As the Gnutella specification v 0.6, a crawler Ping, i.e., a Ping with TTL=2 and hops=0, will get Pongs from all its neighours and all its neighbours' neighbours. We want to know whether the leaf nodes of its neighbours will repsond the Ping. If yes, how can we distinguish who is leaf node and who is ultra peer? If not, how can we know how many leaf nodes a ultra-peer have?