IP mobility support is becoming very important as the

Transcription

1 706 IEEE TRANSACTIONS ON COMPUTERS, VOL. 52, NO. 6, JUNE 2003 A New Scheme for Reducng Lnk and Sgnalng Costs n Moble IP Young J. Lee and Ian F. Akyldz, Fellow, IEEE Abstract IP moblty support s provded by the basc Moble IP protocol. The man drawback of the protocol s that the packets must be routed along the paths longer than the optmal one. Ths s known as the trangle routng problem. Recently, the route optmzaton protocol was proposed to solve the trangle routng problem, whch allows packets to be routed along an optmal path from a correspondent node to a moble node. However, the route optmzaton protocol may cause hgh sgnalng and processng costs. In ths paper, a new scheme for reducng costs n route optmzaton s ntroduced to solve the above problems. Lnk and sgnalng cost functons are ntroduced to capture the trade off between the network resources consumed by the routng, sgnalng, and processng load ncurred by the route optmzaton. In ths new scheme, route optmzaton s performed only when t mnmzes the total cost functon, whch provdes the optmal result from the pont of vew of lnk and sgnalng costs. The smulaton results show that the proposed scheme provdes the best performance. Index Terms Moble IP, trangle routng, route optmzaton. æ 1 INTRODUCTION IP moblty support s becomng very mportant as the Internet s growng fast, and the wreless communcaton technology s advancng. The basc Moble IP protocol [1] was proposed to provde IP moblty support. It ntroduces three new functonal enttes: moble node (MN), home agent (HA), and foregn agent (FA). Although the basc Moble IP protocol proposes a smple and elegant mechansm to provde IP moblty support, there s a major drawback, where each packet destned to the MN must be routed through the HA along an ndrect path. Ths s known as the trangle routng problem. The so-called Route Optmzaton Protocol (IETF RO) [2] was proposed by the IETF to solve the trangle routng problem. When packets are sent from a correspondent node (CN) to an MN, f the CN has a bndng cache entry for the MN, they can be drectly tunneled wthout the help of the HA to the care-of-address (COA) ndcated n the bndng cache. In ths scheme, route optmzaton s acheved by sendng bndng update messages from the HA to the CN. In Moble IPv6 [3], bndng update messages are sent from the MN to the CN. Moreover, the FA smooth handoff scheme [2] allows packets n flght or sent based on the outof-date bndng cache to be forwarded drectly to the MN s new COA. The major drawback of the IETF RO [2] s that there are addtonal control messages such as bndng warnng and bndng update, whch cause communcaton overhead and ntroduce hgh sgnalng and processng load on the network and on certan nodes. Some mechansms such as local anchorng scheme [8], regonal regstraton [4], and. The authors are wth Broadband & Wreless Networkng Laboratory (BWN-LAB), School of Electrcal & Computer Engneerng, Georga Insttute of Technology, Atlanta, GA E-mal: {young, an}@ece.gatech.edu. Manuscrpt receved 3 Oct. 2001; revsed 6 Aug. 2002; accepted 30 Oct For nformaton on obtanng reprnts of ths artcle, please send e-mal to: tc@computer.org, and reference IEEECS Log Number herarchcal management scheme [5] have been proposed to reduce sgnalng costs and communcaton overhead recently. Our work s motvated by the queston: Does route optmzaton need to be performed whenever an MN hands off, and a prevous FA receves packets destned to the MN? What f we perform route optmzaton only when certan condtons are satsfed by dong that? If the route optmzaton s not performed as often as t s n the IETF RO [2], sgnalng and processng load wll be reduced. Ths queston naturally leads to two ssues: 1. how to guarantee that the packets destned to the MN are routed temporarly along a suboptmal path wthout performng route optmzaton, and 2. when to perform route optmzaton. For the frst ssue, the FA smooth handoff scheme [2] gves an answer. By keepng the prevous FAs servng as forwardng ponters untl route optmzaton s performed, we can guarantee that IP datagrams are routed along a suboptmal path. We name ths mechansm a route extenson because t smply extends the routng path from the prevous FAs to the current FA. For example, n Fg. 1, FA1 forwards packets to FA2, FA2 forwards them to FA3, and, fnally, the packets are delvered to the MN through the FA3. In ths paper, we focus on the second ssue. Although the route optmzaton ncreases the network utlzaton by allowng packets to be routed along an optmal path from the CN to the MN, t wll also ncrease the sgnalng load of the network and the processng load of certan nodes. We know from ths fact that there s a trade off between the network resources consumed by the routng path and the sgnalng and processng load ncurred by the route optmzaton. The decson of when to perform route optmzaton needs to be consdered based on the followng: /03/$17.00 ß 2003 IEEE Publshed by the IEEE Computer Socety

2 LEE AND AKYILDIZ: A NEW SCHEME FOR REDUCING LINK AND SIGNALING COSTS IN MOBILE IP 707 wthout route optmzaton. The sgnalng cost functon s denoted by hðy ; Þ, where y s the number of lnks n the shortest path between the current FA and the prevous FA durng the th perod. The total cost functon s then defned as the sum of these two cost functons: Fg. 1. Route extenson. 1. the network resources consumed by the routng path, 2. the sgnalng and the processng load, and 3. the Qualty-of-Servce (QoS) requrements. In IETF RO [2], when an FA receves a tunneled packet and, f t has the bndng cache entry for the MN and does not have the vstor lst entry for ths MN at that pont, the prevous FA then sends a bndng warnng message to the MN s HA advsng t to send a bndng update message to the CN. Regardng ths FA-ntated route optmzaton, we propose that the prevous FA should not send the bndng warnng message to the HA. In our scheme, we propose that route optmzaton should be ntated by the current FA. We develop a mathematcal model to determne when to perform route optmzaton. Lnk cost and sgnalng cost functons are ntroduced to capture the trade off. Our objectve s to fnd a cost effcent scheme for route optmzaton, whch mnmzes the total cost functon defned as the sum of the lnk and sgnalng cost functons. The smulaton results show that the proposed scheme sgnfcantly reduces the sgnalng costs caused by IETF RO and provdes the lowest total costs. The rest of ths paper s organzed as follows: In Secton 2, the mathematcal model s descrbed, and the decson model s provded. In Secton 3, the performance evaluaton s presented. Fnally, we conclude the paper n Secton 4. 2 THE NEW COST EFFICIENT SCHEME 2.1 The Mathematcal Model The decson must be made n the tme nterval ½t ;t þ1 Þ between the current handoff at t and the next handoff at t þ1 whether to perform route optmzaton or not. As stated before, there s a trade off between the network resources utlzed by the routng path and the sgnalng and processng load ncurred by route optmzaton. We ntroduce two cost functons to capture the trade off: the lnk and sgnalng cost functons. The lnk cost functon s denoted by gðx ð ÞÞ, where x ð Þ s the number of lnks n the routng path between the CN and the FA durng the th perod and 2fRO; NROg, where RO s the acton whch performs route optmzaton and NRO s the acton fðx ð Þ;y ; Þ¼gðx ð ÞÞ þ hðy ; Þ: Let ðþ denote a sequence of actons, ð 1 ; 2 ;...; Þ, whch are taken sequentally durng the occurrence of handoffs. We call ths sequence, ðþ, aroute Optmzaton Sequence. Let G ðþ ð1þ denote the accumulatve lnk cost, H ðþ the total the accumulatve sgnalng cost, and F ðþ accumulatve cost under the route optmzaton sequence ðþ, respectvely. Then, G ðþ H ðþ F ðþ ¼ X ¼ X ¼ X ¼ X gðx j ð j ÞÞ: hðy j ; j Þ: fðx j ð j Þ;y j ; j Þ fgðx j ð j ÞÞ þ hðy j ; j Þg; where ðþ ¼ð 1 ;...; Þ. Here, x j ð j Þ and y j are network parameters. When a route optmzaton s performed under the sequence ðþ durng the th perod, a sgnalng cost, hðy ; Þ, s ncurred. In ths case, the sgnalng cost, hðy ; Þ, can be decomposed as follows: hðy ; Þ¼vðy Þþk ð Þ; ð5þ where vðy Þ s a varable sgnalng cost functon, whch s ndependent of, and k ð Þ s a porton of sgnalng cost, whch depends on,.e., k ð Þ¼ wðx0 Þ f ¼ RO ð6þ 0 f ¼ NRO; where wðx 0 Þ s a sgnalng cost functon that depends on x0. In (5), the two terms reflect the cost of sendng a bndng update message from the current FA to the prevous FA, sendng a bndng warnng message from the current FA to the HA, and sendng a bndng update message from the HA to the CN. Here, we assume every cost functon to be lnear. Then, the lnk cost functon, gðx ð ÞÞ, durng the th perod becomes gðx ð ÞÞ ¼ A T x ð Þ; where A represents the average lnk cost per lnk whch captures the bandwdth consumed by the routng path of length x ð Þ, and T represents the sojourn tme of the MN from the th handoff to the next handoff. The varable sgnalng cost functon v durng the th perod becomes vðy Þ¼B y ; ð2þ ð3þ ð4þ ð7þ ð8þ

3 708 IEEE TRANSACTIONS ON COMPUTERS, VOL. 52, NO. 6, JUNE 2003 where B represents the average sgnalng cost per lnk n the path of length y. Thus, we obtan (9) from (4), (5), (7), and (8). F ðþ ¼ X ða T j x j ð j ÞþB y j þ k j ð j ÞÞ; ð9þ where ðþ ¼ð 1 ;...; Þ. 2.2 Optmal Soluton The objectve of ths secton s to fnd the optmal sequence whch we denote as opt ðþ, whch mnmzes the expected value of total cost F ðþ n (9). E½F optðþ ðþ Š¼ mn E½F Š ðþ2 X ¼ mn ða x j ð j ÞE½T j ŠþBy j þ k j ð j ÞÞ; ðþ2 ð10þ where ðþ ¼ð 1 ;...; Þ, and s the set of all possble sequences of ðþ. If a route optmzaton s performed durng the th perod, the shortest path between the CN and the current FA wll be selected as the routng path. Thus, the length of the routng path wll be x 0 n ths case where x0 s the number of lnks n the shortest path between the CN and the current FA durng the th perod. If the route optmzaton s not performed durng the th perod, then an extended path wll be the routng path and, the length of the routng path durng the perod wll be x 1 ð 1 Þþz, where y z ¼ f MN vsts a new FA y 0 ð11þ otherwse; where y 0 s the number of lnks whch wll be reduced after the th handoff. For example, n Fg. 2, y 0 þ1 wll be y f MN handoffs back to FA1 at tme t þ1. Ths stuaton s detaled n Fg. 2, and can be summarzed as follows: x ð Þ¼ x 1ð 1 Þþz f ¼ NRO x 0 ð12þ f ¼ RO; where x 0 x 1ð 1 Þþz. In general, the source routng s not beng adopted n the Internet. Even though t s beng used, network parameters cannot be known completely as networks grow bgger and become more complex [6], [7]. Thus, x ð Þ and x 0 are not avalable n every node. Wthout knowledge of these parameters, (10) cannot be solved. However, t can be easly solved f we restrct our model wthn ntradoman (ntrasubnet) handoff, where y ¼ y j for 6¼ j and make a reasonable assumpton,.e., f handoffs occur n the same doman (subnet), the length of the shortest path between the CN and any FA s the same,.e., x 0 ¼ x0 j for 6¼ j. Ths assumpton s reasonable because the shortest path between the CN and any FA n the same doman wll pass through the man router of the doman. Let -stage denote the decson stage when the decson whether to perform route optmzaton or not s made Fg. 2. Route extenson and route optmzaton. durng the th perod. In the -stage, we can thnk that the routng path has been extended n tmes wthout performng route optmzaton after the last one was performed, where n s an nteger. Thus, n the -stage, x 1 ð 1 Þ¼x 0 n 1 þ Xn z j : ð13þ But, x 0 n 1 ¼ x0 by the above assumpton. Hence, (13) becomes Fnally, (12) becomes x 1 ð 1 Þ¼x 0 þ Xn z j : ð14þ x ð Þ¼ x0 þ Z ;n f ¼ NRO x 0 ð15þ f ¼ RO; where Z ;n ¼ P n j¼0 z j. Note that Z ;n s the length of the path between the COA and the current FA whch s known wthn a doman. Under ths assumpton, the sgnalng cost functon wðx Þ n (6) can be assumed to be constant. Thus, we can restate (6) as follows: k ð Þ¼ K f ¼ RO ð16þ 0 f ¼ NRO; where K s a constant porton of sgnalng cost. We wll sequentally mnmze the expected value Š n (10) wth fxed 1 ; 2 ;...; 1 by solvng E½F ðþ X E½F optðþ Š¼ mn ða x j ð j ÞE½T j ŠþBy j þ k j ð j ÞÞ: 2ðÞ ð17þ Fg. 3 descrbes our decson model. To calculate the total accumulatve cost F ðþ n (4) and make an approprate decson n the -stage, all we need to know are the current

4 LEE AND AKYILDIZ: A NEW SCHEME FOR REDUCING LINK AND SIGNALING COSTS IN MOBILE IP 709 Fg. 3. Decson model. length x 1 ð 1 Þ of the routng path, the length y of the path between two adjacent FAs, and the length x 0 of the shortest path between the CN and the current FA. Let s 1 ¼ ðx 1 ð 1 Þ;x 0 ;y Þ denote the current state vector n the -stage. Our decson can be made only based on the current state vector s 1, and the next state vector s wll be determned based on s 1 and the acton taken n the -stage. From ths fact, we know that our decson model s Markovan,.e., memoryless. Our decson n each stage must consttute the optmal sequence opt ðþ. fðx ðnroþ;y ;NROÞ s the total cost whch wll be ncurred durng the th perod when the acton NRO s taken, and fðx ðroþ;y ;ROÞ s the one whch wll be ncurred durng the th perod when the acton RO s taken. Then, the expected value of total accumulatve cost, E½F optðþ E½F optðþ Š, becomes Š¼E½F optð 1Þ 1 Šþmnðfðx ðnroþ;y ;NROÞ; fðx ðroþ;y ;ROÞÞ: From (18), a decson rule can be found. Decson Rule: f E½fðx ðnroþ;y ;NROÞŠ <E½fðx ðroþ;y ;ROÞŠ then ¼ NRO; else ¼ RO; end f where ð18þ E½fðx ðnroþ;y ;NROÞŠ ¼ A E½T Šx ðnroþþby þ 0 ¼ A E½T Šðx 0 þ Z ;nþþby ; and E½fðx ðroþ;y ;ROÞŠ ¼ A E½T Šx ðroþþby þ K ¼ A E½T Šx 0 þ B y þ K; whch can be derved from (15). The condton E½fðx ðnroþ;y ;NROÞŠ <E½fðx ðroþ;y ;ROÞŠ K AE½T Š. means that A E½T ŠZ ;n <K,.e., Z ;n < Thus, the optmal sequence, opt ðþ, can be obtaned by followng the above decson rule n each decson stage. 3 PERFORMANCE EVALUATION In ths Secton, we evaluate the performance of the proposed scheme, opt ð NÞ, ~ for route optmzaton and compare t wth other schemes, whch are explaned below:. Scheme 1: The proposed optmal sequence s opt ð ~ NÞ.. Scheme 2: Always perform route optmzaton. ARO ð ~ NÞ¼ð 1 ;...; ~ N Þ; where ¼ RO for ¼ 1;...; ~ N.. Scheme 3: Never perform route optmzaton. NRO ð ~ NÞ¼ð 1 ;...; ~ N Þ; where ¼ NRO for ¼ 1;...; N. ~ The sequence ARO ð NÞ ~ represents the IETF RO [2], whle NRO ð NÞ ~ s a heurstc scheme. To obtan numercal results, we assume that N ~ ntradoman handoffs occur durng a sesson and that B s equal to A. The performance metrcs are the total cost per sesson F ð NÞ ~ (9), the sgnalng cost per ~N sesson H ð ~NÞ (3), and the number of route optmzatons ~N per sesson. In our smulaton model, the number of handoffs wthn a doman N ~ s assumed to be a unform random varable whose average value N s assgned durng a sesson, and the sojourn tme of an MN wthn a subnet s assumed to be exponentally dstrbuted. 3.1 The Total Cost The total cost per sesson s the sum of lnk cost and sgnalng cost per sesson. We use (9) to compute the total cost per sesson for sequences opt ð ~ NÞ, ARO ð ~ NÞ, and NRO ð ~ NÞ. In (9) the frst term reflects the network resources utlzed by the routng path durng a sesson, whle the others explan the sgnalng load ncurred by the route optmzaton. The sequence opt ð ~ NÞ s the one whch we can fnd by followng the decson rule of the prevous secton n each decson stage. In Fg. 4, we show the total cost versus the average lnk cost per lnk A durng a sesson. The sequence opt ð ~ NÞ shows the lowest total cost among the gven sequences. The numercal result shows that the total cost under the sequence opt ð ~ NÞ s 20.0 percent lower than that under the sequence ARO ð ~ NÞ, on the average. For each sequence, the total cost ncreases as A does. When the average lnk cost per lnk A s low, no dfference can be observed between the results of opt ð ~ NÞ and NRO ð ~ NÞ because there s no advantage of performng route optmzaton. Under the sequence ARO ð ~ NÞ, however, route optmzaton s performed regardless of A causng the addtonal sgnalng cost. As A ncreases the frequency of route optmzaton becomes hgher, and thus, we can see that the results of opt ð ~ NÞ and ARO ð ~ NÞ converge. In Fg. 5, we show the total cost versus the average number of ntradoman handoffs N durng a sesson. The sequence opt ð ~ NÞ shows the lowest total cost among the

5 710 IEEE TRANSACTIONS ON COMPUTERS, VOL. 52, NO. 6, JUNE 2003 Fg. 4. Total cost versus average lnk cost per lnk A (B ¼ A, K ¼ 3, N ¼ 20). gven sequences. When the frequency of ntradoman handoff s low there s only a slght dfference among the results of the gven sequences. However the results of the sequences dverge and, thus, the gap between the results of opt ð ~ NÞ and ARO ð ~ NÞ becomes bgger as the frequency ncreases. 3.2 The Sgnalng Cost The sgnalng cost per sesson s ncurred by performng route optmzatons durng the sesson. We use (3) to compute the sgnalng cost per sesson for sequences opt ð ~ NÞ, ARO ð ~ NÞ, and NRO ð ~ NÞ. Whenever each handoff occurs, the decson must be made whether to perform route optmzaton or not. If route optmzaton s determned to be performed after a handoff, t causes the addtonal sgnalng cost and (3) captures t. In Fg. 6, we show the sgnalng cost versus average lnk cost per lnk A durng a sesson. As t can be seen n Fg. 6, the sgnalng cost of each sequence ncreases as A does. When A s low, no dfference s seen between the results of the sequences opt ð ~ NÞ and NRO ð ~ NÞ because no route Fg. 6. Sgnalng cost versus average lnk cost per lnk A (B ¼ A, K ¼ 3, N ¼ 20). optmzaton s performed n that perod. It s observed that the sgnalng cost of opt ð NÞ ~ s sgnfcantly reduced compared wth that of ARO ð NÞ. ~ The numercal result shows that the sgnalng cost under the sequence opt ð NÞ ~ s 83.0 percent lower than that under the sequence ARO ð NÞ ~ on the average. In Fg. 7, we show the sgnalng cost versus the average number of ntradoman handoffs N durng a sesson. In ths fgure, t can also be observed that the sgnalng cost of opt ð NÞ ~ s sgnfcantly reduced compared wth that of ARO ð NÞ. ~ Under the sequence ARO ð NÞ, ~ the sgnalng cost grows lnearly as N ncreases because the more frequently ntradoman handoffs occur the more route optmzaton s performed. Under the sequence opt ð NÞ, ~ however, the sgnalng cost grows slghtly as A ncreases because route optmzaton s not always performed whenever ntradoman handoff occurs, whch reduces the sgnalng cost sgnfcantly. Fg. 5. Total cost versus average number of ntradoman handoffs N (A ¼ B ¼ 0:1, K ¼ 3). Fg. 7. Sgnalng cost versus average number of ntradoman handoffs N (A ¼ B ¼ 0:1, K ¼ 3).

6 LEE AND AKYILDIZ: A NEW SCHEME FOR REDUCING LINK AND SIGNALING COSTS IN MOBILE IP 711 Fg. 8. Number of route optmzatons versus average lnk cost per lnk A (B ¼ A, K ¼ 3, N ¼ 20). 3.3 The Number of Route Optmzatons In Fg. 8, we show the number of route optmzatons versus the average lnk cost per lnk A. As A ncreases, the number of route optmzatons under opt ð ~ NÞ grows slowly. When A s low, no route optmzaton s performed under the sequence opt ð ~ NÞ because there s no advantage of performng route optmzaton. The number of route optmzatons ncreases as A does because t s more proftable to perform route optmzaton from the pont of vew of cost. In Fg. 9, we show the number of route optmzatons versus the number of ntradoman handoffs durng a sesson. In ths fgure, t can be observed that the number of route optmzatons under the sequence opt ð ~ NÞ ncreases slowly as the frequency of ntradoman handoff ncreases, whch results n the reducton of the sgnalng cost and the total cost. In our smulaton, the sequence opt ð ~ NÞ shows the best performance compared wth other sequences. The optmal sequence opt ð ~ NÞ reduces the sgnalng cost caused by route optmzaton and provdes the lowest total cost. 4 CONCLUSION In ths paper, we proposed a cost effcent scheme for route optmzaton to reduce the sgnalng cost caused by the route optmzaton. Lnk cost functon represents the network resources utlzed by the routng path, whle sgnalng cost reflects the sgnalng and processng load ncurred by route optmzaton. We presented a Markovan decson model to fnd an optmal sequence for route optmzaton. We restrcted the model to ntradoman handoff to smplfy the decson process. A decson rule s derved from ths model. The optmal sequence opt s obtaned by followng the decson rule n each decson stage. The performance of the optmal sequence opt s compared wth the other sequences ARO and NRO. The smulaton results show that the optmal sequence opt provdes the lowest total costs among the gven sequences. Fg. 9. Number of route optmzatons versus average number of ntradoman handoffs N (A ¼ B ¼ 0:1, K ¼ 3). ACKNOWLEDGMENTS Ths work was supported by the US Natonal Scence Foundaton under Grant No. CCR REFERENCES [1] C. Perkns, IP Moblty Support for IPv4, RFC 3344, Aug [2] C. Perkns and D. Johnson, Route Optmzaton n Moble IP, Internet Draft, Internet Eng. Task Force (IETF), Nov [3] D. Johnson, C. Perkns, and J. Arkko, Moblty Support n IPv6, Internet Draft, Internet Eng. Task Force (IETF), June [4] E. Gustafsson, A. Jonsson, and C. Perkns, Moble IP Regonal Regstraton, Internet Draft, Internet Eng. Task Force (IETF), July [5] H. Solman, C. Castellucca, K. El-Malk, and L. Beller, Herarchcal MIPv6 Moblty Management, Internet Draft, Internet Eng. Task Force (IETF), Oct [6] R. Guern and A. Orda, QoS Routng n Networks wth Inaccurate Informaton: Theory and Algorthms, IEEE/ACM Trans. Networkng, vol. 7, no. 3, pp , June [7] A. Orda, Routng wth End-to-End QoS Guarantees n Broadband Networks, IEEE/ACM Trans. Networkng, vol. 7, no. 3, pp , June [8] J. Ho and I. Akyldz, Local Anchor Scheme for Reducng Sgnalng Costs n Personal Communcatons Networks, IEEE/ ACM Trans. Networkng, vol. 4, no. 5, pp , Oct Young J. Lee receved the BS degree n electrcal engneerng from Seoul Natonal Unversty, Korea, n He also receved the MS degree n electrcal and computer engneerng from the Georga Insttute of Technology n Currently, he s enrolled n the PhD program n the School of Electrcal and Computer Engneerng, Georga Insttute of Technology. Hs research nterests nclude wreless routng protocols and moblty management n the nextgeneraton wreless networks.