An Analysis of the Skype Peer-to-Peer Internet Telephony Protocol

TL;DR

This paper analyzes Skype's peer-to-peer VoIP protocol, focusing on its NAT traversal, call setup, media transfer, and security features through traffic analysis across different network configurations.

Contribution

It provides a detailed technical analysis of Skype's protocol operations and network behavior, revealing insights into its NAT traversal and encryption mechanisms.

Findings

Skype effectively handles NAT and firewall traversal.

It encrypts calls end-to-end for privacy.

Skype's media transfer adapts to various network conditions.

Abstract

Skype is a peer-to-peer VoIP client developed by KaZaa in 2003. Skype claims that it can work almost seamlessly across NATs and firewalls and has better voice quality than the MSN and Yahoo IM applications. It encrypts calls end-to-end, and stores user information in a decentralized fashion. Skype also supports instant messaging and conferencing. This report analyzes key Skype functions such as login, NAT and firewall traversal, call establishment, media transfer, codecs, and conferencing under three different network setups. Analysis is performed by careful study of Skype network traffic.