UWP DatagramsSocket doesn't not fire MessageReceived - sockets

Question

I'm working on a UWP application which should communicate via TCP/UDP to a remote device. My issue is that the UWP app successfully sends UDP messages to the remote device, but does not receive the replies.
Here follows the code extracted from the app (simplified):
async Task TestUdpIP()
{
// Writer to the DatagramSocket
DataWriter writer;
using (var udpClient = new DatagramSocket())
{
try
{
// UDP Socket binding
udpClient.MessageReceived += UdpClient_MessageReceived;
var controllerName = new Windows.Networking.HostName(controllerIpAddress.ToString());
await udpClient.BindEndpointAsync(controllerName, controllerIpPort.ToString());
var remoteHostName = new Windows.Networking.HostName(hostIpAddres.ToString());
await udpClient.ConnectAsync(remoteHostName, remoteHostPort.ToString());
// Create a message to send
string message = "Some message";
// Reset the counter of messages received back from the remote robot
messagesReceived = 0;
// Send the message
writer = new DataWriter(udpClient.OutputStream);
writer.WriteString(message);
await writer.StoreAsync();
// Wait for robot status messages
await Task.Delay(5000);
}
catch
{
}
}
}
void UdpClient_MessageReceived(DatagramSocket sender, DatagramSocketMessageReceivedEventArgs args)
{
// Just increment the number of messages received
messagesReceived++;
}
However the UdpClient_MessageReceived handler does not fire. I'm sure that UDP messages are correctly sent from the UWP app and that the remote device replies back as shown in the following screenshot from Wireshark (the test has been taken on the same PC where the UWP app is running)
.
(IP Address, Port) details are shown in the following list to better explain the picture above
UWP application: (192.168.1.108, 19000) to send and receive.
Remote device: (192.168.1.152, 15999) to receive and (192.168.1.152, 54697) to send
Note: this is a similar question, where the answer says that for whatever reason the DatagramSocket should fire some messages before being able to receive. In my example a message is sent out however the message received handler does not fire anyway.
Note: The UWP app has been granted the internet (client) and the internet (client and server) capabilities
Note: I've tried also this (more readable) approach to bind the inbound/outbound datagram socket on the UWP app obtaining the same results:
// UDP Socket binding
var controllerName = new HostName(controllerIpAddress.ToString());
var remoteHostName = new HostName(hostIpAddres.ToString());
EndpointPair endpointpar = new EndpointPair(controllerName,
controllerIpPort.ToString(),
remoteHostName,
remoteHostPort.ToString());
udpClient.MessageReceived += UdpClient_MessageReceived;
await udpClient.ConnectAsync(endpointpar);
Where is the issue? Many thanks!

Answer 1

Please try to also add the Private Networks (Client & Server) capability in your app's manifest.
On other hand, you can try the Official DatagramSocket sample to see whether the both devices can communicate with each other.

Answer 2

After some experimenting, I came out with the idea of using two different DatagramSocket instances: one to send out UDP messages, and one to listen for incoming messages. With this code, I've been able to send UDP messages to the remote device (as before) and also I've been able to receive UDP messages from the remote device.
async Task TestUdpIP_DifferentPorts()
{
// Writer to the DatagramSocket
DataWriter writer;
// Inbound and outbound DatagramSocket
DatagramSocket udpListener = new DatagramSocket();
DatagramSocket udpSender = new DatagramSocket();
try
{
// String containing the serializaed message
string serializedMessage = "Some message";
var controllerName = new HostName(controllerIpAddress.ToString());
var remoteHostName = new HostName(hostIpAddres.ToString());
// Bind listener
udpListener.MessageReceived += UdpClient_MessageReceived;
await udpListener.BindEndpointAsync(controllerName, controllerIpPort.ToString());
// Connect sender
await udpSender.ConnectAsync(remoteHostName, remoteHostPort.ToString());
// Reset the counter of messages received back from the remote robot
messagesReceived = 0;
// Send the message
writer = new DataWriter(udpSender.OutputStream);
writer.WriteString(JsonConvert.SerializeObject(message));
await writer.StoreAsync();
// Wait for robot status messages
await Task.Delay(1000);
}
catch (Exception ex)
{
// Some exception handling
}
udpSender.Dispose();
udpListener.Dispose();
}
void UdpClient_MessageReceived(DatagramSocket sender, DatagramSocketMessageReceivedEventArgs args)
{
// Just increment the number of messages received
messagesReceived++;
}
With this approach only the IP port (or service name) of the inbound listener needs to be specified. The framework will choose the next available outbound IP port.
Note: If I understand well the DatagramSocket.ConnectAsync(EndpointPair) documentation, the same DatagramSocket instance can be used to send and listen for incoming message, so I could not figure out the need for two different instances. From the documentation:
This ConnectAsync(EndPointPair) method on a DatagramSocket is used to define the local and remote endpoint where datagrams will be sent when using the OutputStream property. This method also restricts remote IP addresses of packets that will be accepted to the remote hostname in the endpointPair parameter. Only incoming packets that match the remote endpoint in the endpointPair parameter will trigger the MessageReceived event on the DatagramSocket.