Peer-to-Peer IP Telephony

In August 2003 a small application called Skype was introduced on the Internet. It could be downloaded for free, and it enabled its users to make VoIP calls to each other free of charge. The most compelling feature of this application, in addition to being free, was that the voice quality was comparable to that of traditional fixed line telephony. Other applications such as MSN and Yahoo messenger had for some time offered its users voice communication, but with poor quality and with one major obstacle, they could not operate when the users where located behind a Network Address Translator . Skype on the other hand, introduced advanced traversals techniques enabling the users to communicate peer-to-peer even when both the caller and the callee are located behind a Network Address Translator. After a general overview of peer-to-peer communication, we present IP telephony and how it can be deployed in peer-to-peer settings. The term “peer-to-peer” is not used as a euphemism for file sharing or other related activities, but in its original architectural sense, that all hosts on the network are logically equals. This was indeed the case in the early days of the Internet, but as the usage of the Internet changed, and the dominating communication form on the Internet became client/server, the end-to-end model of the Internet was broken, disabling peer-to-peer communication. We also describe some of the technology used to traverse Network Address Translators to enable peer-topeer communication, as well as some of the problems associated with these solutions.