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 1024



int 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  loop
    while (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;
}