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UDP UWP server + Mono2x client

I have a really weird issue with communication between server and client using UDP protocol. Client is written in Mono2x (I use Unity 3D as my client) and creates UdpClient class instance:
_udpClient = new UdpClient(9050);
_serverEP = new IPEndPoint(IPAddress.Parse(_serverIp), _serverPort);
My server is UWP application I want to run on Raspberry Pi that is using DatagramSocket:
_udpServer = new DatagramSocket();
_udpServer.MessageReceived += ClientCheck;
await _udpServer.BindServiceNameAsync(port.ToString());
I send data from client to server but with no luck. I checked with TCPView that data is send from my client application but never reaches the server. And now is the weird part. When I receive message from server first (I hardcode port to client), my client is able to send data with a success.
I am using same IPEndPoint to send data from client without any changes after receiving packet from server, it just starts working. Honestly, I have no idea what I could do wrong, so I will be thankful for any advice.

And now is the weird part. When I receive message from server first (I hardcode port to client), my client is able to send data with a success
This is a known issue which is filed based on this related question: https://stackoverflow.com/a/39767527/5254458
It includes the issue's description and temporary workaround.
The corresponding is team is investigating it and I can not guarantee that when the fix will be delivered.

How to listen to a TCP port which is already being listened by another app

I've a plugin which always listening to the port 6002, and i have an ASP.net application which sending messages to the same port and receiving the reply from the plugin on the same port,
Sending is working fine, and the plugin sends a reply on the same port but i don't know how to catch this reply, when i try to listen to the same port using Tcplistener the start method throws this exception : Only one usage of each socket address (protocol/network address/port) is normally permitted,
is there any way to catch the received message
Thanks

It sounds like you are wrongly assuming that the Socket which you get from TcpListener.AcceptSocket can only be used in one direction.
Sockets can actually be bidirectional. You can use Send to send something, and Receive to listen for get the replies. Open one socket, and then use it for both sending and receiving.

In short, no.
Once a port is opened an exception will be thrown if further attempts are made to utilise that same port from a different source - as you are experiencing right now. There isn't a way to get around this.

I've solved this problem using this way ,, I know it's old method but it's working !! :
'/*Variables Initialization*/
dim objSocket, strServicePort, strIpAddr, strResult, strMsgTo, strMsgResponse
strServicePort = "6002"
strIpAddr = "127.0.0.1"
'/* Create a TCP/IP socket. */
objSocket = Server.CreateObject("Intrafoundation.TCPClient.3")
objSocket.ClearLog()
'/* Establish socket connection. */
objSocket.Open (strIpAddr,strServicePort)
objSocket.Timeout=60.0
strMsgTo ="---- Message here ----"
'/* Send request message to plugin */
objSocket.SendRN(strMsgTo)
'/* receive XML Request Message from plugin */
strMsgResponse = objSocket.Recv()
strMsgResponse = Replace(strMsgResponse, vbLf, "")
objSocket.Close()

If you want to inspect traffic you can use winpcap.
edit: I don't think you are asking the right question. In this case the plugin is the server (listening on port 6002) and your ASP.net app is the client listening on some arbitrary port. You only need to bind to a different port in your ASP.net app if it also needs to run as a server with the plugin acting s the client. In this case you should pick a different port even though there are, in fact, ways to make it work when they are both bound to the same port.
In your case though you should just read back responses from the connection you established from the client.

C# - WireShark detects incomming packets but application does not receive them

I've got a strange problem. I have a client sending packets to my server, but my servers UDP socket never receives them. Same thing happens the other way around, if I send and he tries to receive.
Check this image, captured from wireshark:
http://img263.imageshack.us/img263/2636/bokus.png
I hav bound my UDP socket to EndPoint 192.168.2.3, which is the internal IP of the server, and port 9998.
The client sends data to my IP, which should then be forwarded to the local server machine..
As you can see wireshark clearly detects incomming packets for 192.168.2.3 with destination port 9998 but nothing gets delivered!
(...why does it say distinct32 btw in destination port?)

Something else to watch for is make sure any firewall you might running has a rule setup to allow communications on your port 9998.
If I had to guess (would need to see your recieving C# code to know), It looks like you might be trying to receive UDP packets but using TCP protocol on the client side. (Or i might just be misunderstanding some of the text of your screenshot.)
Theres no need to 'listen' for a connection when using UDP. UDP packets don't have a connect/disconnect protocol. Nor do they guarantee that packets are received in the same order they're sent.
try using something along these lines in your C# client and see if you get data.
var udpClient = new System.Net.Sockets.UdpClient(9998);
Byte[] receiveBytes = udpClient.Receive(ref RemoteIpEndPoint);

TCP connection lost problems in c#

In my application, I have a server listening on a port on localhost for client connection. Then I have clients running and establishes connection with the server. I used tcpListner in c# for the server and the clients were written in perl using Inet function.
The connection between clients and server were established without a problem and a message from client is received by the server. However, after about 3 to 4 seconds, the connection is lost. I use netstat -a to see if the status of the connection is still "established". The result is "Close_wait" after I use netstat command on DOS.
How can I debug this problem? Which side is causing the lost of connection?
Thanks
Edit:
$tx_socket = IO::Socket::INET->new( PeerAddr => '127.0.0.1', PeerPort => 9000,
LocalPort => 8000, Proto => 'tcp',
Type =>SOCK_STREAM, Reuse=>1 ) or die("$!\n");
die("init_tx_socket failed!\n") unless $tx_socket; print "Socket good!\n";
$tx_socket->autoflush(1); $tx_socket->print("Hello");
sleep(5);
$tx_socket->print("World");

Capturing network packets with Microsoft Network Monitor or Wireshark, or use System.Net tracing.
Socket issues cannot be easily analyzed and handled without such analysis.

Socket Server Disconnect and Reconnect Buffer Error?

I'm using the following TCP Sockets Server/Client example: http://www.codeguru.com/Csharp/Csharp/cs_network/sockets/article.php/c8781/
I'm also using the following CryptoStream example:
http://www.obviex.com/samples/Encryption.aspx
Problem: Both Server and Clients communicate perfectly until I Stop the Server socket, wait a minute or so, then start the Server socket. Sometimes but not always I recieve a base64 error in the Encryption.aspx on line 'Convert.FromBase64String(cipherText);'...
I know there is incorrect / corrupted data in the buffer probably left-over from stopping the socket. Then the new data comes in and the error occurs.
Q. Will clearing the 'class SocketPacket; solve this issue?
Q. How do I clear the 'class SocketPacketsocketBuffer'?
Other suggestions are greatly appreciated..

One of the things you may find is happening is that one of the sockets is not being closed down properly. The thing with sockets is that you need to make sure they get correctly closed or you set the server socket to reuse the address.
Try tcpview from sysinternals to view the status of sockets. You can also use netstat to view the status of the sockets.