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INTERNET-DRAFT                                             Erik Nordmark
Oct 15, 2004                                            Sun Microsystems
                                                         Marcelo Bagnulo
                                                                    UC3M


                       Multihoming L3 Shim Approach


                   <draft-nordmark-multi6dt-shim-00.txt>


   Status of this Memo


   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.


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


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   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 April 15, 2005.



   Abstract


   This document outlines an approach to solving IPv6 multihoming in
   order to stimulate discussion.


   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.





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


      1.  Introduction.............................................    4
         1.1.  Non-Goals...........................................    4


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


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


      4.  Locators as Upper-layer Identifiers......................    7


      5.  Placement of the multi6 shim.............................    7


      6.  Deferred Context Establishment...........................   10


      7.  Assumptions about the DNS................................   10


      8.  Protocol Walkthrough.....................................   11
         8.1.  Initial Context Establishment.......................   11
         8.2.  Locator Change......................................   11
         8.3.  Concurrent Context Establishment....................   12
         8.4.  Handling Initial Locator Failures...................   12


      9.  Demultiplexing of data packets in multi6 communications..   13
         9.1.  Approaches preventing the existence of ambiguities..   14
            9.1.1.  Pre-agreed identifiers.........................   14
            9.1.2.  N-square addresses.............................   14
         9.2.  Providing additional information to the receiver....   15
            9.2.1.  Flow-label.....................................   15
            9.2.2.  Extension Header...............................   16
         9.3.  Host-Pair Context...................................   16


      10.  IPSEC INTERACTIONS......................................   17


      11.  OPEN ISSUES.............................................   17


      12.  ACKNOWLEDGMENTS.........................................   18


      13.  REFERENCES..............................................   18
         13.1.  Normative References...............................   18
         13.2.  Informative References.............................   18


      AUTHORS' ADDRESSES...........................................   19








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











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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
                    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
                    locators.








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      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 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:


    - 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]






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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 section 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
   implications.


   There has been some discussion of using non-routable locators, such
   as unique-local addresses, as ULIDs in a multihoming solution.  While
   this approach doesn't currently specify all aspects of this, it is
   believed that the approach can be extended to handle such a case.
   For example, the protocol probably needs to handle ULIDs that are not
   initially reachable.  Thus the same mechanism can handle ULIDs that
   are permanently unreachable.  Note that the hard issues with ULIDs is
   how to perform the mappings between them and the locator sets.  With
   routable ULIDs the AAAA resource record set provides this "mapping".
   Non-routable ULIDs would need similar mechanisms, which is probably
   feasible for unique local addresses based on prefixes that are
   centrally assigned.




5.  Placement of the multi6 shim













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                            -----------------------
                            | 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
   packets").


   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
   layer will map to/from different locators.  The multi6 layer
   maintains state, called host-pair context, in order to perform this
   mapping.  The mapping is performed consistently at the sender and the




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   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.  It
   might make sense to merge the context state for multiple ULIDs
   assigned to the same peer host, but 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
   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.










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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
   ULIDs).


   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.















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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 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 triggers, 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 exhanged
        the locator sets, and probably until they rehome the context to
        use different locators, they continue sending and receiving IPv6
        packets as before.



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
      [RFC 2461]





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    - negative advise from the ULP


    - failure of some explicit multi6 "heartbeat" messages


    - local indications such as the local locator becoming invalid [RFC
      2462] 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.


   However, if different ULIDs are used this would result in two
   completely independent contexts between the two hosts following the
   basic content establishment above.



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 where selected to
   be ULIDs are not working and the multi6 shim does not know of
   alternate locators, it has to 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.  However, those are
   optimizations and not required for the protocol to work.





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   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
   exchanged.




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
   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




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   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.




9.1.  Approaches preventing the existence of ambiguities


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 form the initiator node, implying that only upgraded
   nodes will obtain improves 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




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   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
   DNS to prevent them from every 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.



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
   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 multi6 context is
   established, a Flow Label value is associated with this context.
   When a packet is received, the Flow Label value is used as a key to
   determine the context to be used for the demultiplexing operation,
   hence determining the identifiers that have to be presented to the
   upper layers.  Because this approach overloads the Flow Label field,
   it is necessary to have an additional information to determine
   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 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.




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   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].


   Another approach is to extend the context tag to include additional
   fields of the IPv6 header.  The obvious choice would be to extend the
   context tag to the combination of Flow Label, Source Address and
   Destination Address.  In this case, the context tag is composed of
   these three values and it will be used to identify the context.  The
   limitation imposed by this approach is that all the potential source
   and destination addresses have to be known beforehand by the receiver
   in order to be recognized.  This means that before sending packets
   with a new address, the sender has to inform the receiver about the
   new address.


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
   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 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




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   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".




10.  IPSEC INTERACTIONS


   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.




11.  OPEN ISSUES


   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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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.




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-dt-
             multi6-functional-dec-00.txt


     [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, Centrally Assigned Unique Local
             IPv6 Unicast Addresses, draft-ietf-ipv6-ula-central-00.txt


     [MAST] D. Crocker, "MULTIPLE ADDRESS SERVICE FOR TRANSPORT (MAST):
             AN EXTENDED PROPOSAL", draft-crocker-mast-protocol-01.txt,
             October, 2003.





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



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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