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Skype

VIEWS: 22 PAGES: 13

									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.

								
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