你不知道的TCP协议：四次握手！

前言：我们都知道建立一个tcp连接需要进行三次握手，甚至被问到为什么不是四次握手、两次握手

本文将要介绍tcp协议中的四次握手

正文：

当一个客户端向服务端发起tcp连接请求时，先发送一个SYN包，这个请求是单向的，当服务端接收到该SYN包并进行ACK回复时，三次握手正式开始

那么，两端同时向对方发起连接请求会发生什么呢？图例请参照"tcp\ip协议详解卷一 p427"

答案是，两端同时向对方发送SYN包，收到对方的SYN包后各自又向对方发送一个SYN+ACK包，一共四次交互

详细介绍以及图例请参照"tcp\ip协议详解卷一 p427"

代码实现：

#include <stdio.h>
#include <netinet/tcp.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <error.h>
#include <fcntl.h>
#include <arpa/inet.h>#define IP_SIZE   20
#define BUFFER_SIZE 1024int connect_2(int sockfd, char *ip, int port) {struct sockaddr_in addr;memset(&addr, 0, sizeof(struct sockaddr_in));addr.sin_family = AF_INET;addr.sin_port = htons(port);addr.sin_addr.s_addr = inet_addr(ip);if (connect(sockfd, (struct sockaddr*)&addr, sizeof(struct sockaddr_in)) < 0) {return -4;}return sockfd;
}int main(int argc, char **argv) {if(argc < 3) {printf("please enter ip and port correctly\n");return -1;}char *ip = argv[1];int port  = atoi(argv[2]);int sockfd = socket(AF_INET, SOCK_STREAM, 0);struct sockaddr_in addr;memset(&addr, 0, sizeof(struct sockaddr_in));addr.sin_family = AF_INET;addr.sin_port = htons(port);addr.sin_addr.s_addr = htonl(INADDR_ANY);int ret = bind(sockfd, (struct sockaddr*)&addr, sizeof(struct sockaddr_in));if (ret == -1) {perror("bind");return -2;}printf("bind success\n");while(1){ret = connect_2(sockfd, ip, port);if (ret < 0) {usleep(1);continue;}break;}printf("connection established\n\n");char recv_buffer[BUFFER_SIZE] = {0};char send_buffer[BUFFER_SIZE] = {0};// recv + send  loopwhile (1) {printf("enter messages to send:\n");fgets(send_buffer, BUFFER_SIZE, stdin);ret = send(sockfd, send_buffer, strlen(send_buffer), 0);if (ret < 0) {perror("send");} else {printf("%d bytes were sent\n\n", ret);}memset(send_buffer, 0, sizeof(send_buffer));ret = recv(sockfd, recv_buffer, BUFFER_SIZE, 0);if (ret < 0) {perror("recv");break;}else if (ret == 0) {perror("disconnect");break;}else {printf("recv %d bytes : %s\n", ret, recv_buffer);memset(recv_buffer, 0, sizeof(recv_buffer));}}close(sockfd);return 0;
}