/**
* Estimate the <tt>RemoteClock</tt> of a given RTP stream (identified by its SSRC) at a given
* time.
*
* @param ssrc the SSRC of the RTP stream whose <tt>RemoteClock</tt> we want to estimate.
* @param time the local time that will be mapped to a remote time.
* @return An estimation of the <tt>RemoteClock</tt> at time "time".
*/
public RemoteClock estimate(int ssrc, long time) {
ReceivedRemoteClock receivedRemoteClock = receivedClocks.get(ssrc);
if (receivedRemoteClock == null || receivedRemoteClock.getFrequencyHz() == -1) {
// We can't continue if we don't have NTP and RTP timestamps
// and/or the original sender frequency, so move to the next
// one.
return null;
}
long delayMillis = time - receivedRemoteClock.getReceivedTime();
// Estimate the remote wall clock.
long remoteTime = receivedRemoteClock.getRemoteClock().getRemoteTime();
long estimatedRemoteTime = remoteTime + delayMillis;
// Drift the RTP timestamp.
int rtpTimestamp =
receivedRemoteClock.getRemoteClock().getRtpTimestamp()
+ ((int) delayMillis) * (receivedRemoteClock.getFrequencyHz() / 1000);
return new RemoteClock(estimatedRemoteTime, rtpTimestamp);
}
/**
* Inspect an <tt>RTCPCompoundPacket</tt> and build-up the state for future estimations.
*
* @param pkt
*/
public void apply(RTCPCompoundPacket pkt) {
if (pkt == null || pkt.packets == null || pkt.packets.length == 0) {
return;
}
for (RTCPPacket rtcpPacket : pkt.packets) {
switch (rtcpPacket.type) {
case RTCPPacket.SR:
RTCPSRPacket srPacket = (RTCPSRPacket) rtcpPacket;
// The media sender SSRC.
int ssrc = srPacket.ssrc;
// Convert 64-bit NTP timestamp to Java standard time.
// Note that java time (milliseconds) by definition has
// less precision then NTP time (picoseconds) so
// converting NTP timestamp to java time and back to NTP
// timestamp loses precision. For example, Tue, Dec 17
// 2002 09:07:24.810 EST is represented by a single
// Java-based time value of f22cd1fc8a, but its NTP
// equivalent are all values ranging from
// c1a9ae1c.cf5c28f5 to c1a9ae1c.cf9db22c.
// Use round-off on fractional part to preserve going to
// lower precision
long fraction = Math.round(1000D * srPacket.ntptimestamplsw / 0x100000000L);
/*
* If the most significant bit (MSB) on the seconds
* field is set we use a different time base. The
* following text is a quote from RFC-2030 (SNTP v4):
*
* If bit 0 is set, the UTC time is in the range
* 1968-2036 and UTC time is reckoned from 0h 0m 0s UTC
* on 1 January 1900. If bit 0 is not set, the time is
* in the range 2036-2104 and UTC time is reckoned from
* 6h 28m 16s UTC on 7 February 2036.
*/
long msb = srPacket.ntptimestampmsw & 0x80000000L;
long remoteTime =
(msb == 0)
// use base: 7-Feb-2036 @ 06:28:16 UTC
? msb0baseTime + (srPacket.ntptimestampmsw * 1000) + fraction
// use base: 1-Jan-1900 @ 01:00:00 UTC
: msb1baseTime + (srPacket.ntptimestampmsw * 1000) + fraction;
// Estimate the clock rate of the sender.
int frequencyHz = -1;
if (receivedClocks.containsKey(ssrc)) {
// Calculate the clock rate.
ReceivedRemoteClock oldStats = receivedClocks.get(ssrc);
RemoteClock oldRemoteClock = oldStats.getRemoteClock();
frequencyHz =
Math.round(
(float)
(((int) srPacket.rtptimestamp - oldRemoteClock.getRtpTimestamp())
& 0xffffffffl)
/ (remoteTime - oldRemoteClock.getRemoteTime()));
}
// Replace whatever was in there before.
receivedClocks.put(
ssrc,
new ReceivedRemoteClock(
ssrc, remoteTime, (int) srPacket.rtptimestamp, frequencyHz));
break;
case RTCPPacket.SDES:
break;
}
}
}