For more information on TCP three-way handshake, refer to RFC 793. See also tcp_conn_req_max_q.
0 to 4,294,967,296
For applications, such as web servers that might receive excessive connection requests, you can increase the default value to match the incoming rate.
The following explains the relationship between tcp_conn_req_max_q0 and the maximum number of pending connections for each socket.
When a connection request is received, TCP first checks if the number of pending TCP connections (three-way handshake is done) waiting to be accepted exceeds the maximum (N) for the listener. If the connections are excessive, the request is denied. If the number of connections is allowable, then TCP checks if the number of incomplete pending TCP connections exceeds the sum of N and tcp_conn_req_max_q0. If it does not, the request is accepted. Otherwise, the oldest incomplete pending TCP request is dropped.
For information, see tcp_conn_req_max_q0.