# RFC Errata

### 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.