49 C.F.R. § 31.8 – Service of complaint.

(a) Service of a complaint must be made by certified or registered mail or by delivery in any manner authorized by Rule 4(d) of the Federal Rules of Civil Procedure. Service of a complaint is complete upon receipt.
(b) Proof of service, stating the name and address of the person on whom the complaint was served, and the manner and date of service, may be made by—
1. (1) Affidavit of the individual serving the complaint by delivery;
2. (2) A United States Postal Service return receipt card acknowledging receipt; or
3. (3) Written acknowledgment of receipt by the defendant or his or her representative.