ALM-FastReplica: Optimizing the Reliable Distribution of Large Files within CDNs

Transcription

1 ALM-FastReplca: Optzng the Relable Dstrbuton of Large Fles wthn CDNs Ludla Cherkasova Internet Systes and Storage Laboratory HP Laboratores Palo Alto HPL Aprl 4, 005* E-al: CDNs, edge servers, orgn server, relablty, large fle replcaton, content delvery, perforance analyss, scalablty Content Delvery Networks (CDNs) provde an effcent support for servng http and streang eda content whle nzng the network pact of content delvery as well as overcong the server overload proble. For servng the large docuents, software download packages and eda fles, there s an addtonal proble of the orgnal content dstrbuton across the edge servers n CDN nfrastructure. We propose an algorth, called ALM-FastReplca, for optzng replcaton of large fles across the edge servers n CDNs. The orgnal fle s parttoned nto k subfles, and each sub fle s replcated va a correspondngly constructed ultcast tree. Nodes fro the dfferent ultcast trees use addtonal cross nodes connectons to exchange ther correspondng sub fles such that each node eventually receves an entre fle. Ths new replcaton ethod sgnfcantly reduces fle replcaton te, up to 5-5 tes copared to the tradtonal uncast (or pont-to-pont) schea. Snce a sngle node falure n the ultcast tree durng the fle dstrbuton ay pact the fle delvery to a sgnfcant nuber of nodes, t s portant to desgn an algorth whch s able to deal wth node falures. We augent ALM-FastReplca wth an effcent relablty echans, that can deal wth node falures by akng local repar decsons wthn a partcular replcaton group of nodes. Under the proposed algorth, the load of the faled node s shared aong the nodes of the correspondng replcaton group, akng the perforance degradaton gradual. * Internal Accesson Date Only A slghtly shorter paper verson wll be publshed n Proceedngs of the 0th IEEE Syposu on Coputers and Councatons (ISCC'05), 7-30 June 005, La Manga del Mar Menor, Cartagena, Span Approved for External Publcaton Copyrght 005 Hewlett-Packard Developent Copany, L.P.

2 ALM-FastReplca: Optzng the Relable Dstrbuton of Large Fles wthn CDNs Ludla Cherkasova Hewlett-Packard Laboratores 50 Page Mll Road, Palo Alto, CA Abstract Content Delvery Networks (CDNs) provde an effcent support for servng http and streang eda content whle nzng the network pact of content delvery as well as overcong the server overload proble. For servng the large docuents, software download packages and eda fles, there s an addtonal proble of the orgnal content dstrbuton across the edge servers n CDN nfrastructure. We propose an algorth, called ALM-FastReplca, for optzng replcaton of large fles across the edge servers n CDNs. The orgnal fle s parttoned nto k subfles, and each subfle s replcated va a correspondngly constructed ultcast tree. Nodes fro the dfferent ultcast trees use addtonal crossnodes connectons to exchange ther correspondng subfles such that each node eventually receves an entre fle. Ths new replcaton ethod sgnfcantly reduces fle replcaton te, up to 5-5 tes copared to the tradtonal uncast (or pont-to-pont) schea. Snce a sngle node falure n the ultcast tree durng the fle dstrbuton ay pact the fle delvery to a sgnfcant nuber of nodes, t s portant to desgn an algorth whch s able to deal wth node falures. We augent ALM-FastReplca wth an effcent relablty echans, that can deal wth node falures by akng local repar decsons wthn a partcular replcaton group of nodes. Under the proposed algorth, the load of the faled node s shared aong the nodes of the correspondng replcaton group, akng the perforance degradaton gradual. Introducton The aount of streang eda content onlne ncrease everyday. Maor news copanes are aong the largest producers of streang eda content. Sports and usc vdeo stes add ther lon share to popular eda content. Vdeo clps of portant events (SuperBowl, Olypcs, Electon 000, Electon 004) drew unprecedented aounts of traffc. Streang eda can easly becoe a bandwdth hog that overwhels networks and pacts ssoncrtcal applcatons unless the specal content dstrbuton technques are used. Fast and relable dstrbuton of data fro a sngle source to a large nuber of recevers located across the Internet has got a lot of attenton durng last decade. Content Delvery Networks (CDNs) (e.g. Akaa), eploy a dedcated set of achnes (edge servers) to dstrbute content to clents on behalf of the orgn server. In ths soluton, content s replcated at the edge servers (typcally through cachng) closer to the end user, thereby provng content delvery latency whle reducng WAN traffc. For eda fles, t s desrable to replcate these fles at A slghtly shorter paper verson wll be publshed n Proceedng of the 0th IEEE Syposu on Coputers and Councatons (ISCC 005). edge servers n advance, usng the so-called push odel. For large eda fles t s a challengng, resource-ntensve proble, e.g. eda fles can requre sgnfcant bandwdth and download te due to ther large szes: 0 n eda fle encoded at Mbt/s results n a fle of 50 MBytes. Whle transferrng a large fle wth ndvdual pont-to-pont connectons fro an orgnal server can be a vable soluton n the case of lted nuber of rror servers (tenths of servers), ths ethod does not scale when the content needs to be replcated across a CDN wth thousands of geographcally dstrbuted achnes. In our recent work [6], we proposed a new algorth FastReplca for replcatng large fles n the Internet envronent. There are a few basc deas exploted n FastReplca. In order to replcate a large fle aong k nodes (k s n the range of 0-30 nodes), the orgnal fle s parttoned nto k subfles of equal sze and each subfle s transferred to a dfferent node n the group. After that, each node sends ts subfle to the reanng nodes n the group, and the replcaton process s teratvely repeated by takng the nodes wth already replcated fle as the orgn nodes. A fle transfer n FastReplca follows a store-and-forward ode,.e. only after a subfle s receved by a node, ths node starts sendng the correspondng subfle to the subsequent nodes. In ths store-and-forward odel, when the node falure occurs, t ay only pact the content delvery for nodes n the local replcaton group. The recovery algorth s soewhat natural and straghtforward: t only nvolves the nodes wthn the correspondng replcaton group. Snce the natve IP ultcast has not receved wdespread deployent, any ndustral and research efforts shfted to nvestgatng and deployng the applcaton level ultcast, where nodes across the Internet act as nteredate routers to effcently dstrbute content along a predefned esh or tree. A growng nuber of researchers [4, 5, 8,, 4, 9] have advocated ths alternatve approach, where all ultcast related functonalty, ncludng group anageent and packet replcaton, s pleented at end systes. In ths work, we propose to cobne FastReplca deas wth applcaton level ultcast (ALM) for optzng the replcaton of the large fles wthn CDN. We call ths

3 ethod ALM-FastReplca. The orgnal fle s parttoned nto k subfles of equal sze, and each subfle s replcated va a correspondngly constructed ultcast tree usng a fastforward ode,.e. each node starts a fle transfer to the next nodes edately after recevng the frst packet of the correspondng subfle. Nodes fro the dfferent ultcast trees use addtonal cross-nodes connectons to exchange ther correspondng subfles such that each node eventually receves an entre fle. Ths new replcaton ethod sgnfcantly reduces fle replcaton te, up to 5-5 tes copared to the tradtonal ultple uncast ethod. In ultple uncast ethod, the whole fle s transferred n a pont-topont fashon to the frst group of nodes, and after that each node fro the frst group ntates a fle replcaton process to the next group, etc. ALM-FastReplca explots dverse set of Internet paths wthn the replcaton groups to exchange the copleentary content between these nodes. Multple Descrpton Codng (MDC) codes a eda strea nto ultple copleentary descrptons [8, 0, 5]. These descrptons have the property that f ether descrpton s receved t can be used to decode the baselne qualty vdeo, and ultple descrptons can be used to decode proved qualty vdeo. We show how to apply the ALM-FastReplca algorth for an effcent replcaton of ulteda fles encoded wth Multple Descrpton Codng (MDC). Ths new replcaton ethod sgnfcantly reduces the dstrbuton te of MDC-encoded fles to the edge servers n CDN. Under the store-and-forward odel, the node falure ay only pact the content delvery n the local replcaton group. However, under the fast-forward odel, a sngle node falure n a ultcast tree durng the fle dstrbuton ay pact the fle delvery to a sgnfcant nuber of nodes. Thus, t s portant to desgn an algorth whch s able to deal wth node falures. It s a non-trval task because each node n the ultcast tree has a wde fan-out (0 or ore connectons out). A soluton that s based on a straghtforward approach, where the tree below the faled node N faled s reconnected to ts predecessor-node N prev, wll suffer fro poor perforance. Under such a soluton, node N prev becoes a severe bottleneck n the whole syste snce t needs to handle the doubled nuber of connectons. We augent ALM-FastReplca wth an effcent relablty echans, that can deal wth node falures by akng local repar decsons wthn a partcular replcaton group of nodes. Under the proposed algorth, the load of the faled node s shared aong the nodes of the correspondng replcaton group, akng the perforance degradaton practcally unnotceable. The rest of the paper explans these deas n ore detal. Related Work Durng few last years, fast and relable dstrbuton of data fro a sngle source to a large nuber of recevers located across the Internet has been a topc of any research studes. One ethod conssts of accessng ultple servers n parallel to reduce downloadng te or to acheve fault tolerance. The authors n [6], deonstrate proved response te observed by the clent for a large fle download through the dynac parallel access schea to replcated content at rror servers. Dgtal Fountan [] apples Tornado codes to acheve a relable data download. In ther subsequent work [3], the download tes are reduced by havng clent receve a Tornado encoded fle fro ultple rror servers. The target applcaton under ther approach s bulk data transfer. In recent years, overlay networks have becoe an effectve alternatve to IP ultcast for effcent pont to ultpont councaton across the Internet. An nterestng extenson for the end-syste ultcast s ntroduced n [4], where authors, nstead of usng the end systes as routers forwardng the packets, propose that the end-systes do actvely collaborate n nfored anner to prove the perforance of large fle dstrbuton. The an dea s to overcoe the ltaton of the tradtonal servce odels based on tree topologes where the transfer rate to the clent s defned by the bandwdth of the bottleneck lnk of the path fro the server. The authors propose to use addtonal cross-connectons between the end-systes to exchange the copleentary content these nodes have already receved. However, the authors do not explctly partton the orgnal docuent nto a set of copleentary subfles for speedng up the fle delvery. Ther ethod reles on the assupton that any gven par of end-systes has not receved exactly the sae content, and these cross-connectons between the end-systes can be used to reconcle the dfferences n receved content n order to reduce the total transfer te. Other work aed on dstrbuton large fles uses custo overlay protocols n peer-to-peer based fashon. Bt- Torrent [7] s the ost heavly deployed: t pleents peer-to-peer network wth centralzed trackng of avalable clents. However, ths syste has a low reslence to node falures, and could be sgnfcantly proved n ths drecton. Aong acadec ntatves are Bullet [] whch uses a self-organzng overlay esh, and SpltStrea [5] whch uses a collecton of superposed ultcast trees reseblng ALM-FastReplca constructon. SpltStrea supports the streang eda ode, and whle tryng to optze the streang qualty of delvered content, t does not attept to nze the download te. The goal s qute dfferent: t attepts to support the streang of lve eda n peer-topeer fashon. Whle CDNs were orgnally ntended for statc web content, they have been appled for delvery of streang e-

4 da as well. Delverng streang eda over the Internet s challengng due to a nuber of factors such as hgh bt rates, delay and loss senstvty. Most of the current work n ths drecton concentrates on how to prove the eda delvery fro the edge servers (or rror servers) to the end clents. In order to prove streang eda qualty, the latest work n ths drecton [, 3] proposes streang vdeo fro ultple edge servers (or rror stes), and n partcular, by cobnng the benefts of ultple descrpton codng (MDC) wth Internet path dversty. MDC codes a eda strea nto ultple copleentary descrptons. These descrptons have the property that f ether descrpton s receved t can be used to decode the baselne qualty vdeo, and ultple descrptons can be used to decode proved qualty vdeo. One of the basc assuptons n the research papers referred to above s that the orgnal content s already replcated across the edge (rror) servers. The goal of our paper s to address the content dstrbuton wthn ths nfrastructure (and not to the clents of ths nfrastructure). In our work, we propose a ethod to effcently replcate the content (represented by large fles) fro a sngle source to a large nuber of servers n a scalable and relable way. We explot deas of parttonng the orgnal fle and usng dverse Internet paths between the recpent nodes to speedup the dstrbuton of an orgnal large fle over Internet. 3 ALM-FastReplca Let N 0 be a node whch has an orgnal fle F. and letsze(f ) denote the sze of fle n bytes. The proble conssts n replcatng fle F across nodes N,..., N n whle nzng the overall (both average and axu) replcaton te. Orgnal FastReplca algorth [6] works as follows. Let k be a nuber of network connectons chosen for concurrent transfers between a sngle node and ultple recevng nodes. The orgnal set of nodes s parttoned nto replcaton groups, each consstng of k nodes. Fle s dvded n k equal subsequent subfles: F,..., F k. The replcaton process fro N 0 to a frst replcaton group s called FastReplca n the Sall, and t proceeds as follows. Step : Dstrbuton Step. The orgnator node N 0 opens k concurrent network connectons to nodes N,..., of the frst replcaton group, and sends to each recpent node N subfle ( k). Step : Collecton Step. After recevng fle, node N opens k concurrent network connectons to reanng nodes n the group and sends subfle to the (.e. a fle transfer s done va a store-and-forward ode). At ths step, each node N has the followng set of connectons: there are k outgong connectons fro node N : one connecton to each node N ( ) for sendng the correspondng subfle to node N. there are k ncong connectons to node N : one connecton fro each node N ( ) for sendng the correspondng subfle F to node N. At the end of the collecton step, each node receves all subfles F,..., F k coprsng the entre orgnal fle F. After that FastReplca n the sall s appled teratvely, usng nodes N,..., as the orgn nodes to replcate fle F further to new groups of nodes. Exaple. Let k = 0. Inthree algorth teratons (0 0 0), the orgnal fle can be replcated aong 000 nodes. At each teraton, the replcaton process follows FastReplca n the sall,.e. the teraton conssts of steps, each used for transferrng the k -th porton of the orgnal fle F. In ths work, we propose to cobne FastReplca schea wth applcaton level ultcast n the followng way. Let k be a nuber of network connectons chosen for concurrent transfers between a sngle node and ultple recevng nodes. The orgnal set of nodes s parttoned nto replcaton groups, each consstng of k nodes. Let G,..., G k be the correspondng replcaton groups. Let be a targeted nuber of groups coprsng a ultcast tree (accordngly to prevous studes [9], a reasonable value of ay vary n a range of several 0s). Then replcaton groups G,..., G k are arranged n the specal ultcast trees ˆM,M,..., M, each consstng of (or less) groups, where ˆM s called a prary ultcast tree, and M,..., M are called the secondary ultcast trees. To acheve the best perforance results, the values and should be slar: ths wll lead to well-balanced ultcast trees. Fle s dvded n k equal subsequent subfles: F,..., F k where Sze( )= Sze(F ) bytes for each k. k A hgh-level schea of ALM-FastReplca s shown n Fgure. Frst, ALM-FastReplca algorth replcates fle F va the prary ultcast tree ˆM. Once groups Ĝ,..., Ĝ, coprsng the prary ultcast tree ˆM, receve subfles F,..., F k copletely they ntate (ndependently fro each other) transfers of subfles F,..., F k to the secondary ultcast trees M,..., M. Now, we descrbe ALM-FastReplca n ore detal. ). Startng the prary ultcast tree ˆM. Let groups Ĝ,..., Ĝ coprse the prary ultcast tree ˆM. Let Ĝ = {N,..., Nk },. Dstrbuton step. Orgnator node N 0 opens k concurrent network connectons to nodes N,..., of Dependng on the nuber of nodes n the orgnal replcaton set, ALM-FastReplca ay operate by usng only a prary ultcast tree, or ay requre to eploy two-level schea as descrbed n the paper.

5 N 0 G (dstr) G (coll) G G G G Secondary ultcast trees G M Prary ultcast tree G M M Fgure : ALM-FastReplca: a hgh-level, overall schea. group Ĝ, and starts sendng subfle to the correspondng recpent node N ( k). Ths step s represented by box Ĝ(dstr) n Fgure. Collecton step. In group Ĝ, each node N after recevng frst packet of fle, edately starts sendng the fle to the rest of the nodes n group Ĝ (.e. a fle transfer s done va a fast-forward ode). In ths way, each node n group Ĝ wll be recevng all subfles F,..., F k of orgnal fle F. Ths step s represented by box Ĝ(coll) n Fgure. Group councaton step. Councaton aong groups Ĝ and Ĝ follows a dfferent fle exchange protocol defnng another typcal councaton pattern actvely used n the ALM-FastReplca algorth. We denote ths step as a group councaton step. Each node N of group Ĝ after recevng frst packet of fle, edately starts sendng the fle to node N of group Ĝ. After that nodes of group Ĝ perfor the collecton step descrbed above,.e. each node N opens k concurrent network connectons to the rest of the nodes of group Ĝ for transferrng ts subfle. In ths way, each node of group Ĝ wll be recevng all subfles F,..., F k of the orgnal fle F. The councatons aong the nodes n groups Ĝ and Ĝ are shown n ore detal n Fgure. N N N k G F F F k N N N G k F F F k G Slarly, group Ĝ starts councatons wth group Ĝ3 usng group councaton step edately after any node N receves the frst packet of the correspondng fle. Ths replcaton procedure contnues unrollng through the set of correspondng groups n ultcast tree ˆM shown n Fgure 3. In fact, as t s shown n Fgure 3, the ultcast tree ˆM s a collecton of k ultcast sub-trees ˆM F, ˆM F,..., ˆM Fk, where each such sub-tree ˆM F s replcatng the correspondng subfle. At the sae te, nodes fro these dfferent ultcast sub-trees use addtonal cross-connectons be- N N N G k Fgure : Group councaton step between the nodes n groups Ĝ and Ĝ. N F F F k N M F F N M F F k N k N N N k G F F k N N Nk G N N Nk F N F Prary Multcast Tree M N G - N 0 F k N N Multcast Sub-Trees F k k M Fk G G G Dstrbuton Step Collecton Step Group Councaton Step Group Councaton Step Fgure 3: Councatons aong the groups of prary ultcast tree ˆM. tween ther nodes (as shown n Fgure 3) to exchange ther copleentary subfles. ). Startng the secondary ultcast trees M,..., M. When node N of Ĝ receves the entre subfle F n the prary ultcast tree ˆM, then t starts transferrng fle F to group G of the secondary tree M usng the group councaton step. Thus, each group Ĝ ( ) of the prary ultcast tree ˆM ntates the replcaton process of subfles F,..., F k to the next, secondary ultcast tree M ={G,..., G } (see Fgure ). These transfers are asynchronous wthn the group Ĝ ={N,..., }. Ths copletes the descrpton of ALM-FastReplca.

6 4 ALM-FastReplca and MDC Multple Descrpton Codng (MDC) s source codng for ultple channels such that a decoder whch receves an arbtrary subset of the channels ay produce a useful reconstructon [8, 0, 5]. MDC codes a eda strea nto ultple copleentary descrptons. These descrptons have the property that f ether descrpton s receved t can be used to decode the baselne qualty vdeo, and ultple descrptons can be used to decode proved qualty vdeo. Ths s n contrast to conventonal vdeo coders (e.g. MPEG-//4, H.6/3, Mcrosofts and Real Network s propretary coders)., whch produce a sngle strea that does not have these MD propertes; we refer to these ethods as sngle descrpton codng (SDC). MDC refers to a for of copresson where a sgnal s coded nto a nuber of separated bt streas, where the ultple btstreas are referred to as ultple descrptons (MD). These ultple descrptons provde two portant propertes. Frst, each descrpton can be decoded ndependently to gve a usable reproducton of the orgnal sgnal. Second, the ultple descrptons contan copleentary nforaton so that the qualty of the decoded sgnal proved wth the nuber of descrptons that are correctly receved. An portant pont s that each descrpton or MD btstrea s ndependent of each other and s typcally of roughly equal portance. Layered or scalable approaches [7] essentally prortze data and thereby support ntellgent dscardng of the data (the enhanceent data can be lost or dscarded whle stll antanng usable vdeo). However the base-layer btstrea s crtcally portant f t s lost then the other btstrea(s) are useless. MD codng overcoes ths proble by allowng useful reproducton of the sgnal when any descrpton s receved, and wth ncreasng qualty when ore descrptons are receved. In order to prove streang eda qualty, the latest work n ths drecton proposes streang vdeo fro ultple edge servers (or rror stes), and n partcular, by cobnng the benefts of ultple descrpton codng (MDC) [] wth Internet path dversty. Under ths approach, the dfferent descrptons are explctly sent over dfferent routes to a clent. Path dversty explots the fact that whle any network lk ay suffer fro packet loss, there s a uch saller chance that two (or ore) network paths ay suffer sultaneously fro losses. In the orgnal ALM-FastReplca algorth, we partton the orgnal fle n n equal subsequent subfles and apply ALM-FastReplca to replcate the. Ths part of the algorth can be odfed accordngly to the nature of the fle. For exaple, for a eda fle encoded wth MDC, dfferent descrptons can be treated as subfles, and ALM- FastReplca can be appled to replcate the. Ths wll sgnfcantly prove the replcaton te. Takng nto account the nature of MDC (.e. that ether descrpton receved by the recpent node can be used to decode the baselne qualty vdeo), the edge servers n CDN ay start servng the correspondng eda fle once any of ts descrptons s receved. 5 Relable ALM-FastReplca In ths Secton, we extend the ALM-FastReplca algorth to be able to deal wth node falures. Basc algorth presented n Secton 3 s senstve to node falures. Under the fast-forward odel, a sngle node falure n a ultcast tree durng the fle dstrbuton ay pact the fle delvery to a sgnfcant nuber of nodes. For exaple, f node N (fro group Ĝ n ˆM) fals durng the ether dstrbuton or collecton steps descrbed n Secton 3 then ths event ay pact all nodes N,..., n the group Ĝ because each node depends on node N to replcate subfle F. A slar stuaton occurs f a falure of node N happens durng the group councaton step between groups Ĝ and Ĝ: ths falure ay pact all the nodes n the dependent subtree because the nodes n ths subtree should receve subfle fro node N. Thus, t s portant to desgn an algorth whch s able to deal wth node falures. It s a non-trval task because each node n the ultcast tree has a wde fan-out (0 or ore connectons out). Usng a nave approach where the tree below the faled node N faled s reconnected to ts predecessor N prev wll not be a well perforng soluton. Under such a soluton, N prev becoes a severe bottleneck n the whole syste snce t needs to handle the doubled nuber of connectons. Relable ALM-FastReplca proposed below effcently deals wth node falures by akng the local repar decson wthn the partcular group of nodes. It keeps the an structure of the ALM-FastReplca algorth practcally unchanged whle addng the desred property of reslence to node falures. Let us call group Ĝ an ntal group. We wll pay a specal attenton to group Ĝ and node falures n t, because t s the ost vulnerable and crucal group of nodes. Nodes of Ĝ are called ntal nodes. We wll call the rest of the groups subsequent groups, and ther nodes - subsequent nodes. There are several councaton patterns n whch node N ght be nvolved at oent of ts falure: If node N s an ntal node,.e. N = N, t ay fal: durng the dstrbuton step, when node N 0 s transferrng fle to N. Only node N 0 has subfle fle at ths pont. Snce node N s faled durng the fle transfer fro N 0 to N, node N 0 s aware of node N falure.

7 durng the collecton step, when node N has receved frst packet of fle and started to transfer t to the reanng nodes n group Ĝ. durng the group councaton step n the frst ultcast tree ˆM,.e. when node N s transferrng fle to the nodes of group Ĝ. durng the group councaton step to the next ultcast tree M,.e. when node N s transferrng fle to the nodes of group G as shown n Fgure. The crucal dfference of the node falure at ths step s that any node n group Ĝ already has receved subfle. If node N s a subsequent node, t ay fal durng the group councaton step when node N of group G s transferrng fle to the nodes of the next group G +. In the relable ALM-FastReplca algorth, node N of group G sends heart-beat essages to node N of group G. These heart-beat essages are augented wth addtonal nforaton on the correspondng algorth step and the current replcaton lst of nodes correspondng to ths step. Ths nforaton s necessary because of the asynchronous nature of the ALM-FastReplca algorth. For exaple, whle soe of the nodes of group Ĝ can perfor a fle transfer n the prary ultcast tree ˆM (.e. they are stll replcatng ther subfles to the correspondng nodes of group Ĝ), soe faster nodes of the sae group Ĝ ght already have started a fle transfer to the next ultcast tree M (.e. they are replcatng ther subfles to the correspondng nodes of group G as shown n Fgure ). Thus n case of node falure, t s portant to know: whch partcular node n the group has faled; whether the node s ntal or not; whether the correspondng step of the algorth s the dstrbuton, collecton,orgroup councaton step; whch ultcast tree, group and set of recevng nodes are pacted as a result of ths falure. There are dfferent repar procedures dependng on the crcustances under whch the node falure occurred: The ntal node N of group Ĝ fals durng dstrbuton, collecton, orgroup councaton step n the prary ultcast tree ˆM. In ths case, node N 0 s ether aware of the node N falure or receves a essage about ths fro the heartbeat group Ĝ. Snce node N 0 s the root of the overall replcaton procedure, to avod a sngle pont of falure t has a buddy-node ˆN 0 wth rrored nforaton and data. Node N 0 sends a essage to ˆN 0 to open k network connectons to the rest of the nodes n group Ĝ for sendng the ssng fle to each node n the group. Node N fals durng the group councaton step, where node N s ether a subsequent node, or an ntal node perforng the group councaton step to secondary ultcast tree. Fgure 4 depcts a falure of node N n group G whle t was transferrng fle to the rest of the nodes n group G and further to node N + of group G + at the group councaton step of the algorth. - N N - N N N N + N N + N - Faled Node N N F - G Faled Node G - G N + + G + Group Councaton Step Fgure 4: Node N falure durng group councaton step. The repar procedure for node N s shown n Fgure 5. Once node N realzed that node N has faled, node N conveys ths nforaton to the rest of the nodes n group G. The nodes n group G share the addtonal load of transferrng fle to the nodes of group G \ N as shown n Fgure 5. Node N transfers fle to node N + of group G +. - N N - - N - G - Faled Node N N N + N N N + N Faled Node N G G N + + G + Group Councato Step Fgure 5: Repar procedure for node N faled durng group councaton step. The dstrbuton lsts are the sae for all the nodes of group G, and after the repar step, the ALM- FastReplca algorth proceeds n the usual way for the entre subtree orgnated n group G. The nodes n group G contnue to share the addtonal load

8 of transferrng fle to the next, secondary ultcast tree orgnated at a group G or untl node N s repared. Snce the load of faled node N s shared aong k nodes of group G, the perforance degradaton s gradual for the repared porton of the dstrbuton tree. 6 Perforance Evaluaton Let Te (F ) denote the transfer te of fle F fro the orgnal node N 0 to node N as easured at node N.We use transfer te and replcaton te nterchangeably n the text. In our study, we consder the followng two perforance etrcs: Average replcaton te: Te aver = n n Te (F ) = Maxu replcaton te: Te ax = ax{te (F )}, {,,n} replcaton set receve a copy of the orgnal fle, and the prary goal of ALM-FastReplca s to nze the axu replcaton te. However, we are also nterested n understandng the pact of ALM-FastReplca on the average replcaton te Te aver. To deonstrate perforance benefts of ALM-FastReplca, let us consder a case wth 000 replcaton nodes, where each node can open k =0concurrent connectons,.e. there are 00 groups, each consstng of k =0nodes. Let Multple Uncast denote a schea that works teratvely n the followng way. Frst, t transfers the entre fle F fro the orgnal node to a frst group of 0 nodes by sultaneously openng k =0pont-to-pont concurrent network connectons. Then, t uses these 0 nodes (when they receve the entre fle F ) as the orgn nodes and repeats the sae procedure to the new groups of nodes, etc. The orgnal FastReplca algorth works teratvely too. Fle s dvded n k =0equal subsequent subfles and s replcated to a frst group of 0 nodes. Once they receve all the subfles, coprsng fle F, FastReplca uses these 0 nodes as the orgn nodes wth fle F and repeats the sae procedure agan, etc. Thus n three teratons, fle F can be replcated aong 000 nodes. Each teraton conssts of steps (dstrbuton and collecton), each used for transferrng the 0-th porton of the orgnal fle F. The ALM-FastReplca algorth operates as descrbed n Secton 3 usng a prary ultcast tree consstng of 0 replcaton groups, and 9 secondary ultcast trees, each also consstng of 0 replcaton groups. Let BW denote a bandwdth atrx, where BW[][] reflects the avalable bandwdth of the path fro node N to node N (as easured at soe te T ), and let Var be the rato of axu to nu avalable bandwdth along the paths partcpatng n the fle transfers. We call Var a bandwdth varaton. In our analyss, we consder the bandwdth atrx BW to be populated n the followng way: BW[][] =B rando(, Var ), where functon rando(, Var ) returns a rando nteger var: var Var. Whle t s a splstc odel, t helps to reflect a realstc stuaton, where the avalable bandwdth of dfferent lnks can be sgnfcantly dfferent. We wll call ths odel a unfor-rando odel. To perfor a senstvty analyss of how ALM- FastReplca perforance depends on a bandwdth varaton of partcpatng paths, we experented wth a range of dfferent values for Var between and 0. When Var =,t s an dealstc settng, where all of the paths are hoogeneous and have the sae bandwdth B (.e. no varaton n bandwdth). When Var =0, the network paths between the nodes have hghly varable avalable bandwdth wth a possble dfference of up to 0 tes. Usng the unfor-rando odel and ts bandwdth atrx BW, we coputed the average and axu fle replcaton tes under Multple Uncast, FastReplca, ALM- FastReplca for 000 nodes n the replcaton set. After that, we derved the relatve speedup of the fle replcaton te under ALM-FastReplca copared to the replcaton te under the Multple Uncast and FastReplca strateges. Relatve speedup of average fle replcaton te s shown n Fgure 6. Speedup (tes) Speedup: Average Replcaton Te 6 ALM-FR-aganst-MU 4 ALM-FR-aganst-FR Bandwdth Varaton Fgure 6: Unfor-rando odel: speedup n average fle replcaton te under ALM-FastReplca. For Var=, there s no varaton n bandwdth, and all the paths have the sae bandwdth B. Under ths dealstc settng, ALM-FastReplca outperfors FastReplca three

9 tes, and Multple Uncast ffteen tes as shown n Fgure 6. It can be explaned n the followng way. The ALM-FastReplca replcaton process conssts of two steps: ) prary ultcast tree and ) secondary ultcast trees, each used for transferrng the 0-th porton of the orgnal fle F. The FastReplca replcaton process conssts of three teratons. At each teraton, the replcaton process follows FastReplca n the sall,.e. the teraton conssts of steps, each used for transferrng the 0-th porton of the orgnal fle F. Thus overall FastReplca perfors 6 steps, each used for transferrng the 0-th porton of the orgnal fle F. Thus, the replcaton te under ALM-FastReplca s 3 tes better 6 than under FastReplca: 0 / 0 =3. Fnally, the Multple Uncast replcaton process conssts of three teratons, each used for transferrng the entre fle F. Thus, the replcaton te under ALM-FastReplca s 5 tes better than under Multple Uncast: 3 / 0 =5. Whle the perforance benefts of ALM-FastReplca are decreasng for hgher varaton of bandwdth of partcpatng paths, t stll reans very effcent, outperforng FastReplca ore than two tes, and Multple Uncast ore than fve tes n average replcaton te under bandwdth varaton of 0. Maxu replcaton te under ALM-FastReplca s three tes better than under FastReplca, and ffteen tes better than under Multple Uncast ndependent of the values of bandwdth varaton (ntutvely, these are proveents of the worst path n unfor-rando odel, and the coparson of replcaton te for a worst case scenaro under dfferent strateges s slar to dealstc settng case). 7 Concluson Streang eda over the Internet can be sgnfcantly proved wth specally desgned content dstrbuton technques and archtectures. In recent years, the Internet servces has oved fro an archtecture where data obects are located at a sngle orgn server to the archtecture where obects are replcated across ultple, geographcally dstrbuted servers. Clent requests for content are redrected to a best-suted replca rather than the orgn server. For large eda fles, the replcaton process across ths dstrbuted network of servers s a challengng, resource-ntensve proble on ts own. In ths work, we propose a new algorth, called ALM- FastReplca, for an effcent and relable replcaton of eda fles n the Internet envronent. Ths algorth cobnes the orgnal FastReplca deas wth ALM approach for sgnfcantly reducng the replcaton te of the large fles wthn CDN. We augent ALM-FastReplca wth an effcent relablty echans, that can deal wth node falures by akng local repar decsons wthn a partcular replcaton group of nodes. Proposed relablty algorth s constructed n such a way, that n the repared porton of the dstrbuton tree, the perforance degradaton s gradual, snce the load of the faled node s shared aong the nodes of the precedng replcaton group. References [] J. Apostolopoulos, T. Wong, W. Tan, and S. Wee. 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