Review of a research paper on Skype

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Review of a research paper on Skype

• Ashwin Sampath

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Outline

• Introduction
• Motivations of research
• Goals of research
• Adopted techniques
• Analysis
• Conclusions

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Introduction

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

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Motivation

• Verify the functionality and quality claims made
  by Skype.

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Goals

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

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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.

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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.

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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.

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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.

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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.

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Strong points

• Real-time examination of working of Skype.

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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.

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Results

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