Voice over IP

1
Voice over IP

• Skype

2
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

3
Skype products

• Basic skype
• Free phone calls from computer to computer on the
  Internet
• Similar to MSN, Yahoo IM, but better voice
  quality
• 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

4
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

5
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

6
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 clients 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

7
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 callees 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

8
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
• wideband codecs allow 50 8000 Hz to pass
  through
• 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