网络IO-tcp是怎么建立通信的

发表于 更新于 分类于
本文字数： 6.7k 阅读时长 ≈ 6 分钟

学习马士兵课程-内存与IO，通过 netstat、lsof、tcpdump 观察tcp连接时内核在每一步都就行了什么相应的操作。

本文资料来自于马士兵MAC课程-内存与IO。

通过 netstat、lsof、tcpdump 观察 tcp 连接时内核在每一步都就行了什么相应的操作。

阅读本文，你可以知道在编写 java 代码时的每一步对应内核会产生哪些操作，其探究范围在客户端、服务端代码启动到客户端、服务端建立连接。

案例代码

服务器端

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
/**
 * @author: 马士兵教育
 * @create: 2020-05-17 05:34
 * BIO  多线程的方式
 */
public class SocketIOPropertites {


    //server socket listen property:
    private static final int RECEIVE_BUFFER = 10;
    private static final int SO_TIMEOUT = 0;
    private static final boolean REUSE_ADDR = false;
    private static final int BACK_LOG = 2;
    //client socket listen property on server endpoint:
    private static final boolean CLI_KEEPALIVE = false;
    private static final boolean CLI_OOB = false;
    private static final int CLI_REC_BUF = 20;
    private static final boolean CLI_REUSE_ADDR = false;
    private static final int CLI_SEND_BUF = 20;
    private static final boolean CLI_LINGER = true;
    private static final int CLI_LINGER_N = 0;
    private static final int CLI_TIMEOUT = 0;
    private static final boolean CLI_NO_DELAY = false;
/*

    StandardSocketOptions.TCP_NODELAY
    StandardSocketOptions.SO_KEEPALIVE
    StandardSocketOptions.SO_LINGER
    StandardSocketOptions.SO_RCVBUF
    StandardSocketOptions.SO_SNDBUF
    StandardSocketOptions.SO_REUSEADDR

 */


    public static void main(String[] args) {

        ServerSocket server = null;
        try {
            server = new ServerSocket();
            server.bind(new InetSocketAddress(9090), BACK_LOG);
            server.setReceiveBufferSize(RECEIVE_BUFFER);
            server.setReuseAddress(REUSE_ADDR);
            server.setSoTimeout(SO_TIMEOUT);

        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("server up use 9090!");
        try {
            while (true) {

                System.in.read();  //分水岭：

                Socket client = server.accept();  //阻塞的，没有 -1  一直卡着不动  accept(4,
                System.out.println("client port: " + client.getPort());

                client.setKeepAlive(CLI_KEEPALIVE);
                client.setOOBInline(CLI_OOB);
                client.setReceiveBufferSize(CLI_REC_BUF);
                client.setReuseAddress(CLI_REUSE_ADDR);
                client.setSendBufferSize(CLI_SEND_BUF);
                client.setSoLinger(CLI_LINGER, CLI_LINGER_N);
                client.setSoTimeout(CLI_TIMEOUT);
                client.setTcpNoDelay(CLI_NO_DELAY);

                //client.read   //阻塞   没有  -1 0
                new Thread(
                        () -> {
                            try {
                                InputStream in = client.getInputStream();
                                BufferedReader reader = new BufferedReader(new InputStreamReader(in));
                                char[] data = new char[1024];
                                while (true) {

                                    int num = reader.read(data);

                                    if (num > 0) {
                                        System.out.println("client read some data is :" + num + " val :" + new String(data, 0, num));
                                    } else if (num == 0) {
                                        System.out.println("client readed nothing!");
                                        continue;
                                    } else {
                                        System.out.println("client readed -1...");
                                        System.in.read();
                                        client.close();
                                        break;
                                    }
                                }

                            } catch (IOException e) {
                                e.printStackTrace();
                            }

                        }
                ).start();

            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                server.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }
}

客户端

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
/**
 * @author: 马士兵教育
 * @create: 2020-05-17 16:18
 */
public class SocketClient {

    public static void main(String[] args) {

        try {
            Socket client = new Socket("192.168.150.11",9090);

            client.setSendBufferSize(20);
            client.setTcpNoDelay(true);
            OutputStream out = client.getOutputStream();

            InputStream in = System.in;
            BufferedReader reader = new BufferedReader(new InputStreamReader(in));

            while(true){
                String line = reader.readLine();
                if(line != null ){
                    byte[] bb = line.getBytes();
                    for (byte b : bb) {
                        out.write(b);
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

在服务器端启动但客户端未连接时

  • 启动服务器端代码

    1
    2
    javac SocketIOPropertites.java
    java SocketIOPropertites

    此时客户端创建了 serverSocket 并绑定了端口9090，阻塞在 System.in.read() 处

  • netstat

    1
    2
    3
    4
    5
    6
    7
    -a 显示所有状态的socket  
    -n 不做名字解析，不加此参数，80端口会显示成http,127.0.0.1显示成localhost,uid为0显示成root等等    
    -e 显示更多信息如用户，inode
    -p 显示pid和程序名字
    -t 显示tcp链接
    -u 显示udp链接
    -x 显示unix套接字

    通过 netstat 可以看出系统中多了一行监听9090端口的tcp连接

    1
    2
    3
    4
    5
    6
    7
    8
    9
    netstat -natp
    Active Internet connections (servers and established)
    Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
    tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      7631/sshd           
    tcp        0      0 192.168.203.133:22      192.168.203.1:3137      ESTABLISHED 7846/sshd: root@pts 
    tcp        0      0 192.168.203.133:22      192.168.203.1:7070      ESTABLISHED 15711/sshd: root@pt 
    tcp        0     36 192.168.203.133:22      192.168.203.1:6022      ESTABLISHED 7942/sshd: root@pts 
    tcp6       0      0 :::22                   :::*                    LISTEN      7631/sshd           
    tcp6       0      0 :::9090                 :::*                    LISTEN      15853/java

  • lsof

    通过 lsof 观察到运行java代码的进程出现了一个文件描述符处于Listen状态

    1
    2
    3
    4
    lsof -p 15853
    COMMAND   PID USER   FD   TYPE             DEVICE  SIZE/OFF   NODE NAME
    ...
    java    15853 root    6u  IPv6              53184       0t0    TCP *:websm (LISTEN)

客户端请求连接但服务器端未accept

  • 启动客户端

    1
    2
    javac SocketClient.java
    java SocketClient

  • 解决异常java.net.NoRouteToHostException: 没有到主机的路由

    1
    2
    # 关闭防火墙
    systemctl stop firewalld.service

  • tcpdump

    1
    2
    3
    4
    5
    6
    tcpdump -nn -i ens33 port 9090
    tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
    listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes
    15:00:49.829735 IP 192.168.203.132.37058 > 192.168.203.133.9090: Flags [S], seq 493363808, win 29200, options [mss 1460,sackOK,TS val 14267030 ecr 0,nop,wscale 7], length 0
    15:00:49.829764 IP 192.168.203.133.9090 > 192.168.203.132.37058: Flags [S.], seq 2693402151, ack 493363809, win 1152, options [mss 1460,sackOK,TS val 15102986 ecr 14267030,nop,wscale 0], length 0
    15:00:49.829979 IP 192.168.203.132.37058 > 192.168.203.133.9090: Flags [.], ack 1, win 229, options [nop,nop,TS val 14267031 ecr 15102986], length 0

    可以发现已经发生了三次握手

  • netstat

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    netstat -natp
    Active Internet connections (servers and established)
    Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
    tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      7631/sshd           
    tcp        0      0 192.168.203.133:22      192.168.203.1:3137      ESTABLISHED 7846/sshd: root@pts 
    tcp        0      0 192.168.203.133:22      192.168.203.1:7070      ESTABLISHED 15711/sshd: root@pt 
    tcp        0     36 192.168.203.133:22      192.168.203.1:6022      ESTABLISHED 7942/sshd: root@pts 
    tcp6       0      0 :::22                   :::*                    LISTEN      7631/sshd           
    tcp6       1      0 :::9090                 :::*                    LISTEN      16423/java          
    tcp6       0      0 192.168.203.133:9090    192.168.203.132:37058   ESTABLISHED -

    在内核层面已经建立起了socket连接，并且当客户端发出信息后，服务器端是已经被接受了，即使这个连接没有分配给任何一个进程

当服务器端accept后

  • netstat

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    netstat -natp
    Active Internet connections (servers and established)
    Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
    tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      7631/sshd           
    tcp        0      0 192.168.203.133:22      192.168.203.1:3137      ESTABLISHED 7846/sshd: root@pts 
    tcp        0      0 192.168.203.133:22      192.168.203.1:7070      ESTABLISHED 15711/sshd: root@pt 
    tcp        0     36 192.168.203.133:22      192.168.203.1:6022      ESTABLISHED 7942/sshd: root@pts 
    tcp6       0      0 :::22                   :::*                    LISTEN      7631/sshd           
    tcp6       0      0 :::9090                 :::*                    LISTEN      16423/java          
    tcp6       0      0 192.168.203.133:9090    192.168.203.132:37058   ESTABLISHED 16423/java

    该连接已经分配给了对应进程

  • lsof

    1
    2
    3
    4
    lsof -p 16423
    COMMAND   PID USER   FD   TYPE             DEVICE  SIZE/OFF   NODE NAME
    ...
    java    16423 root    7u  IPv6              70977       0t0    TCP CentOS7.6:websm->192.168.203.132:37058 (ESTABLISHED)

    可以发现该进程已经被分配了文件描述符

总结

  • socket本质是一个四元组，通过 CIP_CPORT + SIP_SPORT 能确认唯一socket
  • new ServerSocket() 在内核层面的操作就是开启了一个监听 port 的 socket
  • 当两台服务器之间建立连接时并未将此 socket 分配给进程，即进程并未分配对应的文件描述符 fd，但服务器之间已经可以就行信息传递