tremely high PDV, then increasing the packet rate is one way of attempting to receive enough good messages to lock. In practice, the experience of the authors is that this strategy is rarely successful. Note that the choice of a packet rate may influence the requirements on the oscillator stability. This is because with less data arriving, the oscillator needs to be more stable SO that it is impacted less by jitter. From experience with real deployments, increasing the message rate often does not fix any issues, but reducing it to a low level certainly starts to cause them. 9.3.1 Two-Way Time Error As covered in the earlier section "Packet Selection and Floor Delay," only a subset of the arriving packets may have data fresh enough to accurately solve the time equations. When the SDOs design models for modeling clock behavior, they must specify metrics that (hopefully) reflect packet selec- tion mechanisms used in real implementations. Appendix I of G.8260 provides quite a bit of back- ground on these selection methods. When the timestamped PTP Sync message arrives at the slave, it has been in transit for an interval, which is called the forward delay, fwd. This delay is composed of a fixed but unknown delay (switching time and propagation time) and a random variable component resulting from other factors such as queuing in intermediate equipment. This random component is the variation part of the PDV. For frequency-only recovery (such as with G.8265.1), the fixed delay is not important, but, of course, the random delay (PDV) does cause error. On the other hand, for phase/time, both are im- portant to the time error. In the case of phase/time using two-way traffic, the same effect occurs in the other direction with the Delay_Req message. This delay, drev, is also a combination of a fixed and dynamic error. If the slave were to use the Sync t1 timestamp from the master as a reference, the time error on the slave would be equal to fwd because the slave clock would be behind the master time by the amount of time the 