Asynchronous TCP/IP - Part 1

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Asynchronous TCP/IP - Part 1


3 min read

This is going to be a series of blog posts to understand how to code our TCP Server and clients in C#

I started using, which is a messaging platform and DotNet library called "Alternats." When I saw how well the library worked, I was surprised. I went through the code to see how it works. I was surprised to learn that optimizing the TCP client can improve the system's throughput and latency.

For the most part, zero-copy architecture has become the buzzword for most high-performance architecture. To put this in perspective, you can see the performance of AlterNATS here.


Nats can consume five times as many messages as the original library on the optimized path. Impressive!!

It's well-written code; however, I had to teach myself TCP/IP to use the library. Throughout this series, I will be documenting my findings.

TCP server

Sockets are the class in C# that abstracts any networking layer, and to make a TCP Server component, you need to do two things.

Listen Stage

  • You need to create a Socket that can be done using the following code

      var listenSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
    • All the TCP connection is stream based, so you send and receive a set of bytes to and from the system.
  • Next, you need to Bind to the port or open a port for listening. That can be done using

listenSocket.Bind(new IPEndPoint(IPAddress.Any, 33333));

You need to understand the port ranges before choosing the ports.

Ports with numbers 0–1023 are called system or well-known ports;
ports with numbers 1024-49151 are called user or registered ports,
ports with numbers 49152-65535 are called dynamic, private or ephemeral ports

You need a range between 1024-49151 for the programs

Accept Stage

The third and most important step is to accept connections from the client. This code needs to run forever, so it's usually written in a loop.

while (true) 
   var connectedSocket = await listenSocket.AcceptAsync();
    // Do processing after this.

The most important thing to remember When the server accepts the connection, a new child socket is opened. The listening socket is never interrupted, and the new child socket runs on a different thread.

TCP view

If you run the above code in the console application, a server will be running on port 33333.

How can you verify the same? Tools like TCP View allow you to look at that. You can download the tool at

TCP Client

The TCP client is again a socket that is opened by the client to connect to the TCP server.

 var _clientConnection = new Socket(AddressFamily.InterNetwork, 
 await _clientConnection.ConnectAsync("localhost", 33333);

The interesting thing about this is that the OS opens a new socket in the range of ephemeral ports 49152–65535. Again, you can look at the client using the TCP view, but this time with a different port.

The source code can be found here.

Well, that's all I have learned in a day. I learned following the video series from Stephan Clearly

I hope to learn more and write more about the same. I will keep coming back and updating the series of posts.

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