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.”

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The senators pointed to research from the Wehe testing platform, news of which has surfaced before, that found evidence of throttling by all four carriers on video services ranging from YouTube to Netflix to Amazon Prime.

As noted in a lengthy article on the topic by Ars Technica, Verizon and T-Mobile didn’t immediately respond to questions on the topic, while Sprint said it would look into the matter.

Indeed, Sprint has been previously implicated in Skype throttling allegations by the Wehe testing platform, which is essentially an iOS and Android app developed by researchers at Northeastern University that users can download to test potential throttling situations.

On the Wehe website, the researchers explained how the app works: “Wehe uses your device to exchange Internet traffic recorded from real, popular apps like YouTube and Spotify—effectively making it look as if you are using those apps. As a result, if an Internet service provider (ISP) tries to slow down an YouTube, Wehe would see the same behavior. We then send the same app's Internet traffic, but replacing the content with randomized bytes, which prevents the ISPs from classifying the traffic as belonging to the app. Our hypothesis is that the randomized traffic will not cause an ISP to conduct application-specific differentiation (e.g., throttling or blocking), but the original traffic will. We repeat these tests several times to rule out noise from bad network conditions, and tell you at the end whether your ISP is giving different performance to an app's network traffic.”

As noted by Ars Technica, AT&T disputed the allegations at the heart of the issue, and referred the topic to a CTIA blog post by the association’s CTO posted last month: “In order to manage networks, providers optimize the bandwidth available for a video so that your smartphone gets DVD quality without downloading excess data,” wrote CTIA’s Tom Sawanobori. “The Wehe app compares the data speeds that consumers experience with and without that content provider metadata. If the Wehe app detects a difference in speed, it registers this as ‘differentiation’ and implies this is a violation of ‘net neutrality.’ What the Wehe app is really detecting is basic wireless network management and operators delivering the service consumers choose.

Continued Sawanobori: “What the Wehe app is really detecting is either basic wireless network management (based on consumer choice) or data management practices used by content providers. That’s because content providers have data practices in place—outside the control of providers—that reduce video resolution of data traffic flowing through their sites or apps depending on the consumer’s mobile device.”

Wireless operators’ data-throttling practices have raised questions before. For example, AT&T faced a government lawsuit over the issue in 2014.

Every wireless network operator works to manage traffic to some degree, and in recent years the nation’s top wireless providers have implemented throttling specific to video designed to limit the amount of data traveling over their networks. Most operators now offer a variety of unlimited data pricing plans that handle that throttling differently based on how much customers pay per month.