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:
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:
- 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:
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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