TR-08-2.pdf
``LightFlood: minimizing redundant messages and maximizing scope of
peer-to-peer search"
Song Jiang, Lei Guo, Xiaodong Zhang, and Haodong Wang
IEEE Transactions on Parallel and Distributed Systems, Vol. 19, No. 5,
pp. 601-614, 2008.
Abstract
Flooding 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 LightFlood, 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. LightFlood 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 LightFlood, 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 LightFlood 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 LightFlood
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 LightFlood
can be widely used as a core mechanism for efficient message broadcasting
in P2P systems due to its near-optimal performance.