Issue link: https://read.uberflip.com/i/1146286
112 | Chapter 15: Deconstructing 10 Top Sites In the first half, where there is greater parallelization, the responses have been down- graded from HTTP/1.1 to HTTP/1.0. I discovered this by looking at the HTTP head- ers where the request method specifies HTTP/1.1, whereas the response states HTTP/1.0. GET /_media/aolp_v21/bctrl.gif HTTP/1.1 Host: www.aolcdn.com HTTP/1.0 200 OK For HTTP/1.0, the specification recommends up to four parallel downloads per host- name, versus HTTP/1.1's guideline of two per hostname. Greater parallelization is achieved as a result of the web server downgrading the HTTP version in the response. Typically, I've seen this result from outdated server configurations, but it's also pos- sible that it's done intentionally to increase the amount of parallel downloading. At Yahoo!, we tested this, but determined that HTTP/1.1 had better overall perfor- mance because it supports persistent connections by default (see the section "Keep- Alive" in Chapter B). There are no parallel downloads in much of the second half of AOL's HTTP traffic because most of these requests are scripts. As described in Chapter 6, all other down- loading is blocked while the browser downloads external scripts. This results in a small number of requests spreading out over a longer time period than if they were done in parallel. These scripts appear to be used for ads, but the insertion of the scripts seems ineffi- cient. The scripts come in pairs. The first script contains: document.write('