Following Skype Signaling Footsteps

Skype is beyond any doubt the VoIP application in the current Internet. Its amazing success drawn the attention of telecom operators and the research community, both interested in knowing its internal mechanisms, characterizing its traffic, understanding its users' behavior. One of the most peculiar characteristics of Skype is that it relies on a P2P infrastructure for the exchange of signaling information that is distributed between active peers. Leveraging on the use of an accurate Skype classification engine that we recently proposed, we carry on an experimental study of Skype signaling based on extensive passive measurements collected from our campus LAN. In particular, we focus on the signaling traffic in the attempt of inferring some interesting properties of the overlay maintenance and, possibly, some hints about its structure. Our results show that, despite the signaling bandwidth used by normal peers is exiguous, it may however constitute a very significant portion of the total traffic generated by a Skype client - since, in order to guarantee reachability, Skype application is running most of the time even if no active call is in progress. Skype performs peer discovery and refresh by using a large number of single packets probes - which may be as effective for the purpose of the overlay maintenance as costly from the viewpoint of statefull layer-4 network devices. At the same time, single packet probes constitute only a minor portion of the signaling traffic: therefore, we dig into further details the traffic exchanged among more stable peers in the attempt of learning how the peer selection mechanism works