rtt: Fix extra 1 second delay in RTT calculation #70
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Fix the issue in the rtt() function where it incorrectly adds an extra 1 second delay to the RTT calculation. To ensure the acquisition of atomic seconds and microseconds values, use gettimeofday() instead of time() and get_usec(). Reviewed-by: lishuo02 <[email protected]> Signed-off-by: maoyingming <[email protected]>
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Occasionally hping mis reports large RTT latency (>1000ms) . For more details: #48
Thanks.
Fix the issue in the rtt() function where it incorrectly adds an extra 1 second delay to the RTT calculation. To ensure the acquisition of atomic seconds and microseconds values, use gettimeofday() instead of time() and get_usec().
Trigger command:
hping3 -S -p 6379 -c 1000 192.168.0.7 -i u10000
Error output:
On average, out of every 1000 ping packets, 1 to 2 times the RTT delay exceeds 1 second. This delay of more than 1 second is exactly 1 second longer compared to the delays of the packets before and after it.
tcpdump analysis:
The tcpdump capture shows that the TCP connection with cport 2570 and sport 6379 completed the three-way handshake in a total of 102 microseconds, which did not exceed 1 second.
More detailed log analysis
This issue typically arises when there is a system time change between the two steps of fetching a timestamp. If a carry-over occurs between fetching seconds and microseconds, it indeed results in the seconds being 1 second less than the actual time. Modify the source code to print out some intermediate results within the rtt() function.
case1: The start time is incorrect, the end time is correct, and the time difference is incorrect. （note fixed）
Incorrect start time: 1719999425.000532s Correct start time: 1719999426.000532s
Correct end time: 1719999426.002238s
Incorrect time difference: 1.001706s = 1001.7ms Correct time difference: 0.001706s = 1.7ms
case2: The start time is correct, the end time is incorrect, and the time difference is incorrect (partially fixed).
Correct start time: 1720000176.998808s
Incorrect end time: 1720000176.000237s Correct end time: 1720000177.000237s
Incorrect time difference: -0.998571s Correct time difference: 0.001429s = 1.4ms
This situation has been partially fixed but not completely.
