1. Client-Server Model and Sockets
King Fahd University of Petroleum & Minerals
College of Computer Science & Engineering
Information & Computer Science Department
ICS 343
Fundamentals of Computer Networks
Client-Server Model and Sockets
These slides are based on:
- Understanding Networked Applications, David Messerschmitt
- Introduction to Sockets and TCP/IP Protocols, Leon Jololian and George Blank

2. Copyright 1999, University of California
Aug 26, 2012
Client-Server Model
Client
host
Client
host
Server
host

3. Client Server Model
Client application program running on the local machine requests a service from another application program – server – running on the remote machine.
Commonly server provides service to any client, not a particular client
Generally, a client application program that requests a service should run only when it is needed. A server program providing service should run all the time, as it does not know when its services will be needed.

4. Client Server Model
A client opens the communication channel using IP address of the remote host and the port address of the specific server program running on the host – Active open.
Request-response may be repeated several times, the process is finite.
The client closes the communication channel with an Active close.

5. Client Server Model
A server program opens its door for incoming requests from clients but never initiates a service unless explicitly requested – Passive open.
A server program is infinite – runs unless a problem occurs.
Concurrency in client: two or more clients can run at the same time on a machine – current trend, alternatively: one client must start, run, and terminate before another client may start (iterative).

6. Client Server Model
Concurrency in server: An iterative server can process only one request at a time whereas a concurrent server can process many requests at the same time – share its time.
Connectionless iterative server: the ones that use UDP are iterative, server uses one single port, arriving packets wait in line,
Connection oriented concurrent server: the ones that use TCP are normally concurrent,

7. Client–Server Computing
Process takes place
on the server and
on the client
Servers
Store and protect data
Process requests from clients
Clients
Make requests
Format data on the desktop

8. Copyright 1999, University of California
Aug 26, 2012
Example: Server
Server
Client
Client
client sends message to server
Later, recipient’s client retrieves message from server
Message is stored on POP server

9. Copyright 1999, University of California
Aug 26, 2012
Chat Server
Server
Client
Client
Chat server aggregates typing from all users and sends to all clients
Other user’s clients display aggregated typing from chat server
Chat clients send user’s typing to server

10. Client-Server Vs Peer-To-Peer
Copyright 1999, University of California
Copyright 1999, University of California
Aug 26, 2012
Aug 26, 2012
Client-Server Vs Peer-To-Peer
“I want to
collaborate
with my
colleague”
“I want to access
some information”
Client
Peer-to-peer
Server
Client/server 10

11. Client-Server Vs Peer-To-Peer
Copyright 1999, University of California
Aug 26, 2012
Client-Server Vs Peer-To-Peer
Client-server
Asymmetric relationship
Client predominately makes requests, server makes replies
Peer-to-peer
Symmetric relationship

12. Types of Servers Application Servers Audio/Video Servers Chat Servers
Fax Servers
FTP Servers
Groupware Servers
IRC Servers
List Servers
Mail Servers
News Servers
Proxy Servers
Telnet Servers
Web Servers
Z39.50 Servers

13. Socket
Socket: is a construct that supports network input/output.
An application creates a socket when it needs a connection to a network.
It then establishes a connection to a remote application via the socket.
Communication is achieved by reading data from the socket and writing data to it.
Socket acts as an endpoint.

14. Socket

15. Socket A socket is defined in the OS as a structure.
Simplified socket structure with five fields:
- Family: defines the protocol group (IPv4, IPv6, UNIX domain protocols)
- Type: defines the exchange-type (stream, packet, raw)
- Protocol: set to zero for TCP/ UDP
- Local address: combination of local IP and application port address.
- Remote address: combination of remote IP and application port address.

16. Socket
Stream socket: to be used with connection oriented protocol (TCP)
Datagram socket: to be used with connectionless protocol (UDP)
Raw socket: some protocols (ICMP) directly use the service of IP. Raw sockets are used in these applications.

17. The end