AVTP: Introduce the Clock Reference Format (CRF) elements

Hi Everyone,

This merge request introduces two new elements to the AVTP Plugin. These elements add support for AVTP Clock Reference Format (CRF) as described in Chapter 10 of IEEE 1722-2016 Standard.

CRF is useful in synchronizing events within different systems by distributing a common clock. This is useful in a scenario where there are multiple talkers who are sending data to a single listener which is processing that data. E.g. CCTV cameras on a network sending AVTP video streams to a base station to display on the same screen.

It is assumed that all the systems are already time-synchronized with each other. So, the AVTP Talker essentially adjusts the AVTP Presentation Time so it's phase-locked with the reference clock provided by the CRF stream.

There are 2 different roles of systems which participate in CRF data exchange. A system can either be a CRF Talker, which samples it's own clock and generates a stream of timestamps to transmit over the network, or a CRF Listener, which receives the generated timestamps and recovers the media clocks from these timestamps. The timestamps generated by the talker may not be continuous and the listener might have to interpolate some timestamps to generate the full stream. The number of timestamps to interpolate is mentioned in the CRF stream packet (Refer IEEE 1722-2016 Section 10.4 for packet structure). Only CRF Listener has been implemented going to be supported in this merge request.

The two elements implementing the CRF Listener are used to either synchronize the time with the media stream (avtpcrfsync) or check whether the incoming media stream is synchronized (avtpcrfcheck) with the CRF stream.

For more details about how the elements are implemented, please look into the commit message of the first and second patches in this merge request.

In order to test the elements, we used an example implementation of the CRF Talker application from the libavtp.

Thanks,

Vedang Patel