TCP connection establishment and closing status and data transmission communication process [including PHP socket API test experiment code]

 <?php
/**
 * Created by PhpStorm.
 * User: 1655664358@qq.com
 * Date: 2019/6/19
 * Time: 9:55
 */
error_reporting(E_ALL);
/* Allow the script to hang around waiting for connections. */
set_time_limit(0);
/* Turn on implicit output flushing so we see what we&#39;re getting
 * as it comes in. */
ob_implicit_flush();
$address = &#39;0.0.0.0&#39;;  //要监听的ip地址
$port = 54321;//要监听的端口 
//创建一个socket AF_INET是一个协议族 （一般都有AF_INET,AF_INET6,AF_UNIX c语言提供也是，只不过是地址族），而PHP叫网络协议  
//C语言提供的协议族是PF_INET,PF_INET6,PF_UNIX  
//SOCK_STREAM sock的服务类型  这里是字节流的服务类型用于TCP
if (($sock = socket_create(AF_INET, SOCK_STREAM, SOL_TCP)) === false) {
    echo "socket_create() failed: reason: " . socket_strerror(socket_last_error()) . "\n";
}
//socket选项  ，选项一般在socket创建后设置 用于设置TCP的连接属性
//选项几乎和c差不多一样
//一般来说这些选项我们可以通过修改系统内核来调整 
if (!socket_set_option($sock, SOL_SOCKET, SO_REUSEADDR, 1)) {
    echo &#39;Unable to set option on socket: &#39;. socket_strerror(socket_last_error()) . PHP_EOL;
}
if (!socket_set_option($sock, SOL_SOCKET, SO_REUSEPORT, 1)) {
    echo &#39;Unable to set option on socket: &#39;. socket_strerror(socket_last_error()) . PHP_EOL;
}
//绑定【有的叫命名socket】
if (socket_bind($sock, $address, $port) === false) {
    echo "socket_bind() failed: reason: " . socket_strerror(socket_last_error($sock)) . "\n";
}
//监听 此时服务器进入  LISTEN状态
if (socket_listen($sock, 5) === false) {
    echo "socket_listen() failed: reason: " . socket_strerror(socket_last_error($sock)) . "\n";
}
do {
//接受客户端连接sock  从系统内核接受队列里取  如果取出则双方进入了ESTABLISHED状态
    if (($msgsock = socket_accept($sock)) === false) {
        echo "socket_accept() failed: reason: " . socket_strerror(socket_last_error($sock)) . "\n";
        break;
    }
    /* Send instructions. */
    $msg = "\nWelcome to the PHP Test Server. \n" .
        "To quit, type &#39;quit&#39;. To shut down the server type &#39;shutdown&#39;.\n";
    socket_write($msgsock, $msg, strlen($msg));
    do {
    //读客户端数据【从TCP接收接受缓冲区取】
        if (false === ($buf = socket_read($msgsock, 2048, PHP_NORMAL_READ))) {
            echo "socket_read() failed: reason: " . socket_strerror(socket_last_error($msgsock)) . "\n";
            break 2;
        }
        if (!$buf = trim($buf)) {
            continue;
        }
        if ($buf == &#39;quit&#39;) {
            break;
        }
        if ($buf == &#39;shutdown&#39;) {
        //发送FIN结束报文，正常情况服务器会进入CLOSED状态
            socket_close($msgsock);
            break 2;
        }
        $talkback = "PHP: You said &#39;$buf&#39;.\n";
        socket_write($msgsock, $talkback, strlen($talkback));
        echo "$buf\n";
    } while (true);
    socket_close($msgsock);
} while (true);
socket_close($sock);

tcpdump -A -XX -i lo