The Source MAC Address Field of the received SOAM DM session PDUs.
If mefSoamDmCfgType is dm1DmRx this object indicates the source
address of the dm1DmTx DM session.
This object is only valid for mefSoamDmCfgType set to dm1DmRx. It is
ignored for mefSoamDmCfgType set to dmDmm or dm1DmTx.
This object can only be written at row creation time and cannot be
modified once it has been created.