SSDP Multicast Missing answers from M-SEARCH - Discovery - UPnP

Question

I am trying to receive the UPnP NOTIFY Messages from UPnP Devices in my Network. But when i send the M-SEARCH Message, i sometimes get no Answers. My Code looks like this:

public bool StartListener()
{
  if (this.ssdpSocket == null)
  {
    IPAddress localIpAddress = IPAddress.Any; 
    IPEndPoint localIpEndpoint = new IPEndPoint(localIpAddress, SsdpPort);

    try
    {
      this.ssdpSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
      this.ssdpSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, 1);

      this.ssdpSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReceiveBuffer, 16384); 

      this.ssdpSocket.Bind(localIpEndpoint);
      this.ssdpSocket.SetSocketOption(
        SocketOptionLevel.IP,
        SocketOptionName.AddMembership,
        new MulticastOption(IPAddress.Parse(SsdpMulticastAddress), localIpAddress));

      this.ssdpSocket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastTimeToLive, 2);
      this.ssdpSocket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastLoopback, true);

      this.culture = Thread.CurrentThread.CurrentUICulture;
      this.workerThreadListener = new WorkerThread(this.ssdpSocket, this.HandleSsdpMessage);
      this.workerThreadListener.Start();

      Log.InfoFormat("SSDP server bind successful [{0}]", localIpEndpoint);

      return true;
    }
    catch (Exception exception)
    {
      Log.Info(string.Format("SSDP server bind failed [{0}]", localIpEndpoint), exception);
      throw;
    }
  }

  return false;
}

I have found following answer but for me it is not possible to change the port. Is there an alternative solution?

UPnP Multicast

Answer 1

It's a pure luck this worked for you - rather it worked for reasons different than expected.

You received the NOTIFY messages because you started listening to multicasts as member of the group - UPnP periodically multicast their presence to the neighborhood on 1900, w/o search requests.

On the other hand, when you send M-SEARCH (multicast, presumably), you should stay and listen on the ephemeral port, since unicast responses will be coming there.