time transport by determining an accurate residence time for PTP event messages. The device measures this time (meaning the time taken for it to re- ceive, process, and transmit the PTP event message) and writes this residence time into the correc- tionField of the PTP message. For example, the E2E TC does not terminate or interrupt the Sync and Delay_Req message flows between the master and slave ports but updates a field in the mes- sage that helps the slave clock to recover the time with increased accuracy. A management node is included for completeness because it is not involved in the processing of PTP time information (and therefore is not really a clock) but is a device used to manage, configure, and monitor the clocks throughout the network. However, it is mentioned here because it is defined as a clock type in IEEE 1588-2008, and it processes PTP management messages, although PTP management messages are not used in the telecom profiles. For further reference, the specification for ordinary and boundary clocks is covered in Chapter 9 of IEEE 1588-2008, and transparent clocks are covered in Chapter 10. 7.4.1 Grandmaster (Ordinary) Clocks A grandmaster clock is the ultimate source of time for all the clocks in a PTP domain, whereas a master clock is a clock that is the source of time to which other clocks on a single PTP communica- tions path synchronize. As shown in Figure 7-7, PTP clocks are organized into a master-slave syn- chronization hierarchy, but the clock at the top of the hierarchy of the entire system is the grandmas- ter clock. Remember that because an ordinary clock can have only one port, any ordinary clock must either be the grandmaster clock in a domain hierarchy or be an ordinary slave clock. Ordinary Boundary Grand- slave clock master Ordinary Ordinary slave slave Ordinary M = Master port slave S = Slave port PTP message flows Figure 7-7 Master-Slave and Grandmaster Hierarchy Note from Figure 7-7 that the grandmaster has a mushroom-shaped icon protruding from the right side of the box, re