如何发送消息并在同一个套接字上接收响应

Question

我有以下代码发送多路广播消息，然后等待响应发送到消息来自的地址。如果我观察Wireshark中的流量，我可以看到消息发送正常，并且响应返回到正确的IP和端口，但是套接字永远不会从接收线返回，就像没有收到响应一样。

var multicastAddress = IPAddress.Parse("239.255.255.250"); 
    var multicastPort = 1900; 
    var unicastPort = 1901;   

    using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp)) 
    { 
     socket.Bind(new IPEndPoint(IPAddress.Any, unicastPort)); 
     socket.Connect(new IPEndPoint(multicastAddress, multicastPort)); 
     var thd = new Thread(() => 
      { 
       try 
       { 
        while (true) 
        { 
         var response = new byte[8000]; 
         EndPoint ep = new IPEndPoint(IPAddress.Any, unicastPort); 
         socket.ReceiveFrom(response, ref ep); 
         var str = Encoding.UTF8.GetString(response); 
         Devices.Add(new SsdpDevice() {Location = str}); 
        } 
       } 
       catch 
       { 
        //TODO handle exception for when connection closes 
       } 
      }); 
     socket.Send(broadcastMessage, 0, broadcastMessage.Length, SocketFlags.None); 
     thd.Start(); 
     Thread.Sleep(30000); 
     socket.Close(); 
    } 

我知道我应该使用的插座类的异步方法，需要停止依靠的Thread.Sleep但我只是希望得到一个简单的例子，之前的工作我整理代码。

Answer 1

加文，检查了这一点：

• 不要使用不同的端口。你如何期望在一个上多播并在另一个上接收？
• 不要使用Connect()，多播是无连接消息（就像广播一样）。
• 在Bind()之后，将套接字选项设置为多播。
• 使用SendTo()而不是Send()，在这种情况下不起作用。
• 首先开始接收（即使在阻塞模式下，它是一个不同的端点），然后发送。

和一个简单的工作示例：

var broadcastMessage = Encoding.UTF8.GetBytes("Hello multicast!"); 
var multicastAddress = IPAddress.Parse("239.255.255.250"); 
var signal = new ManualResetEvent(false); 
var multicastPort = 1900; 

using (var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp)) 
{ 
    var multicastEp = new IPEndPoint(multicastAddress, multicastPort); 
    EndPoint localEp = new IPEndPoint(IPAddress.Any, multicastPort); 

    // Might want to set this: 
    //socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, 1); 
    socket.Bind(localEp); 
    socket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.AddMembership, new MulticastOption(multicastAddress, IPAddress.Any)); 
    // May want to set this: 
    //socket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastTimeToLive, 0); // only LAN 
    var thd = new Thread(() => 
     { 
      var response = new byte[8000]; 
      socket.ReceiveFrom(response, ref localEp); 
      var str = Encoding.UTF8.GetString(response).TrimEnd('\0'); 
      Console.WriteLine("[RECV] {0}", str); 
      signal.Set(); 
      Console.WriteLine("Receiver terminating..."); 
     }); 
    signal.Reset(); 
    thd.Start(); 

    socket.SendTo(broadcastMessage, 0, broadcastMessage.Length, SocketFlags.None, multicastEp); 
    Console.WriteLine("[SEND] {0}", Encoding.UTF8.GetString(broadcastMessage)); 
    signal.WaitOne(); 
    Console.WriteLine("Multicaster terminating..."); 
    socket.Close(); 
    Console.WriteLine("Press any key."); 
    Console.ReadKey(); 
}