sockets - Is there a limit on number of tcp/ip connections between machines on linux?

Question

I have a very simple program written in 5 min that opens a sever socket and loops through the request and prints to the screen the bytes sent to it.

I then tried to benchmark how many connections I can hammer it with to try to find out how many concurrent users I can support with this program.

On another machine (where the network between them is not saturated) I created a simple program that goes into a loop and connects to the server machine and send the bytes "hello world".

When the loop is 1000-3000 the client finishes with all requests sent. When the loop goes beyond 5000 it starts to have time outs after finish the first X number of requests. Why is this? I have made sure to close my socket in the loop.

Can you only create so many connections within a certain period of time?

Is this limit only applicable between the same machines and I need not worry about this in production where 5000+ requests are all coming from different machines?

Answer 1

There is a limit, yes. See ulimit.

Also you need to consider the TIMED_WAIT state. Once a TCP socket is closed (by default) the port remains occupied in TIMED_WAIT status for 2 minutes. This value is tunable. This will also "run you out of sockets" even though they are closed.

Run netstat to see the TIMED_WAIT stuff in action.

P.S. The reason for TIMED_WAIT is to handle the case of packets arriving after the socket is closed. This can happen because packets are delayed or the other side just doesn't know that the socket has been closed yet. This allows the OS to silently drop those packets without a chance of "infecting" a different, unrelated socket connection.