The source of Compass’ complaint, which was filed on Monday in a New York federal court, is a new rule from Zillow stating that any home listing publicly marketed to consumers anywhere must be published on Zillow within one day or it won’t be allowed to be published on Zillow at all.

The brokerage firm argues that Zillow is using its platform dominance to stifle competition and control how agents market homes. Two of the nation’s real estate titans are on a collision course.