imum possible time. It means that there was no queuing, no buffering, and no re- source contention in any component in the end-to-end path. On normally functioning networks with no congestion, a reasonable percentage of the packets will cross the network close to this floor time. Others might experience some delay, whereas a few others might experience much longer delay. So, the distribution of the transit time, even on a lightly loaded network, trails off toward long delays, what is referred to as a long tail. For this reason, the methods used to select and qualify the message stream are an important part of the implementation of a PTP servo. So, the engineer needs to know what contributes to PDV, and what can be done to limit it. At its most fundamental, PDV is a characteristic of a network element and how it is designed and imple- mented. The following is a list of items among others that can contribute to the delay floor value upon arrival at a PTP-unaware node: Input processing and packet classification delay Delay in making the forwarding decision and doing the next-hop lookup Time to forward the message across the switch fabric to the output line card Policy application delay (traffic policing that drops or remarks packets and traffic shaping that queues and delays them) Output queuing delay and head-of-line blocking Delays in the mapping and modulation in the transport Propagation delay through the transport Timestamp resolution Clock errors in the PHY and the backplane Figure 9-22 illustrates the difference between PDV behavior that can be tolerated on the slave clock (left side) and that which cannot (right side). Both diagrams are a scatter plot of the transit time of a series of messages (y-axis) over an observation period of 60 seconds (x-axis). Figure 9-22 PDV Accumulation With and Without a Clear Delay Floor A good reference for further information on packet selection and filtering is Appendix I of G.8260, specifically clause I.3, "Packet selection and filtering." Because this is outside the scope 