1. Client Server Model The client machine (or the client process) makes the request for some resource or service, and the server machine (the server process) handles the request and sends the response (result) back to the client. In database server, the client queries the server for records from the database, and the server looks up the records and responds to the client. The user on the client machine may not even know what machine in the network actually has the database. In fact, the database may be distributed across several machines on the network.

2. Client Server Model The Client needs to know the existence and the address of the server. However, the server does not need to know the existence or address of the client prior to the connection. Once a connection is established, both sides can send and receive information.

3. How to establish a connection? Both client and server will construct a socket. A socket is one end of an interprocess communication channel. The process to establish a socket on the client side is different than the process to establish a socket on the server side.

4. How to establish a socket on the client side Create a socket using the socket() system call. Connect the socket to the address of the server using the connect() system call Send and receive data. You can use read() and write() system calls. However, there are different ways to send and receive data.

5. How to establish a socket on the server side Create a socket using the socket() system call. Bind the socket to an address using the bind() system call. For a server socket on the Internet, an address consists of a port number of the host machine. Listen for the connections using the listen() system call. Accept a connection using the accept() system call Send and receive data. You can use read() and write() system calls. However, there are different ways to send and receive data.

6. Address domain and the socket type The client and server processes can communicate with each other only if their sockets are in the same domain and of the same type. Two widely used address domains are:  Unix domain (in which two processes share a common file system).  Internet domain (in which two processes run on any two hosts on the Internet)

7. Unix and Internet domains The address of a socket in the Unix domain is a character string, an entry in the file system. The address of a socket in the Internet domain is the Internet address (IP address) of the host machine. Each socket needs a port number (16 bits unsigned integers) on the host. The lower numbers are reserved in Unix for standard services (e.g. 21 is used as the port number for the FTP server). Generally, port number above 2000 are available.

8. Socket types Two widely used socket types are: Stream socket: treats communications as a continuous stream of characters.It uses TCP Datagram socket: reads entire message at one time. It uses UDP.

9. Sample Client and Server codes Files (client and server) are posted on the Internet. Download these two files (server.c and client.c) Compile them into two executable files, server and client. How to run server?  Start the server first.  If your server is running on a machine called Merlin, you need to pass the port number as an argument (any number between 2000 and 65535).  If the port number is already in use, the server will let you know and exit. Then select another port number.  To run server, type server 3000

10. How to run the client To run the client, you have to pass two arguments:  The name of the host on which the server is running  The port number on which the server is listening for connections.  To run client, type client Merlin 3000

11. A Server on the Internet domain /* A simple server in the internet domain using TCP The port number is passed as an argument */ #include #include // def. of data types used in system calls #include //def. of structures needed for sockets #include // constants and structures for Internet // domain addresses void error(char *msg) // func. Is called when system calls fail { perror(msg); exit(1); }

12. Server Code int main(int argc, char *argv[]) { int sockfd, newsockfd, portno, clilen; // sockfd, newsockfd=>file // descriptor, portno stores the port number on which the server // accepts connections, clilen=> size of the address of the client, n=> no. of char to read or write char buffer[256]; struct sockaddr_in serv_addr, cli_addr; //sockaddr_in => structure containing Internet address int n; if (argc < 2) { fprintf(stderr,"ERROR, no port provided\n"); exit(1); }