TCP Persist Timer
Receiver flow control can set the window to zero
Receiver later sends “window update acks”
But TCP does not transmit acks reliably (I.e. ACKs are not acknowledged) => update acks may be lost and source may be stuck at a zero window value
TCP uses persist timer to query the receiver periodically to find if the window has been increased.
Persist timer always bounded between 5s and 60s. It does exponential backoff like other timers too.
Persist timer never stops until window update or connection terminated