Download Skype overview

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
– 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
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