two-way mechanism, the slave port will now ask for a grant of Delay_Resp messages. Again, the grant port responds with the acknowledgement of the grant of Delay_Resp service. The Announce messages continue to arrive at the requested rate until the duration of the grant has ex- pired, and the slave will then have to again request a flow of Announce messages (granted by the grant port). Remember that, by default, the grant interval requested is 300 seconds, SO the slave port needs to request service for each message type only once every five minutes. Now, because the slave has already received a grant for the Delay_Resp, the slave is then free to send Delay_Req messages to the grant port. The grant port answers each Delay_Req with a Delay_Resp to determine the final timestamp needed for the calculations. 7.4 PTP Clocks There are five basic types of PTP devices: Ordinary clock (OC) Boundary clock (BC) End-to-end transparent clock (E2E TC) Peer-to-peer transparent clock (P2P TC) Management node In the definition from IEEE 1588-2008, the OC is simply a PTP clock that has only one single PTP port, which under normal conditions is in either slave or master state (there are other states pos- sible). Obviously, this means that an OC is located either at the beginning (see grandmaster in the next section) or end (ordinary slave) of a clock hierarchy in a PTP domain. Because the OC can handle time transfer only in one direction from a single PTP port, it cannot be in the middle of the chain. A BC is a clock that has more than one PTP port, some combination of ports in either master, slave (or passive) states. In simple terms, it receives timing information from an upstream master, recov- ers the clock, and distributes it downstream via its master ports. Given that it terminates one PTP flow and reinitiates a new flow between the master port and the slave port, a BC "resets" the PDV of the PTP messages. In modern telecom networks, the BC is normally the most common type of clock. A TC is a clock that assists the accuracy of 