Protocol to be used in ESA Probe messages. 'icmpTimestamp', 'icmpEcho', 'udpEcho' protocols support layer-3 ethernet service assurance. If the remote reflector is an ADVA equipment based on CM family of products, one-way loss measurement, one-way delay measurement, round-trip delay measurement and one-way jitter measurements are computed. Else, only round-trip delay measurements are applicable. 'y1731' protocol can be used for layer-2 ethernet service assurance. One-way loss measurement, one-way and round-trip delay measurement, and one-way jitter measurements are available for this protocol. 'y1731-slm-slr' protocol can be used for layer-2 ethernet service assurance using Synthetic Loss Measurement messages. One-way loss measurement, one-way and round-trip delay measurement, and one-way jitter measurements are available for this protocol.