1. Skype P2P
Kedar Kulkarni
04/02/09

2. Goals Study Skype working Study VOIP using P2P Use of P2P for VOIP
Analyze effects of Churn
Data for future P2P VOIP systems

3. Skype Communication system VOIP IM File Transfer Video User Search
Paid services

4. Skype P2P Not much known Similar to Kazaa Hierarchical P2P
Supernodes/nodes
Login Server

5. Basic Operation Control Traffic / Media Traffic
Nodes connect to supernodes
Behind NAT and Firewalls
Host Cache
Supernode promotion
Supernode Relay
Robust
Encryption

6. Skype User Search Uses Global Index technology
Skype always could find an Online User
Search query sent to SN
SN returns 8 IPs of possible matches
if not found SN sends 24 next. And so on.
For SC behind NAT, SN will process the query
Login Server is the fall back option.

7. Skype Calls To call, callee should be in the friends list
If not, first search is done.
Public SCs
Caller establishes TCP connection with Callee
Caller behind NAT
Signaling information exchanged with SN first
Media flow between Caller and callee

8. Calls Caller and Callee behind NAT
Signaling information exchanged with SN
Media flow through relay

9. Experiments Supernode network activity Supernode and client population
Observed a supernode for 135 days with 13GB of total data.
Supernode and client population
Obtained SN info from host cache
250K total SNs found
Supernode Presence
Observed which SNs were online by sending ping msgs
6000 random SNs at 30 min interval

10. Characterization
Diurnal Behavior
Stable, than skype users clients

11. Characterization
Fraction of supernodes joining or departing the network
Log­log plot of the complimentary CDF of supernode session times.

12. Characterization
Semi­log plot of CDF of bandwidth used by the supernode
Geographic distribution of supernodes

13. Comparison with other IMs

14. Skype Supernode Map
US: 83.7%, Asia8.9%, Europe 7.1%

15. Conclusion Diurnal, work-week behavior. Similar to web browsing
Stability of SNs mitigates churn
SNs use little bandwidth, occasionally relay media and file transfer
Best Mouth to ear latency

16. Papers An Experimental Study of the Skype Peer-to-Peer VoIP System
S. Guha, Neil Daswani, and Ravi Jain
An Analysis of the Skype Peer-to-Peer Internet Telephony Protocol.
S.A.Baset and H.G. Schulzrine