Voice over ip skype Skype



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tarix15.10.2017
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#5080


Voice over IP

  • Skype


Skype

  • The largest and most well know P2P Internet telephony operator

  • Started in 2003, grew in only 1.5 years from nothing to a rapidly expanding Internet telephony operator

  • over 2 million active users as of March 2005



Skype products

  • Basic skype

  • SkypeOut

    • a low cost way to make calls from Skype to traditional landlines or mobile phones.
    • e.g. use SkypeOut to call the United Kingdom you can save up to 80% per minute compared with Bell Canada
  • SkypeIn

    • get your own, regular phone number, receive phone calls from anywhere
    • In Beta test
  • Skype API

    • an open application programming interface (API) that enables third parties to develop their own add-ons for Skype
    • Business users can develop their application systems based on Skype


Skype technology

  • Based on peer-to-peer network

    • minimal network infrastructure
    • Utilize its users’ computers to do the work
    • three types of hosts:
      • ordinary hosts, Skype users
      • super nodes, Skype users with sufficient computing power, memory and network bandwidth
      • login servers, for authentication
    • Skpye owns only the login servers
  • Proprietary protocols



Connect to Skype network

  • Each Skype client maintains a host cache which is a list of IP addresses of known super nodes. Initially empty.

  • super nodes are responsible for locating users, routing calls and providing the clients information about hosts currently connected to the network.

  • A Skype client must connect to the network through super nodes



Connect to Skype network

  • First login

    • When Skype client is run for the first time after intallation, it will communicate with the Skype server (skype.com)
    • After that, the client’s host cache is filled with IP addresses of 7 bootstrap super nodes.
    • Establish a connection with one of them
    • authenticate the user name and password with the Skype login server
    • The host cache is periodically updated with the IP address of new super nodes
  • Subsequent login

    • Choose one super nodes from the host cache and establish a connection to that super node


Call establishment

  • User lookup

    • “Global Index” technology that keeps track of which users have used the network in the near past
    • Skype claims, that any user who has logged in to the network in the last 72 hours can be found
  • After acquiring the callee’s IP address, the caller establishes a connection to the callee and send signaling messages.

  • After the callee answered the call, voice packets are sent directly between the two parties.

  • If one or both of the parties are behind NAT or firewall, they communicate through another Skype online node



Other issues

  • Security

    • All communication is encrypted with Advanced Encryption Standard (AES)
    • The AES encryption keys are transmitted between hosts using the Rivest, Shamir, & Adleman (RSA) algorithm.
  • Voice quality

  • NAT and firewall

    • Skype uses Simple Traversal of User Datagram Protocol (STUN) and Traversal Using Relay NAT (TURN) algorithms to determine the type of firewalls and NAT


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