I am programming with sockets (TcpListener and TcpClient actually) in C#. I wrote a server that accepts client connections and streams data to them.
In order to test scalability, I wrote a test harness that creates a certain number of connections (say 1000) in a loop, connects to the server, and writes whatever data is received to the console.
After the server receives about 1300 connections, the clients' connection attempts start failing with a regular "No connection could be made because the target machine actively refused it" exception. If the clients keep trying, some connections get through, but there are still many of them that don't. I even tried putting in delays, e.g. three simultaneous clients each opening one connection per second to the server, but the problem remains.
My guess was that the listen backlog was becoming full, but given the delays I introduced, I now doubt it. How can this behaviour be explained and solved?
Edit: before anyone else jumps on this question and marks it as duplicate without having read it...
I am using asynchronous sockets using the Asynchronous Programming Model. That's the old BeginXXX/EndXXX pattern, not the new async/await pattern. The APM uses the Thread Pool underneath, so this is not a naive one-thread-per-connection model. The connections are dormant most of the time unless I/O occurs. In that case, the .NET Framework automatically allocates threads to handle this.
Edit 2: The gist of this question, for those who thought it was too [insert silly adjective here], is: why does a server drop connections when under a heavy load? The error message I quoted usually occurs when a connection cannot be established (i.e. when you got the ip/port wrong), but this clearly isn't the case.
