Network Working Group                                      J. Kempf, Ed.
Request for Comments: 3374                                September 2002
Category: Informational


     Problem Description: Reasons For Performing Context Transfers
                 Between Nodes in an IP Access Network

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   In IP access networks that support host mobility, the routing paths
   between the host and the network may change frequently and rapidly.
   In some cases, the host may establish certain context transfer
   candidate services on subnets that are left behind when the host
   moves.  Examples of such services are Authentication, Authorization,
   and Accounting (AAA), header compression, and Quality of Service
   (QoS).  In order for the host to obtain those services on the new
   subnet, the host must explicitly re-establish the service by
   performing the necessary signaling flows from scratch.  In some
   cases, this process would considerably slow the process of
   establishing the mobile host on the new subnet.  An alternative is to
   transfer information on the existing state associated with these
   services, or context, to the new subnet, a process called "context
   transfer".  This document discusses the desirability of context
   transfer for facilitating seamless IP mobility.

Table of Contents

   1.0   Introduction................................................2
   2.0   Reference Definitions.......................................3
   3.0   Scope of the Context Transfer Problem.......................3
   4.0   The Need for Context Transfer...............................4
   4.1   Fast Context Transfer-candidate Service Re-establishment....4
   4.1.1 Authentication, Authorization, and Accounting (AAA).........4
   4.1.2 Header Compression..........................................5
   4.1.3 Quality of Service (QoS)....................................6
   4.2   Interoperability............................................6
   5.0   Limitations on Context Transfer.............................7
   5.1   Router Compatibility........................................7
   5.2   Requirement to Re-initialize Service from Scratch...........7
   5.3   Suitability for the Particular Service......................7
   5.4   Layer 2 Solutions Better....................................7
   6.0   Performance Considerations..................................8
   7.0   Security Considerations.....................................8
   8.0   Recommendations.............................................9
   9.0   Acknowledgements............................................9
   10.0  References.................................................10
   11.0  Complete List of Authors' Addresses........................12
   12.0  Full Copyright Statement...................................14

1.0 Introduction

   In networks where the hosts are mobile, the routing path through the
   network must often be changed in order to deliver the host's IP
   traffic to the new point of access.  Changing the basic routing path
   is the job of a IP mobility protocol, such as Mobile IPv4 [1] and
   Mobile IPv6 [2].  But the success of real time services 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 routing path, the nodes along the new
   path must be prepared to provide similar routing treatment to the IP
   packets as was provided along the old routing path.

   In many cases, the routing treatment of IP packets within a network
   may be regulated by a collection of context transfer-candidate
   services that influence how packets for the host are treated.  For
   example, whether a particular host has the right to obtain any
   routing at all out of the local subnet may depend on whether the host
   negotiated a successful AAA exchange with a network access server at
   some point in the past.  Establishing these services initially
   results in a certain amount of related state within the network and
   requires a perhaps considerable amount of time for the protocol

   exchanges.  If the host is required to re-establish those services by
   the same process as it uses to initially establish them, delay-
   sensitive real time traffic may be seriously impacted.

   An alternative is to transfer enough information on the context
   transfer-candidate service state, or context, to the new subnet so
   that the services can be re-established quickly, rather than require
   the mobile host to establish them from scratch.  The transfer of
   service context 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 can 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 signaling
   overhead.

   In this document, a case is made for why the Seamoby Working Group
   should investigate context transfer.

2.0 Reference Definitions

   Context

      The information on the current state of a service required to re-
      establish the service on a new subnet without having to perform
      the entire protocol exchange with the mobile host from scratch.

   Context Transfer

      The movement of context from one router or other network entity to
      another as a means of re-establishing specific services on a new
      subnet or collection of subnets.

   Context Transfer Candidate Service

      A service that is a candidate for context transfer.  In this
      document, only services that are concerned with the forwarding
      treatment of packets, such as QoS and security, or involve
      granting or denying the mobile host access to the network, such as
      AAA, are considered to be context transfer-candidate services.

3.0 Scope of the Context Transfer Problem

   The context transfer problem examined in this document is restricted
   to re-establishing services for a mobile host that are, in some
   sense, related to the forwarding treatment of the mobile host's

   packets or network access for the mobile host.  It is not concerned
   with actually re-establishing routing information.  Routing changes
   due to mobility are the domain of the IP mobility protocol.  In
   addition, transfer of context related to application-level services,
   such as those associated with the mobile host's HTTP proxy, is also
   not considered in this document, although a generic context transfer
   protocol for transferring the context of services related to
   forwarding treatment or network access may also function for
   application-level services as well.

   An important consideration in whether a service is a candidate for
   context transfer is whether it is possible to obtain a "correct"
   context transfer for the service in a given implementation and
   deployment, that is, one which will result in the same context at the
   new access router as would have resulted had the mobile host
   undergone a protocol exchange with the access router from scratch.
   For some services, the circumstances under which context transfer may
   result in correctness may be very limited [11].

4.0 The Need for Context Transfer

   There are two basic motivations for context transfer:

   1) The primary motivation, as mentioned in the introduction, is the
      need to quickly re-establish context transfer-candidate services
      without requiring the mobile host to explicitly perform all
      protocol flows for those services from scratch.

   2) An additional motivation is to provide an interoperable solution
      that works for any Layer 2 radio access technology.

   These points are discussed in more detail in the following
   subsections.

4.1 Fast Context Transfer-candidate Service Re-establishment

   As mentioned in the introduction, there are a variety of context
   transfer-candidate services that could utilize a context transfer
   solution.  In this section, three representative services are
   examined.  The consequences of not having a context transfer solution
   are examined as a means of motivating the need for such a solution.

4.1.1 Authentication, Authorization, and Accounting (AAA)

   One of the more compelling applications of context transfer is
   facilitating the re-authentication of the mobile host and
   re-establishment of the mobile host's authorization for network
   access in a new subnet by transferring the AAA context from the

   mobile host's previous AAA server to another.  This would allow the
   mobile host to continue access in the new subnet without having to
   redo an AAA exchange with the new subnet's AAA server.  Naturally, a
   security association between the AAA servers is necessary so that the
   mobile host's sensitive authentication information can be securely
   transferred.

   In the absence of context transfer, there are two ways that can
   currently be used for AAA:

   1) Layer 2 mechanisms, such as EAP [3] in PPP [4] or 802.1x [5] can
      be used to redo the initial protocol exchange, or possibly to
      update it.  Currently, there is no general Layer 3 mechanism for
      conducting an AAA exchange between a host and an AAA server in the
      network.

   2) If the mobile host is using Mobile IPv4 (but not Mobile IPv6
      currently), the host can use the AAA registration keys [6]
      extension for Mobile IPv4 to establish a security association with
      the new Foreign Agent.

   Since 2) is piggybacked on the Mobile IPv4 signaling, the performance
   is less likely to be an issue, but 2) is not a general solution.  The
   performance of 1) is likely to be considerably less than is necessary
   for maintaining good real time stream performance.

4.1.2 Header Compression

   In [7], protocols are described for efficient compression of IP
   headers to avoid sending large headers over low bandwidth radio
   network links.  Establishing header compression generally requires
   from 1 to 4 exchanges between the last hop router and the mobile host
   with full or partially compressed headers before full compression is
   available.  During this period, the mobile host will experience an
   effective reduction in the application-available bandwidth equivalent
   to the uncompressed header information sent over the air.  Limiting
   the uncompressed traffic required to establish full header
   compression on a new last hop router facilitates maintaining adequate
   application-available bandwidth for real time streams, especially for
   IPv6 where the headers are larger.

   Context transfer can help in this case by allowing the network entity
   performing header compression, usually the last hop router, to
   transfer the header compression context to the new router.  The
   timing of context transfer must be arranged so that the header
   context is transferred from the old router as soon as the mobile host

   is no longer receiving packets through the old router, and installed
   on the new router before any packets are delivered to or forwarded
   from the mobile host.

4.1.3 Quality of Service (QoS)

   Significant QoS protocol exchanges between the mobile host and
   routers in the network may be required in order to establish the
   initial QoS treatment for a mobile host's packets.  The exact
   mechanism whereby QoS for a mobile host should be established is
   currently an active topic of investigation in the IETF.  For existing
   QoS approaches (Diffsrv and Intsrv) preliminary studies have
   indicated that the protocol flows necessary to re-establish QoS in a
   new subnet from scratch can be very time consuming for Mobile IP, and
   other mobility protocols may suffer as well.

   A method of transferring the mobile host's QoS context from the old
   network to the new could facilitate faster re-establishment of the
   mobile host's QoS treatment on the new subnet.  However, for QoS
   mechanisms that are end-to-end, transferring context at the last hop
   router may be insufficient to completely re-initialize the mobile
   host's QoS treatment, since some number of additional routers in the
   path between the mobile host and corresponding node may also need to
   be involved.

4.2 Interoperability

   A particular concern for seamless handover is that different Layer 2
   radio protocols may define their own solutions for context transfer.
   There are ongoing efforts within 3GPP [8] and IEEE [9] to define such
   solutions.  These solutions are primarily designed to facilitate the
   transfer of Layer 2-related context over a wired IP network between
   two radio access networks or two radio access points.  However, the
   designs can include extensibility features that would allow Layer 3
   context to be transferred.  Such is the case with [10], for example.

   If Layer 2 protocols were to be widely adopted as an optimization
   measure for Layer 3 context transfer, seamless mobility of a mobile
   host having Layer 2 network interfaces that support multiple radio
   protocols would be difficult to achieve.  Essentially, a gateway or
   translator between Layer 2 protocols would be required, or the mobile
   host would be required to perform a full re-initialization of its
   context transfer-candidate services on the new radio network, if no
   translator were available, in order to hand over a mobile host
   between two access technologies.

   A general Layer 3 context transfer solution may also be useful for
   Layer 2 protocols that do not define their own context transfer
   protocol.  Consideration of this issue is outside the scope of the
   Seamoby Working Group, however, since it depends on the details of
   the particular Layer 2 protocol.

5.0 Limitations on Context Transfer

   Context transfer may not always be the best solution for
   re-establishing context transfer-candidate services on a new subnet.
   There are certain limitations on when context transfer may be
   useful. These limitations are discussed in the following subsections.

5.1 Router Compatibility

   Context transfer between two routers is possible only if the
   receiving router supports the same context transfer-candidate
   services as the sending router.  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 router that cannot
   make use of received context should refuse the transfer.  This
   results in a situation no different than a mobile host handover
   without context transfer, and should not be considered an error or
   failure situation.

5.2 Requirement to Re-initialize Service from Scratch

   The primary motivation for context transfer assumes that quickly
   re-establishing the same level of context transfer-candidate service
   on the new subnet is desirable.  And yet, there may be situations
   where either the device or the access network would prefer to
   re-establish or re-negotiate the level of service.  For example, if
   the mobile host crosses administrative domains where the operational
   policies change, negotiation of a different level of service may be
   required.

5.3 Suitability for the Particular Service

   Context transfer assumes that it is faster to establish the service
   by context transfer rather than from scratch.  This may not be true
   for certain types of service, for example, multicast, "push"
   information services.

5.4 Layer 2 Solutions Better

   Context transfer is an enhancement to improve upon the performance of
   a handover for Layer 3 context transfer-candidate services.  Many
   networks provide support for handover at Layer 2, within and between

   subnets.  Layer 3 context transfer may not provide a significant
   improvement over Layer 2 solutions, even for Layer 3 context, if the
   handover is occurring between two subnets supporting the same Layer 2
   radio access technology.

6.0 Performance Considerations

   The purpose of context transfer is to sustain the context
   transfer-candidate services being provided to a mobile host's traffic
   during handover.  It is essentially an enhancement to IP mobility
   that ultimately must result in an improvement in handover
   performance.  A context transfer solution must provide performance
   that is equal to or better than re-initializing the context
   transfer-candidate service between the mobile host and the network
   from scratch.  Otherwise, context transfer is of no benefit.

7.0 Security Considerations

   Any context transfer standard must provide mechanism for adequately
   securely the context transfer process, and a recommendation to deploy
   security, as is typically the case for Internet standards.  Some
   general considerations for context transfer security include:

   - Information privacy: the context may contain information which the
     end user or network operator 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 router, and therefore could potentially affect the
     operation of the access network itself.  The potential threats
     include denial of service and theft of service attacks.

    - Security preservation: part of the context transfer may include
     information pertinent to a security association established between
     the mobile host 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 (e.g., IPSEC [10]) 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.

   Since context transfer requires a trust relationship between network
   entities, the compromise of only one of the network entities that
   transfer context may be sufficient to reduce the security of the
   whole system, if for example the context transferred includes
   encryption keying material.  When the host moves from the compromised
   network entity to an uncompromised network entity in the presence of
   context transfer, the compromised context may be used to decrypt the
   communication channel.  When context transfer is not used, a
   compromise of only one network entity only gives access to what that
   network entity can see.  When the mobile host moves to an
   uncompromised network entity in the absence of context transfer,
   security can be re-established at the new entity.  However, to the
   extent that context transfer happens primarily between routers, the
   security of context transfer will depend on the security of the
   routers.  Any compromise of security on a router that affects context
   transfer may also lead to other, equally serious disruptions in
   network traffic.

   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.

8.0 Recommendations

   The following steps are recommended for Seamoby:

   - Investigation into candidate router-related services for context
     and an analysis of the transfer requirements for each candidate;

   - The development of a framework and protocol(s) that will support
     the transfer of context between the routing nodes of an IP network.

   The context transfer solution must inter-work with existing and
   emerging IP protocols, in particular, those protocols supporting
   mobility in an IP network.

9.0 Acknowledgements

   The editor would like to thank the Seamoby CT design team (listed at
   the end of the document as co-authors), who were largely responsible
   for the initial content of this document, for their hard work, and
   especially Gary Kenward, who shepherded the document through its
   initial versions.

10.0 References

   [1]  Perkins, C., "IP Mobility Support", RFC 3220, January 2002.

   [2]  Johnson, D. and C. Perkins, "Mobility Support in IPv6", Work in
        Progress.

   [3]  Blunk, L. and Vollbrecht, J., "PPP Extensible Authentication
        Protocol (EAP)", RFC 2284, March 1998.

   [4]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
        1661, July 1994.

   [5]  IEEE Std. P802.1X/D11, "Standard for Port based Network Access
        Control", March 2001.

   [6]  Perkins, C., and P. Calhoun, "AAA Registration Keys for Mobile
        IP", Work in Progress.

   [7]  Borman, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu,
        H., Jonsson, L., Hakenberg, R., Koren T., Le, K., Martensson,
        A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T. and H.
        Zheng, "RObust Header Compression (ROHC): Framework and four
        profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001.

   [8]  3GPP TR 25.936 V4.0.0, "Handovers for Real Time Services from PS
        Domain," 3GPP, March 2001.

   [9]  IEEE Std. 802.11f/D2.0, "Draft Recommended Practice for Multi-
        Vendor Access Point Interoperability via an Inter-Access Point
        Protocol Across Distribution Systems Supporting IEEE 802.11
        Operation," July 2001.

   [10] Kent, S. and Atkinson, R., "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [11] Aboba, B. and M. Moore, "A Model for Context Transfer in IEEE
        802", Work in Progress.

11.0 Complete List of Authors' Addresses

   O. Henrik Levkowetz
   A Brand New World
   Osterogatan 1
   S-164 28 Kista
   SWEDEN

   Phone: +46 8 477 9942
   EMail: henrik@levkowetz.com


   Pat R. Calhoun
   Black Storm Networks
   110 Nortech Parkway
   San Jose  CA 95134
   USA

   Phone: +1 408-941-0500
   EMail: pcalhoun@bstormnetworks.com


   James Kempf
   NTT DoCoMo USA Laboratories
   181 Metro Drive, Suite 300
   San Jose, CA 95110
   USA

   Phone: 408-451-4711
   EMail: kempf@docomolabs-usa.com


   Gary Kenward
   Nortel Networks
   3500 Carling Avenue
   Nepean, Ontario  K2G 6J8
   CANADA

   Phone: +1 613-765-1437
   EMail: gkenward@nortelnetworks.com

   Hamid Syed
   Nortel Networks
   100 Constellation Crescent
   Nepean  Ontario K2G 6J8
   CANADA

   Phone: +1 613 763-6553
   EMail: hmsyed@nortelnetworks.com


   Jukka Manner
   Department of Computer Science
   University of Helsinki
   P.O. Box 26 (Teollisuuskatu 23)
   FIN-00014 Helsinki
   FINLAND

   Phone: +358-9-191-44210
   EMail: jmanner@cs.helsinki.fi


   Madjid Nakhjiri
   Motorola
   1501 West Shure Drive
   Arlington Heights  IL 60004
   USA

   Phone: +1 847-632-5030
   EMail: madjid.nakhjiri@motorola.com


   Govind Krishnamurthi
   Communications Systems Laboratory, Nokia Research Center
   5 Wayside Road
   Burlington  MA 01803
   USA

   Phone: +1 781 993 3627
   EMail: govind.krishnamurthi@nokia.com

   Rajeev Koodli
   Communications Systems Lab, Nokia Research Center
   313 Fairchild Drive
   Mountain View  CA 94043
   USA

   Phone: +1 650 625 2359
   EMail: rajeev.koodli@nokia.com


   Kulwinder S. Atwal
   Zucotto Wireless Inc.
   Ottawa  Ontario K1P 6E2
   CANADA

   Phone: +1 613 789 0090
   EMail: kulwinder.atwal@zucotto.com


   Michael Thomas
   Cisco Systems
   375 E Tasman Rd
   San Jose  CA 95134
   USA

   Phone: +1 408 525 5386
   EMail: mat@cisco.com


   Mat Horan
   COM DEV Wireless Group
   San Luis Obispo  CA 93401
   USA

   Phone: +1 805 544 1089
   EMail: mat.horan@comdev.cc


   Phillip Neumiller
   3Com Corporation
   1800 W. Central Road
   Mount Prospect  IL 60056
   USA

   EMail: phil_neumiller@3com.com

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
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