An Adaptive Batched Patch Caching Scheme for Multimedia Streaming *

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1 An Adatve Batched Patch achng Schee for Multeda Streang * Shaohua Qn, Wehong He, Zu L, and Janng Hu School of outer Scence, Beng Unversty of Aeronautcs & Astronautcs, Beng 8,hna {shqn,annghu}@buaa.edu.cn Hengyang Branch of Hunan Unversty, Hengyang 4, hna hwh@hnbc.edu.cn etwork Research enter of Tsnghua Unversty, Beng 84,hna zl@cernet.edu.cn Abstract. Large-scale steang eda alcatons usually consue a sgnfcant aount of server and network resources due to the hgh bandwdth requreents and the long-lved nature of the streang eda obects. In ths aer, we address the roble of effcently streang eda obect to the clents over a dstrbuted nfrastructure consstng of vdeo server and roy caches. We buld on the earler work and roose an adatve batched atch cachng schee, whch tghtly cobne the transsson schedulng wth roy cachng. Ths schee adatvely caches the net segent data at roy fro the ongong entre strea, whch deends on the current batchng nterval that has non-zero requests. We deonstrate the benefts of our schee coare to the classcal streang strateges. Our evaluatons show that ths schee can reduce sgnfcantly the consuton of aggregate bandwdth on backbone lnk wthn uch wder range of request arrval rate. Keywords: Streang eda, Batched atch, Proy cache, Multcast, Server schedulng. Introducton The eergence of the Internet as a ervasve councaton edu, and a ature dgtal vdeo technology have led to a rse of varous networked streang eda alcatons such as vdeo-on-deand, dstance learnng, vdeo gae and vdeo conferencng. As access rovders are rollng out faster last-le connectons, the bottleneck s shftng ustrea to the rovder s backbone, eerng lnks and best-effort Internet. Due to the large sze, long-lved nature of the streang obects, they need to consue uch ore network bandwdth and server syste resource n dstrbuton and delvery. At the sae te, the I/O caacty of the vdeo server and network bandwdth are ossble to be unrestraned enhanceent because of hardware costs ltaton. So, n the rocess of dstrbutng streang eda over the Internet, how to reduce the backbone bandwdth consuton and effcently utlze the vdeo server syste resource have becoe the research hotsot n the area of streang eda alcatons n recent years. Estng research has focused on develong transsson schees that use ultcast or broadcast connectons n nnovatve ways to reduce server and network loads, * Ths work s suorted by the SF of hna under grant 65. J. Vcente and D. Hutchson Eds.: MMS 4, LS 7,. 6 7, 4. IFIP Internatonal Federaton for Inforaton Processng 4

2 64 Shaohua Qn et al. for servng a oular vdeo to ultle asynchronous clents. Batchng [], Patch [], [4], HMSM[] and Otzed Batch Patchng [] are reactve n that the server transts vdeo data only on deand, n resonse to arrvng clent requests. These schees have an underlyng requreent that the ultcast or broadcast connectvty between the server and the clents s avalable. However, IP ultcast deloyent n the Internet has been slow and even today reans severely lted n scoe and reach. Therefore, transsson schees that can suort effcent delvery n such redonantly uncast settngs need to be develoed.. Another attractve soluton for reducng server loads, backbone network traffc and access latences s the use of roy caches. Ths technque has roven to be qute effectve for delverng tradtonal Web obects. However, streang eda obect can be very large, and tradtonal technques for cachng entre obects are not arorate for such eda. achng strateges that have been roosed n recent years [7],[],[],[4] cache a orton of streang eda obect at the roy. These ref and segentaton-based cachng ethods have a nuber of advantages ncludng reducng startu latency and tter on the server-roy ath, whle savng bandwdth usage along that ath. However, they do not take the ssue of the transsson schedulng nto consderaton. Recent work [] cobnes ref cachng wth royasssted reactve transsson schees for reducng the transsson cost of ultle heterogeneous vdeos. Another work [5],[6],[8],[9] cobnes ultcast-based server schedulng wth roy cachng to nze the aggregate bandwdth usage. In artcular, the Batched Patch achng BP roosed n [5] whch caches the atch data at cachng roy n order to ake ore clents to share t, and as thus acheve better erforance. However, when the request arrval rate s very hgh, these schees entoned above are stll consue qute a few syste resources. In ths aer, we buld on early work and roose an adatve batched atch cachng schee knows as ABP whch cobnes dynac cachng at the roy wth scalable transsson schee at the orgn server. In ths schee, the server-roy network connectons only rovde uncast servce, and on the roy-clent ath offers ultcast caablty. By adatve re-cachng the atch data for the ucong request n the net batchng nterval, t acheves lower bandwdth consuton than the BP schee over a wder range of request arrval rate. The rest of ths aer s organzed as follows. In secton, soe revous works n the ultcast and streang eda cachng areas are revewed. In secton, we resent and forulate our schee n detal. Secton 4 shows results that coare the erforance of new schee and BP. Fnally we resent our conclusons and ongong work n secton 5. Prevous Work Streang eda obects over ultcast consues less network bandwdth and oses less of a load on the server than does streang eda obects over ultle uncast channels. Batchng s a sle schedulng strategy based on ultcast. It delays the earler arrval request to wat for uch ore clents, and serves the over a sngle channel. Patchng s certanly one of the ost effcent technques. The server streas the entre vdeo sequentally to the very frst clent. A later clent receves ts future layback data by lstenng to an estng ongong ultcast of the sae vdeo, and the server only transts afresh only the ssng orton atch data.

3 An Adatve Batched Patch achng Schee for Multeda Streang 65 The concet of otzed batch atchng OBP n lterature [] has recently been roosed, whch aed at nzng the average backbone rate. Bascally, clent requests are batched together on an nterval bass before requestng ether a atch or a regular ultcast fro the server. There s an otal atchng wndow after whch t s ore bandwdth effcent to start a new entre strea rather than send atches. Ths schee outerfors other ultcast-based technques such as otal atchng [4] n ters of average backbone rate over a large range of request rates []. The Batched Patch achng BP was bult on the Otzed Batch Patchng dea. Uon receton of a atch, the roy stores t n the buffer for a erod of the atchng wndow sze, so that t s avalable for the last requests of the sae atchng wndow and consequently reduces the bandwdth usage of the etra channel. By cachng the atch data n the roy, ths schee deands the average backbone rate saller than that of OBP schee. But, t needs to retranst all of the atch data. We dffer fro all the above works n that we develo a new echans to refetch the atch data fro the ongong entre strea along the uncast connecton of the server and the roy. At the sae te, the re-fetchng data s cached dynacally at the roy. They can serve the requests of the sae atchng wndow. Moreover, by varyng the sze of the atchng wndow accordng to oularty of the streang eda obect, the nu consuton of the backbone bandwdth can acheve, and lted storage sace of roy can utlze effcently. SERVER Pr oy Pr oy lent l ent lent lent Fg.. Illustraton of roy cache for streang eda. Adatve Batched Patch achng Schee In ths secton, we consder a delverng archtecture of streang eda obect, whch coosed of an orgn server, a set of roes, and a fnte set of eda obects. Each roy s resonsble for a grou of clents as shown n Fg.. We assue relable transssons over the server-roy ath, and the access network roy-clent s lossless and ultcast enabled. We further assue that the clents are always request lay back fro the begnnng of the eda obect. Moreover we ose the roy to lay the role of a clent for server. That s, all the streang eda obect data streaed out of the server are requested by the roy and are thereby forwarded through t. A roy streas the ref drectly to ts clents f a ref of the eda obect s resent locally, and contacts the server for the reander suff of the strea. Otherwse, the roy sends the server a request to start a full strea uncast and ultcasts t to a set of clents. In order to sheld the clent-erceved startu latency, the roy edately sends clent the frst segent data by uncast channel once a request arrves n each batchng nterval. Ths uncast strea wll ternate at

4 66 Shaohua Qn et al. the boundary of the batchng nterval. At ths te, the clent wll on the full strea and the atch data strea that started by the roy va ultcast channel. We net ntroduce the notatons used n ths aer, as resented n Table. We consder a server wth a reostory of M onstant-bt-rate BR eda obects. Let eda obect M s characterzed by ts layback rate r, duraton L, and average access rate λ. Table. Paraeters used n ths aer. Para. Defnton M uber of the eda obects L Length of eda obect sec. r Playback rate of eda obect bs S The cache sze of roy Ω oralzed transsson rate λ Average request arrval rate for obect W Patchng wndow sze for obect µ Total atch data for obect b The batchng nterval. Schee Descrton The basc dea of the adatve batched atch cachng s that whenever a full strea s started, the roes that receve the data fro ths strea allocate a buffer sze of b unts to cache the ucong data. And at the end of each batch n the sae atchng wndow, these roes check to see whether or not there are requests. If there are requests, these roes searately add b unts to the buffer and cache the ongong strea contnuously. Otherwse they sto cachng. Snce we do not cache the data segents at the end of each batch of zero requests, the roy wll need to start an etra channel to afresh the f there are requests n the subsequent batchng ntervals. Full strea Full strea L L W b -b b b b t t - t t b t t Server te A B D E F Proy te Fg.. Tng Dagra for adatve batched atch cachng. Our scenaro s llustrated n Fg., The roy dvdes unforly the te as nto ntervals [t -, t ] of duraton b, Assue a request arrves at the roy at te t [t -, t ], and the ost recent full strea was started at te t. If t s such that t < t W, the roy need to transt atch data of duraton t t b to the clent at te t, also the roy ons the full strea at te t, and ultcasts t to the clents. However,

5 An Adatve Batched Patch achng Schee for Multeda Streang 67 f t > t W, a new full strea wll be started at te t. Snce the frst segent data [, b ] of the streang eda obect has been stored at the roy at te t after the full strea s started, whether the subsequent segent data need to be cached deends on the current batchng ntervals that have no-zero requests before te t. Ths s better elaned wth the followng eale. Suose there are eght batchng ntervals n the atchng wndow, that s W 8b. The full strea was started at te t. Also at te t the roy began to buffer the frst segent data fro ths stea. Suose [t, t ] had requests, then at te t the roy adds b unts to the buffer and caches the ongong strea contnuously. Ths akes the roy can serve the requests arrve at te ntervals [t, t ] and [t, t ], and does not request server for any atch bytes. Suose the net two ntervals do not have any requests, and the roy does not cache the corresondng data segent [b,b ],[ b,4b ]contnuously. If the fourth nterval [t, t 4 ] has requests, at te t 4, the buffer of the roy has only contaned two segent data [, b ] and [b, b ]. The requests arrve n the fourth nterval requres the atches to be 4b -long, and so the roy eands the buffer fro havng b unts to havng 5b unts, and then fetches the ssng atch data [b,b ] and [b,4b ] fro the server by the etra channel whle storng the segent data [4b,5b ] fro the ongong full strea. The rocessng ethod for subsequence ntervals s slar to that for the revew ntervals. At last, whether the eghth nterval [t 7, t 8 ], has requests or not, the roy wll not store the net segent data fro the full strea at te t 8. Our schee has a rearkable characterstc. That s, the ore robablty of the request arrval each batchng nterval has That eans the request arrval rate s very hgh, the lttle atch data need to be transtted afresh becoe. When each batched nterval of atchng wndow has no-zero requests, the re-fetchng echans can assure that the deanded atchng data ust obtanng fro full strea can satsfy all the request arrved n the sae atchng wndow. On the other hand, f only the last batchng nterval has requests, the roy wll need to fetch the atch data u to b va the etra channel.. Schee Analyses and Proble Forulaton For slcty of eoston, we gnore network roagaton latency. In order to derve analytc eressons n evaluatng the erforance of our schee, we frst ake an abstract of our schee as follows: Assue the access rate of eda obect are odeled by Posson rocess wth araeter λ such that - λ e b s the robablty to have an ety batch zero request of duraton b. The roy stores the atch data n the buffer at least for a erod of W b, so that t s avalable for the all requests arrved n the sae atchng wndow. Suose denotes the requests that arrve n the th batchng nterval [t -, t ]. Let,,, be a sequence of ndeendent rando varables wth coon robablty dstrbuton.

6 68 Shaohua Qn et al. Whether the roy need to fetch the atch data by etra channel at te t deends on the values of,,, - and. In artcular f or, the roy does not request server for any atch bytes. Assue the roy request server for the atches sze s ψ n the atchng wndow. It s obvously ψ wll lkely take one aong the values,b,b,, b, we ght as well let denotes the robablty of ψb, that s ψb,,,,,. Wth the above assuton, we can acheve the ean value of η, naely b b Eψ ow we need to deterne. Obvously,,,, ψ b ψ b - - k ψ b ψ

7 An Adatve Batched Patch achng Schee for Multeda Streang 69 Thus the eresson of can be wrtten as:,,,,,. Substtutng equaton. nto equaton, by coutng all the dfferent ossbltes of batchng nterval along the atchng wndow, we get the average nuber of atched data segents, µ at roy. It s gven by µ Eψ b The aggregate transsson rate on the backbone lnk, R, ncludes the transsson of atches µ and the full strea of duraton L fro the server. Its noralzed transsson rate Ω s thus obtaned fro: R L µ Ω 4 r I Where I reresents the nterval duraton between two adacent full streas: I b / λ 5 The average buffer caacty S needed for cachng at roy s then gven by S µ µ r 6 Where µ' denotes the segent data receved fro the full strea. Accordng to the buffer allocatng echans n our schee, µ' s gven by µ b b 7 We now coute the roy network bandwdth. Recall that the roy delver the frst segent data to clents va uncast channel, and forwards the full strea and the other atchng data va ultcast channel. Thus ts noralzed bandwdth B roy s gven by: B λ b b r - ory I 4 uercal Results and oarsons In ths secton, we evaluate the erforance of our roosed schee ABP and contrast t aganst BP n three asects usng nuercal result. Suose the duraton of the streang eda obect s nutes, and ts average lay back rate s.5 Mbs MPEG-. Frst, we eane the transsson quanttes of atchng data that needed to get by the etra channel wth resect to the request arrval rate and the atchng wndow sze. As shown n Fg., adotng the ABP schee, the average nuber of transtted atches decreases radly as the request arrval rate ncreases, and that close to zero. r Ω 8

8 7 Shaohua Qn et al. Ths ndcates that ABP schee only needs to fetch few atch data through the etra channel whle the request arrval rate reaches relatve hgh. But the BP schee needs to fetch all the atch data through the etra channel, and that close to the atchng wndow sze. In other words, whatever the request arrval rate and the atchng wndow change, under the sae condtons, the ABP schee can save uch ore bandwdth of etra channel than the BP schee. Average nuber of transtted atches [n] BP AB P Request arrval rate [req/ n] Fg.. Average nuber of transtted atches through etra channel versus the request arrval rate, b [n]. Secondly, we coare the noralzed backbone rate deanded n the case of transttng one eda obect for both schees ABP and BP by showng n Fg. 4a ~ d. Fro these dagras we see that the noralzed backbone rate requred n the ABP schee s always saller than that n the BP schee at dfferent values of the atchng wndow as well as request arrval rate. Also, as the atchng wndow sze ncreases, the noralzed backbone rate decreases oralzed backbone rate Ω BP ABP oralzed backbone rate Ω BP ABP Request arrval rate [req/n] 4. a Request arrval rate[req/n]. b oralzed backbone rate Ω BP ABP Request arrval rate[req/n] c oralzed backbone rate Ω BP ABP Request arrval rate [req/n] Fg. 4. oralzed backbone rate versus request arrval rate at dfferent values of atchng wndow, b [n]. d

9 An Adatve Batched Patch achng Schee for Multeda Streang 7 Thrdly, we show n Fg.5 a ~ d the buffer requreent at roy to deal wth one eda obect for both schees ABP and BP wth resect to the atchng wndow sze and the request arrval rate. As the atchng wndow sze ncreases, the buffer requreent also ncreases. Moreover, under the sae condtons, both of these schees consue alost the sae buffers at roy, esecally n the hgh request arrval rate. Ths evdence ndcates that ABP schee archves the lower bandwdth savng than the BP schee, whle t does not consue uch ore buffer sze. Average buffer occuancy [MB] 9 8 BP APB Average buffer occuancy [MB] 9 BP ABP Average buffer occuancy [MB] Request arrval rate [req/n] 4 a BP ABP Request arrval rate [req/n] c Average buffer occuancy [MB] Request arrval rate [req/n] b BP ABP Request Arrval rate [req/n] Fg. 5. Average buffer occuancy at roy versus request arrval rate at dfferent values of atchng wndow, b [n]. d oralzed backbone rate Ω 6 ABP schee 5 λ 8 req/ n, b n 4 λ.5 req/n, b n λ 8 req/ n, b n Patchng wndow sze [n] Fg. 6. oralzed backbone rate versus atchng wndows sze. Fnally, Fg.6 shows the evoluton of the noralzed backbone rate Ω versus the duraton of the atchng wndows under dfferent request arrval rate and the batchng nterval by adotng ABP schee. We clearly fnd that the noralzed backbone

10 7 Shaohua Qn et al. rate ay no longer ehbt a nu value for the atchng wndow duraton wthn T. Moreover, the longer the duraton of atchng wndow, the hgher buffer sze requred at the roy. Accordngly, n ractce the ntal value of the atchng wndow can be deterned usng the eresson n lterature [], afterwards, we let the atchng wndow sze to adatvely eand or shorten n ters of request arrval rate. Intutvely, f all batches durng the duraton of the strea have no-zero requests, ths eda obect s really very oular and we ought to eand the atchng wndow n order to uch cache ths entre obect at roy and not have to request fro the server. Wth such a dynacal control of atchng wndow, t nstructs the roy to leent an otal cache for each eda obect, thereby achevng the better tradeoff between backbone bandwdth and storage requreents. 5 onclusons and Ongong Work In ths aer, we roosed an adatve batched atch cachng schee that onts server schedulng and roy cachng aed at reducng the bandwdth streaed fro the server. We forulate the on-deanded nubers of noralzed backbone transsson rate for consderng a artcular streang eda obect. oared wth the tradtonal batched atch-cachng schee, the roosed schee acheves uch ore savngs of backbone bandwdth by adatve re-cachng the atch data fro full strea. In addton, the adatablty of the roosed schee kees on very well wthn wde range of request arrval rate. The nuercal results show that the adatve batched atch cachng s a hghly effcent ethod that allevates bottlenecks for the delvery of streang eda obects. For slcty of eoston, we gnore network roagaton latency throughout the aer. ote that our schee can be easly adated to the scenaro when the network roagaton latency along server-roy ath s not gnorable. In order to hde the transsson delay for a streang eda obect fro the server, the roy cache has to be allocated to accoodate a ref for each eda obect. We are currently nvestgatng the trade-offs between bandwdth reducton and buffer occuancy at roy. We are also elorng the scenaros based on ABP where the roes can cooerate. References. A. Dan, D. Stara, P. Shahabuddn: Schedulng olces for an on-deand vdeo server wth batchng. In Proc. AM Multeda, San Francsco, alforna, K. A. Hua,Y. a, S. Sheu: Patchng: A ultcast technque for true vdeo-on-deand servces, n Proc. AM Multeda, Brtstol UK, P. P. Whte, J. rowcroft: Otzed batch atchng wth classes of servce, AM ouncatons Revew, Vol., o.4, 4. Y.a, K. Hua, K.Vu: Otzng Patchng Perforance. In roc. of AM/SPIE Multeda outng and etworkng, O.Verscheure,.Verkatraan, P. Froassard, L. An: Jont server schedulng and roy cachng for vdeo delvery. outer ouncatons, Vol.5, o. 4, Venkatraan, O. Verscheure, P. Frossard, K. W. Lee: Otal roy anageent for ulteda streang n content dstrbuton networks. In roc. of AM OSSDAV, Ma Beach, FL, USA, May 47 54

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