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An analysis of Skype protocol

An analysis of Skype protocol. Presented by: Abdul Haleem. Agenda. Introduction Entities of Skype Network Key Components of Skype Software The communication process Skype Network Traffic. Introduction. Peer to peer application Chat Call Skype to Skype

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An analysis of Skype protocol

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  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 rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3

  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.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

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