ry Reference In summary, holdover is a commodity with a price, and good holdover requires clocks with a com- bination of expensive oscillators, good design, and benign environmental conditions. For the de- signer of a timing solution, holdover is really a last resort, and the focus should be on a good net- work design to ensure that the application is always served with some reference signal. 9.7 Packet Network Topologies Of course, when using the transport network to carry the timing signals, the critical factor is the characteristics of the transport system being used. But another aspect to consider is the topology and encapsulation of the transport. So, before moving onto the characteristics of the transport itself, there are a few items to understand about topology. Many of these issues have been covered in pre- vious sections, SO this section brings them together as a summary. Chapter 11 also provides more detail on these issues. The following items must be guarded against in decisions on topology and deployment architecture: Once again, PTPoIP over unaware nodes allows uncontrolled PDV, asymmetry, and time error. The use of a hop-by-hop topology reduces this risk, greatly improves end-to-end performance, and allows predictable budgeting of time error. Using multiple stacked labels (for example, with MPLS) may lead to a situation where the PTP messages are not visible to the timestamping logic. In this case, it might be necessary to pop the MPLS labels at the last hop before a PTP-aware clock. Forwarding timing information using pseudowires and tunnels is normally the wrong thing to do. With G.8275.1, the PTP message stream is hop-by-hop, configured on the physical inter- face, and contains no VLAN tags. This means it runs outside the service layer and SO avoids these issues. ESMC packets carrying clock traceability information similarly should not be tunneled. The whole point of ESMC is to carry QL information about the physical signal it is being carried over. Separating the QL information from the signal 