TCP Persist Timer
Receiver flow control can set window to zero
Receiver later sends “window update acks”
But TCP does not transmit acks reliably => 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.