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Versions: 00 01 02 03 04 RFC 6301

Network Working Group                                             Z. Zhu
Internet-Draft                                                      UCLA
Intended status: Informational                               R. Wakikawa
Expires: September 22, 2011                                   TOYOTA ITC
                                                                L. Zhang
                                                                    UCLA
                                                          March 21, 2011


              A Survey of Mobility Support In the Internet
                    draft-zhu-mobility-survey-04.txt

Abstract

   Over the last two decades many efforts have been devoted to
   developing solutions for mobility support over the global Internet,
   which resulted in a variety of proposed solutions.  We conducted a
   systematic survey of the previous efforts to gain an overall
   understanding on the solution space of mobility support.  This
   document reports our findings and identifies remaining issues in
   providing ubiquitous and efficient global scale Internet mobility
   support.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 22, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Basic Components in Mobility Support Protocols . . . . . . . .  4
   4.  Existing Mobility Support Protocols  . . . . . . . . . . . . .  5
     4.1.  Columbia Protocol  . . . . . . . . . . . . . . . . . . . .  6
     4.2.  VIP Protocol . . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  LSR Protocol . . . . . . . . . . . . . . . . . . . . . . .  9
     4.4.  Mobile IP  . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.5.  Hierarchical Mobile IP (HMIP)  . . . . . . . . . . . . . . 12
     4.6.  Fast Handover for Mobile IPv6 (FMIP) . . . . . . . . . . . 12
     4.7.  NEMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.8.  MSM-IP . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.9.  Cellular IP, HAWAII and TIMIP  . . . . . . . . . . . . . . 14
     4.10. E2E and M-SCTP . . . . . . . . . . . . . . . . . . . . . . 15
     4.11. Host Identity Protocol . . . . . . . . . . . . . . . . . . 15
     4.12. IKEv2 Mobility and Multihoming Protocol (MOBIKE) . . . . . 16
     4.13. Connexion and WINMO  . . . . . . . . . . . . . . . . . . . 16
     4.14. ILNPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.15. Global HAHA  . . . . . . . . . . . . . . . . . . . . . . . 18
     4.16. Proxy Mobile IP  . . . . . . . . . . . . . . . . . . . . . 19
     4.17. Back to My Mac . . . . . . . . . . . . . . . . . . . . . . 20
     4.18. LISP-Mobility  . . . . . . . . . . . . . . . . . . . . . . 21
   5.  Different Directions towards Mobility Support  . . . . . . . . 21
     5.1.  Routing-based Approach v.s. Mapping-based Approach . . . . 22
     5.2.  Mobility-aware Entities  . . . . . . . . . . . . . . . . . 23
     5.3.  Operator-Controlled Approach v.s. User-controlled  . . . . 24
     5.4.  Local and Global Scale Mobility  . . . . . . . . . . . . . 25
     5.5.  Other Mobility Support Efforts . . . . . . . . . . . . . . 26
   6.  Discussions  . . . . . . . . . . . . . . . . . . . . . . . . . 27
     6.1.  Deployment Issues  . . . . . . . . . . . . . . . . . . . . 27
     6.2.  Session Continuity and Simultaneous Movements  . . . . . . 28
     6.3.  Trade-offs of Design Choices on Mobility-awareness . . . . 29
     6.4.  Interconnecting Heterogeneous Mobility Support Systems . . 30
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33




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1.  Introduction

   This document reports our findings from a historical survey of the
   Internet mobility research and standardization efforts since the
   early '90s.  Our survey was motivated by two factors.  First,
   supporting mobility over the Internet has been an active research
   area and has produced a variety of solutions; some of which have
   become the Internet standards.  Yet new issues continue to arise and
   new solutions continue to be developed to address them, making one
   wonder how much more we are yet to discover about the problem space
   as well as the solution space.  The second factor is the rapid growth
   in Internet access via mobile devices in recent years, which will
   inevitably lead to new Internet application development in the coming
   years and further underscore the importance of Internet mobility
   support.  We believe that a historical review of all the proposed
   solutions on the table can help us not only identify their
   commonalities and differences, but also clarify remaining issues and
   shed insight on future efforts.

   In the rest of this document, we provide an overview of the mobility
   support solutions from the early results to the most recent
   proposals.  In the process we also discuss the essential components
   in mobility support, analyze the design space.  Through sharing our
   understanding of the current stage of the art, we aim to initiate an
   open discussion about the general direction for future mobility
   support.

   Note that the solutions discussed in this document are proposed
   designs.  They have in many cases been implemented, but only a few
   have been widely deployed in the Internet.


2.  Terminology

   This document uses the following terms to refer to the entities or
   functions that are required in mobility support.  Readers are
   expected to be familiar with RFC 3753 "Mobility Related Terminology"
   [RFC 3753] before reading this document.

   Identifier:  A stable value that can be used to identify a mobile
           node.  Any unique value can be used as an identifier as long
           as it is topologically and geographically independent, i.e.
           remains unchanged when the mobile node roams around.








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   Locator:  The IP address that indicates the mobile node's current
           attachment point to the Internet.  It could be the IP address
           of the mobile node itself, or the IP address of the network
           entity that is currently serving the mobile node.

   Mapping:  In this document, mapping specifically means the mapping
           between a mobile's identifier and its Locator.

   Rendezvous Point (RP):  The place where the mapping is held.  Some
           other functions such as data forwarding may also be co-
           located on the rendezvous point.

   Global Mobility Management:  A system that keeps track each mobile's
           reachability during the mobile's moving, either
           geographically or topologically, in a global scale.

   Local Mobility Management:  A system that keeps track each mobile's
           reachability within a topologically scoped local domain.  It
           keeps the mobile's local movements transparent to all
           entities that are outside of the local scope.

   Operator Controlled Mobility Management:  The mobile node itself is
           unaware of mobility management.  Instead, certain network
           entities, which are controlled by the network operators,
           perform all the mobility related signaling job on behalf of
           the mobile node.

   User Controlled Mobility Management:  The mobile node participates in
           the mobility management.  Typically, the mobile updates its
           reachability information after it changes locations and
           refreshes its reachability at a user-defined frequency.


3.  Basic Components in Mobility Support Protocols

   The basic question in Internet mobility support is how to send data
   to a moving receiver (a mobile in short; here we do not distinguish
   between mobile nodes and mobile subnets).  We call the host who sends
   data to a mobile the Correspondent Node (CN).  To send data to a
   moving receiver M, the CN must have means to obtain M's latest IP
   address (solution type-1), or be able to reach M using a piece of
   stable information, where "stable" means that the information does
   not change as the mobile moves (solution type-2).

   Among the existing solutions, a few fall under type-1 and most of
   them use DNS as the means to provide the CN with the mobile's most
   current IP address information.  The rest of the existing solutions
   fall under type-2, which must provide the function to reach the



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   mobile's dynamically changing location by using that unchanged
   identifier of the mobile known to the CN.  We can summarize all the
   mobility support solutions as essentially involving three basic
   components:

   o  a stable identifier for a mobile;
   o  a locator, which is usually an IP address representing the
      mobile's current location; and
   o  a mapping between the two.

   We show in the next section that different mobility support designs
   are merely different approaches to provide mapping between the
   identifiers and the mobiles' current IP addresses.  In type-1
   solutions, the stable identifier of a mobile is its DNS name, the
   locator is its current IP address, and the DNS server provides the
   mapping function.  In type-2 solutions, because the CN must be able
   to reach the mobile using the stable identifier, the identifier
   itself is typically an IP address; either the network can dynamically
   find a path to reach the mobile, or the IP address leads to the
   "home" of the mobile which knows the mobile's current locator, thus
   can forward the CN's packets to the mobile.  All the type-2 solutions
   face two common issues.  One issue is how to carry out this
   forwarding task, given the original packet sent by the CN has the
   mobile's "home address" as the destination; the other issue is how to
   avoid triangle routing between CN, the home location and the mobile.


4.  Existing Mobility Support Protocols

   In this section, we review the existing mobility support protocols
   roughly in the time order, with a few exceptions where we grouped
   closely related protocols together for writing clarity.  We briefly
   describe each design and point out how it implements the three basic
   mobility support components defined in the last section.

















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   Figure 1 shows a list of mobility support protocols and the time they
   were first proposed.

           +----------------+-----+---------------+-----+
           | Protocol Name  |Year | Protocol Name |Year |
           +----------------+-----+---------------+-----+
           |    Columbia    |1991 |    TIMIP      |2001 |
           +----------------+-----+---------------+-----+
           |      VIP       |1991 |    M-SCTP     |2002 |
           +----------------+-----+---------------+-----+
           |      LSR       |1993 |     HIP       |2003 |
           +----------------+-----+---------------+-----+
           |  Mobile IP     |1996 |   MOBIKE      |2003 |
           +----------------+-----+---------------+-----+
           |     MSM-IP     |1997 |   Connexion   |2004 |
           +----------------+-----+---------------+-----+
           |  Cellular IP   |1998 |    ILNPv6     |2005 |
           +----------------+-----+---------------+-----+
           |      HMIP      |1998 |  Global HAHA  |2006 |
           +----------------+-----+---------------+-----+
           |      FMIP      |1998 |     PMIP      |2006 |
           +----------------+-----+---------------+-----+
           |     HAWAII     |1999 |     BTMM      |2007 |
           +----------------+-----+---------------+-----+
           |      NEMO      |2000 |    WINMO      |2008 |
           +----------------+-----+---------------+-----+
           |      E2E       |2000 | LISP-Mobility |2009 |
           +----------------+-----+---------------+-----+

          Figure 1: A time table of mobility protocol development

4.1.  Columbia Protocol

   This protocol [Columbia] was originally designed to provide mobility
   support on a campus.  A router called Mobile Support Station (MSS) is
   set up in each wireless cell, which serves as the default access
   router for all mobile nodes in that cell.  The identifier for a
   mobile node is an IP address derived from a special IP prefix, and
   the mobile node uses this IP address regardless of to which cell it
   belongs.

   Each MSS keeps a tracking list of mobile nodes that are currently in
   its cell by periodically broadcasting beacons.  The mobile replies
   the MSS with a message containing its stable identifier and its
   previous MSS when it receives the beacon from a new MSS.  The new MSS
   is responsible to notify the old MSS that a mobile has left its cell.
   Each MSS also knows how to reach other MSSes (e.g. all MSSes could be
   in one multicast group, or a list of IP addresses of all MSSes could



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   be statically configured for each MSS).

   When a CN sends a packet to a mobile node, the packet goes to the
   nearest MSS (MC), which either has the mobile node in the same cell
   and can deliver directly, or otherwise broadcast a query to all other
   MSSes and gets a reply from the MSS (MM) with the mobile node.  If it
   is the latter case, MC tunnels the packet to MM, which will finally
   deliver the packet to the mobile node.

   Hence, in this scheme, CN uses the identifier to reach the mobile.
   It largely avoids triangle routing because the router next to CN is
   mobility-aware and can intercept CN's data destined to the mobile and
   forward to destination MSS.  Since a mobile keeps the same IP address
   independent from its movement, mobility does not affect TCP
   connections.

   An illustration of Columbia Approach is shown in Figure 2.

                       +---------+
                       |         |
               .------>|  MSS    |
               |       |         |
               |       +---------+
               | query
               |
            +--------+     query      +--------+
            |        | -------------->|        |
            |  MSS   | <------------- |  MSS   |
            |        |    reply       |        |
            +--------+ ==============>+--------+
               /\          data           ||
               ||                         ||
               ||                         \/
            +--------+                +---------+
            |        |                |         |
            |  CN    |                |  MN     |
            |        |                |         |
            +--------+                +---------+

               ===>: data packets
               --->: signaling packets

                        Figure 2: Columbia Protocol








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4.2.  VIP Protocol

   This design [VIP] has two basic ideas.  First, a packet carries both
   identifier and locator; second, the identifier is an IP address that
   leads to the home network where the mapping is kept.

   The IP header is modified to allow packets sent by a mobile to carry
   two IP addresses: a Virtual IP address (identifier) and a regular IP
   address (locator).  Every time the mobile node changes its location,
   it notifies the home network with its latest IP address.  A mobile's
   virtual address never changes, and can be used to support TCP
   connections independent of mobility.

   To deliver data to a mobile, the CN first uses the mobile's Virtual
   IP address as the destination IP address, i.e. the locator is set to
   be the same as the identifier.  As a result, the packet goes to the
   home network and the home agent redirects the packet to mobile's
   current location by replacing the regular IP destination address
   field with the mobile's current address.

   To reduce triangle routing, the design lets CNs and routers learn and
   cache the identifier-locator mapping carried in the packets from
   mobile nodes.  When a CN receives a packet from the mobile, it learns
   the mobile's current location from the regular IP source address
   field.  The CN keeps the mapping and uses the locator as the
   destination in future exchanges with the mobile.  Similarly, if a
   router along the data path to a mobile finds out that the mapping
   carried in the packet differs from the mapping cached by the router,
   it changes the destination IP address field to its cached value.
   This router caching solution is expected to increase the chance that
   packets destined to the mobile get forwarded to the mobile's current
   location directly, by paying a cost of having all routers examine and
   cache all the mobiles identifier-locator mappings.


















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   Figure 3 shows how VIP protocol works.

                                       ,---.       +-------+
                                      /     \      |  CN   |
                                     ( Router)<====|       |
         +---------+               // \     /      |       |
         |         |              //   `---'       +-------+
         |         |     ,---.   //
         |         |    /     \ //
         | Home    |<--+ Router)
         | Network |    \     /
         |         |     `-+-'\\
         |         |       |   \\   ,---.         +-------+
         |         |       |    \\ /     \=======>|       |
         |         |       +------( Router)<------+  MN   |
         |         |               \     /        |       |
         |         |                `---'         +-------+
         +---------+

                   ===>: data packet
                   --->: location update message

                          Figure 3: VIP Protocol

4.3.  LSR Protocol

   In Loose Source Routing (LSR) protocol [LSR], each mobile has a
   designated router, called Mobile Router, that manages its mobility.
   Mobile Router assigns an IP address (used as an identifier) for each
   mobile it manages and announces reachability to those IP addresses.
   Another network entity in the LSR design is Mobile Access Station
   (MAS), through which a mobile gets its connectivity to the Internet.
   The mobile node reports the IP address of its current serving MAS
   (locator) to its Mobile Router.

   The CN uses the identifier to reach the mobile node in the first
   place.  If the CN and the mobile node are attached to the same MAS,
   the MAS simply forwards packets between the two (in this case CN is
   also mobile); otherwise, the packet from CN is routed to the Mobile
   Router of the mobile.  The Mobile Router looks up the mappings to
   find the serving MAS of the mobile node, and inserts the loose source
   routing (LSR) option into the IP header of the packet with the IP
   address of the MAS on it.  In this way, the packet is redirected to
   the MAS which then delivers the packet to the mobile.  To this point,
   the locator of the mobile node is already included in the LSR option,
   and the two parties can communicate directly by reversing the LSR
   option in the incoming packet.  Hence, the path for the first packet
   from CN to the mobile is: CN->Mobile Router->MAS->mobile node; and



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   then the bi-directional path for the following packets is: mobile
   node<->MAS<->CN.

   The triangle routing is avoided by revealing the mobile's locator to
   the CN in the LSR option.

   Figure 4 shows the basic operation of LSR protocol.

                                      +---------+
                                      |         |
                   ___________________|  CN     |
                  |                   |         |
                  |                   +---------+
                  V                      /\
             +-------+                   ||
             |Mobile |                   ||
             |Router |                   ||
             |       |                   || Reversing LSR
             +---+---+                   ||
                 |                       \/
                 |                    +---------+      +----------+
                 |  LSR Inserted      |         |<====>|          |
                 +------------------->|  MAS    |      |  MN      |
                                      |         |----->|          |
                                      +---------+      +----------+

                        -->: first data packet
                        ==>: following data packets


                          Figure 4: LSR Protocol

4.4.  Mobile IP

   IETF begun standard development in mobility support soon after the
   above three protocols.  The first version of Mobile IP standard was
   developed in 1996.  Later, IETF further made Mobile IPv4 [RFC 3344]
   and Mobile IPv6 [RFC 3775] standards in 2002 and 2004, respectively.
   In 2009, Dual-Stack Mobile IPv4 [RFC 5454] was standardized to allow
   a dual-stack node to use IPv4 and IPv6 home addresses and to move
   between IPv4 and dual stack network infrastructures.

   Although the three documents differs in details, the high-level
   design is similar.  Here we use Mobile IPv6 as an example.  Each
   mobile node has a Home Agent, from which it acquires its Home Address
   (HoA), the identifier.  The mobile node also obtains its locator, a
   Care-of Address (CoA) from its current access router.  Whenever the
   mobile node gets a new CoA, it sends a Binding Update message to



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   notify the Home Agent.  Conceptually Mobile IPv6 design looks similar
   to VIP Protocol, with the mobile's HoA corresponding to the Virtual
   IP Address in VIP, and the CoA corresponding to the regular IP
   address.

   The CN uses the mobile's HoA as the destination IP address when
   sending data to a mobile.  The packets are forwarded to the Home
   Agent , which then encapsulates the packets to mobile node's CoA
   according to the mapping.

   To alleviate triangle routing, the CN, if supports Route
   Optimization, also keeps the mapping between the mobile's HoA and
   CoA.  Thus the CN can encapsulate packets to the mobile directly,
   without going through the Home Agent.  Note that in this case, the
   mobile needs to update its CoA to CNs as well.

   Figure 5 illustrates the data path of Mobile IPv6 without Route
   Optimization.


                              +---+-----+
                              |HoA|DATA |
                              +---+-----+           +-------+
                             +----------------------| CN    |
                             | +------------------->|       |
                             | |                    +-------+
                             | |
                             V |
                          +--------+
                          | Home   |  Mapping: HoA <=> CoA
                          | Agent  |
                          |        |
                          +--------+
                            ||  /\
                            ||  ||                   +-------+
                            ||  +====================|       |
                            ||                       | MN    |
                            +=======================>|       |
                              +-----+---+---+        +-------+
                              |DATA |HoA|CoA|
                              +-----+---+---+


                                      ==>: Tunnel
                                      -->: regular IP

             Figure 5: Mobile IPv6 without Route Optimization




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4.5.  Hierarchical Mobile IP (HMIP)

   HMIP [RFC 5380] is a simple extension to Mobile IP.  It aims to
   improves the performance of Mobile IP by handling mobility within a
   local region locally.  A level of hierarchy is added to Mobile IP in
   the following way.  A Mobility Anchor Point (MAP) is responsible for
   handling the movements of a mobile in a local region.  Simply
   speaking, MAP is the local Home Agent for the mobile node.  The
   mobile node, if it supports HMIP, obtains a Regional CoA (RCoA) and
   registers it with its Home Agent as its current CoA; while RCoA is
   the locator for the mobile in Mobile IP, it is also its regional
   identifier used in HMIP.  At the same time, the mobile obtains a
   Local CoA (LCoA) from the subnet it attaches to.  When roaming with
   the region, a mobile only updates the MAP with the mapping between
   its RCoA and LCoA.  In this way, the handoff performance is usually
   better due to the shorter round-trip time between the mobile and the
   MAP, as compared to the delay between the mobile and its HA.  It also
   reduces the burden of the Home Agents by reducing the frequency of
   sending updates to Home Agents.

4.6.  Fast Handover for Mobile IPv6 (FMIP)

   FMIP [RFC 5568] is another extension to Mobile IP, which reduces the
   Binding Update latency as well as the IP connectivity latency.  It is
   not a fully fledged mobility support protocol; rather, its only
   purpose is to optimize the performance of Mobile IP.

   This goal is achieved by three mechanisms.  First, it enables a
   mobile node to detect that it has moved to a new subnet while it is
   still connected to the current subnet, by providing the new access
   point and the corresponding subnet prefix information.  Second,
   mobile node can also formulate a prospective new care-of address
   (NCoA) when it is still present on the previous link, so that this
   address can be used immediately after it attaches to the new subnet
   link.  Third, to reduce the Binding Update interruption, FMIP
   specifies a tunnel between the previous care-of address (PCoA) and
   the NCoA.  The mobile node send a Fast Binding Update to the previous
   access router (PAR) after the handoff and PAR begins to tunnel
   packets for PCoA to NCoA.  These packets would have been dropped if
   the tunnel were not established.  In the reverse direction, the
   mobile node also tunnels packets to PAR until it finishes the Binding
   Update process (mobile node can only use PCoA now because the binding
   in HA or the correspondent nodes may have not been updated yet).

4.7.  NEMO

   It is conceivable to have a group of hosts moving together.  Consider
   vehicles such as ships, trains, or airplanes which may host a network



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   with multiple hosts attached to.  Because Mobile IP handles mobility
   per host, it is not efficient when handling such mobility scenarios.
   NEMO [RFC 3963], as a backward compatible extension to Mobile IP, was
   introduced in 2000 to provide efficient support for network mobility.

   NEMO introduces a new entity call Mobile Router (note that this is
   different from the "Mobile Router" in LSR protocol).  Every mobile
   network has at least one Mobile Router.  Mobile Router is similar to
   a mobile node in Mobile IP, but instead of having a single HoA, it
   has one or more IP prefixes as the identifier.  After establishing
   bidirectional tunnel with Home Agent, the Mobile Router distributes
   its mobile network's prefixes (namely Mobile Prefixes) through the
   tunnel to Home Agent.  The Mobile Prefix of a mobile network is not
   leaked to its access router (i.e. the access router never knows that
   it can reach the Mobile Prefixes via the Mobile Router).  The Home
   Agent in turn announces the reachability to the Mobile Prefix.
   Packets to and from mobile network flow through the bidirectional
   tunnel between the Mobile Router and the Home Agent to their
   destinations.  Note that mobility is transparent to the nodes in the
   moving network.

4.8.  MSM-IP

   MSM-IP [MSM-IP] stands for Mobility Support using Multicast in IP.
   As one can see from its name, MSM-IP leverages IP multicast routing
   for mobility support.  In IP multicast, a host can join a group
   regardless of to which network it attaches and receive packets sent
   to the group after its join.  Thus mobility is naturally supported in
   the domains where IP multicast is deployed .  Note that MSM-IP does
   not address the issue of feasibility of supporting mobility through
   IP multicast, but rather it simply shows the possibility of using IP
   multicast to provide mobility support, once/if IP multicast is
   universally deployed.

   MSM-IP [MSM-IP] assigns each mobile node a unique multicast IP
   address as the identifier.  When the mobile node moves into a new
   network, it initiates a join to its own address, which makes the
   multicast router in that subnet join the multicast distribution tree.
   Whoever wants to communicate with the mobile node can just send the
   data to the mobile's multicast IP address, and the multicast routing
   will take care of the rest.

   Note that, due to the nature of multicast routing, the mobile node
   can have the new multicast router join the group to cache packets in
   advance before it detaches the old one, resulting in smoother
   handoff.





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4.9.  Cellular IP, HAWAII and TIMIP

   This is a group of protocols that share the common idea of setting up
   host route for each mobile in the local domain.  The mobile retains
   an stable IP address as long as it is within the local domain, and
   this IP address is used as a regional identifier.  The gateway router
   of the local domain will use this identifier to reach the mobile
   node.  All three protocols are intended to work with Mobile IP as a
   local mobility management protocol.  By describing them together we
   can more easily to show the differences by comparison.

   Cellular IP [CIP] handles the local mobility in a network consists of
   Cellular IP routers.  A mobile reports the IP address of the gateway
   for the local network as the RCoA to its Home Agent, and retains its
   locally assigned IP address (the regional identifier) when it roams
   within the Cellular IP network.  The routers in the network monitors
   the packets originated from mobile nodes and maintains a distributed,
   hop-by-hop reverse path for each mobile node.  It utilizes paging
   technique from cellular network to track the location of each mobile:
   idle mobile nodes send dummy packets to the gateway router with a
   relatively low frequency to update their reverse paths in the
   routers.  The out-dated path will not be cleared explicitly after the
   mobile changes its location; instead, it would be flushed by the
   routers if the paging timer expires before next dummy packet comes.
   To reduce the paging cost, only a subset of the routers would set up
   reverse path for the idle mobile nodes.

   When a packet from the CN arrives at the gateway, the gateway
   initiates a controlled flooding query: if a router knows where to
   forward a packet, forward it immediately; otherwise, it forwards the
   packet to all its interfaces except the one from which the packet
   comes.  Due to the paging technique, this will not become a
   broadcast.  Once the mobile receives the query, it replies a route-
   update message to the gateway, and a much more precise reverse path
   is then maintained by the all routers along the data path, via which
   the gateway router forwards packets from CN to the mobile.  Note that
   the timer value for the precise data path is much more smaller than
   the paging timer value, in order to avoid sending duplicate data
   packets to multiple places if the mobile moves during the data
   communication.

   Similarly, HAWAII [HAWAII] also aims to provide efficient local
   mobility support.  Unlike Cellular IP, the route between the gateway
   router and the mobile is always maintained.  When the mobile moves,
   HAWAII dynamically modifies route to the mobile by installing host-
   based forwarding entry on the routers located along the shortest path
   between the old and new base stations of the mobile.  It is possible
   that longer suboptimal routing path will be constructed (e.g. gateway



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   router->old base station->new base station->mobile).  Alternatively,
   a new sub-path between the mobile and the cross-over router can be
   established.  Here, the cross-over router is the router at the
   intersection of two paths, one between the gateway and the old base
   station, and the second between the old base station and the new base
   station.  In HAWAII, the mobile only periodically send refresh
   messages to the base station, and the base station along with other
   routers would take care of the path maintenance.

   TIMIP [TIMIP], which stands for Terminal Independent Mobile IP,
   integrated together the design of Cellular IP and HAWAII.  On one
   hand, it refreshes the routing paths with dummy packets if the mobile
   node is idle.  On the other hand, handoff within a domain results in
   the changes of routing tables in the routers.  Besides, the IP layer
   is coupled with layer 2 handoff mechanisms and special nodes can work
   as Mobile IP proxies for legacy mobiles that do not support Mobile
   IP.  Thus, as long as the mobile roams within the domain, the legacy
   node has the same degree of mobility support as a Mobile IP capable
   node.

4.10.  E2E and M-SCTP

   E2E (End-to-End communication) [E2E] gets the name from its end-to-
   end architecture, and is the first proposal that utilizes existing
   DNS service to track mobile node's current location.  The stable
   identifier here is the domain name of the mobile.  The mobile uses
   Dynamic DNS update to update its current IP address in DNS servers.
   To keep the ongoing TCP connection unaffected by mobility, a TCP
   Migrate option is introduced to allow both ends to replace the IP
   addresses and ports in TCP 4-tuple on the fly.  Thus, the CN can
   query DNS to obtain the current locator of the mobile, and after the
   TCP connection is established, the mobile will be responsible for
   update its locator for this session.

   Inspired by E2E, M-SCTP [M-SCTP] was proposed in 2002.  Similarly, it
   uses Dynamic DNS to track the mobile nodes and allows both ends to
   add/delete IP addresses used in SCTP associations during the move.

4.11.  Host Identity Protocol

   Host Identify Protocol (HIP) [RFC 5201] assigns to each host an
   identifier made of cryptographic keys, and adds a new Host Identity
   layer between transport and network layers.  Host Identities, which
   are essentially public keys, are used to identify the mobile nodes,
   and IP addresses are used only for routing purpose.  In order to
   reuse the existing code, Host Identity Tag (HIT), which is a 128-bit
   hash value of the Host Identity, is used in transport and other upper
   layer protocols.



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   HIP can use DNS as the rendezvous point which holds the mappings
   between HITs and IP addresses.  However, HIP by default uses its own
   static infrastructure Rendezvous Servers, in expectation of better
   rendezvous service.  Each mobile node has a designated Rendezvous
   Server (RVS), which tracks the current location of mobile node.  When
   a CN wants to communicate with mobile node, it queries DNS with
   mobile node's HIT to obtain the IP address of mobile node's RVS, and
   sends out the first packet.  After receiving this first packet, RVS
   relays it to mobile node.  Then mobile node and correspondent node
   can start communication on the direct path.  If the mobile node moves
   to a new address, it notifies CN by sending HIP UPDATE with LOCATOR
   parameter indicating its new IP address (locator).  Meanwhile, it
   also updates the mapping in RVS.

4.12.  IKEv2 Mobility and Multihoming Protocol (MOBIKE)

   MOBIKE [RFC 4555] is an extension to Internet Key Exchange (IKEv2) to
   support mobility and multihoming.  The main purpose of MOBIKE is to
   allow roaming devices to keep the existing IKE and IPsec SAs despite
   of IP address changes.  The mobility support in MOBIKE allows both
   parties to move, but it does not provide a rendezvous mechanism.  In
   other words, simultaneous movement of both parties is not supported.

   MOBIKE allows both parties to have a set of addresses, and the party
   that initiated the IKE_SA is responsible for deciding which pair of
   addresses to use.  During the communication session, if the initiator
   wishes to change the addresses due to movement, it updates the IKE_SA
   with new IP addresses, and also updates the IPsec SAs associated with
   this IKE_SA.  Then it sends an INFORMATIONAL request containing the
   UPDATE_SA_ADDRESSES notification to the other party.  The responder
   then checks the local policy and updates the IP addresses in the
   IKE_SA with the values from the IP header.  It replies the initiator
   with an INFORMATIONAL response, initiates a return routability check
   if it wants to, and updates the IPsec SAs associated with this
   IKE_SA.

   MOBIKE is not a fully fledged mobility protocol, and it does not
   intend to be one.  Nevertheless, through the use of IPsec tunnel
   mode, MOBIKE partially supports mobility as it can dynamically
   updates the tunnel endpoint addresses.

4.13.  Connexion and WINMO

   Connexion [Boeing] was a mobility support service provided by Boeing
   that uses BGP to support network mobility.  Every mobile network is
   assigned a /24 IP address prefix (stable identifier), and the CN uses
   this identifier to reach the moving network, which means that the
   global routing system is responsible for finding a path to the mobile



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   network.  When an airplane moves between its access routers on
   ground, it withdraws its prefix from the previously access router and
   announces the prefix via the new access point.  As a result, the
   location change of the plane is effectively propagated to the rest of
   the world.  However, if the number of moving networks becomes large,
   the amount of BGP updates will also increase proportionally,
   resulting in severe global routing dynamics.

   WINMO [WINMO] (which stands for Wide-Area IP Network Mobility) was
   introduced in 2008 to address the routing update overhead problem of
   Connexion.  Like Connexion, WINMO also assigns each mobile network a
   stable prefix.  However, through two new approaches WINMO can reduce
   the BGP updates overhead for mobile networks by orders of magnitude
   lower than that of Connexion.  First, WINMO uses various heuristics
   to reduce the propagation scope of routing updates caused by mobile
   movements.  Consequently, not every router may know all the mobiles'
   current locations.  Handling this issue led to the second, and more
   fundamental approach taken by WINMO: it adopts the basic idea from
   Mobile IP by assigning each mobile network a "home" in the following
   way.  WINMO assigns each mobile network a prefix out of a small set
   of well defined Mobile Prefixes.  These Mobile Prefixes are announced
   by a small set of Aggregation Routers which also keep track of the
   mobile networks current locations.  Therefore these Aggregation
   Routers play a similar role to Home Agents in Mobile IP, and can be
   counted on as last resort to reach mobile networks globally.

   To prevent frequent iBGP routing updates due to the movement of
   mobile networks within an AS, WINMO also introduces a Home Agent for
   the Mobile Prefixes: only a Designated BGP-speaking Router (DBR) acts
   as the origin of Mobile Prefixes; mobile networks always update the
   addresses of their access routers (intra-AS locators) with DBR, which
   resembles the binding updates in Mobile IP.  Thus, packets destined
   to mobile networks are forwarded to DBR after they enter the border
   of an AS, and DBR will tunnel them to the current locations of mobile
   networks.

   A new BGP community attribute, which includes the mobile network's
   intra-AS locator in each packet, is also defined to eliminate the
   triangle routing problem caused by DBR.  The border routers of the AS
   can tunnel packets directly to the mobile network based on the new
   attribute.

4.14.  ILNPv6

   ILNPv6 [ILNP] stands for Identifier-Locator Network Protocol for
   IPv6.  The ILNPv6 packet header are deliberately made similar to IPv6
   header.  Essentially, it breaks IPv6 address into two components:
   high-order 64 bits as a Locator and low-order 64 bits as an



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   Identifier.  The Identifier identifies a host, instead of an
   interface, and is used in upper-layer protocols (e.g.  TCP, FTP); on
   the other hand, the Locator changes with the movement of the mobile
   node, and a set of Locators can be associated with a single
   Identifier.  Several new DNS RRs are required, among which I
   (Identifier Record) and L (Locator Record) are most important.  As in
   current Internet, the CN will query the DNS about the mobile's domain
   name to determine where to send the packet.  During the movement, the
   mobile node uses Secure Dynamic DNS update to ensure that the Locator
   values stored in DNS are up-to-date.  It also sends Locator Update
   messages to the CNs that are currently communicating with it.  As an
   optimization, ILNPv6 supports soft-handoff, which allows the use of
   multiple Locators simultaneously to achieve smooth transition.
   ILNPv6 also supports mobile networks.

4.15.  Global HAHA

   Global HAHA [HAHA], first proposed in 2006 as an extension to Mobile
   IP, aims to eliminate the triangle routing problem in Mobile IP and
   NEMO by distributing multiple Home Agents globally.  All the Home
   Agents join an IP anycast group and form an overlay network.  The
   same home prefix is announced by all the Home Agents from different
   locations.  Each mobile node can register with any Home Agent that is
   closest to it.  A Home Agent H that accepts the binding request of a
   mobile node M becomes the primary Home Agent for M, and notifies all
   other Home Agents of the binding [M, H], so that the binding
   information databases for all the mobiles in all Home Agents are
   always synchronized.  When a mobile moves, it may switch its primary
   Home Agent to another one that becomes closest to the mobile.

   A correspondent node sends packets to a mobile's Home Address.
   Because of anycast routing, the packets are delivered to the nearest
   Home Agent.  This Home Agent then encapsulates the packets to the IP
   address of the primary Home Agent that is currently serving the
   mobile node, which will finally deliver the packets to mobile node
   after striping off the encapsulation headers.  In the reverse
   direction, this approach works exactly the same as Mobile IP.  If the
   Home Agents are distributed widely, the triangle routing problem is
   naturally alleviated without Route Optimization.












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   The data flow in Global HAHA is shown in Figure 6.

                 +------+             +------+     +-----+
                 | HA   |-------------|  HA  |     |     |
                 |      |             |      |     |  CN |
                 +--+---+      +------+++----+     +-----+
                    |          |       ||             /\
                    |          |       ||             ||
                    |          |       ||             ||
                    |          |       ||             ||
                 +--+---+------+       ||             ||
                 |      |<==============+             ||
                 | HA   |==============================+
                 +-++---+
                   || /\
                   \/ ||
                  +---++-+           ===>: data flow
                  |      |           ----: HA overlay network
                  | MN   |
                  +------+

                                 Figure 6

4.16.  Proxy Mobile IP

   Proxy Mobile IP [RFC 5213] was proposed in 2006 to meet the interest
   of mobile network operators who desire to support mobility in a
   network rather than at mobile devices and to have tighter control on
   mobility support.  Mobility is completely transparent to the mobile
   devices and is provided to legacy IP devices.  PMIP introduces two
   new types of network nodes, Local Mobility Anchor (LMA) and Mobile
   Access Gateway (MAG), which together can support mobility within an
   operator's network without any action taken by the mobile node.  LMA
   serves as a local Home Agent and assigns a local Home Network Prefix
   for each mobile node.  This prefix is the identifier for the mobile
   node within the PMIP domain.  MAGs monitor the attaching and
   detaching events of mobile node, and generates Proxy Binding Update
   to LMA on behalf of mobile node during handoff.  After the success of
   binding, LMA updates mobile node's Proxy-CoA (locator in PMIP domain)
   with the IP address of the MAG that is currently serving mobile node.
   The MAG then emulates mobile node's local Home Link by advertising
   mobile node's local Home Network Prefix in Router Advertisement.
   When roaming in the PMIP domain, mobile node always obtains its local
   Home Prefix, and believes that its on local Home Link.  Within the
   domain, the mobile node is reached by the identifier and LMA tunnels
   packets to the mobile node according to the mapping.





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4.17.  Back to My Mac

   Back to My Mac (BTMM) [BTMM] is an engineering approach to mobility
   support and has been deployed since 2007 with Mac OS leopard release.
   Each user gets a MobileMe account (which includes BTMM service), and
   Apple Inc. provides DNS service for all BTMM users.  The reachability
   information of the user's machine is published in DNS.

   A mobile uses secure DNS update to dynamically refresh its current
   location.  Each host generates an IPv6 ULA [RFC 4193] at boot time,
   which is stored in the DNS database as its topologically independent
   identifier.  The host's current IPv4 address (which is the IPv4
   address of the NAT box if the host is behind a NAT) is stored in a
   SRV resource record [RFC 2782], together with a transport port number
   needed for NAT traversal.  Every node establishes long-lived query
   (llq) session with the DNS server, so that the DNS server can
   immediately notify each node when the answer to its query has
   changed.  A host uses its identifier in transport protocols and
   applications, and uses UDP/IPv4 encapsulation to deliver data packets
   using information learned from the SRV RR.  Note that the locator
   here is the IPv4 address plus the transport port number and that the
   IPv6 address is only for identification purpose.  In fact, it could
   be any form of identifier (e.g. domain name); BTMM chose to IPv6
   address so that its implementation can reuse existing code.

   BTMM is currently used by millions of subscribers.  It is simple and
   easy to deploy.  However, the current applications use BTMM service
   in a "stop-and-reconnect" fashion.  It remains to be seen how well
   BTMM can support continuous communications while hosts are on the
   move, for example as needed for voice calls.





















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   Figure 7 shows the basic architecture of BTMM.

           DDNS update    +--------+  DDNS update
         +--------------->|        |<-------+
         |                |  DNS   |        |
         |      LLQ       |        | LLQ    |
         |    +---------->|        |<----+  |
         |    |           |        |     |  |
         |    |           +--------+     |  |
         |    |                          |  |            +---------+
         |    V                      +---+--+----+       |         |
        ++-------+                   |           +-------|         |
        |Endhost1|     Tunnel        |    NAT    +------>|Endhost2 |
        |        |<=====================================>|         |
        +--------+                   |           |       |         |
                                     +-----------+       +---------+

                                 Figure 7

4.18.  LISP-Mobility

   LISP-Mobility [LISP-Mobility] is a relatively new design.  Its
   designers hope to utilize functions and services provided by LISP
   [LISP], which is designed for Internet routing scalability, to
   support mobility as well.  Conceptually, LISP-Mobility may seem
   similar to some protocols we have mentioned so far, such as ILNPv6
   and Mobile IP.  Light-weight Ingress Tunnel Router and Egress Tunnel
   Router functions are implemented on each mobile node, and all the
   packets to and from the mobile node are processed by the two router
   functions (so the mobile node looks like a LISP site).  Each mobile
   node is assigned a static Endpoint ID , as well as a pre-configured
   Map-Server.  When a mobile node roams into a network and obtains a
   new Routing Locator, it updates its Routing Locator set in the Map-
   Server, and it also clears the cached Routing Locator in the Ingress
   Tunnel Routers or Proxy Tunnel Routers of the CNs.  Thus the CN can
   always learn the up-to-date location of the mobile node by the
   resolution of the mobile node's Endpoint ID, either issued by itself
   or issued after receiving the notification from the mobile node about
   the staled cache.  The data would always travel through the shortest
   path.  Note that both Endpoint IDs and Routing Locators are
   essentially IP addresses.


5.  Different Directions towards Mobility Support

   After studying various existing protocols, we identified several
   different directions for mobility support.




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5.1.  Routing-based Approach v.s. Mapping-based Approach

   All existing mobility support designs can be broadly classified into
   two basic approaches.  The first one is to support mobility through
   dynamic routing.  In such designs, a mobile keeps its IP address
   regardless of its location changes, thus the IP address can be used
   both to identify the mobile and to deliver packets to it.  As a
   result, these designs do not need an explicit mapping function.
   Rather, the routing system must continuously keep track of mobile's
   movements and reflect their current positions in the network on the
   routing table, so that at any given moment packets carrying the
   (stable) receiver's IP address can be delivered to the right place.

   It is also worthwhile to identify two sub-classes in routing-based
   approaches.  One is broadcast based, and the other is path based.
   That is, in the former case, either the mobile's location information
   is actively broadcasted to the whole network or a proactive broadcast
   query is needed to obtain the location information of a mobile (e.g.
   Columbia, Connexion); in the latter case, on the other hand, a host-
   based path is maintained by the routing system instead (e.g.
   Cellular IP, HAWAII, TIMIP).

   Supporting mobility through dynamic routing is conceptually simple;
   it can also provide robust and efficient data delivery, assuming that
   the routing system can keep up with the mobile movements.  However,
   because either the whole network must be informed of every movement
   by every mobile, or otherwise a host-based path must be maintain for
   every mobile host, this approach is feasible only in small scale
   networks with a small number of mobiles; it does not scale well in
   large networks or for large number of mobiles.

   The second approach to mobility support is to provide a mapping
   between a mobile's stable identifier and its dynamically changing IP
   address.  Instead of notifying the world on every movement, a mobile
   only needs to update a single binding location about its location
   changes.  In this approach, if one level of indirection at IP layer
   is used, as in the case of Mobile IP, it has a potential side effect
   of introducing triangle routing; otherwise, if the two end nodes are
   aware of each other's movement, it means that both ends have to
   support the same mobility protocol.

   Yet there is the third case in which the protocols combine the above
   approaches, in the hope of keeping the pros and eliminating some cons
   of the two.  WINMO is a typically protocol in this case.







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   In Figure 8 we show the classification of the existing protocols
   according to the above analysis.


   +---------------+-------------------------------------------+
   |               | VIP, LSR, Mobile IP, HMIP, NEMO,  E2E     |
   | Mapping-based | M-SCTP, ILNPv6, HIP, FMIP, PMIP,          |
   |               | BTMM, GLOBAL HAHA, LISP-Mobility          |
   +---------------+-------------------------------------------+
   |               | Columbia, Connexion                       |
   | Routing-based +-------------------------------------------+
   |               | Cellular IP, HAWAII, TIMIP, MSM-IP        |
   +---------------+-------------------------------------------+
   | Combination   | WINMO                                     |
   +---------------+-------------------------------------------+

                                 Figure 8

5.2.  Mobility-aware Entities

   Among the various design choices, a critical one is how many entities
   are assumed to be mobility-aware; stated in another way, the mobility
   is hidden from which parties.  There are four parties that may be
   involved during a conversation with a mobile: the mobile itself, CN,
   the network, and Home Agent or its equivalent (additional component
   to the existing IP network that holds the mapping).  We mainly focus
   our discussion on mapping-based approach here.

   The first design choice is to hide the mobility from the CN, based on
   the assumption that the CN may be the legacy node that does not
   support mobility.  In this approach, the IP address which is used as
   the mobile's identifier points to the Home Agent or its equivalent
   that keeps track of the mobile's current location.  If a
   correspondent node wants to send packets to a mobile node, it sets in
   the destination field of IP header an IP address which is a mobile's
   identifier.  The packets will be delivered to the location where the
   mapping information of the mobile is kept, and later they will be
   forwarded to the mobile's current location via either encapsulation
   or destination address translation.  Mobile IP and most of its
   extensions, as well as several other protocols fall into this design.

   The second design choice is to hide the mobility from the mobile and
   CN, which is based on a more conservative assumption that both the
   mobile and the CN do not support mobility.  Protocols like PMIP and
   TIMIP adopt this design.  The protocol operations in this design
   resemble those in the first category, but significant difference is
   that, here the mobility related signaling (e.g. update locator to the
   Home Agent) is handled by the entities in the network, rather than



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   the mobile itself.  Hence the mobile blissfully assumes that it is
   always in the same subnet.

   The third one is to let both mobile and the CN to be mobility-aware.
   As a result, the network is not aware of the mobility and no
   additional component is required.  As increasing number of mobile
   devices are connected to Internet (why hide mobility to them), this
   design choice seems to be more and more appealing.  One common
   approach taken by this design is to use DNS to keep track of mobiles'
   current locations.  Mobiles use dynamic DNS updates to keep their DNS
   servers updated with their current locations.  This approach re-
   utilizes the DNS infrastructure, which is ubiquitous and quite
   reliable, and makes the mobility support protocol simple and easy to
   deploy.  Protocols like E2E, ILNP and BTMM fail into this design.
   Although HIP adds special purpose rendezvous servers to the network
   to replace the role of DNS, both mobile and CN are still mobility-
   aware, and hence it is also classified in this category.

   Figure 9 shows the three categories of protocols.


   +-------------+----------------------------------+
   | Design 1    | VIP, LSR, Mobile IP, HMIP, NEMO  |
   |             | Global HAHA                      |
   +-------------+----------------------------------+
   | Design 2    | PMIP, TIMIP                      |
   +-------------+----------------------------------+
   | Design 3    | E2E, M-SCTP, ILNPv6, HIP,        |
   |             | BTMM, LISP-Mobility              |
   +-------------+----------------------------------+


                                 Figure 9

5.3.  Operator-Controlled Approach v.s. User-controlled

   At the time of this writing, cellular networks are providing the
   largest operational global mobility support, using a service model
   that bundles together the device control, network access control and
   mobility support.  The tremendous success of cellular market speaks
   loudly that the current cellular service model is a viable one, and
   is likely to continue into foreseeable future.  Consequently, there
   is a strong advocate in IETF that we continue the cellular way of
   handling mobility, i.e. instead of letting mobile devices participate
   in the mobility related signaling themselves, the network entities
   deployed by the operators should take care of any and all signaling
   process of mobility support.  A typical example along this direction
   is Proxy Mobile IP, in which LMA works together with MAGs to assure



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   the reachability to the mobile using its Home Prefixes, as long as
   the mobile roams within the same provider's domain.

   One main reason for this approach is perhaps backward compatibility.
   By not requiring the participation of mobiles in control signaling
   process, it avoids any changes to the mobile nodes, so that the
   mobile nodes can stay simple and all the legacy nodes can obtain the
   same level of mobility services as the latest mobile devices.
   According to the the claim of 3G vendors and operators, transparent
   mobility support is a key aspect for success as they learn from their
   deployment experience.

   On the other hand, most of the mobility support protocols surveyed in
   this document focus on mobility support only, assuming mobiles
   already obtained network access.  The mobile nodes typically update
   their locations themselves to the rendezvous points chosen by the
   users, and of course only the nodes implementing one of these
   solutions can benefit from mobility support.  However, this class of
   protocols do offer the users and mobile devices with more flexibility
   and freedom, e.g. they can choose whatever mobility services
   available as long as their software support that protocol, and they
   can also tune the parameters to get the services that are most
   suitable to them.

5.4.  Local and Global Scale Mobility

   The works done on mobility management can also be divided according
   to their scale into two categories: local mobility management and
   global mobility management.

   Global mobility management is typically supposed to support mobility
   of unlimited number of nodes in a geographically as well as
   topologically large area.  Consequentially, it pays a lot of
   attentions to the scalability issues.  For the availability concern,
   it also tries to avoid failure of single point.

   Local mobility management on the other hand is designed to work
   together with global mobility management, and thus focuses more on
   performance issues, such as handoff delay, handoff loss, local data
   path and etc.  Since it is typically used in a small scale with not-
   so-large number of mobile nodes, sometimes the designers can use some
   fine-tune mechanisms that are not scale with large network (such as
   host route) to improvement performance.  As a side effect of local
   mobility management, the number of location updates sent by mobile
   nodes to their global rendezvous points is substantially reduced.
   Thus, the existence of local mobility management also contribute to
   the scalability of global mobility management.




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   One problem of the local mobility management is that it often
   requires many infrastructure support, such as MAGs in PMIP, or MAPs
   in HMIP.  These kind of local devices are essentially required in all
   small domains, which can be a huge investment.

   Nevertheless, the mobility managements in two scale make it possible
   for designers to design protocols that fit into specific user
   requirements; it also enables the gradual deployment of local
   enhancement while not losing the ability of global roaming.  The co-
   existence of the two seems to be a right choice in the foreseeable
   future.

   Figure 10 shows the classification of the studied protocols according
   to their serving scale.

   +-----------+-----------------------------------------+
   |           | VIP, LSR, Mobile IP, NEMO, E2E, M-SCTP  |
   |   Global  | HIP, ILNPv6, Connexion, WIMO, BTMM,     |
   |           | MSM-IP, Global HAHA, LISP-Mobility      |
   +-----------+-----------------------------------------+
   |   Local   | Columbia, HMIP, FMIP, Cellular IP,      |
   |           | HAWAII, TIMIP, PMIP                     |
   +-----------+-----------------------------------------+

                                 Figure 10

5.5.  Other Mobility Support Efforts

   Despite the wide spectrum of mobility solutions covered by this
   survey, the list of mobility protocols is not exhaustive.

   GPRS Tunneling Protocol [GTP] is a network-based mobility support
   solution widely used in cellular networks.  Its implementation only
   involves Gateway GPRS Support Node (GGSN) and Serving GPRS Support
   Node (SGSN).  It allows end users of a GSM or UMTS network to move
   from place to place while remaining connected to the Internet as if
   from on location at the GGSN.  It does this by carrying the
   subscriber's data from the subscriber's current SGSN to the GGSN
   which is handling the subscriber's session.  To some extent, it is
   the non-IETF variant of PMIP, with SGSN resembling LMA and GGSN
   resembling MAG, respectively.

   There are also works on application layer mobility support, most
   notably the SIP based mobility support [ALM-SIP].  SIP was initially
   designed as an application signaling protocol for multimedia, and
   later researchers noticed its potential capability for mobility
   support.  When the mobile initiates a session with CN, normal SIP
   signaling procedure is performed to establish the session.  When the



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   mobile moves to a new network while the session is ongoing, it send a
   RE-INVITE message with the existing session but reveals the new IP
   address to the CN.  The home SIP server is also updated with the
   latest location information of the mobile after the move.  However,
   SIP based approach can not maintain the TCP connections when the
   mobile's IP address changes.

   A lot of enhancements to Mobile IPv6 Route Optimization have also
   been developed.  A comprehensive taxonomy and analysis of these
   efforts can be found in [RFC 4651].


6.  Discussions

   In last section we discussed the different directions towards
   mobility support.  We now turn our attention to identify both new
   opportunities and remaining open issues in providing global scale
   mobility support for unlimited number of online mobility devices.  We
   are not trying to identify the solutions to these issues, but rather,
   the goal is to share our opinions and to initiate an open discussion.

6.1.  Deployment Issues

   Among the various protocols we discussed in this document, few have
   been deployed in commercial networks.  There are several reasons to
   explain this situation.

   First, although the research community started to develop mobility
   support protocols 20 years ago, it is until recent years that the
   number of mobiles soars.  Hence, operators did know see the incentive
   of deploying mobility support protocol several years back.  As of
   today, the number of mobiles are still growing by leaps and bounds,
   and there is enough user demand for the operators to seriously
   consider the deployment of mobility support protocols.

   Second, the complexity of most mobility support protocols impedes the
   implementation and hence the deployment in commercial networks.  The
   complexity arises from multiple aspects.  One is the optimizations on
   performance.  And the other is the problem with the use of security
   protocols such as IPsec and IKE.  The discussions regarding to these
   two problems are still ongoing in MEXT working group.  Some
   researchers argue that the research community should design a "barely
   work" version of mobility support protocol first, without considering
   nice performance features and complex security mechanism, roll it out
   in the real world and improve it thereafter.  However, there are
   different views on what are the essential features and which security
   mechanism is better.




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   Third, almost all the mobility support protocols assume that the
   mobile nodes have network connectivity anywhere any time.  In the
   reality, however, it is not always the case.  Nevertheless, wireless
   access is available in more and more places, and it is foreseeable
   that in the near future the coverage of wireless access in different
   forms (WiFi, Wimax, 3G/4G) would be ubiquitous.

6.2.  Session Continuity and Simultaneous Movements

   In order for the users to benefit from the mobility support, it is
   important to keep the TCP sessions un-interrupted by the mobility.
   If the durations of the sessions are short (e.g. web browsing), the
   probability is high that the TCP sessions finish before the handover
   happens; even if the TCP session is interrupted by the handover, the
   cost is usually low (e.g. refresh the web page).  However, if the TCP
   sessions are typically long (e.g. downloading large files, voice
   calls), the interruptions during the handover would become
   unacceptable.

   It's hard to predict tomorrow's applications, but most of the
   mobility support protocols tries to keep the sessions up during the
   movements.  For routing based protocols, session continuity is not a
   problem since the IP address of the mobile never changes.  For other
   protocols, either a stable IP address (e.g.  HoA) or an equivalent
   (e.g.  HIT) is used in transport layer so that the mobility is
   hidden, or the TCP protocol is modified so that both ends can change
   IP addresses while keeping the established session (e.g.  E2E).

   Another concern is the support of simultaneous movements.  In some
   scenarios, only one end is mobile and the other end is always static;
   moreover, the communication between the two is always initiated by
   the mobile end.  A lot of applications as of today fall into this
   category.  Typically, the server side is static and the client is
   mobile; usually, the client would contact the server first.  Hence,
   in these scenarios, the support of simultaneous movements is not a
   requirement.  However, in other scenarios, both ends may be moving at
   the same time.  For example, during a voice call, two mobile nodes
   may experience the handovers simultaneously.  In this case, a
   rendezvous point is necessary to keep the current locations of the
   mobiles so that can find each other after a simultaneous movement.
   Besides, if a static server wants to push information to a mobile
   client, a rendezvous point is also required.

   It is clear that the number of the mobile devices is rapidly growing
   and more mobiles are going to provide content in the near future,
   hence the simultaneous movements scenarios are considered important.
   In fact, almost all the mobility support protocols are equipped with
   rendezvous points, either by adding dedicated components or by



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   leveraging the existing DNS systems.

6.3.  Trade-offs of Design Choices on Mobility-awareness

   The mobility-awareness at two communicating ends is closely related
   to the backward compatibility problem.  The Internet has been running
   for more than two decades, and the scale of the Internet gets so
   large that it is impossible to upgrade the whole system over night.
   As a result, it is also not possible for a mobility support system
   designer to overlook this problem: how to decide the mobility-
   awareness in the protocol design and how important the backward
   compatibility is?

   In the following text we discuss the trade-offs of the design choices
   mentioned in Section 5.2.

   The advantage of the first design choice is that the mobile does not
   lose the ability of communicating with legacy nodes while roaming
   around, i.e. the mobile can benefit from unilateral deployment of
   mobility support.  Another potential advantage is that the static
   nodes do not need to be bothered by the mobility of the mobiles,
   which saves the resources and could be desirable if the CN is a busy
   server.  The disadvantage of this design is also well known: it
   introduces triangle routing, which significantly increases the delays
   in the worst cases.  There are means to remedy the problem, e.g.
   Route Optimization in Mobile IP if CN is mobility-capable, and
   distributing Home Agents as Global HAHA does, at the expense of
   increasing complexity.

   The second design cater to the inertness of the Internet (and the
   users) by keeping everything status quo from the user's point of
   view.  It is like the cellular network, with the smart network and
   dumb terminals.  The advantage is that the legacy nodes can benefit
   from the mobility support without upgrade.  However, the cost is also
   not trivial: the users lose the freedom of control in terms of
   mobility management, and a large number of entities in the network
   needs to be upgraded.

   The third design assumes that the other end is by a large chance also
   mobility capable (as of today, more people are accessing the Internet
   via mobile devices than a desktop), and thus do not provide backward
   compatibility at all; but as a tradeoff, the system design becomes
   much simpler and the data path is always the shortest one.

   We all know that backward compatibility is important in system
   design.  But how important is that?  How much effort should we make
   for this issue?  At least for now, the answer is not clear yet.




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6.4.  Interconnecting Heterogeneous Mobility Support Systems

   As our survey suggests, multiple solutions of mobility support are
   already there today, and it is almost for sure that the mobility
   support systems in the future are going to be heterogeneous.
   However, as of today, the inter-operation between different protocols
   is still problematic.  For example, when a mobile node supporting
   Mobile IP only wants to communicate with another mobile with only HIP
   support, neither of them can benefit from mobility support.

   This situation reminds us the days before IP were adopted.  In that
   time, the hosts in different networks are not able to communicate
   with each other.  It is the IP that merged the networks and created
   the Internet, where each host can freely communicate with any other
   host.  Is it necessary to introduce something like IP to the mobility
   support in the future?  Is it possible to design an architecture, so
   that it glues all the mobility support systems together?  We believe
   the answers to both questions are "yes".

   The basic idea for the solution is simple, as the famous quote says:
   "Every problem in Computer Science can be solved by adding a level of
   indirection".  However, the devil is in the details and we still need
   to figure that out.


7.  Security Considerations

   Since mobility means that the location of a mobile may change at any
   time, thus how to secure such dynamic location updates is a very
   important consideration for all mobility support solutions.  However
   due to the wide range of the solution proposals examined in this
   document, their security aspects also vary over a wide range.  For
   example home-agent based solutions call for secure communications
   between the mobile and its home agent(s).  On the other hand for
   routing based solutions, such as Connexions, the issue becomes one of
   the global routing security.  Similarly, for those solutions that use
   DNS to provide mapping between identifiers and locators, the issue is
   essentially converted to how to secure DNS dynamic updates as well as
   queries.  To keep this survey document both comprehensive as well as
   within a reasonable size, we chose to focus the survey on describing
   and comparing the solutions to the center piece of all mobility
   supports which is the resolution between identifiers and locators.


8.  IANA Considerations

   There are no IANA actions required by this document.




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9.  Informative References

   [ALM-SIP]  Schulzrinne, H. and E. Wedlund, "Application-Layer
              Mobility Using SIP", Mobile Computing and Communications
              Review, 2010.

   [BTMM]     Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
              "Understanding Apple's Back to My Mac Service", draft -
              zhu-mobileme-05.txt, 2010.

   [Boeing]   Andrew, L., "A Border Gateway Protocol 4 (BGP-4)",
              Boeing White Paper, 2006.

   [CIP]      Valko, A., "Cellular IP: A New Approach to Internet Host
              Mobility", ACM SIGCOMM, 1999.

   [Columbia]
              Ioannidis, J., Duchamp, D., and G. Maguire, "IP-based
              Protocols for Mobile Internetworking", ACM SIGCOMM CCR,
              1991.

   [E2E]      Snoeren, A. and H. Balakrishnan, "An End-to-End Approach
              to Host Mobility", ACM Mobicom, 2000.

   [GTP]      "GPRS Tunneling Protocol Across Gn and Gp Interface", 3G
              TS 29.060 v3.5.0.

   [HAHA]     Wakikawa, R., Valadon, G., and J. Murai, "Migrating Home
              Agents Towards Internet-scale Mobility Deployment",
              ACM CoNEXT, 2006.

   [HAWAII]   Ramjee, R., Varadhan, K., and L. Salgarelli, "HAWAII: A
              Domain-based Approach for Supporting Mobility in Wide-are
              Wireless Networks", IEEE/ACM Transcations on Networking,
              2002.

   [ILNP]     Atkinson, R., Bhatti, S., and S. Hailes, "A Proposal for
              Unifying Mobility with Multi-Homing, NAT, and Security",
              MobiWAC '07, 2007.

   [LISP]     Farinacci, D., Fuller, V., Lewis, D., and D. Meyer,
              "Locator/ID Separation Protocol (LISP)",
              draft-farinacci-lisp-12.txt (work in progress), 2009.

   [LISP-Mobility]
              Farinacci, D., Fuller, V., Lewis, D., and D. Meyer, "LISP
              Mobility Architecture", draft-meyer-lisp-mn-04.txt (work
              in progress), 2009.



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   [LSR]      Bhagwat, P. and C. Perkins, "A Mobile Networking System
              Based on Internet Protocol (IP)", Mobile and Location-
              Independent Computing Symposium, 1993.

   [M-SCTP]   Xing, W., Karl, H., and A. Wolisz, "M-SCTP: Design and
              Prototypical Implementaion of An End-to-End Mobility
              Concept", 5th Intl. Workshop on the Internet Challenge,
              2002.

   [MSM-IP]   Mysore, J. and V. Bharghavan, "A New Multicast-based
              Architecture for Internet Host Mobility", ACM Mobicom,
              1997.

   [RFC 2782]
              Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              Specifying the Location of Services (DNS SRV)", RFC 2782,
              2000.

   [RFC 3344]
              Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
              2002.

   [RFC 3753]
              Manner, J. and M. Kojo, "Mobility Related Terminology".

   [RFC 3775]
              Johnson, D., Perkins, C., and J. Arkko, "IP Mobility
              Support in IPv6", RFC 3775, 2004.

   [RFC 3963]
              Devarapalli, V., Wakikawa, R., Peterson, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, 2005.

   [RFC 4193]
              "Unique Local IPv6 Unicast Address", RFC 4193.

   [RFC 4555]
              Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, 2006.

   [RFC 4651]
              Vogt, C. and J. Arkko, "A Taxonomy and Analysis of
              Enhancements to Mobile IPv6 Route Optimization", RFC
               4651, February 2007.

   [RFC 5201]
              Nikander, P., Moskowitz, R., Jokela, P., and T. Henderson,



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              "Host Identity Protocol", RFC 5201, 2008.

   [RFC 5213]
              Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6", RFC 5213, 2008.

   [RFC 5380]
              Soliman, H., Castelluccia, C., Malki, K., and L. Bellier,
              "Hierarchical Mobile IPv6 (HMIPv6) Mobility Management",
              RFC 5380, 2005.

   [RFC 5454]
              Tsirtsis, G., Park, V., and H. Soliman, "Dual-Stack Mobile
              IPv4", RFC 5454, 2009.

   [RFC 5568]
              Koodli, R., "Mobile IPv6 Fast Handovers", RFC 5568, 2009.

   [TIMIP]    Grilo, A., Estrela, P., and M. Nunes, "Terminal
              Independent Mobility For IP", IEEE Communications
              Magazine, 2001.

   [VIP]      Teraoka, F., Yokote, Y., and M. Tokro, "A Network
              Architecture Providing Host Migration Transparency",
              ACM SIGCOMM CCR, 1991.

   [WINMO]    Hu, X., Li, L., Mao, Z., and Y. Yang, "Wide-Area IP
              Network Mobility", IEEE INFOCOM, 2008.


Authors' Addresses

   Zhenkai Zhu
   UCLA
   4805 Boelter Hall, UCLA
   Los Angeles, CA  90095
   US

   Phone: +1 310 993 7128
   Email: zhenkai@cs.ucla.edu











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   Ryuji Wakikawa
   TOYOTA ITC
   465 Bernardo Avenue
   Mountain View, CA  94043
   US

   Email: ryuji@jp.toyota-itc.com


   Lixia Zhang
   UCLA
   3713 Boelter Hall, UCLA
   Los Angeles, CA  90095
   US

   Phone: +1 310 825 2695
   Email: lixia@cs.ucla.edu


































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