





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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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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   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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   [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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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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