P2P Case study: Skype

• inherently P2P: pairs of users communicate.
• proprietary application-layer protocol (inferred via reverse engineering) 
• hierarchical overlay with SNs
• Index maps usernames to IP addresses; distributed over SNs

Peers as relays

• Problem when both Alice and Bob are behind  “NATs”. 
  • NAT prevents an outside peer from initiating a call to insider peer
• Solution:
  • Using Alice’s and Bob’s SNs, Relay is chosen
  • Each peer initiates session with relay. 
  • Peers can now communicate through NATs via relay