er clock. Note that in much documentation and discussion on this topic, a PTP port in master state is referred to as a PTP master port and, similarly, a PTP port in slave state is referred to as a PTP slave port. Just realize that based on the profile or configuration, the port may be limited to adopt only one of several states or might be capable of changing state under PTP control. So, the simplest PTP network has two ordinary clocks, a master clock and a slave clock with one port each, and a PTP message exchange between them. Normally, you would also want the master to have a primary reference clock (PRC) or primary reference time clock (PRTC) as a reference source of time for its timestamps (see "Chapter 3. Synchronization and Timing Concepts"). Boundary clocks contain more than one PTP port, one of which must become a slave port (to get time from an upstream master port) and one or more PTP ports that then become master ports, feed- ing time downstream to slave ports on other clocks. Of course, this means that BCs can be "stacked" in a chain, with one feeding into the next. Transparent clocks are clocks that help maintain the accuracy of the timing signal through the net- work. There are several types of TC available in PTP, which are covered in more detail later in the chapter in the section "Transparent Clocks." For this general overview, suffice it to say that TCs are "transparent" in that they allow the important PTP time messages to pass through them. Figure 7-1 illustrates the interaction of the various clock types. Ordinary Transparent Boundary Grand- slave clock (E2E) clock master Ordinary Boundary M = Master port Boundary slave clock clock S = Slave port Timing flow Figure 7-1 Overview of PTP Clocks 7.3.1 PTP Clock Domains The most basic concept in IEEE 1588 is that of the PTP domain, which at its simplest is a network of clocks where one grandmaster is delivering its time over the hierarchy of clocks belonging to that domain. There can be multiple domains in a network, and a PTP clock can belong to