A SOAP-Oriented Component-Based Framework Supporting Device-Independent Multimedia Web Services Jia Zhang infiNET Solutions Buffalo Grove, IL 60089
[email protected] Abstract A web service is a programmable web application accessible using standard Internet protocols. A threelayer architecture has been suggested for web services: service providers, service brokers, and service requesters. We propose in this paper a SOAP-oriented componentbased framework to support device-independent multimedia web services. Two intelligent agents are introduced and embedded into proxy server and service provider server, respectively. Separating metadata from multimedia content leads to enhance SOAP flexibility. Composite Capability/ Preference Profiles-based user profile management provides an easy and flexible way to split and adapt multimedia services to appropriate composite devices as well as increases the flexibility for users to manage multi-devices. Together with XML/XSL strategy, we ensure the device independency of the system. Utilizing local caches, our agents enable the caching and streaming of multimedia transportation. In addition, our framework seamlessly incorporates cuttingedge technologies relating to web services: SOAP, XML/XSL, and CC/PP.
Keywords
web services, SOAP, XML/XSL, profile
multimedia,
framework,
1. Introduction In the broadest sense, a web service is a programmable web application accessible using standard Internet protocols [15,20]. Roy [14] summarized a three-layer architecture for web services: service providers, service brokers, and service requesters. Service providers register web services on service brokers; service brokers publish registered services; service requesters fetch web services from service brokers. As a consequence, the interface of a web service should be strictly defined in terms of the messages the web service accepts and generates [20]. Serving for web services, Simple Object Access Protocol (SOAP) [16] is becoming a de facto standard. SOAP is a lightweight protocol for exchanging structured and typed
Jen-Yao Chung IBM T. J. Watson Research Yorktown Heights, New York 10598
[email protected] information [1,17]. There are four major categories in SOAP definition: defining the way of using eXtensible Markup Language (XML) [8] to represent data, an extensible message format, how to represent remote procedure calls (RPC) using SOAP message format, and bindings to Hypertext Transfer Protocol (HTTP) [14]. SOAP takes a major step to gravitate software applications toward web and XML. In addition, due to the fact of the advancement of wireless information appliances, a web service will gain more popularity if it is accessible to mobile devices, such as wireless phones and Personal Digital Assistance (PDAs) as well as normal web browsers [11]. Meanwhile, if the same set of information can be delivered to different devices, the system is called to have device independence [4]. However, most sites provide separate applications for different devices [6] hence reusability and maintenance among those different applications remain a problem. Multimedia-oriented web services generally involve transportation of multimedia contents over the web, and management of composite devices for multimedia contents. Multimedia content here implies the content that seamlessly integrates multiple media types in a synchronized and interactive presentation [10]. Caching and streaming are major factors influencing the success of deployment of Internet multimedia services [12]. Without caching a server may experience implosion when it is unable to keep up with an incoming stream of messages [13,21], while every user tries to fetch multimedia information from the server for every request. Caching strategy utilizes a general web caching normally through a proxy server [12]. Streaming paradigm implies that a media file is played out when it is being received over the web [12]. To manage the composite devices for users, Composite Capability/Preference Profiles (CC/PP) [2] specifies an XML and RDF based framework to help define device capabilities and user preferences. It provides a machine understandable and interoperable basis for managing profile metadata [17]. There have been decent amount of research activities in the area of web services. Hitherto, however, all existing efforts address only specific aspects of web services. An additional limitation is that those methods may or may not integrate easily with most current web technologies. Our
Proceedings of the IEEE Fourth International Symposium on Multimedia Software Engineering (MSE’02) 2002114351/02 $17.00 © 2002 IEEE
research objective is to define a comprehensive infrastructure to support multimedia web services. Here we define a SOAP-oriented component-based framework to support device-independent multimedia web services. We accomplish our goal in several ways. First, two intelligent agents are introduced and embedded into proxy server and service broker server respectively. Second, separating metadata from multimedia content leads to enhance SOAP flexibility. Third, CC/PP-based user profile management together with our enhancement provide an easy and flexible way to split and adapt multimedia services to appropriate composite devices, as well as increases the flexibility for users to manage multidevices. Utilizing XML/XSL strategy, we ensure the device independency of the system. Fourth, Utilizing local caches, our agents enable the caching and streaming of multimedia transportation. Finally, our framework seamlessly incorporates cutting-edge technologies relating to web services, such as SOAP, XML/XSL, and CC/PP. The rest of this paper is organized as follows. In Section 2, we discuss related work in web services. In Section 3, we discuss the enhancement of SOAP protocol for multimedia content. In Section 4, we introduce an infrastructure for multimedia web services. In Section 5, we present an experimenting system. In Section 6, we summarize the contributions and innovations, assess limitations and discuss future work directions.
2. Related work In IDDS [10] system, hyperlinks are embedded into HTML files pointing to original multimedia files such as images, audio- and movie-clips. On the screen, the corresponding multimedia information is displayed as thumbnails or icons. The including multimedia pieces and the HTML files might be stored separately in database. Swan [18] encodes multimedia flow as layered signals that are striped across different multicast groups. Vin [19] built a three-layer model for multimedia collaboration: streams layer, sessions layer, and conferences layer. Streams layer manages media communication modulated by access rights; sessions layer represents collections of semantically related media streams; and conferences layer represents temporally related sequences of sessions. Paknikar et al [12] defines a client-side framework to converge caching and streaming Internet multimedia. The architecture consists of a number of caching proxy servers. A central controlling proxy server called broker handles all the initial interactions, and then may transfer control to its sibling servers. The work also defines a layered replacement policy for cashing scalably encoded video objects. Paknikar predicted that Real Time Streaming Protocol (RTSP) would become the de facto standard for Internet A/V caching and streaming. Pham et al. [11] defines a Small Screen/Composite Device (SS/CD) architecture that implements small screen
device focused communication systems. The key component of the architecture is a Smart Gateway (SG) that outsources multimedia information to the most appropriate composite devices to ensure reliable performances, in which a critical part is relative algorithms with a Selection-Device-Assignment-Matrix. However, the work does not involve most current standards and technology. FieldWise [3] relies on a server engine to adapt responses to the capabilities of the client devices and their network connections. Composite Capability/Preference Profiles (CC/PP) [2] was designed to register user preferences and device capabilities. Koskelainen et al. utilizes the mechanism of carrying remote procedure call (RPC) of SOAP to implement commands for conference control system [7]. WebSplitter [4] provides a unified XML framework for multi-device web browsing. The framework defines an XML-based metadata policy file based on CC/PP to enable users to define access privilege groups. With the framework, all web pages are constructed as XML files, with tags specifying mappings to access privileges. A proxy is then adopted to split a web page to different devices. Corresponding XSL style sheets are attached to devices to transform the customized XML to suitable deviceunderstandable languages. MyXML [6] is an XML/XSL based template engine to solve the issue of device independence. The idea is to completely separate content from layout information [5]. However, MyXML introduces a whole set of syntax elements that requires learning curve. JBSL [9] introduces an abstraction layer between presentation and application logic. By dynamically generating user interface “snapshots” and transforming generated description through XSL processor, JBSL provides a framework for mobile devices. Both MyXML and JBSL try to use XML/XSL combination to realize device independence, as the similar work in WebSplitter. As described in the literature review decent amount of research efforts have been conducted in the area of web services. It appears that all existing efforts address only specific aspects of development of web services. An additional limitation is that those methods may or may not integrate easily with current web technologies. Here we seek to provide efficient support for multimedia web services. In contrast with previous approaches, we accomplish this objective by providing a SOAP-oriented component-based framework to support deviceindependent multimedia web services.
3. Enhancement of SOAP for multimedia web services SOAP is on the way to be the de facto standard for web services. For simplicity, it does not support boxcarring and batching of messages, as well as it is a
Proceedings of the IEEE Fourth International Symposium on Multimedia Software Engineering (MSE’02) 2002114351/02 $17.00 © 2002 IEEE
…. Msg00001” Msg00002” Msg00003” Msg00004” Msg00005” ….
Figure 1. Pieces of SOAP messages one-way protocol [16]. If we consider using SOAP to deliver multimedia web services, however, there are some enhancements seem necessary. For example, it is difficult to embed a large multimedia file into one SOAP message. On the contrary, it may be more practical to load a big chunk of information into multiple SOAP messages, with the first message declaring the metadata information. There should also be a way to identify the relationships among those related SOAP messages. In exceptional cases, some of the media files may not be even suitable to be transferred in SOAP messages. Therefore, special
attributes should be provided to identify all of the above situations. Here we suggest several enhancements as follows. We propose a simple workaround of boxcarring and batching of messages for multimedia web services, which introduce several global attributes. Simulating SOAP envelope following real envelope in the real life, we insert into SOAP envelope the information of the sender as the first part. SenderURL is defined to specify sender URL; id is defined to uniquely identify the message; index is defined to identify the index of the message in the whole message box; total is defined to identify the total number of messages in the box. In addition, we introduce a global attribute MustSendBack. Users can use this attribute to contain information that needs to be sent back without any changes. In addition, we propose to always have the first message contains metadata in the body block, which identifies the structure of the complete SOAP response messages. Figure 1 illustrates example pieces of SOAP messages showing the usages of our new terms. The first example is a piece of SOAP request that contains request id “RequestId0001” , which must be sent back. The second example is a piece of SOAP response. We can see that the message is the first of total five messages. Metadata shows the message ids of the other four messages.
4. Proposed infrastructure supporting multimedia web services As Roy [14] summarized, web services extensively adopt a three-layer architecture: service providers, service brokers, and service requesters. In this paper we do not discuss issues about service brokers, since they are to certain degree yellow pages for corresponding service
MWSAgent
SOAP message
ServiceRequester 1
Internet
MM Device
Proxy Server
SOAP message Web Service Provider
MM Device
Figure 2. Proposed MWS infrastructure ServiceRequester 2
Proceedings of the IEEE Fourth International Symposium on Multimedia Software Engineering (MSE’02) 2002114351/02 $17.00 © 2002 IEEE
providers. In addition, service requesters eventually need to connect to service providers to get the requesting services. We make one assumption that, with the help of service brokers, service requesters have already located the service providers that offer the requested services. Meanwhile, since high-speed local area networks (LANs) have been deployed extensively over the world [12], users on such LANs normally access Internet through a proxy server that provides caching facility. Therefore, we make another assumption on such a concept that users invoke web services through their proxy server. We propose an infrastructure supporting multimedia web services (MWS) illustrated in Figure 2. Two intelligent agents are introduced. One is Multimedia Web Service Server Agent (MWSSAgent) that locates at service provider. The other is Multimedia Web Service Agent (MWSAgent) that locates at proxy server on the clients’ LAN. Proxy server therefore becomes an intermediary between service requesters and service providers. In addition, our infrastructure is SOAP-oriented. We believe that our work provides a powerful SOAP-based infrastructure to support multimedia web services. As shown in Figure 2, two service requester machines locating on the same LAN try to access the same web service through their common proxy server. MWSAgent handles the requests, generates the final SOAP message and sends to the service provider through web. MWSSAgent on service provider helps generate the return SOAP message(s) and sends them back to the proxy server. MWSAgent then parses the return messages and decides how to send the multimedia contents to appropriate media devices, such as web browser, audio system, PDA, etc. MWSAgent also tries to hit the requested service in its local cache before connecting to service providers. In this paper, cache is used to imply local storage, therefore it can be either local disk or cache in the memory. We will discuss each of the two agents in detail in the following sections.
4.1 Multimedia Web Services Server Agent (MWSSAgent) A multimedia web service may generate a complex result including multiple multimedia files. Disregarding the fact that SOAP currently can only bind to HTTP while HTTP was not designed for streaming media data [12], it is not efficient and practical to include all information in one SOAP response message to send back to requesters. Therefore, we choose to completely separate metadata from real content. The metadata includes the structure of the set of messages, and links to other media files. The information may be separated in multiple SOAP messages, with the first message containing metadata and specifying the identifications of other messages. The response philosophy MWSSAgent adopts is a lazy-driven norm. That is, it does not always send back all of the
messages at the same time; some messages may stay on service provider until requested particularly. We propose MWSSAgent as an intelligent agent on the service provider site to facilitate multimedia services. Its architecture is illustrated in Figure 3, together with the interactions among its components. The infrastructure contains three functional components - SOAP message parser (MP), SOAP message generator (MG), SOAP message manager (MM) - and a local cache. MP is in charge of parsing incoming SOAP request messages, figuring out the request and sending to MM. MM will first check the cache to see whether the result SOAP messages have already been generated and stored. If the results exist, MM will send back the results through MG – MG needs to generate corresponding return envelopes based on the requests. If not, MM will invoke service backend for the service. MG will then generate the full set of SOAP response messages and store to the local cache, and then send back the first response message including metadata.
Request
Response
SOAP Message Parser(MP)
SOAP Message Generator(MG)
SOAP Message Manager(MM)
Cache
MWSSAgent Web Service Backend
Figure 3. MWSSAgent architecture All of the SOAP response messages will be cached on local disk under control of MM. This caching is important, since published web service providers normally expect large amount of requests. When a SOAP request message comes in, MP will verify whether the request is the first request for a service, or a successive request for other response messages of a particular service. When MG generates the set of response messages, all messages will be uniquely numbered for identification. We adopt caching and streaming algorithms introduced in [12].
4.2 Multimedia (MWSAgent)
Proceedings of the IEEE Fourth International Symposium on Multimedia Software Engineering (MSE’02) 2002114351/02 $17.00 © 2002 IEEE
Web
Services
Agent
device capabilities and user preferences. However, we found that CC/PP definition is too rigid for device capabilities. For example, one device may be capable of accepting one kind of media information with simple transformation. Therefore we extend CC/PP by enabling transformation description to be added to devices. With this extension, CC/PP protocol becomes more powerful of describing device capabilities. User profile manager (UPM) registers user profiles and stores them to user profiles cache. UPM assigns a unique id to every profile, so that one user can have multiple profiles for the purpose of different web services. When a user requests a web service, he needs to specify which profile he wishes to use to receive the service. Service broker will assign a unique id to the request, and UPM will store the pair (request id, profile id) in the cache of request-profile mapping. When response comes in, UPM will pick up the stored corresponding profile from the
We propose MWSAgent as an intelligent agent on the proxy server at client site to support multimedia web services. The component-based architecture is shown in Figure 4. MWSAgent comprises of four functional components: service broker, user profile manager, service delivery manager, and multimedia manager. In addition, there are three caches contained in MWSAgent: one for user profiles; one for request-profile mapping; and one for multimedia information. We will discuss each functional component in detail in the following sections. 4.2.1 User Profile Manager (UPM). Every user can create his profile on the basis of CC/PP format. This profile declares the list of the user’s available resources (devices) and preferences about how his resources will be used. We chose this format on two reasons. One is that CC/PP is built on top of XML technology. The other is that CC/PP was designed particularly for describing
User Profiles
update profile
register personal CC/PP profile
request
Request-Profile Mapping
Service Delivery Manager
User Profile Manager
MWSAgent
Service Broker
Request Manager
SOAP Generator
Internet
SOAP Parser
Service Manger
reply
MManager Multimedia Content Manager
Multimedia Adaptation Manager
Cache
Figure 4. MWSAgent architecture
Proceedings of the IEEE Fourth International Symposium on Multimedia Software Engineering (MSE’02) 2002114351/02 $17.00 © 2002 IEEE
profile cache. Anytime users can change their profiles, add new profiles, or delete some profiles. The granularity of the definition of profile depends on the power of CC/PP and RDF vocabulary. In user’s profile, every resource is declared as a separate item, such as a WAP phone. Each item comprises of two parts. One is the declaration of the hardware. The other one is the declaration of the service, or more directly multimedia resource files that the device can receive. Figure 5 is a piece of a CC/PP profile for a WAP phone. As shown, the declaration of this WAP phone contains two parts. The first part defines its hardware equipment, such as the device name, size of the screen, resolution, etc. The second part defines its acceptable multimedia information, such as image, text, WML, etc. It can be noticed that HTML2WML algorithm will be used to transform receiving HTML code to WML code for the WAP phone. When SOAP response message comes in, UPM will locate the corresponding profile from cache by profile id. … Nokia-3360 30X23mm 101X52Pixels 1.24 … HTML HTML2WML.xsl
Figure 5. A piece of user profile 4.2.2 Multimedia Manager (MManager). Multimedia Manager (Mmanager) contains two sub-components: Multimedia Content Manager (MCM) and Multimedia Adaptation Manager (MAM). MCM manages the caching of multimedia SOAP response messages on the local disk. We adopt caching algorithm introduced in [12]. MAM handles media adaptation if necessary, in order to achieve device independence. In some cases when the destination device is not capable of handling requested multimedia contents, MAM may convert the media accordingly. In some other cases, the destination device may announce to be able to receive some media contents, however some adaptations need to be done by MAM likewise. For instance, a WAP phone could accept HTML code however it needs to be transformed to WML code. MAM accumulates media transformation algorithms, which we
realized using XML/XSL technologies. In accordance with our extension to CC/PP, when a user specifies his profile, he can stipulate the transformation algorithm he prefers. 4.2.3 Service broker. Service broker consists of four subcomponents: request manager, service manager, SOAP generator, and SOAP parser, as shown in Figure 4. SOAP generator helps generate SOAP request messages. SOAP parser helps parse incoming SOAP response messages. Request manager is a request broker of user requests for web services. A user may request a web service when another user from the same LAN has requested the same web service already. Therefore, request manager will first check the local disk through MCM. If result is hit, request manager will pass the control to service manager to send back the information. Otherwise, request manager will assign a unique id to the request, ask UPM to store to request-profile mapping, and then call SOAP generator to generate SOAP request message and send to the service provider over Internet. As a consequence, not only the remote service broker and service provider will have less traffic, but also the response time may be largely shortened. Service manager is invoked when a SOAP response comes back from Internet. It will first call SOAP parser to analyze the content of the result message. If the result contains multimedia information not coming together with the first message, service manager will schedule to prefetch the corresponding media files to enhance the streaming of the media data. All the messages will be sent to MCM to store in local disk before sending back to user, to achieve caching. The source might not be even on-line all the time but its content can always be available at the cache of MWSAgent. Considering scalably encoded or layered video information, for example, such objects have a “base” layer containing essential information, and one or more “enhanced” layers containing higher level information [12]. Service manager will try to download the sub-nodes following the layers. It will try to download the lower layers before downloading higher and more enhanced layers. Meanwhile, service manager will work with UPM to launch the corresponding user profile. And then the control will be passed to SDM to deliver the result to the user. 4.2.4 Service Delivery Manager (SDM). Service delivery manager (SDM) decides the priority and order of the services to be sent back to the users, and decides how to split return information to appropriate devices, as shown in Figure 6. It is important to avoid the performance of a current multiple service requests. A set of criteria is kept at SDM to be used to optimize the order of the delivery. Since all the information is stored in XML-based SOAP messages, SDM explores tags contained in the objects. Only media resource tags are considered and all its attributes are examined. Attributes
Proceedings of the IEEE Fourth International Symposium on Multimedia Software Engineering (MSE’02) 2002114351/02 $17.00 © 2002 IEEE
can give detailed information about the multimedia object, such as size, resolution, constraints, and format, etc. A media resource can be delivered to a user if the user has one device that accepts corresponding media resource.
one includes metadata and the other four each comprises of one media file respectively. Then MWSSAgent stores all five messages on its local cache and sends back the first message. MWSAgent on the proxy server receives the SOAP response message, parses it and figures out that it is the first of total five messages and contains metadata. So that MWSAgent continues to contact with server to fetch the left four multimedia files. From the SOAP envelope, MWSAgent also picks up the (request id, profile id) pair so that the corresponding profile can be fetched from local disk. MWSAgent finds that the first student has one computer and speakers. Therefore, it calls SDM to send text, graphics and video information to the computer and deliver audio chips to the speakers. Suppose then the second student requests the same course information. MWSAgent finds that the course information has already been cached on its local disk. Therefore it directly fetches the student’s profile, and decides to send text information to the cell phone through transformation, and send audio chips to the speakers. Both graphics and video chips will be discarded.
Figure 6. SDM splits multimedia data
5. Example To prove the effectiveness and efficiency of our framework serving for multimedia web services, we constructed an experimental system, which is a distancelearning environment. Figure 7 is a snapshot of the system. One server machine stores course information and serves as service provider. Two students request courses from this server machine as service requesters. The first student possesses one computer and speakers, while the second student has a cell phone and speakers. Both students reside on the same LAN and send out requests through a proxy server machine. We installed MWSSAgent on the server, and MWSAgent on proxy server. As we discussed earlier, we skip service broker layer for simplicity. The two students register their device profiles through UPM on their proxy server. The courses here include multimedia information. Let us consider a scenario of using this system. Suppose the first student specifies his profile and requests a multimedia course, which contains video and audio chips together with text and graphics information. MWSAgent does not find corresponding course content from its local cache, so that is assigns a unique id to the request and stores (request id, profile id) pair in the local disk, and then generates SOAP request message and sends to the server. MWSSAgent on the server generates five SOAP messages, while the first
Figure 7. Snapshot of distance learning system
6. Assessments, innovations and future work We propose in this paper a SOAP-oriented componentbased framework to support device-independent multimedia web services. In the current infrastructure, we adopt caching and replacement algorithms introduce in [12] for both MWSAgent and MESSAgent. Since these two agents serve for different purposes, using the same set of caching and replacement algorithms may not be most efficient. Despite of this limitation that can be resolved by further work, our framework benefits multimedia web services in several ways. Two intelligent agents are introduced and embedded into proxy server and service provider server respectively, to facilitate SOAP-oriented multimedia web services. Separating metadata from multimedia content leads to enhance SOAP flexibility.
Proceedings of the IEEE Fourth International Symposium on Multimedia Software Engineering (MSE’02) 2002114351/02 $17.00 © 2002 IEEE
Introduction of several global attributes to the definition of SOAP envelope enhances SOAP ability of serving for multimedia services. CC/PP-based user profile management together with enhancement of CC/PP specification provides an easy and flexible way to split and adapt multimedia services to appropriate composite devices as well as increases the flexibility for users to manage multi-devices. Together with XML/XSL strategy, we ensure the device independency of the system. Utilizing local caches, our agents enable the caching and streaming of multimedia transportation. In addition, our framework seamlessly incorporates cutting-edge technologies relating to web services: SOAP, XML/XSL, and CC/PP. Our future work includes exploring efficient caching and replacement algorithms for agents on proxy server and service provider respectively. We would also try to bind SOAP to other more multimedia-oriented transportation protocols, such as Real Time Streaming Protocol (RTSP). Finally, we would like to pursue code generation on the two intelligent agents.
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[10] J.V. Ossenbruggen, et al. "Towards Second and Third Generation Web-based Multimedia", Proceedings of the 10th International Conference on World Wide Web, 2001, Hong Kong, pp. 479-488. [11] T. Pham, G. Schneider, and S. Goose, "A Situated Computing Framework for Mobile and Ubiquitous Multimedia Access Using Small Screen and Composite Devices", Proceedings of the 8th ACM International Conference on Multimedia, 2000, Marina del Rey, California, USA, pp. 323331. [12] S. Paknikar, et al. "A Caching and Streaming Framework for Mulitmedia", Proceedings of the 8th ACM International Conference on Multimedia, 2000, Marina del Rey, California, USA, pp. 13-20. [13] S. Pingali, D. Towsley, and J.F. Kurose, “ A Comparison of Sender-Initiated and Receiver-Initiated Reliable Multicast Protocols” , Proceedings of the 1994 Conference on Measurement and Modeling of Computer Systems, 1994, Nashville, Tennessee, USA, pp. 221-230. [14] J. Roy and A. Ramanujan, “ Understanding Web Services” , IEEE IT Professional, Nov. 2001, pp. 69-73. [15] A. Ryman, "Simple object access protocol (SOAP) and Web services", Proceedings of the 23rd International Conference on Software Engineering, 2001, Toronto, Ontario, Canada, pp. 689. [16] Simple Object Access Protocol (SOAP) 1.1, World Wide Web Consortium (W3C), May 2000, http://www.w3.org/TR/SOAP. [17] L, Suryanarayanam and J. Hjelm, "Profiles for the Situated Web” , Proceedings of the 11th International Conference on World Wide Web, 2002, Honolulu, Hawaii, USA, pp. 200-209. [18] A. Swan, S. McCanne, and L.A. Rowe, "Layered Transmission and Caching for the Multicast Session Directory Service", Proceedings of the 6th ACM International Conference on Multimedia, 1998, Bristol, UK, pp. 119-128. [19] H.M. Vin. and M. Chen, “ System Support for Computer Mediated Multimedia Collaborations” , ACM Conference Proceedings on Computer Supported Cooperative Work (CSCW), 1992, pp. 203-209. [20] A. Weiss, "Microsoft’s .NET: Platform in the Clouds", ACM netWorker, Vol. 5, Issue 4, Dec. 2001, pp. 26-31. [21] L.K. Wright, S. McCanne, and J. Lepreau, "A Reliable Multicast Webcast Protocol for Multimedia Collaboration and Caching", Proceedings of the 8th ACM International Conference on Multimedia, 2000, Marina del Rey, California, USA, pp. 2130.
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