ous message. The data that contains outlier values in messages will therefore be ignored and discarded. If too many messages arrive like this, PTP will struggle to recover accurate time. So, the transport of time with PTP requires a small but steady stream of these "lucky" messages to allow a slave to recover time accurately. But the actual transit time does not matter; PTP can work over any practical distance if either the transit is symmetrical or any asymmetry is controlled or compensated for. 7.3.1 Asymmetry Correction You have seen the effect that asymmetry has on the recovered time accuracy on the downstream slave. In many cases, some proportion of any observed asymmetry can be static (meaning that it never changes, even following a node or link restart) and the rest of it will be dynamic. Dynamic asymmetry is mostly caused by changes in network conditions, even where no local reason for it is apparent. An example of static asymmetry could be cable length mismatch between fiber pairs, whereas dynamic asymmetry could arise from traffic conditions causing message queueing in a non-PTP interface upstream of the clock. In the case of total asymmetry, it is possible to measure it with specialized testing equipment that carries onboard a source of time (for example, a global navigation satellite system [GNSS] re- ceiver). The network designer may also be able to estimate the amount of asymmetry because of their understanding of the likely sources of it. For example, a long run of bidirectional (BiDi) fiber will have different forward and reverse path delay because of the different speed of the forward and reverse As in the fiber. Those values can be known or estimated to a considerable degree of accuracy. If the operator can measure or calculate the asymmetry, it is possible to configure this expected asymmetry in the PTP port characteristics SO that the time-recovery algorithm can permanently cor- rect for this error. The only difficulty can be understanding what proportion of the asymmetry is static or constant