There exists a certain level of ambiguity regarding whether network coding can further improve download performance in P2P content distribution systems, as compared to commonly applied heuristics such as rarest first protocols. In this paper, we revisit the problem of broadcasting multiple data blocks from a single source in an overlay network using gossip-like protocols. Our new finding reveals that the marginal benefit of network coding critically depends on the dynamics of network topologies. We show that although network coding is optimal as a block selection mechanism, simple non-coding protocols are close to optimal in complete and random graphs, leading to marginal benefits of network coding. However, network coding demonstrates salient benefits in clustered and time-varying topologies, which are common in real-world systems with ISP-locality mechanisms implemented. Through both theoretical analysis and simulation results, we unveil the underlying reasons behind discrepancies in the power of network coding under different scenarios.
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