Network Working Group
Request for Comments: 4177
Category: Informational

G. Huston
APNIC
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

Abstract

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.

   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 address.

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

Endpoint: 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 protocol.

Locator: 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 services.

Re-homing: The transition of a site between two states of connectedness, due to a change in the connectivity between the site and its transit providers.

Site: 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 protocol.

3. The Multi-Homing Space

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

                           +------+
                           |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    |                 |
                     +-----------+
         + - - - - - - - - - - - - - - - - - - - - +
         
              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
         locator,
      *  changes to packets on the wire, and
      *  the interaction between names, endpoints, and the DNS.

In evaluating various approaches, further considerations also include:

* 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:

Routing: Use the IPv4 multi-homing approach

Mobility: 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 sections.

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 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; 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 minutes.

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 point.

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 section.

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.

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.

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.

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.

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 heartbeat.

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 state.

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 paths.

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 session.

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 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 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 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?

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 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?

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/
              publications/endtoend/endtoend.txt>.
   
   [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
APNIC

EMail:

          gih@apnic.net

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