Voice over IP Skype Skype The largest and

Voice over IP Skype

Skype • The largest and most well know P 2 P 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 – Free phone calls from computer to computer on the Internet – Similar to MSN, Yahoo IM, but better voice quality • Skype. Out – a low cost way to make calls from Skype to traditional landlines or mobile phones. – e. g. use Skype. Out to call the United Kingdom you can save up to 80% per minute compared with Bell Canada • Skype. In – 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 – 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