private void compute(NtpPacket packet, long t4) {
Timestamp time1 = packet.getOriginateTime();
long t1 = time1.timeInMillisValue();
// Receive Time is time request received by server (t2)
Timestamp time2 = packet.getReceiveTime();
long t2 = time2.timeInMillisValue();
// Transmit time is time reply sent by server (t3)
Timestamp time3 = packet.getTransmitTime();
long t3 = time3.timeInMillisValue();
delayIsSet = false;
offsetIsSet = false;
/*
* Round-trip network delay and local clock offset (or time drift) is
* calculated according to this standard NTP equation:
*
* LocalClockOffset = ((ReceiveTimestamp - OriginateTimestamp) +
* (TransmitTimestamp - DestinationTimestamp)) / 2
*
* equations from RFC-1305 (NTPv3) roundtrip delay = (t4 - t1) - (t3 -
* t2) local clock offset = ((t2 - t1) + (t3 - t4)) / 2
*
* It takes into account network delays and assumes that they are
* symmetrical.
*
* Note the typo in SNTP RFCs 1769/2030 which state that the delay is
* (T4 - T1) - (T2 - T3) with the "T2" and "T3" switched.
*/
if (time1.ntpValue() == 0) {
// without originate time cannot determine when packet went out
// might be via a broadcast NTP packet...
if (time3.ntpValue() != 0) {
offset = t3 - t4;
offsetIsSet = true;
}
// else, ERROR: cannot compute delay/offset
} else if ((time2.ntpValue() == 0) || (time3.ntpValue() == 0)) {
if (t4 < t1) {
throw new AssertionError("return-time < originate-time");
}
// without receive or xmit time cannot figure out processing
// time
// so delay is simply the network travel time
delay = t4 - t1;
delayIsSet = true;
// TODO: is offset still valid if rcvNtpTime=0 || xmitNtpTime=0 ???
// Could always hash origNtpTime (sendTime) but if host doesn't set
// it
// then it's an malformed ntp host anyway and we don't care?
// If server is in broadcast mode then we never send out a query in
// first place...
if (time2.ntpValue() != 0) {
// xmitTime is 0 just use rcv time
offset = t2 - t1;
} else if (time3.ntpValue() != 0) {
// rcvTime is 0 just use xmitTime time
offset = t3 - t4;
}
offsetIsSet = true;
} else {
// assert xmitTime >= rcvTime: difference typically < 1ms
if (t3 < t2) {
throw new AssertionError("malformed NTP packet, transmit-time < receive-time");
}
// assert returnTime >= origTime: network delay could not be
// negative
if (t4 < t1) {
throw new AssertionError("return-time < originate-time");
}
long troundtrip = t4 - t1;
// subtract processing time from round-trip network delay
long tprocessing = t3 - t2;
// in normal cases the processing delta is less than
// the total roundtrip network travel time.
if (tprocessing <= troundtrip) {
troundtrip -= tprocessing; // delay = (t4 - t1) - (t3 - t2)
} else {
// if delta - delayValue == 1 ms then it's a round-off error
// e.g. delay=3ms, processing=4ms
if (tprocessing - troundtrip == 1) {
// delayValue == 0 -> local clock saw no tick change but
// destination clock did
if (troundtrip != 0) {
troundtrip = 0;
}
}
}
offset = ((t2 - t1) + (t3 - t4) - troundtrip) / 2;
delayIsSet = true;
offsetIsSet = true;
}
}
