Update last packet time only on change

With large samples like key frame, they could be hundreds of
packets. Updating the last packet time on each of those
packets means the sender report could be off by the amount
of time it takes to send the full sample which could be a few ms.

Author: boks1971

pkg/report/sender_stream.go

@@ -43,8 +43,12 @@ func (stream *senderStream) processRTP(now time.Time, header *rtp.Header, payloa
 	if stream.useLatestPacket || stream.packetCount == 0 || (diff > 0 && diff < (1<<15)) {
 		// Told to consider every packet, or this was the first packet, or it's in-order
 		stream.lastRTPSN = header.SequenceNumber
-		stream.lastRTPTimeRTP = header.Timestamp
-		stream.lastRTPTimeTime = now
+		// update only on first packet of a frame to ensure sender report does not get affected by
+		// processing delay of pushing a large frame which could span multiple packets
+		if header.Timestamp != stream.lastRTPTimeRTP {
+			stream.lastRTPTimeRTP = header.Timestamp
+			stream.lastRTPTimeTime = now
+		}
 	}
 
 	stream.packetCount++