanged.  This means that the context establishment can not be
   deferred, and that there is a rehoming event, with the necessary
   security checks, before the first ULP packets can be successfully

9.  Demultiplexing of data packets in multi6 communications

   The mechanisms for preserving established communications through
   outages that reside in the M6 shim layer manage the multiple
   addresses available in the multihomed node so that a reachable
   address is used in the communication.  Since reachability may vary
   during the communication lifetime, different addresses may have to be
   used in order to keep packets flowing.  However, the addresses
   presented by the M6 shim layer to the upper layer protocols must
   remain constant through the locator changes, so that received packets

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   are recognized by the upper layer protocols as belonging to the
   established communication.  In other words, in order to preserve
   established communications through outages, the M6 shim layer will
   use different locators for exchanging packets while presenting the
   same identifiers for the upper layer protocols.  This means that upon
   the reception of an incoming packet with a pair of locators, the M6
   shim layer will need to translate the received locators to the
   identifiers that are being used by the upper layer protocols in the
   particular communication.  This operation is called demultiplexing.

   For example, if a host has address A1 and A2 and starts communicating
   with a peer with addresses B1 and B2, then some communication
   (connections) might use the pair <A1, B1> as upper-layer identifiers
   and others might use e.g., <A2, B2>.  Initially there are no failures
   so these address pairs are used as locators i.e. in the IP address
   fields in the packets on the wire.  But when there is a failure the
   multi6 shim on A might decide to send packets that used <A1, B1> as
   upper-layer identifiers using <A2, B2> as the locators.  In this case
   B needs to be able to rewrite the IP address field for some packets
   and not others, but the packets all have the same locator pair.

   Either we must prevent this from happening, or provide some
   additional information to B so that it can tell which packets need to
   have the IP address fields rewritten.

   In this section, we will analyze different approaches to perform the
   demultiplexing operation.  The possible approaches can be classified
   into two categories: First, the approaches that prevent the existence
   of ambiguities on the demultiplexing operation i.e. each received
   locator corresponds to one and only one ULP identifier.  Second, the
   approaches that use a context tag to provide additional information
   to the receiver that indicates the identifiers that correspond to the
   locators contained in the packets.

   Note that the sender also needs to be able to demultiplex ICMP errors
   as noted in Section 5.2, however the analysis below does not take
   that added constraint into account.

9.1.  Approaches preventing the existence of ambiguities

   One could think this problem can be avoided if the host never used
   the same locators for different ULIDs when communicating with the
   same peer host.  However, the host can't tell a priori whether two
   peers share an IP stack.  For instance, if A connects to
   with AAAA=B1 and with AAAA=B2 it can't tell whether B1 or
   B2 are assigned to the same IP stack or not until it communicates
   with B1 and B2 and retrieves there complete locator sets.  (And even

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   this might not suffice, since the peer might want to preserve the
   illusion of being two different hosts by returning B3 as an
   alternative locator for B1 and B4 as an alternate for B2.)

9.1.1.  Pre-agreed identifiers

   The simplest approach of this type is to designate one of the
   available addresses as the identifier to be used for all the
   communications while the remaining addresses will only be used as
   locators.  This means that the upper layer protocols will only be
   aware of a single address, the one used as identifier, and all the
   remaining addresses that are used as locators will remain invisible
   to them.  Consequently, only the address that is being used as
   identifier can be returned by the resolver to the applications.  The
   addresses used as locators cannot be returned to the applications by
   the resolver.  So, if no additional information about the role of the
   addresses is placed in the DNS, only the identifier-address can be
   published in the DNS.  This configuration has reduced fault tolerance
   capabilities during the initial contact, since the initiator will
   have only one address available to reach the receiver.  If the
   identifier address placed in the DNS is not reachable, the
   communication will fail.  It would be possible to overcome this
   limitation by defining a new DNS record for storing information about
   address that can be only used as locators.  If such record is
   defined, the initiator can use an alternative locator, even for
   initial contact, while still presenting the address designated as
   identifier to the upper layer protocols.  However, this approach
   requires support from the initiator node, implying that only upgraded
   nodes will obtain improved fault tolerance while legacy nodes that
   don't support the new DNS record will still obtain reduced fault
   tolerance capabilities.

9.1.2.  N-square addresses

   In order to overcome the limitations presented by the previous
   scheme, it is possible to create additional addresses that have a
   pre-determined role.  In this approach, each multihomed node that has
   n prefixes available, will create n^2 addresses, or in other words,
   the node will have n sets of n addresses each.  Each set will contain
   one address per prefix.  So, in each set, one address will be
   designated as identifier while the remaining addresses will be
   designated as locators.  The addresses designated as identifiers will
   have different prefixes in the different sets.  The result is that
   there will be n addresses designated as identifiers, one per
   available prefix, and each identifier-address will have an associated
   set of n-1 addresses that can only be used as locators.  The
   addresses designated as identifiers will be published in the DNS
   while the addresses used as locators must not be AAAA records in the

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   DNS to prevent them from ever being used as ULIDs.  The applications
   will only have knowledge of the first ones, and only the M6 shim
   layer will deal with locators.  The resulting configuration has full
   fault tolerance capabilities since n addresses (one per prefix) will
   be published in the DNS, allowing the usage of different addresses to
   make the initial contact.

   Even though the destination address field rewrite can be inferred
   from the destination locator in both of the above approaches, there
   is still a need to maintain multi6 state at the receiver in order for
   the receiver to tell how and whether to rewrite the source address

9.2.  Providing additional information to the receiver

   When two nodes establish a multi6 enabled communication, a context is
   created at the M6 shim layers of each node.  The context stores
   information about the addresses that are used as identifiers for the
   upper layer protocols and also about the locator set available for
   each node.  In this approach, data packets carry a context tag that
   allows the receiver determine which is the context that has to be
   used to perform the demultiplexing operation.  There are several ways
   to carry the context tag within the data packets.  In this section we
   will explore the following options: the Flow Label, and an Extension

9.2.1.  Flow-label

   A possible approach is to carry the context tag in the Flow Label
   field of the IPv6 header.  This means that when a multi6 context is
   established, a Flow Label value is associated with this context (and
   perhaps a separate flow label for each direction).

   The simplest approach that does this is to have the triple <source
   locator, destination locator, flow label> identify the context at the

   The sender and receiver needs to agree to allocate the flow labels so
   that each context between a pair of IP stacks receives a different
   flow label.  While this might seem simple at first sight, the
   possibilities that different ULIDs refer to the same IP stack, and
   even that different FQDNs refer to the same IP stack, severely
   constrains how flow labels can be allocated.  For instance, when
   communication is initiated from a host X to both foo.example (with
   ULIDs A and B) and bar.example (with ULIDs C and D), then host X
   might think it is communicating to two different IP stacks, when in

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   fact they might be the same IP stack.  Later when the locator sets
   are updated, for instances, after some failure, it might be told that
   ULID A has locators A, B, C, and D.  Hence the two communications
   would need to have separate flow labels for the packets sent from X.

   A protocol can handle this either by having X (in this example)
   allocate the flow label for the packets it is sending, or having the
   intended receiver allocate them.  In the former case, X would need to
   allocate different flow labels for the different ULID pairs, since it
   doesn't know which peers are the same IP stack or not.  In the latter
   case, the intended receiver would pick a flow label which is unique
   i.e., it can disambiguate the two contexts in this case.  This
   implies that the flow label will not be assigned until the
   multihoming protocol has established the context state.

   An added limitation imposed by this approach is that all the
   potential source and destination locators have to be known beforehand
   by the receiver in order to be recognized.  This means that before
   sending packets with a new locator, the sender has to inform the
   receiver about the new locator, while for other approaches it is
   probably possible to start sending packets using a new locator and
   the same context tag in parallel with carrying information about the
   new locator to the peer, if the context tag would be sufficient by
   itself to identify the context i.e., the source locator isn't used to
   identify the context.

   Note that we do not yet understand how beneficial it would be to be
   able to accept packets from unknown source locators (the rules for
   packet injection can probably be more relaxed than for where packets
   are sent, for instance if the context tag matches).  Requiring a
   match on <source locator, destination locator, flow label> would make
   this impossible.  Instead the locator change signaling would need to
   be acknowledged before the peer can start sending using a new source

   An attempt to remove the above limitation would be to try to have the
   receiver only identify the context based on the flow label field,
   i.e., without taking the locators into account in the lookup.  This
   requires constraining flow label allocation for the hosts that
   implement multi6 so that for multi6 packets the receiver wouldn't
   have to compare the locators but only use the flow label.  Due to the
   deferred multi6 capability discovery this would have to apply to all
   flow label assignments on a host which implements multi6.

   It also requires carrying some additional information in the packet
   to identify whether the Flow Label field is actually being used as a
   context tag or not.  In other words, additional information is needed
   to identify multi6 packets from regular IPv6 packets.  This is

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   because, the same Flow Label value that is being used as context tag
   in multi6 enabled communication can be used for other purposes by a
   non-multi6 enabled host, resulting in two communications using the
   same Flow Label value.  The result of this situation would be that
   packets of the non-multi6 enabled communication would be
   demultiplexed using the context associated to the Flow Label value
   carried in the packets.  A possible approach to solve this issue it
   to use an additional bit to identify data packets that belong to
   multi6 capable communications and that have to be demultiplexed using
   the Flow Label value.  However, there are no obvious choices for that
   bit, since all bits of the IPv6 header are currently in use.  A
   possibility would be to use new Next Header values to indicate that
   the packet belongs to a multi6 enabled communication and that the
   Flow Label carries context information as proposed in [NOID].

9.2.2.  Extension Header

   Another approach is to define a new Extension Header to carry the
   context tag.  This context tag is agreed between the involved parties
   during the multi6 protocol initial negotiation.  Following data
   packets will be demultiplexed using the tag carried in the Extension
   Header.  This seems a clean approach since it does not overload
   existing fields.  However, it introduces additional overhead in the
   packet due to the additional header.  The additional overhead
   introduced is 8 octets.  However, it should be noted that the context
   tag is only required when an address other than the one used as
   identifier for upper layer protocols is contained in the packet.
   Packets carrying the addresses that have to be used as identifier for
   the upper layer protocols do not require a context tag, since the
   address contained in the packets is the address presented to the
   upper layers.  This approach would reduce the overhead.  On the other
   hand, this approach would cause changes in the available MTU, since
   packets that include the Extension Header will have an MTU 8 octets

9.3.  Host-Pair Context

   The host-pair context is established on each end of the communication
   when one of the endpoints performs the multi6 signaling (the 4-way
   handshake referred to in [M6FUNC]).

   This context is accessed differently in the transmit and receive
   paths.  In the transmit path when the ULP passes down a packet the
   key to the context state is the tuple <ULID(local), ULID(peer)>; this
   key must identify at most one state record.  In the receive path the
   context must be found based on what is in the packet, be it just the
   locators, or the locators plus some additional "context tag" as

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   discussed above, or just a "context tag".


   As specified, all of ESP, AH, and key management is layered above the
   multi6 layer.  Thus they benefit from the stable ULIDs provided above
   the multi6 layer.  This means the IPsec security associations are
   unaffected by switching locators.

   The alternative would be to layer multi6 above IPsec, but that
   doesn't seem to provide any benefits and it would add the need to
   create different IPsec SAs when the locators change due to rehoming.

   A result of layering multi6 above IPsec is that the multi6 protocol
   can potentially be used to redirect IPsec protected traffic as a
   selective DoS mechanism.  If we somehow could require IPsec for the
   multi6 protocol packets when the ULP packets between the same hosts
   use IPsec, then we could prevent such attacks.

   However, due to the richness in IPsec policy, this would be a bit
   tricky.  If only some protocols or port numbers/selectors are to be
   protected by IPsec per a host's IPsec policy, then how would one
   determine whether multi6 traffic needs to be protected?  Should one
   take the conservative approach that if any packets between the
   hosts/ULIDs need to be protected, then the multi6 traffic should also
   be protected?

   For this to be useful both communicating hosts would need to make the
   same policy decisions, so if we are to take this path there would
   need to some standardization in this area.


   Receive side demultiplexing issue as described above.

   Is it possible to facilitate transition to multi6 using some "multi6
   proxy" at site boundaries until all important hosts in a site have
   been upgraded to support multi6?  Would would be the properties of
   such a proxy?  Would it place any additional requirements on the
   protocol itself?

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   This document was originally produced of a MULTI6 design team
   consisting of (in alphabetical order):  Jari Arkko, Iljitsch van
   Beijnum, Marcelo Bagnulo Braun, Geoff Huston, Erik Nordmark, Margaret
   Wasserman, and Jukka Ylitalo.

   The idea to use a set of locators and not inventing a new identifier
   name space, as well as using the DNS for verification of the
   locators, was first brought up by Tony Li.


13.1.  Normative References

     [M6THREATS] Nordmark, E., and T. Li, "Threats relating to IPv6
             multihoming solutions", draft-ietf-multi6-multihoming-
             threats-00.txt, July 2004.

     [ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6
             Addressing Architecture", RFC 3513, April 2003.

     [IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version
             6 (IPv6) Specification", RFC 2461.

     [M6FUNC] Functional decomposition of the M6 protocol, draft-dt-

     [HBA] Hash Based Addresses (HBA), draft-bagnulo-multi6dt-hba-00.txt

     [M6DET] Jari Arkko, Failure Detection and Locator Selection in
             Multi6, draft-multi6dt-failure-detection-00.txt

13.2.  Informative References

     [NSRG] Lear, E., and R. Droms, "What's In A Name: Thoughts from the
             NSRG", draft-irtf-nsrg-report-09.txt (work in progress),
             March 2003.

     [ULA] R. Hinden, and B. Haberman, Unique Local IPv6 Unicast
             Addresses, draft-ietf-ipv6-unique-local-addr-08.txt

     [ULA-CENTRAL] R. Hinden, and B. Haberman, Centrally Assigned Unique
             Local IPv6 Unicast Addresses, draft-ietf-ipv6-ula-central-

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     [NOID] Erik Nordmark, "Multihoming without IP Identifiers", Oct 27,
             2003, <draft-nordmark-multi6-noid-01.txt>

             AN EXTENDED PROPOSAL", draft-crocker-mast-protocol-01.txt,
             October, 2003.

     [RFC3041]  T. Narten, R. Draves, "Privacy Extensions for Stateless
             Address Autoconfiguration in IPv6", RFC 3041, January 2001.

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

     [RFC3550]  H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
             "RTP: A Transport Protocol for Real-Time Applications",
             July 2003, RFC 3550.

     [INGRESS] C. Huitema, R. Draves, and M. Bagnulo, "Ingress filtering
             compatibility for IPv6 multihomed sites", Oct 2004,


Changes since draft-nordmark-multi6dt-shim-00.txt:

 o Added assumption that something else handles the interaction between
   ingress filtering and source address selection.

 o Clarified things with respect to using ULAs in general, and added
   separate text about centrally assigned ULAs.

 o Added more text about MTU dropping implications and ICMP too big re-

 o Added text specifying how the shim handles the flow label field, and
   the impact on flow setup protocols.

 o Added text about the need for the sender to handle ICMP errors

 o Added text that there might be other protocols than flow setup
   protocols and ICMP errors that might be impacted by the shim.

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 o Added text about IP multicast in a new section.

 o Added clarification in section 8.4 about AAAA records being for a
   service and not a host needing some care.

 o Added a clarification in section 8.4 that learning the different
   locators during initial communication from the DNS potentially has
   different trust issues than learning them from the peer.

 o Clarified the two models of flow label usage for demultiplexing

 o In section 5 clarified that state maintenance is not per ULP

 o In section 5 clarified merging option.

 o Clarified in section 9.1 why it isn't sufficient to avoid using the
   same locators for different ULIDs for the same peer host.

 o Clarified in section 9.1.1/9.1.2 that there is multi6 state at the
   receiver to tell how/whether to rewrite the source address field.

 o Clarified the aspect of section 9.2.1 which talks about not being
   able to use a new locator until the peer has been told of the new

 o Added text about the implications of renumbering and reassignment.

 o Clarified section on flow labels to first talk about the simple case
   of using <source locator, destination locator, flow label> and its
   complexities, and then about the potential to just use the flow label
   by itself to identify the context.


     Erik Nordmark
     Sun Microsystems, Inc.
     17 Network Circle
     Mountain View, CA

     phone: +1 650 786 2921
     fax:   +1 650 786 5896

     Marcelo Bagnulo
     Universidad Carlos III de Madrid
     Av. Universidad 30

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     Leganes, Madrid  28911

     Phone: 34 91 6249500

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   Funding for the RFC Editor function is currently provided by the
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