ck (P2P TC) delay mechanism, which is covered in detail in the sections "Peer-to- Peer Versus End-to-End Delay Mechanisms" and "Transparent Clocks" later in the chapter. Please refer to sections 11.5 and 11.6 of the IEEE 1588-2008 standard for more details on residence time correction and asymmetry correction. 7.3.1 PTP Ports and Port Types PTP has the concept of a "port," which is an interface that connects to the network for the transfer of time; the port is the window from the PTP clock to the network. Any one PTP port will support only a single version of the PTP protocol and use a single transport protocol; however, a physical network interface might support multiple PTP ports. Another important point is that a PTP port can belong to only a single domain and, therefore, the communications path between that port and an- other port can belong to only one domain. PTP ports (per 1588-2008 section 7.5) have several characteristics, the most important of which is the portIdentity, made up from clockIdentity (64-bit number representing the identity of the clock) portNumber (16-bit value counting upward starting from 1) There is also a versionNumber that is 2 for the 2008 version of the standard. There is no need to spend too much time on these fields; they are only mentioned because the port identity and port number can be visible in the command-line interface (CLI) outputs, and both the clock identity and port number can sometimes become a tie-breaking factor in selecting the best master under the best master clock algorithm (BMCA), which is discussed in depth later in the chapter. The PTP port will be in a port state that is governed by the PTP node state machine (and the BMCA), which is outlined in IEEE 1588-2008, section 9.2. Table 7-3 outlines the possible port states (the state always starts off in initializing). Table 7-3 PTP Port States PTP State Meaning Initializing The port is initializing its state and preparing the communication path, and no traffic is flowing. This is the state where the port begins. F