【TCP】Recv-Q和Send-Q 不要在有什么误解了

1 LISTEN状态：表示队列中的连接数量，非LISTEN状态为字节数量；

非 LISTEN 状态

Recv-Q 表示 receive queue 中的 bytes 数量；
Send-Q 表示 send queue 中的 bytes 数值。

2 Recv-Q 正在等待accept的连接数，Send-Q表示全连接队列容量

表示的当前等待服务端调用 accept 来三次握手的 listen backlog 数值，即图中的全连接队列值（最大值为Send_Q+1），当客户端通过connect() 去连接正在 listen() 的服务端时，这些连接会经过半连接队列，接收到ACK后，进入accept queue(全连接队列) ，在里面等待直到被服务端 accept()；

Send-Q 表示的则是最大的 listen backlog 数值，这就是 min(backlog, /proc/sys/net/core/somaxconn)的值

3 详细过程如下图

4 我们实际操作下：观察Recv-Q 的变化

启动服务端执行命令：（注意：在连接事件发生后|accept操作执行前会睡眠Thread.sleep(100000);）

使用NIO作为服务端监听TCP端口：9779

~/recv_q_send_q# /usr/bin/java NIOServer

NOIServer start run in port 9779

可以看到默认Send-Q值为50；

如果并发请求的链接数超多50，多余的怎样？感兴趣的可以自己修改代码测试mark

如果等待时间过长客户端也会收到服务端的拒绝反馈，正常情况下如果超出的连接数多不并且等待时间不多，可以得到服务端的正常响应

启动客户端执行命令：

/usr/bin/java NIOClient

等待100秒后：

5 这就解释了Recv-Q 在服务端debug时为啥会阻断后面的链接接入，因为全连接队列满了，后面的链接只能等待

参考NIO代码：

server端import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;public class NIOServer {private int num;private static final int BLOCK = 4096;private static final ByteBuffer sendB = ByteBuffer.allocate(BLOCK);private static final ByteBuffer receB = ByteBuffer.allocate(BLOCK);private Selector selector;public NIOServer(int port) throws IOException {//开启ServerSocketChannelServerSocketChannel serverSocketChannel = ServerSocketChannel.open();//设置为非阻塞serverSocketChannel.configureBlocking(false);//获取ServerSocketServerSocket serverSocket = serverSocketChannel.socket();//绑定ServerSocket提供服务的端口serverSocket.bind(new InetSocketAddress(port));//开启选择器selector = Selector.open();//将ServerSocketChannel注册到选择器上serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);System.out.println("NOIServer start run in port " + port);}/*** 监听选择器的数据** @throws IOException*/private void listen() throws IOException {//循环监听，事件驱动模式while (true) {//select()阻塞，等待有事件发生时唤醒selector.select();Set<SelectionKey> selectionKeys = selector.selectedKeys();Iterator<SelectionKey> iterator = selectionKeys.iterator();while (iterator.hasNext()) {SelectionKey selectionKey = iterator.next();//处理完后移除该事件iterator.remove();//处理该事件handleKey(selectionKey);}}}/*** 处理选择器的监听事件** @param selectionKey 选择器的监听事件key* @throws IOException*/private void handleKey(SelectionKey selectionKey) throws IOException {ServerSocketChannel serverSocketChannel = null;SocketChannel socketChannel = null;int count = 0;//客户端新连接if (selectionKey.isAcceptable()) {//开启通道连接serverSocketChannel = (ServerSocketChannel) selectionKey.channel();try {System.out.println(System.currentTimeMillis() + "收到请求我在睡眠100秒...");Thread.sleep(100000);} catch (InterruptedException e) {e.printStackTrace();}socketChannel = serverSocketChannel.accept();System.out.println(System.currentTimeMillis() + "accept...");//设置为非阻塞socketChannel.configureBlocking(false);//将通道注册到选择器socketChannel.register(selector, SelectionKey.OP_READ);} else if (selectionKey.isReadable()) {//获取读事件通道socketChannel = (SocketChannel) selectionKey.channel();//清除原先读缓存receB.clear();//读取通道缓存count = socketChannel.read(receB);if (count > 0) {//解析通道缓存数据String receMsg = new String(receB.array(), 0, count);System.out.println("receive from client " + receMsg);//注册切到写事件socketChannel.register(selector, SelectionKey.OP_WRITE);}} else if (selectionKey.isWritable()) {//获取写事件通道socketChannel = (SocketChannel) selectionKey.channel();//清除发送缓存数据sendB.clear();String sendMsg = "num " + num++;//设置待发送的数据sendB.put(sendMsg.getBytes());//准备写sendB.flip();int write = socketChannel.write(sendB);System.out.println("send to client " + sendMsg);//注册切到读事件socketChannel.register(selector, SelectionKey.OP_READ);}}public static void main(String[] args) throws Exception {new NIOServer(9779).listen();}
}
客户端代码：import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;public class NIOClient {private static final int BLOCK = 4096;private static final ByteBuffer sendB = ByteBuffer.allocate(BLOCK);private static final ByteBuffer receB = ByteBuffer.allocate(BLOCK);private SocketChannel socketChannel;private Selector selector;public NIOClient(String ip, int port) throws IOException {//开启通道socketChannel = SocketChannel.open();//设置为非阻塞socketChannel.configureBlocking(false);//开启选择器selector = Selector.open();//将通道注册到选择器socketChannel.register(selector, SelectionKey.OP_CONNECT);//连接服务端socketChannel.connect(new InetSocketAddress(ip, port));}/*** 连接服务器*/public void connect() throws IOException {Set<SelectionKey> selectionKeys;Iterator<SelectionKey> iterator;SelectionKey selectionKey;int index = 0;while (true && index < 1000) {index++;selector.select();selectionKeys = selector.selectedKeys();iterator = selectionKeys.iterator();while (iterator.hasNext()) {selectionKey = iterator.next();handleKey(selectionKey, index);}selectionKeys.clear();}//交互10次后关闭连接socketChannel.close();}/*** 处理选择器监听事件** @param selectionKey*/public void handleKey(SelectionKey selectionKey, int index) throws IOException {SocketChannel client;int count = 0;// 连接事件if (selectionKey.isConnectable()) {System.out.println("client connect......");client = (SocketChannel) selectionKey.channel();if (client.isConnectionPending()) {client.finishConnect();sendB.clear();sendB.put("Hello, Server".getBytes());sendB.flip();client.write(sendB);}client.register(selector, SelectionKey.OP_READ);} else if (selectionKey.isReadable()) {// 读事件client = (SocketChannel) selectionKey.channel();receB.clear();count = client.read(receB);if (count > 0) {String receMsg = new String(receB.array(), 0, count);System.out.println("receive from server " + receMsg);client.register(selector, SelectionKey.OP_WRITE);}} else if (selectionKey.isWritable()) {// 给客户端注册写事件client = (SocketChannel) selectionKey.channel();sendB.clear();String sendMsg = "index " + index;sendB.put(sendMsg.getBytes());sendB.flip();client.write(sendB);System.out.println("send to server " + sendMsg);client.register(selector, SelectionKey.OP_READ);}}// 模拟10个请求，并发10秒后同时执行connectionpublic static void main(String[] args) throws Exception {for(int i = 0; i<10;i++) {final int finalI = i;new Thread(new Runnable() {@Overridepublic void run() {System.out.println("========"+ finalI);try {Thread.sleep(10000);} catch (InterruptedException e) {e.printStackTrace();}try {new NIOClient("127.0.0.1", 9779).connect();} catch (IOException e) {e.printStackTrace();}}}).start();}System.in.read();}}

6 MTU

TCP 协议向网络发包的时候，会把要发的数据先在缓冲区中进行囤积，当囤积到一定尺寸时（MTU），才向网络发送，这样可以最大化利用我们的网络带宽。而传输速度和性能也会变得很快。