Asymmetric networks present a particular problem when attempting to account for network latency during PTP offset calculations between master and slave servers. PTP assumes symmetry in the network and the PTP protocol is not able to detect asymmetry in the network paths between master and slave.
Asymmetry can be present for a number of reasons. For example, a change in network speed might result in a switch using different buffering methods for each direction, causing longer store and forward delays in one direction than the other. See the following figure.
The result is that PTP offsets between master and slave will converge, but will be wrong by a constant offset from the master equal to half the asymmetry. For example with the following transmit times:
- 5 µs master to slave
- 1 µs slave to master
the one-way delay is (5+1)/2 = 3 µs. So:
- 3 µs is added to the time offsets received from the master clock.
- 1PPS will display a mean offset value of (5-3) = 2 µs.
- Actual asymmetry should be double this observed value, so is 4 µs.