====== Tweak retries ======
  * 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.