Socket Programming

Socket Programming is a way of connecting 2 nodes on a network to communicate with each other. One socket (ie; the Server) listens on a particular port at an IP, while the other socket (ie; the Client) reaches out to the other to form a connection.

Socket programming is widely used in instant messaging applications, binary streaming, document collaborations & online streaming platforms.

If we are creating a connection between the client & server using TCP then it has few functionalities like: TCP is suited for applications that require high reliability. It is used by protocols like HTTP, HTTPS, FTP, SMTP & Telnet. TCP rearranges the data packets in the order specified. Therefore, there is a absolute guarantee that the data transferred remains intact in the way it was sent.

TCP does flow control & requires 3 packets to setup a socket communication before any user data can be sent. TCP handles reliability & congestion control. It also does error checking & error recovery.

Image	Description
	TCP Server: Using create(), Creates a TCP Socket. Using bind(), binds the socket to server address. Using listen() it waits for the client to approach the server to make a connection. Using accept() the connection is established between client & server and it is ready to transfer the data. TCP Client: It creates TCP Socket & connects to the server.

Necessity of the header file imported

Header File	Description
stdio.h	Used for standard input/output operations. Used for printf() to display messages in the console or to use getchar() to read server's response from the keyboard.
netdb.h	Used for network database operations. Commonly used for DNS lookups & Network Services.
netinet/in.h	Purpose is for Internet address family structures. - struct sockaddr_in: IPv4 socket address structure - INADDR_ANY: Binds to all available interfaces
stdlib.h	Used for common library functions like exit() to terminate programs with status code
string.h	Used for string manipulation functions like - bzero(): Clear/zero out buffer memory - strncmp(): Compare strings
sys/socket.h	This library contains core socket programming interfaces like socket(), bind(), accept(), listen(), Socket constants like AF_INET & SOCK_STREAM
sys/types.h	Used for system datatypes like socketlen_t
unistd.h	UNIX standard functions like read(), write(), close()
arpa/inet.h	Used for Internet Operations & address conversions. inet_addr converts the IP address string to binary network format.

Main Components of TCP Server Program

• Creates a TCP Socket SOCK_STREAM and with IPV4 addressing AF_INET
• Configures server address & listens on all available network interfaces (INADDR_ANY) and at port 8080

servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(PORT);

• Binds the socket to IP address & port

bind(sockfd, (SA*)&servaddr, sizeof(servaddr))

• Server starts listening for connections. Queue upto 5 pending connections.

listen(sockfd, 5)

• Accepts all incoming client connections & blocks until a client connects. Returns a new socket descriptor for communication

connfd = accept(sockfd, (SA*)&cli, &len);

• Reads message from the client into the buffer

read(connfd, buff, sizeof(buff));

• Display's the client message

printf("From client: %s\t To client : ", buff);

• Reads server response from keyboard

while ((buff[n++] = getchar()) != '\n');

• Sends server response back to the client

write(connfd, buff, sizeof(buff));

• Exits the chat loop if the server types exit

if (strncmp("exit", buff, 4) == 0)

• Closes the socket after chat ends

close(sockfd);

Main Components of TCP Client Program

• Socket creation & Server address Configuration: Creates a TCP socket with IPv4 addressing with PORT number set to 8080
  
  inet_addr() converts string IP to network byte order htons() converts port number to network byte order

sockfd = socket(AF_INET, SOCK_STREAM, 0);
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
servaddr.sin_port = htons(PORT);

• Connect to the server (Client uses connect() instead of bind(), listen(), accept()) Initiates a TCP connection to the server & blocks until the connection is established or fails.

connect(sockfd, (SA*)&servaddr, sizeof(servaddr))

Output