An experimental study of P2P VoIP

Posted by Neil Daswani & Ravi Jain, Google; and Saikat Guha, Cornell University

VoIP (Voice-over-IP) systems are one of the fastest growing means of communication on the Internet, enabling free or low-cost phone calls. But to date, researchers have had little data to work with to learn how to build VoIP systems better. Some of these systems are proprietary, and obtaining data about their operational characteristics has been particularly challenging. For instance, even though the Skype network has tens of millions of users, it has been hard for researchers to benefit from its commercial success.

Data was collected from a Skype 'supernode' running at Cornell. Skype is a Peer-to-Peer (P2P) system in which clients (for example, a home user's PC) communicate directly to exchange voice packets with other clients (also called peers). However, their communication is facilitated by special peers called supernodes that can allow the peers to connect even if they are behind firewalls or other network elements such as NATs (Network Address Translators). P2P in Skype already connects millions of users behind NATs today. Prior to our research, not much has been known about how Skype users and clients behave, and how supernodes are selected or what kinds of demands they place on the network they reside in.

We learned a couple things from the data. For example, we found that Skype users typically keep their client software open during the workday, as opposed to users of file-sharing P2P systems (such as KaZaa) where users typically join and leave the network with much greater frequency. In further contrast to P2P file-sharing applications, which typically tend to be bandwidth hogs, Skype clients and supernodes use relatively little bandwidth and CPU even when they relay VoIP calls. So this means you can run Skype without having it slow down your Internet connection.

You'll find even more results discussed in the paper. In addition to better P2P systems, researchers can use the data to design a better Internet. Based on what we've learned, perhaps researchers can design a next-generation P2P-friendly Internet that is commercially viable.