OCTOBER/NOVEMBER 2005 170278-6648/05/$20.00 © 2005 IEEE

As it unleashes users from the wiredworld, mobile communication is trans-forming the way people connect to theInternet. Eventually, wireless communica-tion systems will migrate to the fourth gen-eration (4G) mobile communication sys-tems, empowering users with high-speeddata access. Meanwhile, as demand formobile services increases, many serviceproviders will find solutions for integratingthird and fourth generation networks.

An all-Internet Protocol (IP) networkwill provide high-speed, seamless

Internet connectivity—even as usersmove from one 4G network domain toanother. With this network, a user willwalk out from his office building and ahandoff will occur to a cellular IP net-work. On city roads, a mobile IP (MIP)handoff will transfer the connection toa different network domain and amicromobility protocol such as hand-off-aware wireless access Internetinfrastructure (HAWAII) or hierarchicalmobile IP (HMIP) may take over.When a user drives on the highway, anad hoc network formed on the flybetween other cars will keep her con-nected to the Internet through a gate-way. Such seamless connectivity is notfar from becoming a reality.

Extensive research is being carried outto develop mobility protocols. Althoughstand-alone applications of such protocolshave surfaced, true 4G Internet facesmany obstacles, including a lack of stan-dardization, security concerns, unreliablehandoff techniques, and performancedegradation. This article introduces themacromobility, micromobility, and ad hocrouting protocols the Internet EngineeringTask Force (IETF) has discussed over thepast few years. The aim is to familiarizereaders with various mobility protocols,

point out differences, and generate inter-est to pursue research in this area.

Macromobility protocolAs designed for wired networks, IP

fails to work for mobile nodes (MNs)because it routes packets to destinationsunder the assumption that the networkis static. For Transmission Control Pro-tocol (TCP), the connections areindexed by a quadruplet, which con-tains the IP addresses and port numbersof the source and destination. MNsacquire a new IP address with everychange in their point of attachment;therefore, mobility becomes impossibleusing the IP standard. To overcome thecomplications faced by IP to support

mobility, MIP was standardized by IETFand supports uninterrupted connectivityfor MNs. MNs use two IP addresses; thehome address remains unchangedwhile the care-of address (CoA)changes with every point of attachment.When the MN moves from the homeagent (HA) to a foreign agent (FA), itregisters the new CoA with its HA. TheHA intercepts all packets destined forthe MN and forwards it to the FA.

Figure 1(a) shows an MN attached toits HA. As the MN moves to a new net-work, it registers the new CoA with itsHA via the FA. When a correspondent-node (CN) sends a packet to the MN, it

is first intercepted by the HA. The HAadds a new IP header to the packet thatcontains the MN’s CoA as the destina-tion address and forwards it to the FA.The process of adding a new IP headerto redirect the packet is called tunnel-ing. By default, IP-in-IP tunneling isused, but generic route encapsulation(GRE) and minimal encapsulation mayalso be used (see references). Figure1(b) shows the MN connected to an FA.The FA removes the tunnel header andforwards the packet to the MN.

For MIP, each time the MN moves toa new network, it needs to perform ahandoff, the process of associating witha new mobility agent once it moves outof the old mobility agents’ domain. If

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VIPUL BAHETY AND RAVI PENDSE

Bridging the generation gap

18 IEEE POTENTIALS

the HA and FA are far away, this regis-tration delay can be considerably high(packet transmission has a delay ofabout 6–9 ms per 1,000 km). Duringthe registration process, all packets willbe dropped because the MN is notattached to any agent. Moreover,sending registration messages on theInternet every time a node migrates toa different agent’s domain can causeexcessive congestion. The situationbecomes worse if the MN exhibits highmobility and if large numbers of thesedevices are in the domain.

Network Mobility (NEMO), an exten-sion to MIP, allows the mobility of anentire network similar to that performedby individual MNs. A mobile router (MR)is implemented in software on a networkrouter and allows entire networks toroam while maintaining connection to theInternet. The MR connects to the globalInternet while nodes attached to the MRcan be mobile or fixed. Figure 1(c) showsMN conncected to the MR and the tunnel-ing mechanism. The MR first connects to

the HA as a MIP node,forming a tunnelbetween the HA and FA(Tunnel 1). Anotherbidirectional tunnel isbuilt between the HAand MR for packets des-tined to nodes connect-ed to the MR (Tunnel2). This is set up whenthe MR sends a bindingupdate to the HA,informing it about itscurrent point of attach-ment. Personal area net-works (PANs) and com-puters deployed invehicles on ships withnetwork mobility areexamples of such sce-narios. Though innova-tive, there are draw-backs to NEMO. Qualityof service (QoS)degrades further be-cause of additional tun-neling overheads. Plus,the formation of multi-ple tunnels causespacket header sizes toincrease, leading tohigher processing costsat mobility agents.

To overcome theproblems of signalingoverhead and exces-sive packet loss, IETF

proposed micromobility protocols thathandle the local movement of MNs. Itis envisioned that while MIP will pro-vide wide-area mobility, micromobilityprotocols such as cellular IP, HAWAII,and HMIP will cater to mobility needsin local wireless access networks.When integrated together with ad hocnetworks, an infrastructure for a global4G-mobility solution can be provided.

Micromobility protocolsMicromobility protocols complement

MIP by reducing signaling overheadsand enabling faster handoffs for seam-less connectivity. These protocols han-dle local movements without any inter-action with MIP. When an MN movesfrom one wireless access network toanother, a MIP handoff occurs. Depend-ing on the application, the size of wire-less access networks may vary, fromWANs to LANs. IP paging techniquesare used by micromobility protocols totrack idle MNs, thereby minimizing sig-naling overhead. This also reduces

bandwidth consumption over the airinterface and the wired world. Anotheradvantage of reducing signaling over-heads is that it helps to conserve thepower reserves of MNs.

Cellular IP can perform handoffs in anumber of ways, and it uses an efficientpaging technique to loosely track MNswhen they are idle, determining theirexact position only when the nodes areactive. To achieve this, it uses two map-pings: paging cache (PC) and routingcache (RC). PCs are used only to searchfor MNs while RCs are used to routedata to the MNs. For this reason, PCshave a much higher timeout value thanRCs. Idle MNs periodically transmit pag-ing-update packets to the closest basestation. This update packet travels tothe gateway router, which connects thewireless access network to the Internet.All intermediate base stations that main-tain a PC update the value for the MN.The gateway router blocks paging-update packets from traveling over theInternet. When IP packets arrive for aMN at the gateway, PCs are used tolocate the host. The IP packet isqueued at the gateway, and a pagingpacket containing the MN’s IP addressis forwarded. The paging packet travelshop by hop in a direction that is thereverse of the last paging-update packet.When the MN receives the paging pack-et, a route-update packet is sent to thegateway router. Packets destined fromthe gateway to the MN are routed alongthe reverse path with the help of RCs.

HMIP uses a hierarchy of FAs tolocally handle registrations. At the topof the hierarchy is a gateway foreignagent (GFA), and at the next level areFAs directly connected to the GFA.When an MN first arrives in a foreignnetwork, it registers the CoA with theHA. This CoA is the GFA’s address, andit does not change as the MN movesfrom one FA under the GFA to theother. Regional registration request andregional registration reply messageshelp MNs register with the GFA andreduce the traffic overhead caused bytransmitting registration messages to thehome network, which might be faraway. The HA is aware of the MN’slocation with respect to the GFA andtunnels any packet destined for the MNto the GFA. The GFA pages for the MNand establishes a path to the currentpoint of attachment.

HAWAII is another routing protocolto handle intradomain mobility. An MNin a new FA’s domain is assigned a

Fig. 1 (a) A mobile node attached to its home agent, (b) amobile node attached to a foreign agent, and (c) amobile node attached to a mobile router

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colocated CoA, which remainsunchanged until the node moves to adifferent domain. The HA is unaware ofthe MN’s movement within the foreigndomain. Path setup messages are usedto dynamically maintain host-based rout-ing entries in selected routers for theMN. HAWAII uses multicasting tech-niques to page for idle mobile hostshaving no recent routing information.Although each micromobility protocoldiffers in the way location databases aremaintained and updated, there aremany similarities in the way these pro-tocols operate. Each of these protocolshave intermediate nodes in the micro-mobility domain that forward packetsand maintain location information. Pag-ing techniques are used to track idlemobile hosts, and next-hop entries forma route to the destination.

Ad hoc routing protocolsIt is impractical to have an infrastruc-

ture with a signal footprint (that is, thecoverage of the signal) everywhere onearth. In areas where there is very littlecommunication infrastructure, mobileusers can communicate by forming adhoc networks. In an ad hoc network,each MN is a router, forwarding packetsbetween other nodes that are in the adhoc network. These self-organizing net-works have varied applications in themilitary, disaster zones, digital commu-nication to remote villages, and wherev-er there is no existing infrastructure.Individual researchers have already pro-posed solutions to provide Internet con-nectivity for ad hoc networks by usinggateways connecting to infrastructurenetworks. Figure 2 shows an ad hocnetwork as part of a 4G network. Pack-ets destined for MN3 from MN1 may gothrough MN2. If the link between MN1and MN2 is broken due to link failureor if MN2 moves away, another routewill be discovered.

Conventional routing protocols can-not be adopted in ad hoc networksbecause of their dynamic nature. More-over, distance-vector and link-staterouting protocols require regular

updates to be broadcast to neighboringrouters. Wireless networks are band-width-constrained variable-capacitylinks, and using protocols designed forthe wired world will result in a lot ofoverhead. Also, MNs may be battery-operated devices, and energy conserva-tion is an important design criteria. Ad

hoc on-demand distance vector(AODV) routing, optimized link staterouting (OLSR), dynamic source routing(DSR), topology dissemination based onreverse-path forwarding (TBRPF), andtemporally ordered routing algorithm(TORA) are some of the protocols thathave been discussed within the IETFlately. Table 1 shows the comparison ofthese routing protocols. Since a detailedanalysis of each of these protocols isnot possible in this article, the main fea-tures of ad hoc networks and differ-ences are outlined, providing the readerenough background to understanddetailed literature on this topic.

Ad hoc routing protocols can bebroadly classified as table-driven (orproactive) routing protocols and on-demand (or reactive) routing protocols.

Table-driven routing protocols maintainan updated entry in the routing tableabout the path from each node to everyother node in the network. Nodesupdate their routing table informationperiodically or whenever the networktopology changes. The advantage ofthis method is that there is no latency in

finding the route to a destination. How-ever, some of these routes may neverbe used, and there will be a waste ofmemory and wireless bandwidth. OLSRand TBPRF are examples of proactiverouting protocols.

On the other hand, on-demand rout-ing protocols maintain routes only tothe nodes for which the source has traf-fic. Whenever a node wants to senddata to a destination, it finds the routeby a process called route discovery.Once the route has been discovered,packets are transmitted along the path.If a link in the path is broken, routemaintenance is needed. These protocolsreduce the control overhead and helpin conserving power for MNs. The dis-advantage of on-demand routing proto-cols is the delay caused in the route

Table 1. Comparison of Some Ad Hoc Routing Protocols.

Protocol Route Entry Computation Update Routing Number Method Method Method Mechanism of Paths

AODV Reactive Distributed Event driven Hop-by-hop MultipleDSR Reactive Distributed Event driven Source MultipleOLSR Proactive Distributed Periodic Hop-by-hop SingleTBRPF Proactive Decentralized Hybrid Hop-by-hop SingleTORA Reactive Distributed Event driven Hop-by-hop Multiple

Fig. 2 IP-based mobility: macromobility, micromobility and ad hoc networks

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20 IEEE POTENTIALS

discovery process. Packets destined tonodes have to be buffered in the sourceuntil a valid route is discovered. AODV,DSR, and TORA are examples of on-demand ad hoc routing protocols.

In a distributed-route computationprotocol, each node in an ad hocnetwork maintains only partial informa-tion about the network topology. Theroute to a destination is computed bysharing this information with othernodes. In a decentralized computation-based protocol, each node in the net-work has complete information aboutthe network topology. With this knowl-edge, the node can compute a route toany destination on its own and doesnot need to collaborate with peernodes. In Table 1, all except TBRPF aredistributed ad hoc routing protocols.

As with routing protocols for wirednetworks, route information must beshared among nodes in an ad hoc net-work. Route information may beshared periodically or due to a changein the network topology. Some proto-cols exclusively use one of the meth-ods, while others share route informa-tion in both cases (hybrid updatemethod). Periodically sharing routeinformation helps to keep the networkconverged and allows new nodes toquickly join the network. If the updateperiod is too large and the networktopology is changing rapidly, the net-work may never converge. However, asmall update period will cause a lot oftraffic overhead in the network. There-fore, it is important that an optimalupdate period be chosen based on thenetwork characteristics. In an event-driven update protocol, whenever anevent such as a topology changeoccurs or a link fails, an update packetis sent. If the network is very dynamic,a lot of traffic will be generated, result-ing in wasted bandwidth.

Protocols use hop-by-hop andsource routing to forward packets todestinations. Nodes using source rout-ing embed the entire route in the pack-et header, and intermediate nodes for-ward packets based on this route. Inhop-by-hop routing a source just for-wards packet to the next hop corre-sponding to the destination. If the routeis too long, packet size for source rout-ing may be too large, resulting in a lotof bandwidth usage. For hop-by-hoprouting, all nodes need to maintainrouting information. This may causerouting loops to be formed. Some rout-ing protocols are capable of finding

multiple routes to a destination. Thishelps in load balancing and providesredundant paths in case routes fail.

Figure 2 shows the relationshipamong macromobility, micromobility,and ad hoc networks. Nodes wanting toconnect to the Internet will have to sendtraffic through the gateway. Within a net-work domain, micromobility will providefaster handoff and better QoS. When anode moves from one mobility agent’sdomain to the other, an MIP handoff willoccur. An MN in an ad hoc network willconnect to the infrastructure network assoon as it receives advertisements from amobility agent. Such seamless integrationwill help provide true 4G mobile com-munication systems.

ConclusionsToday, 3G systems are being imple-

mented on a large scale. Users will beexposed to a broad range of content-rich services, such as multimedia mes-saging, video conferencing, andstreaming media. At the same time,widespread deployments of wirelessLANs provide users with high-speedInternet access in certain places. Suchheterogeneous networks and differentair interfaces will smoothly transitioninto, or be part of, 4G mobile commu-nication systems. It is envisaged thatMIP, micromobility protocols, and adhoc communication will form the corearchitecture in 4G networks.

Read more about it• C. Perkins, “IP Mobility Support for

IPv4,” IETF, RFC 3344, Aug. 2002[Online]. Available: http://www.faqs.org/rfcs/rfc3344.html

• V. Devarapalli, R. Wakikawa, A.Petrescu, and P. Thubert, “Nemo basicsupport protocol,” IETF, RFC 3963, Jan2005 [Online]. Available: http://www.faqs.org/rfcs/rfc3963.html

• T. Ernst, “Network mobility supportgoals and requirements,” IETF, Oct. 2004[Online]. Available: http://www1.ietf.org/proceedings_new/04nov/IDs/draft-ietf-nemo-requirements-03.txt

• X. Zou, B. Ramamurthy, and S.Magliveras, “Routing techniques in wire-less ad hoc networks—classification andcomparison” in Sixth Multiconf. Sys-temics, Cybernetics, Informatics, 2000.

• D.B. Johnson, D.A. Maltz, and Y.Hu, “The dynamic source routing proto-col for mobile ad hoc networks (DSR),”IETF, Internet Draft, July 2004 [Online].Available: http://www.ietf.org/internet-drafts/draft-ietf-manet-dsr-10.txt

• A.T. Campbell, J. Gomez, S. Kim,Z. Turanyi, C-Y Wan, and A. Valko,“Comparison of IP micromobility proto-cols,” IEEE Wireless Commun., vol. 9, no.1, pp. 72–82, Feb. 2002.

• R. Ogier, F. Templin, and M.Lewis, “Topology dissemination basedon reverse-path forwarding (TBRPF),”IETF, Internet Draft, Oct. 2003 [Online].Available: http://www.ietf.org/pro-ceedings/04mar/I-D/draft-ietf-manet-tbrpf-11.txt

• C. Perkins, E. Belding-Royer, andS. Das, “Ad hoc on-demand distancevector (AODV) Routing,” IETF, RFC3561, July 2003 [Online]. Available:http:// www.faqs.org/rfcs/rfc3561.html

• T. Clausen and P. Jacquet, “Opti-mized link state routing protocol(OLSR),” RFC 3626, IETF, RFC 3626, Oct.2003 [Online]. Available: http://www.faqs.org/rfcs/rfc3626.html

• A.T. Campbell, J. Gomez, and A.G.Valkó, “An overview of cellular IP,” inProc. IEEE Wireless Commun. NetworksConf. 1999, New Orleans, LA, vol. 2, pp.606–610.

• A. Pentland and R. Fletcher,“DakNet: Rethinking connectivity indeveloping nations,” Computer, vol. 37,no. 1, pp. 78–83, Jan. 2004.

• D. Farinacci, T. Li, S. Hanks, D.Meyer, nad P. Traina, “Generic routingencapsulation (GRE),” IETF, RFC 2784,Mar. 2000 [Online]. Available: http://www.faqs.org/rfcs/rfc2784.html

• C. Perkins, “Minimal Encapsulationwithin IP,” IETF, RFC 2004, Oct. 1996[Online]. Available: http://www.faqs.org/rfcs/rfc2004.html

About the authorsVipul Bahety has completed his mas-

ter’s degree in electrical engineeringfrom Wichita State University, Kansas,and obtained his bachelor’s degreefrom Walchand Institute of Technologyin India. His research interest lies in thearea of computer networks with specialemphasis on wireless and mobile com-puting. He is working as a system testengineer at Qualcomm, Inc., SanDiego, California. Contact him at vba-hety@qualcomm.com.

Ravi Pendse is the vice presidentof academic affairs and research,Wichita State Cisco Fellow, and direc-tor of the Advanced NetworkingResearch Center at Wichita State Uni-versity, Kansas. His research interestsinclude wireless networks, security,and multiservice over IP. Contact himat ravi.pendse@wichita.edu.