Internet Engineering Task Force Hamid Syed
Internet Draft Gary Kenward
draft-hamid-seamoby-ct-reqs-01.txt Nortel Networks
Expires: September 2001 March, 2001
General Requirements for a Context Transfer Framework
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Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document captures the set of general requirements for context
transfer. These requirements are provided for the replication and
synchronization of the context associated with a mobile node's
traffic between access routers.
1 Introduction
In networks where hosts are mobile, the success of real-time
sensitive services like VoIP telephony, video, etc. depends
heavily on the ability of the network to support seamless handover.
Ideally, seamless means that the handoff will not introduce any
degradation in the quality of the service provided to the user. At
the very least, the user should not perceive any degradation in
service quality during handoff.
The service quality offered at an access router is embodied in the
context of the support provided to the IP traffic. The ability of
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a new access router to support the same service quality after handoff
is determined by the router's built-in capabilities, by the
availability of the necessary router resources, by the availability
of unused bandwidth on the links that the traffic must traverse to
and from the router, and, by the timely available of the service
support context at the router.
The support context referred to here is comprised of the information
necessary to support the all the committed service features, such as
AAA, header compression, Differentiated Services, Integrated Services,
policy enforcement, etc. [2]. This context is initially established
when the service is set-up between the mobile node and the network,
and changes over time as the components supporting the service
features change state.
In order for this context to be available at a new access router
after handoff, it must be replicated from the access router
currently supporting the mobile modes traffic. The replicated context
must represent the most recent support state, if the service is not
to be interrupted or degraded. Thus, when the mobile node's traffic
arrives at a new access router, the replicated context must be
synchronized with the context at the previous access router just
prior to the handoff. For seamless reactive context transfer, the
time scale of this synchronization is roughly on the order of the
allowable incremental delay for forwarding the next packet. For
proactive context transfer, the synchronization latency is on the
order of the average packet inter-arrival time for the mobile node's
traffic.
This document captures the requirements this context transfer in
Support of seamless mobility.
2 Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
3 Terminology
The terminology and the definitions used in the document are for the
most part taken from [2]. This document defines additional
terminology needed to explain the requirements for the transfer of
context. This section presents the new general definitions.
3.1 Coverage Area (CA)
The coverage area for a given AR is defined in terms of the access
points (APs) that are connected to that AR. Each AP forwards traffic
between AR and a given MN.
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For the purpose of describing Context Transfer, there is no need to
assume a given cardinality between ARs and APs. Thus, an AP may be
connected to multiple ARs, and an AR may be connected to multiple
APs. Each AP must be connected to at least one AR, and each AR must
be connected to at least one AP.
3.2 Forwarding Path Handover Scenarios
3.2.1 Break-before-make
Break-before-make is a term used to describe a discontinuity in
connectivity between MN and the access network during a handoff.
With a break-before-make handoff, the forwarding of an MN's traffic
along the current path is discontinued before forwarding of that
traffic is initated along the new path through the network. The old
connection for the traffic flow is "broken" before the new
connection is "made".
For example: an MN moves between the CAs of two ARs. In a
break-before-make scenario, the MN's traffic through the old AR,
and old AP, is stoppe before being redirected through
the new AR/AP pair.
In break-before-make, there is exists some interval where the MN's
traffic cannot be forwarded. The extent of this interval, and the
impact on the IP packets (additional packet drops or buffering
delay) is dependent upon the details of the break-before-make
alogorithm.
This definition of break-before-make is independent of the method
used for or the timing of the context transfer. The context
transfer may still be "proactive" or "reactive" (c.f. below).
3.2.2 Make-before-break
Make-before-break is a term used to describe the continuity of
connectivity between MN and the access network during a handoff.
With a make-before-break handoff, the MN's traffic flow is
established along the new path through the network, before the
old path is released. The new connection for the traffic flow is
"made" before the new connection is "broken".
For example, an MN moves between the CAs of two ARs. In a
make-before-break scenario, the MN's traffic will be forwarded to
the new AR, and the new AP, while the old AR/AP pair continues to
forward traffic. In make-before-break, there is exists some
interval where the MN's traffic traverses both paths. Whether
these two flows contain duplicated packets is dependent upon the
details of the make-before-break alogorithm.
This definition of make-before-break is independent of the method
used for or the timing of the context transfer. The context transfer
may still be "proactive" or "reactive" (c.f. below).
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3.3 Context Transfer Scenarios
3.3.1 Mobile Arrival-Departure Event (MADE)
The MADE is an notification delivered to an AR when the MN enters
its CA. Reception of a MADE indicates that connectivity exists
between the AR and the MN through at least one AP.
3.3.2 Reactive Context Transfer
The context information required to completely supporting an IP
micro-flow is replicated to the access router at the instant when a
packet from that micro-flow arrives at the new access router.
A reactive context transfer can be performed for a make-before-break
or for a break-before-make handoff.
3.3.3 Proactive Context Transfer
The context information required to completely support an IP micro-
flow is replicated to the access router(s), that detect the presence
of MN in its coverage area, in advance of the first packet arrival
to one or any of the ARs.
A proactive context transfer can be performed for a make-before-break
or for a break-before-make handoff.
4 General Requirements for a Context Transfer Framework
This section captures the general requirements for context transfer.
The general requirements cover two functional areas.
- Distributed framework approach
- Context transfer mechanism
4.1 Distributed Framework Approach
An MN may have connectivity to the access network through more
than one access points (AP) at one time. The determination of which
APs are able to communicate with an MN is dependent entirely on the
link characteristics and the layer 2 protocols and services.
The APs able to communicate with an MN may be linked with one or
more ARs. In the scenario where two or more ARs are candidates for
fowarding an MN's traffic, the context for the MN's active
micro-flows must be replicated at every AR.
- The framework MUST support one-to-many context transfer.
To achieve seamless handover, the introduction of additional packet
delays and drops must be avoided. Context transfer will require
some exchange of information and since the context needs to be
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established before an AR can provide the appropriate forwarding
treatment, it is necessary to initiate the transfer well before
forwarding is required to begin.
- The framework MUST support proactive context transfer.
The unpredictability of some channels, and the vagaries of layer 2
handoff mechanism ensure that a proactive approach may not always
be possible. There will be situations where there is no warning,
and an AR requires the context needed to forward traffic
immediately.
- The framework SHOULD support reactive context transfer.
There are various alternative approaches to context transfer, some
of which were reviewed in [2]. The main distinction between these
alternatives begins with the choice of the functional entity or
entities that orchestrate the context transfer (e.g. MN driven
versus network driven, centralized versus distributed.
A single entity or centralized approach to context transfer will
likely suffer from scalability difficulties as the number MN's or
the rate of handovers increases. Moreover, the most current context
information will only be available at the access router(s) actively
supporting an MN's flows. Thus, a centralized approach will first
require retrieving context from an AR before distributing it to
other ARs.
- The framework MUST support a distributed transfer approach
in which the access routers are responsible for transferring
context.
The actual context associated with an MN reflects the service
parameters that were agreed upon between the MN and the access
network when each microflow was established, and the state
variables for the service facilities supporting each microflow.
Various protocols participate in setting up the service support
for a given micro-flow, and many may require state be maintained
for the duration of the session. A few examples of context types
are captured in [2].
It is likely that more than one network entity will be involved in
updating the context due to the interaction of the various
protocols with different network services. The the most relevant
instantiation of the context, however, is that which is local to
the AR and maintained for the purpose of suppor supporting a
microflow. A context transfer approach that uses the active AR as
the source of the context, and delivers the context directly to the
new AR would be the most efficient. The number of entities involved
in the context transfer would simply the number of ARs requiring
the context for a particular microflow. In addition, by implication,
the number of protocol exchanges would be less, as the number of
communicating entities is limited to those same ARs.
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4.2 Context Transfer Protocol
The context transfer protocol is the mechanism for transporting
context information from one AR to another AR. The outcome of a
context transfer will be an up-to-date replication of the
configuration and state information from the source AR at the new
AR.
- The context transfer protocol MUST provide 100% reliable
transfer of the context information. 100% reliable
information transfer means no loss of information and no
induced errors.
- The context transfer protocol MUST deliver the context
without duplication or re-ordering of the information.
- The context transfer protocol MUST transfer the context fast
enough for the information to be meaningful at the receiving
AR.
The context at the AR actually supporting traffic from the MN will
change over time. In addition to the progression of the various
state information, the MN may initiate new microflow(s) or
discontinue existing microflows. The timing of these changes in
context is on the order of the intervals between packet arrivals
in the MN's traffic flow.
- The context transfer protocol MUST provide method for
synchronizing context information when it changes.
- The synchronization of context MUST preserve the integrity,
and thus the meaning, of the context at each AR who has
received the context.
As a corollary, any signaling exchanges required by the context
transfer protocol will introduce additional delay. Protocols such
as TCP [4] and COPS [5] require signalling exchanges, or
"handshakes" between the communicating entities at various stages
of the protocol session.
- The context transfer meachanism SHOULD minimize signaling
overhead when performing an actual context transfer.
The time taken to replicate context depends greatly upon the number
of packet exchanges required to complete a transfer of the context
information. In many situations, such as with a reactive
break-before-make scenario, the context transfer delay becomes a
critical factor in determining whether the service is disrupted or
not.
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- The context transfer MUST complete with minimum number of
protocol exchanges between the source AR and the rest of the
ARs.
- The context transfer protocol MUST sustain the security of
context information.
Similarly, if the context transfer protocol delivers the information
in a form that requires significant processing at the AR before the
context is useable - for example, if the information has to be
re-ordered, then significant delay may be introduced in establishing
the replicated context.
- The context transfer protocol MUST minimize any processing
at the ARs.
A seamless handover of an MN's active sessions requires that there
be at least one AR capable of supporting the MN's traffic. In order
for the handoff to be targetted to the ARs capable of supporting the
MN's traffic, each AR must be able to return the admission status of
the context transfer.
- The context transfer protocol MUST provide for feedback from
each candidate AR of the admission status for each context
transfer attempt.
- The context transfer protocol MUST interwork with the
micro-mobility mechanism [3].
In a situation where a single AR is not available to support the
whole context associated with an MN's traffic, a mechanism could
be provided to negotiate the handover of each of the active
sessions to different ARs.
Similarly, when complete support for a particular micro-flow is not
possible at any AR, it may be perferred that a degraded service be
negotiated over dropping the micro-flow at the time of handoff.
- The context transfer protocol MAY provide a mechanism for
negotiating partial context transfer.
- Any mechanism for partial context transfer MUST interwork
with the micro-mobility mechanism [3].
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5 References
[1] S. Bradner, "keywords for use in RFCs to Indicate Requirement
Levels", RFC2119 (BCP), IETF, March 1997.
[2] The seamoby CT design team, "Context transfer: problem
statement", draft-ietf-seamoby-context-transfer-problem-
stat-00.txt.
[3] The seamoby MM design team, "Micro-mobility: problem
statement", draft-ietf-seamoby-mm-problem-00.txt.
[4] "Transmission Control Protocol", RFC 793, September 1981.
[5] D.Durham et. al, "The COPS (Common open Policy Services)
protocol", RFC2748, January 2000.
6 Acknowledgments
The contents of this draft are a result of the discussions within the
Nortel Networks Advanced Wireless Network Technology Lab and we would
like to thank all the members who contributed in these discussions.
7 Author's Address
Syed, Hamid
100-Constellation Crescent
Nepean, Ontario. K2G 6J8 Phone: 1-613-763-6553
Canada Email: hmsyed@nortelnetworks.com
Kenward, Gary
100-Constellation Crescent
Nepean, Ontario. K2G 6J8 Phone: 1-613-765-1437
Canada Email: gkenward@nortelnetworks.com
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