PTP disciplines your clocks to a grandmaster. Explain how the four-message exchange computes offset and delay, what assumption it makes, and what a 3 microsecond asymmetry between the two directions does to your measured latency.

PTP disciplines your clocks to a grandmaster. Explain how the four-message exchange computes offset and delay, what assumption it makes, and what a 3 microsecond asymmetry between the two directions does to your measured latency.

Approach: Write down the four timestamps, form the two equations for offset and one-way delay, and see which term cancels only when the path is symmetric.

PTP solves offset and delay from four timestamps assuming the path is symmetric, and a 3 microsecond asymmetry puts a fixed 1.5 microsecond error into every clock offset. The master sends Sync at t1 and the slave receives it at t2, so t2 - t1 is the forward delay plus the clock offset. The slave sends Delay_Req at t3 and the master receives it at t4, so t4 - t3 is the reverse delay minus the offset. Adding them gives the round trip and dividing by two gives the mean path delay, and subtracting gives offset as ((t2 - t1) - (t4 - t3))/2. The division by two is where symmetry enters: if the forward path is 3 microseconds longer than the reverse, half of that difference lands directly in the offset, so the slave clock sits 1.5 microseconds away from the grandmaster with no indication that anything is wrong. The consequence is that a measured one-way latency between two hosts is wrong by that amount, and comparisons between two colocated strategies become unreliable at the microsecond scale. Asymmetry comes from different fibre lengths, different switch queueing in each direction and from software stamping, which is why boundary and transparent clocks correct for switch residence time and why stamps are taken in the NIC.

Follow-up: Your grandmaster loses GPS lock and holds over on its local oscillator. How fast does the offset accumulate and what do you stop doing?

Key concepts: ptp, clock offset, path asymmetry, grandmaster.