Transcript skype problem

An analysis of Skype protocol
Presented by:
Abdul Haleem
Agenda
Introduction
Entities of Skype Network
Key Components of Skype Software
The communication process
Skype Network Traffic
Introduction
Peer to peer application
Chat
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 Server
Super 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 Cache
Obfuscation 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 in
Skype to Skype call – Both public IPs
Skype to Skype call - behind NAT
Call 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
rest of
Internet
local network
(e.g., home network)
10.0.0/24
10.0.0.4
10.0.0.1
10.0.0.2
138.76.29.7
10.0.0.3
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