IETF Mobile IP Working Group E. Njedjou
Internet Draft P. Bertin
Document: draft-njedjou-inter-an-handoffs-00.txt France Telecom R&D
P. Reynolds
Orange SA
June 2003
Motivation for Network Controlled Handoffs using IP mobility
between heterogeneous Wireless Access Networks
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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This Internet Draft will expire on December 2003
Copyright Notice
Copyright (c) The Internet Society (2003). All rights reserved.
Abstract
In the near future, multi-interfaces Mobile Nodes will be used for
connecting to the Internet by way of a multitude of Radio Access
Networks including 802.11 based WLANs, GPRS, CDMA2000 and 3G based
cellular networks. Ensuring the non-disrupted flow of real-time
applications data, as well as adhering to subscribed service profiles
while the Mobile Node moves between Access Networks of different
technologies, is an issue that needs to be addressed. It is assumed
that a unified and external IP core network is used to support such a
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multitude of Access Networks. This will probably be the case for a
mobile network operator intending to benefit its subscribers with its
own hot-spots broadband internet access. Consequently, the need
arises to define managed handoff mechanisms between heterogeneous
attachment networks, while providing service continuity to the Mobile
Node.
As such, information necessary for the Mobile Node to performing a
judicious handoff across Wireless Access Networks, will have to be
gathered from the involved Access Networks, transferred across the IP
network that interconnects them, to the operators home network.
This document discusses the desirability of a network controlled
handoff process for optimizing inter-Access Network Mobile Node
mobility. The approach presented provides the means for the operator
home network to achieve the best possible selection of the Mobile
Node target Access Network for handoff, on the basis of information
gathered on the most relevant nodes. It introduces a new function
located in the operator network and referred to as a Mobility
Manager. It also introduces the concepts for implementing such a
handoff process to make it compatible with Mobile IPv6. Other
documents will be needed to specify the protocol structures that are
intended for handling the handoff process hereafter described.
Table of Content
1. Introduction...............................................2
2. Terminology................................................3
2.1. General Terms..............................................4
2.2. Specific terms.............................................5
3. Motivation for a Network Controlled Handoff................5
4. Proposed Concept...........................................7
4.1. Architectural Considerations...............................7
4.2. Protocol Considerations....................................8
5. Scenarios..................................................9
5.1. Scenario 1.................................................9
5.2. Scenario 2................................................10
5.3. Scenario 3................................................10
6. Performances Considerations...............................10
7. Security Considerations...................................11
8. References................................................11
9. Acknowledgments...........................................12
10. Author's Addresses........................................12
11. Intellectual Property Statement...........................12
1. Introduction
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Next generation multi-interfaces Mobile Nodes (MN) will be able to
gain connectivity to the Internet over a multitude of Access Networks
(AN) including 802.11 based WLANs, GPRS, CDMA2000 and 3G based
cellular networks. Ensuring seamless flow of real-time application
traffic as well as adhering to Mobile Node subscribed service
profiles whilst in handoff between such heterogeneous access links is
the challenge.
Currently, one way to handle the Layer 3 (L3) mobility of nodes is to
make use of such protocols as [MIPV4] or [MIPV6]. These protocols are
mainly concerned with describing how a MN can maintain its
connectivity to the Internet after a change of its IP point of
attachment as a result of its mobility. Using these protocols, the MN
is able to attach itself to a variety of ANs regardless of the
underlying link technology. The MN then needs to be efficiently
assisted in choosing or detecting among several available, the one
attachment link suitable for its needs, in the case where a handoff
might be unavoidable to achieve a seamless transfer of the sessions
features.
[FMIPV6] [FMIPV4] provide the means to optimize the L3 handoff
procedures by taking benefit from timely information available at
link-layer, namely Layer 2 (L2) triggers, to anticipate the change of
the MN's Access Router (AR) of attachment before the loss of current
link connectivity. In this way, handoff delays can be reduced as well
as service context information transferred from old to new default
router before the MN connects to the new subnet. Prior to performing
this Fast Handoff procedure to the new AR, it might be useful to
first select the AN where it is better for the MN to attach to, with
respect to the criteria that most satisfy its requirements.
Effectively, in [FMIPV6], for either mobile or network initiated
handoff modes, the handoff management is performed within the ANs
edge, be it in the MN or in the ARs (previous and candidate). Still,
in some situations, from a MN as well as from an AR perspective, the
view of the network might not be large and comprehensive enough to
make the most pertinent decision about the opportunity for the mobile
Node to perform a handoff to a particular AR. It then might appear
more effective to convey the link layer trigger information (or any
other information relevant to take the handoff decision) into the
provider external IP network. In this way, the decision to move the
terminal to a new link could be made relatively to information,
events or situations for which the AN edge would not have had the
knowledge.
2. Terminology
The keywords "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.
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The terminology used in this document is in conformance with that
given in [TERM]. The definitions of some of the terms are recalled
and some additional terms are defined.
2.1. General Terms
Access Point (AP)
An Access Point is a layer 2 device that is connected to the
wired Network and offers the wireless link connection to the MN.
Access Network Router (ANR)
An IP router in the Access Network. An Access Network Router may
include Access Network specific functionalities as QoS.
Access Router (AR)
An Access Network Router residing on the edge of an Access
Network and connected to one or more APs. An AR offers IP
connectivity to Mobile Node.
Access Network (AN)
An IP network which includes one or more Access Network routers
Access Network Gateway (ANG)
An ANR that separates an Access Network from other IP networks.
Capability of AR
A characteristic of the service offered by an AR that may be of
interest to a MN when the AR is being considered as a handoff
candidate.
Candidate AR (CAR)
An AR to which MN has a choice of performing IP-level handoff.
This means that MN has the right radio interface to connect to
an AP that is served by this AR.
GGSN
Gateway GPRS Support Node. A router between the GPRS network and
an external network (i.e, the Internet). The GGSN is an example
of an Access Network Gateway.
Layer 2 Handoff (L2 Handoff)
A process of terminating existing link layer connectivity and
obtaining new one. This handoff alone is transparent to the
routing at the IP layer.
Layer 3 Handoff (L3 Handoff)
A process of terminating existing network layer connectivity and
obtaining new one.
Link Layer Trigger (L2 Trigger)
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Information from L2 that informs L3 of the detailed events
involved in handoff sequencing at L2. L2 triggers are not
specific to any particular L2, but rather represent
generalizations of L2 information available from a wide variety
of L2 protocols
Mobile Node (MN)
An IP node capable of changing its point of attachment to the
network.
Inter-AN Handoff
This handoff occurs when the MN moves to a new AN. This requires
some sort of host mobility across ANs, which typically is
provided by the external IP core.
Intra-AN Handoff
This handoff occurs when the MN changes ARs inside the same AN.
2.2. Specific terms
Network Controlled Handoff (NCH)
In this handoff, the decision is taken by an external network
element
Inter-AN Network Controlled Handoff
A Network Controlled Handoff where the Mobile Node moves to a
new AN
Mobility Manager (MM)
A function that serves for the management of inter-AN mobility
of hosts.
3. Motivation for a Network Controlled Handoff
Integrating several access technologies to a single IP based core
network requires efficient management of mobility and resources among
heterogeneous ANs. In the following, it is assumed that each AN
relies on a given access technology at L2. Further, inter-AN handoffs
is considered as the main issue to be optimized even if the presented
concepts may be further applied to intra-AN handoffs.
In the near future, Mobile Nodes will be able to integrate and manage
different radio access technologies. Using alternatively, or even
simultaneously, those different radio access technologies will
require to provide new services able to dynamically adapt their
features to the capacities of available technologies and resources in
any area. Thus it is necessary to manage, efficiently, seamless
mobility among heterogeneous ANs in a transparent manner for the end
user. This leads one to consider two main issues:
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. to manage efficiently handoff schemes at the IP layer.
. to provide means to select the appropriate AN when at least two
distinct ANs are available for a given Mobile Node. These ANs
can be based on the same or different L2 technologies.
For the first issue, the Mobile IP [MIPV4][MIPV6] protocols provide
an efficient solution to perform IP based handoff applicable over
heterogeneous networks. Mobile IP efficiency can be optimized with
the implementation of Fast Handoff Schemes described in [FMIPV4],
[FMIPV6] and hierarchical approach described in [HMIPV6].
For the second issue, the use of L2 triggers [REQ] needs to be
considered in order to provide the MN with information on current
access attachment availability and capabilities. As defined in [REQ],
L2 triggers can be implemented within a MN or an Access
Router and can be carried within L3 protocols.
Thus handoff triggering is performed either in the MN or the AN,
whereas the handoff process involves several entities located in both
the AN and the Home Network, especially for inter-AN handoffs.
Indeed, with Mobile IP the MN needs to update its association in the
Home Agent located in the Home network. It can be noted that Mobile
IP Home Agent can be either located in Local
Area Networks (for example, corporate LANs), ISP platforms or
operators IP core networks. In the last two cases (the ones
considered here), the MN will never attach itself to its
Home network but move among different Visited Networks that provide
AN facilities.
When managed in the MN and/or AR, AN selection for handoff triggering
can be made only with locally available information such as MN radio
signal strength and link quality on one or several Interfaces, and AR
load and/or capabilities.
However, other information registered in the Home Network can be
relevant for making such selection: for example, user profiles,
global load of Access Networks (acquired for example from ANGs), user
preferences, operator policy, peering agreements between access and
service providers. Then, it appears that AN information, being
restricted to the MN and AR knowledge, provides only a limited view
that may lead to a non-optimal AN selection. Such optimal selection
can be obtained only when mixing different kinds of parameters
available locally and remotely in the home network.
This leads to the consideration of performing handoff triggering in a
specific network handoff control function taking care of several
types of parameters as mentioned above. This specific function could
be, for example, implemented in a home Network element like a Home
Agent (but this is not mandatory) and provided with remote ANs
information reported by the MN, ARs and/or ANGs. Such information can
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be reported through a protocol able also to transport handoff
triggering messages giving the MN the optimal target AN to handoff
to. In the following, the entity responsible for network handoff
assistance function is called the Mobility Manager (MM). Hence, MNs
can periodically report such information as link quality to the MM
which, based on information received from different MNs as well as
several parameters registered in a central database such as user
profiles and operator policy, evaluates for each MN when triggering
inter-AN handoff. It is also necessary to consider in which way
communication can be made possible between ARs or ANGs and the MM to
help in handoff decision by providing complementary information such
as the AR or global AN load.
Moreover, as the MM by performing AN selection, anticipates the MN
movement and potentially the next attachment point, it is able to
provide this information to the relevant entity in AN or Home Network
for preparing changes in path updates. Such optimization would
support planned handoff limiting packet losses.
Finally, it should be outlined that when handoff need evaluation and
decision are completely managed by the MNs, the computation of
overloading information is required and may be limited by MN
processing capabilities, especially for smart mobile devices. Hence,
a Network Controlled handoff scheme limits the computation to be done
by the Mobile Nodes.
4. Proposed Concept
4.1. Architectural Considerations
The considered reference architecture for an AN is taken from [TERM].
It can be noted that depending on the access technology, some of the
considered entities may be present or not. Typically:
. in a GPRS based AN, the ANG can be considered being implemented
at the GGSN so that the rest of the network is viewed as a L2
technology from both the MN and GGSN point of views. Thus,
neither AR nor ANR are present and intra-AN mobility is managed
at L2.
. in a 802.11 based AN, all the networking elements (AR, ANR and
ANG) can be present and allow to manage inter-AR mobility at L3
whether intra-AR mobility is done in L2.
However, this reference architecture permits the hiding of AN
entities from the rest of the external network (IP core and Home
Network), which allows the management of inter-AN mobility between AN
based on the same or even different L2 technologies.
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--- ------ ------- |
--- | <--> | | -------| AR | -------------------| | |
| |--[] --- /------ \ /| ANG |--|
--- AP / \ / | | |
MN / \ / ------- |
(+ mobile --- / ------- |
device(s)) | |---- | ANR | |
--- ------- |
AP / \ |
/ \ ------- |
--- ------ / \| | |
| |-------| AR |---------------------| ANG |--|
--- ------ | | |
AP ------- |
|
Access Network (AN) 1 |
|
Figure 1: Reference Access Network Architecture
The complete reference architecture for managing mobility among
several
ANs is given hereafter.
--------------- -------- -----------
| ----- | ( ) | Home |
| | ANG || ( ) | Agent |
| ----- | ( ) | (HA) |
| AN 1 | ( ) ------------ |
--- | --------------- ( INTERNET ) | Mobility |----
| |--[] ( OR )| Manager |
--- --------------- ( IP CORE NETWORK ) | (MM) |
| ----- | ( ) ------------
| | ANG || ( )
| ----- | ( )
| AN 2 | ( )
--------------- ---------
Figure 2: Reference Architecture for inter-AN Mobility Management
4.2. Protocol Considerations
As already introduced, the considered scheme for inter-AN Mobility
Management relies on a protocol between the MN and the MM. It is also
considered that communication could be provided between ANs and MM
for optimizing network selection with additional information. The
general requirements for such a protocol to support are:
. reporting of locally available information from MN (and maybe
AR) to the MM responsible for handoff control. The reported
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information may include radio link quality measured by the MN on
different ANs through one or several interfaces as well as
capabilities of the ARs, or they may be simple abstracted values
for example ôcan see - cant seeö. In order to avoid unnecessary
signaling, these reports can be sent only when the MN identifies
that current radio link quality is degrading or when new Access
Networks become available.
. delivering of handoff triggering messages from the MM to the MN
when the MM identifies the need for such a handoff to occur.
These trigger messages should include new AN selection
information from the MM to the MN.
. awareness about handoff triggering from the MM to other logical
entity involved in the handoff process such as HA (and maybe AR
or ANG) for making them able to prepare the handoff, for
example, by reserving adequate resources, establishing relevant
tunnels between ARs).
. transporting said information in standard Mobile IP within newly
defined extensions.
5. Scenarios
The following scenarios illustrate some advantages of a Network
Controlled approach compared to the basic scenarios assuming MN or AR
control.
In these scenarios, one makes the assumption that the MN belongs to a
mobile network operator which also provide 802.11 based WLAN access
services in hot Spot areas, a situation likely to be encountered in
the near future. The 802.11 access is always supposed to be better in
terms of access speed. One also considers a multi-interface smart MN,
for example, notebook, PDA, etc.
5.1. Scenario 1
Consider the case of such a MN that has been under 802.11 coverage
for a while. The user of the MN has registered a profile with
broadband Internet access preference. Over time, MNs number increases
within the AN, leading to saturation of the ANG. With a handoff
scheme managed from within the AN edge i.e. between MN and AR, as
long as the load and QoS capabilities of the current AR are
satisfactory to maintain the MN attachment, no handoff need would be
detected.
However, a Mobility Manager located, for instance, inside the
external IP core network of the operator (IP core serving multiple
ANs), could get information that the 802.11 AN overall load, and IP
QoS, are in a critical state and, having an overall visibility of the
situation, decide that the MN should handoff to the GPRS Access
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Network because the capabilities measured on that AN could help
maintain a service level that match its profile.
5.2. Scenario 2
Consider the MN is currently attached to a GPRS AN, and is moving
inside 802.11 coverage. One further assumes that sufficient MNs are
present in the AR serving the coverage area, for it to be overloaded.
A handoff process operated with an AN edge visibility would not
permit the MN to gain attachment to the 802.11 AN because of the
saturation at the AR.
However, A Mobility Manager knowing the profiles priorities of the
MNs currently attached to the 802.11 AR (as they would have been
stored in a home network register), could decide to instruct some of
those MNs with lower demanding needs, to handoff to the GPRS access,
giving capacity for the MN to attach to that AR. In this way, the MN
having higher demanding profiles would be able to benefit from the
WLAN facility.
5.3. Scenario 3
Consider the MN is currently attached to a GPRS network and assume
that it is moving into 802.11 coverage hosting an AN provided by a
service provider having peering agreements with the mobile operator.
The MN will have knowledge from layer 2 triggers information
(acquired from its own or from its current AR) that a 802.11 link is
becoming available. A handoff procedure managed between MN and AR
edge could then prompt the MN to handoff to a target AR of the 802.11
coverage on the sole basis of these L2 triggers information combined
to the capabilities of the ARs.
In the case where the link layer triggers information, and/or
capabilities of the ARs, were reported to a Mobility Manager these
information could be matched to other features like mobile operator
handoff policy, before prompting the terminal to handoff to the
802.11 AN or not.
6. Performances Considerations
The concept introduced below is targeted at optimizing inter-AN
handoff scenarios and is not meant to provide any alternative to
existing related work as done in [FMIPV6]: it is intended to
complement the previous approach with a specific focus on handoff
involving the crossing between heterogeneous AN technologies.
Network Controlled inter-AN handoff as presented here can effectively
be implemented so as to co-exist with Fast Handoffs. In which case,
once the selection of the candidate AN can be indicated by the MM to
the MN, a Fast Handoff process could be initiated with the intent to
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reduce the latency caused by the Mobile IP protocols operation.
Furthermore, it can be noted that the Network Controlled scheme
always applies even when such features as Fast Handoffs are not
supported at the edge of the access network.
However, in this situation, such information as AR capabilities
available with [FMIPV6] might not be profitable in the AN candidate
choice.
7. Security Considerations
It is acknowledged that there are new security threats associated
with the handoff management concept presented above.
Authentication of the local information from MN (L2 triggers,
capabilities of ARs, etc.) needed to MM to assist in handoff decision
needs to be ensured. This will guard against malicious MN pretending
to belong to the home network and requesting unauthorized handoff
services. The MN reports could contain information on ARs that should
be hidden from third parties as eavesdropper could make use of such
information to perform denial of service attacks on these sensitive
network elements.
Handoff decision from MM to MN will have to be authenticated as well
to prevent against false MMs pretending to assist the MN in its
handoff process. This information needs to be encrypted in order to
keep the MN location hidden to any eavesdropper, as the handoff
instruction message will reveal the identification of the next AN
where the MN will handoff to.
Integrity protection is necessary because of the sensitiveness of the
information exchanged between the MM in the home network and the MN.
A modified report on AN characteristics could compromise any targeted
seamless handoff.
There also might be other specific security requirements to be
fulfilled that are not identified in this document. It should
therefore be observed that the architecture and protocols extensions
to be defined with the objective to implement the concept have to
follow the general mechanisms and guidelines available from IETF
security solutions and protocols.
8. References
[MIPV4] "IP Mobility Support", C. Perkins (Editor), RFC 2002, October
1996.
[MIPV6] "Mobility Support in IPv6", D. Johnson, C. Perkins, and Jari
Arkko, draft-ietf-mobileip-ipv6-21.txt, work in progress, February
2003.
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[FMIPV6] "Fast Handoffs for Mobile IPv6", MIPv6 handoff Design Team,
draft-ietf-mobileip-fast-mipv6-06.txt, work in progress, March 2003.
[HMIPV6] "Hierarchical Mobile IPv6 mobility management", H Soliman, C
Castellucia, K El-Maki, L Bellier, draft-ietf-mobileip-hmipv6-07.txt,
work in progress, October 2002.
[REQ] "Requirements for Layer 2 Protocols to Support Optimized
Handoff for IP Mobility" J Kempf Ed, draft-manyfolks-l2-mobilereq-
00.txt, work in progress, January 2000.
[TERM] "Mobility Related Terminology", J. Manner, M. Kojo, draft-
ietf-seamoby-mobility-terminology-01.txt, work in progress,
November 2002.
9. Acknowledgments
10. Author's Addresses
Eric Njedjou
France Telecom R & D
4, Rue du Clos Courtel
35512 CESSON SEVIGNE
Phone: +33 2 99 12 48 78
Email: eric.njedjou@france.telecom.com
Philippe Bertin
France Telecom R & D
4, Rue du Clos Courtel
35512 CESSON SEVIGNE
Phone: +33 2 99 12 41 57
Email: philippe.bertin@france.telecom.com
Paul Reynolds
Orange SA
Bradley Stoke
Bristol BS32 4QJ
Phone: +44 7973 746 050
Email: paul.reynolds@orange.co.uk
11. Intellectual Property Statement
France Telecom is the owner of pending patent applications that may
relate to this Internet Draft. See France Telecom's notice Regarding
Intellectual Property Rights: http://www.ietf.org/ietf/IPR/FRANCE-
TELECOM.txt
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