Individual Submission G. Huston
Internet-Draft Telstra
Expires: October 31, 2004 May 2, 2004
Architectural Approaches to Multi-Homing for IPv6
draft-huston-multi6-architectures-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo provides an analysis of the aspects of multi-homing support
for the IPv6 protocol suite. The purpose of this analysis is to
provide a taxonomy for classification of various proposed approaches
to multi-homing. It is also an objective of this exercise to identify
common aspects of this domain of study, and also to provide a
framework that can allow exploration of some of the further
implications of various architectural extensions that are intended to
support multi-homing.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Multi-Homing Space . . . . . . . . . . . . . . . . . . . 3
3. Requirements and Considerations . . . . . . . . . . . . . . 5
4. Approaches to Multi-Homing . . . . . . . . . . . . . . . . . 6
4.1 Multi-Homing: Routing . . . . . . . . . . . . . . . . . . . 6
4.2 Multi-homing: Identity Considerations . . . . . . . . . . . 7
4.3 Multi-homing: Identity Protocol Element . . . . . . . . . . 9
4.4 Multi-homing: Modified Protocol Element . . . . . . . . . . 11
4.5 Modified Site-Exit and Host Behaviors . . . . . . . . . . . 11
4.6 Approaches to Endpoint Identity . . . . . . . . . . . . . . 13
4.7 Endpoint Identity Structure . . . . . . . . . . . . . . . . 13
5. Common Issues for Multi-Homing Approaches . . . . . . . . . 15
5.1 Triggering Locator Switches . . . . . . . . . . . . . . . . 15
5.2 Session Startup and Maintenance . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . 17
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
Normative References . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . 18
A. Notes on Various approaches . . . . . . . . . . . . . . . . 18
A.1 Host Identity Protocol (HIP) . . . . . . . . . . . . . . . . 18
A.2 Multihoming without IP Identifiers (NOID) . . . . . . . . . 19
A.3 Common Endpoint Locator Pools (CELP) . . . . . . . . . . . . 20
A.4 Weak Identifier Multihoming Protocol (WIMP) . . . . . . . . 21
A.5 Host-Centric IPv6 Multihoming . . . . . . . . . . . . . . . 22
A.6 Summaries of Selected ID/LOC Separation Documents . . . . . 23
A.6.1 New or Updated Documents Since IETF58 . . . . . . . . . . . 24
A.6.2 Older Documents that Remain Active/Interesting . . . . . . . 26
A.6.3 Related Multi-Homing drafts, Status unknown . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . 30
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1. Introduction
The objective of this exercise is to allow various technical
proposals relating to the support of multi-homing environment in IPv6
to be placed within an architectural taxonomy. This is intended to
allow these proposals to be classified and compared in a structured
fashion. It is also an objective of this exercise to identify common
aspects across all proposals within this domain of study, and also to
provide a framework that can allow exploration of some of the further
implications of various architectural extensions that are intended to
support multi-homing. The scope of this study is limited to the IPv6
protocol suite architecture, although reference is made to IPv4
approaches as required.
2. The Multi-Homing Space
The simplest formulation of the multi-homing environment is indicated
in Figure 1.
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+------+
|remote|
| host |
| R |
+------+
|
+ - - - - - - - - - - - +
| Internet Connectivity |
+ - - - - - - - - - - - +
/ \
+---------+ +---------+
| ISP A | | ISP B |
+---------+ +---------+
| Path a | Path b
+ - - - - - - - - - - - - - - - - - - - - +
| multi- | | |
homed +------+ +------+
| site | site | | site | |
| exit | | exit |
| |router| |router| |
| A | | B |
| +------+ +------+ |
| |
| local site connectivity |
|
| +-----------+ |
|multi-homed|
| | host | |
+-----------+
+ - - - - - - - - - - - - - - - - - - - - +
The Multi-Homed Domain
Figure 1
The environment of multi-homing is one that is intended to provide
sufficient support to local hosts so as to allow local hosts to
exchange IP packets with remote hosts, such that this exchange of
packets is to be seamlessly supported across dynamic changes in
connectivity. This implies that if a local multi-homed-aware host
establishes an application session with the remote host using "Path
a", and this path fails, the application session should be mapped
across to "Path b" without requiring any application-visible
re-establishment of the session. In other words, the application
session should not be required to be explicitly aware of underlying
path changes at the level of packet forwarding paths chosen by the
network.
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This simple multi-homing scenario also includes "site-exit' routers,
where the local site interfaces to the upstream Internet transit
providers. The nature of the interactions between the external
routing system and the site-exit routers, and interactions between
the site-exit routers and the local multi-homed host, and the
interactions between local connectivity forwarding and the local host
and site exit routers are not defined a priori in this scenario, as
they form part of the framework of interaction between the various
multi-homing components.
The major characteristic of this scenario is that the address space
used by, and advertised as reachable by, ISP A is distinct from the
address space used by ISP B.
This simple scenario is intended to illustrate the basic multi-homing
environment. Variations of this scenario include additional external
providers of transit connectivity to the local site, complex site
requirements and constraints, where the site may not interface
uniformly to all external transit providers, sequential rather than
simultaneous external transit reachability, communication with remote
multi-homed hosts, multi-way communications, use of host addresses in
a referential context (third party referrals) and the imposition of
policy constraints on path selection. However, the basic scenario is
sufficient to illustrate the major architectural aspects of support
for multi-homing, so this scenario will be used as the reference
model for this analysis.
3. Requirements and Considerations
RFC 3582 [RFC3582] documents some requirements that a multi-homing
approach should attempt to address. These requirements include:
o redundancy
o load sharing
o traffic engineering
o policy constraints
o simplicity of approach
o transport-layer survivability
o DNS compatibility
o packet filtering capability
o scaleability
o legacy compatibility
The reader is referred to [RFC3582] for a complete description of
each of these requirements.
In addition, work in progress
[draft-lear-multi6-things-to-think-about] documents further
considerations for IPv6 multi-homing. Again, the reader is referred
to this document for the detailed enumeration of these
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considerations. The general topic areas considered in this study
include:
o interaction with routing systems,
o aspects of a split between end-point-identifier and forwarding
locator,
o changes to packets on the wire, and
o the interaction between names, endpoints and the DNS.
In considering various approaches, further consideration also
include:
o the role of helpers and agents in the approach,
o modifications to host behaviors,
o the required trust model to support the interactions, and
o the nature of potential vulnerabilities in the approach.
4. Approaches to Multi-Homing
There appear to be four generic forms of architectural approaches to
this problem, namely:
o Routing
Use the IPv4 multi-homing approach
o New Protocol Element
Insertion of a new element in the protocol stack that manages a
persistent identity for the session
o Modify a Protocol Element
Modify the Transport or IP protocol stack element in the host in
order to support dynamic forwarding locator change
o Modified Site-Exit Router / Local Host interaction
Modify the site-exit router and local forwarding system to allow
various behaviors including source-based forwarding, site-exit
hand-offs, and address rewriting by site-exit routers
These approaches will be described in detail in the following
sections.
4.1 Multi-Homing: Routing
The approach used in IPv4 for multi-homing support is to preserve the
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
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providers, or from some third party provider not current directly
connected to the local site. Alternatively the address space may be a
distinct address block obtained by direct assignment from a Regional
Internet Registry as Provider Independent space. Each host within the
local site is uniquely addressed from the site's address prefix.
All transit providers for the site accept a prefix advertisement from
the multi-homed site, and advertise this prefix globally in the
inter-domain routing table. When connectivity between the local site
and an individual transit provider is lost, normal operation of the
routing protocol will ensure that the routing advertisement
corresponding to this particular path will be withdrawn from the
routing system, and those remote domain domains who had selected this
path as the best available will select another candidate path as the
best path. Upon restoration of the path, the path is re-advertised in
the inter-domain routing system. Remote domains will undertake a
further selection of the best path based on this re-advertised
reachability information. Neither the local or the remote host need
to have multiple addresses, nor undertake any form of address
selection. The path chosen for forward and reverse direction path
flows is a decision made by the routing system.
This approach generally meets all the criteria for multi-homing
approaches with one noteable 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 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 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.
4.2 Multi-homing: Identity Considerations
The intent of multi-homing in the IPv6 domain is to achieve a
comparable functional outcome for multi-homed sites without an
associated additional load being imposed on the routing system. The
overall intent of IPv6 is to provide a scaleable 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
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deployment and underlying technology capabilities. Within the
inter-domain routing space, the basic approach used in IPv4 and IPv6
is to attempt to align address deployment with network topology, so
that address aggregation can be used to create a structured hierarchy
of the routing space.
Within this constraint of topological-based address deployment and
provider aggregatable addressing architectures, the local site that
is connected to multiple providers is delegated addresses from each
of these providers' address blocks. In the example network in Figure
1, the local multi-homed host will concievably be addressed in two
ways: one using transit provider A's address prefix and the other
using transit provider B's address prefix.
If remote host R is to initiate a communication with the local
multi-homed host, it would normally query the DNS for an address for
the local host. In this context the DNS would return 2 addresses (One
using the A prefix and the other using the B prefix). The remote host
would select one of these addresses and send a packet to this
destination address. This would direct the pact 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 teh local multi-homed host hat 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, the ISPs A and B may be using reverse
unicast address filtering on source addresses of packets passed to
the ISP, as a means of prevention of source address spoofing. This
implies that if the multi-homed address selects a source address from
address prefix A, and the local routing to R selects a best path via
ISP B, then ISP B's ingress filters will discard the packet.
Within this addressing structure there is no form of routing-based
repair of certain network failues. If the link between the local site
and ISP A fails, there is no change in the route advertisements made
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by ISP A to its external routing peers. Even though the multi-homed
sitines to be reachable via ISP B, packets directed to the site using
ISP A's prefix will be discarded by ISP A as the destination is
unreachable. The implication here is that if the local host wishes to
maintain a session across such events it needs to communicate to
remote host R that it is possible to switch to using a destination
address for the multi-homed host that is based on ISP B' address
prefix.
In an aggregated routing environment multiple transit paths to a host
imply multiple address prefixes for the host, where each possible
transit path is identified by an address for the host. The
implication of this constraint on multi-homing is that paths being
passed to the local multi-homed site via transit provider ISP A must
use a forwarding-level destination IP address drawn from ISP A's
advertised address prefix set that maps to the multi-homed host.
Equally, packets being passed via the transit of ISP B must use a
destination address drawn from ISP B's address prefix set. The
further implication here is that path selection (ISP A vs ISP B
transit for incoming packets) is an outcome of the process of
selecting an address for the destination host.
The architectural consideration here is that in the conventional IP
protocol architecture the assumption is made that the transport-layer
endpoint identity is the same identity used by the internet-layer
forwarding layer, namely the IP address.
If multiple forwarding paths are to be supported for a single
transport session, and path selection is to be decoupled from the
functions of transport session initiation and maintenance, then the
corollary of this requirement in architectural terms appears to be
that some changes are required in the protocol architecture to
decouple the concepts of identification of the endpoint and
identification of the location and associated path selection for the
endpoint. This change in the protocol architecture would permit a
transport session to use an invariant endpoint identity value to
initiate and maintain a session, while allowing the forwarding layer
to dynamically change paths and associated endpoint locator
identities without impacting on the operation of the session, nor
would such a decoupled concept of identities and locators add any
incremental load to the inter-domain routing system.
Some generic approaches to this form of separation of endpoint
identity and locator value are described in the following sections.
4.3 Multi-homing: Identity Protocol Element
One approach to this objective is to add a new element into the model
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of the protocol stack.
The presentation to the upper level protocol stack element (ULP)
would use endpoint identifiers to uniquely identify both the local
stack and the remote stack. This will provide the ULP with stable
identifiers for the duration of the ULP session.
The presentation to the lower level protocol stack element (LLP)
would be of the form of a locator. This implies that the protocol
stack element would need to maintain a mapping of endpoint identifier
values to locator values. In a multi-homing context one of the
essential characteristics of this mapping is that it needs to be
dynamic, in that environmental triggers should be able to trigger a
change in mappings, which in turn would correspond to a change in the
paths (forward and/or reverse) used by the endpoints to traverse the
network. In this way the ULP session is defined by a peering of
endpoint identifiers that remain constant throughout the lifetime of
the ULP session, while the locators may change to maintain end-to-end
reachability for the session.
The operation of the new protocol stack element (termed here the
"endpoint identity protocol stack element", or "EIP") is to establish
a synchronized state with its remote counterpart. This would allow
the stack elements to exchange a set of locators that may be used
within the context of the session. A change in the local binding
between the current endpoint identity value and a locator will cause
a change in the source locator value used in the forwarding level
packet header. The actions of the remote EIP upon receipt of this
packet with the new locator is to firstly recognize this locator as
part of an existing session, and, upon some trigger condition, to
change its session view of the mapping of the remote endpoint
identity to the corresponding locator, and use this locator as the
destination locator in subsequent packets passed to the LLP.
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
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locator is undertaken within the EIP stack element.
4.4 Multi-homing: Modified Protocol Element
As an alternative to insertion of a new protocol stack element into
the protocol architecture, an alternative approach is to modify an
existing protocol stack element to include the functionality
performed by the EIP element. This modification could be undertaken
within the transport protocol stack element, or within the
internetworking stack element. The functional outcome from these
modifications would be to create a mechanism to support the use of
multiple locators within the context of a single endpoint-to-endpoint
session.
Within the transport layer, this functionality can be achieved, for
example, by the binding of a set of locators to a single session, and
then communicating this locator set to the remote transport entity.
This would allow the local transport entity to switch the mapping to
a different locator for either the local endpoint or the remote
endpoint while maintaining the integrity of the ULP session.
Within the IP level this functionality could be supported by a form
of dynamic rewriting of the packet header as it is processed by the
protocol element. Incoming packets with the source and destination
locators in the packet header are mapped to packets with the
equivalent endpoint identifiers in both fields, and the reverse
mapping is performed to outgoing packets passed from the transport
layer. Mechanisms that support direct rewriting of the packet header
are potential candidates in this approach, as are various forms of
packet header transformations of encapsulation, where the original
endpoint identifier packet header is preserved in the packet and an
outer level locator packet header is wrapped around the packet as it
is passed through the internetworking protocol stack element.
In all these scenarios, there are common issues of what state is
kept, by which part of the protocol stack, how state is maintained
with additions, removals of locator bindings, and does only one piece
of code have to be aware of the endpoint / locator split or do
multiple protocol elements have to be modified? For example, if the
functionality is added at the internetworking (IP) layer, there is no
context of an active transport session, so that removal of identity /
locator state information for terminated sessions needs to be
triggered by some additional mechanism from the transport layer to
the internetworking layer.
4.5 Modified Site-Exit and Host Behaviors
The above approaches all assume that the hosts are explicitly aware
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of the multi-homed environment and use modified protocol behavior to
support multi-homing functionality. A further approach to this
objective is to split this functionality across a number of network
elements and potentially perform packet header rewriting from a
persistent endpoint identity value to a locator value at a remote
point.
One possible approach proposes the use of site-exit routers to
perform some form of packet header manipulation as packets are passed
out from the local multi-homed site to a particular transit provider.
The local site routing system will select the best path to a
destination host based on the remote hosts's locator value. The local
host will write its endpoint identity as the source address of the
packet. When the packet reaches a site-exit router, the site-exit
router will rewrite the source field of the packet to a corresponding
locator that selects a reverse path through the same transit ISP when
the locator is used as a destination locator by the remote host. In
order to preserve session integrity there is a need for a
corresponding reverse transformation to be undertaken on incoming
packets, where the destination locator has to be mapped back to the
host's endpoint identifier. There are a number of considerations
whether this is best performed at the site exit router on packet
ingress to the site, or by the local host.
Packet header rewriting by remote network elements has a large number
of associated considerations, and documentation relating to the
considerations of the use of Network Address Translators ([NAT
Considerations] contains much of this material.
An alternative for packet header rewriting on site exit is for the
host to undertake the endpoint-to-locator mapping, using one of the
approaches outlined above. The consideration here is that there is
some significant deployment of unicast reverse path filtering in
Internet environments as a counter-measure to source address
spoofing. Using the example in Figure 1, if a host selects a locator
drawn from the ISP B address prefix, and local routing directs that
packet to site-exit router A, then if the packet is passed to ISP A,
the this would be discarded by such filters. Various approaches have
been proposed to modify the behavior of the site forwarding
environment all with the end effect that packets using a source
locator drawn from the ISP B address prefix are passed to site-exit
router B. These approaches include forms of source address routing
and site-exit router hand-over mechanisms, as well as augmentation of
the routing information between site-exit routers and local
multi-homed hosts, so that the choice of locator by the local host
for the remote host is consistent with the current local routing
state for the local site to reach the remote host.
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4.6 Approaches to Endpoint Identity
Both of the above mechanisms assume some form of exchange of
information that allows both parties to the communication to be aware
of the remote endpoint identity and the associated mapping to
locators. There are a number of choices in terms of the way in which
this information exchange can be implemented.
The first such possible approach is termed here a 'conventional'
approach, where the mode of operation is in terms of encapsulating
the protocol data unit (PDU) passed from the ULP with additional data
elements that specifically refer to the function of the endpoint
identity protocol stack element. The compound data element is passed
to the LLP as its PDU. The corresponding actions on receipt of a PDU
from a LLP is to extract the fields of the data unit that correspond
to the EIP function, and pass the reminder of the PSU 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.
Another approach is to allow the EIP to communicate using a separate
communications channel, where the EIP generates dedicated messages
that are directed to its peer EIP, and passes these PDUs to the LLP
independently of the PDUs that are passed top the EIP from the ULP.
This allows the EIP to exchange information and synchronize state
with the remote EIP semi-independently of the ULP protocol exchange.
As a part of the EIP function is to transform the ULP PDU to include
locator information there is an associated requirement to ensure that
the EIP peering state remains synchronized to the exchange of ULP
PDUs, so that the remote EIP can correctly recognize the locator to
endpoint mapping for each active session.
Another potential approach here is to allow the endpoint to locator
mappings to be held at a third party point. This model is already
used for supporting the name to IP address mappings performed by the
Domain Name system, where the mapping is obtained by reference to a
third party, namely a DNS resolver. A similar form of third party
mapping between endpoints and a locator set could be supported
through the use of the DNS, or a similar third party referential
mechanism. Rather than have each party exchange endpoint to locator
mappings, this approach would see this mapping being obtained as a
result of a lookup for a DNS Endpoint to Locator set map contained as
DNS Resource Records, for example.
4.7 Endpoint Identity Structure
The previous section has used the term "endpoint identity" without
examining what form this identity may take. There are a number of
salient considerations regarding the structure and form of this
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identity that should be enumerated within an architectural overview
of this space.
One possible form of an identity is the use of identity tokens lifted
from the underlying protocol's "address space". In other words an
endpoint identity is a special case instance of an IPv6 protocol
address. There are a number of advantages in using this form of
endpoint identity, observing that the suite of IP protocols and
associated applications already manipulate IP addresses. The
essential difference in a domain that distinguishes between endpoint
identity and locator is that the endpoint identity parts of the
protocol would operate on those addresses that assume the role of
endpoint identities, and the EIP mapping function would undertake a
mapping from an endpoint "address" to a set of potential locator
"addresses", and also undertake a reverse mapping from a locator
"address" to the distinguished endpoint identifier "address". The
address space is hierarchically structured, permitting a suitably
efficient mapping to be performed in both directions, and the
underlying semantics of addresses in the context of public networking
includes the necessary considerations of global uniqueness of
endpoint identity token values.
It is possible to take this approach further and allow the endpoint
identifier to also be a valid locator. This would impliy 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
prefix, 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
address values which are not part of the global unicast locator
space, allowing applications and protocol elements to distinguish
between endpoint identity values and locators based on address prefix
value. It is also possible to allow the endpoint identity and locator
space to overlap, and distinguish between the two identity realms by
the context of usage rather than by a prefix comparison.
It is also feasible to use the fully qualified domain name (FQDN) as
an endpoint identity, undertaking a similar mapping as described
above, using the FQDN as the lookup "key". The implication here is
that there is no default 'address' that is to be associated with the
endpoint identifier.
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.
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It is also an option to consider a new structured identity space
which is not generated through the reuse of IPv6 address values nor
drawn from the FQDN. Given that the address space would need to be
structured in such a fashion that permits it to be used as a lookup
key to obtain the corresponding locator set, the obvious question in
such an option is what additional or altered characteristics would be
used in such an endpoint identity space that would distinguish it
from either of the above approaches?
Instead of structured tokens that double as lookup keys to obtain
mappings from endpoint identities to locator sets, the alternative is
to use an unstructured token space, where individual token values are
drawn opportunistically for use within a multi-homed session context.
Here the semantics of the endpoint identity are subtly changed. The
endpoint identity is not a persistent alias or reference to the
identity of the endpoint, but a means to allow an EIP to confirm that
two locators are part of the same mapped locator set for an endpoint.
In this context the unstructured opportunistic endpoint identifier
values are used in determining locator equivalence rather than in
some form of lookup function.
5. Common Issues for Multi-Homing Approaches
The above overview encompasses a very wide range of potential
approaches to multi-homing, and each particular approach necessarily
has an associated set of considerations regarding its applicability.
There are, however, a set of considerations that appear to be common
across all approaches, and they are examined in further detail in
this section.
5.1 Triggering Locator Switches
Ultimately, regardless of the method of generation, a packet
generated from a local multi-homed host to a remote host must have a
source locator in the IP packet that is passed into the transit
network. In a multi-homed situation the local multi-homed host has a
number of self-referential locators that are equivalent aliases in
almost every respect. The difference between locators is the
inference that at the remote end the choice of locator may determine
the path used to send a packet back to the local multi-homed host.
The issue here is how does the local host make a selection of the
"best" source locator to use? Obviously the parameters of this
selection include the objective to select a locator that represents a
currently viable path from the remote host to the local multi-homed
host. Local routing information for the multi-homed host does not
include this reverse path information. Equally, the local host does
not necessarily know of any additional policy constraints that apply
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to the remote host that may result in a remote host's preference to
use one locator over another for the local host. Considerations of
unicast reverse path forwarding filters also indicate that the
selection of a source locator should result in the packet being
passed to a site-exit router that is connected to the associated ISP
transit provider, and that the site-exit router passes the packet to
the associated ISP.
If the local multi-homed host is communicating with a remote
multi-homed host, the local host may have some discretion in the
choice of a destination locator. The considerations relating to the
selection of a destination locator include considerations of local
routing state (to ensure that the chosen destination locator reflects
a viable path to the remote endpoint), policy constraints that may
determine a "best" path to the remote endpoint. In such situations it
may also be the case that the source address selection should also be
considered in relation to the destination locator selection.
Another common issue is the consideration of the point when a locator
is not considered to be viable, and the consequences to the transport
session state.
o Transport Layer Triggers
A change in state for a currently used path to another path could
be triggered by indications of packet loss along the current path,
or by transport session timeouts, assuming an internal signalling
mechanism between the transport stack element and the locator pool
management stack element.
o Routing Triggers
Alternatively, in the absense of local transport triggers, the
site exit router could communicate failure of the outbound
forwarding path in the case where the remote host is multi-homed
with an associated locator set. Conventional routing would be
incapable of detecting a failure in the inbound forwarding path,
so there are some limitations in the approach of using routing
triggers to change locator bindings.
o Heartbeat Triggers
An alternative to these approaches is the use of a session
heartbeat protocol, where failure of the heartbeat would cause the
session to seek a new locator binding that would re-establish the
heartbeat.
The sensitivity of the locator-switch trigger is a consideration
here. A very fine-grained sensitivity of the locator switch trigger
may generate false triggers arising from short-term transient path
congestion, while coarse-grained triggers may impose an undue
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performance penalty on the session due to an extended time to detect
a path failure.
5.2 Session Startup and Maintenance
The next issue if that of the difference between the initial session
startup mode of operation and the maintenance of the session state.
In a split endpoint identifier / locator environment there needs to
be at least one initial locator associated with an endpoint
identifier in order to establish an initial connection between the
two hosts. This locator could be loaded into the DNS in a
conventional fashion, or, if the endpoint identifier is a
distinguished address value, the initial communication could be
established using the endpoint identifier in the role of a locator
(i.e. using this as a conventional address).
The initial actions in establishing a session would be simular. If
the session is based on specification of a FQDN, the FQDN is first
mapped to an endpoint identity value, and this endpoint identity
value could then be mapped to a locator set. The locators in this set
are then candidate locators for use in establishing an initial
synchronized state between the two hosts. Once the state is
established it is then possible to update the initial locator set
with the current set of useable locators. This update could be part
of the initial synchronization actions, or deferred until required.
This leads to the concept of the use of a 'distinguished' locator
that acts as the endpoint identifier, and a pool of alternative
locators that are associated with this 'home' locator. This
association may be statically defined, using referential pointers in
a third party referral structure (such as the DNS), or dynmically
added to the session through the actions of the endpoint identity
protocol stack element, or both.
6. 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 draft, but to provide a distinct document that describes the
security considerations of this domain. Subsequent revisions of this
draft will refer the reader to this yet-to-be-drafted document.
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7. Acknowledgements
The author acknowledges the exensive contribution of Margaret
Wasserman in preparing the original draft of the summary of current
approaches to multi-homing.
Normative References
Author's Address
Geoff Huston
Telstra
Appendix A. Notes on Various approaches
These notes were orginally drafted by Margaret Wasserman. The notes
on various approaches are non-exclusive, i.e. solutions not reviewed
or mentioned here are not ruled out of discussion. Also the review
comments are not comprehensive, and the selection reflects the time
constraints of the contributors to this section than any qualititive
judgement on any of the approaches. The author is desirous, in future
revisions of this draft, in augmenting this selection of reviewed
approaches.
A.1 Host Identity Protocol (HIP)
HIP is an ID/Locator separation mechanism intended to solve a much
wider problem space than site multi-homing. HIP uses cryptographic
identifiers termed Host Identity Tags (HITs) at the application
layer, which are mapped to locators (IP Addresses) by a HIP protocol
stack layer that interfaces between the transport layer and IP
internetwork layer.
The essential characteristic of HIP is it use of opportunistic
identity generation, as it uses a cyptographic host identifier as the
basis of the persistent identity. The transport session cab be agile
across locators, or even across IP protocol versions, as the HIT is
used to determine session integrity. allowing the hosts to determine
what packets legitimately form part of the session.
HIP is proposed as a new protocol element, located at layer 3.5 (i.e.
above the internetwork IP layer and below the transport layer). The
presentation to the transport layer uses 128 bit hash values (the
HIT) in place of IP addresses, while the presentation to the internet
layer uses conventional IP addresses.
Being opportunistic and unstructured, the HIT space is not an
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efficient search space, nor can a HIT be used as a unique search key.
HITs are part of an an equivalence function, to allow each host to
determine that an incoming packet is part of an established session.
HITs cannot be used as an identity value in a conventional referral
sense (HostA wants to tell HostB to talk to HostC). While an
application could pass a HIT to a third-party (and legacy
applications would unknowingly do so), the third party would have no
way to map that HIT to a locator (an IP address) as HIP does not
include any global HIT->Locator mapping mechanism.
Summary:
o New Protocol Stack Element
o Layer 3.5 (Above IP, below Transport
o Unstructured, opportunistic identity values (non-referential)
o DNS rendezvous
o No Locator exchange protocol
Current IETF Documents:
o draft-moskowitz-hip-arch
o draft-moskowitz-hip
o draft-nikander-hip-mm
o draft-nikander-esp-beet-mode
A.2 Multihoming without IP Identifiers (NOID)
NOID proposes an approach for endpoint identifier and locator
separation where the endpoint identifier space is drawn from the
locator space. Instead of creating a new namespace for endpoint
identifiers, the endpoint identifier may be chosen from the set of
locators that can be used to reach a given endpoint. Until an event
occurs that modifies the list of usable locators, the initial
endpoint identifier value can serve as a locator.
NOID uses next-header values in the IPv6 header to indicate whether a
given packet should be processed by the NOID layer. At a conceptual
level, NOID adds a layer to the middle of IP above most IP
processing, but below IPSec, fragmentation and reassembly functions.
NOID makes use of the global DNS as a mapping system between IDs and
Locators. A node who wishes to communicate with another node can use
the FQDN to get a list of possible locators (IP Addresses). That node
will then choose one of the locators to use as an Application-level
ID (AID).
NOID offers some support for application referrals. If Host A passes
an AID to Host B that is supposed to point to Host C, Host B should
be able to do a reverse DNS lookup to map the AID to an FQDN and then
use the FQDN to get the complete set of locators. However, for this
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to be effective, nodes would need to have both forward and reverse
DNS entries. There might also be a need to dynamically update the DNS
as a node becomes reachable or unreachable at certain locators.
Summary:
o New Protocol Stack Element
o Layer 3 (Inserted in the upper part of IP, below IPSEC and
fragementation / reassembly
o Identity values based on locator set
o DNS rendezvous
o Identity peering protocol
Current IETF Documents:
o draft-nordmark-multi6-noid
o draft-templin-isnoid
A.3 Common Endpoint Locator Pools (CELP)
CELP explores the concept of sharing information about locator
reachability between transport-layer "multi-addressing" mechanisms
(such as SCTP and DCCP) and Internet-layer multiaddressing
mechanisms, referred to in the draft as "wedge-layer approaches"
(such as NOID). (This concept was originally discussed on the MULTI6
mailing list under the name 'SLAP'.)
The motivation behind CELP is that muliple multiaddressing mechanisms
may be used (by different applications or for different connections)
on a single endpoint, and that it would beneficial for those
mechanisms to share information about the reachability of the IP
addresses in a given locator pool. If a transport-layer mechansim,
such as SCTP, could share its knowledge regarding the reachability of
a certain locator, it might be possible to minimize or elimate
Internet-layer control packets that are used to maintain that
information at the Internet layer. In some ways, this is similar to
IPv6 Neighbor Discovery's use of upper layer advice regarding
neighbor reachability to avoid sending unncecessary ND packets.
This document offers a definition of the term "endpoint" that refers
to a locator pool that may have a smaller scope than an entire IP
node (i.e. a given locator pool may only contain a subset of the
locators available on an IP node).
The CELP document is more of a consideration of approach than an
actual proposal for a solution. It doesn't specify in detail how it
would work with any particular transport-layer or Internet-layer
multiaddressing mechanisms. However, it is an approach that could be
applied to many combinations of solutions.
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Summary:
o Considerations relating to sharing locator reachability
information across session instances.
Current IETF Documents:
o draft-crocker-celp
A.4 Weak Identifier Multihoming Protocol (WIMP)
WIMP is an endpoint identifier / locator separation protocol that is
heavily focused on mitigating the threats outlined in work in
progress on security threats in multi-homing scenarios
[draft-nordmark-multi6-threats-00.txt]. The WIMP approach uses
divided secrets and hash chaining to ensure that new locators are
supplied by the same node that supplied the original locator.
WIMP uses a separate name space for 128-bit non-routable IDs that are
never used in packets on the network. These IDs are locally generated
for both local and remote nodes by hashing a nonce (for the
initiator's endpoint identity) or the FQDN (for the responder's
endpoint identity). (The approach assumes a requirement that all
responders will have a FQDN.)
The WIMP protocol introduces a WIMP layer that maps between IDs and
locators based on internal state. The WIMP layer is conceptually
located within the network layer, above most IP processing and below
IPsec, fragmentation/reassembly and destination options, similar to
NOID.
Communication between two end-points requires establishment of a WIMP
session. Once the session is established, it can be used for multiple
simultaneous or sequential connections to the same end-point. During
WIMP session establishment, WIMP introduces a separate header into
the data packets, between the IP and TCP/UDP headers that contains
information about the WIMP session. The WIMP session establishment
packets can optionally be piggy-backed on data packets. WIMP does not
introduce a separate header into all IPv6 packets. Instead, once a
WIMP session is established, the IPv6 FlowID is used to hold an
identifier for the WIMP host-pair context associated with a given
packet.
WIMP is intended to provide a solution to some of the security
concerns, particularly regarding connection hijacking, that have been
raised for some other endpoint identity / locator separation
mechanisms.
Reviewers of WIMP have raised some questions of this approach,
particularly concerning the use of an optional header while operating
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below IP fragmentation. The piggy-backing mechanism requires that the
packets not be fragmented, but it doesn't explain how upper layers
will become aware of the MTU limitations on those packets and/or how
this mechanism would interact with Path MTU discovery. Like HIP, WIMP
makes no provision to handle application-level referrals and does not
contain a mechanism for global endpoint identifier to locator
mapping. It has also been pointed out that it is interesting to
consider whether the WIMP approach to security, hash chaining, could
be applied to other endpoint identity / locator separations
mechanisms, such as NOID.
Summary:
o New Protocol Stack Element
o Layer 3 (Inserted in the upper part of IP, below IPSEC and
fragementation / reassembly
o Identity values based on hash of FQDN
o Identity peering protocol
Current IETF Documents:
o draft-ylitalo-multi6-wimp
A.5 Host-Centric IPv6 Multihoming
Host-Centric Multihoming is, in some ways, the simplest way to
address the IPv6 site multihoming problem. The concept is that every
host in the multihomed site is configured with multiple prefixes that
correspond to different service providers. Each host configures
addresses within those prefixes and selects among those addresses
when connecting to a remote host. This configuration is automated
using Router Renumbering and IPv6 Address Autoconfiguration. However,
this simple solution Layer 3 (inserted in the upper part of IP, below
IPSEC and fragementation / reassembly has several practical
limitations and drawbacks, and this draft attempt to address them.
In particular, the Host-Centric Multihoming proposal attempts to
address the "site exit issue". Hosts cannot control the exit path
that their packets will take from the local site, so hosts with
multiple addresses may use a source IP address from one ISP on
packets that end-up being routed through a different ISP. In many
cases, the ISPs will run ingress filtering and will discard those
packets.
One solution to the site exit problem is to changes the ISP ingress
filters to accept all of the source address prefixes that are used
within the site. Other approaches are to perform source-based routing
within the site, to deploy a single site-exit router or to structure
the network so that all exit routers are connected to a single DMZ
network that employs source-based routing. A virtual DMZ can be
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constructed by configuring a mesh of tunnels between all exit
routers, tunneling packets to the correct exit router based on source
address. Each of these solutions has operational drawbacks and/or
introduces inefficiencies.
This proposal suggests another solution to the site exit problem
called "source address discovery". Based on Path MTU discovery, this
mechanism involves adding extra information to the ICMP Destination
Unreachable message that the packet was discarded due to an ingress
filter. This extra information indicates what address prefix should
be used to pass the ingress filter. Rather than adding a field to the
ICMP message, this extra information is communicated via the source
address that the route Layer 3 (Inserted in the upper part of IP,
below IPSEC and fragementation / reassembly).
It also proposes a "superior" alternative called "exit router
discovery", which allows hosts to specify which exit router will be
used for each packet. Instead of sending ICMP error messages when
ingress filtering causes packets to be discarded, the exit router
will send the equivalent of a redirect message and future packets
with the same source/destination address pair will be tunneled to the
indicated exit router. This mechanism involves tunneling to a
site-exit anycast address that is derived from the sites' prefixes.
The draft primary focuses on the specification of this "superior"
approach, largely ignoring some pertinent questions such as: Will
residential and enterprise-level IPv6 routers reall support anycast
routing?
One important thing to note about the host-centric multihoming
solution is that it doesn't appear to provide any ability for
transport connections to survive a change in the topology that causes
a host to become unreachable at an address that is currently used as
a connection end-point. It also does not offer any support for legacy
applications that do application-level referrals, requiring that a
full set of locators be exchanged as part of the referral.
A.6 Summaries of Selected ID/LOC Separation Documents
This section summarizes a set of selected ID/Loc separation
documents. The selection includes documents that appear to be active,
and this section provides a very short summary of each one. The first
sub-section lists documents that are new or updated since IETF 58 and
the second sub-section lists older documents that remain active. The
documents in each sub-section are listed alphabetically by draft
filename.
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A.6.1 New or Updated Documents Since IETF58
o TLC-FM: Transport Layer Common Framework for Multihoming
draft-arifumi-multi6-tlc-fm
This draft proposes a transport-layer mechanism for ID /Locator
mapping. There is a conceptual layer within the transport layer
that provides support for common multihoming functions. It is
compatible with the use of Mobile IPv6 (MIP6) to provide
mobility support.
In TLC-FM, like SCTP, the ID consists of a collection of
locators that may be used to reach a given host. It employs
transport-level clues (such as TCP retransmissions) to decide
when to switch locators. For UDP connections, ICMP error
messages or application-level hints are necessary.
This mechanism is not well enough specified to fully evaluate
it, but it doesn't appear to offer any support for
application-level referrals.
o Multi-Homing Tunnel Broker (MHTB)
draft-bagnulo-multi6-mhtb
This document defines an enhancement to RFC 3178, IPv6
Multihoming Support at Site Exit Routers, to reduce the
administrative overhead of maintaining a configured tunnel for
each multihoming association. However, this draft does not
address another major drawback of the RFC 3178 approach, that
it does not protect against the complete failure of one or more
connected ISPs.
o Framework for Common Endpoint Locator Pools (CELP)
draft-crocker-celp
Dave Crocker and Avri Doria's CELP draft, reviewed in the
previous section.
o Multi-Homing: the SCTP Solution
draft-coene-multi6-sctp
One confusing question about the direction of this work is why
SCTP is being discussed as a "solution" to site multihoming,
when a clear requirement for a site multihoming solutions is
the ability to support existing TCP-based and UDP-based
applications. This document isn't really a proposal, though --
it consists of answers to the questions posted in Eliot Lear's
"Things MULTI6 Developers Should Think About" draft, and does
not discuss how SCTP does (or doesn't) address the requirements
outlined in the Multi6 requirements RFC.
An interesting thing about this proposal is that it claims that
SCTP is not an ID/Loc separation mechanism, however in some
academic sense it actually is. The ID is the group of available
IP addresses, and the locator is whichever address is currently
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being used for communication. SCTP also experiences the same
complexities as other proposals (AKA NOID, CELP) that use a
pool of locators as the ID -- How do you choose which locator
to use? And how do you detect when a member of the pool becomes
invalid for use as a locator? So, while it isn't actually a
solution for site multihoming, SCTP may provide some useful
experiences and mechanisms that may apply to a class of
possible solutions.
o Host Identity Protocol (HIP) Rendezvous Mechanisms
draft-eggert-hip-rendezvous-00.txt
This is an overview draft that discusses the concept of HIP
rendezvous mechanisms to improve the applicability of HIP for
mobility and multihoming. This is a survey document that
outlines the problem and discusses different type of solutions
to the problem.
o Host-Centric IPv6 Multihoming
draft-huitema-multi6-hosts
Draft by Christian Huitema and others, described above.
o Things MULTI6 Developers Should Think About
draft-lear-multi6-things-to-think-about
Eliot Lear's efforts to collect a set of practical questions
that should be considered for all MULTI6 protocols.
o Host Identity Protocol (HIP)
draft-moskowitz-hip
This is the base protocol specification for HIP. Along with the
HIP architecture, these documents form the basis of the HIP
work.
o Consideration on HIP Based IPv6 Multi-Homing
draft-nikander-multi6-hip
Pekka Nikander's document that submits HIP as a solution for
the MULTI6 problem space.
o 8+8 Addressing for IPv6 End to End Multihoming
draft-ohta-multi6-8plus8
o Threats Relating to Transport Layer Protocols Handling Multiple
Addresses
draft-ohta-multi6-threats
o Multihoming Using IPv6 Addressing Derived from AS Numbers
draft-savola-multi6-asn-pi
This draft provides a mechanism for organizations that have
been assigned a 16-bit AS number to use that number to
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auto-generate a globally routable, provider-independent address
prefix.
o Problem Statement: HIP Operation over Network Address Translators
draft-stiemerling-hip-nat
Summarizes the problems with running HIP and IPsec-based data
transmission across NATs.
o Operational Approach to Achieve IPv6 Multihomed Network
draft-toyama-multi6-operational-site-multihoming
This document proposes to support site multihoming in IPv6 by
assigning additional /32 prefixes and AS numbers to "groups" of
providers who will provide multihomed /48 prefixes to their
mutual customers.
It is currently unclear to the reviewer how/if this proposal
would work and/or scale since it seems to involve two different
providers advertising the same /32 and the same AS number into
the default free zone. It requires some type of peering "to
share prefix assignments" between ISPs, and the diagram shows
some type of connection between the ISPs, but I don't know what
the details of that connection are.
It also has the potential to more quickly exhaust the AS number
space and to result in a substantially larger number of routes
in default free routers, since the number of "groups" could
scale exponentially with the number of providers.
o Crypto Based Host Identifiers (CBHI)
draft-van-beijnum-multi6-cbhi
This draft defines a crytographic mechanism for generating host
identifiers. It is intended for use with other protocols that
require host identifiers, such as ODT (see below).
o On Demand Tunneling for Multihoming (ODT)
draft-van-beijnum-multi6-odt
This draft discusses an automatic tunnelling-based solution for
multihoming.
o Weak Identifier Multihoming Protocol (WIMP)
draft-ylitalo-multi6-wimp
WIMP proposal, described above.
A.6.2 Older Documents that Remain Active/Interesting
o RFC 3582: Goals for IPv6 Site-Multihoming Architectures
o Choices for Multiaddressing
draft-crocker-mast-analysis
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o What's In a Name: Thoughts from the NSRG
draft-irtf-nsrg-report
o A Roadmap for Multihoming in IPv6
draft-kurtis-multi6-roadmap
o Host Identity Protocol (HIP) Architecture
draft-moskowitz-hip-arch-05.txt
o End-Host Mobility and Multi-Homing with Host Identity Protocol
(HIP)
draft-nikander-hip-mm
o Threats Relating to IPv6 Multihoming Solutions
draft-nordmark-multi6-threats-00.txt
o Multihoming without IP Identifiers (NOID)
draft-nordmark-noid
Erik Nordmark's NOID specification, described above.
A.6.3 Related Multi-Homing drafts, Status unknown
This is a list of ID/Loc separation and/or MULTI6 documents, listed
alphabetically by draft name.
o Extension Header for Site-Multi-homing Support
draft-bagnulo-multi6-mhexthdr
o Application of the MIPv6 Protocol to the Multi-Homing Problem
draft-bagnulo-multi6-mnm
o Multiple Address Service for Transport (MAST): An Extended
Proposal
draft-crocker-mast-proposal
o NAROS : Host-Centric IPv6 Multihoming with Traffic Engineering
draft-de-launois-multi6-naros
o Application and Use of the IPv6 Provider Independent Global
Unicast Format
draft-hain-ipv6-pi-addr-use
o Simple Dual Homing Experiment
draft-huitema-multi6-experiment-00.txt
o Host-Centric IPv6 Multihoming
draft-huitema-multi6-hosts
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o IPv4 Multihoming
draft-ietf-multi6-v4-multihoming
This documents how multi-homing is supported at present in the
IPv4 protocol domain.
o Multihoming in IPv6 by Multiple Announcement of Longer Prefixes
draft-kurtis-multihoming-longprefix
o Multihoming using 64-bit Crypto-based IDs
draft-nordmark-multi6-cb64
o Strong Identity Multihoming using 128-bit Identifiers (SIM/
CBID128)
draft-nordmark-multi6-sim
o IPv6 Address Assignment and Route Selection for End-to-End
Multihoming
draft-ohira-assign-select-e2e-multihome
o Hierarchical IPv6 Subnet ID Autoconfiguration for Multi-Address
Model Multi-Link Multihoming Site
draft-ohira-multi6-multilink-auto-prefix-assign
o Hierarchical IPv6 Subnet ID Autoconfiguration for Multi-Address
Model Multi-Link Multihoming Site
draft-ohira-multi6-multilink-auto-prefix-assign
o The Architecture of End to End Multihoming
draft-ohta-e2e-multihoming-05.txt
o 8+8 Addressing for IPv6 End to End Multihoming
draft-ohta-multi6-8plus8-00.txt
o Threats Relating to Transport Layer Protocols Handling Multiple
Addresses
draft-ohta-multi6-threats-00.txt
o Multihomed ISPs and Policy Control
draft-ohta-multihomed-isps-00.txt
o GAPI: A Geographically Aggregatable Provider Independent Address
Space to Support Multihoming in IPv6
draft-py-multi6-gapi
o Multi Homing Translation Protocol (MHTP
draft-py-multi6-mhtp-01.txt
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o Multihoming Using IPv6 Addressing Derived from AS Numbers
draft-savola-multi6-asn-pi-01.txt
o IPv6 Site Multihoming: Now What?
draft-savola-multi6-nowwhat
o Operation of NOID Multihoming Protocol on ISATAP Nodes
draft-templin-isnoid
o LIN6: A Solution to Multihoming and Mobility in IPv6
draft-teraoka-multi6-lin6
o Operational Approach to achieve IPv6 multihomed network
draft-toyama-multi6-operational-site-multihoming-00.txt
o Two Prefixes in One Address
draft-van-beijnum-multi6-2pi1a-00.txt
o Crypto Based Host Identifiers
draft-van-beijnum-multi6-cbhi-00.txt
o Provider-Internal Aggregation based on Geography to Support
Multihoming in IPv6
draft-van-beijnum-multi6-isp-int-aggr-01.txt
o On Demand Tunneling For Multihoming
draft-van-beijnum-multi6-odt-00.txt
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Internet-Draft Multi6 Architectures May 2004
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Internet-Draft Multi6 Architectures May 2004
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