Events Calendar

PhD Defense - Tobias Flach

Title: Detecting and Mitigating Root Causes for Slow Web Transfers
Time: Sept 8 (Thursday), 11am - 1pm
Location: EEB 248

PhD Candidate: Tobias Flach

Committee:

Ramesh Govindan
Ethan Katz-Bassett
John Heidemann
Konstantinos Psounis

Abstract:

One of the key goals for Web service providers is the quick delivery of their content to customers.
Minimizing the latency between a user's service request and the delivery of the corresponding content is of paramount importance for Web services like search, shopping, or video streaming. This is motivated by the fact that users have a low tolerance for delays. Past studies verified a link between increasing latency for content delivery and corresponding reductions in user engagement and provider revenue. As a result content providers go to great lengths to minimize latency by improving their infrastructure, communication protocols, and proximity to the users. However, end-to-end latency can still suffer from other network limitations, some of which have their root causes outside of a content provider's control domain.

In this thesis we strive to get a better understanding of the performance-limiting factors that affect Web transfers. In addition we explore techniques to mitigate these factors. For this we conducted multiple measurement studies dissecting Web transfers from different angles to find and analyze the root causes for poor performance.

First, we present two measurement studies investigating how the Transmission Control Protocol (TCP) can introduce delays that adversely affect Web transfers. We use large-scale measurements that we obtained at Google frontends across the globe as well as through the widely distributed M-Lab measurement platform for this task. We start by evaluating how packet loss affects Google's content delivery and show that especially short-lived connections suffer from packet loss in the network. We then discuss the design, deployment, and evaluation of algorithms tailored reduce the frequency and impact of the costly losses.
As a follow-up we present a methodology to break down the delay incurred by a packet into components attributable to propagation delay and cross-traffic, loss recovery, and queuing. Moreover we investigate the degree to which queuing delays slow TCP's loss recovery. We find that many of the flows see packet delivery times of one second or more, with large regional differences, and with queuing being a key cause of delay.

Second, we take a look at structural limitations affecting Web latency. Specifically we analyze the impact of path inflation in mobile carriers where traffic between content providers and mobile customers is taking geographically circuitous routes. We attribute these pathologies to root causes like a lack of ingress points between a carrier's network and the wider Internet as well as limited peering arrangements with content providers. Based on longitudinal data we show that performance in some carriers improved over time with other regions continuing to suffer from path inflation.

Third, we look at a particular type of third-party interference as a contributor to delay. We analyze the prevalence and impact of traffic policing, a traffic management technique used to enforce pre-configured throughput limits on connections by dropping excess packets. Based on global-scale measurements taken at Google frontends we show that a substantial number of connections with packet loss are affected by policing. Moreover we demonstrate that policing negatively impacts user quality of experience. We conclude by designing and testing solutions for content providers and the policing ISPs to mitigate the negative effects of policers.

Location: Hughes Aircraft Electrical Engineering Center (EEB) - 248

Audiences: Everyone Is Invited

Contact: Lizsl De Leon