Efficient and highly available peer discovery: A case for independent trackers and gossiping

Tracker-based peer-discovery is used in most commercial peer-to-peer content distribution systems, as it provides performance benefits compared to distributed solutions, and facilitates the control and monitoring of the overlay. But a tracker is a central point of failure, and its deployment and maintenance incur costs; hence an important question is how high tracker availability can be achieved at low cost. We investigate highly available, low overhead peer discovery, using independent trackers and a simple gossip protocol. This work is a step towards understanding the trade-off between the overhead and the achievable peer connectivity in highly available distributed overlay-management systems for peer-to-peer content distribution. We propose two protocols that connect peers in different swarms efficiently with a constant, but tunable, overhead. The two protocols, Random Peer Migration (RPM) and Random Multi-Tracking (RMT), employ a small fraction of peers in a torrent to virtually increase the size of swarms. We develop analytical models of the protocols based on renewal theory, and validate the models using both extensive simulations and controlled experiments. We illustrate the potential value of the protocols using large-scale measurement data that contains hundreds of thousands of public torrents with several small swarms, with limited peer connectivity. We estimate the achievable gains to be up to 40% on average for small torrents.