SYN-SENT: The duration of this state can be controlled by TCP_SYNCNT or tcp_syn_retries.
SYN-RECV: It's usually hidden from application. It is tuned by tcp_synack_retries.
Idling ESTABLISHED connection, will never notice any issues. A solution is to use TCP keepalives.
Busy ESTABLISHED connection, adheres to tcp_retries2 setting, and ignores TCP keepalives.
Zero-window ESTABLISHED connection, adheres to tcp_retries2 setting, and ignores TCP keepalives.