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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: April 29, 2010                                       TOYOTA ITC
                                                                L. Zhang
                                                        October 26, 2009

              A Survey of Mobility Support In the Internet

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
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   This Internet-Draft will expire on April 29, 2010.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   Over the last two decades many research efforts have been devoted to
   developing solutions for mobility support over the global Internet,
   which resulted in a variety of proposed solutions.  In this draft we
   conducted a systematic survey of the previous efforts to gain an
   overall understanding on the solution space of mobility support.
   This draft reports our finding and identifies remaining issues in
   providing ubiquitous and efficient global scale mobility support.

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  . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Sony Protocol  . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  LSR Protocol . . . . . . . . . . . . . . . . . . . . . . .  8
     4.4.  Mobile IP  . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.5.  MSM-IP . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.6.  Cellular IP, HAWAII and TIMIP  . . . . . . . . . . . . . . 12
     4.7.  Hierarchical Mobile IP (HMIP)  . . . . . . . . . . . . . . 13
     4.8.  NEMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.9.  E2E and M-SCTP . . . . . . . . . . . . . . . . . . . . . . 14
     4.10. Host Identity Protocol . . . . . . . . . . . . . . . . . . 15
     4.11. Connexion and WINMO  . . . . . . . . . . . . . . . . . . . 15
     4.12. ILNP . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.13. Global HAHA  . . . . . . . . . . . . . . . . . . . . . . . 17
     4.14. Proxy Mobile IP  . . . . . . . . . . . . . . . . . . . . . 18
     4.15. Mobile Me  . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.16. LISP-Mobility  . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Different Directions towards Mobility Support  . . . . . . . . 20
     5.1.  Routing-based Approach v.s. Mapping-based Approach . . . . 21
     5.2.  IP Layer Indirection v.s. Mobility Exposure  . . . . . . . 22
     5.3.  Operator-Controlled Approach v.s. User-controlled  . . . . 24
     5.4.  Local and Global Scale Mobility  . . . . . . . . . . . . . 24
   6.  Discussions  . . . . . . . . . . . . . . . . . . . . . . . . . 26
     6.1.  Backward Compatibility . . . . . . . . . . . . . . . . . . 26
     6.2.  Undisrupted TCP Connection . . . . . . . . . . . . . . . . 27
     6.3.  Interconnecting Heterogeneous Mobility Support Systems . . 28
     6.4.  Flat-id Based Routing  . . . . . . . . . . . . . . . . . . 29
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31

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1.  Introduction

   This draft 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 motivation is the rapid
   growth in Internet access via mobile devices in recent years, which
   will inevitably lead to new Internet application development in
   coming years and further underscore the importance of Internet
   mobility support.  We believe that a historical review of all the
   proposed solutions so far can help us not only identify their
   commonalities and differences, but also shed insight on future

   In the rest of this document, we provide an overview of the mobility
   support solutions from the early works 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.

2.  Terminology

   This document uses a number of terms to refer to the entities or
   functions that are required in mobility support.  Readers are also
   encouraged to scan [RFC3753] before reading this document.

   Identifier:  A stable piece of information that can be used to
           identify a mobile node.  Anything could be used as an
           identifier as long as it remains unchanged when the mobile
           node roams around.

   Locator:  The IP address that indicates the mobile node's current
           location.  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 it's Locator.

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   Rendezvous Point:  The place where the mapping is held.  Some other
           functions such as data forwarding may also be placed on the
           rendevzvous point.

   Global Mobility Management:  The mobility management in a global
           scale.  It keeps the mobile's reachability during the
           mobile's long distance moving, either geographically or

   Local Mobility Management:  The mobility management within a
           topologically local domain.  It tries to keep the mobile's
           local movements transparent to the network entity that
           manages the mobile's mobility in a global scale.  It also
           tries to improve the handoff performance.

   Operator Controlled Mobility Management:  The mobile node does not
           get involved in mobility management.  Instead, the network
           entities, which are controlled by the network operators, do
           all the mobility related signalling job on behalf of the
           mobile node.

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

3.  Basic Components in Mobility Support Protocols

   The basic question in mobility support is how to make data reach a
   moving receiver (a mobile in short; here we do not distinguish
   between mobile nodes and mobile subnets).  Whoever sending data to a
   mobile must be able to identity the receiver via a piece of stable
   information, where "stable" means that the information does not
   change as the mobile moves.  However if the sender's knowledge about
   the mobile does not change while the mobile moves, some means must
   exist to find the mobile's dynamically changing location by using
   that unchanged identifier of the mobile known to the sender.

   The above intuitive reasoning leads to the following observation--
   mobility support essentially involves three basic components:

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

   Although mobility support can be provided through multiple different
   ways, we show in the next section that different mobility support

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   designs are merely different ways to choose mobile identifiers and
   different approaches to provide mapping between the identifiers and
   the mobiles' current IP addresses.

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, for the sake
   of convenience, we grouped closely related protocols together.  We
   briefly describe each design and point out how it implements the
   three basic mobility support components defined in the last section.

   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 |
           |      Sony      |1991 |    M-SCTP     |2002 |
           |      LSR       |1993 |     HIP       |2003 |
           |  Mobile IP     |1996 |   Connexion   |2004 |
           |     MSM-IP     |1997 |     ILNP      |2005 |
           |  Cellular IP   |1998 |  Global HAHA  |2006 |
           |      HMIP      |1998 |     PMIP      |2006 |
           |     HAWAII     |1999 |  Mobile Me    |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 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.  A mobile node obtains an IP address from

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   a special block of IP addresses and keeps it regardless of its
   current location.  Each MSS keeps a list of mobile nodes that are
   currently in its cell.  When a correspondent node sends a packet to a
   mobile node, the packet is first routed to the MSS that is closest to
   the correspondent node.  This MSS will deliver the packet directly to
   the mobile node if the mobile node happens to be in its cell, or
   otherwise sends a query message to all other MSSs on the campus to
   find out the current location of the mobile node.  Each MSS in turn
   checks its tracking list of mobile nodes, and sends a reply if the
   target mobile node is on the list.  The MSS which sends the query can
   now forward the packet to the MSS that serves the mobile node.

   Here, the three basic components are:
   o  Identifier: the mobile node's IP address from the special IP
      address block;
   o  Locator: the IP address of the serving MSS;
   o  Mapping: the mapping between the identifier and locator is not
      established a prior, but discovered in reaction to packets through

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   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

4.2.  Sony Protocol

   In this protocol, the IP header is modified to allow packets sent by
   a mobile to carry two IP addresses: a Virtual IP address and a
   conventional IP address.  The mobile obtains its Virtual IP address
   from its home network, and the conventional IP address from its
   access router.  Every time the mobile node changes its location, it
   sends a special packet containing the mapping between its Virtual IP
   address and conventional IP address to its home network, which stores
   the mapping.  Routers in the middle as well as the correspondent
   hosts can also cache a set of mappings for mobile nodes.

   To deliver data to a mobile, the correspondent node uses the mobile's
   Virtual IP address as the destination IP address.  If a router along
   the way from the correspondent node to the mobile's home network
   happens to have a mapping for mobile, it modifies the destination IP
   address field and forwards the packets to the mobile's current
   location; otherwise, the packet will flow to the mobile's home
   network, which then chnages the packet's destination address to the
   mobile's current location according to the mapping and delivers the

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   packets to the mobile.  Note that the destination IP address field
   can be modified again if the packet encounters a router with more
   recent mapping.

   The three basic components are easy to identify:
   o  Identifier: the mobile node's Virtual IP address;
   o  Locator: the mobile node's conventional IP address;
   o  Mapping: the binding between the mobile's Virtual IP address and
      conventional IP address, which is sent by the mobile to its home
      network and may also be cached in routers and correspondent host.

   Figure 3 shows how sony protocol works.

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

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

                          Figure 3: Sony Protocol

4.3.  LSR Protocol

   In LSR each mobile has a designated router, called Mobile Router,
   that manages its mobility.  Mobile Router assigns an IP address for
   each mobile it manages and announce reachability to those IP
   addresses.  Another network entity required is Mobile Access Station
   (MAS) which connects to the mobile at its current location.  The
   mobile node always reports the current serving MAS to its Mobile

   If the correspondent node and the mobile node are attached to the
   same MAS, then the MAS simply forwards packets between the two;
   otherwise, the packet sent by the correspondent node is routed to the

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   Mobile Router who assinged the IP address to 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.  This way,
   the packet is redirect to the MAS which then delivers the packet to
   the mobile.  After that, the mobile node reverses the LSR and replies
   to the correspondent node; the correspondent node, similarly, sends
   the subsequent packets directly through MAS (i.e. without going
   through the Mobile Router) by reversing the LSR.

   Let's identify the three basic components for this protocol:
   o  Identifier: the mobile's IP address assigned by its Mobile Router;
   o  Locator: the IP address of the serving MAS;
   o  Mapping: the mapping between the above two kept at the Mobile

   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 [RFC3220]
   and Mobile IPv6 [RFC3775] standards in 2002 and 2004, respectively.

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   Let's look at Mobile IPv6 first.  Each mobile node has a Home Agent,
   from which it acquires its Home Address (HoA).  Unlike LSP Protocol,
   the mobile node also obtains a Care-of Address (CoA) from its current
   access router.  Whenever the mobile node changes its point of
   attachment and gets a new CoA, it sends a Binding Update message to
   notify the Home Agent about the new CoA.  Conceptually Mobile IPv6
   design looks similar to Sony Protocol, with the mobile's HoS
   corresponding to the Virtual Address in Sony, and the CoA
   corresponding to the Conventional IP address.

   The correspondent node uses the mobile's HoA as the destination IP
   address when sending to a mobile.  The packets are forwarded to the
   Home Agent since Home Agent is announcing the reachability to the
   HoA.  Home Agent then encapsulates the packets to mobile node's CoA
   according to the mapping.  If the correspondent node supports Route
   Optimization, it will also keep the mapping between the mobile's HoA
   and CoA; the mobile will also update the correspondent node when it
   changes the CoA.  Thus the correspondent node can encapsulate packets
   to the mobile directly, without going through the Home Agent.

   The three basic components in Mobile IP are:
   o  Identifier: the mobile node's HoA;
   o  Locator: the mobile node's CoA;
   o  Mapping: the binding between HoA and CoA.  Home Agent always holds
      the mapping; correspondent nodes may also store the mappings if
      they support Route Optimization.

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   Figure 5 illustrates the data path of Mobile IPv6 without Route

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

                                      ==>: Tunnel
                                      -->: regular IP

             Figure 5: Mobile IPv6 without Route Optimization

4.5.  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 if IP multicast is
   universally 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.  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

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   communicate with the mobile node can just send the data to the
   mobile's multicast IP address.

   In order to make this approach deployable in wide-area networks, a
   hierarchical location management service is also proposed.  In the
   location server for each mobile node, the IP address of the multicast
   router that serves mobile node is stored.  Thus, a new multicast
   router can always query the location server to find out how to join
   the multicast distribution tree.

   Also 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.

   The three basic components here are:
   o  Identifier: the mobile node's multicast IP address;
   o  Locator: the IP address of the multicast router that is currently
      serving the mobile node;
   o  Mapping: the mapping between the above two; this mapping is stored
      in location server.

4.6.  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.  They are all
   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 gateways, which are essentially special routers that
   integrate location tracking service with routing.  A mobile reports
   the local gateway node's IP address as the regional CoA to its Home
   Agent, and retains its locally assigned IP address when it roams
   within the Cellular IP network.  The network monitors the packets
   originated from mobile node and maintains a distributed, hop-by-hop
   reverse path which can be used to route packets back to the mobile
   node.  It utilizes paging technique from cellular network to track
   the location of each mobile by sending dummy packets with a
   relatively low frequency to allow mobile nodes to update their
   location information.  When a packet from correspondent node arrives
   at the gateway, the gateway initiates a controlled flooding query if
   it does not know the exact location of the mobile.  Once the
   communication between the mobile and correspondent nodes starts
   going, a much more precise reverse path can be maintained.

   Similarly, HAWAII [HAWAII] also aims to provide efficient local

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   mobility support but with two main differences from Cellular IP.
   First, HAWAII uses path setup update messages to establish and update
   host-based routing entries for the mobile nodes in selective routers
   within the domain, so that packets arriving at the domain border
   router can reach the mobile host with limited disruption.  Second,
   unlike the controlled floodingin CIP, it uses IP multicast to page
   idle mobile nodes when they have incoming packets arrived at the
   border router and there are no routing entries available for these

   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 there is
   no traffic for the mobile node.  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 nodes that do
   not support Mobile IP.  Thus, as long as it roams within the domain,
   a legacy node has the same degree of mobility support as a Mobile IP
   capable node.

   Given the similarity of the three protocols, the implementation of
   the three basic components in them are listed as below:
   o  Identifier: the mobile node's IP address that are unchanged when
      it roams within the domain;
   o  Locator: the network node that gives network access to the mobile
   o  Mapping: no explicit mapping; host-based entries in local routers
      instead tracks the mobiles' locations.

4.7.  Hierarchical Mobile IP (HMIP)

   HMIP [RFC4140] is a simple extension to Mobile IP.  It aims to
   improves the performance of MIP 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 movement 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 and
   registers it with its Home Agent as its current CoA; at the same
   time, the mobile also obtains a Local CoA from the subnet it attaches
   to.  When roaming with the region, a mobile only updates the MAP with
   the mapping between its Regional CoA and Local CoA.  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 reduced the overhead of Mobile IP by
   reducing the frequency of sending updates to Home Agents.

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   The basic components here are:
   o  Identifier: the mobile node's HoA;
   o  Locator: the mobile node's Local CoA;
   o  Mapping: the mapping between HoA and Local CoA, stored in MAP.

4.8.  NEMO

   It is concervable to have a group of hosts moving together.  Consider
   vehicles such as ships, trains, or airplanes which may host a network
   with multiple hosts attached to.  Because Mobile IP handles mobility
   per host, it is not efficient when handling such mobility scenarios.
   NEMO [RFC3963], 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 via which it can be reached.
   Mobile Router is similar to a mobile node in original Mobile IP, but
   with additional functions.  After establishing bidirectional tunnel
   with Home Agent, the Mobile Router will distribute its mobile
   network's pefixes (namely Mobile Prefixes) through the tunnel to Home
   Agent.  The Mobile Prefix of a mobile network is not leaked to its
   point of attachment (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 to their
   destinations.  Note that mobility is transparent to the nodes in the
   moving network.

   Hence, in this protocol, the basic components are:
   o  Identifier: the Mobile Prefixes;
   o  Locator: the CoA of the Mobile Router;
   o  Mapping: the mapping between two, stored in the Home Agent.

4.9.  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.  A mobile uses
   Dynamic DNS update to save 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.  Migration security is protected
   based on token and sequence number.

   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.

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   The three components hence are:
   o  Identifier: the domain name of the mobile node;
   o  Locator: the current IP address of the mobile node;
   o  Mapping: the DNS records kept in the Dynamic DNS server.

4.10.  Host Identity Protocol

   Host Identify Protocol (HIP) [RFC5201] 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.  For the
   convenience of reusing existing code, Host Identity Tag (HIT), a 128-
   bit hash value of the Host Identity, is used in transport and other
   upper layer protocols.

   HIP uses DNS as the rendezvous point which holds the mappings between
   host identifiers and IP addresses.  In addition, HIP also defined 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 correspondent node 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 (I1) of Base
   Exchange; Base Exchange is a four-packet handshake process for the
   purpose of establishing an IPSec Encapsulated Security Payload and
   Security Association pair.  After receiving this first packet, RVS
   relays it to mobile node.  Then mobile node and correspondent node
   can finish Base Exchange and start communication directly.  If mobile
   node moves to a new address, it notifies correspondent node by
   sending HIP UPDATE with LOCATOR parameter indicating its new IP

   The basic components here are:
   o  Identifier: Host Identity or Host Identity tag of a mobile node;
   o  Locator: the current IP address of the mobile node;
   o  Mapping: the mapping between the two, either kept in Dynamic DNS
      server or in Rendezvous Server.

4.11.  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.  When an airplane moves between its
   access points to Internet, it withdraws its prefix from the
   previously connected access point 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

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   number of moving networks becomes large, the amount of BGP updates
   will also increase propertionally, resulting in severe global routing

   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 within an AS: only a designated BGP speaking
   router (DBR) acts as the origin of Mobile Prefixes; mobile networks
   always update their addresses of attachment points 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 location of mobile
   networks.  A new BGP community attribute, named Packet State, is also
   defined to eliminate the triangle routing problem caused by DBR by
   informing the correspondent node of the mobile's CoA, an approach
   similar to Route Optimization in Mobile IP.

   The basic components in Connexion are:
   o  Identifier: the prefix of a moving network that does not change
      during movement;
   o  Locator: the current router that the mobile network connects to;
   o  Mapping: the mapping between the above two is implemented as BGP
      updates to track the location of the mobile network.

   The basic components in WINMO are:
   o  Identifier: the Mobile Prefix of a moving network;
   o  Locator: the point of attachment to the Internet;
   o  Mapping: mapping between the two, kept in Aggregation Routers and

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4.12.  ILNP

   ILNP [ILNP] stands for Identifier/Locator Network Protocol.  It
   splits IPv6 address into two parts: high-order 64 bits as a Locator
   field and low-order 64 bits as an Identifier field.  Locator is used
   for IP packet delivery, while Identifier identifies a host and is
   used by all upper layer protocols, same as HIT in HIP.  During the
   movement, a mobile uses Dynamic DNS Update to update the binding
   between Locator and Identifier; it also sends the updated Locator to
   correspondent nodes using a new ICMP Locator Update message.

   The basic components in ILNP are clear:
   o  Identifier: the low-order 64 bits of the mobile's IPv6 address;
   o  Locator: the high-order 64 bits of the mobile's IPv6 address;
   o  Mapping: the mapping between the two, kept in Dynamic DNS server
      and correspondent node.

4.13.  Global HAHA

   Global HAHA [HAHA], first proposed in 2006, 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 avoided without Route Optimization.

   Thus, the basic components in Global HAHA are:
   o  Identifier: the HoA of the mobile node;
   o  Locator: the CoA of the mobile node (in the primary Home Agent);
      the IP address of primary Home Agent (in other Home Agents).

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   o  Mapping: the mapping between the two.

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

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

                                 Figure 6

4.14.  Proxy Mobile IP

   Proxy Mobile IP [RFC5213] was proposed in 2006, largely to meet the
   interest of mobile network operators who desire to have tighter
   control on mobility support.  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.  MAGs monitor the attaching and leaving 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 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.

   Here, the basic components are:
   o  Identifier: the Home Network Prefix of a mobile node;

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   o  Locator: the Proxy-CoA, which is the IP address of the current
   o  Mapping: the mapping between the two kept in LMA.

4.15.  Mobile Me

   Mobile Me [BTMM] is a pragmatic approach to mobility support and has
   been deployed since 2007 with Mac OS leopard release.  Each user gets
   a MobileMe account, and Apple Inc. provides DNS service for all
   Mobile Me users.  A mobile uses secure DNS update to dynamically
   refresh its current location.  MobileMe assigns each host a special
   IPv6 address as the host identifier, which is stored in the DNS
   database as an AAAA RR.  The host's current IP address is stored in a
   SRV [RFC2782] resource record, 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
   automatically notify each node when the answer to its previous query
   has changed.  This approach avoids the need for frequent queries to
   get up-to-date location of mobile nodes.  A host uses its identifier
   (the IPv6 address) in transport protocols and applications, and uses
   UDP/IPv4 encapsulation to deliver data packets based on the SRV RR
   from DNS reply.  Note that the IPv6 address is only for
   identification purpose.  In fact, it could be any form of identifier
   (e.g.  HIT in HIP, domain name, etc.); MobileMe chose to IPv6 address
   so that its implementation can reuse existing code.

   Mobile ME has millions of subscribers, which is perhaps the first
   large-scale commercial host mobility support in Internet as of today.
   It is simple and easy to deploy.  However, the current applications
   use MobileMe service in a "stop-and-reconnect" fashsion.  It remains
   to be seen how well MobileMe can support continuous communications
   while hosts are on the move, for example as needed for voice calls.

   We can also list the basic components for Mobile Me:
   o  Identifier: the IPv6 address of a mobile node;
   o  Locator: the IPv4 address (and port in the case of NAT) of a
      mobile node;
   o  Mapping: the mapping between them, kept in Dynamic DNS server.

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   Figure 7 shows an example communication scenario of Mobile Me.

                         +----------+      dynamic DNS update
                 llq     |          |<------------------+
               +-------->|  DNS     |                   |
               |         |          | <------------+    |
               |         +----------+       llq    |    |
               |                                   |    |
               |                                   |    |
               V                                   V    |
           +-------+                 ,---.        +-----+-+
           |       |  UDP/IPv4      /     \       |       |
           | Mac1  |<=============>(  NAT  )<---->|  Mac2 |
           |       |    tunnel      \     /       |       |
           +-------+                 `---'        +-------+

                                 Figure 7

4.16.  LISP-Mobility

   LISP-Mobility [LISP-Mobility] is a relatively new design, in which
   the designer hopes to utilize LISP[LISP], which is designed for
   routing scalability, to support mobility as well.  Conceptually, LISP
   may seem similar to some protocols we have mentioned so far, such as
   ILNP and Mobile IP.  Light-weight Ingress Tunnel Router and Egress
   Tunnel Router are implemented on each mobile node, encapsulating/
   decapsulating the packets to and from the mobile node.  Each mobile
   node is assigned a static Endpoint ID, which is used in upper layer
   protocols, as well as a pre-configed 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 also in the Ingress
   Tunnel Routers or Proxy Tunnel Routers of the correspondent nodes.
   Thus the correspondent node can always learn the location of the
   mobile node via the resolution of its Endpoint ID, and the data would
   always travel through the shortest path.  Note that both Endpoint IDs
   and Routing Locators are essentially IP addresses.

   It is trivial to list the basic components in this protocol:
   o  Identifier: the Endpoint ID of the mobile node;
   o  Locator: the Routing Locator the mobile node;
   o  Mapping: the mapping between them, kept in Map-Server or the
      Ingress Tunnel Router of the Correspondent.

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.

   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 the whole network must be informed of every movement by every
   mobile, 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 need
   to be aware of each other's movement, which 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.

                                  ,-''           `--.
                               ,-'                   `-.
                             ,'           +----+        `.
               ,-------.    /   +----+    |ILNP|  +----+  \
            ,+--------+ `-,'    |Sony|    +----+  |HMIP|   `.
          ,' |Columbia|  / `.   +----+            +----+     \
        ,'   +--------+ /    `.            +---+   +------+   \
       /               ;       \    +----+ |E2E|   |M-SCTP|    :
      / +---------+    ;+-----+ \   |NEMO| +---+   +------+    :
     ;  |Connexion|   ; |WINMO|  :  +----+          +---------+ :
     ;  +---------+   | +-----+  :           +---+  |Mobile Me| |
    ;    +-----+      |           :  +---+   |HIP|  +---------+ |
    |    |TIMIP|      | +------+  |  |LSR|   +---+              |
    :    +-----+      : |MSM-IP|  ;  +---+      +---------+     ;
     :   +------+      :+------+ ;    +----+    |Mobile IP|    ;
     :   |HAWAII|      :         ;    |PMIP|    +---------+    ;
      \  +------+       \       /     +----+ +-----------+    /
       \   +-----------+ \     /             |Global HAHA|   /
        `. |Cellular IP|  `. ,'              +-----------+ ,'
          `+-----------+   ,'    +-------------+          /
            `-.         ,-'  `.  |LISP-Mobility|        ,'
               `-------'       '-+-------------+     ,-'
                                  `--.           _.-'
         Routing-based                 Mapping-based

                                 Figure 8

5.2.  IP Layer Indirection v.s. Mobility Exposure

   One critical choice in mapping-based systems is the choice of
   rendezvous point, i.e. where to locate the mapping function and
   whether or not the rendezvous point provides other functions except
   storing mapping information.

   Several mobility support designs provide the mapping function at IP
   layer, where both the identifier and the current location of a mobile
   are represented by IP addresses.  In this approach, the IP address
   which is used as the mobile's identifier points to the rendezvous
   point that keeps track of the mobile's current location.  Such
   designs offer an advantage of hiding the mobility from correspondent
   nodes through one level of indirection.  When a correspondent node
   sends packets to an IP address which is a mobile's identifier, the

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   packets will be delivered to the location where the mapping
   information of the mobile is kept (e.g. the Home Agent in Mobile IP),
   and later they will be forwarded to the mobile's current location via
   either encapsulation or destination address translation.

   Although this one level of indirection at IP layer makes mobility
   transparent, it has a potential side effect of introducing triangle
   routing: the path taken by the packets via the rendezvous point can
   be much longer than the direct path between the correspondent and the
   mobile's current location.  Besides, as increasing number of mobile
   devices are connected to Internet (why hide mobility to them), some
   mobility solutions have opted to expose mobility to both ends and let
   them communicate directly.  One common approach taken by these
   protocols is to use DNS for the mapping function 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.

   Figure 9 shows the two categories of mobility protocols according to
   their choices of rendezvous point.  Note, however, protocols like
   Mobile IP can also expose mobility to the other end if Route
   Optimization is supported.

                    ,-'         `-.
                  ,' +---------+   `.                   ,-----.
                 /   |Mobile IP|     \               ,-'       `-.
               ,'    +---------+      `.            /   +----+    \
              ;                         :         ,'    |ILNP|     `.
              ; +----+                  :        ;      +----+       :
             /  |HMIP|   +-----------+   \       ;          +------+ :
            ;   +----+   |Global HAHA|    :     /   +---+   |M-SCTP|  \
            |            +-----------+    |    ;    |E2E|   +------+   :
            |   +---+                     |    |    +---+ +---------+  |
            |   |LSR|        +----+       |    |          |Mobile Me|  |
            :   +---+ +----+ |NEMO|       ;    |  +---+   +---------+  |
             \        |Sony| +----+      /     :  |HIP|                ;
              :       +----+            ;       \ +---+               /
              :                         ;        :   +-------------+ ;
               `.       +----+        ,'         :   |LISP-Mobility| ;
                 \      |PMIP|       /            `. +-------------+'
                  `.    +----+     ,'               \             /
                    '-.         ,-'                  `-.       ,-'
                       `-------'                        `-----'
                   IP Layer Indirection            Mobility Exposure

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                                 Figure 9

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

   At the time when this draft was written, the largest global mobility
   support today is provided by cellular networks, 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 for foreseeable future.  As a result, there is
   a strong advocate in IETF that we continue the cellular way of
   handling mobility, i.e. the mobile do not necessarily need to
   participate in the mobility related signaling; instead, the network
   entities deployed by the operators will take care of any signaling
   process of mobility support.  A typical example is Proxy Mobile IP,
   in which LMA work together with MAGs, assuring that the mobile always
   obtains its Home Prefixes as long as it roams within the domain.

   The 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, which essentially
   means that the mobile nodes can stay simple and all the legacy nodes
   can obtain the same level of mobility services as the most fancy
   mobile devices.  According the the claim of 3G vendors and operators,
   this is a key aspect as they learn from their deployment experience.

   On the contrary, most mobility support protocols so far focus on
   mobility support only.  And the mobile nodes typically need to update
   their locations by themselves to the rendezvous points chosen by the
   user.  In these protocols, only the node implementing them can
   benefit from mobility support.  However, the users get 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.

   Which one is better?  No one knows.  We expect them to co-exist as
   they do today.  However, as the technology advances and the hand-hold
   devices becomes more and more powerful, the latter approach seems to
   be a much simpler way to move forward with.

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

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   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 shorten handoff delay, reduce handoff
   loss, short local data path and etc.  Since it is typically used in a
   small scale with no-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

   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.

   Neverthe less, 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

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

              ,-------.          _.-''               `---.
           ,-+--------+`-.   ,-''  +---+    +---------+   `--.
         ,'  |Columbia|   `,'      |HIP|    |Mobile IP|       `.
       ,'    +--------+  ,' `.     +---+    +---------+         `.
      /      +----+     /     \                                   \
     /       |PMIP|   ,'+-----+\    +---+    +------+    +----+    `.
    ;        +----+  ;  |WINMO| :   |E2E|    |MSM-IP|    |NEMO|      :
    ;+----+          ;  +-----+ :   +---+    +------+    +----+      :
   ; |HMIP| +-----+ ;            :                                    :
   | +----+ |TIMIP| +-----------+|   +------+     +---+               |
   :        +-----+ |Global HAHA|;   |M-SCTP|     |LSR|    +----+     ;
    :  +-----------++-----------+    +------+     +---+    |ILNP|    ;
    :  |Cellular IP| :          ;           +---------+    +----+    ;
     \ +-----------+  `.       /   +----+   |Mobile Me|            ,'
      \       +------+  \     /    |Sony|   +---------+           /
       `.     |HAWAII|   `. ,'     +----+ +---------+           ,'
         `.   +------+    ,'.             |Connexion|         ,'
           `-.         ,-'   `--.         +---------+     _.-'
              `-------'          `---.               _.-''
      Local Mobility Management     Global Mobility Management

                                 Figure 10

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.  Backward Compatibility

   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 not possible for a
   mobility support system designer to overlook this problem: how
   important the backward compatibility is?

   As one can expect, different designers have different opinions.

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   Some assume 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.  The
   examples of protocols fall into this categories could be HIP, Mobile
   Me and etc.

   Some take a more conservative approach.  They do not want to lose the
   ability of communicating with legacy nodes during the movement, and
   they also do not want the rest of the world (meaning, the static
   nodes) to be bothered by the mobility of mobile nodes.  Thus they add
   a level of indirection at IP layer, and hide the mobility of the
   mobile nodes.  The mobile node thus can benefit from mobility support
   even when communicating with legacy node.  And when the other end
   also supports mobility, it is also possible to achieve the shortest
   data path, but with additional complexity.  The Mobile IP related
   protocols fall into this category.

   Others want to cater to the inertness of the Internet (and the users)
   and keep everything status quo, at least from the users' point of
   view.  And thus they take backward compatibility more serious and
   hide the mobility completely, even for the mobile node itself.  Proxy
   Mobile IP represents this approach.

   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.

6.2.  Undisrupted TCP Connection

   TCP is the most widely used transport layer protocol in the Internet,
   and the majority of the data traffic today is TCP traffic.  In order
   for the users to benefit from the mobility support, it is of vital
   importance to keep the established TCP connections undisrupted during
   the mobile's movement.

   Basically there are three approaches to do this: 1) make the mobile
   node keep its IP address regardless of its point of attachment to the
   Internet; 2) add a level of indirection and use a static IP address
   in the TCP pseudo header; 3) modify the TCP protocol.

   The routing-based protocols typically make the mobile node keep a
   stable IP address and thus the first approach is the right one for
   them.  Proxy Mobile IP, however, also takes the first approach.  No
   overhead or complexity is introduced in order to maintain TCP
   connections in this approach.

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   A large number of protocols take the second approach.  This approach
   could be achieved via a forwarding agent, as in the case of Mobile
   IP; or the two end nodes can directly set up a tunnel and use a
   stable IP address in TCP pseudo headers, as in the case of Mobile Me.
   In order to maintain the TCP connections undisrupted via this
   approach, an extra IP header is appended to every data packets, and
   the possible forwarding agent also introduce problems such as single
   point of failure, triangle routing, bottleneck of bandwidth and
   processing power, etc.

   A few protocols also take the third approach.  In this approach, the
   TCP protocol itself is modified, either to 1) put an identifier
   rather than an IP address as connection id or to 2) allow the TCP
   connections to change the IP address of both ends.  HIP, ILNP uses
   designated ID in TCP layer (they can be in the format of IP address
   though), while E2E allows using dynamically changing IP addresses.
   This approach faces the difficulty of changing the TCP protocol
   itself, which will introduce serious backward compatibility problem.

   What is the best choice for the future?  Although it seems that most
   people favor the second approach, whether or not it is the right
   direction is still uncertain.

6.3.  Interconnecting Heterogeneous Mobility Support Systems

   As our survey suggests, multiple solutions of mobility support are
   already running 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, both of them can not 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 answer to both questions is 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.

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6.4.  Flat-id Based Routing

   Up until now, all our discussions are based on an implication that
   the underlying global routing systems is not fundamentally changed,
   that is, the location of the mobile node is always indicated by an IP
   address.  However, recently there are a lot of works that challenge
   this "classic" idea.  Flat-id based routing schemes [VRR] [ROFL][VIR]
   are especially popular.

   With flat-id based routing, mobility as well as multihoming are
   naturally incorporated.  And most of these protocols are
   mathematically elegant.  However, although the notion of flat-id
   space has been widely adopted in peer-to-peer network, it seems that
   the Internet is not going to undertake such revolutionary changes
   (from hierarchical IP Address based routing to flat-id based routing)
   in the foreseeable future due to the inertness of the extremely huge
   system and also the backward compatibility problem.

   Nevertheless, whether or not flat-id based routing will be a good
   solution to the mobility support problem, or perhaps more generally,
   the routing problem is not determined yet.

7.  References

   [BTMM]     Nakamoto, R., Zhu, Z., Lau, D., and B. Allen,
              "Understanding Apple's Back to My Mac Service", UCLA CS217
              Project, 2009.

   [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.

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

   [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,

   [I-TCP]    Bakre, A. and B. Badrinath, "I-TCP: Indirect TCP for

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              Mobile Hosts", Proceedings of the 15th International
              Conference on Distributed Computing System, 1995.

   [ILNP]     Atkinson, R., "An Overive of the Identifier-Locator
              Network Protocol", Research Note, 2005.

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

              Farinacci, D., Fuller, V., Lewis, D., and D. Meyer, "LISP
              Mobility Architecture", draft-meyer-lisp-mn-00.txt (work
              in progress), 2009.

   [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,

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

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

   [MSOCKS]   Ferguson, P. and D. Senie, "MSOCKS: An Architecture for
              Transport Layer Mobility", IEEE INFOCOM, 1998.

   [RFC2002]  Perkins, C., "IP Mobility Support", RFC 2002,
              October 1996.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              Specifying the Location of Services (DNS SRV)", RFC 2782,

   [RFC3007]  Willington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, 2000.

   [RFC3220]  Perkins, C., "IP Mobility Support for IPv4", RFC 3220,

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   [RFC3753]  Manner, J. and M. Kojo, "Mobility Related Terminology".

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "IP Mobility
              Support in IPv6", RFC 3775, 2004.

   [RFC3963]  Devarapalli, V., Wakikawa, R., Peterson, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, 2005.

   [RFC4140]  Soliman, H., Castelluccia, C., Malki, K., and L. Bellier,
              "Hierarchical Mobile IPv6 Mobility Management (HMIPv6)",
              RFC 4140, 2005.

   [RFC5201]  Nikander, P., Moskowitz, R., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, 2008.

   [RFC5213]  Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6", RFC 5213, 2008.

   [ROFL]     Caesar, M., Condie, T., Kannan, J., Lakshminarayanan, K.,
              Stoica, I., and S. Shenker, "ROFL: Routing on Flat
              Labels", ACM  Sigcomm, 2006.

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

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

   [VIR]      Lu, G., Jain, S., Chen, S., and Z. Zhang, "Virtual Id
              Routing", ACM  MobiArch, 2008.

   [VRR]      Caesar, M., Castro, M., Nightingale, E., O'Shea, G., and
              A. Rowstron, "Virtual Ring Routing: Network Routing
              Inspired by DHTs", ACM  Sigcomm, 2006.

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

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Authors' Addresses

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

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

   Ryuji Wakikawa
   465 Bernardo Avenue
   Mountain View, CA  94043

   Email: ryuji@jp.toyota-itc.com

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

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

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