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How to keep a TCP connection open and perform multiple Writes/Reads in C# .NET?

There are multiple posts that describe the performance benefit of keeping a TCP connection open, instead of closing and opening each time you need to read or write. For example:
Best practice: Keep TCP/IP connection open or close it after each transfer?
I'm communicating with an RPC based device that takes json commands. The example I have from the device vendor opens and closes a connection each time they send a command. This is what I currently do via TcpClient in a using statement, but I'd like to see if there's anyway I could improve upon what I've already done. In fact, I had attempted this when starting the project, but couldn't figure out how to do so, so closed each time out of frustration and necessity. My latest experiment using sockets because all posts indicate doing so as a necessity for lower level control:
public class Connection
{
private Socket tcpSocket = null;
public string IpAddress = "192.168.0.30";
public int Port = 50002;
public Connection(string ipAddress, int port)
{
this.IpAddress = ipAddress;
this.Port = port;
}
public void Connect()
{
DnsEndPoint ipe = new DnsEndPoint(this.IpAddress, this.Port);
Socket tempSocket =
new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
tempSocket.Connect(ipe);
if (tempSocket.Connected)
{
this.tcpSocket = tempSocket;
this.tcpSocket.NoDelay = true;
this.tcpSocket.
//this.tcpSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive,true);
Console.WriteLine("Successfully connected.");
}
else
{
Console.WriteLine("Error.");
}
}
public void Disconnect()
{
this.tcpSocket.Disconnect(true);
this.tcpSocket.Dispose();
Console.WriteLine("Successfuly closed.");
}
public string SendCommand()
{
string path = #"C:\Users\me\Desktop\request.json";
string request = File.ReadAllText(path);
Byte[] bytesSent = Encoding.UTF8.GetBytes(request);
this.tcpSocket.Send(bytesSent);
this.tcpSocket.Shutdown(SocketShutdown.Send);
var respBytes = ReceiveAll();
string s = System.Text.Encoding.UTF8.GetString(respBytes, 0, respBytes.Length);
return s;
}
public byte[] ReceiveAll()
{
var buffer = new List<byte>();
var currByte = new Byte[1];
var byteCounter = this.tcpSocket.Receive(currByte, currByte.Length, SocketFlags.None);
while (this.tcpSocket.Available > 0)
{
currByte = new Byte[1];
byteCounter = this.tcpSocket.Receive(currByte, currByte.Length, SocketFlags.None);
if (byteCounter.Equals(1))
{
buffer.Add(currByte[0]);
}
}
return buffer.ToArray();
}
}
Console app:
static void Main(string[] args)
{
Connection s = new Connection();
s.Connect();
Console.WriteLine(s.SendCommand());
Console.WriteLine(s.SendCommand());
Thread.Sleep(5000);
s.Disconnect();
Console.ReadKey();
}
This approach works once. The first time I call send command. It doesn't the second time (throws an exception), because I call socket.Shutdown() on Send in my SendCommand(). I do so because of this post:
TCPClient not receiving data
However, there doesn't seem to be a way to re-enable the ability to Send after calling Shutdown(). So now I just don't know if it's even possible to keep a tcp connection open if you have to both read and write. Moreover, I can't really find a useful example online. Does anyone know how to do so in .NET? Is this even possible?

TCP/IP is a streaming protocol. To pass messages with it you need a “framing protocol” so peers can determine when a message is finished.
One simple way to signal the end of a message is to close the socket when you’ve sent the last byte. But this prevents socket reuse. See the evolution of HTTP for an example of this.
If this is what your device does, there’s no way to reuse a socket.

If it is possible to keep the connection open for more messages depends on the application protocol. There is no way to enforce this if the protocol does not supports it. Thus, ask the vendor or look into the protocol specification (if it exists) for information if and how this is supported.
However, there doesn't seem to be a way to re-enable the ability to Send after calling Shutdown().
There is no way. TCP write shutdown means that one does not want to send any more information. It is impossible to take this back. If the protocol supports multiple message exchanges then it needs to have a different way to detect the end of a message than calling Shutdown.

How do I detect that my attempt to send data via TLS secured socket has failed?

This sample app creates a client-server connection via a TLS secured socket and sends some data over it:
static void Main(string[] args)
{
try
{
var listenerThread = new Thread(ListenerThreadEntry);
listenerThread.Start();
Thread.Sleep(TimeSpan.FromSeconds(1));
var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.IP);
socket.Connect("localhost", Port);
var rawStream = new NetworkStream(socket);
using (var sslStream = new SslStream(rawStream, false, VerifyServerCertificate))
{
var certificate = new X509Certificate(CertsPath + #"test.cer");
var certificates = new X509CertificateCollection(new[] { certificate });
sslStream.AuthenticateAsClient("localhost", certificates, SslProtocols.Tls, false);
using (var writer = new StreamWriter(sslStream))
{
writer.WriteLine("TEST");
writer.Flush();
Thread.Sleep(TimeSpan.FromSeconds(10));
}
}
socket.Shutdown(SocketShutdown.Both);
socket.Disconnect(false);
Console.WriteLine("Success! Well, not really.");
}
catch (Exception exc)
{
Console.WriteLine(exc);
}
}
private static bool VerifyServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
return true;
}
static void ListenerThreadEntry()
{
try
{
var listener = new TcpListener(IPAddress.Any, Port);
listener.Start();
var client = listener.AcceptTcpClient();
var serverCertificate = new X509Certificate2(CertsPath + #"\test.pfx");
var sslStream = new SslStream(client.GetStream(), false);
sslStream.AuthenticateAsServer(serverCertificate, false, SslProtocols.Tls, false);
client.Close(); // terminate the connection
using (var reader = new StreamReader(sslStream))
{
var line = reader.ReadLine();
Console.WriteLine("> " + line);
}
}
catch (Exception exc)
{
Console.WriteLine(exc);
}
}
The trick is that the connection is terminated from the server side immediately after handshake. And the problem is that the client side knows nothing about it; I'd expect the client side to raise an exception when it tries to send data over the closed connection, but it doesn't.
So, the question is: how do I detect such cases, when the connection was interrupted and data didn't really reach the server?

It is not possible to know which packets have arrived under the TCP model. TCP is a stream-oriented protocol, not a packet-oriented protocol; that is, it behaves like a bi-directional pipe. If you write 7 bytes, and then write 5 bytes, it's possible the other end will just get 12 bytes all at once. Worse, TCP's reliable delivery only guarantees that if the data arrives, it will do so in the correct order without duplication or rearrangement, and that if the data does not arrive, it will be resent.
If the connection is broken unexpectedly, TCP does not guarantee that you will know exactly what data was lost, nor is it reasonably possible to provide that information. The only thing the client knows is "I never received an acknowledgement for byte number N [and presumably not for the previous n bytes either], despite resending them multiple times." That is not enough information to determine whether byte N (and the other missing bytes) arrived at the server. It's possible that they did arrive and then the connection dropped, before the server could acknowledge them. It is also possible that they did not arrive at all. TCP cannot provide you with this information, because only the server knows it, and you are not connected to the server any longer.
Now, if you close the socket properly, using shutdown(2) or the .NET equivalent, then data in flight will be pushed through if possible, and the other end will error out promptly. Generally, we try to ensure that both sides agree on when to shutdown the connection. In HTTP, this is done with the Connection: Close header, in FTP with the BYE command, and so on. If one side shuts down unexpectedly, it may still cause data to be lost, because shutdown does not normally wait for acknowledgements.

Check if a port is open

I can't seem to find anything that tells me if a port in my router is open or not.
Is this even possible?
The code I have right now doesn't really seem to work...
private void ScanPort()
{
string hostname = "localhost";
int portno = 9081;
IPAddress ipa = (IPAddress) Dns.GetHostAddresses(hostname)[0];
try
{
System.Net.Sockets.Socket sock =
new System.Net.Sockets.Socket(System.Net.Sockets.AddressFamily.InterNetwork,
System.Net.Sockets.SocketType.Stream,
System.Net.Sockets.ProtocolType.Tcp);
sock.Connect(ipa, portno);
if (sock.Connected == true) // Port is in use and connection is successful
MessageBox.Show("Port is Closed");
sock.Close();
}
catch (System.Net.Sockets.SocketException ex)
{
if (ex.ErrorCode == 10061) // Port is unused and could not establish connection
MessageBox.Show("Port is Open!");
else
MessageBox.Show(ex.Message);
}
}

Try this:
using(TcpClient tcpClient = new TcpClient())
{
try {
tcpClient.Connect("127.0.0.1", 9081);
Console.WriteLine("Port open");
} catch (Exception) {
Console.WriteLine("Port closed");
}
}
You should probably change 127.0.0.1 to something like 192.168.0.1 or whatever your router's IP address is.

A better solution where you can even specify a timeout:
using System;
using System.Net.Sockets;
// ...
bool IsPortOpen(string host, int port, TimeSpan timeout)
{
try
{
using(var client = new TcpClient())
{
var result = client.BeginConnect(host, port, null, null);
var success = result.AsyncWaitHandle.WaitOne(timeout);
client.EndConnect(result);
return success;
}
}
catch
{
return false;
}
}
And, in F#:
open System
open System.Net.Sockets
let isPortOpen (host: string) (port: int) (timeout: TimeSpan): bool =
try
use client = new TcpClient()
let result = client.BeginConnect(host, port, null, null)
let success = result.AsyncWaitHandle.WaitOne timeout
client.EndConnect result
success
with
| _ -> false
let available = isPortOpen "stackoverflow.com" 80 (TimeSpan.FromSeconds 10.)
printf "Is stackoverflow available? %b" available

There is no way to know if the port is forwarded in your router, except if there is a program listening on that port.
As you may see in the Clinton answer, the .Net class being used is TcpClient and that is because you are using a TCP socket to connect to. That is the way operating systems make connections: using a socket. However, a router just forwards the packets (layer 3 of the OSI Model) in or out. In your case, what your router is doing is called: NAT. It is one public IP shared by a one or more private IPs. That´s why you are making a port forwarding.
There may be a lot of routers in the path of the packets, and you will never know what had happened.
Let´s imagine you are sending a letter in the traditional way. Perhaps you can write in the letter that the receiver must answer, in order to check he/she is there (you and the receiver are the sockets). If you receive an answer you will be sure he/she is there, but if you don´t receive anything you don´t know if the mailman (in your case the router) forgot to deliver the letter, or the receiver hadn´t answered. You would also never know if the mailman has asked a friend to deliver that letter. Moreover, the mailman won´t open the letter in order to know he/she may answer because you are waiting for a reply. All you may do is wait some time to receive the answer. If you don´t receive anything in that period you will assume that the receiver isn´t where you sent the letter. That is a "timeout".
I saw an answer mentioning the nmap software. It´s really a very good and complex soft, but I think it will work in the same way. If there is no app listening in that port, there is no way to know if it is open or not.
Please, let me know if I was clear.

If you're connecting to the loopback adapter — localhost or 127.0.0.1 (there's no place like 127.0.0.1!), you're unlikely to ever go out to the router. The OS is smart enough to recognize that it's a special address. Dunno if that holds true as well if you actually specify your machine's "real" IP address.
See also this question: What is the purpose of the Microsoft Loopback Adapter?
Also note that running traceroute localhost (tracert localhost in Windows) shows that the only network node involved is your own machine. The router is never involved.

Other than BeginConnect you can also use ConnectAsync (added in .NET Framework 4.5 I think?).
TcpClient client = null;
try {
client = new TcpClient();
var task = client.ConnectAsync(host, port);
if (task.Wait(timeout)) {//if fails within timeout, task.Wait still returns true.
if (client.Connected) {
// port reachable
}
else
// connection refused probably
}
else
// timed out
}
catch (Exception ex) {
// connection failed
}
finally {
client.Close();
}
Full project is here because paping refuses to run and I couldn't find another "ping host:port" tool to my likes.

A port forward on the router cannot be tested from inside the LAN, you need to connect from the WAN (internet) side to see if a port forward is working or not.
Several internet sites offer services to check if a port is open:
What's My IP Port Scanner
GRC | ShieldsUP!
If you want to check with your own code, then you need to make sure the TCP/IP connection is rerouted via an external proxy or setup a tunnel. This has nothing to do with your code, it's basic networking 101.

public static bool PortInUse(int port)
{
bool inUse = false;
IPGlobalProperties ipProperties = IPGlobalProperties.GetIPGlobalProperties();
IPEndPoint [] ipEndPoints = ipProperties.GetActiveTcpListeners();
foreach(IPEndPoint endPoint in ipEndPoints)
{
if(endPoint.Port == port)
{
inUse = true;
break;
}
}
return inUse;
}

For me, I needed something blocking until the connection to the port is available or after a certain amount of retries. So, I figured out this code:
public bool IsPortOpen(string host, int port, int timeout, int retry)
{
var retryCount = 0;
while (retryCount < retry)
{
if (retryCount > 0)
Thread.Sleep(timeout);
try
{
using (var client = new TcpClient())
{
var result = client.BeginConnect(host, port, null, null);
var success = result.AsyncWaitHandle.WaitOne(timeout);
if (success)
return true;
client.EndConnect(result);
}
}
catch
{
// ignored
}
finally { retryCount++; }
}
return false;
}
Hope this helps!

also you can use ConnectAsync like
public async Task<bool> IsIPAndPortOpen(string hostOrIPAddress, int port, TimeSpan timeOut)
{
try
{
using (var client = new TcpClient())
{
var ct = new CancellationTokenSource(timeOut).Token;
await client.ConnectAsync(hostOrIPAddress, port, ct);
return true;
}
}
catch
{
return false;
}
}

public string GetAvailablePort()
{int startingPort=1000;
string portnumberinformation = string.Empty;
IPEndPoint[] endPoints;
List<int> portArray = new List<int>();
IPGlobalPr`enter code here`operties properties = IPGlobalProperties.GetIPGlobalProperties();`enter code here`
//getting active tcp listners
endPoints = properties.GetActiveTcpListeners();
portArray.AddRange(from n in endPoints
where n.Port >= startingPort
select n.Port);
portArray.Sort();
for (int i = 0; i < portArray.Count; i++)
{
if (check condition)
{
do somting
}
}
return portnumberinformation;
}

If it is Router the simplest way to check it through online services like
Port Checker
Port Forwarding Test
You can also try using telenet to chek wether port is accessible or not
telenet [ip-address] [port]

TcpClient communication with server to keep alive connection in c#?

I've this TcpClient code which works fine. It connects to perl server on linux system and receives anything that server sents to it. Works nicely.
public static void Main() {
foreach (ProtocolConnection tcpConnection in TcpConnectionsList) {
ProtocolConnection connection = tcpConnection;
ThreadPool.QueueUserWorkItem(_ => {
ThreadTcpClient(connection);
ManualResetEventTcp.Set();
});
}
... Some code...
}
public static void TcpConnect(ProtocolConnection varConnection) {
int retryCountSeconds = varConnection.RetryEverySeconds*Program.MilisecondsMultiplier;
int count = 0;
while (true) {
try {
using (var client = new TcpClient(varConnection.IpAddress.ToString(), varConnection.Port) { NoDelay = true })
using (var stream = client.GetStream()) {
var data = new Byte[256];
while (!Program.PrepareExit) {
Int32 bytes = stream.Read(data, 0, data.Length);
string varReadData = Encoding.ASCII.GetString(data, 0, bytes).Trim();
if (varReadData != "" && varReadData != "PONG") {
VerificationQueue.EnqueueData(varReadData);
Logging.AddToLog("[TCP][" + varConnection.Name + "][DATA ARRIVED]" + varReadData);
} else {
Logging.AddToLog("[TCP]" + varReadData);
}
}
}
} catch (Exception e) {
if (e.ToString().Contains("No connection could be made because the target machine actively refused it")) {
Logging.AddToLog("[TCP][ERROR] Can't connect to server (" + varConnection.Name + ") " + varConnection.IpAddress + ":" + varConnection.Port );
} else {
Logging.AddToLog(e.ToString());
}
}
DateTime startTimeFunction = DateTime.Now;
do {
Thread.Sleep(1000);
} while (((DateTime.Now - startTimeFunction).TotalSeconds < retryCountSeconds));
}
}
However in certain conditions I'm having some problems with it:
My work connection often drops connection after some idle time so I've implemented in server so when it receives PING it responds with PONG. I can send PING with UDP to server and it will respond with PONG on tcp but i would prefer built-in way into tcp client so it does send PING every 60 seconds or so. Even if UDP solution would be acceptable I have no timeout on string varReadData = Encoding.ASCII.GetString(data, 0, bytes).Trim(); so when PONG doesn't arrive my client doesn't even notice it anyway. It just keeps waiting ... which brings me to..
My other problem is that at some point string varReadData = Encoding.ASCII.GetString(data, 0, bytes).Trim(); this is waiting for data all the time. When server crashes or disconnects my client i don't even notice that. I would like server to have some kind of timeout or check if connection is active. If it's not active it should try to reconnect.
What would be simplest way to fix this TcpClient ? How do i implement both way communication making sure that if server drops my connections or my net gets disconnected client will notice it and reestablish connection ?

It's not Encoding.ASCII.GetString(data, 0, bytes).Trim(); that blocks forever, it's the stream.Read()
If you're reading, you can't easily distinguish between the server(or any NAT gateway inbetween) dropping your connection , and the case where the server simply doesn't have anything to send you. Atleast in the case where the TCP FIN/RST packets doesn't reach your client in case of failure, or a NAT gateway silently dropping your connection.
What you can do;
Set a Send/ReceiveTimeout , and ping the server if a timeout occurs, or implement your own heartbeat messages over your TCP connection. Reestablish or take other actions if you don't receive a heartbeat within a reasonable time.
Set the TCP keepalive option, and rely on that to tell you if the server is gone. See code here.
The last point will tell you if the tcp connection fails, it won't tell you if the server has somewhat failed - e.g. if you CTRL+Z your perl server, it'll just sit there not doing anything as the tcp window closes , so you might need to implement your own heatbeat messges to cover such a case too if you need to.

You should get rid of the UDP heartbeat attempt and put in a real TCP heartbeat. "Pinging" the server using UDP is almost meaningless.
Your protocol is also missing message framing.
Read both of those linked articles carefully (especially message framing). The protocol you're currently using does need serious revision.

udpclient.close doesn't always close the socket?

I'm using the code below, it seem sometime the socket is not released
How did I found that?
by using process explorer(from sysinternal),
proprieties on the application
then going into TCP/IP tab.
I can see the port being used by typing "netstat -a" into a console
My problem is, after a while (like 5 weeks) there is like 40 port used by the application while it should be zero.
Anyone know why it does that?
public void Connect()
{
try {
// Resolve server address
IPHostEntry hostadd = Dns.GetHostEntry(TimeServer);
IPEndPoint EPhost = new IPEndPoint(hostadd.AddressList[0], 123);
//Connect the time server
UdpClient TimeSocket = new UdpClient();
TimeSocket.Connect(EPhost);
TimeSocket.Send(SNTPData, SNTPData.Length);
SNTPData = TimeSocket.Receive(ref EPhost);
TimeSocket.Close();
if (!IsResponseValid())
{
throw new Exception("Invalid response from " + TimeServer);
}
} catch(SocketException e)
{
throw new Exception(e.Message);
}
}

You're blocking forever on:
SNTPData = TimeSocket.Receive(ref EPhost);
If the socket never receives a packet it will sit there waiting until the process dies.
You'll need to close the socket by calling TimeSocket.Close() on a different thread or by setting a timeout on the receive using SetSocketOption.