An Analysis of the Skype Peer-to-Peer Internet Telephony Protocol Salman Abdul Baset and Henning Schulzrinne Internet Real-Time Lab, Columbia University http://www.cs.columbia.edu/~salman/skype/

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An Analysis of the Skype Peer-to-Peer Internet
Telephony Protocol Salman Abdul Baset and
Henning Schulzrinne Internet Real-Time Lab,
Columbia Universityhttp//www.cs.columbia.edu/sa
lman/skype/

• The Skype Components
• Port
• No default listening port. Randomly chooses a
  port upon installation.
• Host cache (HC)
• IP address and port number of online Skype nodes
• Maximum size 200 entries
• Codecs (GlobalIPSound)
• Wideband codecs (50-8,000 Hz)
• iLBC (packet size 20 and 30 ms bitrate 15.2
  kb/s and 13.3 kb/s)

Skype Functions Summary
Call Establishment
Public NAT Firewall
Login 10 KB(3-7 secs) 11 KB(3-7 secs) 7 KB(30-35 secs)
Search 1-2 KB(3-4 secs) 1-2 KB(5-6 secs) 5-7 KB(10-15 secs)
Call establishment 6 KB 8 KB 8 KB

• Super Nodes
• 8,153 login attempts over four days
• 35 hostnames had a .edu suffix
• 102 universities
• 894 unique SNs
• Unique SN IP distribution US 83.7, Asia
  8.9, Europe 7.1
• Top 20 nodes received 43.8 of the total
  connections

Experimental Setup

• The Skype Network
• Ordinary Host (OH)
• A Skype client
• Super node (SN)
• A Skype client
• Has public IP address, sufficient bandwidth,
  CPU and memory
• Bootstrap super nodes (maintained by Skype)
• Used when running Skype for the first time
• A total of seven nodes hard-coded in Skype
  executable
• Login server
• Stores Skype ids, passwords and buddy lists
• Used at login for authentication
• Version 1.4 212.72.49.141 and 195.215.8.141

• Skype Relay Experiments
• Jan 20th to Feb 5th 2006.. Caller and callee
  machines in IRT lab
• 8,822 successful call attempts and 4,904 unique
  relay IP addresses
• 60 of the calls routed through caller SN
• Relay nodes in 51 countries

Total successful calls 8,882 Total successful calls 8,882 Total successful calls 8,882
of successful calls between of successful calls between of successful calls between
Relay distribution 800am and 759pm EST 800pm and 759am EST
Global 44.66 (3,985) 55.34 (4,937)
Asia 3.3 (131) 7.9 (392)
Australia 0.05 (2) 0.2 (10)
Europe 42.9 (1,709) 38.51 (1,901)
North America 52.42 (2,089) 52.17 (2,576)
South America 0.14 (54) 1.18 (58)
Skype, MSN, Yahoo and Google Talk
Application version Memory usage before call (caller, callee) Memory usage after call (caller, callee) Process priority before call Process priority during call Mouth-to-ear latency
Skype 1.4.0.84 19 MB, 19 MB 21 MB, 27 MB Normal High 96ms
MSN 7.5 25 MB, 22 MB 34 MB, 31 MB Normal Normal 184ms
Yahoo 7.0 beta 38 MB, 34 MB 43 MB, 42 MB Normal Normal 152ms
GTalk 1.0.0.80 9 MB, 9 MB 13 MB, 13 MB Normal Normal 109ms