snmp bandwidth calculation

[snmp5152]

I am trying to debug an issue where the interface bandwidth graph is spiky despite sending constant traffic. The cacti is configured to poll the 64bit counter every 300 seconds. Due to some known problem, the snmp response from the router may not be immediate. During some conditions the response time may vary from 5 to 10 seconds.

So I use the following formula to calcuate the bandwidth from the 64 bit octet counters

(Delta between current poll and last poll counter *8)/(Delta of time between the last and current poll)

What I want to know is how the Delta of time is calculated? Does it take the delta of the time between when the snmp query was sent out by the cacti server or does it take the delta between the timestamps when the snmp response was received?

So lets assume the following timestamps

0 - Snmp query sent out of the cacti server - t1
10 - snmp response sent by router - t2
300 - next snmp query sent out of the cacti server -t3
305 - snmp response sent by router - t4

So will the delta of time in this case be 300 seconds (t3-t1) or 295 seconds (t4-t2) or 290 seconds (t3-t2).


Thanks,