Modern distributed control systems comprise multiple intelligent devices capable of performing complex time and mission-critical tasks both independently of each other or partly jointly with each other. To do so, they strongly depend on an accurate common notion of time as well as a reliable shared communication medium for timely data exchange. Traditional legacy communication technologies (field bus systems) supported time transport to a certain extent or provided at least a common frequency. Due to its numerous undisputed advantages, Ethernet has become the only viable communication medium effectively replacing such systems. Being inherently asynchronous time and frequency transfer has to be accomplished using a packet-based approach when moving to Ethernet. After explaining the basic principles of packet-based time transfer, the most common standards are explained compared with each other with respect to their intended application domains. Special emphasis will be put on the Precision Time Protocol (PTP) as defined in the underlying IEEE 1588 standard and its variant IEEE 802.1AS used for time-sensitive networks. Maintaining a highly accurate common notion of time under all circumstances is a crucial prerequisite for most distributed systems. Although PTP has proven to provide sub-microsecond accuracies, it can cope only with a limited number of error conditions. This paper describes all major sources that can either deteriorate the accuracy or cause a total loss of synchronization altogether. Selected countermeasures and enhancements are presented, which can greatly improve the resilience of PTP against errors as well as malicious attacks. This paper concludes by presenting the selected measurements' results of a novel proposed method.