Olivier Langlois's blog

Upon reception of RST segment, the receiving side will immediately abort the connection. This statement has more implications than just meaning that you will not be able to receive or send any more data to/from this connection. It also implies that any unread data still in the TCP reception buffer will be lost. This information can be found in TCP/IP Internetworking volume 2 and Unix Network Programming Volume 1 third edition but in my opinion those books do not put enough emphasis on that detail and if you are not reading these books to find this exact detail, it might slip away from your attention.

Now, what are the conditions to receive a RST segment? The easiest way is to enable the SO_LINGER option with a timeout value of 0 on a server socket. As soon as the connection will be closed by the server, it will send a RST. You can learn more about SO_LINGER in the book Unix Network Programming Volume 1 third edition. The other possibility is if a server receives data after having closed the connection. If you enable the LINGER option, sending RST is the expected behavior but you can easily be bitten by the second possibility and here is an example on how it can happen.

Imagine a HTTP server that limits the number of simultaneous connections. Upon accepting a new connection, it might compare a global connection counter against a configured limit and if the counter has reached the limit then immediately send back a 503 error and close the connection. Do you see the problem? The server is closing the connection before receiving the HTTP request. There is a high probability that the client will never receive the 503 reply because a RST segment will immediately follow the reply.

A not too good solution would be to call shutdown(SHUT_WR) before closing the socket. What this will do is force the server to send a FIN segment before the RST segment. I have tried this solution and this seems to greatly improve the probability that the client application will receive the data before receiving the RST segment.

A better way is to program the server to read the request even if it has no intention to actually process it. This ensures that no RST will be sent unintentionally.

There remains one question. What exactly means data reception on a closed connection? Does that only include new received TCP segment or could data still in the TCP receive buffer of a closing connection be considered as data arrived after the connection has been closed? My intuition is that this question is open to interpretation and the TCP behavior will vary from one platform to the other. However, even if your platform does not react with a RST if a TCP connection receive buffer is not empty when it is closing it; my opinion is that you should not rely on that since this is a race condition. You could be closing the connection before or after receiving the client request depending on the RTT (Round Time Trip) of the connection.