As long as I know, when a connection request SYN segment arrives, TCP will response with an SYN&ACK segment and mark the connection incomplete(SYN_RCVD) and put it into an incomplete queue, waiting for the ACK to complete the connection.
But I think it won't always stay in the queue, is there a timeout on it?
On my Ubuntu system:
# cat /proc/sys/net/ipv4/tcp_synack_retries
5
A tcp_synack_retries variable is responsible for controlling the number of retransmissions in Linux operating system. Its default value is set to 5 for most Linux operating systems, which causes the half-open connection to be removed after 3 minutes. In the below table there are calculations for other values.
Value Time of retransmission Total time to keep half-open connections in the backlog queue
1 in 3rd second 9 seconds
2 in 3rd and 9th second 21 seconds
3 in 3rd , 9th and 21st second 45 seconds
