1. More about Skype

2. Overview Any node with a public IP address having sufficient CPU, memory and network bandwidth is a candidate to become a super node A peer cannot control whether it becomes a SN An ordinary host must connect to a super node and must register itself with the Skype login server

3. Techniques used in Skype Firewall and NAT traversal –SC uses a variation of the STUN and TURN protocols to determine the type of NAT and firewall Global decentralized user directory Intelligent routing (fast switching) Security (encryption) Super-simple UI Impact on fixed-line operator and mobile phone operator

4. More Details The Skype protocol is encrypted, so it is not possible to truly “reverse engineer” it, but guess what it does –A case study: Baset and Schulzrinne, “An analysis of the Skype peer- to-peer Internet telephony protocol”, Infocom 2006 Super node –A few special “bootstrap” SNs help SC find login server –A SN helps a SC determine whether it is behind NAT or UDP- blocking firewall –A SN helps a SC search a user Any user logged in during the last 72 hours can be found This is validated by the paper –A SN helps two SC behind firewalls to relay voice packets

5. Number of SN seen After 8163 successful logins

6. Distribution of SNs Note, majority (83.7%) found in US