Understand backlog Argument in listen() Command of Linux

In linux system, backlog specify the accepted queue size (the number of established status packets). If the accepted queue is full, further SYN RECEIVED packets will be ignored. If you are using TCP, then TCP will simply resend this packet, for the sender does not receive ACK. But for UDP, you will deal with it in another way.

For further reading, here is a good blog post.