Timer and a while loop - does the timer interrupt execution

[thekioskman]

I read in an older post that the timer will not work if you call the timer and then enter a conditional while loop that could not end. For example the below code:

#78

addTimer(1 second)
while( ack_not_received == True){
  do_nothing() 
} 

timeCallback(){
  ack_not_recevied = False;
  timed_out = True;
}

I'm wondering if there is a specific reason that the timer callback cannot interrupt the while loop. Also if this style of timeout doesn't work, I'm not sure how we would implement a packet time out. Is there any hint you could give in that case?

[Pusnow]

KENS follows single-threaded event-driven model, so you should avoid busy-waiting (while( ack_not_received == True)) to proceed.

Rather than busy-waiting, the original function must return, and later the remaining tasks should be handled in timerCallback().

[thekioskman]

Ok, I see. So it would be ok to design out function to return after a constant amount of time on each call. Regardless of when the response arrives. I also noticed there was not specification for how long the timer should be, is there a recommended timeout?

[Pusnow]

It depends on types of timeout.

For retransmission timeout, see:

KENSv3/Documentation/rfc/rfc793.txt

Lines 2609 to 2634 in eba97b6

	Retransmission Timeout
	Because of the variability of the networks that compose an
	internetwork system and the wide range of uses of TCP connections the
	retransmission timeout must be dynamically determined. One procedure
	for determining a retransmission time out is given here as an
	illustration.
	An Example Retransmission Timeout Procedure
	Measure the elapsed time between sending a data octet with a
	particular sequence number and receiving an acknowledgment that
	covers that sequence number (segments sent do not have to match
	segments received). This measured elapsed time is the Round Trip
	Time (RTT). Next compute a Smoothed Round Trip Time (SRTT) as:
	SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)
	and based on this, compute the retransmission timeout (RTO) as:
	RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]
	where UBOUND is an upper bound on the timeout (e.g., 1 minute),
	LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
	a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
	factor (e.g., 1.3 to 2.0).

[thekioskman]

gotcha, and I'm assuming we use c++ chrono to measure the RTT of the acks