What happens when two internet users in the same home or coffee shop log in simultaneously? Does one user watch streaming videos in 4K, while the other's video comes through grainy and glitchy? Does one user's webpage load in the blink of an eye, while the other user stares at a progress bar slowly inching forward?

A new study from researchers in Carnegie Mellon University's CyLab Security and Privacy Institute and Computer Science Department (CSD) aims to find out. The Prudentia project explores how popular internet services — from Netflix to Google Drive — perform when sharing the same internet connection.

"You can imagine an internet connection like a water pipe to your house or apartment building," said Justine Sherry, the A. Nico Habermann Associate Professor of Computer Science and lead investigator on the project. "If you have a lot of people who all want to take showers at the same time, you need a big pipe. If there's not enough water to go around, residents on the ground floor may have great water pressure while residents a story up see their water come through at a drip. Internet bandwidth is just like water pressure — if your provider sells you 100Mbps, that capacity needs to be shared by all users."

Sherry and her students, led by CSD Ph.D. student Adithya Abraham Philip, studied the degree to which popular services equally share capacity.

"Some services simply don't coexist well with others," Philip said. "For example, the popular file sharing service Mega leaves its competitors with as little as 16% of their fair share of bandwidth, leading to grainy videos and slow file transfers."

According to the Prudentia team, the problem arises in part because the internet does not explicitly allocate transmission rates between competing internet services. The decision about how fast to transfer data and when to send it is instead left to service providers. Seemingly minor decisions about whether to download a collection of photos all at once or one after another can have a significant impact on how much capacity an application takes for itself or leaves for others.

In their work, the researchers describe some services as contentious and others as sensitive. Sensitive services slow down quickly in the presence of competition when they could possibly take a little more bandwidth for themselves. The researchers point to YouTube as a sensitive service, as it gifts competing services on average 13% to 32% more bandwidth than their fair share.

"The challenges in ensuring equitable sharing between services are subtle," Sherry said. "Many of the providers we spoke to have been surprised to learn that their applications aren't friendly to competition or are backing off too easily."

Philip presented a research paper describing the results of the project, which was funded by grants from CyLab and the National Science Foundation, at the Association for Computing Machinery's Special Interest Group on Data Communication (ACM SIGCOMM) 2024 Conference this month in Sydney. Results from the Prudentia project, which runs experiments continually, are available on the project's website.