Context and Micro-mobility Routing Working Group J. Kempf,
Editor
Internet Draft
draft-ietf-seamoby-context-transfer-problem-stat-
03.txt
Expires: April, 2002
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
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
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Context Transfer Problem Statement October 2001
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 routing-related
services on subnets that are left behind when the host moves.
Examples of such services are AAA, header compression, and 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........................4
4.0 The Need for Context Transfer................................4
4.1 Fast Routing-Related 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)...............................5
4.2 Interoperability............................................6
5.0 Limitations on Context Transfer..............................6
5.1 Router Compatibility........................................6
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...................................7
7.0 Security Considerations......................................7
8.0 Recommendations..............................................8
9.0 Acknowledgements.............................................8
10.0 References..................................................8
11.0 Complete List of Authors' Addresses.........................9
12.0 Full Copyright Statement...................................11
13.0 Funding Acknowledgement....................................12
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
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Context Transfer Problem Statement October 2001
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 routing-related 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 routing-
related 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 routing-
related 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 routing-related service
required to re-establish the routing-related 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 routing related services
on a new subnet or collection of subnets.
Routing-related Service
A modification to the default routing treatment of packets to and
from the mobile host. Initially establishing routing-related
services usually requires a protocol exchange with the mobile
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Context Transfer Problem Statement October 2001
host. An example of a routing-related service is header
compression. The service may also be indirectly related to
routing, for example, security. Security may not affect the
forwarding decision of all intermediate routers, but a packet may
be dropped if it fails a security check (can't be encrypted,
authentication failed, etc.). Dropping the packet is basically a
routing decision.
3.0 Scope of the Context Transfer Problem
The context transfer problem examined in this document is restricted
to re-establishing routing-related services for a mobile host. It is
not concerned with re-establishing routing itself. 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 routing-related services may also function
for application-level services as well.
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 routing-related 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 Routing-Related Service Re-establishment
As mentioned in the introduction, there are a variety of routing-
related 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
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Context Transfer Problem Statement October 2001
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
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Context Transfer Problem Statement October 2001
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 routing-related 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 routing-related 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 routing-related 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
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Context Transfer Problem Statement October 2001
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 routing-related 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 routing-related 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 routing-related
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 routing-related 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.
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Context Transfer Problem Statement October 2001
- 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.
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 draft as co-authors), who were largely responsible
for the initial content of this draft, for their hard work, and
especially Gary Kenward, who shepherded the draft through its
initial versions.
10.0 References
[1] Perkins, C., editor, "IP Mobility Support," RFC 2002, October,
1996.
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Context Transfer Problem Statement October 2001
[2] Johnson, D., and Perkins, C., "Mobility Support in IPv6," draft-
ietf-mobileip-ipv6-14.txt, a work in progress.
[3] Blunk, L., and Vollbrecht, J., "PPP Extensible Authentication
Protocol (EAP)," RFC 2284, March, 1998.
[4] Simpson, W., editor, "The Point-to-Point Protocol (PPP)," STD
51, July, 1994.
[5] IEEE Std. P802.1X/D11, "Standard for Port based Network Access
Control," March, 2001.
[6] Perkins, C., and Calhoun, P., "AAA Registration Keys for Mobile
IP", draft-ietf-mobileip-aaa-key-08.txt, a work in progress.
[7] Borman, C., editor, "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.
11.0 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
250 Cambridge Ave.
Palo Alto CA 94306
USA
Phone: +1 650-617-2932
EMail: pcalhoun@bstormnetworks.com
James Kempf
NTT DoCoMo USA Laboratories
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Context Transfer Problem Statement October 2001
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
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Context Transfer Problem Statement October 2001
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
12.0 Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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Context Transfer Problem Statement October 2001
the copyright notice or references to the Internet Society or other
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13.0 Funding Acknowledgement
Funding for the RFC editor function is currently provided by the
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Kempf, editor Informational - Expires April 2002 [Page 12]
