Context and Micro-mobility Routing Working Group         O. H. Levkowetz
    Internet Draft                                                      ABNW
    draft-ietf-seamoby-context-transfer-problem-               P. R. Calhoun
    stat-02.txt                                        Sun MicroSystems, Inc
    Expires: December 2001                                        G. Kenward
                                                                  H. M. Syed
                                                             Nortel Networks
                                                                   J. Manner
                                                      University of Helsinki
                                                                 M. Nakhjiri
                                                            G. Krishnamurthi
                                                                   R. Koodli
                                                                 K. S. Atwal
                                                            Zucotto Wireless
                                                                   M. Thomas
                                                                    M. Horan
                                                                     COM DEV
                                                                P. Neumiller
                                                                   June 2001
          Problem Description: Reasons For Performing Context Transfers
                    Between Nodes in an IP Access Network
     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.
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        There are a large number of IP access networks that support mobility
        of hosts. For example, wireless Personal Area Networks (PANs),
        wireless LANs, satellite WANs and cellular WANs. The nature of this
        mobility is such that the communication path to the host changes
        frequently and rapidly.
        This Internet-Draft describes the problem encountered during
        mobility that requires the exchange of IP context between different
        forwarding nodes in an access network.
     Table of Contents
        1 Introduction...................................................2
        2 Reference Definitions..........................................3
        3 The Need for Context Transfer..................................4
         3.1 Scope of the Context Transfer Solution .....................5
         3.2 Reference Architectures ....................................6
        4 A Description of Context.......................................6
         4.1 Feature ....................................................6
         4.2 Feature Context ............................................7
         4.3 Microflow Context ..........................................7
         4.4 Common Context .............................................7
         4.5 Examples of Feature Context ................................8
        5 Context Transfer and Mobility..................................8
        6 Forwarding Node Capabilities...................................8
        7 Inter-technology Context Transfers.............................8
        8 Applicability..................................................9
        9 Performance Considerations.....................................9
        10 Security Considerations.......................................9
        11 Recommendations..............................................10
     1 Introduction
        There are a large number of IP access networks that support hosts
        that are mobile. For example, wireless Personal Area Networks
        (PANs), wireless LANs, satellite WANs and cellular WANs. The nature
        of this mobility is such that the communication path to a host may
        change frequently and rapidly.
        In networks where the hosts are mobile, the forwarding path through
        the access network must often be redirected in order to deliver the
        host's IP traffic to the new point of access. The success of a time
        sensitive service such as VoIP telephony, video, etc., in a mobile
        environment depends heavily upon the minimization of the impact of
        this traffic redirection. In the process of establishing the new
        forwarding path, the nodes along the new path must be prepared to
        provide similar forwarding treatment to the IP packets.
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        The information required in support of a specific forwarding
        treatment provided to IP traffic is part of the context for that
        traffic. This context is comprised of configuration and state
        information. To minimize the impact of a path change on an IP flow,
        the context used at the forwarding nodes along the original path,
        must be replicated at the forwarding nodes along the new path.
        The transfer of context information may be advantageous in
        minimizing the impact of host mobility on, for example, AAA, header
        compression, QoS, Policy, and possibly sub-IP protocols and services
        such as PPP. Context transfer at a minimum will be used to replicate
        the configuration information needed to establish the respective
        protocols and services. In addition, it may also provide the
        capability to replicate state information, allowing stateful
        protocols and services at the new node to be activated along the new
        path with less delay and less signalling overhead.
     2 Reference Definitions
        Mobile Node
        An IP node capable of changing its point of attachment to the
        network. The mobile node is usually an end user host, but may be a
        forwarding node for a network that is itself mobile.
        Forwarding Node
        A node that supports IP forwarding within the access network. This
        includes all routers, gateways, servers and other nodes within the
        network that have IP protocol stacks and forward IP traffic to and
        from a mobile node.
        A forwarding node will be connected to one or more other forwarding
        nodes. Some forwarding nodes will be connected to zero or more
        mobile nodes. The forwarding nodes that connect to mobile nodes will
        do so through a layer two entity that supports wireless
        One useful definition for the fundamental unit of IP service is the
        microflow. RFC 2475 [1] defines a microflow as:
        A single instance of an application-to application flow of packets
        destined to or originated from a mobile device (MN or MH), which is
        identified by source address, source port, destination address,
        destination port and protocol id.
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        RFC 2207 [2] provides a definition of a microflow that applies to
        IPSec packets. It is based on the IPSec SPI found in the AH or ESP
        headers. Other methods for identifying for IP microflows may be
        defined in the future, e.g. the Ipv6 flow label.
     3 The Need for Context Transfer
        In networks where the hosts are mobile, the forwarding path through
        the access network must often be redirected in order to deliver the
        host's IP traffic to the new point of access. The success of a time
        sensitive service such as VoIP telephony, video, etc., in a mobile
        environment depends heavily upon the minimization of the impact of
        this traffic redirection. In the process of establishing the new
        forwarding path, the nodes along the new path must be prepared to
        provide similar forwarding treatment to the IP packets.
        Figure 1 and 2 below illustrate an example of a handover scenario. A
        mobile node, MN7, is shown attached to the Internet via an access
        network. The forwarding nodes, FN1 through to FN7 comprise a portion
        of this access network. FN1 and FN7 are the last forwarding nodes
        before the untethered link to MN7. This last link allows the nodes
        to move while maintaining connectivity to the edge of the access
        network - e.g. the last link is wireless. For simplicity, many
        details of a real access network topology have been left out of the
        Prior to handoff, the path for MN7's traffic traverses FN1 and FN2.
        It is presumable that FN1 and FN2 have been configured to provide
        specific forwarding services for MN7's traffic, and that some of
        these services may require state information to be maintained.
                                       MN7's traffic
                              -------                 -------
              [ MN7 ]<= = = =>| FN1 |=================| FN2 |========
                              -------                /-------
                                            | FN4 |
                                          /         \
                                         /           \
                                 -------/             \-------
                            - - -| FN7 |               | FN6 |----
                                 -------               -------
                         Figure 1: Path Prior to Handover
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        When MN7's movement results in the link to FN1 to be replaced by a
        link to FN7, MN7's traffic must be re-directed along a new
        forwarding path, illustrated in Figure 2 as the path through FN7,
        FN4 and FN2. In many mobile networks, this handover can occur
        frequently, and sometimes with little or no warning.
                               -------                -------
                          - - -| FN1 |----------------| FN2 |========
                               -------              //-------
                                                   //   MN7's traffic
                                            -------/    <----------->
                                            | FN4 |
                           MN7's traffic  //        \
                           <-----------> //          \
                                  -------/            \-------
                  [ MN7 ]<= = = =>| FN7 |              | FN6 |----
                                  -------              -------
                 Figure 2: Path Re-Directed After Handover
        If the forwarding treatment provided to MN7's traffic is to be
        sustained, the support conditions at FN1 and FN2 must be reproduced
        at FN7 and FN4. At the very least, this requires that FN7 and FN4 be
        configured to provide the appropriate support to the traffic flow.
        However, establishing forwarding support along the new path through
        the use of IP signalling protocols would be relatively time
        consuming. In addition, given that some forwarding services may
        maintain state information, simply configuring FN7 and FN4 would not
        circumvent disruption of the services.
        Given that FN7 and FN4 support the same functionality as FN1, it
        should be possible to reproduce the needed support along the new
        forwarding path by capturing the salient information at FN1 as it
        exists just prior to the handover, and then use that information to
        replicate the state of FN1 at FN7 and FN4.
     3.1 Scope of the Context Transfer Solution
        The information required to produce the necessary forwarding
        treatments at a node is referred to as the support "context". In
        order to replicate the context of one forwarding node at another
        forwarding node, an open, standard solution for context transfer
        must be developed.
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        In addition, for a forwarding node to be able to interpret the
        transferred context, both the syntax and the semantics of the
        context information must be defined. Definition of the actual
        context information will be specific to the support feature being
        replicated, however, a generic container format of the payload of
        context transfer messages is required for inter-operability.
     3.2 Reference Architectures
        The concept of the "forwarding node" is used here to describe the
        general context transfer problem. In different networks, the actual
        context transfer may take place between different entities within
        the network. In some cases the preferred context transfer peers
        might be the routers at the edge of the access network. In other
        situations, it may be more appropriate to transfer context between
        wireless access points that also support forwarding of IP packets.
        The context transfer solution will be applicable to any nodes that
        support forwarding of a mobile nodes IP traffic.
     4 A Description of Context
        The term `                 `context'                          ' pertains to all information required by a
        forwarding node specific to supporting the IP traffic for a
        particular mobile node.
        The context for a mobile node can be partitioned into three
                - information required to provide generally support to the
                  mobile node;
                - information required to provide common support to the
                  mobile node's traffic;
                - information required to support specific features of the
                  forwarding treatment for an IP microflow.
     4.1 Feature
        A feature is an aspect of the IP forwarding treatment that is
        instantiated for each mobile node, and which may support one or more
        of the mobile nodes microflows. A feature must include associated
        configuration information: at the very least, some identification
        information for the mobile node, such as the mobile node's IP
        address. A feature may have associated state information.
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        In other words, for the purpose of supporting mobility the features
        of interest all have context pertinent to the forwarding of a
        specific mobile nodes IP packets. Examples of features include:
        header compression, security, subscriber policy, and quality of
        Each feature is, in general, supported by an amalgamation of one or
        more protocols and associated local support mechanisms. As an
        example, header compression is a feature that consists of various
        "mechanisms" such as IP/TCP header compression and IPv6/UDP/RTP
        header compression, operating according to well-defined "protocols".
        Thus, an instance of a feature reflects the state associated with
        protocols and the corresponding mechanisms.
     4.2 Feature Context
        Feature context is the condition or state of an instance of a
        particular feature. It represents the current evolution of the
        behaviour of that feature instance. At a given instant in time,
        feature context is represented by the values of the associated data
        Some of the data elements associated with a feature are time
        invariant and, thus, represent the configuration of the feature
        instance. Other data elements, which are often referred to as the
        "state variables" for the feature instance, change value over time
        as a consequence of the various operations performed in support of
        the feature instance.
        A feature context is believed to be the smallest indivisible unit of
        context that can be transferred between forwarding nodes.
     4.3 Microflow Context
        A microflow context is the collection of the various feature
        contexts associated with a single IP microflow. It is the smallest
        unit of context that must be transferred in order to re-establish
        support for the microflow at a forwarding node.
     4.4 Common Context
        Some of the associated feature contexts may be applicable to more
        then one of the IP microflows comprising the mobile node's traffic.
        An example of this might be the authentication state for the mobile.
        This type of feature context is also required to support each
        microflow, but because of the multiplicity of the relationship to
        the mobile node's microflows, it would seem prudent to transfer one
        instance of the context, independent of the microflow context.
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     4.5 Examples of Feature Context
        Some examples of feature contexts that are candidates for context
        transfer are given below. This list is provided solely to illustrate
        the nature of the context transfer problem domain.
                - encryption context;
                - authentication context;
                - authorization context;
                - accounting context;
                - header compression context;
                - quality of service context;
                - policy enforcement context;
                - multicast group membership;
                - buffer state;
                - sub-network layer context (e.g. PPP context).
     5 Context Transfer and Mobility
        Context transfer is an enhancement to a mobility solution that is
        clearly most effective when used on a local scale between forwarding
        nodes within a subnetwork. However, with due consideration to the
        nature of the context and the requirements for transfer of that
        context, context transfer should be applicable between any two
        similar forwarding nodes.
     6 Forwarding Node Capabilities
        Context transfer between two forwarding nodes is worthwhile only if
        the receiving node's capabilities are similar enough to those of the
        sending node that the received context can be utilized. This does
        not mean that the two nodes are identical in their implementation,
        nor does it even imply that they must have identical capabilities.
        A forwarding node that cannot make use of received context could,
        and should, refuse the transfer. This would result in a situation no
        different then mobile handover without context transfer.
     7 Inter-technology Context Transfers
        The context transfer described here operates at the IP layer, and
        should be applicable to situations where the forwarding nodes are
        supporting different wireless link technologies.
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        An exception to this would be where sub-IP layer context is an
        intrinsic part of the forwarding treatment. In this situation, an
        interworking solution must be developed for sub-IP context transfer
        between the two technologies. This issue is outside the scope of the
        Seamoby working group.
     8 Applicability
        The primary motivation for developing an IP context transfer assumes
        that service sustainment during handover is desirable. And yet,
        there may be situations where either the user or the access network
        would prefer to re-establish or re-negotiate a service or a service
        feature. For example, if the mobile node crosses administration
        domains where the operational policies change, negotiation of a
        different level of service may be required by the respective
        More important, the concept of context transfer also assumes that
        the service is worth sustaining through a variety of handover
        scenarios, and that context transfer is the best approach. The
        former may not be true for certain types of service - for example,
        multicast, "push" information services. In addition, even if service
        level sustainment is desirable, context transfer may not be the most
        cost efficient solution.
        Alternatives to context transfer for sustaining services during
        handover are out of scope for the Seamoby working group. The
        possibility of other solutions is only noted here to emphasize that
        context transfer is but one of possibly many approaches.
        Finally, context transfer is an enhancement at the IP level to
        improve upon the performance of a handover between IP forwarding
        nodes. Many networks provide support for handover at the link layer,
        within and between subnetworks, and context transfer may not provide
        a significant improvement over the these link layer solutions.
     9 Performance Considerations
        The purpose of context transfer is to sustain the services being
        provided to a mobile node's traffic during handover. It is
        essentially an enhancement to a mobility solution that ultimately
        must result in an improvement in handover performance. The various
        aspects of performance must be a prime consideration in the
        development of the context transfer solution.
     10 Security Considerations
        Some of the considerations for context transfer security include:
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                - information privacy: the context may contain information
                  which the end user would prefer to keep hidden from
                  unauthorized viewers.
                - transfer legitimacy: a false or purposely corrupted
                  context transfer could have a severe impact upon the
                  operation of the receiving forwarding node, and therefore
                  could potentially affect the operation of the access
                  network itself; the potential threats include denial of
                  service and theft of service attacks; context transfer
                  must only occur between forwarding nodes that have an
                  established trust relationship.
                - security preservation: part of the context transfer may
                  include information pertinent to a security association
                  established between the mobile node and another entity on
                  the network; for this security association to be preserved
                  during handover, the transfer of the security context
                  must include the appropriate security measures.
        It is expected that the measures used to secure the transport of
        information between peers in an IP network should be sufficient for
        context transfer. However, given the above considerations, there
        may be reason to provide for additional security measures beyond the
        available IETF solutions.
        The context transfer investigation must identify any novel security
        measures required for context transfer that exceed the capabilities
        of the existing or emerging IETF solutions.
     11 Recommendations
        The Seamoby context transfer design team recommends the following:
                - investigation into candidates for context transfer - in
                  particular, feature context transfer, and an analysis
                  of the transfer requirements for each candidate;
                - the development of a framework and protocol(s) that will
                  support the transfer of configuration and state context
                  between the forwarding nodes of an IP network.
        The context transfer solution must interwork with existing and
        emerging IP protocols, in particular, those protocols supporting
        mobility in an IP network.
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        [1] Blake, et al., "An Architecture for Differentiated Services",
            RFC 2475, December 1998.
        [2] Berger & O'Malley, "RSVP Extensions for IPSEC Data Flows",
            RFC 2207, September 1997.
     Authors' Addresses
        O. Henrik Levkowetz
        A Brand New World
        Osterogatan 1
        S-164 28 Kista
        Phone: +46 8 477 9942
        Pat R. Calhoun
        Network and Security Research Center, Sun Labs
        15 Network Circle
        Menlo Park  CA 94025
        Phone: +1 650-786-7733
        Gary Kenward
        Nortel Networks
        3500 Carling Avenue
        Nepean, Ontario  K2G 6J8
        Phone: +1 613-765-1437
        Hamid Syed
        Nortel Networks
        100 Constellation Crescent
        Nepean  Ontario K2G 6J8
        Phone: +1 613 763-6553
        Jukka Manner
        Department of Computer Science, University of Helsinki
        P.O. Box 26 (Teollisuuskatu 23)
        FIN-00014 Helsinki
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        Phone: +358-9-191-44210
        Madjid Nakhjiri
        1501 West Shure Drive
        Arlington Heights  IL 60004
        Phone: +1 847-632-5030
        Govind Krishnamurthi
        Communications Systems Laboratory, Nokia Research Center
        5 Wayside Road
        Burlington  MA 01803
        Phone: +1 781 993 3627
        Rajeev Koodli
        Communications Systems Lab, Nokia Research Center
        313 Fairchild Drive
        Mountain View  CA 94043
        Phone: +1 650 625 2359
        Kulwinder S. Atwal
        Zucotto Wireless Inc.
        Ottawa  Ontario K1P 6E2
        Phone: +1 613 789 0090
        Michael Thomas
        Cisco Systems
        375 E Tasman Rd
        San Jose  CA 95134
        Phone: +1 408 525 5386
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        Mat Horan
        COM DEV Wireless Group
        San Luis Obispo  CA 93401
        Phone: +1 805 544 1089
        Phillip Neumiller
        3Com Corporation
        1800 W. Central Road
        Mount Prospect  IL 60056
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