Socket Programming Lecture Notes

Socket Programming Lecture Date 10/04/2025 Goals We plan to learn the following from this lecture: What is a socket? The client-server model TCP socket API UDP socket API Concurrent server design Byte order Key Concepts Sockets: Endpoints for network communication, identified by an...

Socket Programming Lecture Date 10/04/2025 Goals We plan to learn the following from this lecture: What is a socket? The client-server model TCP socket API UDP socket API Concurrent server design Byte order Key Concepts Sockets: Endpoints for network communication, identified by an IP address and port number. Client-Server Model: A common architecture where a server listens for connections from clients. Protocols: Rules that govern how data is transmitted (e.g., TCP for reliable, ordered data, UDP for faster, unordered data). Functions: Socket programming typically involves functions like socket(), bind(), listen(), accept(), connect(), send(), recv(), and close(). Socket Programming Socket programming allows applications to communicate over a network by establishing a connection between two endpoints, represented by sockets, using protocols like TCP or UDP. It's a fundamental concept for building network applications. Types of Sockets: Stream Socket (TCP): Reliable, connection-oriented Datagram Socket (UDP): Fast, connectionless "Think of a socket as a phone — it connects you to another person (device) to talk (send/receive data)."  The Client-Server Model Client Server A network communication model where: Requests Provides service service Client initiates the request Passive Active, always Server provides services or data listening Characteristics: Temporary Always ready connection 1. socket() Purpose: Creates a new socket (communication endpoint). int sockfd = socket(int domain, int type, int protocol); domain: Address family (e.g., AF_INET for IPv4) type: Socket type (SOCK_STREAM for TCP, SOCK_DGRAM for UDP) protocol: Usually 0 for default Used by: Both Server & Client Returns: A file descriptor (socket ID), or -1 on error. 2. bind() Purpose: Assigns an IP address and port to the socket. bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen); Required for server to define where to listen (IP + Port). Not usually needed for clients unless binding to a specific port/IP. Used by: Server 3. listen() Purpose: Marks the socket as passive, ready to accept incoming connections. listen(int sockfd, int backlog); backlog: Number of pending connections the queue will hold. Used by: Server 4. accept() Purpose: Accepts a new incoming connection on a listening socket. int clientfd = accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen); Blocks until a connection is made. Returns a new socket descriptor for the client. Used by: Server 5. connect() Purpose: Initiates a connection to a server socket. connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen); Used by client to connect to server’s IP & port. Returns 0 on success, -1 on error. Used by: Client 6. send() Purpose: Sends data over a connected socket. send(int sockfd, const void *buffer, size_t length, int flags); Sends bytes of data. flags is often set to 0. Used by: Both Server & Client 7. recv() Purpose: Receives data from a connected socket. recv(int sockfd, void *buffer, size_t length, int flags); Waits for data from peer. Returns number of bytes received. Used by: Both Server & Client 8. close() Purpose: Closes the socket and releases resources. close(int sockfd); Used by: Both Server & Client Server Example Flow Summary int sockfd = socket(AF_INET, SOCK_STREAM, 0); bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)); listen(sockfd, 5); int clientfd = accept(sockfd, (struct sockaddr*)&client_addr, &addrlen); recv(clientfd, buffer, size, 0); send(clientfd, response, size, 0); close(clientfd); close(sockfd); Client Example Flow Summary int sockfd = socket(AF_INET, SOCK_STREAM, 0); connect(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)); send(sockfd, message, size, 0); recv(sockfd, buffer, size, 0); close(sockfd); TCP Socket API As shown in the figure, the steps for establishing a TCP socket on the client side are the following: Create a socket using the socket() function; Connect the socket to the address of the server using the connect() function; Send and receive data by means of the read() and write() functions. Close the connection by means of the close() function. The steps involved in establishing a TCP socket on the server side are as follows: Create a socket with the socket() function; Bind the socket to an address using the bind() function; Listen for connections with the listen() function; Accept a connection with the accept() function system call. This call typically blocks until a client connects with the server. Send and receive data by means of send() and receive(). Close the connection by means of the close() function. UDP client-server. UDP client-server. As shown in the Figure, the steps of establishing a UDP socket communication on the client side are as follows: Create a socket using the socket() function; Send and receive data by means of the recvfrom() and sendto() functions. The steps of establishing a UDP socket communication on the server side are as follows: Create a socket with the socket() function; Bind the socket to an address using the bind() function; Send and receive data by means of recvfrom() and sendto(). Coming Byte order Concurrent Servers