TCP sockets

杨文联

1. What is a socket?
2. What is TCP?
3. Client-server Architecture.
4. Implementation
5. Further Reading
6. Summary

1

A socket is a structure that allows communication between different processes on the same computer or different computers. A socket allows us to communicate by sending and receiving data over the network. In simple terms, it is a way for a computer to talk to other computers on the network.

2

TCP refers to the Transmission Control Protocol. It is one of the main protocol used for communication over the internet. Some of the features of this protocol are:

• It is a connection-oriented communication protocol.
• It used three-way handshake to establish reliable connections.
• TCP gurantees the delivery of the data packets.

3

A client-server architecture is a model in computer networking, where the server provides some service to the client. In this architecture, the client computer sends a request to the server computer through a network (internet). The server accepts this request and sends the required data to the client.

In simple terms, we can say

Server is a remote computer which provides some services.

Client is the one that request the server for these services.

The server-side process can be broken down into the following steps:

socket() – create TCP socket.

bind() – bind the TCP socket to the server address (ip and port).

listen() – waiting for the clients.

accept() – connection is established between the client and server.

recv() and send () – communicate with each other.

close() – close the connection from the client.

The client-side process can be broken down into the following steps:

socket() – create TCP socket.

connect() – connect to the server.

recv() and send () – communicate with each other.

close() – close the connection.

We begin by including all the required header files, defining the IP (Internet Protocol) address and the port number. We also define the required variable used later in this program.

Here is an example of a simple TCP client-server communication code in C for Linux:

Client side code (tcp_client.c):

```c

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/socket.h>

#include <netinet/in.h>

#include <arpa/inet.h>

#define PORT 12345

int main() {

int client_socket;

struct sockaddr_in server_addr;

char message[100];

client_socket = socket(AF_INET, SOCK_STREAM, 0

if (_socket < 0) {

perror("socket creation failed");

exit(EXIT_FAILURE);

}

memset(&server_addr, '0', sizeof(server_addr));

server_addr.sin_family = AF_INET;

server_addr.sin_port = htons(PORT);

if (inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr) <= 0) {

perror("Invalid address/ Address not supported");

exit(EXIT_FAILURE);

}

if (connect(client_socket, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {

perror("Connection failed");

exit(EXIT_FAILURE);

}

strcpy(message, "Hello, server!");

send(client_socket, message, strlen(message), 0);

close(client_socket);

return 0;

}

```

Server side code (tcp_server.c):

```c

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/socket.h>

#include <netinet/in.h>

#define PORT 12345

int main() {

int server_socket, client_socket;

struct sockaddr_in server_addr, client_addr;

char message[100];

server_socket = socket(AF_INET, SOCK_STREAM, 0);

if (server_socket < 0) {

perror("socket creation failed");

exit(EXIT_FAILURE);

}

memset(&server_addr, '0', sizeof(server_addr));

server_addr.sin_family = AF_INET;

server_addr.sin_addr.s_addr = htonl(INADDR_ANY);

server_addr.sin_port = htons(PORT);

if (bind(server_socket, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {

perror("bind failed");

exit(EXIT_FAILURE);

}

if (listen(server_socket, 5) < 0) {

perror("listen failed");

exit(EXIT_FAILURE);

}

client_socket = accept(server_socket, (struct sockaddr *)&client_addr, (socklen_t*)&client_addr);

read(client_socket, message, sizeof(message));

printf("Client message: %s\n", message);

close(client_socket);

close(server_socket);

return 0;

}

```

These are basic examples to get you started with TCP client-server communication in C for Linux.

TCP server

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <unistd.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>

#define PORT 8888

#define MAX_CONNECTIONS 10

int main() {

int server_fd, new_socket, valread;

struct sockaddr_in address;

int addrlen = sizeof(address);

char buffer[1024] = {0};

int client_sockets[MAX_CONNECTIONS] = {0};

if ((server_fd = socket(AF_INET,

SOCK_STREAM, 0)) == 0)

{

perror("Socket creation failed");

exit(EXIT_FAILURE);

}

address.sin_family = AF_INET;

address.sin_addr.s_addr = INADDR_ANY;

address.sin_port = htons(PORT);

if (bind(server_fd, (struct sockaddr *)&address,

sizeof(address)) < 0) {

perror("Bind failed");

exit(EXIT_FAILURE);

}

if (listen(server_fd, MAX_CONNECTIONS) < 0) ｛

perror("Listen failed");

exit(EXIT_FAILURE);

}

fd_set readfds;

while(1) {

FD_ZERO(&readfds);

FD_SET(server_fd, &readfds);

for (int i = 0; i < MAX_CONNECTIONS; i++) {

if (client_sockets[i] > 0) {

FD_SET(client_sockets[i], &readfds);

}

}

if (select(FD_SETSIZE, &readfds, NULL, NULL, NULL) < 0) {

perror("Select failed");

exit(EXIT_FAILURE);

}

if (FD_ISSET(server_fd, &readfds)) {

if ((new_socket = accept(server_fd,

(struct sockaddr *)&address,

(socklen_t*)&addrlen)) < 0) {

perror("Accept failed");

exit(EXIT_FAILURE);

}

for (int i = 0; i < MAX_CONNECTIONS; i++) {

if (client_sockets[i] == 0) {

client_sockets[i] = new_socket;

break;

}

}

}

for (int i = 0; i < MAX_CONNECTIONS; i++) {

if (FD_ISSET(client_sockets[i], &readfds)) {

valread = read(client_sockets[i], buffer, 1024);

printf("Client message: %s\n", buffer);

char *response = "Message received";

send(client_sockets[i], response, strlen(response), 0);

}

}

}

return 0;

}