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RFC 4177 - Architectural Approaches to Multi-homing for IPv6

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Network Working Group                                          G. Huston
Request for Comments: 4177                                         APNIC
Category: Informational                                   September 2005

           Architectural Approaches to Multi-homing for IPv6

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This memo provides an analysis of the architectural 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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The Multi-Homing Space . . . . . . . . . . . . . . . . . . . .  5
   4.  Functional Goals and Considerations  . . . . . . . . . . . . .  7
   5.  Approaches to Multi-Homing . . . . . . . . . . . . . . . . . .  7
       5.1.  Multi-Homing: Routing  . . . . . . . . . . . . . . . . .  8
       5.2.  Multi-Homing: Mobility . . . . . . . . . . . . . . . . .  9
       5.3.  Multi-homing: Identity Considerations  . . . . . . . . . 12
       5.4.  Multi-homing: Identity Protocol Element  . . . . . . . . 14
       5.5.  Multi-homing: Modified Protocol Element  . . . . . . . . 15
       5.6.  Modified Site-Exit and Host Behaviors  . . . . . . . . . 16
   6.  Approaches to Endpoint Identity  . . . . . . . . . . . . . . . 17
       6.1.  Endpoint Identity Structure  . . . . . . . . . . . . . . 18
       6.2.  Persistent, Opportunistic, and Ephemeral Identities  . . 20
       6.3.  Common Issues for Multi-Homing Approaches  . . . . . . . 23
             6.3.1.  Triggering Locator Switches  . . . . . . . . . . 23
             6.3.2.  Locator Selection  . . . . . . . . . . . . . . . 26
             6.3.3.  Layering Identity  . . . . . . . . . . . . . . . 27
             6.3.4.  Session Startup and Maintenance  . . . . . . . . 29
             6.3.5.  Dynamic Capability Negotiation . . . . . . . . . 31
             6.3.6.  Identity Uniqueness and Stability  . . . . . . . 31
   7.  Functional Decomposition of Multi-Homing Approaches  . . . . . 32
       7.1.  Establishing Session State . . . . . . . . . . . . . . . 32
       7.2.  Re-homing Triggers . . . . . . . . . . . . . . . . . . . 33
       7.3.  Re-homing Locator Pair Selection . . . . . . . . . . . . 33
       7.4.  Locator Change . . . . . . . . . . . . . . . . . . . . . 34
       7.5.  Removal of Session State . . . . . . . . . . . . . . . . 34
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 34
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
   10. Informative References . . . . . . . . . . . . . . . . . . . . 34

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.  Terminology

   Care-of Address (CoA)
      A unicast routeable address associated with a mobile node while
      visiting a foreign link; the subnet prefix of this IP address is a
      foreign subnet prefix.  Among the multiple care-of addresses that
      a mobile node may have at any given time (e.g., with different
      subnet prefixes), the one registered with the mobile node's home
      agent for a given home address is called its "primary" care-of

   Correspondent Node (CN)
      A peer node with which a mobile node is communicating.  The
      correspondent node may be either mobile or stationary.

      A term for the identity for a network host.  This is normally
      assumed to be a constant or long-lived association.

   Endpoint Identity Protocol Stack Element (EIP)
      An added element in a protocol stack model that explicitly manages
      the association of locators to endpoints.

   Home Address (HoA)
      A unicast routeable address assigned to a mobile node, used as the
      permanent address of the mobile node.  This address is within the
      mobile node's home link.  Standard IP routing mechanisms will
      deliver packets destined for a mobile node's home address to its
      home link.  Mobile nodes can have multiple home addresses, for
      instance, when there are multiple home prefixes on the home link.

   Lower Layer Protocol (LLP)
      The lower-level protocol in the protocol stack model relative to
      the protocol layer being considered.  In the Internet
      architecture, the LLP of the transport protocol is the Internet
      Protocol, and the LLP of the application protocol is the transport

      The term "locator" is used as the location token for a network
      host.  This is a network-level address that can be used as a
      destination field for IP packets.

   Mobile Node
      A node that can change its point of attachment from one link to
      another, while still being reachable via its home address.

   Multi-Homed Site
      A site with more than one transit provider.  "Site multi-homing"
      is the practice of arranging a site to be multi-homed such that
      the site may use any of its transit providers for connectivity

      The transition of a site between two states of connectedness, due
      to a change in the connectivity between the site and its transit

      An entity autonomously operating a network using IP.

   Site-Exit Router
      A boundary router of the site that provides the site's interface
      to one or more transit providers.

   Transit Provider
      A provider that operates a site that directly provides
      connectivity to the Internet to one or more external sites.  The
      connectivity provided extends beyond the transit provider's own
      site.  A transit provider's site is directly connected to the
      sites for which it provides transit.

   Upper Layer Protocol (ULP)
      The upper-level protocol in the protocol stack model relative to
      the protocol layer being considered.  In the Internet
      architecture, the ULP of the Internet Protocol is the transport
      protocol, and the ULP of the transport protocol is the application

3.  The Multi-Homing Space

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

                           | 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         |
         |           +-----------+                 |
         |           |   host    |                 |
         + - - - - - - - - - - - - - - - - - - - - +

              Figure 1: The Multi-Homed Domain

   The environment of multi-homing 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
   transparently 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.  Established sessions should survive dynamic changes in
   network-level reachability.

   There are also considerations of providing mechanisms to support
   sustained site visibility to support session establishment.
   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.  This also implies that local attempts to initiate
   a communication with remote hosts should take into account the
   current connectivity state in undertaking locator selection and
   setting up initial locator sets.

   In addition, there is the potential consideration of being able to
   distribute the total traffic load across a number of network paths
   according to some predetermined policy objective.  This may be to
   achieve a form of traffic engineering, support for particular
   quality-of-service requirements, or localized load balancing across
   multiple viable links.

   This simple multi-homing scenario also includes "site-exit" routers,
   where the local site interfaces to the upstream Internet transit
   providers.  The interactions between the external routing system and
   the site-exit routers, the 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 simple site multi-homing 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 may 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; multiway communications; use of host addresses in a
   referential context (third-party referrals); and the imposition of
   policy constraints on path selection.  However, the basic simple site
   multi-homing scenario is sufficient to illustrate the major

   architectural aspects of support for multi-homing, so this simple
   scenario will be used as the reference model for this analysis.

4.  Functional Goals and Considerations

   RFC 3582 [RFC3582] documents some goals that a multi-homing approach
   should attempt to address.  These goals include:

      *  redundancy
      *  load sharing
      *  traffic engineering
      *  policy constraints
      *  simplicity of approach
      *  transport-layer survivability
      *  DNS compatibility
      *  packet filtering capability
      *  scaleability
      *  legacy compatibility

   The reader is referred to [RFC3582] for a complete description of
   each of these goals.

   In addition, [thinks] 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:

      *  interaction with routing systems,
      *  aspects of a split between endpoint-identifier and forwarding
      *  changes to packets on the wire, and
      *  the interaction between names, endpoints, and the DNS.

   In evaluating various approaches, further considerations also

      *  the role of helpers and agents in the approach,
      *  modifications to host behaviours,
      *  the required trust model to support the interactions, and
      *  the nature of potential vulnerabilities in the approach.

5.  Approaches to Multi-Homing

   There appear to be five generic forms of architectural approaches to
   this problem, namely:

         Use the IPv4 multi-homing approach

         Use the IPv6 Mobility approach

      New Protocol Element
         Insert a new element in the protocol stack that manages a
         persistent identity for the session

      Modify a Protocol Element
         Modify the transport or IP protocol stack element in the host
         in order to support dynamic changes to the forwarding locator

      Modified Site-Exit Router/Local Host interaction
         Modify the site-exit router and local forwarding system to
         allow various behaviours 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

5.1.  Multi-Homing: Routing

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

   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; those remote domains that 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 nor the remote host need to have
   multiple addresses or to 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: scaleability.  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, for example, 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-domain routing protocol architecture.

   One other goal, transport-layer surviveability, is potentially at
   risk in this approach.  Dynamic changes within the network trigger
   the routing system to converge to a new stable distributed forwarding
   state.  This process of convergence within the distributed routing
   system may include the network generating unstable transient
   forwarding paths, as well as taking an indeterminate time to
   complete.  This in term may trigger upper-level protocol timeouts and
   possible session resets.

5.2.  Multi-Homing: Mobility

   Preserving established communications through movement is similar to
   preserving established communications through outages in multi-homed
   sites as both scenarios require the capability of dynamically
   changing the locators used during the communication while
   maintaining, unchanged, the endpoint identifier used by Upper Layer
   Protocol (ULP).  Since MIPv6 protocol [RFC3775] already provides the
   required support to preserve established communications through
   movement, it seems worthwhile to explore whether it could also be
   used to provide session survivability in multi-homed environments.

   MIPv6 uses a preferred IP address, the Home Address (HoA), as a
   stable identifier for the mobile node (MN).  This identifier is then
   dynamically mapped to a valid locator (Care-of Address, or CoA) that
   corresponds to the current attachment point within the network
   topology.  When the MN is at the Home Network, the HoA is used both
   as locator and as identifier.  When the MN is not at the Home
   Network, the HoA is used as an identifier, and the CoA is used as
   locator.  A relaying agent (Home Agent) placed in the Home Network is
   used to forward packets addressed to the HoA to the current location,
   specified by the CoA.  After each movement, the MN must inform its
   Home Agent of the new CoA and optionally inform those entities with
   which it has established communications (Correspondent Nodes, or
   CNs).  The mapping between the HoA and the current CoA is conveyed
   using Binding Update (BU) messages.

   When the BU message is exchanged between the MN and the Home Agent,
   it is possible to assume the existence of a pre-established Security
   Association that can be used to protect the binding information.
   However, when the BU message is exchanged between the MN and the CN,
   it is not possible to assume the existence of such a Security
   Association.  In this case, it is necessary to adopt an alternative
   mechanism to protect the binding information contained in the
   message.  The selected mechanism is called the Return Routeability
   procedure, and the background for its design is detailed in [rosec].
   The goal of the mechanism is to allow the CN to verify that the MN
   that is claiming that an HoA is currently located at a CoA is
   entitled to make such claim; this essentially means that the HoA was
   assigned to the MN, and that the MN is currently located at the CoA.
   In order to verify these updates, the CN sends two different secrets,
   one to the claimed HoA and another one to the claimed CoA.  If the MN
   receives both secrets, this means that the Home Agent located at the
   Home Network has a trust relationship with the MN, that it has
   forwarded the secret sent to the HoA, and that the MN is receiving
   packets sent to the CoA.  By including authorisation information
   derived from both secrets within the BU message, the MN will be able
   to prove to the CN that the claimed binding between the HoA and the
   CoA is valid.

   The lifetime of the binding that is created in the CN using
   authorisation information obtained through the Return Routeability
   procedure is limited to 7 minutes, in order to prevent time-shifted
   attacks [rosec].  In a time-shifted attack, an attacker located along
   the path between the CN and the MN forges the Return Routeability
   packet exchange.  The result of such an attack is that the CN will
   forward all the traffic addressed to the HoA to the CoA selected by
   the attacker.  The attacker can then leave the position along the
   path, but the effects of the attack will remain until the binding is
   deleted, shifting in time the effect of the attack.  By limiting the

   lifetime of the binding in the CN, the effect of this attack is
   reduced to 7 minutes, because after that period a new Return
   Routeability procedure is needed to extend the binding lifetime.  It
   should be noted that the Return Routeability procedure is vulnerable
   to "man-in-the-middle" attacks, since an attacker located along the
   path between the CN and the MN can forge the periodic Return
   Routeability packet exchange.

   The possible application of the MIPv6 protocol to the multi-homing
   problem would be to use BU messages to convey information in advance
   about alternative addresses that could be used following an outage in
   the path associated with the currently used address.

   In this scenario, the multi-homed host adopts the MN role and the
   host outside the multi-homed site adopts the CN role.  When a
   communication is established between the multi-homed host and the
   external host, the address used for initiating the communication is
   used as an HoA.  The communication continues using this address as
   long as no outage occurs.  If an outage occurs and the HoA becomes
   unreachable, an alternative address of the multi-homed node is used
   as a CoA.  In this case, the multi-homed node sends a BU message to
   the external host, informing it about the new CoA to be used for the
   HoA, so that the established communication can be preserved using the
   alternative address.  However, such a BU message has to be validated
   using authorisation information obtained through the Return
   Routeability procedure, which implies that the binding lifetime will
   be limited to a fixed period of no more than 7 minutes.  The result
   is that the binding between the HoA and the new CoA will expire after
   this interval has elapsed, and then the HoA will be used for the
   communication.  Since the HoA is unreachable because of the outage,
   the communication will be interrupted.  It should be noted that it is
   not possible to acquire new authorisation information by performing a
   new Return Routeability procedure, because it requires communication
   through the HoA, which is no longer reachable.  Consequently, a
   mechanism based on the MIPv6 BU messages to convey information about
   alternative addresses will preserve communications only for 7

   The aspect of MIPv6 that appears to present issues in the context of
   multi-homing is the Return Routeability procedure.  In MIPv6,
   identity validity is periodically tested by return routeability of
   the identity address.  This regular use of a distinguished locator as
   the identity token cannot support return reachability in the
   multi-homing context, in the event of extended failure of the path
   that is associated with the identity locator.

5.3.  Multi-homing: Identity Considerations

   The intent of multi-homing in the IPv6 domain is to achieve an
   outcome that is comparable to that of multi-homed IPv4 sites using
   routing to support multi-homing, without an associated additional
   load being imposed on the IPv6 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-aggregateable 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 two 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
   response, and hence this choice of source address will direct the

   reverse path from R to the local host.  Secondly, 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 preventing
   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 a destination address for the multi-homed host that is based on
   ISP B's address prefix.  In the event that the local host wishes to
   initiate a session at this point, then it may need to use an initial
   source locator that reflects the situation that the only viable
   destination address to use is the one that is based on ISP B's
   address prefix.  It may be the case that the local host is not aware
   of this return routeability constraint, or it may not be able to
   communicate this information directly to R, in which case R needs to
   discover or be passed this information in other ways.

   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 forwarding layer, namely the IP address.

   If multiple forwarding paths are to be supported for a single
   transport session and if path selection is to be decoupled from the
   functions of transport session initiation and maintenance, then the

   corollary 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 end-to-end architectural model
   [e2e].  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.  Such a
   decoupling of the concepts of identities and locators would not 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.

5.4.  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 be 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.  This 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) will establish a
   synchronised state with its remote counterpart.  This will 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 change 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 recognise 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.

   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.

5.5.  Multi-homing: Modified Protocol Element

   As an alternative to insertion of a new protocol stack element into
   the protocol architecture, an existing protocol stack element could
   be modified to include the functionality performed by the EIP
   element.  This modification could be undertaken within the transport
   protocol stack element or within the internet protocol 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 single-endpoint-to-single-endpoint communication.

   Within the transport layer, this functionality could be achieved, for
   example, by binding 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.  Other potential

   candidates are various forms of packet header transformations using
   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 internet
   protocol stack element.

   There are common issues in all these scenarios: what state is kept,
   which part of the protocol stack keeps this state, how state is
   maintained with additions and removals of locator bindings, and
   whether only one piece of code is 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.

5.6.  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 behaviour 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

   One possible approach uses site-exit routers to perform some form of
   packet header manipulation as packets are passed 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 host'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, a corresponding reverse transformation
   must be undertaken on incoming packets: 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 when the packet is passed into the site, or by the
   local host.

   Packet header rewriting by remote network elements has a large number
   of associated security considerations.  Any packet rewriting
   mechanism has to provide proper protection against the attacks
   described in [threats], in particular against redirection attacks.

   An alternative for packet header rewriting at the site-exit point 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 a significant deployment of unicast reverse-path filtering
   in Internet environments as a counter-measure to source address
   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 a packet passed to ISP A would be
   discarded by such filters.  Various approaches have been proposed to
   modify the behaviour 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.

6.  Approaches to Endpoint Identity

   Both the approach of the addition of an identity protocol element and
   the approach of modification of an existing protocol element assume
   some form of exchange of information that allows both parties to the
   communication to be aware of the other party's endpoint identity and
   the associated mapping to locators.  There are a number of possible
   approaches for implementing this information exchange.

   The first such possible approach, termed here a "conventional"
   approach, encapsulates the protocol data unit (PDU) passed from the
   ULP with additional data elements that specifically refer to the
   function of the EIP.  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 remainder 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 tunnelling 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 and, in this
   case, is identity information.

   Another approach is to allow the EIP to communicate using a separate
   communications channel, where an EIP generates dedicated messages
   that are directed to its peer EIP, and it passes these PDUs to the
   LLP independently of the PDUs that are passed to the EIP from the

   ULP.  This allows an EIP to exchange information and synchronise
   state with the remote EIP semi-independently of the ULP protocol
   exchange.  As one 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 synchronised
   to the exchange of ULP PDUs, so that the remote EIP can correctly
   recognise the locator-to-endpoint mapping for each active session.

   Another potential approach here is to allow the endpoint-to-locator
   mappings to be held by a third party.  This model is already used for
   supporting the name-to-IP address mappings performed by the Domain
   Name System (DNS), 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 obtain this mapping as a result of a
   lookup for a DNS Endpoint-to-Locator set map contained as DNS
   Resource Records, for example.

6.1.  Endpoint Identity Structure

   The previous section has used the term "endpoint identity" without
   examining what form this identity may take.  A number of salient
   considerations regarding the structure and form of this identity
   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, since the suite of IP protocols and associated
   applications already manipulates 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".  It would also undertake a reverse mapping from
   a locator "address" to the distinguished endpoint identifier
   "address".  The IP address space is hierarchically structured,
   permitting a suitably efficient mapping to be performed in both
   directions.  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
   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 that 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 spaces
   to overlap, and to distinguish between the two realms by the context
   of usage rather than by a prefix comparison.  However, this reuse of
   the locator token space for 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 to 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 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
   for using 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 is that
   there is no default "address" associated with the endpoint
   identifier, as the FQDN can be used in the context of session
   establishment and a DNS query can be 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
   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 that
   is neither generated through the reuse of IPv6 address values nor
   drawn from the FQDN.  Given that the address space would need to be
   structured to permit its use as a lookup key to obtain the
   corresponding locator set, the obvious question 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.
   If such unstructured tokens are used in a limited context, then 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 it is 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.

6.2.  Persistent, Opportunistic, and Ephemeral Identities

   The considerations in the previous section highlight 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 in which this
   form of identity is required, in support of concurrent sessions as
   well as 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
   the identity is used as a lookup key to establish a set of locators
   that are associated with the identified endpoint.  It 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 strong 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.  Such mechanisms generally imply a significant
   overhead in terms of administrative tasks.

   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 ensured.  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 that are communicating, and the scope is 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, for example.  In this situation, there is no form of
   reference to 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 represent an approach to identity
   that minimises 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, within an
   application's data payload, or as a key 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 must use existing
   mechanisms for session startup, and the adoption of a session-based
   identity and associated locator switch agility becomes a negotiated
   session capability.

   On the other hand, 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.

   In the context of support for multi-homing configurations, use of
   ephemeral identities in the context of locator equivalence appears to
   represent a viable approach that allows a negotiated use of multiple
   locators within the context of communication between a pair of hosts
   in most contexts of multi-homing.  However, ephemeral identities
   offer little more in terms of functionality.  They cannot be used in
   referential contexts, cannot be used to initiate communications,
   provide limited means of support for various forms of mobility, and
   impose some constraints on the class of multi-homed scenarios that
   can be supported.  Ephemeral identities are generated in the context

   of an established communication state, and the implication in terms
   of multi-homing is that the two end points need to have discovered
   through existing mechanisms a viable pair of locators prior to
   generating an ephemeral identity binding.  The implication is that
   there is some form of static "home" for the end points that is
   discovered by conventional referential lookup.

   The use of a persistent identity space that supports dynamic
   translation between an equivalent set of locators and one or more
   equivalent identity values offers the potential for greater
   flexibility in applications.  Depending on how the mapping between
   identities and locators is managed, this may extend beyond
   multi-homing configuration to various contexts of nomadism and
   mobility as well as service-specific functions.  However, it remains
   an open question as to the nature of secure mapping mechanisms that
   would be needed in the more general context of identity-to-locator
   mapping, and it is also an open question as to how the mapping
   function would relate to viable endpoint-to-endpoint connectivity.
   It is a common aspect of identity realms that the most critical
   aspect of the realm is the nature of the resolution of the identity
   into some other attribute space.

   It appears reasonable to observe that, within certain constraints,
   multi-homing does not generically require the overhead of a fully
   distinct persistent identity space and the associated identity
   resolution functionality, and, if the nature of the multi-homing
   space in this context is to use a token to allow efficient detection
   of locator equivalence for session surviveability, then ephemeral
   identities appear to be an adequate mechanism.

6.3.  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 is, however, a set of considerations that appear to be common
   across all approaches.  They are examined in further detail in this

6.3.1.  Triggering Locator Switches

   Ultimately, regardless of the method of generation, a packet
   generated from a local multi-homed host to a remote host must carry a
   source locator when it 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 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, an objective is 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 any additional policy constraints that
   apply to the remote host and 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) and policy constraints that may
   determine a "best" path to the remote endpoint.  It may also be the
   case that the source address selection should be considered in
   relation to the destination locator selection.

   Another common issue is 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
      by transport-level signalling or by transport session timeouts,
      assuming an internal signalling 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 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 that the remote host is multi-homed
      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 reestablish the

   o  Link Layer Triggers

      Where supported, link layer triggers could be used as a direct and
      immediate signal of link availability, where a "Link Down"
      indication indicates the unavailability of a particular link
      [iab-link].  The limitation of this approach is that a link level
      indication is not a network broadcast event, and only the link's
      immediately-connected devices receive the link transition signal.
      While this approach may be relevant to the degenerate case of a
      multi-homed site composed of a single host, in the case of a
      multi-host site the link indication would need to be used by the
      site-exit router to generate one of the above indications for the
      host to be triggered for a locator change.  In this case this is a
      conventional form of router detection of link status.

   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.  The objectives for sensitivity to triggers may be
   very different depending on the transport session being used.  There
   is no doubt that any session would need a trigger to re-home if its
   path to the locator fails, but for some transports, moving, and
   triggering transport-related changes, may be far less desirable than
   reducing the sensitivity of the trigger and waiting to see if the
   triggering stimulus achieves a threshold level.

   This problem is only partly solved by models with an internal
   signalling mechanism between the transport stack element and the
   locator pool management stack element, because of non-failure

   triggers coming from other stacks, and because of transport issues
   such as use of resource reservation.  As an example, consider the
   case of a session with reservations established by RSVP or NSIS, when
   a routing change has just caused adaptive updates to the reservation
   state in a number of elements along its path.  The transport protocol
   using the path is likely to see some delays or timeouts, and its
   reaction to these events may be a trigger for a locator change, which
   is likely to mean another reservation update.  This chaining of
   reservation updates may represent a high overhead.  The implication
   here is that individual transport protocols may have to tune any
   feedback they give as a locator change trigger, so that they don't
   respond to certain forms of transient routing change delays (not
   knowing their cause) with a locator change trigger.  It should also
   be noted that different transport protocols have rather different
   behaviors and hooks for management.

6.3.2.  Locator Selection

   The selection of a locator to use for the remote end is obviously
   constrained by the current state of the topology of the network, and
   the primary objective of the selection process is to choose a viable
   locator that allows the packet to reach the intended destination
   point.  The selection of a source locator can be considered as an
   indication of preference to the remote end of a preferred locator to
   use for the local end.  However, where there are two or more viable
   locators that could be used, the selection of a particular locator
   may be influenced by a set of additional considerations.

   The selection of a particular locator from a viable locator set
   implies a selection of one particular network path in preference to
   other viable paths.  An implication of this host-based locator
   selection process is that path selection and, by inference, traffic
   engineering functions are not constrained to a network-based
   operation of path manipulation through adjustment of forwarding state
   within network elements.  There is a consequent interaction between
   the locator selection process and traffic engineering functions.  The
   use of an address selection policy table, as described in RFC 3484
   [RFC3484], is relevant to the selection process.

   The element that performs the locator selection, either as a protocol
   element within the host or as a selection undertaken at a site-exit
   router, also determines traffic policy, so the choice of using remote
   packet locator rewriting or host based locator selection shifts the
   policy capability from one element to the other.

   If hosts perform this policy determination, then a more fine-grained
   outcome may be achievable, particularly if the anticipated traffic
   characteristics of the application can be signalled to the locator

   selection process.  A further consideration appears to be that hosts
   may require additional information 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.  These considerations raise the broader
   issue of traffic engineering being a network function entirely
   independent of host function or an outcome of host interaction with
   the network.

   In the latter case, there is also the consideration of whether the
   host is to interact with the network, and, if so, how this
   interaction is to be signalled to hosts.

6.3.3.  Layering Identity

   The consideration of triggering locator switch highlights the
   observation that differing information and context are 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, and 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 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 layers.

   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 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 it 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.  This is a considerable constraint in the case of UDP,
   where the UDP transport protocol has no inherent notion of a session

   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
   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 signalling 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 signalling of when all
   flows to and from a particular destination have 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.  In constructing an
   identity element at this level of the protocol stack, it would appear
   necessary to ensure that an adequate amount of information is being
   passed between the transport protocol, internet protocol, and
   identity protocol elements, to ensure that the identity protocol
   element is not forced into making possibly inaccurate assumptions
   about the current state of active sessions or end-to-end network

   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.  It is also
   challenging to determine 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

   synchronising locator switches and IP packet fragmentation.
   Consideration of IPSec is also 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
   recognisable in the correct context.

6.3.4.  Session Startup and Maintenance

   The next issue is 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
   synchronised state between the two hosts.  Once the state is
   established, it is possible to update the initial locator set with
   the current set of useable locators.  This update could be part of
   the initial synchronisation actions, or deferred until required.

   This leads to the concept 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 EIP, 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

   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, 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, 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.  If the other end is also multi-homed
   (and therefore has multiple locators), then the active end may need
   to cycle through all possible destination locators for each source
   locator.  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.

6.3.5.  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
   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.

6.3.6.  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 reassigned 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 [RFC3041] provides a rationale for such a use of multiple

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

7.  Functional Decomposition of Multi-Homing Approaches

   The following sections provide a framework for the characterisation
   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.

7.1.  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 identity protocol element need to create a mapping from the
   upper-level protocol's local and remote identity tokens into an
   identity token that identifies the session?  If so, then 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 capabilities in its protocol
   stack?  If the remote end cannot, does the multi-homing capable
   protocol element report a session establishment failure to the
   upper-level protocol or silently fall back to a non-multi-homed
   protocol operation?

   How do the endpoints discover the locator set available for each
   other endpoint (locator discovery)?

   What mechanisms are used to perform locator selection at each end,
   for the local selection of source and destination locators?

   What form of mechanism is used to ensure that the selected site exit
   path matches the selected packet source locator?

7.2.  Re-homing Triggers

   What are common denominator goals of re-homing triggers?  What are
   the objectives that triggers conservatively should meet across all
   types of sessions?

   Are there transport session-specific triggers?  If so, then what
   state changes within the network path should be triggers for all
   transport sessions, and what state changes are triggers only for
   selected transport sessions?

   What triggers are used to identify that a switch of locators is

   Are the triggers based on the end-to-end transport session and/or on
   notification of state changes within the network path from the

   What triggers can be used to indicate the direction of the failed
   path in order to trigger the appropriate locator repair function?

7.3.  Re-homing Locator Pair Selection

   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

   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

7.4.  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 synchronisation of locator change
   and transport session behaviour?

7.5.  Removal of Session State

   How is identity/locator binding state removal synchronised with
   session closure?

   What binding information is cached for possible future use?

8.  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 [threats].

9.  Acknowledgements

   The author acknowledges the assistance from the following reviewers:
   Brian Carpenter, Kurtis Lundqvist, Erik Nordmark, Iljitsch van
   Beijnum, Marcelo Bagnulo, John Loughney, Thierry Ernst, Joe Touch,
   Michael Patton, Ted Hardie, and Allison Mankin.

10.  Informative References

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

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

   [RFC3582]  Abley, J., Black, B., and V. Gill, "Goals for IPv6
              Site-Multihoming Architectures", RFC 3582, August 2003.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [iab-link] Aboba, B., Ed., "Architectural Implications of Link Layer
              Indications", Work in Progress, January 2005.

   [e2e]      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/

   [rosec]    Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
              Nordmark, "Mobile IP version 6 Route Optimization Security
              Design Background", Work in Progress, October 2004.

   [thinks]   Lear, E., "Things MULTI6 Developers should think about",
              Work in Progress, January 2005.

   [threats]  Nordmark, E. and T. Li, "Threats relating to IPv6
              multi-homing solutions", Work in Progress, January 2005.

Author's Address

   Geoff Huston

   EMail: gih@apnic.net

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