Skype Relay Calls: Measurements and Experiments Wookyun Kho, Salman Abdul Baset, Henning Schulzrinne Department of Computer Science Columbia University
[email protected], {salman,hgs}@cs.columbia.edu Abstract—Skype, a popular peer-to-peer VoIP applica- Our work makes two contributions. First, we have tion, works around NAT and firewall issues by routing determined that although Skype exhibits geographical calls through the machine of another Skype user with locality in relay selection, the median one way call unrestricted connectivity to the Internet. We describe an latency is not insignificant, suggesting that further tuning experimental study of Skype video and voice relay calls of Skype relay selection mechanism could be beneficial. conducted over a three-month period, where approxi- mately 18 thousand successful calls were made and over Second, we discuss factors impacting the success rate of nine thousand Skype relay nodes were found. We have Skype calls. This result can be used for improved future determined that the relay call success rate depends on protocol design. the network conditions, the presence of a host cache, The remainder of the paper is organized as follows. and caching of the callee’s reachable address. From our Section II describes the experimental setup and Sec- experiments, we have found that Skype relay selection tion III-A discusses factors affecting success rate of relay mechanism can be further improved, especially when both calls. In Section III-B, we discuss the geographical, ISP, caller and callee are behind a NAT. and autonomous system distribution of relay nodes and Index Terms—Skype, relay. relay calls and online presence of relay nodes.