cy it can distribute. To achieve microsecond-level or better precision, time transfer needs a more accurate implementation, such as that used in PTP. PTP is a much more accurate implementa- tion of time transfer because it is deployed with built-in hardware support and accurate timestamp- ing. Much more is to be said on PTP in the following chapters of the book, with a detailed descrip- tion in Chapter 7. Another common use case is in the Operations, Administration, and Maintenance (OAM) functions for network management. Besides link utilization, one piece of information that a network manager is interested in is link latency. A complete snapshot of the latency for every link in the network would allow the network manager to use advanced routing protocols such as segment routing (SR) and Flex-Algo to route packets based on the lowest-latency path. There are mechanisms to determine the link delay using packets with timestamps. Basically, the router sends out a packet with a timestamp and measures the time it takes to be returned. This gives an estimate of the round-trip time for that packet, known as the two-way delay measurement (2DM). From that, the one-way delay measurement (1DM) between two nodes can be estimated by assuming that the forward delay is equal to the reverse delay and halving the 2DM value. The preceding assumption may be erroneous, as many conditions might arise (congestion for one) where a packet in one direction may traverse a link faster than in the other direction. This difference between the transit time of a packet in the forward and reverse directions is known as asymmetry, and you will read a lot more about it in later chapters. So, asymmetry invalidates our assumption and therefore results in our one-way delay estimate being inaccurate. A more accurate way to measure 1DM is to send a timestamped packet to a target node. The receiver/listener timestamps that packet on receipt and reports the timestamp to the sender. For this method to deliver an accurate 1DM value, we require that the tim