Video Proxy System for a Large-scale VOD System (DINA)

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1 Vdeo Proxy System for a Large-scale VOD System (DINA) KWUN-CHUNG CHAN #, KWOK-WAI CHEUNG *# #Department of Informaton Engneerng *Centre of Innovaton and Technology The Chnese Unversty of Hong Kong SHATIN, HONG KONG SAR Emal: kcchan9@e.cuhk.edu.hk, kwcheung@cntec.cuhk.edu.hk Abstract: - We propose a vdeo proxy system whch ams at reducng the backbone network loadng and central server loadng by cachng the vdeo clps n the proxy servers. Unlke the tradtonal replacement algorthms, the proposed replacement algorthm consders the combnaton of the estmated popularty, access frequency and recency of a vdeo clp wth the use of a prorty functon. Moreover, a two-stage replacement polcy s ntroduced to mnmze the start-up latency. The smulaton results show that the proposed vdeo proxy system can effectvely reduce the network and server loadng. Key-Words: - Vdeo Proxy, Vdeo Cachng, Proxy Server, VOD, Two-stage replacement polcy 1. Introducton A large-scale Vdeo-on-Demand (VOD) system (e.g. [1]) needs to support a large number of users to vew any vdeo programs and perform any VCR-lke nteractve functons such as fast forward, slow moton, etc at any tme. Thus, there are heavy demands on the central server and backbone bandwdth. However, t s usually very expensve to ncrease the backbone bandwdth. At the same tme, t s much easer and cheaper to ncrease the local area network bandwdth. Therefore, the cachng of vdeo content at the local dstrbuton pont s regarded as a soluton. It utltes the local area network bandwdth and dsk space n the proxy server to reduce the backbone network and central server loadng. Moreover, the start-up latency can be reduced as the proxy servers are closer to the clents. Many algorthms have been proposed to predct the popularty of vdeo contents so that the most popular contents can be cached n the proxy servers. Effcent use of the dsk space s also another mportant consderaton for a proxy system. In ths paper, we propose a vdeo proxy system for a dstrbuted large-scale VOD system. The proposed replacement algorthm consders the combnaton of the estmated popularty, access frequency and recency of a vdeo clp. A two-stage replacement polcy s employed to mnmze the start-up latency experenced by the clents. We wll demonstrate that the proposed cachng system can gve a better performance than the tradtonal cachng algorthms. The rest of the paper s organzed as follows. Secton 2 ntroduces some related works n meda cachng. A bref overvew of the VOD system archtecture s gven n Secton 3. Secton 4 descrbes the detals of our vdeo proxy system. The smulaton results are presented n Secton 5. Fnally, we conclude n Secton Related Works Recently, many researches on the cachng of meda stream have been performed [2-8]. Ref [2] suggested a prefx cachng technque to store the ntal frames of popular clps. It s to perform workahead smoothng and reduce the start-up latency. In Ref [3], t s suggested that a meda object should be parttoned nto two parts. Usually, the front-end partton s stored n the cache near the clent. The rear-end partton s stored n the cache away from the clent and wll be transferred to the cache closer to the clent f the meda s accessed frequently. The sze of the cached meda should be proportonal to the popularty of that meda [4]. It s to reduce the replacement overhead and ncrease byte ht rate. 3. System Archtecture A dstrbuted large-scale VOD system s shown n Fg. 1 [1]. Ths system s composed of sx man components: Vdeo Server Cluster (VSC), Clent Statons (), Backbone Network, Local Dstrbuton Networks (), Dstrbuted

2 Interactve Server () and Dstrbuted Proxy Server (). A bref overvew of these components s as follows, Vdeo Server Cluster It stores and delvers the vdeo stream to the clents. The vdeos are transmtted n CBR streams. Clent Staton It receves the vdeo stream from the network and then plays t at the dsplay equpment of the clent. Backbone Network It carres the multcast and uncast stream from the VSC to. Local Dstrbuton Network It carres the vdeo streams from VSC, and to each. Dstrbuted Interactve Server It s responsble for provdng nteractve functons and error recovery to [9]. Fully nteractve functons can be acheved by effcent use of the multple-speed uncast streams and pre-processed vdeo content. Dstrbuted Proxy Server It s the man component of our vdeo proxy system, whch s responsble for cachng the frequently accessed contents. In order to utlze the dsk space more effcently and acheve hgher bandwdth reducton, the cached contents n one can be shared wth other s. VSC Router Fg. 1. System archtecture Backbone Network In ths archtecture, the vdeo contents are dvded nto two types, namely hot and non-hot contents. The hot contents are delvered by the multcast streams because they are expected to be vewed by many users at the same tme. Batchng [10] s deployed to generate multcast stream at each fxed batch nterval n a round-robn manner. Snce one multcast stream can support all the user requests wthn a batchng nterval, the backbone bandwdth loadng can be reduced sgnfcantly. Bascally all hot contents are cached n the and wll not be subject to any replacement polcy except for changes ssued by the operator. The hot contents n the are used for provdng varous nteractve functons. For the non-hot contents, they are carred by the uncast streams. The replacement polcy wll choose the hgher prorty contents and cache them n the proxy server. These contents are expected to be re-accessed agan wthn a short perod, so that the can serve the future requests wthout accessng the VSC or consumng backbone bandwdth. 4. Vdeo Proxy System The purpose of t s to reduce the backbone network bandwdth, central server loadng and start-up latency. 4.1 Prorty Functon The prorty functon s to determne the prorty of cached contents n the proxy server at any tme. The accessed frequency and recency were the mportant parameters n web cachng. It s beleved that these parameters can stll be appled for the contnuous streamng meda, though they are not enough. Thus, an estmated popularty parameter s ntroduced to predct the popularty of a vdeo content. The use of vdeo object characterstcs as cachng parameters has also been suggested n [5]. The estmated popularty can be estmated by ts ratng, stars, drectors, etc of the vdeo content. The popularty functon of a cached content at any tme P (t) s defned as follows, P ( t ) ( P ( t = P ( t ) + E N + α N total )e β ( t t ) = 0 for the frst access ) (1) where E s the estmated popularty of a vdeo clp N s the number of tmes vdeo clp s accessed wthn a perod N total s the total number of vdeo clps accessed wthn a perod t s the tme that vdeo clp was accessed α s the bas value to the vdeo clp s accessed frequency β s the bas value to the vdeo clp s accessed recency Ths prorty functon ncludes the estmated popularty, access frequency and recency as an ndcator to predct the popularty of a vdeo object. Moreover, ths functon provdes flexblty for the system to bas to any of the parameters.

3 4.2 Two-Stage Replacement Polcy In the, the dsk s parttoned nto two regons, namely the set and the prefxed regons. The set regon s responsble for storng an entre vdeo object (lke a move), whle the prefxed regon s for storng the ntal porton of a vdeo object. The replacement polcy s performed when the newly requested object s not n the proxy server and there s not enough free space to store that newly requested object. Then, the prorty of each cached object n the proxy server wll be calculated by the prorty functon descrbed n Eq. 1. The lowest prorty objects wll be removed untl there s enough free space to hold the new objects. If there are more than one object that have the same prorty value, the less recently accessed object wll be removed. In ths cachng system, the replaced object wll not be removed completely. Instead, the ntal porton of the object wll be transferred from the set regon to the prefx regon and remaned n the proxy server. The purpose s to mnmze the start-up latency for ths object when t s re-accessed. The mnmum amount of an object S needed to be remaned s shown n Eq. 2. Wth ths porton of the vdeo object, the clent wll experence the mnmum latency and can vew the object as f t s comng from the. It s because the can serve that clent mmedately wthout watng for the arrval of the object from remote ste. S (T T )* r (2) + where T s the transmsson delay between s T VSC s the transmsson delay between the and VSC r s the transmsson rate of the vdeo object When there s a need to store a new object s ntal porton but there s not enough free space, the above replacement polcy wll be performed. The lowest prorty objects prefx wll be removed untl enough space s freed up Cachng Polcy When there s a new request from the clents, the drectly attached wll search for the requested vdeo object. If there s a match for the whole object, the wll delver the requested object to the clents. However, f there s only a match for the prefx of the object, the also delvers that object s prefx mmedately. And at the same tme, the wll make a request for the clents to other s. If VSC there s a match n one of the s, the remanng porton of that object wll be delvered from that to the requestng and then to the clent. However, f no contans that object, the requestng wll request the VSC to get that object s remanng porton. Meanwhle, the requestng wll cache that vdeo object n ts dsk. That object s prefx wll also be transferred from the prefx regon to the set regon so that t can merge wth the remanng porton of the object. If there s not enough free space for that object, the replacement polcy wll be performed. If there s no match for the object s prefx, then the clents need to wat untl the requested object s obtaned from other or VSC. wll also free up the needed space to cache ths object. 5. Performance Evaluaton In our smulaton, we have evaluated the performance of our proposed cachng algorthm. It wll also be compared wth other well-known algorthms such as and. 5.1 Smulaton Envronment In our smulaton, the VSC contans 400 vdeo objects and they are dvded nto 10 sets wth 10 levels of estmated popularty. The vdeo object length ranges from 90 mn to 120 mn. The objects are encoded n MPEG-I format and are transmtted n CBR wth 1.5Mbps. The user requests are generated n exponental dstrbuton. Zpf dstrbuton s employed to model the user access pattern. The backbone bandwdth from VSC to can support up to 100 channels and the local dstrbuton bandwdth from to clents can support up to 200 channels. The transmsson delay between clent and, and, and VSC s assumed to be 5 ms, 10 ms and 20 ms respectvely. Unless otherwse specfed, the dsk space n each s of the total space n VSC and the skewness parameters for Zpf s Performance Metrc Snce the ultmate purpose for the vdeo proxy system s to reduce the backbone bandwdth, the backbone bandwdth reducton rato wll serve as the performance metrc n our smulaton. If there s no vdeo proxy, the central server needs to handle all the clent requests. Thus, one unt of bandwdth used

4 by the cached object can reduce one unt of backbone bandwdth. Thus, the bandwdth reducton rato s defned as, BWcached = 10 (3) BW + BW cached server where BW cached s the bandwdth consumed by the cached object n BW server s the bandwdth consumed by the un-cached object that needed to be delvered from VSC 5.3 Results Fg. 2 shows the comparson of our cachng algorthm wth and under dfferent dsk space of the. As expected, the bandwdth reducton ncrease as the amount of proxy space ncrease. It s because larger proxy space means more vdeo objects can be cached and more clent requests can be served from. In addton, t shows that our proposed cachng algorthm gves better performance than the other two algorthms. It can reduce nearly and more backbone bandwdth than and respectvely. 6 5% 15% Dsk Space (% of VSC dsk space) Fg. 2. Dfferent algorthm performance under dfferent dsk space of the In Fg. 3, t shows that our proposed cachng algorthm can also acheve hgher bandwdth reducton rato under dfferent request nter-arrval tme. It reduces nearly and more backbone bandwdth than and respectvely. The relatonshp between the performance of dfferent algorthms and user access pattern s shown n Fg. 4. It can be seen that our algorthm can acheve better performance under dfferent user access pattern. It s also shown that the performance and the dfference n performance between the algorthms decreases as the skewness of Zpf dstrbuton ncrease. It s because at hgh skewness, the access pattern for the vdeo objects becomes more unform. It means that the popularty of each s nearly the same. So the cached objects are less lkely to be re-accessed wthn a short perod of tme. On the other hand, when the user requests are concentrated on some of the vdeo objects at low skewness, all the algorthms are able to choose and cache the more popular objects. Thus, more backbone bandwdth can be saved by provdng these cached objects to the clents Request Inter-arrval tme (mn) Fg. 3. Dfferent algorthm performance under dfferent request nter-arrval tme Skewness Parameter of Zpf Dstrbuton Fg. 4. The relatonshp between performance of dfferent algorthm and user access pattern The reducton of the backbone bandwdth versus dfferent number of s s shown n Fg. 5. It shows that the bandwdth reducton rato ncreases wth the ncreasng number of s. It s because each can share ts cached object wth other s. Thus, most of the clent requests can be satsfed wthout askng for VSC. In addton, t also shows that the ncrease n reducton rato becomes saturated when the number of s s large. It s because some of the requests are for the very unpopular objects that are not cached n the for

5 a long tme. Each tme they are requested, may not be able to fnd them and need to ask the VSC for them Number of s Fg. 5. Performance wth dfferent number of s Fg. 6 shows that the two-stage replacement polcy can effectvely reduce the start-up latency. It s because the can use the prefx stored to serve most of the clent requests mmedately. Thus, t can compensate for the searchng tme needed for gettng the requested object n other s and VSC. Start-up latency (ms) Number of s Wthout Prefx Wth Prefx Wthout Proxy Fg. 6 Effect on start-up latency for the two-stage replacement polcy 6. Concluson In ths paper, we have presented a Vdeo Proxy System for a large-scale VOD system. Our proposed replacement algorthm consders the combnaton of the estmated popularty, access frequency and recency of a vdeo clps. Moreover, a two-stage replacement polcy s ntroduced to mnmze the start-up latency. The smulaton results show that our vdeo proxy system can gve a better performance n terms of reducng the start-up latency, backbone network and central server loadng. References [1] C.H. Ng, K.C. Chan, K.W. Chan, C.H. Chan, C.Y. Tam and K.W. Cheung, DINA System for a Large-scale Fully nteractve VOD System based on Hybrd Multcast-uncast Streamng, Proceedngs of IEEE-PCM 2000 [2] Sen, S.; Rexford, J.; Towsley, D., Proxy Prefx Cachng for Multmeda Streams, Proceedngs of INFOCOM '99, Volume: 3, 1999, pp [3] Y.W. Park; K.H. Baek; K.D. Chung, Reducng Network Traffc Usng Two-layered Cache Servers for Contnuous Meda Date on the Internet, Proceedngs of the 24th Computer Software and Applcatons Conference, 2000, pp [4] E.J. Lm; S.H Park; H.O. Hong; K.D. Chung, A proxy Cachng Scheme for Contnuous Meda Streams on the Internet, Proceedngs of the 15th Internatonal Conference on Informaton Networkng, 2001, pp [5] Sonah, B.; Ito, M.R. Cache Transparency n VOD Systems: Takng Advantage of Vewers Flexblty, Proceedngs of the Thrd Internatonal Conference on Computatonal Intellgence and Multmeda Applcatons, 1999, pp [6] Y. Wang; Z.L. Zhang; Du, D.H.C.; D. Su A Network-Conscous Approach to End-to-End Vdeo Delvery over Wde Area Networks Usng Proxy Servers Proceedngs of INFOCOM '98, Volume: 2, 1998, pp [7] Rejae, R.; Haobo Yu; Handley, M.; Estrn, D, Multmeda Proxy Cachng Mechansm for Qualty Adaptve Streamng Applcaton n the Internet, Proceedngs of INFOCOM 2000, Volume: 2, 2000, pp [8] Bommaah, E.; Guo, K.; Hofmann, M.; Paul, S, Desgn and Implementaton of a Cachng System for Streamng Meda over the Internet, Proceedngs of Sxth Real-Tme Technology and Applcatond Symposum, RTAS 2000, pp [9] K.C. Chan and K.W. Cheung, Performance Analyss on Dstrbuted Interactve Server n a Large-scale Fully Interactve VOD System (DINA), Proceedngs of the 15th Internatonal Conference on Informaton Networkng, 2001, pp [10] A. Dean, D. Staram, and P. Shahabuddn, Schedulng Batchng, Proceedngs of ACM Multmeda 94, pp ,