A trio of Democratic senators are raising questions about nationwide carriers’ data-throttling practices.

“We write to express our concern that mobile carriers may be inappropriately throttling and prioritizing internet traffic from common mobile apps without the knowledge of their customers,” wrote Sens. Edward Markey, D-Mass., Richard Blumenthal, D-Conn., and Ron Wyden, D-Ore., in their letter addressed to the CEOs of AT&T, Verizon, T-Mobile and Sprint. The senators’ full letter is available here. “All online traffic should be treated equally, and internet service providers should not discriminate against particular content or applications for competitive advantage purposes or otherwise.”