But since you are here, the odds are not inconceivable that you're an IP telephony geek. If you see yourself through the pixels I've just typed, then read, and click, on.
The two links point to papers authored in part by Columbia University Dept. of Computer Science chair Prof. Henning Schulzrinne. Here you go: An Analysis Of The Skype Peer-To-Peer Internet Telephony Protocoland Peer-To-Peer Internet Telephony Using SIP.
I went there for you, and have read both papers.
According to Prof. Schulzrinne's Abstract, the first paper "analyzeskey Skype functions such as login, NAT (Network Address Translation) and firewall traversal, call establishment, media transfer, codecs and conferencing under three different network setups."
The second paper proposes apeer-to-peer architecture for SIP-based telephony. TheProf.'s formulation includes provisions for call setup, user registration, offline message delivery, voice mail, video mail and multi-party conferencing.