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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


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice

   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


8  Full Copyright Statement

   "Copyright (C) The Internet Society (date). All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
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   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organisations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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