详解UDP协议格式及在java中的使用

时间:2022-01-05 22:23:57

UDP是面向无连接的通讯协议,由于通讯不需要连接,所以可以实现广播发送。UDP通讯时不需要接收方确认,属于不可靠的传输,可能会出现丢包现象,实际应用中要求程序员编程验证。

UDP适用于DNS、视频音频等多媒体通信、广播通信(广播、多播)。例如我们常用的QQ,就是一个以UDP为主,TCP为辅的通讯协议。

UDP报文格式如下:

详解UDP协议格式及在java中的使用

UDP首部有8个字节,由4个字段构成,每个字段都是两个字节,

  • 源端口:数据发送方的端口号.
  • 目的端口:数据接收方的端口号。
  • 长度:UDP数据报的整个长度(包括首部和数据),其最小值为8(只有首部)。
  • 校验和:检测UDP数据报在传输中是否有错,有错则丢弃。

可以使用nc发送UDP数据包:echo hello | nc -uv 127.0.0.1 9999

用tcpdump抓取到的数据包如下(注意先运行tcpdump,然后再执行nc命令):

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# tcpdump -i lo -X udp port 9999
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 262144 bytes
11:19:39.267912 IP localhost.45666 > localhost.distinct: UDP, length 6
    0x0000: 4500 0022 5914 4000 4011 e3b4 7f00 0001 E.."Y.@.@.......
    0x0010: 7f00 0001 b262 270f 000e fe21 6865 6c6c .....b'....!hell
    0x0020: 6f0a     o.
... ...

说明:

  1. 源端口:0xb262,十进制的45666。
  2. 目的端口:0x270f,十进制的9999。
  3. 长度:0x000e,14个字节的报文长度。
  4. 校验和:0xfe21。

bio之单播

单播就是一对一通信。

服务器端代码如下:

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package com.morris.udp.bio.single;
 
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
 
public class Server {
 public static void main(String[] args) throws IOException {
 
 DatagramSocket datagramSocket = new DatagramSocket(9999);
 
 byte[] bytes = new byte[1024];
 DatagramPacket datagramPacket = new DatagramPacket(bytes, bytes.length);
 datagramSocket.receive(datagramPacket);
 System.out.println("receive from client: " + new String(bytes));
 
 byte[] req = "hello client".getBytes();
 DatagramPacket resp = new DatagramPacket(req, req.length, datagramPacket.getSocketAddress());
 datagramSocket.send(resp);
 
 }
}

客户端代码如下:

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package com.morris.udp.bio.single;
 
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetSocketAddress;
 
public class Client {
 public static void main(String[] args) throws IOException {
 DatagramSocket datagramSocket = new DatagramSocket();
 
 byte[] req = "hello server".getBytes();
 DatagramPacket datagramPacket = new DatagramPacket(req, req.length, new InetSocketAddress("127.0.0.1", 9999));
 
 datagramSocket.send(datagramPacket);
 
 datagramSocket.receive(datagramPacket);
 
 System.out.println("receive from server: " + new String(datagramPacket.getData()));
 }
}

客户端和服务端的代码几乎一致,只不过接收和发送数据的顺序不一致,receive和send都欧式阻塞方法。

bio之广播

广播:同一网段所有主机都能接收,前提是端口要开启监听。

只需要将单播的例子中客户端发送数据的IP修改为255.255.255.255即可,具体修改如下:

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DatagramPacket datagramPacket = new DatagramPacket(req, req.length, new InetSocketAddress("255.255.255.255", 9999));

bio之多播(组播)

多播数据报套接字类用于发送和接收IP多播包。MulticastSocket是一种DatagramSocket,它具有加入Internet上其他多播主机的“组”的附加功能。

多播组通过D类IP地址和标准UDP端口号指定。D类IP地址在224.0.0.0和239.255.255.255的范围内。地址224.0.0.0被保留,不应使用。

可以通过首先使用所需端口创建MulticastSocket,然后调用joinGroup(InetAddress groupAddr)方法来加入多播组。

服务器端代码如下:

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package com.morris.udp.bio.multicast;
 
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.MulticastSocket;
 
public class Server {
 public static void main(String[] args) throws IOException {
 
 InetAddress group = InetAddress.getByName("228.5.6.7");
 MulticastSocket s = new MulticastSocket(6789);
 s.joinGroup(group);
 
 byte[] buf = new byte[1000];
 DatagramPacket recv = new DatagramPacket(buf, buf.length);
 s.receive(recv);
 
 System.out.println("receive : " + new String(buf));
 s.leaveGroup(group);
 }
}

客户端代码如下:

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package com.morris.udp.bio.multicast;
 
import java.io.IOException;
import java.net.*;
 
public class Client {
 public static void main(String[] args) throws IOException {
 String msg = "Hello";
 InetAddress group = InetAddress.getByName("228.5.6.7");
 MulticastSocket s = new MulticastSocket();
 s.joinGroup(group);
 DatagramPacket hi = new DatagramPacket(msg.getBytes(), msg.length(), group, 6789);
 s.send(hi);
 s.leaveGroup(group);
 }
}

NIO实现单播

服务器端代码如下:

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package com.morris.udp.nio;
 
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.DatagramChannel;
 
public class Server {
 public static void main(String[] args) throws IOException {
 
 DatagramChannel datagramChannel = DatagramChannel.open();
 datagramChannel.bind(new InetSocketAddress(9999));
 // datagramChannel.configureBlocking(false);
 
 ByteBuffer byteBuffer = ByteBuffer.allocate(128);
 SocketAddress receive = datagramChannel.receive(byteBuffer);
 
 byteBuffer.flip();
 byte[] bytes = new byte[byteBuffer.remaining()];
 byteBuffer.get(bytes);
 System.out.println("receive from client: " + new String(bytes));
 
 byteBuffer.clear();
 byteBuffer.put("hello client".getBytes());
 
 datagramChannel.send(byteBuffer, receive);
 }
}

客户端代码如下:

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package com.morris.udp.nio;
 
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.DatagramChannel;
 
public class Client {
 public static void main(String[] args) throws IOException {
 
 DatagramChannel datagramChannel = DatagramChannel.open();
 // datagramChannel.configureBlocking(false);
 
 String req = "hello server";
 ByteBuffer byteBuffer = ByteBuffer.allocate(req.length());
 byteBuffer.put(req.getBytes());
 byteBuffer.flip();
 
 datagramChannel.send(byteBuffer, new InetSocketAddress("127.0.0.1", 9999));
 
 datagramChannel.receive(byteBuffer);
 byteBuffer.flip();
 byte[] bytes = new byte[byteBuffer.remaining()];
 byteBuffer.get(bytes);
 System.out.println("receive from server: " + new String(bytes));
 }
}

Netty实现单播

服务器端代码如下:

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package com.morris.udp.netty.single;
 
import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOption;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.DatagramPacket;
import io.netty.channel.socket.nio.NioDatagramChannel;
import io.netty.util.CharsetUtil;
 
public class Server {
 
 private static final int port = 8899;
 
 public static void main(String[] args) throws InterruptedException {
 
 NioEventLoopGroup group = new NioEventLoopGroup();
 try {
  Bootstrap bootstrap = new Bootstrap();
  bootstrap.group(group).channel(NioDatagramChannel.class)
   .handler(new SimpleChannelInboundHandler<DatagramPacket>() {
   @Override
   protected void channelRead0(ChannelHandlerContext ctx, DatagramPacket msg) throws Exception {
    // 接收数据
    System.out.println(msg.content().toString(CharsetUtil.UTF_8));
    // 发送数据
    ctx.writeAndFlush(new DatagramPacket(Unpooled.copiedBuffer("hello client", CharsetUtil.UTF_8), msg.sender()));
    ctx.close();
   }
   });
 
  bootstrap.bind(port).sync().channel().closeFuture().await();
 } finally {
  group.shutdownGracefully();
 }
 
 }
}

客户端代码如下:

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package com.morris.udp.netty.single;
 
import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.Unpooled;
import io.netty.channel.Channel;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOption;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.DatagramPacket;
import io.netty.channel.socket.nio.NioDatagramChannel;
import io.netty.util.CharsetUtil;
 
import java.net.InetSocketAddress;
 
public class Client {
 
 public static void main(String[] args) throws InterruptedException {
 NioEventLoopGroup group = new NioEventLoopGroup();
 
 try {
  Bootstrap bootstrap = new Bootstrap();
  bootstrap.group(group).channel(NioDatagramChannel.class)
   .handler(new SimpleChannelInboundHandler<DatagramPacket>() {
   @Override
   protected void channelRead0(ChannelHandlerContext ctx, DatagramPacket msg) throws Exception {
    // 接收数据
    System.out.println(msg.content().toString(CharsetUtil.UTF_8));
    ctx.close();
   }
   });
 
  Channel channel = bootstrap.bind(0).sync().channel();
 
  // 发送数据
  channel.writeAndFlush(new DatagramPacket(Unpooled.copiedBuffer("hello server", CharsetUtil.UTF_8), new InetSocketAddress("127.0.0.1", 8899)));
 
  if (!channel.closeFuture().await(30 * 1000)) {
  System.err.println("查询超时");
  }
 } finally {
  group.shutdownGracefully();
 }
 
 }
}

Netty实现广播

只需要将netty实现的单播的客户端代码做如下修改:

1.增加option:

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.option(ChannelOption.SO_BROADCAST, true)

2.将IP地址修改为广播地址255.255.255.255

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channel.writeAndFlush(new DatagramPacket(Unpooled.copiedBuffer("hello server", CharsetUtil.UTF_8), new InetSocketAddress("255.255.255.255", 8899)));

底层实现

recvfrom负责接收UDP数据,其函数声明如下:

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ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);

sendto负责发送UDP数据,其函数声明如下:

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ssize_t sendto(int sockfd, const void *buf, size_t len, int flags, const struct sockaddr *dest_addr, socklen_t addrlen);

下面通过对bio之单播的例子所产生的系统调用进行跟踪:

启动服务器端服务Server:

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# strace -ff -o out java Server

然后使用nc命令充当客户端进行连接:echo hello | nc -uv 127.0.0.1 9999

产生的系统调用中关键信息如下:

socket(AF_INET6, SOCK_DGRAM, IPPROTO_IP) = 4
bind(4, {sa_family=AF_INET6, sin6_port=htons(9999), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, 28) = 0
recvfrom(4, "hello\n", 1024, 0, {sa_family=AF_INET6, sin6_port=htons(7361), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, [28]) = 6
write(1, "receive from client: hello\n\0\0\0\0\0"..., 1045) = 1045
write(1, "\n", 1)
sendto(4, "hello client", 12, 0, {sa_family=AF_INET6, sin6_port=htons(7361), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, 28) = 12

可见发送和接收数据确实使用了上面的系统调用,另外上面的系统调用中并没有listen函数,不需要监听端口,再次验证UDP是面向无连接的。

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原文链接:https://blog.csdn.net/u022812849/article/details/109843694