c# - 如何在同一个套接字上发送消息和接收响应

Question

我有以下代码发送多播消息，然后等待响应发送到消息的来源地址。如果我在 Wireshark 中观察流量，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，但是套接字从未从接收线路返回，就像没有接收到响应一样。

    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();
    }

我知道我应该在套接字类上使用异步方法并且需要停止依赖 Thread.Sleep 但我只想在整理代码之前让一个简单的示例工作。

Answer 1

加文，检查一下:

• 不要使用不同的端口。您希望如何在一台上多播并在另一台上接收？
• 不要使用Connect()，多播是无连接消息传递(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()，这在这种情况下不起作用。
• 首先开始接收(即使是阻塞模式，也是不同的端点)，然后发送。

还有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();
}