by a MEP to the other MEPs in a maintenance entity by using a multicast address. The periodicity of these heartbeat messages can be altered over a range from 300 per second to 1 every 10 minutes depending on the application. The CC message provides a number of features: Loss of continuity, which occurs when a MEP does not receive CC messages over a configured period of time Discovery of missing or unexpected MEPs Service cross-connect or service ID mismatch Detection of forwarding loops The Evolution of Ethernet To verify the bidirectional connectivity between a MIP and a MEP, loopback messages (which are analogous to ping messages) are sent from the MEP to the MIP. This provides a diagnostic tool that can simply be sent once or as often as required. It can be sent to succes- sive MIPs to determine the location of a fault. A link trace message can also be used to identify the path between MIPs and MEPs, providing a sequence from source to sink. Both standards also provide a number of other message types. At the present time there is little in the way of operator experience with either 802.1ag or Y.1731, but it is expected that they will bring significant improvements to network operations. The Evolution of Ethernet As Ethernet has evolved it has faced a number of scaling challenges because of its nature. An excellent overview of many existing capabilities can be found in Clark and Hamilton (2007). Its underlying behavior is derived from its origins as a shared-channel broadcast network technology. As Myers (2004) notes, this model is simple: Hosts could be attached and re-attached at any location on the network; no manual configuration was required; and any host could reach all other hosts on the network with a single broadcast message. Over the years, as Ethernet has been almost completely transformed, this service model has remained remark- ably unchanged. It is the need to support broadcast as a first-class service in today's switched environ- ment that plagues Ethernet's scalability and reliability. The reaso