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Review of a research paper on Skype Ashwin Sampath.

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1 Review of a research paper on Skype Ashwin Sampath

2 Outline Introduction Motivations of research Goals of research Adopted techniques Analysis Conclusions

3 Introduction An Analysis of the Skype Peer to Peer Internet Telephony Protocol - Salman A Baset & Henning Schulzrinne

4 Motivation Verify the functionality and quality claims made by Skype.

5 Goals Analyze the basic functionalities of Skype Analyze the network traffic generated by Skype under different network configurations

6 Techniques Adopted Reverse-engineering based approach. Run Skype clients on windows machines. Three different network setups with machines behind NAT and firewall. Analyze network traffic using Ethereal. Tune network bandwidth using NetPeeker.

7 Background of Skype P2P voip client supporting voice and text based conversation, buddy lists etc. Based on Kazaa. Overlay p2p network consisting of ordinary and Super Nodes (SN). Ordinary node connects to network through a Super Node. Each user registers with a central server. User information propagated in a decentralized fashion. Uses a variant of STUN to identify the type of NAT and firewall.

8 Background of Skype continued.. Skype client listens on configured TCP and UDP ports. Each client maintains a list of super nodes in the Host Cache. Buddy list is local to a machine. All Skype messages are encrypted.

9 Key observations Initial startup and login sequences are different from subsequent sequences. Usage of TCP port 80 enables client to reach super node even through firewalls. Skype login server might be fixed but not hardcoded. Identity of “Bootstrap” nodes hardcoded. Client needs to maintain a constant TCP connection with atleast one super node. Skype client continuously discovers and builds the list of Skype nodes.

10 Key observations continued.. When the client is behind a UDP-restricting firewall, the login takes significantly more time. Search results are cached at intermediate nodes. Call signaling is done over TCP, messages are preferably transported over UDP. In the presence of NAT or firewalls, calls between caller and callee are routed by an intermediate node. Silence suppression is not supported in order to maintain UDP bindings and TCP congestion window size. Reasonable call quality requires atleast 2kbps. During conferencing traffic always passes through one node which can become a single point of failure.

11 Strong points “Real-time” examination of working of Skype.

12 Weak Points Observations are inconclusive. eg : Caching of search results. Organization of figures is bad. Figures contradictory to text explanation eg : Figure 11 shows UDP message exchange and media transport over UDP. Poor conclusions.

13 Results Growing popularity of Skype because of better voice quality, ability to work behind firewalls and NAT and ease of use.


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