A reliable multicast webcast protocol for multimedia collaboration and caching

Large-scale, multi-point, multimedia conferencing applications designed to facilitate long-distance collaboration are enjoying growing popularity. Usually composed of real-time audio, video and shared-drawing applications, these collaborative environments help render the geographical location of collaborators irrelevant. To complement these existing collaborative applications, it would be useful to have the ability to distribute documents synchronously over the World Wide Web (WWW). One model for synchronized information dissemination within the Web is webcasting in which data are simultaneously distributed to multiple destinations. The WWW's traditional unicast client/server communication model suffers, however, when applied to webcasting; solutions which require many clients to simultaneously fetch data from the origin server using the client/server model will likely cause server and link overload.

A number of webcasting solutions have been proposed. Many have limited scalability because they are based on unicast while others use multicast for more scalable data delivery but require server modification or have rigid architectures. We believe that successful webcasting solutions will provide scalable, reliable delivery yet still be compatible with the existing Web infrastructure.

In this paper we describe a webcast design that improves upon previous designs by leveraging application level framing (ALF) design methodology. We build upon the Scalable Reliable Multicast (SRM) framework, which is based upon ALF, to create a custom protocol to meet webcast's scalability needs. We employ the protocol in an architecture consisting of two reusable components: a webcache component and a browser control component. We have implemented our design using the MASH multimedia application toolkit and a SRM protocol module called libsrm. We present the results of a simple performance evaluation and report on lessons learned while using MASH and libsrm.