A count of MPCP frames passed by the MAC sublayer to the
MAC Control sublayer. This counter is incremented when a
ReceiveFrame function call returns a valid frame with:
(1) a lengthOrType field value equal to the reserved
Type for 802.3_MAC_Control as specified in 31.4.1.3, and
(2) an opcode indicating a MPCP frame.
This attribute is relevant for an OLT and an ONU.