Skype
Bruce Maggs
Gratuitous Quote of the Day
"There have been members of the Maggs family in
south east Suffolk since the great subsidy of 1327
but they were of no great distinction either then or
afterwards.“
-- from Allan Farquar Bottomley, "Introduction," in
the Southwold Diary of James Maggs, 1818-1876,
edited by Allan Farquar Bottomley, Volume I 1818-1848, (Suffolk: Published for the Suffolk
Records Society by the Boydell Press, 1983), p.1.
2
References
• S. A. Baset and H. G. Schulzrinne, ”An
Analysis of the Skype Peer-to-Peer
Internet Telephony Protocol,”
Proceedings of the 25th IEEE
International Conference on Computer
Communications (INFOCOMM),
April 2006, pp. 1-11.
• Wikipedia article
3
Skype
• Developed by Danes Niklas Zennstrom
•
•
•
•
and Janus Friis, creators of Kazaa,
Skype, and Joost, and a team of software
developers in Estonia.
First peer-to-peer VOIP application.
12,049,795 people on-line today, 276M
accounts in Q4 2007, $115M Q4 revenue
Now owned by eBay.
Protocol not publically disclosed
4
Architecture
Login Server
Super Node (SN)
Skype Client (SC)
5
Supernodes
• Any client with a public IP address and
sufficient performance can be a
supernode
• Skype claims at most 40kbits/sec
upload/download (?)
6
Login Protocol
• SC connects to SN listed in host cache
•
•
•
•
•
(stored locally in Windows registry), up to 200
entries
On first use, SC uses to bootstrap SNs
SC then authenticates with login server via
user name and password
If access to login server is blocked, login is
routed through SN
SC also checks with skype.com via HTTP for
latest software version
Mysterious ICMP messages to various
continents
7
NAT and Firewall Traversal
• UDP preferred for voice packets
• SC chooses sending/listening port at
random
• Uses ports 80, 443 if random port is
blocked
• Tries UDP first, then TCP
• Uses SN as a relay station, if necessary
8
Encryption
• All communications are encrypted!
• 256-bit AES (Advanced Encryption
Standard, a.k.a. Rijndael) symmetric
cryptosystem
• 1024-bit RSA to exchange private keys
• User public keys certified with1536 or
2048-bit certificates
9
Finding a User
• Buddy list stored locally in earlier
versions, now centrally
• Skype claims to find users logged on
within past 72 hours
• User can simultaneously login in at
multiple locations
• Messages, calls, sent to all
instantiations of user
10
“Global Index” Technology
•
•
•
•
SC queries its SN about user location
SN responds with 8 IP addresses
If SC asks again, SN provides 16 more
Search faster the second time, evidence
that search results are cached at
intermediate nodes
• Rest of the protocol is a bit mysterious
11
Telephony
• iLBC, iSAC, and iPCM codecs developed by
•
•
•
•
•
GlobalIPSound, other codecs
67-byte packets at 40kbits/sec, needs
16kbits/sec minimum
Frequency band approximately 50-8000Hz
PSTN uses uncompressed 56kbits/sec, 8000
samples per sec (max 4000Hz), nonuniform
intervals
Skype keeps transmitting during silence!
Calls on hold send less frequent keepalive
messages
12
Bridging to PSTN
• SkypeIn and SkypeOUT servers for
making PSTN-to-PC and PC-to-PSTN
calls
• To use SkypeIn, local phone number
assigned to PC
13
Conferencing
A
A+C
C
B
B
A+ B
C
Most powerful machine (A) chosen as
conference host.
14