A large number N of mobile users (think about spectators in a large football stadium) are interested in downloading the same content of size F (e.g., a video stream following a scoring drive). The ISP has a total bandwidth of B which can be used to communicate content to the mobile nodes. If this bandwidth B is split across all N users, the download delay would be around F ∗ N/B. For large N, this delay was deemed unacceptable. As a solution, the ISP is contemplating the use of a hybrid protocol, whereby it pushes the content to some of the mobile nodes (seeders) while also allowing these nodes to relay that content to other nodes, until the content reaches all nodes in the network. In particular, it is assumed that each mobile device has two modes of communication – using a direct link to the provider (through a cellular connection to the ISP) and using peer-to-peer communication with neighboring mobile devices (through an 802.11 channel). Moreover, each mobile device is able to measure and report back to the ISP the set of other mobile devices within communication range and the bandwidth to each such device.
(a) How might the ISP decide on the appropriate number of “seeders” to use?
(b) Assuming that the ISP wants to use K seeders, how might it choose these seeders? Rather than sending the entire content to a set of nodes (seeders), the ISP may view the set of mobile nodes as a swarm, and thus send different “pieces” of the content to different “leechers”.
(c) What is different about a swarming solution in this setting, compared to one in a wired networking setting?
(d) How might the ISP go about distributing pieces of the content to specific leechers under such setting?