along with troubleshooting diagnostics (such as loopbacks) that are run when needed, are collectively called FM. Upon detecting a fault, FM may trigger control-plane functions such as APS or reroute, and management-plane functions such as collection of fault statistics, setting off alarms, notification of technical staff, etc. In the following subsection we discuss those OAM functions that monitor less critical operational parameters and which generally only trigger management-plane statistics gathering. 6 Enabling Technologies Ethernet, once without any OAM now has multiple standardized FM mechanisms. In particular connectivity FM (IEEE 802.1ag) and (ITU-T Y.1731) define CC heartbeats, as well as loop-back and link trace mechanisms. The ITU-T version further defines forward and backward defect indications, locking for diagnostics, and messaging channels for APS. Another ITU-T standard (ITU-T Y.1564) specifies the use of FM when commissioning a new service. IP and MPLS have a FM protocol known as BFD (RFC 5880). Originally a simple keep-alive and loop-back (known in BFD as echo) mechanism between two adjacent routers, BFD has expanded to become a full-featured FM protocol, especially in the context of MPLS-TP, which additionally uses LSP-ping for on-demand diagnostics (RFC 6426). 6.6.5.4.7 Performance Monitoring As mentioned in Section 6.5, 5G OAM has been extended to cover performance monitoring for measurement of QoS parameters of the transport network. Thus, while CC is critical for any application (no communications-based application can properly function without communications), such parameters as one-way delay, round-tip-delay, PDV, and PLR may be critical for proper functioning of some, but not all applications. The ITU-T version of Ethernet OAM (Y.1731) supports both FM and performance mon- itoring, while the IEEE version (802.1ag) supports only FM. IP has one-way and two-way measurement mechanisms, known respectively as One-Way Active Measurement Protocol (OWAMP) (RFC 4656) and Two-Way Active Measurement