timestamps in the following formula: meanPathDelay=(t2-t1t4-t3)2 = - Note that devices that do not support peer-to-peer are not allowed between P2P TCs (because this would skew the results of the calculation). It therefore follows that P2P and E2E TCs should not co- exist in the same PTP domain. Also, the P2P delay mechanism is restricted to topologies where each P2P port communicates PTP messages with at most one other such port (point to multipoint does not work). One way to think of it is that it is purely a "hop-by-hop" mechanism. So, in summary, the difference between the two mechanisms can be summarized as shown in Figure 7-12. End-to-End Ordinary Boundary Grand- slave clock master Delay_Req/Delay_Resp Delay_Req/Delay_Resp Ordinary Transparent Grand- slave clock (E2E) master Delay_Req/Delay_Resp Delay_Req/Delay_Resp Peer-to-Peer Ordinary Boundary Grand- slave clock master Pdelay_Req/Pdelay_Resp Pdelay_Req/Pdelay_Resp Ordinary Transparent Grand- slave clock (P2P) master Pdelay_Req/Pdelay_Resp Pdelay_Req/Pdelay_Resp Figure 7-12 End-to-End Versus Peer-to-Peer There are many detailed examples of these message flows, for each of the E2E and P2P cases with both one- step and two-step clocks, in IEEE 1588-2008 Annex C, "Examples of residence and asymmetry corrections." 7.4.1 Management Nodes PTP management nodes are mainly concerned with the processing of PTP management messages, which are used between the management nodes and clocks. These messages are used to query and update the PTP datasets maintained by clocks as well as configure PTP clocks and perform monit- oring and fault management. In summary, these messages perform actions such as GET and SET on a range of any one of about 50 TLV entities. Some of these TLVs apply to the whole clock and others apply only to a port on the clock. An example of a clock TLV is DEFAULT_DATA_SET, which is used to retrieve (GET) one of the datasets on the clock (see the earlier section "PTP Datasets"). An example of a port TLV is ANNOUNCE_RECEIPT_TIMEOUT, which is used to ret