Hi everyone,
I have a question about the calculation of the roundtrip delay in ms for a wireshark´s capture. I read in the RFC-3550 that the roundtrip delay is (A - LSR - DLSR) , I´ve being asking and I was told that A can be taken from the frame timestamp, but I dont have clear where I can take this data
(A) from, I mean which frame is it?
Here I attached some fields from a capture, If you could help me identify the data I need in order to understand how you calculate the roundtrip delay in ms, I´ll be very thankful!
Frame 756 (174 bytes on wire, 174 bytes captured)
Timestamp, MSW: 3346780003 (0xc77bcf63)
Timestamp, LSW: 210803180 (0x0c9099ec)
MSW and LSW as NTP timestamp: Jan 20, 2006 21:06:43,0491 UTC
RTP timestamp: 0
Last SR timestamp: 14635237 (0x00df50e5)
Delay since last SR timestamp: 251396
Frame matching Last SR timestamp: 447
Roundtrip Delay(ms): 5
I was reading some old mails in the developers mailing list for Ethereal and it said:
"You can't do the calculation using just one frame. And bear in mind that the calculation Ethereal does is relative to the point of capture (although in many cases you may be capturing on the same machine as the client).
Look below, the following line is present: "MSW and LSW as NTP timestamp: Feb 24, 2036 17:42:56,0000 UTC "
This is done by the function ntp_fmt_ts() in packet-ntp.c
.
However, this isn't important for calculating the rountrip, as it is relative. We are comparing:
- the observed difference between the current SR and the one the DLSR refers to
- the time in the DLSR (which is the number of 1/65536 ths of asecond)"
If I want to calculate the roundtrip delay as a Quality of Service Parameter for VoIP calls, how should I implement this measures?, and how do I manage this roundtrip delays for every call?, how often are these reports sent (RTCP)?. I mean, should I calculate the average roundtrip delay for "n" frames for each call? or how?, sorry... I´m confused...
Thanks for your time!
Best regards,
Andreina
