A read-only value that identifies the source_address
parameter of the last MPCPDUs passed to the MAC Control.
This value is updated on reception of a valid frame with
(1) a destination Field equal to the reserved multicast
address for MAC Control specified in [802.3ah] Annex
31A, (2) lengthOrType field value equal to the reserved
Type for MAC Control as specified in [802.3ah] Annex
31A. (3) an MPCP subtype value equal to the subtype
reserved for MPCP as specified in [802.3ah] Annex 31A.
This attribute is relevant for an OLT and an ONU.