UDP协议格式以及在java中的使用

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：

在这里插入图片描述
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）：

# 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 60x0000:  4500 0022 5914 4000 4011 e3b4 7f00 0001  E.."Y.@.@.......0x0010:  7f00 0001 b262 270f 000e fe21 6865 6c6c  .....b'....!hell0x0020:  6f0a                                     o.
... ...

说明：

源端口：0xb262，十进制的45666。
目的端口：0x270f，十进制的9999。
长度：0x000e，14个字节的报文长度。
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：

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：

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：

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：

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：

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：

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：

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：

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>() {@Overrideprotected 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：

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>() {@Overrideprotected 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：

增加option：

.option(ChannelOption.SO_BROADCAST, true)

将IP地址修改为广播地址255.255.255.255：

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：

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：

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：

# 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。