46 U.S.C. § 60503

(a) General Authority .— On receiving satisfactory proof from the government of a foreign country that it has suspended, in any part, the imposition of discriminating duties for any class of vessels owned by citizens of the United States or goods imported in those vessels, the President may proclaim a reciprocal suspension of discriminating duties for the same class of vessels owned by citizens of that country or goods imported in those vessels. (b) Effective and Expiration Dates .— A suspension under this section takes effect retroactively from the date the President received the proof from the foreign government, and expires when that government stops granting the reciprocal suspension.