rfc4177
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).
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.
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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
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1. Introduction
The objective of this analysis is to allow various technical
proposals relating to the support of multi-homing environment in IPv6
to be placed within an architectural taxonomy. This is intended to
allow these proposals to be classified and compared in a structured
fashion. It is also an objective of this exercise to identify common
aspects across all proposals within this domain of study, and also to
provide a framework that can allow exploration of some of the further
implications of various architectural extensions that are intended to
support multi-homing. The scope of this study is limited to the IPv6
protocol suite architecture, although reference is made to IPv4
approaches as required.
2. 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.
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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.
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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
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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
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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:
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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
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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.
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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
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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.
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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
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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
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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
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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
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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.
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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
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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.
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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
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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
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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
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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
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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
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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.
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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
heartbeat.
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
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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
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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
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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
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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
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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
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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
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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?
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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?
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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.
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[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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ERRATA
