A STUDY OF THE SCALABILITY AND PERFORMANCE OF MULTI-LEVEL HIERARCHY FOR SCALABLE MOBILITY MANAGEMENT IN WIRELESS IPv6 NETWORKS

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1 A STUDY OF THE SCALABILITY AND PERFORMANCE OF MULTI-LEVEL HIERARCHY FOR SCALABLE MOBILITY MANAGEMENT IN WIRELESS IPv6 NETWORKS E.N. Onwua and Z. Nu State Key Lab on Mcrowave and Dgtal Communcaton Department of Electronc Engneerng, Tsnghua Unversty, Bejng 184 P.R Chna. Emal: Abstract: Ths paper presents a study on the benefts of mult-level herarchy for scalable moblty management n the wreless moble IP context. Mcro moblty management s consdered. The analyss too advantage of the localty n movement and call actvtes of moble users. It was dscovered that whether mult-level herarchy s benefcal or not depends on the type of management employed n the moblty management. When herarchcal structure s used n a mcro moblty regon, the system scales wth more levels of herarchy, but f the structure used s not herarchcal, mult-level herarchy s less scalable because the root presents a bottlenec to the system. Keywords: Moble IP, IPv6, mcro moblty management, herarchy, scalablty. 1. INTRODUCTION The Internet has been accused of beng a vctm of ts own success. Ths speas of the level of attracton t presents to users. Currently, wreless access to the Internet s attractng a lot of attenton both from users, researchers and vendors. In addton, moblty management protocols for Internet devces are beng defned and standardzed. It s beng predcted, wth good reason, that wreless access to the Internet may outstrp all other forms of access n the near future, and that moble users may expect the same levels of servce qualty as wrelne users. All these developments spea of the need for scalable and hgh performance wreless networs. The general moblty management problem n the Internet can be summarzed as ng and routng problem. Ths s because n the tradtonal IP networ, pacet routng s dependent. A host s IP conssts of two parts: networ dentfcaton part and host dentfcaton part. A pacet destned to an Internet host s routed to the networ where the host s attached and onwards to the host [1]. Therefore when a host moves away from ts home networ, t wll be dffcult to route pacets to t wthout addtonal moblty related operatons. Moble IPv6 (MIPv6) [2] s the base moblty support protocol that s beng standardzed by the IETF. Ths protocol ntroduces networ overhead n terms of delay, pacet loss and sgnalng when a moble (MN) changes pont of attachment frequently, such as when t moves about n a local regon. MIPv6 therefore, performs well for macro-moblty, but when used for mcro-moblty management, t mpacts on performance and scalablty. Ths necesstates another protocol to effectvely manage user moblty n a mcro regon. Recently, a number of mcro moblty protocols have been proposed n lterature, and are beng consdered n the IETF- WG. These protocols are concerned wth the scalablty and performance ssues n managng user moblty n a local regon where a user wll frequently mgrate from one access pont to another. Though the desgn am of the protocols s the same, they nevertheless dffer n desgn approach. Because herarchcal moblty management reduces the performance mpact of moblty by handlng local mgratons locally, the mcro moblty protocols generally employ a herarchy of moblty-aware routers, but they dffer n ther management and routng. In order to select from the many mcro moblty protocols, the IETF-WG mapped out a selecton crtera, whch s documented as an Internet Draft named Requrements for localzed moblty management [3], [4]. In one of the crtera, mult-level herarchy vs. flat herarchy n [3] (sldes verson), they needed a quantfable metrcs to judge whether there s performance gans n mult-level herarchy vs-à-vs flat herarchy. In [4] (sldes verson), no specfc recommendaton was made on number of levels of herarchy, but t s requred that the mcro moblty protocol should be able to scale. In ths paper, we present an attempt to quantfy the scalablty and performance benefts of mult-level herarchcal archtecture n IP mcro moblty management networs. The rest of ths paper s arranged thus: In Secton 2 we present a general structure of the mcro moblty protocols proposed n lterature, n Secton 3 we create a scenaro to ad n comparson, n Secton 4 we present an analytcal model, Secton 5 presents numercal results, whle Secton 6 concludes the paper. 2. STRUCTURE OF MICRO MOBILITY OTOCOLS Accordng to the defnton of mcro moblty protocols, when a MN s away from home, the Internet nown by ts HA no longer reflects ts exact pont of attachment. Rather, t represents the of a gateway that s common to potentally large numbers of networ access ponts (AP) [5]. In most of the

2 mcro moblty protocols, the doman s extended by attachng the Aps to moblty routers (MR) that are arranged n herarchcal format. In order to route pacets to MNs actual pont of attachment, each of these MRs mantans moblty bndng entres for all the MNs under ther coverage areas. To forward a pacet, each MR needs to examne the destnaton of the pacet and chec ts moblty bndng entres. If a moblty bndng for the MN to whch the pacet s destned exsts (generally n the form of MN- next-hop-router), t wll forward the pacet accordngly. Ths mples that, ths mappng or moblty bndng for each MN wll exst from the gateway, through the MRs, down to the AP to whch the MN s attached. There are two broad classes of management and routng used by mcro moblty. They are tunnelng and moble specfc routng [5]. In the tunnelng approach, moblty bndng exsts n the form of COAs, so that pacets have to be routed by encapsulatng the orgnal pacet n a new pacet whose destnaton s the COA. All the MRs along the way to the MN need to decapsulate the pacet n order to see the orgnal, search ts entres for moblty bndng, rencapsulate and forward the pacet. The pacet eventually reaches the AR (or the lowest-level FA) that then delvers the pacet. In the moble specfc routng approach, COAs are not used to dentfy MNs n the mcro moblty doman. Rather, MNs are dentfed by ther home es, and moble-specfc moblty bndngs are mantaned n the MRs. Encapsulated pacets enterng the doman are decapsulated at the gateway. Wthn the doman, pacets need not be encapsulated, they are routed accordng to the moble specfc entres n the respectve MR n the herarchy down to the to the AR where the moble s attached. In all the mcro moblty protocols, locaton nformaton s created and mantaned ether by mplct or explct sgnalng. Herarchy actually exsts, but only n the arrangement, and ths s common to all the mcro moblty protocols n lterature. It therefore, does not gve enough room for comparson. Let us, for the purposes of ths study, call ths type of herarchcal structure scenaro-1. In order to carry out ths study, we need to create a second scenaro based on a dfferent herarchcal structure, and let us call t scenaro-2. The next Secton creates scenaro A SCENARIO FOR COMPARISON Recall that one of the reasons the fxed Internet scaled to accommodate many hosts s due to ts herarchcal structure. Ths made t possble to eep the routng table entres manageable. In an attempt to explot ths characterstc n the moblty management problem space, we create scenaro-2 as a herarchcal structure based on Internet ng herarchy. Suppose that an IPv6 subnet space s assgned to mcro moblty doman. And that a porton of the 16-bt SLA ID feld s subdvded to create further ng herarches as shown n fg.1. Each MR on each level wll be aware of ts herarchcal poston and eep approprate routng table entres. Then, es of all Hgher levels SLA ID Interface ID Fg.1 possble prefx mass for a 4-level herarchy MNs regstered n a gven zone are wthn the prefx space of the MRs n ther lne of herarchy up to the gateway. In ths scenaro, the HA functonalty s mplemented n the leaf s whch serve as access routers (AR). Therefore, when a MN moves from access router A to access router B n Fg.2, a moblty bndng s created for t n AR B and MR C. Thus, upln/downln Pacets are stll routed to ths MN by prefx up to C, whch now routes the pacet accordng to the moblty mappng to AR B. Ths way, moblty bndngs need not exst n the hgher-level MRs except when necessary. A MN move D C HR E H I B MN F = Prefx Route HR = Host Route Fg.2 llustraton of management and routng Snce most user movements are lely to be localzed, moblty bndng wll only need to exst n the lower-level MRs. Thus, the farther a MN moves from ts home cell, the hgher n the herarchy wll ts locaton nformaton be placed. Where moblty bndng does not exst for MN, routng wll be based on prefx. In ths scenaro, gven the low percentage of users who wll lely wander far from home cell, one may expect the hgherlevel MRs to be less congested. Also, gven that the number of entres n the hgher-level MRs do not depend on the number of MNs n the doman, but on how far mobles move from home cells, the system could be expanded by addng new s at the lower-levels. 4. ANALYTICAL MODEL In order to create a model to estmate how scalable the two herarchcal scenaros are, we use a 2-, 3-, and 4-level herarchy as a case study wth the leaf s as level- and root as level-3 (note that 1-level s a trval case). Recall that the MRs G J K

3 eep a sort of database where route nformaton are stored. The operatons they perform nclude search, nsert or/and forward. Among these operatons, the search operaton consumes the greatest tme; t depends on the number of database entres. A pacet may experence some delay at each t wll traverse before completng servce, so that, the total delay a gven pacet suffered s the sum of each ndvdual delay component at the s t traversed, ncludng the lns. Thus, the maxmum delay mposed by the system on an arbtrary pacet may be estmated as the summaton of the average delays from the wreless ln to the root, ths may be represented as: 1 max = d wl + 2 d n + 2 = = 1 D 2 d + d (1) Where d wl s the wreless ln delay, d n s the delay at, and d l s the delay on wred-ln. Ths delay dstrbuton s shown n fg.3. MN L- L-1 L-2 L-3 MN L- L-1 L-2 L-3 p p 1 l n q 1 because we assume the wred ln capacty n the access networ s never exceeded, no queue forms. λ 1 λ 1 λ λ 1 jj 1 λ µ w µ µ l µ 1 µ jj 1 µ jj Wreless ln L- Wred ln L-1 Fg.4 llustratng networ of queues for our study model L--1 λ jj Due to the localty n moble user movement, wth some probablty q 1 a MN wll have moblty bndng only n level- and the level-1 s, see fg.3. Wth probablty q 2, t wll have moblty bndng from level- up to level-2 s, and wth probablty q, t wll have bndng from level- up to the root. Also, due to the localty n call actvtes, wth probablty p, a data pacet orgnates and termnates wthn a cell, wth probablty p 1, t orgnates and termnates wthn a level-1 coverage area, wth probablty p 2, t orgnates and termnates n a level-2 coverage area, and wth probablty p, t gets up to the root before leavng the system [7]. L- p 2 q 2 The average servce request arrvng at each n a j-level systems s generalzed as: d wl d l1 d l2 d n d n1 d n2 d l3 d n3 p 3 d wl d l1 d l2 d n d n1 d n2 (a) (b) Fg.3 Delay dstrbuton along the levels of the herarchy (a) routng, (b) update To estmate the delay components, we assume, for smplcty, that data pacet and BU arrval rates and servce rates at the leaf s (or ARs) follow the Posson process wth λ p, λ u, µ respectvely so, the total arrval rate at an AR s λ = 2(λ u +λ p ). Further, we model the servce s (ncludng the lns) n the herarchy as a networ of queues, see Fg.4. Assume that the departure processes from each AR follow the Posson process and therefore, feed the level-1 s wth a Posson arrval rate λ 1 = N 1 λ, and the departure processes from the level-1 s feed the level-2 s wth Posson arrval of rate λ 2 =N 2 λ, agan, level-2 s feed level-3 wth a Posson arrval process of λ 3 =N 3 λ where N 1, N 2 and N 3 are the number of ARs feedng nto the level-1, level-2 and level-3 s respectvely. Wth these assumptons, our system obeys Jacson theory [6]. Therefore, we can model the servce s as M/M/1 queung system wth servce rates µ 1, µ 2, and µ respectvely. We model the wreless ln as M/M/1 system wth servce rate µ w, and the wred ln s modeled as M/M/ wth servce rate µ l, ths s d l3 q 3 d n3 λj = N {2λ + [ 1 u = = 1 (1 p )](2 p } = λ j N {[ (1 q )](2 q ) λu + j = 1 j = j 1 = λ = N { (2) [ (1 p )](2 ) λ } (3) 1 = 1 (1 q p ) λ + u 1 = p j=1,2, =,1, j (1 p ) λ } Where λ s the servce arrval rate, subscrpt j represents the maxmum number of levels n a gven system, and subscrpt represents the th level n a j-level system. If λ p and λ u are nown, we can calculate the average delay at each servce wth the formula: p (4)

4 D s = (5) 1 λs Where s s average servce tme at the th servce, see Fg. 4. Therefore, the maxmum delay could be estmated as: Dmax D = 1 = (6) Note that due to larger number of moblty entres n scenaro-1, servce tme at each s expected to be longer than n scenaro-2 because of the longer search tme ncurred. 5. NUMERICAL RESULTS For numercal evaluaton, we estmate a relatvely practcal stuaton that s lely to be obtanable n the near future, when IPv6 networ s fully deployed..e., traffc pattern n cty pedestran zone, wth hgh termnal densty, small cells, low moblty, and relatvely slow servers. Therefore, λ u s estmated usng the flud flow moblty model as n [7]: λ u = ρv4l/36π (/s) and λ p = ρra/36 (/s) Where ρ s termnal densty (term/km 2 ), v s average termnal velocty, l s lateral length of cells (square cells), A s area of a cell, and r s average data pacet arrval rate per termnal (pt/term/hr). To nvestgate the scalablty and delay performance due to number of levels, we evaluate Eqs (2)-(6) for j=1, 2 and 3 for both scenaros (snce 1-level, j=, s a trval case). In (5) we assume s =s 1 =s 2,=s 3.. See Table I for parameter values. The results are shown n Fgs 5-8. maxmum delay (s) MN densty Fg.5 scalablty performance for 2-level system for scenaro 1and 2 (l=8m, Table I Parameters used for evaluaton parameter J=1: N 1 J=2: N 1,N 2 J=3: N 1,N 2,N 3 p p 1 p 2 value , 128 8, 32, parameter Scenaro-1: s Scenaro-2: s s w s l q 1 q 2 value parameter l r v value.8/1.km 2 5.6/1Km/hr These results show that mult-level herarchcal desgn mproves networ scalablty, but the degree of scalablty depends on the type of herarchcal structure used. Fgs 5 and 6 show that herarchcal desgn based on ng herarchy (scenaro-2) s about three tmes more scalable than that based on arrangement (scenaro-1) when herarchy s ncreased from 2- to 3-level. Ths s because, although herarchy provdes for dstrbuton and data parttonng, scenaro-1 s less scalable wth ncrease n herarchcal level because the root maxmum delay(s) MN densty Fg.6 scalablty performance for 3-level system for scenaro 1and 2 (l=8m, maxmum delay (s) herarchy by MN densty Fg.7 scalablty performance for 4-level system for scenaro 1and 2 (l=8m,

5 maxmum delay (s) herachy by MN densty Fg.8 scalablty performance for 4-level system for scenaro 1and 2 (l=1m, v=1km/hr) causes a bottlenec to the system, therefore t provdes a lmt to the system both n capacty and maxmum acceptable delay. Scenaro-2 does not suffer from ths lmtaton because there s no bottlenec. Fg.7 shows that for 4-level herarchy, scenaro-1 s almost saturated whle scenaro-2 s far from saturaton. Fg.8 shows that for bgger cell and faster termnal moblty, scenaro 1 s stll much more scalable than scenaro Address parttonng may consume space, but ths should be consdered a mnor problem n the face of the benefts t wll attract, especally as wreless Internet access may eventually outstrp the wrelne access. Proper management wll also reduce the mpact of parttonng. Reference: 1. D.E. Comer, Internetworng wth TCP/IP vol.1, Prentce hall Internatonal, D.B. Johnson and C. Perns, Moblty support n Ipv6, Internet Draft, draft-etf-moblep-pv6-19.txt, Nov. 22; wor n progress. 3. C. Wllams, Localzed moblty management requrements for IPv6 Internet Draft, draft-wllams-moblep-lmm-requrements-.txt, November 21, wor n progress. 4. C. Wllams, Localzed moblty management requrements for IPv6 Internet Draft, draft-etf-moblep-lmm-requrements-1.txt, March 22, wor n progress. 5. A.T. Campbell, et al, Comparson of IP mcrmoblty protocols, IEEE wreless communcaton, Feb L. Klenroc, Queueng systems, volume 1, Wley- nterscence publcaton, X. Qu and V.O.K L, Performance Analyss of PCS Moblty Management Database System, Proc. Of the 4 th Int. Conf. on Computer communcatons and networs (IC3N 95), Sept. 1995, pp CONCLUSION Ths paper attempted to answer the queston as to whether there s any performance gans n mult-level herarchy vs-à-vs flat herarchy (.e. a fxed herarchy). From our study we could mae the followng conclusons: Mult-herarchy mproves scalablty and delay performance of moble networ systems because t allows for route nformaton dstrbuton and data parttonng. more number of herarches could ncrease scalablty. The degree of scalablty achevable depends on the type of herarchcal structure employed. A herarchcal structure based on arrangement only, whch forces every MN n a doman to have moblty bndng n the root may seze to scale n the future because the capacty and delay performance of such a system depends on the capacty and delay performance of the root. A herarchcal desgn based on ng structure for IPv6 may scale better because the root does not present a bottlenec therefore, the doman may be fanned out n the future by addng more lower-level s. Herarchcal levels may be ncreased by n the future by further parttonng partcularly n IPv6.