"Failure to provide this information may lead us to pursue legal remedies,'' Verizon general counsel Randal Milch said in a letter to Netflix general counsel David Hyman.
In mid-May, Netflix started displaying messages on the screen for some customers when a video is buffering that say there is congestion on the network of Verizon or another Internet service providers.
Netflix said on Thursday the messages are meant to provide customers more information about their service, similar to its publication of a speed index that ranks Internet service providers.
"This is about consumers not getting what they paid for from their broadband provider,'' Netflix spokesman Jonathan Friedland said. "We are trying to provide more transparency, just like we do with the ISP Speed Index, and Verizon is trying to shut down that discussion."