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Review of a research paper on Skype. Ashwin Sampath. Outline. Introduction Motivations of research Goals of research Adopted techniques Analysis Conclusions. Introduction. An Analysis of the Skype Peer to Peer Internet Telephony Protocol - Salman A Baset & Henning Schulzrinne.
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Review of a research paper on Skype Ashwin Sampath
Outline • Introduction • Motivations of research • Goals of research • Adopted techniques • Analysis • Conclusions
Introduction An Analysis of the Skype Peer to Peer Internet Telephony Protocol - Salman A Baset & Henning Schulzrinne
Motivation • Verify the functionality and quality claims made by Skype.
Goals • Analyze the basic functionalities of Skype • Analyze the network traffic generated by Skype under different network configurations
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.
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.
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.
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.
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.
Strong points • “Real-time” examination of working of Skype.
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.
Results • Growing popularity of Skype because of better voice quality, ability to work behind firewalls and NAT and ease of use.