We study how to optimize throughput of large transfers across richly connected, adaptive overlay networks, focusing on the potential of collaborative transfers between peers to supplement ongoing downloads. First, we make the case for an erasure-resilient encoding of the content, using the digital fountain paradigm. Such an approach affords reliability and a substantial degree of application-level flexibility, as it seamlessly accommodates connection migration and parallel transfers while providing resilience to packet loss. We explain the history of this paradigm, focusing on recent advances in coding that allow efficient implementations of digital fountains. We also describe our previous work showing the effectiveness of digital fountains for reliable multicast and parallel downloading.

In the setting of collaborative transfers on overlay networks, there is an additional consideration since sets of encoded symbols acquired by peers during downloads may overlap substantially. We describe a collection of useful algorithmic tools for efficient estimation, summarization, and approximate reconciliation of sets of symbols between pairs of collaborating peers, all of which keep messaging complexity and computation to a minimum. Through simulations and experiments on a prototype implementation, we demonstrate the performance benefits of our informed content delivery mechanisms.