((a)) ** .—** A carrier, a broker, or a freight forwarder providing transportation or service subject to jurisdiction under chapter 135 shall designate, in writing, an agent by name and post office address on whom service of notices in a proceeding before, and of actions of, the Secretary may be made.
((b)) ** .—** A motor carrier providing transportation under this part shall also file the designation with the appropriate authority of each State in which it operates. The designation may be changed at any time in the same manner as originally made.
((c)) ** .—** A notice to a motor carrier, freight forwarder, or broker shall be served personally or by mail on the motor carrier, freight forwarder, or broker or on its designated agent. Service by mail on the designated agent shall be made at the address filed for the agent. When notice is given by mail, the date of mailing is considered to be the time when the notice is served. If a motor carrier, freight forwarder, or broker does not have a designated agent, service may be made by posting a copy of the notice at the headquarters of the Department of Transportation.