Skype relay calls: Measurements and experiments

Skype, a popular peer-to-peer VoIP application, works around NAT and firewall issues by routing calls through the machine of another Skype user with unrestricted connectivity to the Internet. We describe an experimental study of Skype video and voice relay calls conducted over a three-month period, where approximately 18 thousand successful calls were made and over nine thousand Skype relay nodes were found. We have determined that the relay call success rate depends on the network conditions, the presence of a host cache, and caching of the callee’s reachable address. From our experiments, we have found that Skype relay selection mechanism can be further improved, especially when both caller and callee are behind a NAT.