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An analysis of Skype protocol
Presented by:Abdul Haleem
Agenda
IntroductionEntities of Skype NetworkKey Components of Skype SoftwareThe communication processSkype Network Traffic
Introduction
Peer to peer applicationChat Call
Skype to Skype Skype to telephone/mobile network Telephone/mobile network to Skype
User search (Find people)
Key Entities
Skype Client (ordinary host)Login ServerSuper Node
Key Entities
Skype Client (ordinary host)
An ordinary host is a Skype application that can be used to place voice calls and send text messages
Login Server
It is the only centralized server User names and passwords are stored at the
login server. User authentication at login is also done at this server.
This server also ensures that login names are unique across the Skype name space
An ordinary host must register itself with the Skype login server
Super Node
Super nodes are the end points where Skype clients connect to.
Any node with a public IP address having sufficient CPU, memory, and network bandwidth is a candidate to become a super node.
Key Components
Host CacheObfuscation Layer
Host Cache
Every Skype client maintains a local list of super node IP address and port pairs. It is called Host Cache
Refreshed regularly Upon startup the client reads an entry from the
host cache, and tries to connect to this SN. If connection fails (the SN is offline; it is no
longer part of the network, etc) it reads next entry and tries to connect to it.
Obfuscation Layer
Everything that is being transferred across the network is being encrypted to ensure privacy.
Uses AES algorithm for encryption.
communication process
Logging inSkype to Skype call – Both public IPsSkype to Skype call - behind NATCall Between Skype and PSTN
Logging in
Client authenticates its user name and password with the login server
advertises its presence to other peers determines the type of NAT it is behind discovers online Skype nodes with public IP
addresses
Logging in
Get an IP and port of a super node from HC Send UDP packets to check availability TCP connection on provided IP and port. TCP connection on provided IP and port 80
(HTTP port). TCP connection on provided IP and port 443
(HTTPS port) Try next super node (total 5 attempts)
Logging in
Skype to Skype call – Both public IPs
If both caller and receiver are on public IPs then they establish a call through a direct TCP connection with each other.
Media is transferred using UDP.
Skype to Skype call - behind NAT
NAT prevents an outside peer from initiating a call to insider peer
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network(e.g., home network)
10.0.0/24
rest ofInternet
Skype to Skype call - behind NAT
Solution to NAT problem Every client is connected to its super node
which is non-NATed. Client A asks its super node that she wants to
call B. Super node of A informs super node of B who in
turn informs B. If B accepts the call then a third non-NATed
peer is chosen which relays data between A and B.
Skype to Skype call - behind NAT
Solution to NAT problem
Call Between Skype and PSTN
For Skype out (Skype to PSTN), the application initially contacts the super node and then the PSTN gateway at port 12340.
The gateway servers are a separate part of the architecture and not a part of the overlay network.
For Skype in (PSTN to Skype) the opposite is used.
Skype Network Traffic
UDP Packets For keep alive messages To discover available super nodes
TCP Packets For establishing connection with super node For user authentication with login server
References
An Analysis of the Skype Peer-to-Peer Internet Telephony Protocol – By Salman A. Baset and Henning Schulzrinne Department of Computer Science Columbia University, New York NY 10027 {salman,hgs}@cs.columbia.edu September 15, 2004
http://www.mjalali.com/blog/Skype101– Skype Architecture–Work in Progress.htm
http://www.skype.com/Skype P2P telephony explained.htm http://en.wikipedia.org/wiki/Talk:Skype_protocol http://technologyevangelist.com/2008/05/A bit about how Skype works.htm
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