Network Working Group H. Chan (Ed.)
Internet-Draft Huawei Technologies
Intended status: Informational October 19, 2010
Expires: April 22, 2011
Problem statement for distributed and dynamic mobility management
draft-chan-distributed-mobility-ps-00
Abstract
Mobility solutions deployed with centralized mobility anchoring in
existing hierarchical mobile networks are more prone to the following
problems or limitations compared with distributed and dynamic
mobility management: (1) Routing via a centralized anchor is often
longer, so that those mobility protocol deployments that lack
optimization extensions results in non-optimal routes, affecting
performance; whereas routing optimization may be an integral part of
a distributed design. (2) As mobile network becomes more flattened
centralized mobility management can become more non-optimal,
especially as the content servers in a content delivery network (CDN)
are moving closer to the access network; in contrast, distributed
mobility management can support both hierarchical network and more
flattened network as it also supports CDN networks. (3) Centralized
route maintenance and context maintenance for a large number of
mobile hosts is more difficult to scale. (4) Scalability may worsen
when lacking mechanism to distinguish whether there are real need for
mobility support; dynamic mobility management, i.e., to selectively
provide mobility support, is needed and may be better implemented
with distributed mobility management. (5) Deployment is complicated
with numerous variants and extensions of mobile IP; these variants
and extensions may be better integrated in a distributed and dynamic
design which can selectively adapt to the needs. (6) Excessive
signaling overhead should be avoided when end nodes are able to
communicate end-to-end; capability to selectively turn off signaling
that are not needed by the end hosts will reduce the handover delay.
(7) Centralized approach is generally more vulnerable to a single
point of failure and attack often requiring duplication and backups,
whereas a distributed approach intrinsically mitigates the problem to
a local network so that the needed protection can be simpler.
Status of this Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions used in this document . . . . . . . . . . . . . . 5
3. Centralized versus distributed mobility management . . . . . . 6
3.1. Centralized mobility management . . . . . . . . . . . . . 6
3.2. Distributed mobility management . . . . . . . . . . . . . 7
4. Problem statement . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Non-optimal routes . . . . . . . . . . . . . . . . . . . . 7
4.2. Network architecture evolution . . . . . . . . . . . . . . 9
4.3. Centralized route and mobility context maintenance . . . . 9
4.4. Need versus no need for mobility support . . . . . . . . . 9
4.5. Numerous variants and extensions of MIP . . . . . . . . . 10
4.6. Peer-to-peer communication . . . . . . . . . . . . . . . . 10
4.7. Single point of failure and attack . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Co-authors and Contributors . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
In the past decade a fair number of mobility protocols have been
standardized. Although the protocols differ in terms of functions
and associated message format, we can identify a few key common
features:
presence of a centralized mobility anchor providing global
reachability and an always-on experience;
extensions to optimize handover performance while users roam
across wireless cells;
extensions to enable the use of heterogeneous wireless interfaces
for multi-mode terminals (e.g. cellular phones).
The presence of the centralized mobility anchor allows a mobile
device to be reachable when it is not connected to its home domain.
The anchor, among other tasks, ensures forwarding of packets destined
to or sent from the mobile device. As such, most of the deployed
architectures today have a small number of centralized anchors
managing the traffic of millions of mobile subscribers. Coompared
with a distributed approach, a centralized approach have several
issues or limitations affecting performance and scalability, which
require costly network dimensioning and engineering to fix them.
To optimize handovers for mobile users, the base protocols have been
extended to efficiently handle packet forwarding between the previous
and new points of attachment. These extensions are necessary when
applications impose stringent requirements in terms of delay.
Notions of localization and distribution of local agents have been
introduced to reduce signalling overhead. Unfortunately today we
witness difficulties in getting such protocols deployed, often
leading to sub-optimal choices.
Moreover, all the availability of multi-mode devices and the
possibility to use several network interfaces simultaneously have
motivated the development of more new protocol extensions.
Deployment will be further complicated with so many extensions.
Mobile users are, more than ever, consuming Internet content, and
impose new requirements on mobile core networks for data traffic
delivery. When this traffic demand exceeds available capacity,
service providers need to implement new strategies such as selective
traffic offload (e.g. 3GPP work items LIPA/SIPTO) through alternative
access networks (e.g. WLAN). Moreover, the localization of content
providers closer to the Mobile/Fixed Internet Service Providers
network requires taking into account local Content Delivery Networks
(CDNs) while providing mobility services.
As long as demand exceeds capactity, both offloading and CDN
techniques could benefit from the development of more flat mobile
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architectures (i.e., fewer levels of routing hierarchy introduced
into the data path by the mobility management system). This view is
reinforced by the shift in users!_ traffic behavior, aimed at
increasing direct communications among peers in the same geographical
area. The development of truly flat mobile architectures would
result in anchoring the traffic closer to point of attachment of the
user and overcoming the suboptimal routing issues of a centralized
mobility scheme.
While deploying [Paper-Locating.User] today!_s mobile networks,
service providers face new challenges. More often than not, mobile
devices remain attached to the same point of attachment, in which
case specific IP mobility management support is not required for
applications that launch and complete while connected to the same
point of attachment. However, the mobility support has been designed
to be always on and to maintain the context for each mobile
subscriber as long as they are connected to the network. This can
result in a waste of resources and ever-increasing costs for the
service provider. Infrequent mobility and intelligence of many
applications suggest that mobility can be provided dynamically, thus
simplifying the context maintained in the different nodes of the
mobile network.
The proposed work will address two complementary aspects of mobility
management procedures: the distribution of mobility anchors to
achieve a more flat design and the dynamic activation/deactivation of
mobility protocol support as an enabler to distributed mobility
management. The former has the goal of positioning mobility anchors
(HA, LMA) closer to the user; ideally, these mobility agents could be
collocated with the first hop router. The latter, facilitated by the
distribution of mobility anchors, aims at identifying when mobility
must be activated and identifying sessions that do not impose
mobility management -- thus reducing the amount of state information
to be maintained in the various mobility agents of the mobile
network. The key idea is that dynamic mobility management relaxes
some constraints while also repositioning mobility anchors; it avoids
the establishment of non optimal tunnels between two anchors
topologically distant.
This document discusses the issues with centralized IP mobility
management compared with distributed and dynamic mobility management.
A companion document [dmm-senario] discusses the use case senarios.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
3. Centralized versus distributed mobility management
Mobility management functions may be implemented at different layers
of the OSI stack. At the IP layer, they may reside in the network or
in the mobile node. In particular, network-based solution resides in
the network only. It therefore enables mobility for hosts and
network applications which lack mobility support in them but are
already in deployment.
At the IP layer, a mobility management protocol to achieve session
continuity are typically based on the principle of distinguishing
between session identifier and routing address and maintaining a
mapping between them. With Mobile IP, the home address takes the
role of session identifier whereas the care-of-address takes the role
of routing address, and the binding between them is maintained at the
mobility anchor, i.e., the home agent.
Mobility management functions in the network may be centralized or
distributed, as is explained in the next two subsections.
3.1. Centralized mobility management
With centralized mobility management, the mapping information is kept
at a centralized mobility anchor, and packets destined to the mobile
node are routed via this anchor. That is, such mobility management
systems are centralized in both the data plane and the control plane.
Existing mobility solutions leverage on centralized mobility
anchoring in a hierarchical architecture. Examples of such
centralized mobility anchors are the home agent (HA) and local
mobility anchor (LMA) in mobile IP [RFC3775] and proxy mobile IP
[RFC5213] respectively. Current mobile networks such as UMTS
network, CDMA network, and 3GPP SAE network also use centralized
mobility management.
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UMTS 3GPP SAE MIP/PMIP
+------+ +------+ +------+
| GGSN | | P-GW | |HA/LMA|
+------+ +------+ +------+
/\ /\ /\
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
+------+ +------+ +------+ +------+ +------+ +------+
| SGSN | | SGSN | | S-GW | | S-GW | |FA/MAG| |FA/MAG|
+------+ +------+ +------+ +------+ +------+ +------+
Figure 1. Centralized mobility management.
3.2. Distributed mobility management
The mobility management functions may be distributed to multiple
locations in different networks, so that a mobile node in any of
these networks may be served by a close by mobility function (MF).
+------+ +------+ +------+ +------+
| MF | | MF | | MF | | MF |
+------+ +------+ +------+ +------+
|
----
| MN |
----
Figure 2. Distributed mobility management.
Distributed mobility management may be partially distributed, i.e.,
only the data plane is distributed, or fully distributed where both
data plane and control plane are distributed. These different
approaches are described in [I-D.dmm-scenario].
4. Problem statement
This section describes the problems or limitations in a centralized
mobility approach and compares it against the distributed approach.
4.1. Non-optimal routes
Routing via a centralized anchor often results in a longer route.
Figure 3 shows two cases of non-optimized routes. In the first case,
the mobile node and the correspondent node are close to each other
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but are both far from the mobility anchor. In the second case, the
mobile node is getting content from a local content delivery network
(CDN). In both cases, packets destined to the MN have to be routed
via the mobility anchor, which is not in the shortest path.
MIP/PMIP
+------+
|HA/LMA|
+------+
/\ \ \ +---+
/ \ \ \ |CDN|
/ \ \ \ +---+
/ \ \ \ |
/ \ \ \ |
+------+ +------+ +------+ +------+
|FA/MAG| |FA/MAG| |FA/MAG| |FA/MAG|
+------+ +------+ +------+ +------+
| |
---- ----
| CN | | MN |
---- ----
Figure 3. Non-optimized route when communicating with CN and when
accessing local content.
In a distributed mobility management design, there are numerous
anchors in different networks so that packets may be routed via a
nearby mobility function, as shown in Figure 4.
+---+
|CDN|
+---+
|
|
+------+ +------+ +------+ +------+
| MF | | MF | | MF | | MF |
+------+ +------+ +------+ +------+
| |
---- ----
| CN | | MN |
---- ----
Figure 4. Mobile node in any network is served by a close by
mobility function.
With the centralized mobility anchor design, route optimization
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extensions to mobility protocols are therefore needed, and those
mobility protocol deployments that lack such optimization extensions
will encounter non-optimal routes, which affect performance. In
contrast, route optimization may be an integral part of a distributed
design.
4.2. Network architecture evolution
The centralized mobility management is currently deployed to support
the existing hierarchical networks. It leverages on the hierarchical
architecture. However, the volume of wireless data traffic continues
to increase exponentially. The data traffic increase would require
too much costly capacity upgrade of centralized architectures. It is
thus predictable that the data traffic increase will soon overload
the centralized data anchor point, e.g., P-GW in 3GPP/SAE. In order
to address this issue, a trend in the evolution of mobile networks is
to distribute the network functions close to the access gateways.
These network functions can be the content servers in a Content
Delivery Network (CDN) but also the data anchor point.
As mobile network becomes more flattened the centralized mobility
management can become non-optimal.
Mobility management deployment with distributed architecture is then
needed to support the more flattened network and the CDN networks.
In addition, such distributed design may likely be able to also
support the hierarchical network.
4.3. Centralized route and mobility context maintenance
Routes are set up to enable session continuity when handover occurs.
Mobility contexts are also maintained to route via a mobility anchor.
Setting up such routes and maintaining the mobility context is more
difficult to scale in a centralized design with a large number of
mobile hosts. Distributing the route maintenance function and the
mobility context maintenance function among different networks can be
more scalable.
4.4. Need versus no need for mobility support
The problem of centralized route and mobility context maintenance is
aggravated when the via routes are set up for many more mobile nodes
that are not going to physically move out of a radio cell. For
example, the user may be communicating at home, in one's office, or
at a cafe. Much more resources are also wasted to maintain the
mobility context of these routes. Similarly, intelligent
applications that do not need network-layer mobility support coexist
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with applications lacking such intelligence.
Dynamic mobility management, i.e., to dynamically set up the via
routes only for mobile nodes that actually undergoes handover and
lacks higher-layer mobility support, is needed. With distributed
mobility anchors, the mechanism to support dynamic mobility may then
also be distributed. Therefore, dynamic mobility and distributed
mobility may complement each other and may be integrated.
4.5. Numerous variants and extensions of MIP
Mobile IP (MIP) already has numerous variants and extensions
including PMIP, FMIP, HMIP, and there may be more to come.
Deployment can then become complicated, especially if
interoperability with existing deployments is an issue.
Because the distributed mobility management needs to work both with
network architectures of hierarchical networks as well as flattened
networks, our design needs to be flexible enough to support different
networks. In addition, one goal of dynamic mobility management is
the capability to selectively turn on and off mobility support and
certain different mobility signaling. Such flexibility in the design
provides a good foundation to integrate the different mobility
variants. Some additional extensions to the base protocols may then
be needed to improve the integration.
4.6. Peer-to-peer communication
As MIPv6 Route Optimization (RO) mode enables a more efficient data
packets exchange than the bidirectional tunneling (BT) mode, it is
expected to be used whenever possible unless the mobile node is not
interested in disclosing its topological location, i.e., care-of
address, for the CN (e.g., for privacy reasons) or some other network
constraints are put in place.
However, MIPv6 RO mode requires exchanging a significant amount of
signaling messages in order to establish, and periodically refresh a
bidirectional security association (BSA) between the mobile node and
the correspondent node (CN). While the mobility signaling exchange
impacts the overall handoff latency, the BSA is needed to
authenticate the binding update and acknowledgment messages (note
that the latter is not mandatory). In addition, the amount of
mobility signaling messages increases further when both endpoints are
mobile.
A dynamic mobility management capability to turn off these signaling
when they are not needed will enable the RO mode between two mobile
endpoints at minimum or no cost. It will reduce the handoff latency
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owing to the removal of the extra signaling. It will encourage its
adoption and large scale deployment.
4.7. Single point of failure and attack
A centralized anchoring architecture is generally more vulnerable to
a single point of failure or attack, requiring duplication and
backups of the support functions.
A distributed mobility management architecture has intrinsically
mitigated the problem to a local network which is then of a smaller
scope. In addition, the availability of such functions in
neighboring networks has already provided the needed architecture to
support protection.
5. Security Considerations
TBD
6. IANA Considerations
None
7. Co-authors and Contributors
This problem statement document is a joint effort among the following
participants in a design team. Each individual has made significant
contributions to this work.
Dapeng Liu: liudapeng@chinamobile.com
Pierrick Seite: pierrick.seite@orange-ftgroup.com
Hidetoshi Yokota: yokota@kddilabs.jp
Charles E. Perkins: charles.perkins@tellabs.com
Melia Telemaco: telemaco.melia@alcatel-lucent.com
Hui Deng: denghui@chinamobile.com
Elena Demaria: elena.demaria@telecomitalia.it
Zhen Cao: caozhen@chinamobile.com
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Wassim Michel Haddad: Wassam.Haddad@ericsson.com
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[I-D.dmm-scenario]
Yokota, H., Seite, P., Demaria, E., and Z. Cao, "Use case
scenarios for Distributed Mobility Management",
draft-yokota-dmm-scenario-00 (work in progress),
October 2010.
[Paper-Locating.User]
Kirby, G., "Locating the User", Communication
International, 1995.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
Author's Address
H Anthony Chan (editor)
Huawei Technologies
1700 Alma Dr. Plano, TX 75075, USA
Email: h.a.chan@ieee.org
-
Dapeng Liu
China Mobile
Unit2, 28 Xuanwumenxi Ave, Xuanwu District, Beijing 100053, China
Email: liudapeng@chinamobile.com
-
Pierrick Seite
France Telecom - Orange
4, rue du Clos Courtel, BP 91226, Cesson-Sevigne 35512, France
Email: pierrick.seite@orange-ftgroup.com
-
Hidetoshi Yokota
KDDI Lab
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2-1-15 Ohara, Fujimino, Saitama, 356-8502 Japan
Email: yokota@kddilabs.jp
-
Charles E. Perkins
Tellabs Inc.
3590 N. 1st Street, Suite 300, San Jose, CA 95134, USA
Email: charles.perkins@tellabs.com
-
Melia Telemaco
Alcatel-Lucent Bell Labs
Email: telemaco.melia@alcatel-lucent.com
-
Wassim Michel Haddad
Ericsson
300 Holger Dr, San Jose, CA 95134, USA
Email: Wassam.Haddad@ericsson.com
-
Elena Demaria
Telecom Italia
via G. Reiss Romoli, 274, TORINO, 10148, Italy
Email: elena.demaria@telecomitalia.it
-
Hui Deng
China Mobile
Unit2, 28 Xuanwumenxi Ave, Xuanwu District, Beijing 100053, China
Email: denghui@chinamobile.com
-
Zhen Cao
China Mobile
Unit2, 28 Xuanwumenxi Ave, Xuanwu District, Beijing 100053, China
Email: caozhen@chinamobile.com
-
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