1. An analysis of Skype protocol Presented by: Abdul Haleem

2. Agenda Introduction Entities of Skype Network Key Components of Skype Software The communication process Skype Network Traffic

3. Introduction Peer to peer application Chat Call  Skype to Skype  Skype to telephone/mobile network  Telephone/mobile network to Skype User search (Find people)

4. Key Entities Skype Client (ordinary host) Login Server Super Node

5. Key Entities

6. Skype Client (ordinary host)  An ordinary host is a Skype application that can be used to place voice calls and send text messages

7. Login Server  It is the only centralized server  User names and passwords are stored at the login server. User authentication at login is also done at this server.  This server also ensures that login names are unique across the Skype name space  An ordinary host must register itself with the Skype login server

8. Super Node  Super nodes are the end points where Skype clients connect to.  Any node with a public IP address having sufficient CPU, memory, and network bandwidth is a candidate to become a super node.

9. Key Components Host Cache Obfuscation Layer

10. Host Cache  Every Skype client maintains a local list of super node IP address and port pairs. It is called Host Cache  Refreshed regularly  Upon startup the client reads an entry from the host cache, and tries to connect to this SN.  If connection fails (the SN is offline; it is no longer part of the network, etc) it reads next entry and tries to connect to it.

11. Obfuscation Layer  Everything that is being transferred across the network is being encrypted to ensure privacy.  Uses AES algorithm for encryption.

12. communication process Logging in Skype to Skype call – Both public IPs Skype to Skype call - behind NAT Call Between Skype and PSTN

13. Logging in  Client authenticates its user name and password with the login server  advertises its presence to other peers  determines the type of NAT it is behind  discovers online Skype nodes with public IP addresses

14. Logging in  Get an IP and port of a super node from HC  Send UDP packets to check availability  TCP connection on provided IP and port.  TCP connection on provided IP and port 80 (HTTP port).  TCP connection on provided IP and port 443 (HTTPS port)  Try next super node (total 5 attempts)

15. Logging in

16. Skype to Skype call – Both public IPs  If both caller and receiver are on public IPs then they establish a call through a direct TCP connection with each other.  Media is transferred using UDP.

17. Skype to Skype call - behind NAT  NAT prevents an outside peer from initiating a call to insider peer 10.0.0.1 10.0.0.2 10.0.0.3 10.0.0.4 138.76.29.7 local network (e.g., home network) 10.0.0/24 rest of Internet

18. Skype to Skype call - behind NAT Solution to NAT problem  Every client is connected to its super node which is non-NATed.  Client A asks its super node that she wants to call B.  Super node of A informs super node of B who in turn informs B.  If B accepts the call then a third non-NATed peer is chosen which relays data between A and B.

19. Skype to Skype call - behind NAT Solution to NAT problem

20. Call Between Skype and PSTN  For Skype out (Skype to PSTN), the application initially contacts the super node and then the PSTN gateway at port 12340.  The gateway servers are a separate part of the architecture and not a part of the overlay network.  For Skype in (PSTN to Skype) the opposite is used.

21. Skype Network Traffic UDP Packets  For keep alive messages  To discover available super nodes TCP Packets  For establishing connection with super node  For user authentication with login server

22. References An Analysis of the Skype Peer-to-Peer Internet Telephony Protocol – By Salman A. Baset and Henning Schulzrinne Department of Computer Science Columbia University, New York NY 10027 {salman,hgs}@cs.columbia.edu September 15, 2004 http://www.mjalali.com/blog/Skype101– Skype Architecture–Work in Progress.htm http://www.mjalali.com/blog/Skype101– Skype Architecture–Work in Progress.htm http://www.skype.com/Skype P2P telephony explained.htm http://en.wikipedia.org/wiki/Talk:Skype_protocol http://technologyevangelist.com/2008/05/A bit about how Skype works.htm