NTP Timestamp in RTCP Sender Report is Incorrect

Question

I'm using Amazon's sample code to upload a rtsp stream from an IP camera to Kinesis video streams. Code found here: https://github.com/awslabs/amazon-kinesis-video-streams-producer-sdk-cpp/blob/master/samples/kvs_gstreamer_sample.cpp

I would like to get the NTP time from the camera for each frame. My understanding is that the first step in doing this is reading the RTCP sender report to get the synchronizing time for the camera's RTP and NTP times.

To do that, I've added a callback on receiving RTCP packets like so:

g_signal_connect_after(session, "on-receiving-rtcp", G_CALLBACK(on_rtcp_callback), data);

Then, in my callback function after getting the SR packet from the buffer, I try to get the two timestamps:

gst_rtcp_packet_sr_get_sender_info (packet, ssrc, ntptime, rtptime, packet_count, octet_count);

When comparing the 'ntptime' and 'rtptime' variables I get here, with what I see in Wireshark, the rtp times match perfectly. However, the NTP time I get in my C++ code is very wrong, it shows a time from about a month ago, while the Wireshark packet shows an NTP time that appears correct.

Is there some setting causing gstreamer to overwrite the NTP time in my sender report packets, and if so, how do I disable that setting?

Answer 1

It turns out the NTP time provided gst_rtcp_packet_sr_get_sender_info is not in any format I've seen before. To convert to a meaningful timestamp you have to use gst_rtcp_ntp_to_unix which then gives you a unix time that actually makes some sense.

Answer 2

let's say gst_rtcp_packet_sr_get_sender_info function does not provide direct NTP clock value, instead it provides guint32+guint32 bytes(time_s + fractions).

juts in case, I will share this tiny code snipped that may be helpfull to others to convert these bytes to nano-second UTC time.

#include <gst/rtp/gstrtcpbuffer.h>
...
GstClockTime ntp_epoch_ns = gst_rtcp_ntp_to_unix(rtcp_sr_ntp);