title: RTP timestamp in data packets vs RTCP SR packets
body: <p>I'm trying to understand the difference between the RTP timestamp as it occurs in RTP data packets vs as it is used in RTCP Sender Report (SR) packets.</p>
  <p>For the RTP timestamp in data packets I have established that:</p>
  <ul>
  <li>They are not based on wall-clock time but represent more of a counter</li>
  <li>They typically have a random offset chosen at the beginning of a session</li>
  <li>For simple standard audio codecs they typically increment by 160 (roughly speaking 1000ms / 20ms * 160samples = 8000 bit rate) and this increment also includes not sent silent packets</li>
  </ul>
  <p>For the RTP timestamp in RTCP sender report packets I originally thought they were just a snapshot of the current RTP timestamp of the data packets and in conjunction with the NTP timestamp (which is typically wall-clock) could be used to calculate the wall-clock time of further incoming RTP packets which is also what I understood from <a href="https://www.ece.rutgers.edu/%7Emarsic/books/CN/projects/wireshark/ws-project-4.html" rel="nofollow noreferrer">this analysis on the subject</a>.</p>
  <p>However a sentence of the <a href="https://datatracker.ietf.org/doc/html/rfc3550#section-6.4.1" rel="nofollow noreferrer">RFC 3550 Section 6.4.1</a> makes me wonder about that assumption:</p>
  <blockquote>
  <p>Note that in most cases this timestamp will not be equal to the RTP timestamp in any adjacent data packet.</p>
  </blockquote>
  <p>This ruins my assumption, because I assumed that the SR packet would contain an RTP timestamp that is found in a data packet that has just been sent by the same source. Unfortunately the next sentences are pretty much meaningless for me (maybe this is an English language barrier but it sound like non-helpful nonsense to me)</p>
  <blockquote>
  <p>Rather, it MUST be
  calculated from the corresponding NTP timestamp using the
  relationship between the RTP timestamp counter and real time as
  maintained by periodically checking the wallclock time at a
  sampling instant.</p>
  </blockquote>
  <p>Could you clarify for me how the RTP timestamp in an RTCP SR packet can be calculated?</p>