Network Working Group J. Kempf, Ed.
Request for Comments: 4830 DoCoMo USA Labs
Category: Informational April 2007
Problem Statement for Network-Based Localized
Mobility Management (NETLMM)
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 IETF Trust (2007).
Abstract
Localized mobility management is a well-understood concept in the
IETF, with a number of solutions already available. This document
looks at the principal shortcomings of the existing solutions, all of
which involve the host in mobility management, and makes a case for
network-based local mobility management.
Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. The Local Mobility Problem ......................................4
3. Scenarios for Localized Mobility Management .....................7
3.1. Large Campus ...............................................7
3.2. Advanced Cellular Network ..................................7
3.3. Picocellular Network with Small But Node-Dense Last
Hop Links ..................................................8
4. Problems with Existing Solutions ................................8
5. Advantages of Network-based Localized Mobility Management .......9
6. Security Considerations ........................................10
7. Informative References .........................................10
8. Acknowledgements ...............................................11
9. Contributors ...................................................12
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RFC 4830 NETLMM Problem Statement April 2007
1. Introduction
Localized mobility management has been the topic of much work in the
IETF. The experimental protocols developed from previous works,
namely Fast-Handovers for Mobile IPv6 (FMIPv6) [13] and Hierarchical
Mobile IPv6 (HMIPv6) [18], involve host-based solutions that require
host involvement at the IP layer similar to, or in addition to, that
required by Mobile IPv6 [10] for global mobility management.
However, recent developments in the IETF and the Wireless LAN (WLAN)
infrastructure market suggest that it may be time to take a fresh
look at localized mobility management.
First, new IETF work on global mobility management protocols that are
not Mobile IPv6, such as Host Identity Protocol (HIP) [16] and IKEv2
Mobility and Multihoming (MOBIKE) [4], suggests that future wireless
IP nodes may support a more diverse set of global mobility protocols.
While it is possible that existing localized mobility management
protocols could be used with HIP and MOBIKE, some would require
additional effort to implement, deploy, or in some cases, even
specify in a non-Mobile IPv6 mobile environment.
Second, the success in the WLAN infrastructure market of WLAN
switches, which perform localized management without any host stack
involvement, suggests a possible paradigm that could be used to
accommodate other global mobility options on the mobile node while
reducing host stack software complexity, expanding the range of
mobile nodes that could be accommodated.
This document briefly describes the general local mobility problem
and scenarios where localized mobility management would be desirable.
Then problems with existing or proposed IETF localized mobility
management protocols are briefly discussed. The network-based
mobility management architecture and a short description of how it
solves these problems are presented. A more detailed discussion of
goals for a network-based, localized mobility management protocol and
gap analysis for existing protocols can be found in [11]. Note that
IPv6 and wireless links are considered to be the initial scope for a
network-based localized mobility management, so the language in this
document reflects that scope. However, the conclusions of this
document apply equally to IPv4 and wired links, where nodes are
disconnecting and reconnecting.
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1.1. Terminology
Mobility terminology in this document follows that in RFC 3753 [14],
with the addition of some new and revised terminology given here:
WLAN Switch
A WLAN switch is a multiport bridge Ethernet [8] switch that
connects network segments but also allows a physical and logical
star topology, which runs a protocol to control a collection of
802.11 [6] access points. The access point control protocol
allows the switch to perform radio resource management functions
such as power control and terminal load balancing between the
access points. Most WLAN switches also support a proprietary
protocol for inter-subnet IP mobility, usually involving some kind
of inter-switch IP tunnel, which provides session continuity when
a terminal moves between subnets.
Access Network
An access network is a collection of fixed and mobile network
components allowing access to the Internet all belonging to a
single operational domain. It may consist of multiple air
interface technologies (for example, 802.16e [7], Universal Mobile
Telecommunications System (UMTS) [1], etc.) interconnected with
multiple types of backhaul interconnections (such as Synchronous
Optical Network (SONET) [9], metro Ethernet [15] [8], etc.).
Local Mobility (revised)
Local Mobility is mobility over an access network. Note that
although the area of network topology over which the mobile node
moves may be restricted, the actual geographic area could be quite
large, depending on the mapping between the network topology and
the wireless coverage area.
Localized Mobility Management
Localized Mobility Management is a generic term for any protocol
that maintains the IP connectivity and reachability of a mobile
node for purposes of maintaining session continuity when the
mobile node moves, and whose signaling is confined to an access
network.
Localized Mobility Management Protocol
A protocol that supports localized mobility management.
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Global Mobility Management Protocol
A Global Mobility Management Protocol is a mobility protocol used
by the mobile node to change the global, end-to-end routing of
packets for purposes of maintaining session continuity when
movement causes a topology change, thus invalidating a global
unicast address of the mobile node. This protocol could be Mobile
IP [10] [17], but it could also be HIP [16] or MOBIKE [4].
Global Mobility Anchor Point
A node in the network where the mobile node maintains a permanent
address and a mapping between the permanent address and the local
temporary address where the mobile node happens to be currently
located. The Global Mobility Anchor Point may be used for
purposes of rendezvous and possibly traffic forwarding.
Intra-Link Mobility
Intra-Link Mobility is mobility between wireless access points
within a link. Typically, this kind of mobility only involves
Layer 2 mechanisms, so Intra-Link Mobility is often called Layer 2
mobility. No IP subnet configuration is required upon movement
since the link does not change, but some IP signaling may be
required for the mobile node to confirm whether or not the change
of wireless access point also resulted in the previous access
routers becoming unreachable. If the link is served by a single
access point/router combination, then this type of mobility is
typically absent. See Figure 1.
2. The Local Mobility Problem
The local mobility problem is restricted to providing IP mobility
management for mobile nodes within an access network. The access
network gateways function as aggregation routers. In this case,
there is no specialized routing protocol (e.g., Generic Tunneling
Protocol (GTP), Cellular IP, Hawaii, etc.) and the routers form a
standard IP routed network (e.g., OSPF, Intermediate System to
Intermediate System (IS-IS), RIP, etc.). This is illustrated in
Figure 1, where the access network gateway routers are designated as
"ANG". Transitions between service providers in separate autonomous
systems, or across broader, topological "boundaries" within the same
service provider, are excluded.
Figure 1 depicts the scope of local mobility in comparison to global
mobility. The Access Network Gateways (ANGs), GA1 and GB1, are
gateways to their access networks. The Access Routers (ARs), RA1 and
RA2, are in access network A; RB1 is in access network B. Note that
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it is possible to have additional aggregation routers between ANG GA1
and ANG GB1, and the access routers if the access network is large.
Access Points (APs) PA1 through PA3 are in access network A; PB1 and
PB2 are in access network B. Other ANGs, ARs, and APs are also
possible, and other routers can separate the ARs from the ANGs. The
figure implies a star topology for the access network deployment, and
the star topology is the primary interest since it is quite common,
but the problems discussed here are equally relevant to ring or mesh
topologies in which ARs are directly connected through some part of
the network.
Access Network A Access Network B
+-------+ +-------+
|ANG GA1| (other ANGs) |ANG GB1| (other ANGs)
+-------+ +-------+
@ @ @
@ @ @
@ @ @ (other routers)
@ @ @
@ @ @
@ @ @
+------+ +------+ +------+
|AR RA1| |AR RA2|(other ARs) |AR RB1| (other ARs)
+------+ +------+ +------+
* * *
* * * * *
* * * * *
* * * * *
* * * * *
* * * (other APs) * * (other APs)
/\ /\ /\ /\ /\
/AP\ /AP\ /AP\ /AP\ /AP\
/PA1 \ /PA2 \ /PA3 \ /PB1 \ /PB2 \
------ ------ ------ ------ ------
+--+ +--+ +--+ +--+
|MN|----->|MN|----->|MN|-------->|MN|
+--+ +--+ +--+ +--+
Intra-link Local Global
(Layer 2) Mobility Mobility
Mobility
Figure 1. Scope of Local and Global Mobility Management
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As shown in the figure, a global mobility protocol may be necessary
when a mobile node (MN) moves between two access networks. Exactly
what the scope of the access networks is depends on deployment
considerations. Mobility between two APs under the same AR
constitutes intra-link (or Layer 2) mobility, and is typically
handled by Layer 2 mobility protocols (if there is only one AP/cell
per AR, then intra-link mobility may be lacking). Between these two
lies local mobility. Local mobility occurs when a mobile node moves
between two APs connected to two different ARs.
Global mobility protocols allow a mobile node to maintain
reachability when the MN's globally routable IP address changes. It
does this by updating the address mapping between the permanent
address and temporary local address at the global mobility anchor
point, or even end to end by changing the temporary local address
directly at the node with which the mobile node is corresponding. A
global mobility management protocol can therefore be used between ARs
for handling local mobility. However, there are three well-known
problems involved in using a global mobility protocol for every
movement between ARs. Briefly, they are:
1) Update latency. If the global mobility anchor point and/or
correspondent node (for route-optimized traffic) is at some
distance from the mobile node's access network, the global
mobility update may require a considerable amount of time. During
this time, packets continue to be routed to the old temporary
local address and are essentially dropped.
2) Signaling overhead. The amount of signaling required when a
mobile node moves from one last-hop link to another can be quite
extensive, including all the signaling required to configure an IP
address on the new link and global mobility protocol signaling
back into the network for changing the permanent to temporary
local address mapping. The signaling volume may negatively impact
wireless bandwidth usage and real-time service performance.
3) Location privacy. The change in temporary local address as the
mobile node moves exposes the mobile node's topological location
to correspondents and potentially to eavesdroppers. An attacker
that can assemble a mapping between subnet prefixes in the mobile
node's access network and geographical locations can determine
exactly where the mobile node is located. This can expose the
mobile node's user to threats on their location privacy. A more
detailed discussion of location privacy for Mobile IPv6 can be
found in [12].
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These problems suggest that a protocol to localize the management of
topologically small movements is preferable to using a global
mobility management protocol on each movement to a new link. In
addition to these problems, localized mobility management can provide
a measure of local control, so mobility management can be tuned for
specialized local conditions. Note also that if localized mobility
management is provided, it is not strictly required for a mobile node
to support a global mobility management protocol since movement
within a restricted IP access network can still be accommodated.
Without such support, however, a mobile node experiences a disruption
in its traffic when it moves beyond the border of the localized
mobility management domain.
3. Scenarios for Localized Mobility Management
There are a variety of scenarios in which localized mobility
management is useful.
3.1. Large Campus
One scenario where localized mobility management would be attractive
is a campus WLAN deployment, in which the geographical span of the
campus, distribution of buildings, availability of wiring in
buildings, etc. preclude deploying all WLAN access points as part of
the same IP subnet. WLAN Layer 2 mobility could not be used across
the entire campus.
In this case, the campus is divided into separate last-hop links,
each served by one or more access routers. This kind of deployment
is served today by WLAN switches that coordinate IP mobility between
them, effectively providing localized mobility management at the link
layer. Since the protocols are proprietary and not interoperable,
any deployments that require IP mobility necessarily require switches
from the same vendor.
3.2. Advanced Cellular Network
Next-generation cellular protocols, such as 802.16e [7] and Super
3G/3.9G [2], have the potential to run IP deeper into the access
network than the current 3G cellular protocols, similar to today's
WLAN networks. This means that the access network can become a
routed IP network. Interoperable localized mobility management can
unify local mobility across a diverse set of wireless protocols all
served by IP, including advanced cellular, WLAN, and personal area
wireless technologies such as UltraWide Band (UWB) [5] and Bluetooth
[3]. Localized mobility management at the IP layer does not replace
Layer 2 mobility (where available) but rather complements it. A
standardized, interoperable localized mobility management protocol
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for IP can remove the dependence on IP-layer localized mobility
protocols that are specialized to specific link technologies or
proprietary, which is the situation with today's 3G protocols. The
expected benefit is a reduction in maintenance cost and deployment
complexity. See [11] for a more detailed discussion of the goals for
a network-based localized mobility management protocol.
3.3. Picocellular Network with Small But Node-Dense Last-Hop Links
Future radio link protocols at very high frequencies may be
constrained to very short, line-of-sight operation. Even some
existing protocols, such as UWB [5] and Bluetooth [3], are designed
for low transmit power, short-range operation. For such protocols,
extremely small picocells become more practical. Although picocells
do not necessarily imply "pico subnets", wireless sensors and other
advanced applications may end up making such picocellular type
networks node-dense, requiring subnets that cover small geographical
areas, such as a single room. The ability to aggregate many subnets
under a localized mobility management scheme can help reduce the
amount of IP signaling required on link movement.
4. Problems with Existing Solutions
Existing solutions for localized mobility management fall into two
classes:
1) Interoperable IP-level protocols that require changes to the
mobile node's IP stack and handle localized mobility management as
a service provided to the mobile node by the access network.
2) Link specific or proprietary protocols that handle localized
mobility for any mobile node but only for a specific type of link
layer, for example, 802.11 [6].
The dedicated localized mobility management IETF protocols for
Solution 1 are not yet widely deployed, but work continues on
standardization. Some Mobile IPv4 deployments use localized mobility
management. For Solution 1, the following are specific problems:
1) The host stack software requirement limits broad usage even if the
modifications are small. The success of WLAN switches indicates
that network operators and users prefer no host stack software
modifications. This preference is independent of the lack of
widespread Mobile IPv4 deployment, since it is much easier to
deploy and use the network.
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RFC 4830 NETLMM Problem Statement April 2007
2) Future mobile nodes may choose other global mobility management
protocols, such as HIP or MOBIKE. The existing localized mobility
management solutions all depend on Mobile IP or derivatives.
3) Existing localized mobility management solutions do not support
both IPv4 and IPv6.
4) Existing host-based localized mobility management solutions
require setting up additional security associations with network
elements in the access domain.
Market acceptance of WLAN switches has been very large, so Solution 2
is widely deployed and continuing to grow. Solution 2 has the
following problems:
1) Existing solutions only support WLAN networks with Ethernet
backhaul and therefore are not available for advanced cellular
networks or picocellular protocols, or other types of wired
backhaul.
2) Each WLAN switch vendor has its own proprietary protocol that does
not interoperate with other vendors' equipment.
3) Because the solutions are based on Layer 2 routing, they may not
scale up to a metropolitan area or local province, particularly
when multiple kinds of link technologies are used in the backbone.
5. Advantages of Network-based Localized Mobility Management
Having an interoperable, standardized localized mobility management
protocol that is scalable to topologically large networks, but
requires no host stack involvement for localized mobility management
is a highly desirable solution. The advantages that this solution
has over Solutions 1 and 2 above are as follows:
1) Compared with Solution 1, a network-based solution requires no
localized mobility management support on the mobile node and is
independent of global mobility management protocol, so it can be
used with any or none of the existing global mobility management
protocols. The result is a more modular mobility management
architecture that better accommodates changing technology and
market requirements.
2) Compared with Solution 2, an IP-level network-based localized
mobility management solution works for link protocols other than
Ethernet, and for wide area networks.
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RFC 4831 [11] discusses a reference architecture for a network-
based, localized mobility protocol and the goals of the protocol
design.
6. Security Considerations
Localized mobility management has certain security considerations,
one of which -- the need for security from access network to mobile
node -- was discussed in this document. Host-based localized
mobility management protocols have all the security problems involved
with providing a service to a host. Network-based localized mobility
management requires security among network elements that is
equivalent to what is needed for routing information security, and
security between the host and network that is equivalent to what is
needed for network access, but no more. A more complete discussion
of the security goals for network-based localized mobility management
can be found in [11].
7. Informative References
[1] 3GPP, "UTRAN Iu interface: General aspects and principles", 3GPP
TS 25.410, 2002,
http://www.3gpp.org/ftp/Specs/html-info/25410.htm.
[2] 3GPP, "3GPP System Architecture Evolution: Report on Technical
Options and Conclusions", TR 23.882, 2005,
http://www.3gpp.org/ftp/Specs/html-info/23882.htm.
[3] Bluetooth SIG, "Specification of the Bluetooth System",
November, 2004, available at http://www.bluetooth.com.
[4] Eronen, P., "IKEv2 Mobility and Multihoming Protocol (MOBIKE)",
RFC 4555, June 2006.
[5] IEEE 802.15 WPAN High Rate Alternative PHY Task Group 3a (TG3a),
http://www.ieee802.org/15/pub/TG3a.html.
[6] IEEE, "Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) specifications", IEEE Std. 802.11, 1999.
[7] IEEE, "Amendment to IEEE Standard for Local and Metropolitan
Area Networks - Part 16: Air Interface for Fixed Broadband
Wireless Access Systems - Physical and Medium Access Control
Layers for Combined Fixed and Mobile Operation in Licensed
Bands", IEEE Std. 802.16e-2005, 2005.
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RFC 4830 NETLMM Problem Statement April 2007
[8] IEEE, "Carrier sense multiple access with collision detection
(CSMA/CD) access method and physical layer specifications", IEEE
Std. 802.3-2005, 2005.
[9] ITU-T, "Architecture of Transport Networks Based on the
Synchronous Digital Hierarchy (SDH)", ITU-T G.803, March, 2000.
[10] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[11] Kempf, J., Ed., "Goals for Network-Based Localized Mobility
Management (NETLMM)", RFC 4831, April 2007.
[12] Koodli, R., "IP Address Location Privacy and Mobile IPv6:
Problem Statement", Work in Progress, February 2007.
[13] Koodli, R., "Fast Handovers for Mobile IPv6", RFC 4068, July
2005.
[14] Manner, J. and M. Kojo, "Mobility Related Terminology", RFC
3753, June 2004.
[15] Metro Ethernet Forum, " Metro Ethernet Network Architecture
Framework - Part 1: Generic Framework", MEF 4, May, 2004.
[16] Moskowitz, R. and P. Nikander, "Host Identity Protocol (HIP)
Architecture", RFC 4423, May 2006.
[17] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August
2002.
[18] Soliman, H., Castelluccia, C., El Malki, K., and L. Bellier,
"Hierarchical Mobile IPv6 Mobility Management (HMIPv6)", RFC
4140, August 2005.
8. Acknowledgements
The authors would like to acknowledge the following for particularly
diligent reviewing: Vijay Devarapalli, Peter McCann, Gabriel
Montenegro, Vidya Narayanan, Pekka Savola, and Fred Templin.
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RFC 4830 NETLMM Problem Statement April 2007
9. Contributors
Kent Leung
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: kleung@cisco.com
Phil Roberts
Motorola Labs
Schaumberg, IL
USA
EMail: phil.roberts@motorola.com
Katsutoshi Nishida
NTT DoCoMo Inc.
3-5 Hikarino-oka, Yokosuka-shi
Kanagawa,
Japan
Phone: +81 46 840 3545
EMail: nishidak@nttdocomo.co.jp
Gerardo Giaretta
Telecom Italia Lab
via G. Reiss Romoli, 274
10148 Torino
Italy
Phone: +39 011 2286904
EMail: gerardo.giaretta@tilab.com
Marco Liebsch
NEC Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
Phone: +49 6221-90511-46
EMail: marco.liebsch@ccrle.nec.de
Editor's Address
James Kempf
DoCoMo USA Labs
181 Metro Drive, Suite 300
San Jose, CA 95110
USA
Phone: +1 408 451 4711
EMail: kempf@docomolabs-usa.com
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RFC 4830 NETLMM Problem Statement April 2007
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