Multicast protocols for scalable on-demand download

Previous scalable protocols for downloading large, popular files from a single server include batching and cyclic multicast. With batching, clients wait to begin receiving a re quested file until the beginning of its next multicast transmission, which collectively serves all of the waiting cl ients that have accumulated up to that point. With cyclic multicast, the file data is cyclically transmitte d on a multicast channel. Clients can begin listening to the channel at an arbitrary point in time, and continue listening until all of the file data ha s been received. This paper first develops lower bounds on the average and maximum clie nt delay for completely downloading a file, as functions of the average server bandwidth used to serve r equests for that file, for systems with homogeneous clients. The results show that neither cyclic multicast nor batc hing consistently yields performance close to optimal. New hybrid download protocols are proposed that achieve within 15% of the optimal maximum delay and 20% of the optimal average delay in homogeneous systems. For heterogeneous systems in which clients have widely-varying a chievable reception rates, an additional design question concerns the use of high rate transmissions, which can decreas e delay for clients that can receive at such rates, in addition to low rate transmissions that can be received by all clients. A new scalable download protocol for such systems is proposed, and its performance is compared to that of alternative protocols as well as to new lower bounds on maximum client delay. The new protocol achieves within 25% of the optimal maximum client delay in all scenarios considered.