A Survey of Mobility Support In the Internet
draft-zhu-mobility-survey-02.txt

Abstract

Over the last two decades many efforts have been devoted to developing solutions for mobility support over the global Internet, which resulted in a variety of proposed solutions. 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.

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Table of Contents

1.  Introduction
2.  Terminology
3.  Basic Components in Mobility Support Protocols
4.  Existing Mobility Support Protocols
    4.1.  Columbia Protocol
    4.2.  Sony Protocol
    4.3.  LSR Protocol
    4.4.  Mobile IP
    4.5.  MSM-IP
    4.6.  Cellular IP, HAWAII and TIMIP
    4.7.  Hierarchical Mobile IP (HMIP)
    4.8.  NEMO
    4.9.  E2E and M-SCTP
    4.10.  Host Identity Protocol
    4.11.  Connexion and WINMO
    4.12.  ILNP
    4.13.  Global HAHA
    4.14.  Proxy Mobile IP
    4.15.  Back to My Mac
    4.16.  LISP-Mobility
5.  Different Directions towards Mobility Support
    5.1.  Routing-based Approach v.s. Mapping-based Approach
    5.2.  IP Layer Indirection v.s. Mobility Exposure
    5.3.  Operator-Controlled Approach v.s. User-controlled
    5.4.  Local and Global Scale Mobility
6.  Discussions
    6.1.  Deployment Issues
    6.2.  Backward Compatibility
    6.3.  Interconnecting Heterogeneous Mobility Support Systems
7.  References
§  Authors' Addresses

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 shed insight on future efforts.

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 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] (Manner, J. and M. Kojo, “Mobility Related Terminology,” .) 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 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.

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:

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

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:

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

We show in the next section that different mobility support 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 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.

Figure 1 (A time table of mobility protocol development) 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 |
        +----------------+-----+---------------+-----+

4.1.  Columbia Protocol

This protocol [Columbia] (Ioannidis, J., Duchamp, D., and G. Maguire, “IP-based Protocols for Mobile Internetworking,” 1991.) 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 the IP address from a special block of IP addresses and keeps it regardless of its current location. Each MSS keeps a tracking 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 then delivers 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:

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

An illustration of Columbia Approach is shown in Figure 2 (Columbia Protocol).

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

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

4.2.  Sony Protocol

In this protocol [Sony] (Teraoka, F., Yokote, Y., and M. Tokro, “A Network Architecture Providing Host Migration Transparency,” 1991.), 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 along the path can also cache a set of mappings for mobile nodes. So does the correpondent node (so only the first packet may need to traverse through a triangle path).

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 the mobile, it modifies the destination IP address field and forwards the packets to the mobile's current location; otherwise, the packet traverses to the mobile's home network, where the packet's destination address then is changed to the mobile's current location according to the mapping. 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:

• Identifier: the mobile node's Virtual IP address;
• Locator: the mobile node's conventional IP address;
• 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 nodes.

Figure 3 (Sony Protocol) shows how Sony protocol works.

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

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

4.3.  LSR Protocol

In Loose Source Routing (LSR) based protocol, 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 announces reachability to those IP addresses. Another network entity required is Mobile Access Station (MAS) through which the mobile gets its conectivity to the Internet. The mobile node always reports the current serving MAS to its Mobile Router.

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 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. 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:

• Identifier: the mobile's IP address assigned by its Mobile Router;
• Locator: the IP address of the serving MAS;
• Mapping: the mapping between the above two kept at the Mobile Router.

Figure 4 (LSR Protocol) shows the basic operation of LSR protocol.

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

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

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] (Perkins, C., “IP Mobility Support for IPv4,” 2002.) and Mobile IPv6 [RFC3775] (Johnson, D., Perkins, C., and J. Arkko, “IP Mobility Support in IPv6,” 2004.) standards in 2002 and 2004, respectively.

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 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 HoA 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 also keeps 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:

• Identifier: the mobile node's HoA;
• Locator: the mobile node's CoA;
• 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.

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

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


                                   ==>: Tunnel
                                   -->: regular IP

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] (Mysore, J. and V. Bharghavan, “A New Multicast-based Architecture for Internet Host Mobility,” 1997.) 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 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 the 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:

• Identifier: the mobile node's multicast IP address;
• Locator: the IP address of the multicast router that is currently serving the mobile node;
• 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] (Valko, A., “Cellular IP: A New Approach to Internet Host Mobility,” 1999.) handles the local mobility in a network consists of Cellular IP routers that integrate location tracking service with routing. A mobile reports the IP address of the gateway for the local network as the regional CoA to its Home Agent, and retains its locally assigned IP address when it roams within the Cellular IP network. The routers in 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 allowing mobile nodes to send dummy packets with a relatively low frequency to update their location information. When a packet from the correspondent node 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 communication between the mobile and correspondent nodes starts going, a much more precise reverse path can be maintained by the routers along the path.

Similarly, HAWAII [HAWAII] (Ramjee, R., Varadhan, K., and L. Salgarelli, “HAWAII: A Domain-based Approach for Supporting Mobility in Wide-are Wireless Networks,” 2002.) also aims to provide efficient local mobility support but with two main differences from Cellular IP. First, it dynamically sets up routes to each mobile by installing host-based forwarding entry in specific routers, usually the ones located along the shortest path between the old and new base stations of the mobile. Second, it uses IP multicast, instead of controlled flooding, to page idle mobile nodes when they have incoming packets arrived at the border router and there are no routing entries available for these addresses.

TIMIP [TIMIP] (Grilo, A., Estrela, P., and M. Nunes, “Terminal Independent Mobility For IP,” 2001.), 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:

• Identifier: the mobile node's IP address that are unchanged when it roams within the domain;
• Locator: the network node that gives network access to the mobile node;
• Mapping: no explicit mapping; host-based entries in local routers instead tracks the mobiles' locations.

4.7.  Hierarchical Mobile IP (HMIP)

HMIP [RFC4140] (Soliman, H., Castelluccia, C., Malki, K., and L. Bellier, “Hierarchical Mobile IPv6 Mobility Management (HMIPv6),” 2005.) 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 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 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 reduces the burden of the Home Agents by reducing the frequency of sending updates to Home Agents.

The basic components here are:

• Identifier: the mobile node's HoA;
• Locator: the mobile node's Local CoA;
• Mapping: the mapping between HoA and Local CoA, stored in MAP.

4.8.  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 with multiple hosts attached to. Because Mobile IP handles mobility per host, it is not efficient when handling such mobility scenarios. NEMO [RFC3963] (Devarapalli, V., Wakikawa, R., Peterson, A., and P. Thubert, “Network Mobility (NEMO) Basic Support Protocol,” 2005.), 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 with additional functions. 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.

Hence, in this protocol, the basic components are:

• Identifier: the Mobile Prefixes;
• Locator: the CoA of the Mobile Router;
• Mapping: the mapping between two, stored in the Home Agent.

4.9.  E2E and M-SCTP

E2E (End-to-End communication) 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 secret tokens and sequence numbers.

Inspired by E2E, M-SCTP [M‑SCTP] (Xing, W., Karl, H., and A. Wolisz, “M-SCTP: Design and Prototypical Implementaion of An End-to-End Mobility Concept,” 2002.) 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.

The three components hence are:

• Identifier: the domain name of the mobile node;
• Locator: the current IP address of the mobile node;
• Mapping: the DNS records kept in the Dynamic DNS server.

4.10.  Host Identity Protocol

Host Identify Protocol (HIP) [RFC5201] (Nikander, P., Moskowitz, R., Jokela, P., and T. Henderson, “Host Identity Protocol,” 2008.) 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), a 128-bit hash value of the Host Identity, is used in transport and other upper layer protocols.

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 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 ESP 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 the mobile node moves to a new address, it notifies correspondent node by sending HIP UPDATE with LOCATOR parameter indicating its new IP address. Meanwhile, it also updates the mapping in RVS.

The basic components here are:

• Identifier: Host Identity or HIT of a mobile node;
• Locator: the current IP address of the mobile node;
• Mapping: the mapping between the two, either kept in Dynamic DNS server or in Rendezvous Server.

4.11.  Connexion and WINMO

Connexion [Boeing] (Andrew, L., “A Border Gateway Protocol 4 (BGP-4),” 2006.) 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 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] (Hu, X., Li, L., Mao, Z., and Y. Yang, “Wide-Area IP Network Mobility,” 2008.) (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 fo their access routers with DBR, which resembles the binding updates in Mobile IP. Thus, packets destined to mobile networks are forwarded to DBR after they enter the border of an AS, and DBR will tunnel them to the current locations of mobile networks. A new BGP community attribute, named Packet State, is also defined to eliminate the triangle routing problem caused by DBR by including the mobile network's COA in each packet.

The basic components in Connexion are:

• Identifier: the prefix of a moving network that does not change during movement;
• Locator: the current router that the mobile network connects to;
• 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:

• Identifier: the Mobile Prefix of a moving network;
• Locator: the point of attachment to the Internet;
• Mapping: mapping between the two, kept in Aggregation Routers and DBR.

4.12.  ILNP

ILNP [ILNP] (Atkinson, R., “An Overive of the Identifier-Locator Network Protocol,” 2005.) 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:

• Identifier: the low-order 64 bits of the mobile's IPv6 address;
• Locator: the high-order 64 bits of the mobile's IPv6 address;
• Mapping: the mapping between the two, kept in Dynamic DNS server and correspondent node.

4.13.  Global HAHA

Global HAHA [HAHA] (Wakikawa, R., Valadon, G., and J. Murai, “Migrating Home Agents Towards Internet-scale Mobility Deployment,” 2006.), 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:

• Identifier: the HoA of the mobile node;
• Locator: the CoA of the mobile node (in the primary Home Agent); the IP address of primary Home Agent (in other Home Agents).
• 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   |
               +------+

4.14.  Proxy Mobile IP

Proxy Mobile IP [RFC5213] (Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K., and B. Patil, “Proxy Mobile IPv6,” 2008.) 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 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 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:

• Identifier: the Home Network Prefix of a mobile node;
• Locator: the Proxy-CoA, which is the IP address of the current MAG;
• Mapping: the mapping between the two kept in LMA.

4.15.  Back to My Mac

Back to My Mac (BTMM) [BTMM] (Nakamoto, R., Zhu, Z., Lau, D., and B. Allen, “Understanding Apple's Back to My Mac Service,” 2009.) 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] (, “Unique Local IPv6 Unicast Address,” .), 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] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for Specifying the Location of Services (DNS SRV),” 2000.) 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 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.

We can also list the basic components for BTMM:

• Identifier: the IPv6 address of a mobile node;
• Locator: the IPv4 address (and port in the case of NAT) of a mobile node;
• Mapping: the mapping between them, kept in Dynamic DNS server.

Figure 7 shows the basic architecture of BTMM.

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

4.16.  LISP-Mobility

LISP-Mobility [LISP‑Mobility] (Farinacci, D., Fuller, V., Lewis, D., and D. Meyer, “LISP Mobility Architecture,” 2009.) is a relatively new design, in which the designer hopes to utilize LISP[LISP] (Farinacci, D., Fuller, V., Lewis, D., and D. Meyer, “Locator/ID Separation Protocol (LISP),” 2009.), 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-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 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:

• Identifier: the Endpoint ID of the mobile node;
• Locator: the Routing Locator the mobile node;
• 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.

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.

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, ILNP, HIP, PMIP, Global HAHA,     |
|               | BTMM, LISP-Mobility                       |
+---------------+-------------------------------------------+
|               | Columbia, Connexion, Celluar IP,          |
| Routing-based | HAWAII, TIMIP                             |
+---------------+-------------------------------------------+
| Combination   | WINMO, MSM-IP                             |
+---------------+-------------------------------------------+

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 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.

+-------------+----------------------------------+
| Indirection | Sony, LSR, Mobile IP, HMIP, NEMO |
|             | Global HAHA, PMIP, LISP-Mobility |
+-------------+----------------------------------+
| Exposure    | E2E, M-SCTP, ILNP, HIP,          |
|             | BTMM                             |
+-------------+----------------------------------+

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 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 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 future.

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

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

6.  Discussions

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

6.1.  Deployment Issues

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

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

Second, the complexity of most mobility support protocols impedes the implementation and hence the deployment in commerical 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.

6.2.  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 also 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.

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, BTMM 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.

6.3.  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, 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 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

Authors' Addresses