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/
The Skype Components
SkypeIn /
SkypeOut
servers
Skype login server
Message exchange with the
login server during login
 Port
• No default listening port. Randomly chooses a port upon
installation.
Skype Functions Summary
Public
Login
Search
 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)
• iSAC (packet size: 30-60 ms bitrate: 10-32 kb/s)
• G.729 for SkypeOut?
ordinary host (SC)
super node (SN)
neighbor relationships in the
Skype network
Call
establishment
NAT
10 KB
(3-7 secs)
1-2 KB
(3-4 secs)
6 KB
11 KB
(3-7 secs)
1-2 KB
(5-6 secs)
8 KB
Call Establishment
Firewall
7 KB
(30-35 secs)
5-7 KB
(10-15 secs)
8 KB
Relay
124B (4)
45B (4)
TCP
TCP
Caller
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
Callee (NAT+firewall)
TCP
51B (3)
TCP
117B (4)
Relay
Callee
Media:TCP
 Ordinary Host (OH)
INTERNET
• A Skype client
 Super node (SN)
B (public IP)
A (public IP)
• A Skype client
• Has public IP address, ‘sufficient’ bandwidth,
INTERNET
CPU and memory
 Bootstrap super nodes (maintained by Skype) A (private
B (public IP)
port-restricted NAT
IP)
• Used when running Skype for the first time
• A total of seven nodes hard-coded in Skype
INTERNET
executable
Login server
A (private IP) port-restricted NAT
port-restricted NAT B (private IP)
• Stores Skype id’s, passwords and buddy lists
UDP-blocking firewall
UDP-blocking firewall
Skype Relay Experiments
• Used at login for authentication
• Version 1.4: 212.72.49.141 and 195.215.8.141
 Jan 20th to Feb 5th 2006.. Caller and callee machines in IRT lab
 8,822 successful call attempts and 4,904 unique relay IP
Skype, MSN, Yahoo and Google Talk
addresses
 60% of the calls routed through caller SN
Application Memory usage Memory usage Process
Process
Mouth-to Relay nodes in 51 countries
version
before call
after call (caller, priority
priority
ear latency
(caller, callee)
callee)
before call
during call
Caller
TCP
TCP
Media:TCP
N1, N2, N3
19B
19B
TCP
TCP
Callee
19B
19B
Caller and callee on the average exchange 3 msg/s
over TCP with N1, N2 and N3 after call has been
established.
1.4.0.84
19 MB, 19 MB
21 MB, 27 MB Normal
High
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
9 MB, 9 MB
13 MB, 13 MB Normal
Normal
Total successful calls 8,882
% of successful calls between
Relay
distribution
Global
Internet
Skype
1.0.0.80
Caller
SN
713B (7)
3464B (8)
Experimental Setup
The Skype Network
GTalk
Caller (NAT+firewall)
TCP
TCP
96ms
55.34% (4,937)
3.3% (131)
7.9% (392)
0.05% (2)
0.2% (10)
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)
Australia
Europe
NAT
109ms
Caller Skype
Callee Skype
8:00pm and 7:59am
EST
44.66% (3,985)
Asia
NAT
8:00am and 7:59pm
EST