LocalTree: An Efficient Algorithm for Mobile Peer-to-Peer Live Streaming

Transcription

1 LocalTree: An Effcent Algorthm for Moble Peer-to-Peer Lve Streamng Bo Zhang S.-H. Gary Chan Department of Comp. Sc. & Eng. The Hong Kong Un. of Sc. & Tech. Clear Water Bay, Hong Kong Emal: {zhangbo, Gene Cheung Natonal Insttute of Informatcs Htotsubash, Chyoda-ku Tokyo, Japan Emal: Edward Y. Chang Drector of Research Google Research Bejng , Chna Emal: Abstract Moble peer-to-peer (P2P) technque s a promsng approach to provde lve vdeo streamng. In moble P2P streamng, moble devces relay ther receved packets n a mult-hop manner by means of broadcastng n a secondary channel (such as W-F or bluetooth). We consder the general scenaro that both wreless channel and nodes may not be relable,.e., packets may be lost n channel transmsson and a node may forward ts receved packets probablstcally (due to, for examples, ts power and ncentve concerns). Under ths scenaro, we study how to acheve broadcastng mnmzng the energy consumpton n the network whle meetng a certan stream qualty requrement (n terms of receved packet loss rate). We frst formulate the problem under study, and propose and study a dstrbuted algorthm called LocalTree whch addresses the problem. LocalTree takes advantage of stable clusters of users to optmze the constructon of ts streamng overlay. It combnes the strengths of both tree-based and mesh-based algorthms, and s smple and effectve. Smulaton results show that LocalTree performs smlar to tree-based algorthm for a rather stable network, and exhbts robustness smlar to unstructured algorthm f the network s dynamc. I. Introducton In tradtonal lve vdeo streamng over moble networks, each clent actvely pulls the stream from a content server over a wreless wde area network (WWAN). Ths approach suffers from the weakness that the base staton can easly become a bottleneck as the number of users ncreases. Moreover, t s fnancally expensve, because vdeo s usually of hgh btrate and servce provders often charge pullers based on data usage. Wth the advancement of moble devce capactes n terms of memory and processng, cooperatve moble peer-to-peer (P2P) streamng has been proposed to costeffectvely overcome the above problems. In such approach, peers wthn a geographcal area form an ad-hoc group va ther (free) secondary channel (such as W-F or Bluetooth). A node pulls packets of the vdeo stream from the base staton, and shares the receved packets Ths work was supported, n part, by the General Research Fund from the Research Grant Councl of the Hong Kong Specal Admnstratve Regon, Chna (611209), the Hong Kong Innovaton and Technology Fund (ITS/097/09FP), and Google Moble 2014 and Faculty Research Awards. wth others by means of broadcastng. In such a way, the streamng cost s amortzed among all the peers n the network. In ths paper, we consder vdeo broadcastng n a moble network. We consder the realstc scenaro that connectons between peers may not be relable, and hence relayed/broadcast packets may be lost n the wreless channel (due to, for example, channel fadng). Furthermore, a broadcast node may not relay all ts receved packets to ts neghbors, due to ts power or ncentve concerns. Under these condtons, broadcastng meetng a certan vdeo qualty requrement (n terms of loss rate) s an mportant ssue. We frst formulate the problem to address the above. For power conservaton and network lfetme extenson, our objectve s to mnmze the energy used n packet transmsson n the network. Gven the problem formulaton, we then propose an effcent and dstrbuted algorthm called LocalTree. In LocalTree, stable groups of users (e.g., connected users movng n smlar drecton wth each other) are dentfed n a dstrbuted manner, and then a local tree s constructed for each of such groups. (Indeed stable groups do exst n realty as observed n [1], [2], whch show that moble nodes often exhbt correlated physcal movements.) For unstable peers and for nter-group stream dstrbuton, LocalTree adapts unstructured algorthm to construct a streamng mesh. In LocalTree, peers are organzed nto two ters, the base ter and the tree ter. Peers are frst connected n a smple unstructured mesh n the base ter. Groups of relatvely stable nodes are then dentfed (by the node and lnk condtons). They are then connected followng a modfed tree constructon algorthm n the tree ter. Fgure 1 shows an example of a network resulted from usng LocalTree. Node 1 pulls the stream from the base staton. The connectons n the base ter (unstructured mesh) are ndcated by the dotted lnes. Nodes 2 and 3 are the roots of the two dsjont local trees C1 andc2 as ndcated by sold lnes at the tree ter. Whle most of the packets are dstrbuted along the trees n C1 and /11/$ IEEE

2 The paper s organzed as follows. We frst formulate the problem under study n Secton II. LocalTree, our proposed two-ter dstrbuted algorthm, s presented n Secton III. We present llustratve smulaton results and concluson n Sectons IV and V, respectvely. Fg. 1. Illustraton of two localtrees formed n a moble network. C2, n order to meet the QoS requrement, there are some supplementary packet transmssons as ndcated also by the dotted lnes. To the best of our knowledge, ths work s the frst to consder the more realstc scenaro of lnk and node relablty to acheve QoS for moble P2P streamng. Our approach s that the base ter provdes basc robustness, whle the tree ter performs further optmzaton. Our contrbutons are summarzed below: We formulate the problem under study, whch s lve stream dstrbuton to mnmze energy consumpton meetng a certan packet loss requrement under the condtons that connectons and nodes may not be relable. We propose LocalTree, a dstrbuted and effcent algorthm to address the problem. We have conducted extensve smulaton study on LocalTree. Our results show that LocalTree s ndeed effcent,.e., t meets the QoS requrement by achevng robustness smlar to mesh when the network s dynamc, and optmalty smlar to a global tree when the network s stable. In general prevous work on the overlay topology for moble P2P lve streamng can be dvded nto two categores: unstructured mesh and global tree. In unstructured mesh, each node makes ts own relay decson based on local nformaton [3]. They are not generally optmal n terms of energy consumpton. Global tree, on the other hand, seeks to buld an optmal spannng tree for stream dssemnaton [4]. However, t s not effcent for a network wth node and lnk dynamcs because of ts centralzed nature and ts complexty. Multcast Overlay Spannng Tree (MOST) connects Mult- Pont Relay (MPR) nodes selected by OLSR protocol to form an overlay spannng tree []. However ths s not energy-effcent, because there are many relays (MPRs), snce each node chooses ts own MPR set ndependently. Seldom has any moble P2P streamng work consdered how to acheve QoS under the (realstc) scenaro that both nodes and lnks may not be relable, as we consder here. II. Problem Formulaton A. Prelmnares We model the network as a connected graph G(V, E), where V s the set of all peers, and E s the set of the drected edges correspondng to the transmsson range between peers,.e., (, j) Eff peer can drectly reach peer j (we call peer j a neghbor of peer ). A sngle source node p pulls the content from the base staton and re-broadcasts t to the other peers, where p V. (the selecton of p s outsde the scope of ths paper; an approach has been dscussed n [3].) One-hop broadcast s assumed n ths paper. 1 Note that G may change over tme due to nodes jon/leave/falure or moblty. We defne the followng ndependent parameters to model the network, for, j V: b {0, 1} s the varable ndcatng whether node s a broadcaster or not,.e., b = 1 f node s a broadcaster, and 0 otherwse. w [0, 1] s the relay probablty that node relays ts receved packets through broadcastng. Note that even f a node s a selected broadcaster, t may not re-broadcast all ts receved packets due to ts concerns on, for examples, battery levels and ncentve. w hence captures the relablty of the node. c,j [0, 1] s the success probablty of packet transmsson on lnk (, j), where (, j) E. c,j hence captures the relablty of the connecton between and j. Both w and c,j hence are the stablty parameters of the network. ε [0, 1] s the packet loss rate at node. To meet a certan loss rate requrement, such loss rate should be less than a certan value ˆε,.e., ε ˆε, V. Note that our formulaton s suffcently general to capture node moblty effect through parameters w and c,j. For a hghly dynamc network, both w and c,j may be low. On the other hand, f the network s rather stable and nodes are wllng to share, both w and c,j can be close to 1. (Tradtonal works on global tree assume the specal case both that w and c,j are 1.) B. Problem Formulaton By defnton, we requre b p = w p = 1andε p = 0 for source node p. A packet s receved at node, where p. Let peer j be a neghbor of. Clearly, the event that the packet s successfully receved at j s gven by the occurrence of the followng ndependent events: 1) receves the 1 One-hop broadcast means delverng a packet to all drected neghbors as defned n E.

3 packet wthout error. Ths s wth probablty (1 ε ), whch s, gven meetng ts QoS requrement, at least (1 ˆε ); 2) s the broadcaster and relays the packet. Ths s of probablty b w ; and 3) the broadcast packet reaches j, whch s wth probablty c,j. Therefore, the loss rate at j from s at most 1 (1 ˆε)b w c,j. (1) As the neghbors are ndependent, to meet the QoS requrement we need, j V, ( ) 1 (1 ˆε)b w c,j ˆε. (2) Algorthm A Packet forwardng for the base ter Gven: Node receves packet k whch falls wthn ts buffermap wndow f B k B then Wat for a delay d t ; Update B k ; f B k B then Broadcast packet k; end f end f :(,j) E In order to mnmze the energy consumpton (and hence maxmzng the network lfetme), our optmzaton objectve s to select broadcasters n constructng a cooperatve network to mnmze the packet transmssons,.e., mn b w, (3) V subject to Equaton (2). Note that, due to QoS requrement, n our optmal soluton a peer j may have multple supplyng parents n the constructed overlay. Ths means that the resultant overlay may be a drected acyclc graph (DAG). III. LocalTree We now present n detal our dstrbuted algorthm LocalTree to address the problem. The proposed algorthm conssts of two parts, one for the base ter and the other for the tree ter. A. Algorthm for the Base Ter We frst construct an unstructured mesh as the base ter. The reason for ncludng an unstructured overlay s to accommodate tme-varyng changes and node/connecton unrelablty of the network topology. In the base ter, peers utlze only local neghbor nformaton to make ndependent dstrbuted decsons on whether to rebroadcast a packet or not. A peer perodcally broadcasts ts buffermap, whch s smply a btmap ndcatng whether t has receved the correspondng packet ID or not. As a result, a node knows the receved states of ts neghbors. For node, we defne the occupancy factor of packet k as the fracton of ts neghbors wth the packet,.e., B k # of neghbors have receved packet k =. (4) Total # of neghbors of Clearly node, once recevng a packet k, has to decde whether to broadcast t or not. In order to reduce the number of broadcasters (and hence energy consumpton), t may broadcast only f B k s below a certan threshold B. To avod collson, t should wat for a certan tme d t to see f any better node wll have broadcast the same packet. Obvously, a better node s the one wth lower occupancy factor of the packet, because ts broadcast can beneft hgher fracton of neghbors. Therefore, d t should be monotoncally ncreasng wth B k. An example of d t s d t = ηb k, where η s the maxmum delay for such operaton. We summarze n Algorthm A the detaled steps for the base ter accordng to the descrpton above. B. Algorthm for the Tree Ter The algorthm for the base ter constructs a mesh. It provdes a network for further optmzaton of the energy consumpton. Gven that some clusters of users may exhbt stablty (n terms of the nodes and ther connectons), the base-ter mesh s further optmzed by the tree-ter algorthm. Recall that there s a puller p n the network. A node contnuously updates ts hop dstance to p, whch can be done by takng the mnmum of the hopcounts to p of the most recent packets t receved. If t re-broadcasts a receved packet, t ncrements the hop count by 1 and pggybacks ths nformaton to the packet. A stable hop dstance of node sgnal that ts neghborhood may be stable and hence a local tree may be formed. In ths case, node clams tself to be the local tree root by sendng a LocalRoot message contanng ts ID, a unque localtree d, and a TTL controllng the broadcast scope and hence tree depth. In order to avod collsons and to favor the nodes closer to p to clam the tree root earler, the sendng of the LocalRoot message at node s delayed by a certan tme proportonal to s hop dstance. Durng ths delay, the followng condtons may occur: There s no other LocalRoot message receved. In ths case, node becomes the local root. There s another LocalRoot message receved. In ths case, node examnes the hop dstance of the node ID of ths message. If the hop dstance of s lower, t gnores the LocalRoot message; Otherwse, t needs to decde whether ts upstream forwarder j of the message s stable or not. Ths s done by examnng the loss rate due to j,.e., 1 (1 ɛ j )w j c j,. If t s lower than αˆɛ, for

4 6. LocalTree Energy used Fg. 2: Illustraton of local tree formaton some 0 α 1, j s consdered stable and the node jons the root-clamer. Otherwse, node gnore the LocalRoot message. Once node decdes to jon a root, t frst drops ts own pendng LocalRoot message (f one exsts). It then perodcally sends ts stablty parameters (w and c,j ) and lnk-state nformaton to the root. A root hence collects lnk-state nformaton and stablty parameters from all the joned nodes. It computes the local tree usng ths nformaton. The computed result s then dssemnated to all ts joned members. For the local tree computaton, we adapt a modfed greedy algorthm proposed n [6]. The proposed algorthm tres to greedly grow a mnmum connected domnatng set (MCDS) from a gven vertex of a graph. The modfcaton s that, nstead of smply markng each node as covered or uncovered, each node remans uncovered and keeps evaluatng Equaton (2) untl the QoS s met. In general, the algorthm results n a DAG. For the specal case where w and c,j are both equal to 1, the result reduces to a maxmum leaf tree. We llustrate n Fgure 2 how a local tree s formed. Frst, node R clams tself to be the local tree root, and advertses hs clam. Second, nearby nodes who wsh to jon reply wth ther stablty parameters and lnkstate nformaton,.e, drect neghbors of the sender. Thrd, node R computes an optmzed tree wth all the collected nformaton. Fnally, the computaton result s dstrbuted, and a local tree (possbly supplemented wth a mesh) s formed. IV. Illustratve Smulaton Results In our smulaton, a certan number of nodes are randomly placed n a square area (200x200 unts), each wth a certan transmsson range (0 unts). One of them s randomly assgned the puller. Unless otherwse stated, we use the followng baselne parameters: number of nodes = 2, B = 0.33, w U[0.7, 1.0], c,j U[0.8, 1.0], ˆε = 0.0, α = 0.7, and buffermap sze = 40. Because the performance depends on the product of w c,j,we let Γ,j = 1 w c,j, whch can be nterpreted as the packet loss probablty from to j. Obvously the network s not relable for hgh value of E[Γ,j ] Network Sze Fg. 3: Energy cost vs. network sze. We evaluate usng the followng metrcs: Energy cost for stream dstrbuton, whch s measured by the average of b w over all smulaton V runs. Number of broadcasters, whch s the mean number of nodes relayng packet over all smulaton runs. Packet loss rate, whch s the rato between the number of lost packets and the total number of transmtted packets, averaged over all nodes. Note that some packets may not be receved n tme before the buffermap wndow shfts to a new poston. In ths case these packets mss ther playback deadlne and are consdered lost. We compare LocalTree wth two other algorthms: a modfed global tree algorthm where lnk-state nformaton s globally for each node to compute a global optmal DAG, and a generc unstructured mesh algorthm usng base-ter algorthm. Fgure 3 shows the energy cost for dfferent network sze. For ths smulaton both w and c,j are set to be 1,.e., the energy cost s equal to the number of broadcasters used. As we can see, for the global tree algorthm, the consumed energy does not senstvely ncrease wth the number of nodes. Ths s because, gven a fxed sze of area, more nodes do not necessarly mean more broadcasters. However, for the unstructured algorthm, ths cost ncreases wth larger network sze. Ths s because more nodes are lkely to trgger unnecessary and neffcent relays. LocalTree performs much better than the unstructured algorthm, due to ts optmzaton by the proposed tree ter. We plot n Fgure 4 the numbers of broadcasters versus network unrelablty gven by E[Γ,j ]. Global tree algorthm chooses the least number of broadcasters n stable networks. However n nstable networks (.e, when E[Γ,j ] ncreases), ths number rses quckly. Ths s because each node s more lkely to select be a broadcaster. Increase n transmsson unrelablty only margnally

5 Number of broadcasters Local Tree Number of broadcasters E[ Γ ],j Fg. 4: Total number of broadcasters occurred vs. E[Γ,j ] E[w ] Fg. 6: Total number of broadcasters vs. E[w ]. Loss rate 10% % LocalTree E[Γ ],j Fg. : Packet loss rate vs. E[Γ,j ]. affects unstructured algorthm, because a broadcaster s more lkely to cover more nodes. LocalTree combnes the strengths of both schemes,.e., t exhbts smlar optmalty as global tree n stable network, whle achevng smlar performance as a mesh n nstable network. Fgure llustrates packet loss rates versus E[Γ,j ]. Loss rate of global tree algorthm s the lowest n stable network, but rses drastcally as the nodes/connectons becomes unrelable. LocalTree scheme performs smlarly to global tree n stable network, and quckly converges to unstructured approach n nstable network. Fgure 6 shows the number of broadcasters versus E[w ], where w are dstrbuted unformly wth upper bound 1. c,j for all lnks are fxed to 1. The number of broadcasters decreases wth w, because nodes are more lkely to share all ts receved packets. Such drop s qute sgnfcant ntally, showng the mportance of node sharng n reducng the number of broadcasters n the network. V. Concluson In ths paper, we have consdered the problem of vdeo broadcastng n moble P2P network, under the (realstc) scenaro that both nodes and lnks may not be relable. The objectve s acheve broadcastng meetng a certan loss requrement. We have formulated the problem, and presented LocalTree, a scalable and energy-effcent algorthm, to address the problem. The algorthm operates wth two ters. In the base ter, moble nodes follow a fully dstrbuted and ndependent relay algorthm, whle n the tree ter, relatvely stable groups are dentfed and an optmzed local DAG s computed. Wth the two-ter operaton, LocalTree s able to adapt dfferent network dynamcs. We have conducted extensve smulaton study on LocalTree performance. Smulaton results show that Local- Tree acheves optmalty smlar to tree-based algorthm for a rather stable network, and exhbts robustness smlar to unstructured algorthm f the network s dynamc. References [1] K. Blakely and B. Lowekamp, A structured group moblty model for the smulaton of moble ad hoc networks, n MobWac 04: Proceedngs of the second nternatonal workshop on Moblty management & wreless access protocols. New York, NY, USA: ACM, October 2004, pp [2] D. Cullo, V. Martna, M. Garetto, and E. Leonard, Impact of correlated moblty on delay-throughput performance n moble adhoc networks, n INFOCOM 10: Proceedngs of the 29th conference on Informaton communcatons. Pscataway, NJ, USA: IEEE Press, 2010, pp [3] M.-F. Leung and S.-H. G. Chan, Broadcast-based peer-to-peer collaboratve vdeo streamng among mobles, Broadcastng, IEEE Transactons on, vol. 3, no. 1, pp , march [4] O. Badarneh, M. Kadoch, and A. ElHakeem, Multlayered vdeo multple trees multcast algorthms for heterogeneous wreless ad hoc networks, n Network Computng and Applcatons, NCA 08. Seventh IEEE Internatonal Symposum on, jul. 2008, pp [] G. Rodolaks, A. M. Nam, and A. Laout, Multcast overlay spannng tree protocol for ad hoc networks, n WWIC 07: Proceedngs of the th nternatonal conference on Wred/Wreless Internet Communcatons. Berln, Hedelberg: Sprnger-Verlag, 2007, pp [6] S. Guha and S. Khuller, Approxmaton algorthms for connected domnatng sets, Algorthmca, vol. 20, pp , 1998, /PL [Onlne]. Avalable: