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Versions: 00

INTERNET-DRAFT                                             Erik Nordmark
July 9, 2005                                            Sun Microsystems
                                                         Marcelo Bagnulo
                                                                    UC3M

                       Multihoming L3 Shim Approach

                     <draft-ietf-shim6-l3shim-00.txt>

   Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet Draft expires January 9, 2006.


   Abstract

   This document specifies a particular approach to IPv6 multihoming.
   The approach is based on using a multihoming 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 affect the packet format for the data packets.













































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   Contents

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

      2.  Terminology..............................................    5
         2.1.  Notational Conventions..............................    7

      3.  Overview.................................................    7

      4.  Locators as Upper-layer Identifiers......................    8
         4.1.  IP Multicast........................................    8
         4.2.  Renumbering Implications............................    9

      5.  Placement of the multihoming shim........................    9
         5.1.  Shim Implications on Flow Label Usage...............   12
         5.2.  Shim Implications on ICMP errors....................   12
         5.3.  Other Shim Protocol Implications....................   13
         5.4.  MTU Implications....................................   13

      6.  Deferred Context Establishment...........................   14

      7.  Assumptions about the DNS................................   14
         7.1.  DNS and Unique-local Addresses......................   15

      8.  Protocol Walkthrough.....................................   15
         8.1.  Initial Context Establishment.......................   15
         8.2.  Locator Change......................................   16
         8.3.  Concurrent Context Establishment....................   17
         8.4.  Handling Initial Locator Failures...................   17

      9.  Demultiplexing of data packets in shim6 communications...   18
         9.1.  Approaches preventing the existence of ambiguities..   19
            9.1.1.  Pre-agreed identifiers.........................   19
            9.1.2.  N-square addresses.............................   20
         9.2.  Providing additional information to the receiver....   20
            9.2.1.  Flow-label.....................................   21
            9.2.2.  Extension Header...............................   23
         9.3.  Host-Pair Context...................................   23

      10.  IPSEC INTERACTIONS......................................   24

      11.  OPEN ISSUES.............................................   24

      12.  ACKNOWLEDGMENTS.........................................   25

      13.  REFERENCES..............................................   25



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         13.1.  Normative References...............................   25
         13.2.  Informative References.............................   26

      14.  CHANGE LOG..............................................   27

      AUTHORS' ADDRESSES...........................................   29



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 Preserve established communications through failures, for example,
      TCP connections and application communications using UDP.

    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.  But it is not a goal to try to make communication
   survive a renumbering event (which causes all the locators of a host
   to change to a new set of locators).  This proposal does not attempt
   to solve, perhaps related, problems such as host multihoming or host



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   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
   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.

   Furthermore, a solution following the outline in this document and
   companion shim6 WG documents will most likely end up assuming that a
   host can affect the exit path from the site.  In particular, should
   the host send packets with all combinations of its addresses in the
   IP source address field and all of the peers addresses in the
   destination field, that with high probability that set of packets
   ends up using multiple exits from the site (going via different
   ISPs), when the site is connected to multiple ISPs.  (Of course, the
   network admin might have disabled this somehow, but in order for the
   multihoming solution to find a working path it needs to have the
   ability to explore different exits somehow.)

   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



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                    interface.  128 bits.  This document only uses the
                    "address" term in the case where it isn't specific
                    whether it is a locator or a identifier.

      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 name for an IP layer endpoint (stack
                    name in [NSRG]).  The transport endpoint name is a
                    function of the transport protocol and would
                    typically include the IP identifier plus a port
                    number.  NOTE: This proposal does not specify any
                    new form of IP layer identifier, but still separates
                    the identifying and locating properties of the IP
                    addresses.

      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
                    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.

                    Since the ULID is just one of the IP
                    locators/addresses of the node, there is no need for
                    a separate name space and allocation mechanisms.

      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
                    locators for packets on the wire.

      FQDN        - Fully Qualified Domain Name

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






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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, the exact
   placement of the multihoming shim in the protocol stack, and an
   outline of a protocol for a protocol for establishing and and
   managing the necessary state in the multihoming shim.

   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:

    - can prevent redirection attacks [HBA]

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

    - can detect whether or not a peer supports the shim6 protocol
      [M6FUNC, M6DET]

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












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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 multihoming 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
   implications.

   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-
   shim6 aware, IPv6 hosts potentially trying to communicate to the
   (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



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   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 try to 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).

   However, this might be an overkill.  Even when an IPv6 prefix is
   retired and reassigned to some other site, there is still a very
   small probability that another host in that site picks the same 128
   bit address (whether using DHCPv6, stateless address
   autoconfiguration, or picking a random interface ID [RFC 3041]).
   Should the identical address be used by another host, then there
   still wouldn't be a problem until that host attempts to communicate
   with the same host with which the initial user of the IPv6 address
   was communicating.



5.  Placement of the multihoming shim












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                            -----------------------
                            | Transport Protocols |
                            -----------------------

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

                         ---------------------
                         | shim6 shim layer |
                         ---------------------

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

   Figure 1: Protocol stack

   The proposal uses an multihoming shim layer within the IP layer,
   i.e., below the ULPs, as shown in figure 1, in order to provide ULP
   independence.  Conceptually the multihoming 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 shim6 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 multihoming
   shim on the receiver, as "shim6 packets" (analogous to "ESP packets"
   or "TCP packets").

   Layering AH and ESP above the multihoming 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.  The MOBIKE WG
   is looking at ways to have IPsec security associations survive even
   though the IP addresses changes, which is a different approach.

   Layering the fragmentation header above the multihoming 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
   multihoming 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.




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   Applications and upper layer protocols use ULIDs which the shim6
   layer will map to/from different locators.  The shim6 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 shim6 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) |
      |   multihoming shim       |          |   multihoming 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



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   once the state has been established.  In order for the receiver to
   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
   context.


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
   Locator, 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
   shim6 aware so that they can perform the signaling for all the tuples
   of {Flow Label, Source Locator, Destination Locator} 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 shim6 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



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   ICMP error presumably has some implications on the packet format and
   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
   packets.

   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 shim6 some packets, such as those sent after a
   locator failure, would be subject to a reduction in the available
   MTU.

   The MTU can also change because after a rehoming event a different
   path (with a different MTU) is being used to exchange packets between
   the peers.  So, after a rehoming event, the need to include an
   additional extension header, and the usage of a different path affect
   the MTU.  However, it should be noted that the shim6 is aware of the
   path change and that the MTU discovery techniques already have a
   mechanism to cope with changes in the path MTU when the path changes.

   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 too 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



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   TCP will adapt their segment size to the available MTU.



6.  Deferred Context Establishment

   The protocol will use some context establishment exchange in order to
   setup shim6 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 shim6 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 shim6 capable hosts, the
   content of those records are the locators (which also serve as
   ULIDs).

   However, the approach does not assume that all the AAAA records for a
   given name refer to the same host.  For instance, the AAAA records
   could point at different hosts providing the same service.  This is
   handled by the context establishment mechanism allowing each host to
   pass its locators to the peer.  This set contains a non-null subset
   of the locators presented in the AAAA record set and it may contain
   additional locators, not contained in 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 shim6 is used or not.

   When the reverse DNS tree is populated so that any locator for the
   host can be used to find a FQDN and that FQDN can be used to find all
   the locators of the host, then this property can be used by the



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   applications when doing referals by the application only passing one
   locator to the peer and having the peer application do the DNS
   lookups to find the other locators.  However, such a mechanism does
   not work when the reverse DNS is not setup to satisfy this property.
   Suggestions for how to handle the general case of application
   referals is captured in [APP-REFER]

   Note that in some cases, even when the reverse DNS is populated, it
   can be hard to satisfy the above property.  For instance, a home site
   using two "consumer" ISPs each providing DNS service including
   reverse DNS.  In this case there might be no way for the site to
   control what goes in the DNS for the reverse tree.
7.1.  DNS and Unique-local Addresses

   Earlier we've mentioned that the protocol might provide the basic
   mechanism to use Unique-local addresses [ULA] 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
   and forward tree being consistent, but also to find the full set of
   locators from the ULID, the same way as outlined in the previous
   section.  But again, this doesn't handle the general case of
   application referals.

   For ULAs that are not centrally allocated it is not likely to be
   possible to register them in the global DNS, thus this possibility
   does not exist.



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., www.example.com might include AAAA
        records for multiple hosts.) The default address selection
        [RFC3484] would make that set into an ordered list.  The
        application would typically try to connect using the first
        locator in the list i.e., ULID(B) = L1(B).  The application is
        prepared to try the other locators should the first one fail.




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    2.  The ULP creates "connection" state between ULID(A)=L1(A) and
        ULID(B) and sends the first packet down to the IP/shim6 shim
        layer on A.  L1(A) was picked using regular source address
        selection mechanisms [RFC3484].  Note that should communication
        fail using the initial locator/ULID pair, there has to be a
        mechanism to retry with both different destination locators and
        different source locators.

    3.  The packet passes through the shim6 layer, which has no state
        for ULID(B).  A local policy will be used to determine when, if
        at all, to attempt to setup shim6 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
        will fail if the peer does not support the shim6 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
        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 advice from the ULP (akin to the NUD advice in
      [RFC2461]

    - negative advice from the ULP

    - failure of some explicit shim6 "heartbeat" messages



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    - 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.


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 shim6 layer does not know of
   alternate locators, it has no other choice than to have the
   application try a different ULID.  Note that the mechanism need to be
   able to try both different source ULIDs as well as different
   destination ULIDs, to make sure all combinations are explored.

   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.  Also, the
   current default address selection mechanism [RFC3484] doesn't have a
   mechanism to try different source addresses for a single destination
   address; it can only cycle through different destination addresses
   with each destination address being used with a single source



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   address.

   It is possible to optimize initial locator failures when the shim6
   layer already has alternate locators for the peer.  This might be the
   case when the two hosts have already communicated, or 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 shim6 layer know alternate locators for the peer, it needs
   to perform the shim6 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
   exchanged.



9.  Demultiplexing of data packets in shim6 communications

   The mechanisms for preserving established communications through
   outages that reside in the multihoming shim layer manage the multiple
   locators available in the multihomed node so that a reachable locator
   is used in the communication.  Since reachability may vary during the
   communication lifetime, different locators may have to be used in
   order to keep packets flowing.  However, the locators presented by
   the shim layer to the upper layer protocols must remain constant
   through the locator changes, so that received packets 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 shim layer will use different
   locators for exchanging packets while presenting the same ULIDs to
   the upper layer protocols.  This means that upon the reception of an
   incoming packet with a pair of locators, the shim layer will need to
   translate the received locators to the ULIDs 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



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   with a peer with addresses B1 and B2, then some communication
   (connections) might use the pair <A1, B1> as ULID 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 shim6 layer on
   A might decide to send packets that used <A1, B1> as ULIDs 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 ULID.  Second, the approaches
   that use a context tag to provide additional information to the
   receiver that indicates the ULIDs 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 www.foo.com
   with AAAA=B1 and www.bar.com 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
   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 ULID 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 ULID, and all the remaining addresses that



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   are used as locators will remain invisible to them.  Consequently,
   only the ULID 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 ULID 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 ULID 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 addresses 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 ULID 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 ULID while the remaining addresses will be designated
   as locators.  The ULIDs will have different prefixes in the different
   sets.  The result is that there will be n ULIDs, one per available
   prefix, and each ULID will have an associated set of n-1 addresses
   that can only be used as locators.  The ULIDs will be published in
   the DNS while the addresses usable only as locators must not be AAAA
   records in the DNS to prevent them from ever being used as ULIDs.
   The applications and default address selection [RFC3484] will only
   have knowledge of the ULIDs, and only the 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.


9.2.  Providing additional information to the receiver

   When two nodes establish a shim6 enabled communication, a context is
   created at the shim layers of each node.  The context stores
   information about the ULIDs and also about the locator set available
   for each node.  In this approach, data packets carry a context tag



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   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 Header.

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 shim6 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 <Flow
   Label, Source Locator, Destination Locator> identify the context at
   the receiver.

   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 H to both foo.example (with
   ULIDs A and B) and bar.example (with ULIDs C and D), then host H
   might think it is communicating to two different IP stacks, when in
   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 H.

   A protocol can handle this either by having H (in this example)
   allocate the flow label for the packets it is sending, or having the
   intended receiver allocate them.  In the former case, H 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.

   A modification to this suggested on the mailing list, which does not
   need establish the context state before the flow label is assigned,
   is to start the communication with unmodified flow label usage (could
   be zero, or as suggested in [RFC 3697]).  The packets sent after a
   failure when a different locator pair is used would use a completely
   different flow label, and this flow label can be allocated as part of
   the shim context establishment.  Since it is allocated during the



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   context establishment, the receiver of the "failed over" packets can
   pick a flow label of its choosing, without any performance impact,
   and respecting that for each locator pair, the flow label value used
   for a given locator pair doesn't change due to the operation of the
   multihoming shim.

   A, perhaps minor, limitation imposed by overloading the flow label
   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 reception 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
   locator.

   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 shim6 so that for shim6 packets the receiver wouldn't have
   to compare the locators but only use the flow label.  Due to the
   deferred shim6 capability discovery this would have to apply to all
   flow label assignments on a host which implements shim6.

   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 shim6 packets from regular IPv6 packets.  This is
   because, the same Flow Label value that is being used as context tag
   in shim6 enabled communication can be used for other purposes by a
   non-shim6 enabled host, resulting in two communications using the
   same Flow Label value.  The result of this situation would be that
   packets of the non-shim6 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 shim6 capable
   communications and that have to be demultiplexed using the Flow Label



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   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 shim6 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 shim6 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 a locator other than the one used as ULID
   is contained in the packet.  Packets where both the source and
   destination address fields contain the ULIDs do not require a context
   tag, since no rewriting is necessary at the receiver.  This approach
   would reduce the overhead.  On the other hand, this approach would
   cause changes in the available MTU for some packets, since packets
   that include the Extension Header will have an MTU 8 octets shorter.


9.3.  Host-Pair Context

   The host-pair context is established on each end of the communication
   when one of the endpoints performs the shim6 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
   discussed above, or just a "context tag".













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10.  IPSEC INTERACTIONS

   As specified, all of ESP, AH, and key management is layered above the
   shim6 layer.  Thus they benefit from the stable ULIDs provided above
   the shim6 layer.  This means the IPsec security associations are
   unaffected by switching locators.

   The alternative would be to layer shim6 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 shim6 above IPsec is that the shim6 protocol can
   potentially be used to redirect IPsec protected traffic as a
   selective DoS mechanism.  If we somehow could require IPsec for the
   shim6 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 shim6 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 shim6 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.



11.  OPEN ISSUES

   Receive side demultiplexing issue as described above.

   How is the shim6 state managed, in particular, what mechanism is used
   for removing the state?  There seems to be two choices:

    - a harder state approach which relies on some "CLOSE" message
      exchange (in combination with timers). An example of this is in
      [HIP-BASE].

    - a soft-state mechanism where a node can discard the shim6 state at
      any time, combined with an error message "I have no state for you"
      that triggers the peer to reestablish the context.

   Related to the state management and the demultiplexing issues is how
   the protocol detects a loss of context state, which can occur due to



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   a complete state loss (a host crashing and rebooting) or due to the
   shim garbage collecting the shim state even though the peer might
   continue to rely on it.

   The actual packet formats for the shim6 protocol.

   Is it possible to facilitate transition to shim6 using some "shim6
   proxy" at site boundaries until all important hosts in a site have
   been upgraded to support shim6?  Would would be the properties of
   such a proxy?  Would it place any additional requirements on the
   protocol itself?



12.  ACKNOWLEDGMENTS

   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.

   Greg Daley suggested that the flow label approach can be more easily
   used if different flow label values are used for the different
   locator pairs.



13.  REFERENCES

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-ietf-
             shim6-functional-dec-00.txt




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     [HBA] Hash Based Addresses (HBA), draft-ietf-shim6-hba-00.txt

     [M6DET] Jari Arkko, Failure Detection and Locator Selection in
             Multi6, draft-ietf-shim6-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-
             00.txt

     [APP-REFER] Shim6 Application Referral Issues, July 2005, <draft-
             ietf-shim6-app-refer-00.txt>

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

     [MAST] D. Crocker, "MULTIPLE ADDRESS SERVICE FOR TRANSPORT (MAST):
             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,
             <draft-huitema-multi6-ingress-filtering-00>

     [RFC3484] R. Draves, "Default Address Selection for Internet
             Protocol version 6 (IPv6)", February 2003, RFC 3484.

     [RFC3697] J. Rajahalme, A. Conta, B. Carpenter, S. Deering, "IPv6
             Flow Label Specification", March 2004, RFC 3697.



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     [HIP-BASE] Robert Moskowitz, "Host Identity Protocol", draft-ietf-
             hip-base-03.txt





14.  CHANGE LOG


Changes since draft-ietf-multi6-l3shim-00.txt:

 o Renamed to draft-ietf-shim6-l3shim

 o Renamed protocol and layer from "multi6" to "shim6".

 o Using "address" vs. "locator" and "ULID" more consistently and
   carefully.

 o Made it more clear that the ULID is just an IPv6 address. (Requested
   on mailing list.)

 o In "Renumbering Implications" added text to point out the small
   probability of there being a problem. (Requested on mailing list.)

 o Extended the assumption about ingress filtering and exit selection.
   (Requested on mailing list.)

 o Added clarification to MTU implications.  (Requested on mailing
   list.)

 o Clarified what Centrally assigned ULAs can do which regular IPv6
   addresses can't do with respect to the DNS.  (Requested on mailing
   list.)

 o Added suggestion from mailing list that one can use different flow
   label for the communication when ULIDs=locators, and when they are
   different.

 o Listed a few more open issues.


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



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   separate text about centrally assigned ULAs.

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

 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.

 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
   connection.

 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
   locator.

 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.






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AUTHORS' ADDRESSES

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

     phone: +1 650 786 2921
     fax:   +1 650 786 5896
     email: erik.nordmark@sun.com

     Marcelo Bagnulo
     Universidad Carlos III de Madrid
     Av. Universidad 30
     Leganes, Madrid  28911
     SPAIN

     Phone: 34 91 6249500
     EMail: marcelo@it.uc3m.es
     URI:   http://www.it.uc3m.es

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Disclaimer of Validity



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   This document and the information contained herein are provided on an
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