YouLighter: An Unsupervised Methodology to Unveil YouTube CDN Changes

TL;DR

YouLighter is an unsupervised method that uses passive measurements to identify and compare changes in YouTube's CDN structure over time, helping ISPs optimize user experience despite limited visibility.

Contribution

The paper introduces YouLighter, a novel unsupervised approach and a new metric, Constellation Distance, to detect and quantify CDN changes from passive measurements.

Findings

Detected sudden CDN structure changes over 10 months

Identified small cache policy alterations affecting QoE

Unveiled YouTube CDN structure without active probing

Abstract

YouTube relies on a massively distributed Content Delivery Network (CDN) to stream the billions of videos in its catalogue. Unfortunately, very little information about the design of such CDN is available. This, combined with the pervasiveness of YouTube, poses a big challenge for Internet Service Providers (ISPs), which are compelled to optimize end-users' Quality of Experience (QoE) while having no control on the CDN decisions. This paper presents YouLighter, an unsupervised technique to identify changes in the YouTube CDN. YouLighter leverages only passive measurements to cluster co-located identical caches into edge-nodes. This automatically unveils the structure of YouTube's CDN. Further, we propose a new metric, called Constellation Distance, that compares the clustering obtained from two different time snapshots, to pinpoint sudden changes. While several approaches allow comparison between the clustering results from the same dataset, no technique allows to measure the similarity of clusters from different datasets. Hence, we develop a novel methodology, based on the Constellation Distance, to solve this problem. By running YouLighter over 10-month long traces obtained from two ISPs in different countries, we pinpoint both sudden changes in edge-node allocation, and small alterations to the cache allocation policies which actually impair the QoE that the end-users perceive.