I Sent It: Where Does Slow Data Go to Wait?

Emerging applications like virtual reality (VR), augmented reality (AR), and 360-degree video aim to exploit the unprecedentedly low latencies promised by technologies like the tactile Internet and mobile 5G networks. Yet these promises are still unrealized. In order to fulfill them, it is crucial to understand where packet delays happen, which impacts protocol performance such as throughput and latency. In this work, we empirically find that sender-side protocol stack delays can cause high end-to-end latencies, though existing solutions primarily address network delays. Unfortunately, however, current latency diagnosis tools cannot even distinguish between delays on network links and delays in the end hosts. To close this gap, we present ELEMENT, a latency diagnosis framework that decomposes end-to-end TCP latency into endhost and network delays, without requiring admin privileges at the sender or receiver. We validate that ELEMENT achieves more than 90% accuracy in delay estimation compared to the ground truth in different production networks. To demonstrate ELEMENT's potential impact on real-world applications, we implement a relatively simple user-level library that uses ELEMENT to minimize delays. For evaluation, we integrate ELEMENT with legacy TCP applications and show that it can reduce latency by up to 10 times while maintaining throughput and fairness. We finally demonstrate that ELEMENT can significantly reduce the latency of a virtual reality application that needs extremely low latencies and high throughput.