On Peer-to-Peer Media Streaming Λ

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1 On eer-to-eer Media Streaming Λ Dongyan Xu y, Mohamed Hefeeda, Suanne Hambruch, Bharat Bhargava Department of Computer Science urdue Univerity, Wet Lafayette, IN 797 fdxu, hefeeda, eh, Abtract In thi paper, we tudy a peer-to-peer media treaming ytem with the following characteritic: () it treaming capacity grow dynamically; () peer do not exhibit erverlike behavior; () peer are heterogeneou in their bandwidth contribution; and () each treaming eion may involve multiple upplying peer. Baed on thee characteritic, we invetigate two problem: () how to aign media data to multiple upplying peer in one treaming eion and () how to fat amplify the ytem total treaming capacity. Our olution to the firt problem i an optimal media data aignment algorithm OTS pp, which reult in minimum buffering delay in the conequent treaming eion. Our olution to the econd problem i a ditributed differentiated admiion control protocol DAC pp. By differentiating between requeting peer with different outbound bandwidth, DAC pp achieve fat ytem capacity amplification; benefit all requeting peer in admiion rate, waiting time, and buffering delay; and create an incentive for peer to offer their truly available out-bound bandwidth.. Introduction Although there have been ignificant reearch effort in peer-to-peer ytem during the pat two year [,,, 5, ], one category of peer-to-peer ytem ha o far received le attention: the peer-to-peer media treaming ytem. The major difference between a general peer-topeer ytem and a peer-to-peer media treaming ytem lie in the data haring mode among peer: the former ue the open-after-downloading mode, while the latter ue the play-while-downloading mode. More pecifically, in a peer-to-peer media treaming ytem, a ubet of peer own a certain media file, and they tream the media file to Λ Thi work wa upported in part by NSF grant CCR-99889, CCR-, CCR-7, and CCR-788, and CERIAS. y Correponding author. requeting peer. On the other hand, the requeting peer playback and tore the media data during the treaming eion, and they become upplying peer of the media file after the treaming eion. In thi paper, we aume the following four characteritic of a peer-to-peer media treaming ytem: the firt three are hared by all peerto-peer ytem, while the lat one i unique in peer-topeer media treaming ytem: () A peer-to-peer media treaming ytem i elfgrowing. With requeting peer later becoming upplying peer, the ytem total capacity will be amplified: the more peer it erve, the larger the capacity it will have. () A peer-to-peer media treaming ytem i erverle. A peer i not uppoed to exhibit erver-like behavior, uch a opening a large number of imultaneou connection. () eer are heterogeneou in their out-bound bandwidth contribution to the ytem. Thi heterogeneity may be caued either by different acce network connecting the peer, or by different willingne of the peer to contribute. () The upplying-peer/requeting-peer relation i typically many-to-one, intead of one-to-one a in the general peer-to-peer ytem. Since the out-bound bandwidth offered by a upplying peer may be le than the original playback rate of the media data, it i neceary to involve multiple upplying peer in one real-time treaming eion. We identify two new problem ariing in the above ytem. To the bet of our knowledge, thi i the firt in-depth tudy on thee problem in the context of peer-topeer media treaming. The firt problem i the media data aignment for a multi-upplier peer-to-peer treaming eion. More pecifically, given a requeting peer and a et of upplying peer with heterogeneou out-bound bandwidth offer, we how how to aign a ubet of the media data to each upplying peer. The econd problem i the fat amplification of the peer-to-peer treaming capacity. Intuitively, among multiple requeting peer, ervice priority hould be given to thoe who promie higher out-bound bandwidth offer, becaue they will contribute more to the peer-topeer treaming capacity after becoming upplying peer. roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

2 We how how to realize uch a differentiated admiion policy, and that fat capacity amplification will ultimately benefit all peer. In thi paper, we propoe an algorithm OTS pp that compute the optimal media data aignment for each peerto-peer treaming eion. The aignment will lead to the minimum buffering delay experienced by the requeting peer. We alo propoe a ditributed differentiated admiion control protocol DAC pp, to be executed by both upplying and requeting peer. Compared with the current non-differentiated admiion control mechanim, rotocol DAC pp achieve () fater amplification of peer-topeer ytem capacity; () higher admiion rate and fewer rejection (before a peer i admitted) among all requeting peer; and () horter average buffering delay among all admitted requeting peer. Furthermore, for () and (), the protocol alo differentiate between requeting peer with different out-bound bandwidth promie, creating an incentive for them to offer their truly available bandwidth. The ret of the paper i organized a follow: we firt define our peer-to-peer media treaming model in Section. Section and preent our olution to the two problem, repectively. Section 5 preent our imulation reult. Section 6 compare our work with related work. Finally, Section 7 conclude thi paper. eer-to-eer Media Streaming Model In thi ection, we define a peer-to-peer media treaming model and tate our aumption: () Role of peer For a media data item, requeting peer are the peer that requet the data. Once the peer-topeer treaming eion i over, a requeting peer become a upplying peer. To avoid erver-like behavior, each upplying peer participate in at mot one peer-to-peer treaming eion at any time. We alo aume that there are ome eed upplying peer, which obtain the media data from ome external ource. () Bandwidth of peer Let R denote the playback rate of the media data. We aume that each requeting peer r i willing and able to et aide an in-bound bandwidth of R in ( r )=R to receive the treaming ervice. However, the out-bound bandwidth R out ( ) offered by a upplying peer ha one of the following value: R ; R ; R R 8 :::. N () Clae of peer We claify the peer into N clae, according to the N poible value of their out-bound bandwidth offer. More pecifically, a peer willing to offer out-bound bandwidth R n (» n» N) i called a cla-n We alo aume that each peer ha ufficient torage to tore the entire media file. Thi pecial et of value prevent media data aignment from becoming the N-hard binpacking-like problem. peer. We alo aume that the lower the n, thehigher the cla. () Capacity of the peer-to-peer treaming ytem We define the capacity a the total number of peer-topeer treaming eion that can be imultaneouly provided by the ytem. Since a peer-to-peer treaming eion involve multiple upplying peer whoe R out ( ) add up to R, the capacity of the ytem at time t can be computed (t) (Rout()) a C y (t) = b R c ( (t) i the et of upplying peer in the ytem at t). (5) Segment of media data We aume that the media data can be partitioned into mall equential egment of equal ize. We alo aume that the media tream i of Contant-Bit-Rate (CBR) and therefore, the playback time ffit of each egment i the ame (ffit i typically in the magnitude of econd). Optimal Media Data Aignment In thi ection, we tudy the problem of media data aignment. Baed on the model in Section, the problem can be tated a follow: For a requeting peer r and a et of upplying peer ; ;:::m,ifr = R in ( r )= m i= R out( i ), determine () the media data egment to be tranmitted by i (» i» m) and () the playback tart time for r. The goal i to enure a continuou playback, with minimum buffering delay at r. We define the buffering delay a the time interval between the tart of media data egment tranmiion and the tart of playback at r. A hown in Figure, different media data aignment lead to different buffering delay. The requeting peer i r ; and the upplying peer are ; ; ; with out-bound bandwidth of R, R, R 8, and R 8, repectively. In Aignment I, i aigned media data egment 8k, 8k +, 8k +, 8k +(k =; ; ; :::); i aigned egment 8k +, 8k +5; i aigned egment 8k +6;and i aigned egment 8k +7. The tart time of playback at r i 5ffit. Therefore, the buffering delay achieved by Aignment I i 5ffit. However, if Aignment II i ued, the buffering delay will be reduced to ffit. We propoe an algorithm OTS pp, which compute the optimal media data aignment that lead to the minimum buffering delay. The algorithm i executed by the requeting peer. After computing the media data aignment, it will initiate the peer-to-peer treaming eion by notifying each participating upplying peer of the correponding aignment. The upplying peer will then tart the tranmiion of it aigned media data egment. The peudo-code of Algorithm OTS pp i hown in Figure. Suppoe that the m upplying peer have been orted in decending order according to their out-bound roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

3 Tranmiion of egment δ t layback of egment by r (a) Aignment I Tranmiion of egment 7 Time OTS pp( r ; f ; ::: m g) f i = n ; while (i ) f for j =to m if the aignment to j i not complete f Aign egment i to j ; i = i ; g g g Figure. Algorithm OTS pp compute an optimal media data aignment, which achieve the minimum buffering delay in the conequent peer-topeer treaming eion. The minimum buffering delay i T min buf = m Λ ffit. 5 δ t layback of egment by r (b) Aignment II Time Figure. Different media data aignment lead to different buffering delay bandwidth offer; and that the lowet cla among them i cla-n. The algorithm compute the aignment of the firt n egment; and the aignment repeat itelf every n egment for the ret of the media file. In fact, Aignment II in Figure i computed by OTS pp: after the firt while iteration, egment 7, 6, 5, are aigned to,,, and, repectively; and and are done with the aignment. After the econd while iteration, egment, areaignedto and, repectively; and i done. During the lat two while iteration, egment and are aigned to - each in one iteration. The optimality of Algorithm OTS pp i tated in Theorem, which give a (omewhat urpriingly) imple form of the minimum buffering delay. The proof of Theorem can be found in []. Theorem Given a et of m upplying peer f ; ::: m g and a requeting peer r, if we have R = m R in ( r ) = i= R out( i ), then Algorithm OTS pp will Fat Sytem Capacity Amplification In thi ection, we tudy the problem of fat capacity amplification of the entire ytem. Thi will alo anwer the quetion of how to elect a et of upplying peer for a peer-to-peer treaming eion, which i not mentioned in Section. Recall that one of the mot exciting property of a peer-to-peer treaming ytem i that it capacity dynamically grow. However, no previou work ha addreed thi problem in the context of peer-to-peer media treaming with peer bandwidth heterogeneity. Conider the cenario hown in Figure. Suppoe at time t, there are four upplying peer in the ytem: two cla- peer and and two cla- peer and. According to the ytem capacity definition, the capacity of the peer-to-peer treaming ytem at t i b( R R + R + + R )=R c =. Thi mean that the ytem can admit one requeting peer at t. Now, uppoe there are three requeting peer: two cla- peer r and r and one cla- peer r. If we admit r at t, the capacity will till be at t + T (T i the duration of the peerto-peer treaming eion), and r and r will have to be admitted one after another at t + T and t + T, repectively. However, if r i admitted at t, the ytem capacity will grow to at t + T,andboth r and r can be admitted at t + T. Furthermore, we define the waiting time of a requeting peer a the interval between it firt treaming requet and the earliet time it can be admitted. The average waiting time incurred by the firt admiion equence i ( + T +T )= = T ; while it i (T + T +)= = T in the econd cae. The above example ugget that a differentiated admiion policy which favor higher-cla requeting peer will roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

4 Sytem capacity r r r r r r t t + T t t + T + T Sytem capacity r r r (a) Admitting r! r! r t t + T t + T r r (b) Admitting r! ( r + r ) Figure. Different admiion deciion lead to different growth of treaming capacity lead to a fater amplification of the peer-to-peer ytem capacity, and will ultimately benefit requeting peer of all clae. Other requirement for uch a differentiated admiion policy include: () it hould not tarve the lower-cla requeting peer, even in the hort term; () it hould be enforced in a purely ditributed fahion; and () it hould be differentiating uch that the higher the outbound bandwidth pledged by a requeting peer, the greater the poibility that it will be admitted, and the horter the waiting time and buffering delay it will experience. Thi differentiation will create an incentive to encourage requeting peer to contribute it truly available out-bound bandwidth to the peer-to-peer treaming ytem. Our olution i a ditributed admiion control protocol DAC pp. rotocol DAC pp ha two key feature. Firt, each upplying peer individually decide whether or not to participate in a treaming eion requeted by a requeting peer. The deciion i made in a probabilitic fahion, There i, however, an important aumption: ince the bandwidth commitment i made when a requeting peer requet treaming ervice, there mut be a mechanim to enforce the bandwidth commitment after the requeting peer become a upplying peer. Thi mechanim i aumed to exit in the peer-to-peer oftware intalled in each peer. Time Time with different probability value applied to different clae of requeting peer, and the probabilitie are dynamically adjuted. Second, we propoe a new technique called reminder: under certain condition (to be detailed hortly), a requeting peer r may end a reminder to a buy upplying peer, reminding not to elevate it admiion preference to requeting peer of clae lower than that of r. rotocol DAC pp involve operation of both upplying peer and requeting peer.. DAC pp - Supplying eer Each upplying peer maintain an admiion probability vector <r[];r[]:::; r[n ] >. r[i] (» i» N) will be applied to cla-i requeting peer: if a cla-i requeting peer contact for treaming ervice and i not buy participating in another treaming eion, will grant the requet with probability r[i]. Suppoe itelf i a cla-k peer, then the value in the probability vector of i determined a follow: (a) Initially, when become a upplying peer, it probability vector i initialized a follow: For» i» k, we initialize r[i] = :. For k < i» N, we initialize r[i] =. The intuition behind thi initialization i: i k ince i a cla-k peer itelf, it will favor requeting peer of cla-k and higher by alway granting their treaming requet. However, for requeting peer of lower clae, it will exponentially decreae the admiion probability. We call cla i a favored cla of,if currently ha r[i] =:. For example, for a cla- upplying peer (and uppoe N =), it initial admiion probability vector i < :; :; :5; :5 >, and it initial favored clae are clae and. (b) If ha been idle, then it probability vector will be updated after a timeout period of T out. The update i performed a follow: for each k < i» N, r[i] = r[i] Λ. Thi mean that will elevate the admiion probabilitie of lower-cla requeting peer, if it ha not been erving any requeting peer in the pat period of T out. If remain idle, the update will be performed after every period of T out, until every probability in it probability vector i., i.e. every cla i favored cla. (c) If ha jut finihed erving in a peer-topeer treaming eion, will update it probability vector a follow: ffl If during the treaming eion, it did not receive any requet from a requeting peer of it favored cla, will elevate the admiion probability of the lower clae, imilar to the update in (b): for each k<i» N, r[i] =r[i] Λ. ffl If during the eion, it received at leat one requet from a requeting peer of it favored cla, the requet roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

5 wa not granted becaue wa buy. Under a certain condition (to be decribed in Section.), the requeting peer left a reminder to. Suppoe ^k i the highet favored cla of requeting peer() which left a reminder, then for» i» ^k, r[i] =:;and for ^k <i» N, r[i] = i ^k. In the firt cae, relaxe the admiion preference, becaue it ha not been requeted by any peer of it current favored clae. In the econd cae, tighten the admiion preference, becaue there have been reminder from requeting peer of it favored clae which hould have been erved, had not been buy.. DAC pp - Requeting eer Each requeting peer r firt obtain a lit of M randomly elected candidate upplying peer via ome peerto-peer lookup mechanim. We aume that the cla of each candidate i alo obtained. r then directly contact the candidate upplying peer - from high to low clae: ffl r will be admitted, if r i able to obtain permiion from enough upplying peer (among the M candidate) uch that: () they are neither down nor buy with another treaming eion; () they are willing to provide the treaming ervice (i.e. having paed the probabilitic admiion tet); and () their aggregated out-bound bandwidth offer i R um = R. r will then execute Algorithm OTS pp to compute the media data aignment, trigger the participating upplying peer, and the peer-to-peer treaming eion will begin. ffl r will be rejected, if r i not able to get permiion from enough upplying peer that atify all three condition above. However, r will leave a reminder to a ubetw of the buy candidate. W i determined a follow: from high-cla to low-cla buy candidate, the firt few that atify the following condition will belong to W : () the candidate currently favor the cla of r ; and () the aggregated out-bound bandwidth offer of the candidate in W i equal to (R R um ). Each (buy) candidate in W keep the reminder ; and when it current treaming eion i over, it will ue thi reminder to update it probability vector, a decribed in Section.. Note that a reminded upplying peer may not in the future erve exactly the ame requeting peer which left the reminder. Intead, we propoe reminder a a ditributed mechanim to realize differentiated and adaptive admiion For example, by querying a centralized directory erver a in Napter [], or by uing a ditributed lookup ervice uch a Chord []. control, baed on the current overall requet/upply ituation in the peer-to-peer treaming ytem. ffl If r i admitted, when the treaming eion i over, it will become a upplying peer. If r i rejected, it will backoff for at leat a period of T bkf before making the requet again. Furthermore, it backoff period will become T bkf E x bkf after the xth rejection. 5 erformance Study 5. Simulation Setup In thi ection, we how the excellent performance of rotocol DAC pp via extenive imulation reult. We imulate a peer-to-peer media treaming ytem with a total of 5, peer. Initially, there are only eed upplying peer, while the other 5, peer are requeting peer. Each eed upplying peer i a cla- peer, and it poee a copy of a popular video file. The how time of the video i 6 minute. The 5, requeting peer belong to clae,,, and, and their ditribution i %, %, %, and %, repectively. arameter in rotocol DAC pp are et a follow: M =8-each requeting peer probe 8 randomly elected candidate upplying peer; T out = min - each idle upplying peer elevate the admiion probabilitie of lower-cla requeting peer every minute; and T bkf =min; E bkf =-afterthe ith rejection, a requeting peer will backoff for Λ i minute before retry. For comparion, we alo imulate a non-differentiated admiion control protocol NDAC pp, in which the admiion probability vector of each upplying peer i alway < :; :; :; : >. NDAC pp alo have the ame value for parameter M;T bkf,ande bkf. We imulate a period of hour. During the firt 7 hour, the 5, peer make their firt treaming requet. We imulate four different arrival pattern of firt-time treaming requet: attern ha contant arrival; attern ha gradually increaing, then gradually decreaing arrival; attern ha burty arrival followed by lowerandcontant arrival; and attern ha periodic burty arrival with low and contant arrival between burt (detailed pecification are given in []). 5. Simulation Reult () Sytem capacity amplification We firt compare the ytem capacity amplification achieved by DAC pp and NDAC pp. Figure how the growth of the peerto-peer ytem capacity with the elape of time, under firt-time treaming requet arrival attern and 5. 5 Reult for other arrival pattern are decribed in []. roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

6 rotocol DAC pp achieve ignificantly fater ytem capacity growth than NDAC pp, epecially during the firt 7 hour when the requeting peer make their firt treaming requet. By the end of the -hour period, the ytem capacity achieved by DAC pp ha reached at leat 95% of the maximum capacity if all 5, peer become upplying peer. We alo oberve that after the firt 7 hour, the ytem capacity growth low down (under both protocol), becaue all requet are now retry requet, and no new requeting peer are coming. Total ytem capacity DAC_pp NDAC_pp 5(b). Even for the cla- requeting peer, thi i alo true except for the firt few hour. Thi obervation indicate that DAC pp benefit all clae of requeting peer with repect to admiion rate. Accumulative admiion rate (%) 8 6 cla cla cla 6 8 (a) Arrival attern, uing DAC pp Total ytem capacity (a) Arrival attern DAC_pp NDAC_pp Accumulative admiion rate (%) 8 6 cla cla cla 6 8 (b) Arrival attern, uing NDAC pp 6 8 Figure 5. er-cla accumulative requet admiion rate (b) Arrival attern Figure. Sytem capacity amplification uing DAC pp and NDAC pp () Requet admiion rate Figure 5 how the per-cla requet admiion rate (accumulative over time) achieved by DAC pp and NDAC pp, under arrival pattern. We firt oberve that by uing DAC pp, different clae of requeting peer have different admiion rate (Figure 5(a)): the higher the cla, the higher the admiion rate. On the contrary, rotocol NDAC pp doe not differentiate (Figure 5(b)), reulting in imilar admiion rate among all clae. Furthermore, we oberve that for requeting peer of clae,, and, their requet admiion rate in Figure 5(a) are contantly higher than thoe in Figure () Average buffering delay Similar to (), DAC pp alo achieve both differentiation and overall improvement, in the apect of buffering delay experienced by requeting peer of different clae. The reult are hown in Figure 6. Recall that the buffering delay of a peer-topeer treaming eion i equal to ffit multiplied by the number of participating upplying peer (Theorem ). On the other hand, in DAC pp, if a requeting peer i admitted, it i likely that the higher the cla it belong to, the higher the clae the participating upplying peer belong to, due to the rule each upplying peer determine it favored clae. We can then infer that in DAC pp, the higher the cla of an admitted requeting peer, the fewer the number of participating upplying peer, and therefore, the lower the buffering delay experienced by the requeting peer. Furthermore, the average buffering delay of each cla in Figure 6(a) i contantly lower than that in Figure 6(b). roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

7 Accumulative average buffering delay (y * delta t) Accumulative average buffering delay (y * delta t) cla cla cla 6 8 (a) Arrival attern, uing DAC pp cla cla cla 6 8 (b) Arrival attern, uing NDAC pp Avg. rejection attern attern Cla.77/.7.9/.5 Cla.9/.75.9/.6 Cla./.7.59/. Cla.5/.7.6/.6 Table. er-cla average number of rejection before admiion ( DAC pp / NDAC pp ) to dynamically adjut their favored clae of requeting peer, in repone to the requet arrival rate change (under arrival attern ). The y-axi repreent the lowet cla of requeting peer, favored by each cla of upplying peer. We oberve that for each cla of upplying peer, the degree of admiion differentiation change over time, roughly following the change in the (firt-time) requet arrival rate (recall that attern ha periodic burty arrival). More pecifically, the higher the cla of upplying peer, the more enitive they are to the change in requet arrival rate. Finally, when there are not new requet arrival, and the ytem capacity ha grown ignificantly, all clae of upplying peer relax their admiion preference to all clae of requeting peer, i.e. the lowet favored cla of requeting peer i, for all clae of upplying peer. Figure 6. er-cla accumulative average buffering delay (the actual delay i y Λ ffit) () Average waiting time Similar to () and (), DAC pp alo achieve both differentiation and overall improvement, in the apect of waiting time experienced by requeting peer of different clae. Table how the average (over the entire period of hour) number of rejection before admiion experienced by each cla of requeting peer, under arrival attern and. Given an average number of rejection x, the average waiting time can be computed a T bkf Λ E x bkf. Again, we oberve that the higher the cla of admitted requeting peer, the fewer the average number of rejection each of them experience. Furthermore, for each cla, the average number of rejection achieved by DAC pp i fewer than that achieved by NDAC pp. (5) Adaptivity of differentiation We now take a cloer look at DAC pp adaptivity of admiion differentiation, baed on the dynamic requet/upply ituation in the peerto-peer ytem. Recall that DAC pp ue the elevate-aftertimeout technique to relax the differentiation; while it ue the reminder technique to tighten the differentiation. In Figure 7, we how that upplying peer ue thee technique Lowet favored cla (average) 5 cla cla cla 6 8 (a) Arrival pattern Figure 7. Lowet cla of requeting peer, favored by each cla of upplying peer (non-accumulative, averaged every hour (6) Impact of protocol parameter on performance Finally, we tudy the impact of parameter M;T out,and E bkf on the performance of DAC pp: Figure 8 how the impact of M and T out on the ytem capacity amplification; while Figure 9 how the impact of the backoff exponential factor E bkf on the requet admiion rate, all under arrival attern. In each tudy, the parameter except the one being tudied remain the ame a before. roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

8 In Figure 8(a), the number of candidate upplying peer probed by a requeting peer i et to, 8, 6, and, repectively. The ytem capacity grow ignificantly lower when M =, becaue four candidate are too few to identify ufficient number of qualified upplying peer to erve the requeting peer. If we increae M, the ytem capacity will grow much fater. However, when M i greater than 8, the impact of M quickly decreae. Therefore, having alargem doe not improve the ytem capacity growth ignificantly. On the other hand, it may increae the probing overhead and traffic. In Figure 8(b), different time-out period to relax the admiion differentiation of an idle upplying peer i tried. The reult indicate that T out hould not be too hort. The explanation i: having a hort time-out period may make an idle upplying peer relax it admiion preference too oon to lower-cla requeting peer. Therefore, it may mi the chance to erve the one of higher clae, when both lowercla and higher-cla requeting peer are preent. Total ytem capacity 8 6 M = M = 8 M = 6 M = that exponential backoff of requeting peer doe not help to increae the requet admiion rate. On the contrary, the higher the E bkf, the lower the overall admiion rate. In fact, the contant backoff (E bkf = ) cheme achieve ignificantly higher admiion rate. Although not yet fully explored, one poible explanation i: The capacity of a peer-to-peer ytem i elf-growing intead of fixed. Therefore, a more aggreive retry policy may actually help to increae the ytem capacity fater, and hence improve the overall admiion rate. On the other hand, in a ytem with fixed capacity (uch a a traditional client-erver ytem), client may have to perform conervative backoff, in order to achieve a high overall admiion rate. Accumulative overall admiion rate (%) 8 6 E_bkf = E_bkf = E_bkf = E_bkf = 6 8 Figure 9. Impact of E bkf on overall requet admiion rate Total ytem capacity (a) Impact of M T_out = min T_out = min T_out = min T_out = 6 min T_out = min 6 8 (b) Impact of T out Figure 8. Impact of M and T out on ytem capacity amplification In Figure 9, the backoff exponential factor E bkf i et to,,, and, repectively. It i intereting to oberve In ummary, rotocol DAC pp achieve differentiation toward different clae of requeting peer - not only in their admiion probabilitie, but alo in the waiting time and buffering delay they experience. Moreover, the degree of differentiation i adaptive: it change according to the current requet/upply ituation. 6 Related Work eer-to-peer file haring ytem have gained great popularity in recent year. Repreentative peer-to-peer ytem on the Internet include Napter [], Gnutella [], and Freenet []. Thee ytem hare the ame goal of de-centralized data exchange and dynamic growth of ytem capacity. However, they differ in their data lookup/dicovery cheme. For example, Napter employ centralized directory erver, while Gnutella [] ue controlled query flooding. The data haring mode of mot current peer-to-peer ytem i the open-afterdownloading mode, not the play-while-downloading (or treaming) mode a tudied in thi paper (however, there are exception, uch a C-tar [] to be decribed later). In the pat two year, peer-to-peer ytem have alo attracted tremendou attention from the reearch community. roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE

9 Firt, there have been meaurement baed tudie of the exiting peer-to-peer ytem. In [], a detailed meaurement tudy of Napter and Gnutella i preented. The tudy reveal ignificant degree of heterogeneity in the peer bandwidth availability; and it ugget that future peer-topeer ytem mut have built-in incentive for peer to tell the truth about their bandwidth information. Thee obervation have partly motivated our peer-to-peer treaming model and olution in thi paper. Beide meaurement tudie of current peer-to-peer ytem, new peer-to-peer architecture have alo been propoed. Thee architecture focu on different apect of a fully ditributed and calable peer-to-peer ytem. For example, CAN [8], Chord [], and atry [9] are ditributed peer-to-peer lookup ervice, while AST [] and OceanStore [6] are peerto-peer peritent torage ervice. Our work on peer-topeer media treaming complement thee reult: on one hand, we do not tudy the problem of peer-to-peer data lookup and torage management; on the other hand, the exiting reult do not addre the two new problem in thi paper. Finally, everal cheme of multi-ource media treaming have been propoed. In [7], a ditributed video treaming ytem i preented, where each eion involve multiple replicated video erver. However, it doe not conider the problem of ytem capacity amplification, becaue it i till a client-erver ytem intead of a peerto-peer ytem. C-tar [] i a commercial multi-ource treaming ervice. Similar to our work, the capacity of the C-tar ditribution network grow over time. However, C-tar doe not differentiate between upplier of different out-bound bandwidth capability. In [5], an architecture called SpreadIt i propoed for treaming live media over a peer-to-peer network. It focue on the dynamic contruction of a multicat tree among peer requeting a live media. However, SpreadIt i not intended for the aynchronou treaming of tored media data. Alo it doe not deal with bandwidth heterogeneity and admiion differentiation. 7 Concluion eer-to-peer media treaming ytem are expected to become a popular a the peer-to-peer file haring ytem. In thi paper, we tudy two key problem ariing from peer-to-peer media treaming: the aignment of media data to multiple upplying peer involved in a peer-topeer treaming eion; and fat capacity amplification of the entire peer-to-peer treaming ytem. Our olution to the firt problem i Algorithm OTS pp, which compute optimal media data aignment for peer-to-peer treaming eion. Our olution to the econd problem i the fully ditributed DAC pp protocol. By differentiating between requeting peer according their clae, DAC pp () achieve fat ytem capacity amplification, () benefit all requeting peer in admiion rate, waiting time, and buffering delay, and () create an incentive for peer to offer their truly available out-bound bandwidth. Our extenive imulation reult demontrate the excellent performance of DAC pp. Reference [] C-tar. [] Gnutella. [] Napter. [] I. Clarke, O. Sandberg, B. Wiley, and T. Hong. Freenet: A Ditributed Anonymou Information Storage and Retrieval Sytem. roceeding of Workhop on Deign Iue in Anonymou and Unobervability, July. [5] H. Dehpande, M. Bawa, and H. Garcia-Molina. Streaming Live Media over a eer-to-eer Network. Stanford Databae Group Technical Report (-), Aug.. [6] J. Kubiatowicz, D. Bindel, Y. Chen, S. Czerwinki,. Eaton, D. Geel, R. Gummadi, S. Rhea, H. Weatherpoon, W. Weimer, C. Well, and B. Zhao. Oceantore: An Architecture for Global-State eritent Store. roceeding of ASLOS, Nov.. [7] T. Nguyen and A. Zakhor. Ditributed Video Streaming Over Internet. roceeding of SIE/ACM MMCN,Jan.. [8] S. Ratnaamy,. Franci, M. Handley, R. Karp, and S. Shenker. A Scalable Content-Addreable Network. roceeding of ACM SIGCOMM, Aug.. [9] A. Rowtron and. Druchel. atry: Scalable Ditributed Object Location and Routing for Large-Scale eer-to-eer Sytem. roceeding of IFI/ACM Middleware, Nov.. [] A. Rowtron and. Druchel. Storage Management and Caching in AST, a Large-Scale, eritent eer-to-eer Storage Utility. roceeding of ACM SOS, Oct.. [] S. Saroiu,. Gummadi, and S. Gribble. A Meaurement Study of eer-to-eer File Sharing Sytem. roceeding of SIE/ACM MMCN, Jan.. [] I. Stoica, R. Morri, D. Karger, F. Kaahoek, and H. Balakrihnan. Chord: A Scalable eer-to-eer Lookup Service for Internet Application. roceeding of ACM SIGCOMM, Aug.. [] D. Xu, M. Hefeeda, S. Hambruch, and B. Bhargava. On eer-to-eer Media Streaming. urdue Computer Science Technical Report, Apr.. [] B. Yang and H. Garcia-Molina. Comparing Hybrid eer-to- eer Sytem. roceeding of VLDB, Sept.. [5] B. Zhao, J. Kubiatowicz, and A. Joeph. Tapetry: An Infratructure for Fault-Reilient Wide-Area Location and Routing. UC Berkeley Computer Science Technical Report (CSD--), Apr.. roceeding of the nd International Conference on Ditributed Computing Sytem (ICDCS ) 6-697/ $7. IEEE