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Versions: 00 01 draft-ietf-multi6-architecture

Individual Submission                                          G. Huston
Internet-Draft                                                   Telstra
Expires: December 27, 2004                                 June 28, 2004


           Architectural Approaches to Multi-Homing for IPv6
                draft-huston-multi6-architectures-01.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
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   This Internet-Draft will expire on December 27, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This memo provides an analysis of the aspects of multi-homing support
   for the IPv6 protocol suite.  The purpose of this analysis is to
   provide a taxonomy for classification of various proposed approaches
   to multi-homing.  It is also an objective of this exercise to
   identify common aspects of this domain of study, and also to provide
   a framework that can allow exploration of some of the further
   implications of various architectural extensions that are intended to
   support multi-homing.

Document Revision Notes



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   The following changes have been made to the draft:


      00 to 01:

      Section 2: The Multi-Homing Space
         Added text to include consideration of session initiation in
         the face of changes to the connectivity topology, and a note
         about the potential to consider traffic engineering across
         multiple paths.

      Section 3: Functional Goals and Considerations
         Changed 'requirements' to 'goals'.

      Section 5.1 Endpoint Identity Structure
         Added consideration of disambiguating locators and identities
         when identities are drawn from the same address space as
         locators.  Added text about identities drawn from PA space and
         the problems this raises.  Also added text about disambiguating
         DNS FQDN pseudo-anycast from DNS-based multi-homing with
         equivalent locator sets.

      Section 5.2 Persistent, Opportunistic and Ephemeral Identities
         New section added to the draft considering the implications of
         these three approaches to identity.

      Section 5.3.1 Triggering Locator Switches
         Added section on ICMP triggers.

      Section 5.3.2 Layering Identity
         New section added, considering the implications of placing
         endpoint identity functionality in the transport or intenetwork
         protocol elements, or as a wedge element, conceptually placed
         between these two elements.

      Section 6. Functional Decomposition of Multi-Homing Approaches
         New section added.

   The following comments have yet to be integrated into the draft:

      Comparison with MIPv6:
         Related experiences with MIPv6 and its approach to the identity
         / locator split and the differences between this and the
         approaches proposed with multi-homing.







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      Is Traffic Engineering  within scope?
         Is consideration of Traffic Engineering part of this
         architectural approach? If so what aspects of Traffic
         Engineering are relevant here and why? [The consideration here
         appears to be that hosts need much more information at hand if
         they are to make locator address selection decisions based on
         some form of metric of relative load currently being imposed on
         select components of a number of end-to-end network paths, and
         it raises the entire issue of Traffic Engineering being a
         network function independent of host function or an outcome of
         host interaction with the network, and if the host is to
         interact with the network how is this interaction to be
         signalled?]

      Appendix A Notes on various approaches
         This needs to be placed into a distinct document (and updated).



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  The Multi-Homing Space . . . . . . . . . . . . . . . . . . . .  5
   3.  Functional Goals and Considerations  . . . . . . . . . . . . .  7
   4.  Approaches to Multi-Homing . . . . . . . . . . . . . . . . . .  8
     4.1   Multi-Homing: Routing  . . . . . . . . . . . . . . . . . .  8
     4.2   Multi-homing: Identity Considerations  . . . . . . . . . . 10
     4.3   Multi-homing: Identity Protocol Element  . . . . . . . . . 12
     4.4   Multi-homing: Modified Protocol Element  . . . . . . . . . 13
     4.5   Modified Site-Exit and Host Behaviors  . . . . . . . . . . 14
   5.  Approaches to Endpoint Identity  . . . . . . . . . . . . . . . 15
     5.1   Endpoint Identity Structure  . . . . . . . . . . . . . . . 16
     5.2   Persistent, Opportunistic and Ephermeral Identities  . . . 18
     5.3   Common Issues for Multi-Homing Approaches  . . . . . . . . 20
       5.3.1   Triggering Locator Switches  . . . . . . . . . . . . . 20
       5.3.2   Layering Identity  . . . . . . . . . . . . . . . . . . 22
       5.3.3   Session Startup and Maintenance  . . . . . . . . . . . 24
       5.3.4   Dynamic Capability Negotiation . . . . . . . . . . . . 25
       5.3.5   Identity Uniqueness and Stability  . . . . . . . . . . 26
   6.  Functional Decomposition of Multi-Homing Approaches  . . . . . 27
     6.1   Establishing Session State . . . . . . . . . . . . . . . . 27
     6.2   Rehoming Triggers  . . . . . . . . . . . . . . . . . . . . 27
     6.3   Rehoming Locator Pair Selection  . . . . . . . . . . . . . 27
     6.4   Locator Change . . . . . . . . . . . . . . . . . . . . . . 28
     6.5   Removal of Session State . . . . . . . . . . . . . . . . . 28
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
   9.1   Normative References . . . . . . . . . . . . . . . . . . . . 29
   9.2   Informative References . . . . . . . . . . . . . . . . . . . 29
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 29
   A.  Notes on Various approaches  . . . . . . . . . . . . . . . . . 29
     A.1   Host Identity Protocol (HIP) . . . . . . . . . . . . . . . 30
     A.2   Multihoming without IP Identifiers (NOID)  . . . . . . . . 31
     A.3   Common Endpoint Locator Pools (CELP) . . . . . . . . . . . 31
     A.4   Weak Identifier Multihoming Protocol (WIMP)  . . . . . . . 32
     A.5   Host-Centric IPv6 Multihoming  . . . . . . . . . . . . . . 34
     A.6   Summaries of Selected ID/LOC Separation Documents  . . . . 35
       A.6.1   New or Updated Documents Since IETF58  . . . . . . . . 35
       A.6.2   Older Documents that Remain Active/Interesting . . . . 38
       A.6.3   Related Multi-Homing drafts, Status unknown  . . . . . 39
       Intellectual Property and Copyright Statements . . . . . . . . 42








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1.  Introduction

   The objective of this analysis is to allow various technical
   proposals relating to the support of multi-homing environment in IPv6
   to be placed within an architectural taxonomy.  This is intended to
   allow these proposals to be classified and compared in a structured
   fashion.  It is also an objective of this exercise to identify common
   aspects across all proposals within this domain of study, and also to
   provide a framework that can allow exploration of some of the further
   implications of various architectural extensions that are intended to
   support multi-homing.  The scope of this study is limited to the IPv6
   protocol suite architecture, although reference is made to IPv4
   approaches as required.

2.  The Multi-Homing Space

   A simple formulation of the multi-homing environment is indicated in
   Figure 1.

































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                           +------+
                           |remote|
                           | host |
                           |  R   |
                           +------+
                              |
                    + - - - - - - - - - - - +
                    | Internet Connectivity |
                    + - - - - - - - - - - - +
                         /            \
                   +---------+    +---------+
                   | ISP A   |    |  ISP B  |
                   +---------+    +---------+
                       | Path A        | Path B
         + - - - - - - - - - - - - - - - - - - - - +
         | multi-      |               |           |
           homed   +------+         +------+
         | site    | site |         | site |       |
                   | exit |         | exit |
         |         |router|         |router|       |
                   |  A   |         |  B   |
         |         +------+         +------+       |
                      |                |
         |         local site connectivity         |
                           |
         |           +-----------+                 |
                     |multi-homed|
         |           |   host    |                 |
                     +-----------+
         + - - - - - - - - - - - - - - - - - - - - +


   The Multi-Homed Domain

                                Figure 1

   The environment of multi-homing is one that is intended to provide
   sufficient support to local hosts so as to allow local hosts to
   exchange IP packets with remote hosts, such that this exchange of
   packets is to be seamlessly supported across dynamic changes in
   connectivity.  Session resilience implies that if a local
   multi-homed-aware host establishes an application session with the
   remote host using "Path A", and this path fails, the application
   session should be mapped across to "Path B" without requiring any
   application-visible re-establishment of the session.  In other words,
   the application session should not be required to be explicitly aware
   of underlying path changes at the level of packet forwarding paths
   chosen by the network.



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   In addition to this objective of session resilience across network
   reachability changes, there are also considerations of providing
   mechanisms to support site visibility in the face of dynamic changes
   in external reachability.  Sustained site visibility implies that
   external attempts to initiate a communication with hosts within the
   site will succeed as long as there is at least one viable path
   between the external host and the multi-homed site.

   In addition there is the potential consideration of being able to
   distribute traffic load across a number of network paths according to
   some pre-determined objective, as a form of traffic engineering.

   This simple multi-homing scenario also includes "site-exit' routers,
   where the local site interfaces to the upstream Internet transit
   providers.  The nature of the interactions between the external
   routing system and the site-exit routers, and interactions between
   the site-exit routers and the local multi-homed host, and the
   interactions between local connectivity forwarding and the local host
   and site exit routers are not defined a priori in this scenario, as
   they form part of the framework of interaction between the various
   multi-homing components.

   The major characteristic of this scenario is that the address space
   used by, and advertised as reachable by, ISP A is distinct from the
   address space used by ISP B.

   This simple scenario is intended to illustrate the basic multi-homing
   environment.  Variations of this scenario include additional external
   providers of transit connectivity to the local site, complex site
   requirements and constraints, where the site may not interface
   uniformly to all external transit providers, sequential rather than
   simultaneous external transit reachability, communication with remote
   multi-homed hosts, multi-way communications, use of host addresses in
   a referential context (third party referrals) and the imposition of
   policy constraints on path selection.  However, the basic scenario is
   sufficient to illustrate the major architectural aspects of support
   for multi-homing, so this scenario will be used as the reference
   model for this analysis.

3.  Functional Goals and Considerations

   RFC 3582 [1] documents some goals that a multi-homing approach should
   attempt to address.  These goals include:
   o  redundancy
   o  load sharing
   o  traffic engineering
   o  policy constraints




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   o  simplicity of approach
   o  transport-layer survivability
   o  DNS compatibility
   o  packet filtering capability
   o  scalability
   o  legacy compatibility
   The reader is referred to [1] for a complete description of each of
   these goals.

   In addition, [2] documents further considerations for IPv6
   multi-homing.  Again, the reader is referred to this document for the
   detailed enumeration of these considerations.  The general topic
   areas considered in this study include:
   o  interaction with routing systems,
   o  aspects of a split between end-point-identifier and forwarding
      locator,
   o  changes to packets on the wire, and
   o  the interaction between names, endpoints and the DNS.

   In evaluating various approaches, further consideration also include:
   o  the role of helpers and agents in the approach,
   o  modifications to host behaviors,
   o  the required trust model to support the interactions, and
   o  the nature of potential vulnerabilities in the approach.

4.  Approaches to Multi-Homing

   There appear to be four generic forms of architectural approaches to
   this problem, namely:
   o  Routing
      Use the IPv4 multi-homing approach
   o  New Protocol Element
      Insertion of a new element in the protocol stack that manages a
      persistent identity for the session
   o  Modify a Protocol Element
      Modify the Transport or IP protocol stack element in the host in
      order to support dynamic forwarding locator change
   o  Modified Site-Exit Router / Local Host interaction
      Modify the site-exit router and local forwarding system to allow
      various behaviors including source-based forwarding, site-exit
      hand-offs, and address rewriting by site-exit routers

   These approaches will be described in detail in the following
   sections.

4.1  Multi-Homing: Routing

   The approach used in IPv4 for multi-homing support is to preserve the



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   semantics of the IPv4 address as both an endpoint identifier and a
   forwarding locator.  For this to work in a multi-homing context it is
   necessary for the transit ISPs to announce the local site's address
   prefix as a distinct routing entry in the inter-domain routing
   system.  This approach could be used in an IPv6 context, and, as with
   IPv4, no modifications to the IPv6 architecture are required to
   support this approach.

   The local site's address prefix may be a more specific address prefix
   drawn from the address space advertised by one of the transit
   providers, or from some third party provider not current directly
   connected to the local site.  Alternatively the address space may be
   a distinct address block obtained by direct assignment from a
   Regional Internet Registry as Provider Independent space.  Each host
   within the local site is uniquely addressed from the site's address
   prefix.

   All transit providers for the site accept a prefix advertisement from
   the multi-homed site, and advertise this prefix globally in the
   inter-domain routing table.  When connectivity between the local site
   and an individual transit provider is lost, normal operation of the
   routing protocol will ensure that the routing advertisement
   corresponding to this particular path will be withdrawn from the
   routing system, and those remote domain domains who had selected this
   path as the best available will select another candidate path as the
   best path.  Upon restoration of the path, the path is re-advertised
   in the inter-domain routing system.  Remote domains will undertake a
   further selection of the best path based on this re-advertised
   reachability information.  Neither the local or the remote host need
   to have multiple addresses, nor undertake any form of address
   selection.  The path chosen for forward and reverse direction path
   flows is a decision made by the routing system.

   This approach generally meets all the goals for multi-homing
   approaches with one notable exception: scalability.  Each site that
   multi-homes in this fashion adds a further entry in the global
   inter-domain routing table.  Within the constraints of current
   routing and forwarding technologies it is not clearly evident that
   this approach can scale to encompass a population of multi-homed
   sites of the order of 10**7 such sites.  The implication here is that
   this would add a similar number of unique prefixes into the
   inter-domain routing environment, which in turn would add to the
   storage and computational load imposed on inter-domain routing
   elements within the network.  This scale of additional load is not
   supportable within the current capabilities of the IPv4 global
   Internet, nor is it clear at present that the routing capabilities of
   the entire network could be expanded to manage this load in a
   cost-effective fashion, within the bounds of the current inter-



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   domain routing protocol architecture.

4.2  Multi-homing: Identity Considerations

   The intent of multi-homing in the IPv6 domain is to achieve a
   comparable functional outcome for multi-homed sites without an
   associated additional load being imposed on the routing system.  The
   overall intent of IPv6 is to provide a scalable protocol framework to
   support the deployment of communications services for an extended
   period of time, and this implies that the scaling properties of the
   deployment environment remain tractable within projections of size of
   deployment and underlying technology capabilities.  Within the
   inter-domain routing space, the basic approach used in IPv4 and IPv6
   is to attempt to align address deployment with network topology, so
   that address aggregation can be used to create a structured hierarchy
   of the routing space.

   Within this constraint of topological-based address deployment and
   provider aggregatable addressing architectures, the local site that
   is connected to multiple providers is delegated addresses from each
   of these providers' address blocks.  In the example network in Figure
   1, the local multi-homed host will conceivably be addressed in two
   ways: one using transit provider A's address prefix and the other
   using transit provider B's address prefix.

   If remote host R is to initiate a communication with the local
   multi-homed host, it would normally query the DNS for an address for
   the local host.  In this context the DNS would return 2 addresses
   (One using the A prefix and the other using the B prefix).  The
   remote host would select one of these addresses and send a packet to
   this destination address.  This would direct the packet to the local
   host along a path through A or B, depending on the selected address.
   If the path between the local site and the transit provider fails,
   then the address prefix announced by the transit provider to the
   inter-domain routing system will continue to be the provider's
   address prefix.  The remote host will not see any change in routing,
   yet packets sent to the local host will now fail to be delivered.
   The question posed by the multi-homing problem is: "If the remote
   host is aware of multi-homing, how could it switch over to using the
   equivalent address for the local multi-homed host that transits the
   other provider?"

   If the local multi-homed host wishes to initiate a session with
   remote host R, it needs to send a packet to R with a valid source and
   destination address.  While the destination address is that of R,
   what source address should the local host use? There are two
   implications for this choice.  Firstly the remote host will, by
   default use this source address as the destination address in its



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   response, and hence this choice of source address will direct the
   reverse path from R to the local host.  Secondly, the ISPs A and B
   may be using some form of reverse unicast address filtering on source
   addresses of packets passed to the ISP, as a means of prevention of
   source address spoofing.  This implies that if the multi-homed
   address selects a source address from address prefix A, and the local
   routing to R selects a best path via ISP B, then ISP B's ingress
   filters will discard the packet.

   Within this addressing structure there is no form of routing-based
   repair of certain network failures.  If the link between the local
   site and ISP A fails, there is no change in the route advertisements
   made by ISP A to its external routing peers.  Even though the
   multi-homed site continues to be reachable via ISP B, packets
   directed to the site using ISP A's prefix will be discarded by ISP A
   as the destination is unreachable.  The implication here is that if
   the local host wishes to maintain a session across such events it
   needs to communicate to remote host R that it is possible to switch
   to using a destination address for the multi-homed host that is based
   on ISP B' address prefix.

   In an aggregated routing environment multiple transit paths to a host
   imply multiple address prefixes for the host, where each possible
   transit path is identified by an address for the host.  The
   implication of this constraint on multi-homing is that paths being
   passed to the local multi-homed site via transit provider ISP A must
   use a forwarding-level destination IP address drawn from ISP A's
   advertised address prefix set that maps to the multi-homed host.
   Equally, packets being passed via the transit of ISP B must use a
   destination address drawn from ISP B's address prefix set.  The
   further implication here is that path selection (ISP A vs ISP B
   transit for incoming packets) is an outcome of the process of
   selecting an address for the destination host.

   The architectural consideration here is that in the conventional IP
   protocol architecture the assumption is made that the transport-layer
   endpoint identity is the same identity used by the internet-layer
   forwarding layer, namely the IP address.

   If multiple forwarding paths are to be supported for a single
   transport session, and path selection is to be decoupled from the
   functions of transport session initiation and maintenance, then the
   corollary of this requirement in architectural terms appears to be
   that some changes are required in the protocol architecture to
   decouple the concepts of identification of the endpoint and
   identification of the location and associated path selection for the
   endpoint.  This is a fundamental change in the semantics of an IP
   address in the context of the role of the endpoint address within the



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   end-to-end architectural model [3].  This change in the protocol
   architecture would permit a transport session to use an invariant
   endpoint identity value to initiate and maintain a session, while
   allowing the forwarding layer to dynamically change paths and
   associated endpoint locator identities without impacting on the
   operation of the session, nor would such a decoupled concept of
   identities and locators add any incremental load to the inter-domain
   routing system.

   Some generic approaches to this form of separation of endpoint
   identity and locator value are described in the following sections.

4.3  Multi-homing: Identity Protocol Element

   One approach to this objective is to add a new element into the model
   of the protocol stack.

   The presentation to the upper level protocol stack element (ULP)
   would use endpoint identifiers to uniquely identify both the local
   stack and the remote stack.  This will provide the ULP with stable
   identifiers for the duration of the ULP session.

   The presentation to the lower level protocol stack element (LLP)
   would be of the form of a locator.  This implies that the protocol
   stack element would need to maintain a mapping of endpoint identifier
   values to locator values.  In a multi-homing context one of the
   essential characteristics of this mapping is that it needs to be
   dynamic, in that environmental triggers should be able to trigger a
   change in mappings, which in turn would correspond to a change in the
   paths (forward and/or reverse) used by the endpoints to traverse the
   network.  In this way the ULP session is defined by a peering of
   endpoint identifiers that remain constant throughout the lifetime of
   the ULP session, while the locators may change to maintain end-to-end
   reachability for the session.

   The operation of the new protocol stack element (termed here the
   "endpoint identity protocol stack element", or "EIP") is to establish
   a synchronized state with its remote counterpart.  This would allow
   the stack elements to exchange a set of locators that may be used
   within the context of the session.  A change in the local binding
   between the current endpoint identity value and a locator will cause
   a change in the source locator value used in the forwarding level
   packet header.  The actions of the remote EIP upon receipt of this
   packet with the new locator is to firstly recognize this locator as
   part of an existing session, and, upon some trigger condition, to
   change its session view of the mapping of the remote endpoint
   identity to the corresponding locator, and use this locator as the
   destination locator in subsequent packets passed to the LLP.



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   From the perspective of the IP protocol architecture there are two
   possible locations to insert the EIP into the protocol stack.

   One possible location is at the upper level of the transport
   protocol.  Here the application program interface (API) of the
   application level protocols would interface to the EIP element, and
   use endpoint identifiers to refer to the remote entity.  The EIP
   would pass locators to the API of the transport layer.

   The second approach is to insert the EIP between the transport and
   internet protocol stack elements, so that the transport layer would
   function using endpoint identifiers, and maintain a transport session
   using these endpoint identifiers.  The IP or internetwork layer would
   function using locators, and the mapping from endpoint identifier to
   locator is undertaken within the EIP stack element.

4.4  Multi-homing: Modified Protocol Element

   As an alternative to insertion of a new protocol stack element into
   the protocol architecture, an alternative approach is to modify an
   existing protocol stack element to include the functionality
   performed by the EIP element.  This modification could be undertaken
   within the transport protocol stack element, or within the
   internetworking stack element.  The functional outcome from these
   modifications would be to create a mechanism to support the use of
   multiple locators within the context of a single endpoint-to-endpoint
   session.

   Within the transport layer, this functionality can be achieved, for
   example, by the binding of a set of locators to a single session, and
   then communicating this locator set to the remote transport entity.
   This would allow the local transport entity to switch the mapping to
   a different locator for either the local endpoint or the remote
   endpoint while maintaining the integrity of the ULP session.

   Within the IP level this functionality could be supported by a form
   of dynamic rewriting of the packet header as it is processed by the
   protocol element.  Incoming packets with the source and destination
   locators in the packet header are mapped to packets with the
   equivalent endpoint identifiers in both fields, and the reverse
   mapping is performed to outgoing packets passed from the transport
   layer.  Mechanisms that support direct rewriting of the packet header
   are potential candidates in this approach, as are various forms of
   packet header transformations of encapsulation, where the original
   endpoint identifier packet header is preserved in the packet and an
   outer level locator packet header is wrapped around the packet as it
   is passed through the internetworking protocol stack element.




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   In all these scenarios, there are common issues of what state is
   kept, by which part of the protocol stack, how state is maintained
   with additions, removals of locator bindings, and does only one piece
   of code have to be aware of the endpoint / locator split or do
   multiple protocol elements have to be modified? For example, if the
   functionality is added at the internetworking (IP) layer, there is no
   context of an active transport session, so that removal of identity /
   locator state information for terminated sessions needs to be
   triggered by some additional mechanism from the transport layer to
   the internetworking layer.

4.5  Modified Site-Exit and Host Behaviors

   The above approaches all assume that the hosts are explicitly aware
   of the multi-homed environment and use modified protocol behavior to
   support multi-homing functionality.  A further approach to this
   objective is to split this functionality across a number of network
   elements and potentially perform packet header rewriting from a
   persistent endpoint identity value to a locator value at a remote
   point.

   One possible approach proposes the use of site-exit routers to
   perform some form of packet header manipulation as packets are passed
   out from the local multi-homed site to a particular transit provider.
   The local site routing system will select the best path to a
   destination host based on the remote hosts's locator value.  The
   local host will write its endpoint identity as the source address of
   the packet.  When the packet reaches a site-exit router, the
   site-exit router will rewrite the source field of the packet to a
   corresponding locator that selects a reverse path through the same
   transit ISP when the locator is used as a destination locator by the
   remote host.  In order to preserve session integrity there is a need
   for a corresponding reverse transformation to be undertaken on
   incoming packets, where the destination locator has to be mapped back
   to the host's endpoint identifier.  There are a number of
   considerations whether this is best performed at the site exit router
   on packet ingress to the site, or by the local host.

   Packet header rewriting by remote network elements has a large number
   of associated considerations, and documentation relating to the
   considerations of the use of Network Address Translators [4] contains
   much of this material.

   An alternative for packet header rewriting on site exit is for the
   host to undertake the endpoint-to-locator mapping, using one of the
   approaches outlined above.  The consideration here is that there is
   some significant deployment of unicast reverse path filtering in
   Internet environments as a counter-measure to source address



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   spoofing.  Using the example in Figure 1, if a host selects a locator
   drawn from the ISP B address prefix, and local routing directs that
   packet to site-exit router A, then if the packet is passed to ISP A,
   the this would be discarded by such filters.  Various approaches have
   been proposed to modify the behavior of the site forwarding
   environment all with the end effect that packets using a source
   locator drawn from the ISP B address prefix are passed to site-exit
   router B.  These approaches include forms of source address routing
   and site-exit router hand-over mechanisms, as well as augmentation of
   the routing information between site-exit routers and local
   multi-homed hosts, so that the choice of locator by the local host
   for the remote host is consistent with the current local routing
   state for the local site to reach the remote host.

5.  Approaches to Endpoint Identity

   Both of the above mechanisms assume some form of exchange of
   information that allows both parties to the communication to be aware
   of the remote endpoint identity and the associated mapping to
   locators.  There are a number of choices in terms of the way in which
   this information exchange can be implemented.

   The first such possible approach is termed here a 'conventional'
   approach, where the mode of operation is in terms of encapsulating
   the protocol data unit (PDU) passed from the ULP with additional data
   elements that specifically refer to the function of the endpoint
   identity protocol stack element.  The compound data element is passed
   to the LLP as its PDU.  The corresponding actions on receipt of a PDU
   from a LLP is to extract the fields of the data unit that correspond
   to the EIP function, and pass the reminder of the PDU to the ULP.
   The EIP operates in an "in-band" mode, communicating with its remote
   peer entity through additional information wrapped around the ULP
   PDU.  This is equivalent to generic tunneling approaches where the
   outer encapsulation of the transmitted packet contains location
   address information, while the next level packet header contains
   information that is to be exposed and used at the location endpoints,
   being, in this case, identity information.

   Another approach is to allow the EIP to communicate using a separate
   communications channel, where the EIP generates dedicated messages
   that are directed to its peer EIP, and passes these PDUs to the LLP
   independently of the PDUs that are passed to the EIP from the ULP.
   This allows the EIP to exchange information and synchronize state
   with the remote EIP semi-independently of the ULP protocol exchange.
   As a part of the EIP function is to transform the ULP PDU to include
   locator information there is an associated requirement to ensure that
   the EIP peering state remains synchronized to the exchange of ULP
   PDUs, so that the remote EIP can correctly recognize the locator to



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   endpoint mapping for each active session.

   Another potential approach here is to allow the endpoint to locator
   mappings to be held at a third party point.  This model is already
   used for supporting the name to IP address mappings performed by the
   Domain Name system, where the mapping is obtained by reference to a
   third party, namely a DNS resolver.  A similar form of third party
   mapping between endpoints and a locator set could be supported
   through the use of the DNS, or a similar third party referential
   mechanism.  Rather than have each party exchange endpoint to locator
   mappings, this approach would see this mapping being obtained as a
   result of a lookup for a DNS Endpoint to Locator set map contained as
   DNS Resource Records, for example.

5.1  Endpoint Identity Structure

   The previous section has used the term "endpoint identity" without
   examining what form this identity may take.  There are a number of
   salient considerations regarding the structure and form of this
   identity that should be enumerated within an architectural overview
   of this space.

   One possible form of an identity is the use of identity tokens lifted
   from the underlying protocol's "address space".  In other words an
   endpoint identity is a special case instance of an IPv6 protocol
   address.  There are a number of advantages in using this form of
   endpoint identity, observing that the suite of IP protocols and
   associated applications already manipulate IP addresses.  The
   essential difference in a domain that distinguishes between endpoint
   identity and locator is that the endpoint identity parts of the
   protocol would operate on those addresses that assume the role of
   endpoint identities, and the endpoint identity / locator mapping
   function would undertake a mapping from an endpoint "address" to a
   set of potential locator "addresses", and also undertake a reverse
   mapping from a locator "address" to the distinguished endpoint
   identifier "address".  The address space is hierarchically
   structured, permitting a suitably efficient mapping to be performed
   in both directions, and the underlying semantics of addresses in the
   context of public networking includes the necessary considerations of
   global uniqueness of endpoint identity token values.

   It is possible to take this approach further and allow the endpoint
   identifier to also be a valid locator.  This would imply the
   existence of a 'distinguished' or 'home' locator, and other locators
   could be dynamically mapped to this initial locator peering as
   required.  The drawback of this approach is that the endpoint
   identifier is now based on one of the transit provider's address
   prefixes, and a change of transit provider would necessarily require



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   a change of endpoint identifier values within the multi-homed site.

   An alternative approach for address-formatted identifiers is to use
   distinguished identity address values which are not part of the
   global unicast locator space, allowing applications and protocol
   elements to distinguish between endpoint identity values and locators
   based on address prefix value.

   It is also possible to allow the endpoint identity and locator space
   to overlap, and distinguish between the two identity realms by the
   context of usage rather than by a prefix comparison.  However, this
   reuse of the locator token space as identity tokens has the potential
   to create the anomalous situation where a particular locator value is
   used as an identity value by a different endpoint.  It is not clear
   that the identity and locator contexts can be clearly disambiguated
   in every case, which is a major drawback to this particular approach.

   If identity values are to be drawn from the protocol's address space
   it would appear that the basic choice is to either draw these
   identity values from a different part of the address space, or use a
   distinguished or home address as both a locator and an identity.
   This latter option, that of using a locator as the basis of an
   endpoint identity on a locator, when coupled with a
   provider-aggregated address distribution architecture leads to the
   outcome of a multi-homed site using a provider-based address prefix
   as a common identity prefix.  As with locator addresses in the
   context of a single-homed network, a change of provider connectivity
   implies a consequent renumbering of identity across the multi-homed
   site.  If avoiding such forced renumbering is a goal here, there
   would be a preference in drawing identity tokens from a pool that is
   not aligned with network topology.  This may point to a preference
   from this sector to use of identity token values that are not drawn
   from the locator address space.

   It is also feasible to use the fully qualified domain name (FQDN) as
   an endpoint identity, undertaking a similar mapping as described
   above, using the FQDN as the lookup "key".  The implication here is
   that there is no default 'address' that is to be associated with the
   endpoint identifier, as the FQDN can be used in the context of
   session establishment, and a DNS query used to establish a set of
   initial locators.  Of course it is also the case that there may not
   necessarily be a unique endpoint associated with a FQDN, and in such
   cases if there were multiple locator addresses associated with the
   FQDN via DNS RRs, shifting between locators may imply directing the
   packet to a different endpoint where there is no knowledge of the
   active session on the original endpoint.

   The syntactic properties of these two different identity realms have



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   obvious considerations in terms of the manner in which these
   identities may be used within PDUs.

   It is also an option to consider a new structured identity space
   which is not generated through the reuse of IPv6 address values nor
   drawn from the FQDN.  Given that the address space would need to be
   structured in such a fashion that permits it to be used as a lookup
   key to obtain the corresponding locator set, the obvious question in
   such an option is what additional or altered characteristics would be
   used in such an endpoint identity space that would distinguish it
   from either of the above approaches?

   Instead of structured tokens that double as lookup keys to obtain
   mappings from endpoint identities to locator sets, the alternative is
   to use an unstructured token space, where individual token values are
   drawn opportunistically for use within a multi-homed session context.
   Here the semantics of the endpoint identity are subtly changed.  The
   endpoint identity is not a persistent alias or reference to the
   identity of the endpoint, but a means to allow the identity protocol
   element to confirm that two locators are part of the same mapped
   locator set for a remote endpoint.  In this context the unstructured
   opportunistic endpoint identifier values are used in determining
   locator equivalence rather than in some form of lookup function.

5.2  Persistent, Opportunistic and Ephermeral Identities

   The consideration in the previous section highlights one of the major
   aspects of variance in the method of supporting a split between
   identity and location information.

   One form uses a persistent identity field, by which it is inferred
   that the same identity value is used in all contexts where this form
   of identity is required, in support of concurrent sessions, and in
   support of sequential sessions.  This form of identity is intended to
   remain constant over time and over changes in the underlying
   connectivity.  It may also be the case that this identity is
   completely distinct from network topology, so that the same identity
   is used irrespective of the current connectivity and locator
   addressing used by the site and the host.  In this case the identity
   is persistent, and the identity value can be used as a reference to
   the endpoint stack.  This supports multi-party referrals, where if
   parties A and B establish a communication, B can pass A's identity to
   a third party C, who can then use this identity value to be the
   active party in establishing communication to A.

   If persistent identifiers are to be used to initiate a session, then
   it follows that the identity is used as a lookup key to establish a
   set of locators that are associated with the identified endpoint.  It



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   is desirable that this lookup function be deterministic, reliable,
   robust, efficient and trustable.  The implication of this is that
   such identities must be uniquely assigned, and experience in identity
   systems points to a string preference for a structured identity token
   space that has an internal hierarchy of token components.  These
   identity properties have significant commonality with those of
   unicast addresses and domain names.  The further implication here is
   that persistent structured identities also rely on the adoption of
   well-ordered distribution and management mechanisms to preserve their
   integrity and utility.

   As noted in the previous section, an alternative form of identity is
   an unstructured identity space, where specific values are drawn from
   the space opportunistically.  In this case the uniqueness of any
   particular identity value is not assured in all cases.  The use of
   such identities as a lookup key to establish locators is also
   altered, as the unstructured nature of the space has implications
   relating to the efficiency of the lookup, and the authenticity of the
   lookup is weakened due to the inability to assure uniqueness of the
   identity key value.  A conservative approach to unstructured
   identities limits their scope of utility, such as per-session
   identity keys.  In this scenario the scope of the selected identity
   is limited to the parties who are communicating, and limited to the
   duration of the communication session.  The implication of this
   limitation is that the identity is a session-level binding point to
   allow multiple locators to be bound to the session, and the identity
   cannot be used as a reference to an endpoint beyond the context of
   the session.  Such opportunistic identities with explicitly limited
   scope do not require the adoption of any well-ordered mechanisms of
   token distribution and management.

   Another form of identity is an ephemeral form, where a session
   identity is a shared state between the endpoints, established without
   the exchange of particular token values that take the role of
   identity keys.  This could take the form of a defined locator set, or
   the form of a session key derived from some set of shared attributes
   of the session, as two examples here.  In this situation there is no
   form of reference or use of an identifier as a means of initiating a
   session.  The ephemeral identity value has a very limited role in
   terms of allowing each end to reliably determine the semantic
   equivalence of a set of locators within the context of membership of
   a particular session.

   The latter two forms of identity represents a approach to identity
   that minimizes management overhead, and provides mechanisms that are
   limited in scope to supporting session integrity.  This implies that
   support for identity functions in other contexts and at other levels
   of the protocol stack, such as within referrals, in the use of



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   identities within an application's data payload, or as a key used to
   initiate a communication session with a remote endpoint would need to
   be supported by some other identity function.  Such per-session
   limited scope identities imply that the associated multi-homing
   approaches use existing mechanisms for session startup, and the
   adoption of a session-based identity and associated locator switch
   agility is a negotiated session capability.  The use of a persistent
   identity as a session initiation key implies that identity is part of
   the established session state, and locator agility can be an
   associated attribute of the session, rather than a subsequent
   negotiated capability.  In a heterogeneous environment where such
   identity capability is not uniformly deployed this would imply that
   if a session cannot be established with a split identity locator
   binding, the application should be able to back off to a conventional
   session startup by mapping the identity to a specific locator value
   and initiating a session using such a value.  The reason why the
   application may want to be aware of this distinction is that if the
   application wishes to use self-referential mechanisms within the
   application payload, it would appear to be appropriate to use an
   identity-based self-reference only in the context of a session where
   the remote party was aware of the semantic properties of this
   referential tag.

   In terms of functionality and semantics opportunistic identities form
   a superset of ephemeral identities, although their implementation is
   significantly different.  Persistent identities support a superset of
   the functionality of opportunistic identities, and again the
   implementations will differ.

5.3  Common Issues for Multi-Homing Approaches

   The above overview encompasses a very wide range of potential
   approaches to multi-homing, and each particular approach necessarily
   has an associated set of considerations regarding its applicability.

   There are, however, a set of considerations that appear to be common
   across all approaches, and they are examined in further detail in
   this section.

5.3.1  Triggering Locator Switches

   Ultimately, regardless of the method of generation, a packet
   generated from a local multi-homed host to a remote host must have a
   source locator in the IP packet that is passed into the transit
   network.  In a multi-homed situation the local multi-homed host has a
   number of self-referential locators that are equivalent aliases in
   almost every respect.  The difference between locators is the
   inference that at the remote end the choice of locator may determine



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   the path used to send a packet back to the local multi-homed host.
   The issue here is how does the local host make a selection of the
   "best" source locator to use?  Obviously the parameters of this
   selection include the objective to select a locator that represents a
   currently viable path from the remote host to the local multi-homed
   host.  Local routing information for the multi-homed host does not
   include this reverse path information.  Equally, the local host does
   not necessarily know of any additional policy constraints that apply
   to the remote host that may result in a remote host's preference to
   use one locator over another for the local host.  Considerations of
   unicast reverse path forwarding filters also indicate that the
   selection of a source locator should result in the packet being
   passed to a site-exit router that is connected to the associated ISP
   transit provider, and that the site-exit router passes the packet to
   the associated ISP.

   If the local multi-homed host is communicating with a remote
   multi-homed host, the local host may have some discretion in the
   choice of a destination locator.  The considerations relating to the
   selection of a destination locator include considerations of local
   routing state (to ensure that the chosen destination locator reflects
   a viable path to the remote endpoint), policy constraints that may
   determine a "best" path to the remote endpoint.  In such situations
   it may also be the case that the source address selection should also
   be considered in relation to the destination locator selection.

   Another common issue is the consideration of the point when a locator
   is not considered to be viable, and the consequences to the transport
   session state.
   o  Transport Layer Triggers

      A change in state for a currently used path to another path could
      be triggered by indications of packet loss along the current path
      transport-level signaling, or by transport session timeouts,
      assuming an internal signaling mechanism between the transport
      stack element and the locator pool management stack element.
   o  ICMP Triggers

      Path failure within the network may generate an ICMP Destination
      Unreachable ICMP packet being directed back to the sender.  Rather
      than sending this signal to the transport level as an indicator of
      session failure, the IP layer should redirect the notification
      identity module as a trigger for a locator switch.
   o  Routing Triggers

      Alternatively, in the absence of local transport triggers, the
      site exit router could communicate failure of the outbound
      forwarding path in the case where the remote host is multi-homed



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      with an associated locator set.  Conventional routing would be
      incapable of detecting a failure in the inbound forwarding path,
      so there are some limitations in the approach of using routing
      triggers to change locator bindings.
   o  Heartbeat Triggers

      An alternative to these approaches is the use of a session
      heartbeat protocol, where failure of the heartbeat would cause the
      session to seek a new locator binding that would re-establish the
      heartbeat.

   The sensitivity of the locator-switch trigger is a consideration
   here.  A very fine-grained sensitivity of the locator switch trigger
   may generate false triggers arising from short-term transient path
   congestion, while coarse-grained triggers may impose an undue
   performance penalty on the session due to an extended time to detect
   a path failure.

5.3.2  Layering Identity

   The consideration of triggering locator switch highlights the
   observation that differing information and context is present in each
   layer of the protocol stack.  This impacts on how identity / locator
   bindings are established, maintained and expired.

   These impacts include questions of what amount of state is kept, by
   which element of the protocol stack, at what level of context
   (dynamic or fixed, and per session or per host).  It also includes
   considerations of state maintenance, such as how stale or superfluous
   state information is detected and removed.  Does only one piece of
   code have to be aware of this identity/locator binding or do multiple
   transport protocols have to be altered to support this functionality?
   If so, are such changes common across all transport protocols, or do
   different protocols require different considerations in their
   treatment of this functionality?

   It is noted that the set of approaches considered here include
   proposals to place this functionality within the IP layer, with the
   end-to-end transport protocol layer and as a shim between the IP and
   transport protocol layer.

   Placing this identity functionality at the transport protocol layer
   implies that the identity function can be tightly associated with a
   transport session.  In this approach session startup can trigger the
   identity / locator initial binding actions and transport protocol
   timeouts can be used as triggers for locator switch actions.  Session
   termination can trigger expiration of local identity / locator
   binding state.  Where per-session opportunistic identity token values



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   are being used, the identity information can be held within the
   overall session state.  In the case of persistent identity token
   values the implementation of the identity can also choose to use
   per-session state, or may choose to pool this information across
   multiple sessions in order to reduce overheads of dynamic discovery
   of identity / locator bindings for remote identities in the case of
   multiple sessions to the same remote endpoint.

   One of the potential drawbacks of placing this functionality within
   the transport protocol layer is that it is possible that each
   transport protocol will require a distinct implementation of identity
   functionality.

   An alternative approach is to use a distinct protocol element placed
   between the transport and internet layers of the protocol stack.  The
   advantage of this approach is that it would offer a consistent form
   of mapping between identities and locators for all forms of transport
   protocols.  However this protocol element would not be explicitly
   aware of sessions and would either have to discover the appropriate
   identity / locator mapping for all identity-addressed packets passed
   from the transport protocol later, irrespective of whether such a
   mapping exists and whether this is part of a session context, or have
   an additional mechanism of signaling to determine when such a mapping
   is to be discovered and applied.  At this level there is also no
   explicit knowledge of when identity / locator mapping state is no
   longer required, as there is no explicit signaling of when all flows
   to and from a particular destination has stopped and resources
   consumed in supporting state can be released.  Also, such a protocol
   element would not be aware of transport level timeouts, so that
   additional functionality would need to be added to the transport
   protocol to trigger a locator switch at the identity protocol level.
   Support of per-session opportunistic identity structure is more
   challenging in this environment, as the transport protocol layer is
   used to store and manipulate per-session state.

   It is also possible to embed this identity function within the
   internet protocol layer of the protocol stack.  As noted in the
   previous section, per session information is not readily available to
   the identity module, so that opportunistic per-session identity
   values would be challenging to support in this approach, as well as
   determining when identity / locator state information should be set
   up and released.  It would also appear necessary to signal transport
   level timeouts to the identity module as a locator switch trigger.
   Some attention needs to be given in this case to synchronizing
   locator switches and IP packet fragmentation, and consideration of
   IPSEC is necessary in this case, in order to avoid making changes to
   the address field in the IP packet header that trigger a condition at
   the remote end where the packet is not recognizable in the correct



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   context.

5.3.3  Session Startup and Maintenance

   The next issue is that of the difference between the initial session
   startup mode of operation and the maintenance of the session state.

   In a split endpoint identifier / locator environment there needs to
   be at least one initial locator associated with an endpoint
   identifier in order to establish an initial connection between the
   two hosts.  This locator could be loaded into the DNS in a
   conventional fashion, or, if the endpoint identifier is a
   distinguished address value, the initial communication could be
   established using the endpoint identifier in the role of a locator
   (i.e.  using this as a conventional address).

   The initial actions in establishing a session would be similar.  If
   the session is based on specification of a FQDN, the FQDN is first
   mapped to an endpoint identity value, and this endpoint identity
   value could then be mapped to a locator set.  The locators in this
   set are then candidate locators for use in establishing an initial
   synchronized state between the two hosts.  Once the state is
   established it is then possible to update the initial locator set
   with the current set of useable locators.  This update could be part
   of the initial synchronization actions, or deferred until required.

   This leads to the concept of the use of a 'distinguished' locator
   that acts as the endpoint identifier, and a pool of alternative
   locators that are associated with this 'home' locator.  This
   association may be statically defined, using referential pointers in
   a third party referral structure (such as the DNS), or dynamically
   added to the session through the actions of the endpoint identity
   protocol stack element, or both.

   If opportunistic identities are used, where the identity is not a
   fixed discoverable value but one that is generated in the context of
   a session then additional actions must be performed at session
   startup.  In this case there is still the need for defined locators
   that are used to establish a session, but then an additional step is
   required to generate session keys and exchange these values in order
   to support the identity equivalence of multiple locators within the
   ensuing session.  This may take the form of a capability exchange and
   an additional handshake and associated token value exchange within
   the transport protocol if an in-band approach is being used, or it
   may take the form of a distinct protocol exchange at the level of the
   identity protocol element, performed out-of-band from the transport
   session.




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   Some approaches are capable of a further distinction, namely that of
   initial session establishment and that of establishment of additional
   shared state within the session to allow multiple locators to be
   treated as being bound to a common endpoint identity.  It is not
   strictly necessary that such additional actions be performed at
   session startup, but it appears that such actions need to be
   performed prior to any loss of end-to-end connectivity on the
   selected initial locator, so that any delay in this additional state
   exchange does increase the risk of session disruption due to
   connectivity changes.

   This raises a further question of whether the identity / locator
   split is a capability negotiation performed per session or per remote
   end, or whether the use of a distinguished identity value by the
   upper level application to identify the remote end triggers the
   identity / locator mapping functionality further down in the protocol
   stack at the transport level, and that this is performed without any
   further capability negotiation within the session.

   Within the steps related to session startup there is also the
   consideration that the passive end of the connection follows a
   process where it may need to verify the proposed new address
   contained in the source address of incoming packets before using it
   as a destination address for outgoing packets.  It is not necessarily
   the case that the sender's choice of source address reflects a valid
   path from the receiver back to the source.  While using this offered
   address appears to offer a low overhead response to connection
   attempts, if this response fails the receiver may need to discover
   the full locator set of the remote end through some locator discovery
   mechanism in order to establish whether there is a viable locator
   that can use a forwarding path that reaches the remote end.

   Alternatively, the passive end would use the initially offered
   locator and if this is successful leave it to the identity modules in
   each stack to exchange information to establish the current complete
   locator set for each end.  This approach implies that the active end
   of a communication needs to cycle through all of its associated
   locators as source addresses until it receives a response or exhausts
   its locator set.  While this may extend the time to confirm that no
   path exists to the remote end, it has the potential to improve the
   characteristics of the initial exchange against denial of service
   attacks that could force the remote end to engage in a high volume of
   spurious locator lookups.

5.3.4  Dynamic Capability Negotiation

   The common aspect of these approaches is that they all involve
   changes to the end-to-end interaction, as both endS of the



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   communication need to be aware of this separation.  The implication
   is that this form of support for multi-homing is relatively sweeping
   in its scope, as the necessary changes to support multi-homing extend
   beyond changes to the hosts and/or routers within the multi-homed
   site and encompass changes to the IPv6 protocol itself.  It would be
   prudent when considering these changes to evaluate associated
   mechanisms that allow the communicating endpoints to discover each
   other's capabilities and only enable this form of split identity /
   locator functionality when it is established that both ends can
   support it.

   It is a corollary of this form of negotiated capability that it is
   not strictly necessary that only one form of functionality can be
   negotiated in this way.  If the adoption of a particular endpoint
   identity / locator mapping scheme is the outcome of a negotiation
   between the endpoints then it would be possible to negotiate to use
   one of a number of possible approaches.  There is some interaction
   between the approach used and the form of endpoint identity, and some
   care needs to be taken that any form of acceptable outcome of the
   endpoint identity capability negotiation is one that allows the upper
   level application to continue to operate.

5.3.5  Identity Uniqueness and Stability

   When considering the properties of long-lived identities, it is
   reasonable to assume that the identity assignation is not necessarily
   one that is permanent and unchangeable.  In the case of structured
   identity spaces the identity value reflects a distribution hierarchy.
   There are a number of circumstances where a change of identity value
   is appropriate.  For example, if an endpoint device is moved across
   administrative realms of this distribution hierarchy it is likely
   that the endpoint's identity value will be re-assigned to reflect the
   new realm.  It is also reasonable to assume that an endpoint may have
   more than one identity at any point in time.  RFC 3014 [5] provides a
   rationale for such a use of multiple identities.

   If an endpoint's identity can change over time, and if an endpoint
   can be identified by more than one identity at any single point in
   time, then some further characteristics of endpoint identifiers
   should be defined.  These relate to the constancy of an endpoint
   identity within an application, and the question of whether a
   transport session relies on a single endpoint identity value, and, if
   so, whether an endpoint identity can be changed within a transport
   session, and under what conditions the old identity can continue to
   be used following any such change.  If the endpoint identity is a
   long-lived reference to a remote endpoint, and if multiple identities
   can exist for a single unique endpoint, then the question arises as
   to whether applications can compare identities for equivalence, and



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   whether it is necessary for applications to recognize the condition
   where different identities refer to the same endpoint.  These
   identities may be used within applications within a single host, or
   may be identifies being used on applications on different hosts.

6.  Functional Decomposition of Multi-Homing Approaches

   The following sections provide a framework for the characterization
   of multi-homing approaches through a decomposition of the functions
   associated with session establishment, maintenance and completion in
   the context of a multi-homed environment.

6.1  Establishing Session State

      What form of token is passed to the transport layer from the upper
      level protocol element as an identification of the local protocol
      stack?
      What form of token is passed to the transport layer from the upper
      level protocol element as an identification of the remote session
      target?
      What form of token is used by the upper level protocol element as
      a self-identification mechanism for use within the application
      payload?
      Does the transport need to translate the upper level interface
      token into an identity token that identifies the session? Is this
      translation performed before or after the initial session packet
      exchange handshake?
      How does the session initiator establish that the remote end of
      the session can support the multi-homing version of the transport
      protocol? If not, does the transport session report a session
      establishment failure to the upper level protocol, or silently
      fall back to a non-multi-homed version of the transport protocol?
      What form of mechanism is used to ensure that the selected site
      exit path matches the selected packet source locator?

6.2  Rehoming Triggers

      What triggers are used to identify that a switch of locators is
      desirable?
      Are the triggers based on the end-to-end transport session and/or
      on notification of state changes within the network path from the
      network?
      What triggers can be used to indicate the direction of the failed
      path in order to trigger the appropriate locator repair function?

6.3  Rehoming Locator Pair Selection





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      What parameters are used to determine the selection of a locator
      to use to reference the local endpoint?
      If the remote endpoint is multi-homed, what parameters are used to
      determine the selection of a locator to use to reference the
      remote endpoint?
      Must a change of an egress site exit router be accompanied by a
      change in source and / or destination locators?
      How can new locators be added to the locator pool of an existing
      session?

6.4  Locator Change

      What are the preconditions that are necessary for a locator
      change?
      How can the locator change be confirmed by both ends?
      What interactions are necessary for synchronization of locator
      change and transport session behavior?

6.5  Removal of Session State

      How is identity / locator binding state removal synchronized with
      session closure?
      What binding information is cached for possible future use?

7.  Security Considerations

   There are a significant number of security considerations that result
   from the action of distinguishing within the protocol suite endpoint
   identity and locator identity.

   It is not proposed to enumerate these considerations in detail within
   this document, but to reference a distinct document that describes
   the security considerations of this domain [6].

8.  Acknowledgements

   The author acknowledges the exensive contribution of Margaret
   Wasserman in preparing the original draft of the summary of current
   approaches to multi-homing.

   The author acknowledges the assistance from the following reviewers:
   Brian Carpenter, Kurtis Lundqvist, Erik Nordmark, Iljitsch van
   Beijnum, Marcelo Bagnulo and Joe Touch.

9.  References






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9.1  Normative References

9.2  Informative References

   [1]  Abley, J., Black, B. and V. Gill, "Goals for IPv6
        Site-Multihoming Architectures", RFC 3582, August 2003.

   [2]  Lear, E., "Things MULTI6 Developers should think about", Work in
        progress: Internet Drafts
        draft-lear-multi6-things-to-think-about-03.txt, May 2004,
        <http://bgp.potaroo.net/ietf/idref/draft-lear-multi6-things-to-think-about/>
        .

   [3]  Saltzer, J., Reed, D. and D. Clark, "End-to-End Arguments in
        System Design", ACM TOCS Vol 2, Number 4, pp 277-288, November
        1984,
        <http://web.mit.edu/Saltzer/www/publications/endtoend/endtoend.txt>
        .

   [4]  Srisuresh, P. and M. Holdrege, "IP Network Address Translator
        (NAT) Terminology and Considerations", RFC 2663, August 1999.

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

   [6]  Nordmark, E. and T. Li, "Threats relating to IPv6 multi-homing
        solutions", Work in progress: Internet Drafts
        draft-nordmark-multi6-threats-02.txt, June 2004,
        <http://bgp.potaroo.net/ietf/idref/draft-nordmark-multi6-threats/>
        .


Author's Address

   Geoff Huston
   Telstra

Appendix A.  Notes on Various approaches

   These notes were orginally drafted by Margaret Wasserman.  The notes
   on various approaches are non-exclusive, i.e.  solutions not reviewed
   or mentioned here are not ruled out of discussion.  Also the review
   comments are not comprehensive, and the selection reflects the time
   constraints of the contributors to this section than any qualititive
   judgement on any of the approaches.  The author is desirous, in
   future revisions of this draft, in augmenting this selection of
   reviewed approaches.




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A.1  Host Identity Protocol (HIP)

   HIP is an ID/Locator separation mechanism intended to solve a much
   wider problem space than site multi-homing.  HIP uses cryptographic
   identifiers termed Host Identity Tags (HITs) at the application
   layer, which are mapped to locators (IP Addresses) by a HIP protocol
   stack layer that interfaces between the transport layer and IP
   internetwork layer.

   The essential characteristic of HIP is it use of opportunistic
   identity generation, as it uses a cyptographic host identifier as the
   basis of the persistent identity.  The transport session cab be agile
   across locators, or even across IP protocol versions, as the HIT is
   used to determine session integrity.  allowing the hosts to determine
   what packets legitimately form part of the session.

   HIP is proposed as a new protocol element, located at layer 3.5 (i.e.
   above the internetwork IP layer and below the transport layer).  The
   presentation to the transport layer uses 128 bit hash values (the
   HIT) in place of IP addresses, while the presentation to the internet
   layer uses conventional IP addresses.

   Being opportunistic and unstructured, the HIT space is not an
   efficient search space, nor can a HIT be used as a unique search key.
   HITs are part of an an equivalence function, to allow each host to
   determine that an incoming packet is part of an established session.
   HITs cannot be used as an identity value in a conventional referral
   sense (HostA wants to tell HostB to talk to HostC).  While an
   application could pass a HIT to a third-party (and legacy
   applications would unknowingly do so), the third party would have no
   way to map that HIT to a locator (an IP address) as HIP does not
   include any global HIT->Locator mapping mechanism.

   Summary:
   o  New Protocol Stack Element
   o  Layer 3.5 (Above IP, below Transport
   o  Unstructured, opportunistic identity values (non-referential)
   o  DNS rendezvous
   o  No Locator exchange protocol

   Current IETF Documents:
   o  draft-moskowitz-hip-arch
   o  draft-moskowitz-hip
   o  draft-nikander-hip-mm
   o  draft-nikander-esp-beet-mode






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A.2  Multihoming without IP Identifiers (NOID)

   NOID proposes an approach for endpoint identifier and locator
   separation where the endpoint identifier space is drawn from the
   locator space.  Instead of creating a new namespace for endpoint
   identifiers, the endpoint identifier may be chosen from the set of
   locators that can be used to reach a given endpoint.  Until an event
   occurs that modifies the list of usable locators, the initial
   endpoint identifier value can serve as a locator.

   NOID uses next-header values in the IPv6 header to indicate whether a
   given packet should be processed by the NOID layer.  At a conceptual
   level, NOID adds a layer to the middle of IP above most IP
   processing, but below IPSec, fragmentation and reassembly functions.

   NOID makes use of the global DNS as a mapping system between IDs and
   Locators.  A node who wishes to communicate with another node can use
   the FQDN to get a list of possible locators (IP Addresses).  That
   node will then choose one of the locators to use as an
   Application-level ID (AID).

   NOID offers some support for application referrals.  If Host A passes
   an AID to Host B that is supposed to point to Host C, Host B should
   be able to do a reverse DNS lookup to map the AID to an FQDN and then
   use the FQDN to get the complete set of locators.  However, for this
   to be effective, nodes would need to have both forward and reverse
   DNS entries.  There might also be a need to dynamically update the
   DNS as a node becomes reachable or unreachable at certain locators.

   Summary:
   o  New Protocol Stack Element
   o  Layer 3 (Inserted in the upper part of IP, below IPSEC and
      fragementation / reassembly
   o  Identity values based on locator set
   o  DNS rendezvous
   o  Identity peering protocol

   Current IETF Documents:
   o  draft-nordmark-multi6-noid
   o  draft-templin-isnoid

A.3  Common Endpoint Locator Pools (CELP)

   CELP explores the concept of sharing information about locator
   reachability between transport-layer "multi-addressing" mechanisms
   (such as SCTP and DCCP) and Internet-layer multiaddressing
   mechanisms, referred to in the draft as "wedge-layer approaches"
   (such as NOID).  (This concept was originally discussed on the MULTI6



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   mailing list under the name 'SLAP'.)

   The motivation behind CELP is that muliple multiaddressing mechanisms
   may be used (by different applications or for different connections)
   on a single endpoint, and that it would beneficial for those
   mechanisms to share information about the reachability of the IP
   addresses in a given locator pool.  If a transport-layer mechansim,
   such as SCTP, could share its knowledge regarding the reachability of
   a certain locator, it might be possible to minimize or elimate
   Internet-layer control packets that are used to maintain that
   information at the Internet layer.  In some ways, this is similar to
   IPv6 Neighbor Discovery's use of upper layer advice regarding
   neighbor reachability to avoid sending unncecessary ND packets.

   This document offers a definition of the term "endpoint" that refers
   to a locator pool that may have a smaller scope than an entire IP
   node (i.e.  a given locator pool may only contain a subset of the
   locators available on an IP node).

   The CELP document is more of a consideration of approach than an
   actual proposal for a solution.  It doesn't specify in detail how it
   would work with any particular transport-layer or Internet-layer
   multiaddressing mechanisms.  However, it is an approach that could be
   applied to many combinations of solutions.

   Summary:
   o  Considerations relating to sharing locator reachability
      information across session instances.

   Current IETF Documents:
   o  draft-crocker-celp

A.4  Weak Identifier Multihoming Protocol (WIMP)

   WIMP is an endpoint identifier / locator separation protocol that is
   heavily focused on mitigating the threats outlined in work in
   progress on security threats in multi-homing scenarios
   [draft-nordmark-multi6-threats-00.txt].  The WIMP approach uses
   divided secrets and hash chaining to ensure that new locators are
   supplied by the same node that supplied the original locator.

   WIMP uses a separate name space for 128-bit non-routable IDs that are
   never used in packets on the network.  These IDs are locally
   generated for both local and remote nodes by hashing a nonce (for the
   initiator's endpoint identity) or the FQDN (for the responder's
   endpoint identity).  (The approach assumes a requirement that all
   responders will have a FQDN.)




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   The WIMP protocol introduces a WIMP layer that maps between IDs and
   locators based on internal state.  The WIMP layer is conceptually
   located within the network layer, above most IP processing and below
   IPsec, fragmentation/reassembly and destination options, similar to
   NOID.

   Communication between two end-points requires establishment of a WIMP
   session.  Once the session is established, it can be used for
   multiple simultaneous or sequential connections to the same
   end-point.  During WIMP session establishment, WIMP introduces a
   separate header into the data packets, between the IP and TCP/UDP
   headers that contains information about the WIMP session.  The WIMP
   session establishment packets can optionally be piggy-backed on data
   packets.  WIMP does not introduce a separate header into all IPv6
   packets.  Instead, once a WIMP session is established, the IPv6
   FlowID is used to hold an identifier for the WIMP host-pair context
   associated with a given packet.

   WIMP is intended to provide a solution to some of the security
   concerns, particularly regarding connection hijacking, that have been
   raised for some other endpoint identity / locator separation
   mechanisms.

   Reviewers of WIMP have raised some questions of this approach,
   particularly concerning the use of an optional header while operating
   below IP fragmentation.  The piggy-backing mechanism requires that
   the packets not be fragmented, but it doesn't explain how upper
   layers will become aware of the MTU limitations on those packets and/
   or how this mechanism would interact with Path MTU discovery.  Like
   HIP, WIMP makes no provision to handle application-level referrals
   and does not contain a mechanism for global endpoint identifier to
   locator mapping.  It has also been pointed out that it is interesting
   to consider whether the WIMP approach to security, hash chaining,
   could be applied to other endpoint identity / locator separations
   mechanisms, such as NOID.

   Summary:
   o  New Protocol Stack Element
   o  Layer 3 (Inserted in the upper part of IP, below IPSEC and
      fragementation / reassembly
   o  Identity values based on hash of FQDN
   o  Identity peering protocol

   Current IETF Documents:
   o  draft-ylitalo-multi6-wimp






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A.5  Host-Centric IPv6 Multihoming

   Host-Centric Multihoming is, in some ways, the simplest way to
   address the IPv6 site multihoming problem.  The concept is that every
   host in the multihomed site is configured with multiple prefixes that
   correspond to different service providers.  Each host configures
   addresses within those prefixes and selects among those addresses
   when connecting to a remote host.  This configuration is automated
   using Router Renumbering and IPv6 Address Autoconfiguration.
   However, this simple solution Layer 3 (inserted in the upper part of
   IP, below IPSEC and fragementation / reassembly has several practical
   limitations and drawbacks, and this draft attempt to address them.

   In particular, the Host-Centric Multihoming proposal attempts to
   address the "site exit issue".  Hosts cannot control the exit path
   that their packets will take from the local site, so hosts with
   multiple addresses may use a source IP address from one ISP on
   packets that end-up being routed through a different ISP.  In many
   cases, the ISPs will run ingress filtering and will discard those
   packets.

   One solution to the site exit problem is to changes the ISP ingress
   filters to accept all of the source address prefixes that are used
   within the site.  Other approaches are to perform source-based
   routing within the site, to deploy a single site-exit router or to
   structure the network so that all exit routers are connected to a
   single DMZ network that employs source-based routing.  A virtual DMZ
   can be constructed by configuring a mesh of tunnels between all exit
   routers, tunneling packets to the correct exit router based on source
   address.  Each of these solutions has operational drawbacks and/or
   introduces inefficiencies.

   This proposal suggests another solution to the site exit problem
   called "source address discovery".  Based on Path MTU discovery, this
   mechanism involves adding extra information to the ICMP Destination
   Unreachable message that the packet was discarded due to an ingress
   filter.  This extra information indicates what address prefix should
   be used to pass the ingress filter.  Rather than adding a field to
   the ICMP message, this extra information is communicated via the
   source address that the route Layer 3 (Inserted in the upper part of
   IP, below IPSEC and fragementation / reassembly).

   It also proposes a "superior" alternative called "exit router
   discovery", which allows hosts to specify which exit router will be
   used for each packet.  Instead of sending ICMP error messages when
   ingress filtering causes packets to be discarded, the exit router
   will send the equivalent of a redirect message and future packets
   with the same source/destination address pair will be tunneled to the



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   indicated exit router.  This mechanism involves tunneling to a
   site-exit anycast address that is derived from the sites' prefixes.
   The draft primary focuses on the specification of this "superior"
   approach, largely ignoring some pertinent questions such as: Will
   residential and enterprise-level IPv6 routers reall support anycast
   routing?

   One important thing to note about the host-centric multihoming
   solution is that it doesn't appear to provide any ability for
   transport connections to survive a change in the topology that causes
   a host to become unreachable at an address that is currently used as
   a connection end-point.  It also does not offer any support for
   legacy applications that do application-level referrals, requiring
   that a full set of locators be exchanged as part of the referral.

A.6  Summaries of Selected ID/LOC Separation Documents

   This section summarizes a set of selected ID/Loc separation
   documents.  The selection includes documents that appear to be
   active, and this section provides a very short summary of each one.
   The first sub-section lists documents that are new or updated since
   IETF 58 and the second sub-section lists older documents that remain
   active.  The documents in each sub-section are listed alphabetically
   by draft filename.

A.6.1  New or Updated Documents Since IETF58

   o  TLC-FM: Transport Layer Common Framework for Multihoming
      draft-arifumi-multi6-tlc-fm
         This draft proposes a transport-layer mechanism for ID /Locator
         mapping.  There is a conceptual layer within the transport
         layer that provides support for common multihoming functions.
         It is compatible with the use of Mobile IPv6 (MIP6) to provide
         mobility support.
         In TLC-FM, like SCTP, the ID consists of a collection of
         locators that may be used to reach a given host.  It employs
         transport-level clues (such as TCP retransmissions) to decide
         when to switch locators.  For UDP connections, ICMP error
         messages or application-level hints are necessary.
         This mechanism is not well enough specified to fully evaluate
         it, but it doesn't appear to offer any support for
         application-level referrals.

   o  Multi-Homing Tunnel Broker (MHTB)
      draft-bagnulo-multi6-mhtb
         This document defines an enhancement to RFC 3178, IPv6
         Multihoming Support at Site Exit Routers, to reduce the
         administrative overhead of maintaining a configured tunnel for



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         each multihoming association.  However, this draft does not
         address another major drawback of the RFC 3178 approach, that
         it does not protect against the complete failure of one or more
         connected ISPs.

   o  Framework for Common Endpoint Locator Pools (CELP)
      draft-crocker-celp
         Dave Crocker and Avri Doria's CELP draft, reviewed in the
         previous section.

   o  Multi-Homing: the SCTP Solution
      draft-coene-multi6-sctp
         One confusing question about the direction of this work is why
         SCTP is being discussed as a "solution" to site multihoming,
         when a clear requirement for a site multihoming solutions is
         the ability to support existing TCP-based and UDP-based
         applications.  This document isn't really a proposal, though --
         it consists of answers to the questions posted in Eliot Lear's
         "Things MULTI6 Developers Should Think About" draft, and does
         not discuss how SCTP does (or doesn't) address the requirements
         outlined in the Multi6 requirements RFC.
         An interesting thing about this proposal is that it claims that
         SCTP is not an ID/Loc separation mechanism, however in some
         academic sense it actually is.  The ID is the group of
         available IP addresses, and the locator is whichever address is
         currently being used for communication.  SCTP also experiences
         the same complexities as other proposals (AKA NOID, CELP) that
         use a pool of locators as the ID -- How do you choose which
         locator to use? And how do you detect when a member of the pool
         becomes invalid for use as a locator? So, while it isn't
         actually a solution for site multihoming, SCTP may provide some
         useful experiences and mechanisms that may apply to a class of
         possible solutions.

   o  Host Identity Protocol (HIP) Rendezvous Mechanisms
      draft-eggert-hip-rendezvous-00.txt
         This is an overview draft that discusses the concept of HIP
         rendezvous mechanisms to improve the applicability of HIP for
         mobility and multihoming.  This is a survey document that
         outlines the problem and discusses different type of solutions
         to the problem.

   o  Host-Centric IPv6 Multihoming
      draft-huitema-multi6-hosts
         Draft by Christian Huitema and others, described above.

   o  Things MULTI6 Developers Should Think About
      draft-lear-multi6-things-to-think-about



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         Eliot Lear's efforts to collect a set of practical questions
         that should be considered for all MULTI6 protocols.

   o  Host Identity Protocol (HIP)
      draft-moskowitz-hip
         This is the base protocol specification for HIP.  Along with
         the HIP architecture, these documents form the basis of the HIP
         work.

   o  Consideration on HIP Based IPv6 Multi-Homing
      draft-nikander-multi6-hip
         Pekka Nikander's document that submits HIP as a solution for
         the MULTI6 problem space.

   o  8+8 Addressing for IPv6 End to End Multihoming
      draft-ohta-multi6-8plus8

   o  Threats Relating to Transport Layer Protocols Handling Multiple
      Addresses
      draft-ohta-multi6-threats

   o  Multihoming Using IPv6 Addressing Derived from AS Numbers
      draft-savola-multi6-asn-pi
         This draft provides a mechanism for organizations that have
         been assigned a 16-bit AS number to use that number to
         auto-generate a globally routable, provider-independent address
         prefix.

   o  Problem Statement: HIP Operation over Network Address Translators
      draft-stiemerling-hip-nat
         Summarizes the problems with running HIP and IPsec-based data
         transmission across NATs.

   o  Operational Approach to Achieve IPv6 Multihomed Network
      draft-toyama-multi6-operational-site-multihoming
         This document proposes to support site multihoming in IPv6 by
         assigning additional /32 prefixes and AS numbers to "groups" of
         providers who will provide multihomed /48 prefixes to their
         mutual customers.
         It is currently unclear to the reviewer how/if this proposal
         would work and/or scale since it seems to involve two different
         providers advertising the same /32 and the same AS number into
         the default free zone.  It requires some type of peering "to
         share prefix assignments" between ISPs, and the diagram shows
         some type of connection between the ISPs, but I don't know what
         the details of that connection are.
         It also has the potential to more quickly exhaust the AS number
         space and to result in a substantially larger number of routes



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         in default free routers, since the number of "groups" could
         scale exponentially with the number of providers.

   o  Crypto Based Host Identifiers (CBHI)
      draft-van-beijnum-multi6-cbhi
         This draft defines a crytographic mechanism for generating host
         identifiers.  It is intended for use with other protocols that
         require host identifiers, such as ODT (see below).

   o  On Demand Tunneling for Multihoming (ODT)
      draft-van-beijnum-multi6-odt
         This draft discusses an automatic tunnelling-based solution for
         multihoming.

   o  Weak Identifier Multihoming Protocol (WIMP)
      draft-ylitalo-multi6-wimp
         WIMP proposal, described above.

A.6.2  Older Documents that Remain Active/Interesting

   o  RFC 3582: Goals for IPv6 Site-Multihoming Architectures

   o  Choices for Multiaddressing
      draft-crocker-mast-analysis

   o  What's In a Name: Thoughts from the NSRG
      draft-irtf-nsrg-report

   o  A Roadmap for Multihoming in IPv6
      draft-kurtis-multi6-roadmap

   o  Host Identity Protocol (HIP) Architecture
      draft-moskowitz-hip-arch-05.txt

   o  End-Host Mobility and Multi-Homing with Host Identity Protocol
      (HIP)
      draft-nikander-hip-mm

   o  Threats Relating to IPv6 Multihoming Solutions
      draft-nordmark-multi6-threats-00.txt

   o  Multihoming without IP Identifiers (NOID)
      draft-nordmark-noid
         Erik Nordmark's NOID specification, described above.







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A.6.3  Related Multi-Homing drafts, Status unknown

   This is a list of ID/Loc separation and/or MULTI6 documents, listed
   alphabetically by draft name.
   o  Extension Header for Site-Multi-homing Support
      draft-bagnulo-multi6-mhexthdr

   o  Application of the MIPv6 Protocol to the Multi-Homing Problem
      draft-bagnulo-multi6-mnm

   o  Multiple Address Service for Transport (MAST): An Extended
      Proposal
      draft-crocker-mast-proposal

   o  NAROS : Host-Centric IPv6 Multihoming with Traffic Engineering
      draft-de-launois-multi6-naros

   o  Application and Use of the IPv6 Provider Independent Global
      Unicast Format
      draft-hain-ipv6-pi-addr-use

   o  Simple Dual Homing Experiment
      draft-huitema-multi6-experiment-00.txt

   o  Host-Centric IPv6 Multihoming
      draft-huitema-multi6-hosts

   o  IPv4 Multihoming
      draft-ietf-multi6-v4-multihoming
         This documents how multi-homing is supported at present in the
         IPv4 protocol domain.

   o  Multihoming in IPv6 by Multiple Announcement of Longer Prefixes
      draft-kurtis-multihoming-longprefix

   o  Multihoming using 64-bit Crypto-based IDs
      draft-nordmark-multi6-cb64

   o  Strong Identity Multihoming using 128-bit Identifiers (SIM/
      CBID128)
      draft-nordmark-multi6-sim

   o  IPv6 Address Assignment and Route Selection for End-to-End
      Multihoming
      draft-ohira-assign-select-e2e-multihome

   o  Hierarchical IPv6 Subnet ID Autoconfiguration for Multi-Address
      Model Multi-Link Multihoming Site



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      draft-ohira-multi6-multilink-auto-prefix-assign

   o  Hierarchical IPv6 Subnet ID Autoconfiguration for Multi-Address
      Model Multi-Link Multihoming Site
      draft-ohira-multi6-multilink-auto-prefix-assign

   o  The Architecture of End to End Multihoming
      draft-ohta-e2e-multihoming-05.txt

   o  8+8 Addressing for IPv6 End to End Multihoming
      draft-ohta-multi6-8plus8-00.txt

   o  Threats Relating to Transport Layer Protocols Handling Multiple
      Addresses
      draft-ohta-multi6-threats-00.txt

   o  Multihomed ISPs and Policy Control
      draft-ohta-multihomed-isps-00.txt

   o  GAPI: A Geographically Aggregatable Provider Independent Address
      Space to Support Multihoming in IPv6
      draft-py-multi6-gapi

   o  Multi Homing Translation Protocol (MHTP
      draft-py-multi6-mhtp-01.txt

   o  Multihoming Using IPv6 Addressing Derived from AS Numbers
      draft-savola-multi6-asn-pi-01.txt

   o  IPv6 Site Multihoming: Now What?
      draft-savola-multi6-nowwhat

   o  Operation of NOID Multihoming Protocol on ISATAP Nodes
      draft-templin-isnoid

   o  LIN6: A Solution to Multihoming and Mobility in IPv6
      draft-teraoka-multi6-lin6

   o  Operational Approach to achieve IPv6 multihomed network
      draft-toyama-multi6-operational-site-multihoming-00.txt

   o  Two Prefixes in One Address
      draft-van-beijnum-multi6-2pi1a-00.txt

   o  Crypto Based Host Identifiers
      draft-van-beijnum-multi6-cbhi-00.txt





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   o  Provider-Internal Aggregation based on Geography to Support
      Multihoming in IPv6
      draft-van-beijnum-multi6-isp-int-aggr-01.txt

   o  On Demand Tunneling For Multihoming
      draft-van-beijnum-multi6-odt-00.txt













































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