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RFC 5905, "Network Time Protocol Version 4: Protocol and Algorithms Specification", June 2010

Source of RFC: ntp (int)

Errata ID: 3125
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT

Reported By: Richard Walters
Date Reported: 2012-02-16
Held for Document Update by: Brian Haberman

Section A.5.1.1 says:

        /*
         * Calculate offset, delay and dispersion, then pass to the
         * clock filter.  Note carefully the implied processing.  The
         * first-order difference is done directly in 64-bit arithmetic,
         * then the result is converted to floating double.  All further
         * processing is in floating-double arithmetic with rounding
         * done by the hardware.  This is necessary in order to avoid
         * overflow and preserve precision.
         *
         * The delay calculation is a special case.  In cases where the
         * server and client clocks are running at different rates and
         * with very fast networks, the delay can appear negative.  In
         * order to avoid violating the Principle of Least Astonishment,
         * the delay is clamped not less than the system precision.
         */
        if (p->pmode == M_BCST) {
                offset = LFP2D(r->xmt - r->dst);
                delay = BDELAY;
                disp = LOG2D(r->precision) + LOG2D(s.precision) + PHI *
                    2 * BDELAY;
        } else {
                offset = (LFP2D(r->rec - r->org) + LFP2D(r->dst -
                    r->xmt)) / 2;
                delay = max(LFP2D(r->dst - r->org) - LFP2D(r->rec -
                    r->xmt), LOG2D(s.precision));
                disp = LOG2D(r->precision) + LOG2D(s.precision) + PHI *
                    LFP2D(r->dst - r->org);
        }
        clock_filter(p, offset, delay, disp);

It should say:

        /*
         * Calculate offset, delay and dispersion, then pass to the
         * clock filter.  Note carefully the implied processing.  The
         * first-order difference is done directly in 64-bit arithmetic,
         * then the result is converted to floating double.  All further
         * processing is in floating-double arithmetic with rounding
         * done by the hardware.  This is necessary in order to avoid
         * overflow and preserve precision.
         *
         * The delay calculation is a special case.  In cases where the
         * server and client clocks are running at different rates and
         * with very fast networks, the delay can appear negative.  In
         * order to avoid violating the Principle of Least Astonishment,
         * the delay is clamped not less than the system precision.
         */
        if (p->pmode == M_BCST) {
                offset = LFP2D(r->xmt - r->dst);
                delay = BDELAY;
                disp = LOG2D(r->precision) + LOG2D(s.precision) + PHI *
                    2 * BDELAY;
        } else {
                offset = (LFP2D(r->rec - r->org) + LFP2D(r->xmt -
                    r->dst)) / 2;
                delay = max(LFP2D(r->dst - r->org) - LFP2D(r->xmt -
                    r->rec), LOG2D(s.precision));
                disp = LOG2D(r->precision) + LOG2D(s.precision) + PHI *
                    LFP2D(r->dst - r->org);
        }
        clock_filter(p, offset, delay, disp);

Notes:

Calculations of 'offset' and 'delay' have terms that are incorrectly swapped. In the calculation of 'offset', term 'r->dst' should be 'r->xmt', and term 'r->xmt' should be 'r->dst'. In the calculation of 'delay', term 'r->rec' should be 'r->xmt', and term 'r->xmt' should be 'r->rec'.

See the text from section 8:

"In the figure, the first packet transmitted by A contains only the
origin timestamp t1, which is then copied to T1. B receives the
packet at t2 and copies t1 to T1 and the receive timestamp t2 to T2.
At this time or some time later at t3, B sends a packet to A
containing t1 and t2 and the transmit timestamp t3. All three
timestamps are copied to the corresponding state variables. A
receives the packet at t4 containing the three timestamps t1, t2, and
t3 and the destination timestamp t4. These four timestamps are used
to compute the offset and delay of B relative to A, as described
below.

...

"The four most recent timestamps, T1 through T4, are used to compute
the offset of B relative to A

theta = T(B) - T(A) = 1/2 * [(T2-T1) + (T3-T4)]

and the round-trip delay

delta = T(ABA) = (T4-T1) - (T3-T2)."

Noting that, from the perspective of A at time t4:

T1 = t1 = origin timestamp (r->org)
T2 = t2 = receive timestamp (r->rec)
T3 = t3 = transmit timestamp (r->xmt)
T4 = t4 = destination timestamp (r->dst)

An alternative correction would be to change the '+' to a '-' in the calculation of 'offset', and change the '-' to a '+' in the calculation of 'delay'. However, this would deviate from the operators used in the formulas from section 8.

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