884 IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 11, NO. 6, DECEMBER 2003Proxy-Assisted Techniques for DeliveringContinuous Multimedia StreamsLixin Gao, Zhi-Li Zhang, and Don Towsley, Fellow, IEEEAbstract—We present a proxy-assisted video delivery architec-ture that can simultaneously reduce the resources requirements atthe central server and the service latency experienced by clients(i.e.,endusers).Undertheproposed video deliveryarchitecture,wedevelopandanalyze twonovelproxy-assistedvideostreamingtech-niques for on-demand delivery of video objects to a large numberof clients. By taking advantage of the resources available at theproxy servers, these techniques not only significantly reduce thecentral serverand network resource requirements, but are also ca-pable of providing near-instantaneous serviceto a large number ofclients. We optimize the performance of our video streamingarchi-tecturebycarefullyselectingvideo deliverytechniques forvideosofvarious popularity and intelligently allocating resources betweenproxy servers and the central server. Through empirical studies,we demonstrate the efficacy of the proposed proxy-assisted videostreaming techniques.I. INTRODUCTIONTHE past few years have seen a dramatic growth of multi-media applications whichinvolvevideo streaming over theInternet. Server and network resources (in particular, server I/Obandwidth and network bandwidth) have proven to be a majorlimiting factor in the widespread usage of video streaming overthe Internet. In order to support a large population of clients,techniques that efficiently utilize server and network resourcesare essential. In designing such techniques, another importantfactor that must be taken into consideration is the service la-tency, i.e., the time a client has to wait until the object he/shehas requested is started to playback. The effectiveness of a videodelivery technique must be evaluated in terms of both the serverand network resources required for delivering a video object andthe expected service latency experienced by clients. Clearly, the“popularity” or access pattern of video objects (i.e., how fre-quently a video object is accessed in a given time period) playsan important role in determining the effectiveness of a video de-livery technique.Manuscript received March 7, 2002; revised April 30, 2002; approved byIEEE/ACM TRANSACTIONS ON NETWORKING Editor M. Ammar. The work ofL. Gao was supported in part by the National Science Foundation under GrantANI-9977555 and NSF CAREER Award Grant ANI-9875513. The work ofZ.-L. Zhang was supported in part by the National Science Foundation underNSF CAREER Award Grant NCR-9734428 and NSF Grant ANI-9903228. Thework of D. Towsley was supported in part by the National Science Foundationunder Grant ANI-9805185.L. Gao is with the Department of Electrical and Computer Engi-neering, University of Massachusetts, Amherst, MA 01060 USA (e-mail:[email protected]).Z.-L. Zhang is with the Department of Computer Science and Engi-neering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail:[email protected]).D. Towsley is with the Department of Computer Science, University of Mass-achusetts, Amherst, MA 01030 USA (e-mail: [email protected]).Digital Object Identifier 10.1109/TNET.2003.820423In thispaper, we propose a proxy-assisted videostreaming ar-chitecture that takes advantage of the resources (processing anddisk storage) available at proxy servers to significantly reducethe server and (backbone wide-area) network resource require-ments, while at the same time providing near-instantaneous ser-vice to clients. The central server multicasts video objects pe-riodically, using, for example [13], source-specific multicast.Proxy servers are strategically placed between, say, local accessnetworks and the backbone wide-area network. A proxy serverstores a fixed number of initial frames or a “prefix” of the mul-timedia stream [12] so as to serve the future requests: when anew request arrives, the client joins an on-going multicast groupto retrieve the multicast stream from the central server and re-trieves the missing initial frames from the proxy server. Themissing portion of the prefix is delivered by the proxy usinga unicast channel and played back immediately by the client.Hence, the proxy server reduces the service latency experiencedby the user by unicasting a prefix of the multimedia stream.This proxy-assisted video delivery environment has severaladvantages over the traditional stand-alone video server envi-ronment. First, since it requires only network resources betweenthe proxy and the client, latency reduction is achieved withoutincreasing the demand on backbone network resources. Second,unlike the proxy caching schemes proposed for conventionalWeb objects such as text and image objects, the proxy needs tostore only prefixes of the multimedia streams. Thus it is feasibleeven with the large data volume typically associated with mul-timedia objects. Third, since the proxy server delivers only theprefixes and is only responsible for a limited number of clients,the I/O bandwidth requirement imposed on the proxy server isnot significant.Under the proposed proxy-assisted video delivery archi-tecture, we develop a novel video delivery technique calledproxy-assisted catching, which can efficiently utilize serverand proxy resources while providing near instantaneous serviceto clients. This technique is particularly suitable for “hot”(i.e., frequently access) video objects. The effectiveness of thetechnique is achieved through the intelligent integration ofthe “server-push” and “client-pull” video delivery paradigms.Using this technique, the server periodically “broadcasts” avideo object via a number of dedicated multicast channels.A client who wishes to watch the video immediately joinsan appropriate multicast channel without waiting for thebeginning of the next broadcast period. At the same time, theclient sends a request to a proxy server to retrieve the missingprefix of the video object. Using a smart broadcast schemesuch as the Greedy Disk-conserving Broadcast (GDB) scheme[7], we present an analytical framework to determine design1063-6692/03$17.00 © 2003 IEEEGAO et al.: PROXY-ASSISTED TECHNIQUES FOR DELIVERING CONTINUOUS MULTIMEDIA STREAMS 885parameters such as the size of the prefix stored in the proxyserver. Furthermore, we show that the total resource requiredby the central server and proxy servers combined is close to
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