Review of a research paper on Skype

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Review of a research paper on Skype

  • Ashwin Sampath


  • Introduction

  • Motivations of research

  • Goals of research

  • Adopted techniques

  • Analysis

  • Conclusions



  • Verify the functionality and quality claims made by Skype.


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.


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

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