Due to the complex diversity of contemporary Internet applications, computer network measurements have gained considerable interest during the recent years. Since they supply network research, development and operations with data important for network traffic modelling, performance and trend analysis etc., the quality of these measurements affect the results of these activities and thus the perception of the network and its services. One major source of error is the timestamp accuracy obtained from measurement hardware and software. On this background, we present a method that can estimate the timestamp accuracy obtained from measurement hardware and software. The method is used to evaluate the timestamp accuracy of some commonly used measurement hardware and software. Results are presented for the Agilent J6800/J6830A measurement system, the Endace DAG 3.5E card, the Packet Capture Library (PCAP) either with PF_RING or Memory Mapping, and a RAW socket using either the kernel PDU timestamp (ioctl) or the CPU counter (TSC) to obtain timestamps.

1. www.rws.com/moravia
How to timestamp
accurately
August 2020

2. First impressions
› In this first video you are going to see a stanza that, according to the
coloring of the lines, is apparently well synched.
› Remember that this is only to be used as a guide, but there’re more
effective ways to review the timestamps accuracy.

4. Start times
If you pay closer attention to the start times, when we run Ctrl+L in each line,
you’ll see that:
› Lines 1 and 3 are early. I.e. there’s a brief silence before the first sound.
› Lines 2 and 4 are late. I.e. there’s a portion of the first sound missing.
Remember that the start time must be the on the first sound of the first word
of the line, including prolonged sounds.

6. Start times
There are two options to fix start times:
› Start the song from the top and re-timestamp to run the checks again
› Use Ctrl+plus/minus buttons to move the line 100 milliseconds.
Remember that just clicking on plus/minus buttons will move timestamps by 1
second.

8. To sum up
› Check that you can’t hear any instrumental music or silence before the
first sound of the first word of the line and that you can hear the complete
first sound.
› If you press Ctrl+L a couple of times in a row and you can hear the singer
“stubbering” the first sound of the first word, your timestamps are on
spot.

9. End times
› Delivering accurate end times is just as important as delivering accurate
start times.
› The line coloring is very helpful. However, it’s important to learn when
that shading is showing accurate end times.
If you take a look at the next video you’ll see that end times look quite
accurate.

11. End times
If you’d review each line closely, you’ll see that:
› Lines 1 and 2 are early or cut short. I.e. the last sound of the last word is
not heard.
› Lines 3 and 4 are late. I.e. there’s a brief silence after the last word (or
even the beginning of the following line).
Remember that the end time must be right after the last sound of the last
word of the line, including prolonged sounds which can be clearly heard.

13. End times
The best way to check the end times is to create a new line using
Ctrl+Intro/Enter.
› The new line will have as start time the exact same timestamp than the end
time in the line above.
› When the end time is early you’ll hear the last sound of the last word in the
new line when pressing Ctrl+L.

15. End times
› When the end time is late, you’ll see that the coloring lasts significantly
longer than the singer’s voice.
› To fix it, you’ll need to move your timestamps back enough to make them
match exactly with the end of the line. Always check that you didn’t go to
far by creating a new line.

17. Final comparison
Correct
Incorrect

18. Final summary
› Please take into account that all tasks that are not accurately timestamped
will be reopened for corrections.
› An acceptable margin of error is of less tan 100 milliseconds since the tool
allows to correct by this margin.
› This process might be time-consuming the first couple of times. However,
timestamping accuracy has always been a request to work on this project.