Issue No.05 - May (2008 vol.19)
<em>Flooding</em> is a fundamental file search operation in unstructured peer-to-peer (P2P) file sharing systems, in which a peer starts the file search procedure by broadcasting a query to its neighbors, who continue to propagate it to their neighbors. This procedure repeats until a time-to-live (TTL) counter is decremented to 0. Flooding can seriously limit system scalability because the number of redundant query messages grows exponentially during the message propagation. Our study shows that more than 70% of the generated messages are redundant in a flooding with a TTL of 7 in a moderately connected Gnutella network. Existing efforts to address this issue have been focused on limiting the use of the flooding operation. We propose a new flooding scheme, called <em>LightFlood</em>, with the objective of minimizing the number of redundant messages and retaining a similar message propagating scope as that of the standard flooding. In the scheme, each peer keeps track of the connectivities of every immediate and next indirect neighbor peers, which can be acquired locally. <em>LightFlood</em> identifies the neighbor with the highest connectivity, and uses the link to that neighbor to form a sub-overlay within the existing P2P overlay. In <em>LightFlood</em>, flooding is divided into two stages. The first stage is a standard flooding with a limited number of TTL hops, where a message can spread to a sufficiently large scope with a small number of redundant messages. In the second stage, message propagating is only conducted along the sub-overlay, significantly reducing the number of redundant messages. Our analysis and simulation experiments show that the <em>LightFlood</em> scheme provides a low-overhead broadcast facility that can be effectively used in P2P search. For example, compared with standard flooding with 7 TTL hops, we show that <em>LightFlood</em> with an additional 2 to 3 hops can reduce up to 69% of the flooding messages, and retain the same flooding scope. We believe that <em>LightFlood</em> can be widely used as a core mechanism for efficient message broadcasting in P2P systems due to its near-optimal performance.
Peer-to-Peer System, File Searching, Overlay Network, Query Flooding.
Lei Guo, Xiaodong Zhang, Song Jiang, "LightFlood: Minimizing Redundant Messages and Maximizing Scope of Peer-to-Peer Search", IEEE Transactions on Parallel & Distributed Systems, vol.19, no. 5, pp. 601-614, May 2008, doi:10.1109/TPDS.2007.70772