Adaptive and Lazy Segmentation Based Proxy Caching for Streaming Media Delivery

Transcription

1 Adaptive ad Based Proxy Cachig for Streamig Media Delivery Sogqig Che Dept. of Computer Sciece College of William ad Mary Williamsburg, VA Susie Wee Mobile ad Media System Lab Hewlett-Packard Laboratories Palo Alto, CA 9434 Bo She Mobile ad Media System Lab Hewlett-Packard Laboratories Palo Alto, CA 9434 Xiaodog Zhag Dept. of Computer Sciece College of William ad Mary Williamsburg, VA ABSTRACT Streamig media objects are ofte cached i segmets. Previous segmet-based cachig strategies cache segmets with costat or expoetially icreasig legths ad typically favor cachig the begiig segmets of media objects. However, these strategies typically do ot cosider the fact that most accesses are targeted toward a few popular objects. I this paper, we argue that either the use of a predefied segmet legth or the favorable cachig of the begiig segmets is the best cachig strategy for reducig etwork traffic. We propose a adaptive ad lazy segmetatio based cachig mechaism by delayig the segmetatio as late as possible ad determiig the segmet legth based o the cliet access behaviors i real time. I additio, the admissio ad evictio of segmets are carried out adaptively based o a accurate utility fuctio. The proposed method is evaluated by simulatios usig traces icludig oe from actual eterprise server logs. Simulatio results idicate that our proposed method achieves a 3% reductio i etwork traffic. The utility fuctios of the replacemet policy are also evaluated with differet variatios to show its accuracy. Categories ad Subject Descriptors H.4.m [Iformatio System]: Miscellaeous Geeral Terms Algorithms, Experimetatio Keywords Streamig media delivery,, Proxy Cachig Permissio to make digital or hard copies of all or part of this work for persoal or classroom use is grated without fee provided that copies are ot made or distributed for profit or commercial advatage ad that copies bear this otice ad the full citatio o the first page. To copy otherwise, to republish, to post o servers or to redistribute to lists, requires prior specific permissio ad/or a fee. NOSSDAV 3, Jue 1 3, 23, Moterey, Califoria, USA. Copyright 23 ACM /3/6...$ INTRODUCTION Proxy cachig has bee show to reduce etwork traffic ad improve cliet-perceived startup latecy. However, the proliferatio of multimedia cotet makes cachig difficult. Due to the large sizes of typical multimedia objects, a full-object cachig strategy quickly exhausts the cache space. Two techiques are typically used to overcome this problem, amely prefix cachig ad segmet-based cachig. Prefix cachig [17] works well whe most cliets access the iitial portios of media objects as oted i [4, 5]. It also reduces startup latecy by immediately servig the cached prefix from the proxy to the cliet while retrievig subsequet segmets from the origi server. I prefix cachig, the determiatio of the prefix size plays a vital role i the system s performace. Segmet-based cachig methods have bee developed for icreased flexibility. These methods also cache segmets of media objects rather tha etire media objects. Typically two types of segmetatio strategies are used. The first type uses uiformly sized segmets. For example, authors i [14] cosider cachig uiformly sized segmets of layer-ecoded video objects. The secod type uses expoetially sized segmets. I this strategy, media objects are segmeted with icreasig legths; for example, the segmet legth may double [19]. This strategy is based o the assumptio that later segmets of media objects are less likely to be accessed. A combiatio of these methods ca be foud i [2], i which costat legths ad expoetially icreased legths are both cosidered. This type of method also favors the begiig segmets of media objects. The prefix ad segmetatio-based cachig methods discussed above have greatly improved media cachig performace. However, they do ot address the followig cosideratios: 1) Cliet accesses to media objects typically represet a skewed patter: most accesses are for a few popular objects, ad these objects are likely to be watched i their etirety or ear etirety. This is ofte true for movie cotet i a VoD eviromet ad traiig videos i a corporatio eviromet. A heuristic segmet-based cachig strategy with a predefied segmet size, expoetial or uiform, always favorably caches the begiig segmets of media objects ad does ot accout for the fact that most accesses are targeted to a few popular objects. 2) The access characteristics of media objects are dyamically chagig. The media object s popularity ad mostwatched portios may vary with time. For example, some objects

2 may be popular for a iitial time period where most users access etire objects. The, as the time goes o, there may be fewer requests for these objects ad there may be fewer user accesses to the later portios of the objects. I this sceario, usig a fixed strategy of cachig several early segmets may ot work, sice durig the iitial time period this may overload the etwork as later segmets eed to be retrieved frequetly; the durig the later time, cachig all the iitial segmets may become wasteful of resources. The lack or pooress of adaptiveess i the existig proxy cachig schemes may reder proxy cachig to be ieffective. 3) The uiform or the expoetial segmetatio methods always use the fixed base segmet size to segmet all the objects through the proxy. However, a proxy is always exposed to objects with a wide rage of sizes from differet categories ad the access characteristics to them ca be quite diverse. Without a adaptive scheme, a overestimate of the base segmet legth may cause a iefficiet use of cache space, while a uderestimate may cause icreased maagemet overhead. I this paper, we propose a adaptive ad lazy segmetatio based cachig strategy, which resposively adapts to the real time accesses ad lazily segmets objects as late as possible. Specifically, we desig a aggressive admissio policy, a lazy segmetatio strategy, ad a two-phase iterative replacemet policy. The proxy system supported by the proposed cachig strategy has the followig advatages: 1) It achieves maximal etwork traffic reductio by favorably cachig the popular segmets of media objects, regardless of their positios withi the media object. If most of the cliets ted to watch the iitial portios of these objects, the iitial segmets are cached. 2) It dyamically adapts to chages i object access patters over time. Specifically, it performs well i commo scearios i which the popularity characteristics of media objects vary over time. The system automatically takes care of this situatio without assumig a priori access patter. 3) It adapts to differet types of media objects. Media objects from differet categories are treated fairly with the goal of maximizig cachig efficiecy. Specifically, the adaptiveess of our proposed method falls ito two areas. 1) The segmet size of each object is decided adaptively based o the access history of this object recorded i real time. The segmet size determied i this way more accurately reflects the cliet access behaviors. The access history is collected by delayig the segmetatio process. 2) Segmet admissio ad evictio policies are adapted i real time based o the access records. A utility fuctio is derived to maximize the utilizatio of the cache space. Effectively, the cache space is favorably allocated to popular segmets regardless of whether they are iitial segmets or ot. Both sythetic ad real proxy traces are used to evaluate the performace of our proposed method. We show that (1) the uiform segmetatio method achieves a similar performace result as the expoetial segmetatio method o average; (2) our proposed adaptive ad lazy segmetatio strategy outperforms the expoetial ad the uiform segmetatio methods by about 3% i byte hit ratio o average, which represets a server workload ad etwork traffic reductio of 3%. The rest of the paper is orgaized as follows. The desig of the adaptive ad lazy segmetatio based cachig system is preseted i Sectio 2. Performace evaluatio is preseted i Sectio 3 ad further evaluatio is preseted i Sectio 4. We evaluate the utility fuctio of the replacemet policy i Sectio 5 ad make cocludig remarks i Sectio Related Work Proxy cachig of streamig media has bee explored i [17, 6, 2, 1, 13, 14, 15, 19, 8, 18, 12, 2]. Prefix cachig ad its related protocol cosideratios as well as partial sequece cachig are studied i [17, 7, 6]. It had bee show that prefix/suffix cachig is worse tha expoetial segmetatio i terms of cachig efficiecy i [19]. Studies have also show that it is appropriate to cache popular media objects i their etirety. Video stagig [2] reduces the peak or average badwidth requiremets betwee the server ad proxy chael by cosiderig the fact that coded video frames have differet sizes depedig o the scee complexity ad codig method. Specifically, if a coded video frame exceeds a predetermied threshold, the the frame is cut such that a portio is cached o the proxy while the other portio remais o the server, thus reducig or smoothig the badwidth required betwee the two. I [13, 14, 15], a similar idea is proposed for cachig scalable video, ad this is doe i a maer that co-operates with the cogestio cotrol mechaism. The cache replacemet mechaism ad cache resource allocatio problems are studied accordig to the popularity of video objects. I [1], the algorithm attempts to partitio a video ito differet chuks of frames with alteratig chuks stored i the proxy, while i [11], the algorithm may select groups of o-cosecutive frames for cachig i the proxy. The cachig problem for layer-ecoded video is studied i [8]. The cache replacemet of streamig media is studied i the [18, 12]. 2. ADAPTIVE AND LAZY SEGMENTATION BASED CACHING SYSTEM This sectio describes our proposed segmetatio-based cachig algorithm. I our algorithm, each object is fully cached accordig to the proposed aggressive admissio policy whe it is accessed for the first time. The fully cached object is kept i the cache util it is chose as a evictio victim by the replacemet policy. At that time, the object is segmeted usig the lazy segmetatio strategy ad some segmets are evicted by the first phase of the two-phase iterative replacemet policy. From the o, the segmets of the object are adaptively admitted by the aggressive admissio policy or adaptively replaced as described i the secod phase of the twophase iterative replacemet policy. For ay media object accessed through the proxy, a data structure cotaiig the followig items is created ad maitaied. This data structure is called the access log of the object. T 1: the time istace the object is accessed for the first time; T r: the last referece time of the object. It is equal to T 1 whe the object is accessed for the first time; L sum: the sum of the duratio of each access to the object; : the umber of accesses to the object; L b : the legth of the base segmet; s: the umber of the cached segmets of the object. Quatities T r, ad s are dyamically updated upo each access arrival. Quatity L sum is updated upo each sessio termiatio. Quatity L b is decided whe the object is segmeted. I additio, the followig quatities ca be derived from the above items ad are used as measuremets of access activities to each object. I our desig, T c is used to deote the curret time istace. At time istace T c, we deote the access frequecy F as T r T 1, ad deote the average access duratio L avg as Lsum. Both of these quatities are also updated upo each access arrival.

3 We ow preset the three major modules of the cachig system. The aggressive admissio policy is preseted i sectio 2.1. Sectio 2.2 describes the lazy segmetatio strategy. Details of the two-phase iterative replacemet policy are preseted i sectio Aggressive Admissio Policy For ay media object, cache admissio is evaluated each time it is accessed with the followig aggressive admissio policy. If there is o access log for the object, the object is accessed for the first time. Assumig the full legth of the object is kow to the proxy, sufficiet cache space is allocated through a adaptive replacemet algorithm as described i sectio 2.3. The accessed object is subsequetly cached etirely regardless of the request s accessig duratio. A access log is also created for the object ad the recordig of the access history begis. If a access log exists for the object (ot the first time), but the log idicates that the object is fully cached, the access log is updated. No cache admissio is ecessary. If a access log exists for the object (ot the first time), but the log idicates that the object is ot fully cached, the system aggressively cosiders cachig the ( s + 1)th segmet if L avg 1 a (s +1) L b, where a is a costat determied by the replacemet policy (see sectio 2.3). The iequality idicates that the average access duratio is icreasig to the extet that the cached s segmets ca ot cover most of the requests while a total of s + 1 segmets ca. Therefore, the system should cosider the admissio of the ext ucached segmet. The fial determiatio of whether this ucached segmet is fially cached or ot is determied by the replacemet policy (see sectio 2.3). (I our system, a = 2, that is, whe Lsum s+1 L 2 b is true, the ext ucached segmet of this object is cosidered to be cached.) I summary, usig aggressive admissio, the object is fully admitted whe it is accessed for the first time. The the admissio of this object is cosidered segmet by segmet. 2.2 Strategy The key of the lazy segmetatio strategy is as follows. Oce there is o cache space available ad thus cache replacemet is eeded, the replacemet policy calculates the cachig utility of each cached object (see sectio 2.3). Subsequetly, the object with the smallest utility value is chose as the victim if it is ot active (o request is curretly accessig it). If the victim object is fully cached, the proxy segmets the object as follows. The average access duratio L avg at curret time istace is calculated. It is used as the legth of the base segmet of this object, that is, L b = L avg. Note that the value of L b is fixed oce it is determied. The object is the segmeted uiformly based o L b. After that, the first a segmets are kept i the cache, while the remaiig segmets are evicted (see sectio 2.3). The umber of cached segmets, s, is updated i the access log of the object accordigly. If a later request demads more tha the cached umber of segmets of this object, data of legth L b (except for the last segmet) is prefetched from the server. I cotrast with existig segmetatio strategies, i which segmetatio is performed whe the object is accessed for the first time, the lazy segmetatio strategy delays the segmetatio process as late as possible, thus allowig the proxy to collect a sufficiet amout of accessig statistics to improve the accuracy of the segmetatio for each media object. By usig the lazy segmetatio strategy, the system adaptively sets differet base segmet legths for differet objects accordig to real time user access behaviors. 2.3 Two-Phase Iterative Replacemet Policy The replacemet policy is used to select cache evictio victims. We desig a two-phase iterative replacemet policy as follows. First of all, a utility fuctio is derived to help the victim selectio process. Several factors are cosidered to predict future accesses. The average umber of accesses; the average duratio of accesses; the legth of the cached data (could be the whole object, or could be some segmets), which is the cost of the storage; ad the probability of the future access. I additio to the above factors used to predict the users future access behaviors, the two-phase iterative replacemet policy cosiders the possibility of future accesses as follows: the system compares the T c T r, the time iterval betwee ow ad the most recet access, ad the Tr T 1, the average time iterval for a access happeig i the past. If T c T r > Tr T 1, the possibility that a ew request arrives soo for this object is small. Otherwise, it is more likely that a request may be comig soo. Ituitively, the cachig utility of a object is proportioal to the average umber of accesses, the average duratio of accesses ad the probability of accesses. I additio, it is iversely proportioal to the size of the occupied cache space. Therefore, the cachig utility fuctio of each object is defied as follows: f 1( Lsum ) f 2(F ) p 2 MIN{1, Tr T1 f 3( s L b ) p 3 T c T r } where f 1( Lsum ) represets the average duratio of future access; f 2(F ) represets the average umber of future accesses;, (1) MIN{1, Tr T 1 T c T r } deotes the possibility of future accesses; ad f 3( s L b ) is the cost of disk storage. Equatio 1 ca be simplified as L sum T r T 1 MIN{1, Tr T1 T c T r } s L b (2) whe = 1, p 2 = 1 ad p 3 = 1. Compared with the distace-sesitive utility fuctio 1 T c T r 1 (T c T r) i (i represets the i th segmet, is the estimated frequecy) used i the expoetial segmetatio method [19] which favorably caches segmets closer to the begiig of media objects, the proposed utility fuctio provides a more accurate estimatio based o the popularity of segmets regardless of their relative positios i the media object. This helps to esure that less popular segmets get evicted from the cache. Give the defiitio of the utility fuctio, we desig a twophase iterative replacemet policy to maximize the aggregated utility value of the cached objects. Upo object admissio, if there is ot eough cache space, the system calculates the cachig utility of each object curretly i the cache. The object with the smallest utility value is chose as the victim ad partial cached data of this object is evicted i oe of the two phases as follows.

4 First Phase: If the access log of the object idicates that the object is fully cached, the object is segmeted as described i sectio 2.2. The first a (a = 2) segmets are kept ad the rest segmets are evicted right after the segmetatio is completed. Therefore, the portio of the object left i cache is of legth 2 L b. Give that L b = L avg at this time istace, the cached 2 segmets cover a ormal distributio i the access duratio. Secod Phase: If the access log of the object idicates that the object is partially cached, the last cached segmet of this object is evicted. The utility value of the object is updated after each replacemet ad this process repeats iteratively util the required space is foud. The desig of the two-phase iterative replacemet policy reduces the chaces of makig wrog decisios of the replacemet, ad gives fair chaces to the replaced segmets so that they ca be cached back ito the proxy agai by the aggressive admissio policy if they become popular agai. I additio, the iterative ature of the replacemet procedure esures that the aggregated utility value of the cached objects is maximized. Note that eve after a object is fully replaced, the system still keeps its access log. If ot, whe the object is accessed agai, it should be fully cached agai. Sice media objects ted to have dimiishig popularity as the time goes o, if the system caches the object i full agai, it results i a iefficiet use of the cache space. Our desig ehaces the resource utilizatio by avoidig this kid of situatio. 3. PERFORMANCE EVALUATION A evet-drive simulator is implemeted to evaluate the performace of the expoetial segmetatio, the uiform segmetatio, ad our proposed adaptive ad lazy segmetatio techiques by usig sythetic ad real traces. For the adaptive ad lazy segmetatio strategy, Equatio 2 is used as the utility fuctio. The expoetial segmetatio strategy always reserves a portio of cache space (1%) for begiig segmets, ad leaves the rest for later segmets. The utility fuctio used is as described i sectio 2.3 for the replacemet of later segmets, while the LRU policy is used for the begiig segmets. The oly differece betwee the uiform segmetatio ad the expoetial segmetatio method is as follows. Istead of segmetig the object expoetially, the uiform segmetatio strategy segmets the object with a costat legth. Sice the expoetial segmetatio strategy always caches the first 6 segmets as i [19], for a fair compariso, the uiform segmetatio strategy always caches the same legth of first several segmets of media objects. Thus whether the expoetially icreasig segmet legth plays a importat role ca also be evaluated. The byte hit ratio is defied as how may bytes are delivered to the cliet from the proxy directly, ormalized by the total bytes the cliet requested. It is used as the major metric to evaluate the reductio of the etwork traffic to the server ad the disk badwidth utilizatio o the server. The delayed start request ratio is defied as how may requests amog the total do ot have a startup latecy sice the iitial portio of the requested object is cached o the proxy. It is used to idicate the efficiecy of these techiques i reducig the user perceived startup latecy. The average umber of cached objects per time uit deotes that the average umber of objects whose segmets are partially or fully cached. It is used to idicate whether the method favorably caches the begiig segmets of a large umber of differet objects or favorably caches the popular segmets of a small umber of differet objects. 3.1 Workload Summary Table 1 lists some kow properties of sythetic traces ad a actual eterprise trace. Trace Num of Num of Size λ α Rage Duratio Name Request Object (GB) (miute) (day) WEB VOD PART REAL Table 1: Workload Summary WEB ad VOD deote the traces for the Web ad VoD eviromet with complete viewig, while PARTIAL deotes the trace for the Web eviromet with partial viewig. These sythetic N traces assume a Zipf-like distributio (p i = f i/ i fi, fi = 1/i α ) for the popularity of media objects. They also assume the request arrival follows the Poisso distributio (p(x, λ) = e λ (λ) x /(x!), x =, 1, 2...). REAL deotes the trace extracted from server logs of HP Corporate Media Solutios, coverig the period from April 1 through April 1, Evaluatio o Complete Viewig Traces Figure 1 shows the performace results from simulatios usig the WEB trace. Lazy segmetatio refers to our proposed adaptive ad lazy segmetatio method. Expoetial segmetatio refers to the expoetial segmetatio method. Uiform segmetatio (1K) refers to the uiform segmetatio method with 1KB sized segmets, while uiform segmetatio (1M) refers to the uiform segmetatio method with 1MB sized segmets 1. Evidet from Figure 1(a), lazy segmetatio achieves the highest byte hit ratio. Whe the cache size is 1%, 2% ad 3% of the total object size, the byte hit ratios of lazy segmetatio ad expoetial segmetatio are more tha 5% ad 13%, 67% ad 39%, 75% ad 29%, respectively. The absolute performace gap is more tha 3% o average ad gradually decreases with the icrease of available cache space. O average, uiform segmetatio achieves a similar result as expoetial segmetatio, which idicates that the expoetially icreased legth does ot have a obvious effect o the byte hit ratio. Figure 1(b) shows that i terms of the delayed start request ratio, uiform segmetatio (1K) achieves the best result, while expoetial segmetatio is raked secod. This is expected sice both of them always favorably cache the begiig segmets of media objects. Lazy segmetatio achieves the worst percetage amog the three. The results idicate that the achieved high byte hit ratio is paid at the expese of a high delayed start request ratio. Figure 1(c) shows the average umber of cached objects per time uit. Lazy segmetatio always has the least umber of objects cached o average, while it always achieves the best byte hit ratio. The results implicitly idicate that favorably cachig the begiig segmets of media objects is ot efficiet i reducig etwork traffic to the server ad disk badwidth utilizatio o the server. The results of the VOD trace show i Figure 2 show similar treds as those of WEB. The byte hit ratio of lazy segmetatio is improved by 28, 24 ad 1 percetage poits over expoetial segmetatio whe the cache size is 1%, 2% ad 3% of the total object size correspodigly. Sice WEB ad VOD are the complete viewig scearios, results from simulatios usig these two traces demostrate that i 1 I the followig cotext, we also use them to represet their correspodig strategies for brevity.

5 Figure 1: WEB: (a) Byte Hit Ratio, (b) Delayed Start Request ratio, ad (c) Figure 2: VOD: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) terms of the byte hit ratio, it is more appropriate to cache the popular segmets of objects istead of favorably cachig the begiig segmets of a media object. It also implicitly shows that the two-phase iterative replacemet policy of our proposed method ca successfully idetify the popular objects whe compared with the distace-sesitive replacemet policy used i the the expoetial segmetatio techique. 3.3 Evaluatio o Partial Viewig Traces Figure 3 shows the performace results from simulatios usig PARTIAL. 8% of requests i PARTIAL oly access 2% of the object. Show i Figure 3(a), lazy segmetatio achieves the best byte hit ratio: whe the cache size is 1%, 2% ad 3% of the total object size, the byte hit ratio icreases by 28, 42, ad 7 percetage poits, respectively, over the expoetial segmetatio method. Compared with results of WEB, the improvemet of our proposed method over the expoetial segmetatio method is reduced due to the 8% partial viewig sessios. Agai, uiform segmetatio (1K) achieves a similar byte hit ratio as that of expoetial segmetatio o average. Figure 3(b) shows the delayed start request ratio. As expected, the lowest is achieved by uiform segmetatio (1K) while expoetial segmetatio still gets the secod lowest percetage. Lazy segmetatio achieves the highest. This cofirms that the higher byte hit ratio is paid by a higher delayed start request ratio. Figure 3(c) shows the average umber of cached objects of PAR- TIAL. It shows that lazy segmetatio always has the least umber of objects cached, while it always achieves the highest byte hit ratio. The results further idicate that favorably cachig the begiig segmets of media objects is ot effective for alleviatig the bottleecks of deliverig streamig media objects. For the real trace REAL, Figure 4(a) shows the byte hit ratio as a fuctio of icreased cache size. Whe the cache size is 2%, 3%, 4% ad 5% of the total object size, the byte hit ratio icreases of lazy segmetatio are 31, 28, 29, 3 percetage poits, respectively, over expoetial segmetatio. The average performace improvemet is about 3 percetage poits. The treds show i Figure 4(a) are cosistet with the previous oes. Figure 4(b) shows the delayed start request ratio for the REAL trace. It shows that lazy segmetatio has similar results as expoetial segmetatio. Its performace eve exceeds that of expoetial segmetatio whe the cache size is 1% ad 2% of the total cache size. This is due to the ature of partial viewig i REAL. I our proposed method, much cache space is available for the begiig segmets of objects. The result reflects the adaptiveess of our proposed method. Figure 4(c) shows that cosistet with previous evaluatios, our proposed method still has the least umber of objects cached o average while achievig the highest byte hit ratio. The results of REAL show that lazy segmetatio achieves the highest byte hit ratio ad early the lowest delayed start request ratio. The adaptiveess of our proposed method show i this evaluatio cofirms our aalysis i Sectio 1. All these performace results show that: (1) i terms of byte hit ratio (reductios of server workload ad etwork traffic), our proposed adaptive ad lazy segmetatio method always performs best with the least umber of objects cached; (2) uiform segmetatio (1K) achieves a similar result o both the byte hit ratio ad the delayed start request ratio as the expoetial segmetatio method o average. The performace results also idicate that favorably cachig the begiig segmets of media objects is ot effective i alleviatig

6 Figure 3: PART: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) Figure 4: REAL: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) the bottleecks for deliverig streamig media ad expoetially icreasig segmet legth does ot have a obvious advatage over costat segmet legth i terms of byte hit ratio. Uiform segmetatio with other base segmet legths are also tested. They achieve similar performace results as those of uiform segmetatio (1M) i byte hit ratio ad worse results i delayed start request ratio. 4. ADDITIONAL RESULTS I Sectio 3, the adaptive ad lazy segmetatio strategy is evaluated comparatively with the expoetial segmetatio ad the uiform segmetatio strategies. We have leared that geerally the adaptive ad lazy segmetatio strategy achieves a higher byte hit ratio. We also kow that the adaptive ad lazy segmetatio strategy does ot reserve space for begiig segmets of media objects, while the expoetial segmetatio ad uiform segmetatio strategies do reserve space for begiig segmets (1% of the total cache size i the experimets). Thus oe may argue that the higher byte hit ratio achieved by lazy segmetatio comes from freeig the reserved space. To examie whether this is true or ot, two groups of experimets are desiged ad performed based o the chages of either the lazy segmetatio strategy, either the expoetial ad uiform segmetatio strategies. These experimets are used to evaluate whether the freeig of reserved space has a sigificat impact o the byte hit ratio improvemet achieved by lazy segmetatio. 4.1 Small Cache Size for Firstly, we use a smaller cache space for lazy segmetatio. It meas that the available cache space for the adaptive ad lazy segmetatio strategy is the same as the cache space for the expoetial segmetatio strategy other tha the reserved part. I these experimets, the reserved portio for the expoetial segmetatio strategy is set to 1%, thus the totally available cache space for lazy segmetatio is oly 9% of the cache space for expoetial segmetatio. The remaiig 1% is reserved for o use. Figure 5 shows the correspodig results usig the WEB trace. Compared with Figure 1, the achieved byte hit ratio of lazy segmetatio does decrease a little. However, it still achieves the highest byte hit ratio amog all the strategies as show i Figure 5(a). I Figure 5(b), the delayed start request ratio achieved by lazy segmetatio is eve worse compared to Figure 1(b), due to the decrease of the total available cache size. Figure 5(c) shows that due to the case whe oly 9% space is available for lazy segmetatio, the average umber of cached objects is less tha the total umber of objects whe the cache size is 1% of the total object size. The results usig the VOD trace is show i Figure 6. All the treds idicated o Figure 6 are similar to those o Figure 5, with smaller chages correspodigly. The smaller chages are due to the loger duratios of the VOD trace. Compared with Figure 3, the variatios of results usig the trace PARTIAL i Figure 7 are more sigificat o byte hit ratio ad delayed start request ratio. Due to the partial viewig ature of this trace, the performace results are more sesitive to the chages of the available cache size for the adaptive ad lazy segmetatio strategy. However, lazy segmetatio still has a sigificat advatage over the others i achievig a high byte hit ratio. Note that i Figure 7, the average umber of cached objects reaches the total umber of objects whe the cache size is 1%. This is differet i Figure 5 ad Figure 6. Figure 8 shows the correspodig results usig the REAL trace. The byte hit ratio, delayed start request ratio, ad average umber

7 Figure 5: WEB: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) Figure 6: VOD: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) Figure 7: PART: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) Figure 8: REAL: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c)

8 Figure 9: WEB: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) Figure 1: VOD: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) (c) of cached objects chage little for lazy segmetatio. We believe this is due to the fact that the REAL trace has may prematurely termiated sessios. I summary, the results of these experimets show that the highest byte hit ratio achieved by lazy segmetatio is ot from freeig the reserved space. 4.2 Elimiatig Reserved Space for Uiform ad I previous sectio, we have altered the available cache space for lazy segmetatio to show that the freeig of reserved space does ot chage the coclusio we made. I this sectio, we desig aother group of experimets for this purpose. I these experimets, the available cache space for the uiform ad expoetial segmetatio strategies ad the lazy segmetatio strategy are the same. However, for uiform ad expoetial segmetatio, o cache space is reserved for the begiig segmets. Followig the origial strategy, for the uiform ad expoetial segmetatio strategies, the first several segmets will be cached whe the object is iitially accessed while the rest will ot. Oce the rest of a object is accessed agai, it is cosidered to be cached accordig to its cachig utility as defied i [19]. The begiig segmets ad the remaiig segmets compete together for the cache space whe they eed it. These comparisos ca provide more isights ito whether the byte hit ratio improvemet of our proposed adaptive ad lazy segmetatio strategy comes from the reserved cache space. Figure 9 shows the correspodig results usig the WEB trace. Compared with Figure 1, the uiform ad expoetial segmetatio strategies have similar performace results, ad the lazy segmetatio strategy still achieves the highest byte hit ratio amog all the strategies as show i Figure 9(a). I Figure 9(b), the delayed start request ratio achieved by the uiform ad expoetial segmetatio strategies are almost same. Figure 9(c) shows that the average umber of cached objects for uiform ad expoetial segmetatio are close to each other. The results usig the VOD trace is show i Figure 1. All the treds idicated o Figure 1 are similar to those show o Figure 9. We show the results usig the PARTIAL trace i Figure 11. It is iterestig to see that lazy segmetatio has a larger umber of cached objects o average whe the cache size icreases from 3% i Figure 11(c). The reaso behid this is that a large portio of the sessios are termiated earlier. I Figure 11(a), we fid that lazy segmetatio still achieves the highest byte hit ratio. The correspodig results usig the trace REAL are show i Figure 12. Agai, the results do ot show sigificat impact of the reserved space o the byte hit ratio improvemet for the uiform ad expoetial segmetatio strategies. 5. EVALUATION OF THE REPLACEMENT UTILITY FUNCTION I the previous evaluatio, we always use Equatio 2 as the utility fuctio for lazy segmetatio. To examie the effects of the variat utility fuctio o system performace, we vary ad p 2 i Equatio 1 to simulate the differet weights of the frequecy ad the average access duratio. To simulate differet weights of storage space, we vary p 3 i Equatio 1. The correspodig results are preseted i this sectio. Figure 13 shows the performace results usig the WEB trace. Figure 13(a) shows that the byte hit ratio chages slightly whe the cachig utility is chagig with the available cache size. Figure

9 Figure 11: PART: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) Figure 12: REAL: (a) Byte Hit Ratio, (b) Delayed Start Request Ratio, ad (c) ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p Figure 13: WEB: Variat Utility Fuctios of Replacemet Policy ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p Figure 14: VOD: Variat Utility Fuctios of Replacemet Policy 13(b) idicates that the delayed start request ratio has larger variatios whe the cache size icreases, especially whe the cache size icrease from 4% to 9% of the total object size. It also shows that for lazy segmetatio, a better delayed start request ratio ca be achieved oce the priority of the cache space cosumptio is icreased. The results from the VOD trace i Figure 14 idicate similar treds as before. Figure 14(a) shows that the byte hit ratio varies widely compared to those of the WEB trace while Figure 14(b) idicates the chages of the delayed start request ratio are ot as sigificat as those of the WEB trace. Figure 15 shows the results usig the PARTIAL trace, which is a partial viewig case of the WEB trace. Geerally, the treds show i Figure 15 are similar to those of WEB. However, Figure 15(b) does show larger variatios of the delayed start request ratio whe the cache space is icreasig. This idicates that the PARTIAL trace is more sesitive to the storage space cosumptio. Figure 16 shows the results usig the REAL trace. It idicates similar treds as show i Figure 15. As show o Figure 16, the icrease of the priority o storage space cosumptio will worse ad fluctuate the delayed start request ratio for the lazy segmetatio strategy. This is due to the large umber of early termiated sessios. Through all these experimets, we fid the performace of the adaptive ad lazy segmetatio strategy ca be adjusted depedig o available cache space to a certai extet. However, i geeral, icreasig the weights of average access duratio ad frequecy has less impact o the performace results. 6. CONCLUSION We have proposed a streamig media cachig proxy system based

10 ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p Figure 15: PART: Variat Utility Fuctios of Replacemet Policy ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p ,p 2,P 3.5,p 2.5,p 3,p 3,p 3,p 2,p 3.5,p 2,p 3,p 2,p 3,p 2,p Figure 16: REAL: Variat Utility Fuctios of Replacemet Policy o a adaptive ad lazy segmetatio strategy with a aggressive admissio policy ad two-phase iterative replacemet policy. The proposed system is evaluated by simulatios usig sythetic traces ad a actual trace extracted from eterprise media server logs. Compared with a cachig system usig uiform ad expoetial segmetatio methods, the byte hit ratio achieved by the proposed method is improved by 3% o average, which idicates a 3% reductio i the server workload ad etwork traffic. Additioal evaluatios show that the improvemet i byte hit ratio of the lazy segmetatio is ot from the freeig of reserved space. The results show that the adaptive ad lazy segmetatio strategy is a highly efficiet segmet-based cachig method that alleviates bottleecks for the delivery of streamig media objects. We are curretly ivestigatig the trade-offs betwee etwork traffic reductio ad cliet startup latecy. 7. ACKNOWLEDGMENT We would like to thak aoymous reviewers for their helpful commets o this paper. The work is supported by NSF grat CCR ad a grat from Hewlett-Packard Laboratories. 8. REFERENCES [1] E. Bommaiah, K. Guo, M. Hofma ad S. Paul, Desig ad Implemetatio of a Cachig System for Streamig Media over the Iteret, IEEE Real Time Techology ad Applicatios Symposium, May 2. [2] Y. Chae, K. Guo, M. Buddhikot, S. Suri, ad E. Zegura, Silo, Raibow, ad Cachig Toke: Schemes for Scalable Fault Tolerat Stream Cachig, IEEE Joural o Selected Areas i Commuicatios, Special Issue o Iteret Proxy Services, Vol. 2, p , Sept. 22. [3] S. Che, B. She, S. Wee ad X. Zhag, Adaptive ad Lazy Segmetatio Based Proxy Cachig for Streamig Media Delivery, HPCS Lab Tech. Report TR-3-2, College of William ad Mary, Ja., 23. [4] L. Cherkasova ad M. Gupta, Characterizig Locality, Evolutio, ad Life Spa of Accesses i Eterprise Media Server Workloads, NOSSDAV 22, Miami, FL, May 22. [5] M. Chesire, A. Wolma, G. Voelker ad H. Levy, Measuremet ad Aalysis of a Streamig Media Workload, Proc. of the 3rd USENIX Symposium o Iteret Techologies ad Systems, Sa Fracisco, CA, March 21. [6] M. Y.M. Chiu ad K. H.A Yeug, Partial Video Sequece Cachig Scheme for VOD Systems with Heteroeeous Cliets IEEE Trasactios o Iducstrial Electroics, 45(1):44-51, Feb [7] S. Gruber, J. Rexford ad A. Basso, Protocol cosidertatios for a prefix-cachig for multimedia streams, Computer Network, 33(1-6): , Jue 2. [8] J. Kagasharju, F. Hartato, M. Reisslei ad K. W. Ross, Distributig Layered Ecoded Video Through Caches, Proc. of IEEE INFOCOM 1, Achorage, AK, USA, 21. [9] S. Lee, W. Ma ad B. She, A Iteractive Video Delivery ad Cachig System Usig Video Summarizatio, Computer Commuicatios, vol. 25, o. 4, pp , Mar. 22. [1] W.H. Ma ad H.C. Du, Reducig Badwidth Requiremet for Deliverig Video over Wide Area Networks with Proxy Server, Proc. of Iteratioal Cofereces o Multimeida ad Expo., 2, vol. 2, pp [11] Z. Miao ad A. Ortega, Scalable Proxy Cachig of Video Uder Storage Costraits, IEEE Joural o Selected Areas i Commuicatios, vol. 2, p , Sept. 22. [12] M. Reisslei, F. Hartato ad K. W. Ross, Iteractive Video Streamig with Proxy Servers, Proc. of IMMCN, Atlatic City, NJ, Feb. 2. [13] R. Rejaie, M. Hadely ad D. Estri, Quality Adaptatio for Cogestio Cotrolled Video Playback over the Iteret, Proc. of ACM SIGCOMM 99, Cambridge, MA, Sept [14] R. Rejaie, M. Hadley, H. Yu ad D. Estri, Proxy Cachig Mechaism for Multimedia Playback Streams i the Iteret, Proc. of WCW 99, Apr [15] R. Rejaie, H. Yu, M. Hadely ad D. Estri, Multimedia Proxy Cachig Mechaism for Quality Adaptive Streamig Applicatios i the Iteret, Proc. of IEEE INFOCOM, Tel-Aviv, Israel, March 2. [16] S. Se, L. Gao, J. Rexford ad D. Towsley, Optimal Patchig Schemes for Efficiet Multimedia Streamig, NOSSDAV 99, Baskig Ridge, NJ, Jue [17] S. Se, K. Rexford ad D. Towsley, Proxy Prefix Cachig for Multimedia Streams, Proc. IEEE INFOCOM 99, New York, USA, March [18] R. Tewari, H. Vi, A. Da ad D. Sitaram, Resource-based Cachig for Web Servers, Proc. SPIE/ACM Coferece o Multimeida Computig ad Networkig, Ja [19] K. Wu, P. S. Yu ad J. L. Wolf, Segmet-based Proxy Cachig of Multimedia Streams, WWW 21, pp [2] Z.L. Zhag, Y. Wag, D.H.C. Du ad D. Su, Video Stagig: A Proxy-server Based Approach to Ed-to-ed Video Delivery over Wide-area Networks, IEEE Trasactios o Networkig, Vol. 8, o. 4, pp , Aug. 2.