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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: December 22, 2010                                    TOYOTA ITC
                                                                L. Zhang
                                                           June 20, 2010

              A Survey of Mobility Support In the Internet


   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.  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.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 December 22, 2010.

Copyright Notice

   Copyright (c) 2010 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
   publication of this document.  Please review these documents

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   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.  Sony Protocol  . . . . . . . . . . . . . . . . . . . . . .  7
     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
   6.  Discussions  . . . . . . . . . . . . . . . . . . . . . . . . . 26
     6.1.  Deployment Issues  . . . . . . . . . . . . . . . . . . . . 26
     6.2.  Session Continuity and Simultaneous Movements  . . . . . . 27
     6.3.  Trade-offs of Design Choices on Mobility-awareness . . . . 28
     6.4.  Interconnecting Heterogeneous Mobility Support Systems . . 29
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32

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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 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

2.  Terminology

   This document uses the following terms to refer to the entities or
   functions that are required in mobility support.  Readers should also
   read RFC3753 "Mobility Related Terminology" [RFC3753] before reading
   this document.

   Identifier:  A stable value that can be used to identify a mobile
           node.  Anything could be used as an identifier as long as it
           is topologically and geographically independent, i.e. 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.

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

   Rendezvous Point (RP):  RP is 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 signalling 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 via 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
   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

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   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.  Two common issues face
   all the type-2 solutions.  One issue is how to carry out this
   forwarding, 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 the sake of convenience).  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 |
           |      Sony      |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 be statically configured for each

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   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

   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

   This design [Sony] 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.

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   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 Sony protocol works.

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

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

                          Figure 3: Sony 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.  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 then the

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   bi-directional path for the following packets is: mobile

   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 [RFC3220]
   and Mobile IPv6 [RFC3775] standards in 2002 and 2004, respectively.
   In 2009, Dual-Stack Mobile IPv4 [RFC5454] 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 Sony Protocol, with the mobile's HoA corresponding to the Virtual
   IP Address in Sony, and the CoA corresponding to the regular IP

   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

                              |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 [RFC4140] 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 [RFC4068] 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, it's 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 [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.  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 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 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

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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

   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

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) [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.  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 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

   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

   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

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 avoided 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 [RFC5213] was proposed in 2006 to meet the interest
   of mobile network operators who desire to support legacy mobile
   devices and 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.  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 a pragmatic 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, and only
   accessible to the subscriber.

   A mobile uses secure DNS update to dynamically refresh its current
   location.  Each machine generates a random IPv6 ULA [RFC4193], which
   is stored in the DNS database as a topologically independent
   identifier.  The host's current IPv4 address (which in many cases is
   the IPv4 address of the NAT box) 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 query has changed.  This approach
   avoids the need for frequent pollings to get up-to-date location of
   mobile nodes.  A host uses its identifier in transport protocols and
   applications, and uses UDP/IPv4 encapsulation to deliver data packets
   based on what 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.  HIT in HIP, domain name, etc.); BTMM chose
   to IPv6 address so that its implementation can reuse existing code.

   BTMM 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
   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, 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
   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.

   |               | Sony, 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    | Sony, 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 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 works together with MAGs, assuring that the mobile

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   always obtains its Home Prefixes as long as it roams within the

   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 the answer.  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
   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

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   the scalability of global mobility management.

   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

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

   |           | Sony, 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

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

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   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.

   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

   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

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   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
   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 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

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   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.

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.  References

   [BTMM]     Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
              "Understanding Apple's Back to My Mac Service", draft -
              zhu-mobileme-01.txt, 2010.

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   [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.

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

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

   [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.

              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,

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   [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,

   [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.

   [RFC4068]  Koodli, R., "Fast Handover for Mobile IPv6", RFC 4068,

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

   [RFC4193]  "Unique Local IPv6 Unicast Address", RFC 4193.

   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, 2006.

   [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.

   [RFC5454]  Tsirtsis, G., Park, V., and H. Soliman, "Dual-Stack Mobile

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              IPv4", RFC 5454, 2009.

   [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.

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

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

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

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