Mind theDelay: TheAdverse Effects of Delay-Based TCP onHTTP

The last three decades have seenmuch evolution inweb and network protocols: amongst them, a transition fromHTTP/1.1 toHTTP/2 and a shift from loss-based to delay-based TCP congestion control algorithms.Thispaperargues that these twotrendscomeatoddswithone another, ultimatelyhurtingwebperformance.Usinga controlled synthetic study, we show how delay-based congestion control protocols (e.g., BBR and CUBIC +Hybrid Slow Start) result in the underestimationof the available congestionwindow inmobilenetworks, andhow that dramatically hampers the effectiveness of HTTP/2. To quantify the impact of such finding in the current web, we evolve the web performance toolbox in two ways. First, we develop Igor, a client-side TCP congestion control detection tool that can differentiate between loss-based and delay-based algorithms by focusing on their behavior during slow start. Second, we develop a Chromium patch which allowsfine-grainedcontrol on theHTTPversion tobeusedperdomain. Using these newweb performance tools, we analyze over 300 real websites and find that 67% of sites relying solely on delay-based congestion control algorithms have better performance with HTTP/1.1.