[Network] Three-Way Handshake in TCP

TCP的three-way handshake是在哪一個階段發生的?

environment:
* foo: 192.168.104.1, 執行server.c
* bar: 192.168.104.100, 執行client.c

foo$ vim server.c

#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main()
{
    int serverfd, clientfd;
    struct sockaddr_in server;
    struct sockaddr_in client;
    socklen_t len;
    server.sin_family = AF_INET;
    server.sin_port = htons(8080);
    server.sin_addr.s_addr = inet_addr("192.168.104.1");
    serverfd = socket(AF_INET, SOCK_STREAM, 0); 
    bind(serverfd, (struct sockaddr*)&server, sizeof(server));
    listen(serverfd, 10);
    len = sizeof(client);
    clientfd = accept(serverfd, (struct sockaddr*)&client, &len);
    close(clientfd);
    close(serverfd);
    return 0;
}

foo$ gcc -Wall -g server.c -o server

bar$ vim client.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main()
{
    int serverfd;
    struct sockaddr_in server;
    serverfd = socket(AF_INET, SOCK_STREAM, 0); 
    server.sin_family = AF_INET;
    server.sin_port = htons(8080);
    server.sin_addr.s_addr = inet_addr("192.168.104.1");
    connect(serverfd, (struct sockaddr*)&server, sizeof(server));
    close(serverfd);

    return 0;
}

bar$ gcc -Wall -g client.c -o client

單步執行server.c, 在accept()被block住

foo$ cgdb ./server

這時還沒有送出任何的封包

單步執行client.c到connect()

bar$ cgdb ./client

bar在connect()時進行three-way handshake,與foo傳送SYN,SYN/ACK,ACK封包

bar關閉socket,送出FIN封包(若由client關閉socket,則由foo送出FIN封包)