This is a static archive of our old Q&A Site. Please post any new questions and answers at ask.wireshark.org.

rtp.timestamp units

0

What are the units of rtp.timestamp? When I capture RTP traffic, I have a field under Real-Time Transport Protocol that shows TimeStamp = 2218552874.

What are the units of this number (sec, ms, usec, nsec)? And where does it come from? Mac Layer? Libpcap? System clock?

asked 12 Dec '11, 11:10

AminGho's gravatar image

AminGho
51●4●4●8
accept rate: 0%

edited 30 Dec '11, 14:06

helloworld's gravatar image

helloworld
3.1k●4●20●41

One Answer:

0

From RFC 3550:

timestamp: 32 bits

The timestamp reflects the sampling instant of the first octet in
the RTP data packet.  The sampling instant MUST be derived from a
clock that increments monotonically and linearly in time to allow
synchronization and jitter calculations (see Section 6.4.1).  The
resolution of the clock MUST be sufficient for the desired
synchronization accuracy and for measuring packet arrival jitter
(one tick per video frame is typically not sufficient).  The clock
frequency is dependent on the format of data carried as payload
and is specified statically in the profile or payload format
specification that defines the format, or MAY be specified
dynamically for payload formats defined through non-RTP means.

So there is no default timestamp unit.

answered 12 Dec ‘11, 13:05

Jaap's gravatar image

Jaap ♦
11.7k●16●101
accept rate: 14%

Dear Jaap,

Thanks for you answer. Just to confirm, so there is no direct translation to any time measurement units from RTP timestamp!?

How about the MAC Timestamp in the Radio tap hearder? it says that the unit is in usec but for example what does this number means in usec : 84818688

Does it mean it took this much time (84818688 usec) to receive the packet? I doubt it because that’s too long?

I really appreciate your help and comments on this.

Regards, Amin

(13 Dec ‘11, 01:12) AminGho
1

Just to confirm, so there is no direct translation to any time measurement units from RTP timestamp!?

Well, there can be. From the quote:

  The clock frequency is dependent on the format of data carried as payload
and is specified statically in the profile or payload format specification
that defines the format, or MAY be specified dynamically for payload
formats defined through non-RTP means.

So, for instance, the PCMA codec has a fixed sample clock of 8kHz. That results in a timetick of 125 µs, for that given codec.

(13 Dec ‘11, 02:21) Jaap ♦