IE8 and JavaScript Download Concurrency

by Dylan SchiemannMarch 11th, 2008

In the recent post IE 8: 6 Connections Per Host, several of the comments suggested that Firefox and other browsers only allow two connections per host, when in fact they allow 8 by default.

In a recent post, IE 8 Speeds Things Up, by Steve Souders of YSlow! fame (and a fellow SXSW 2008 attendee), he points out that IE 8 is the first browser to download scripts in parallel, yet execute them serially, whereas “Firefox and IE7 and earlier won’t start any parallel downloads while downloading an external script.”

While this is not the rate-limiting step for most Comet-based apps, it is something that will significantly improve initial page load times for non-cached resources.

One Response to “IE8 and JavaScript Download Concurrency”

  1. Steve Souders Says:

    Thanks for the mention, Dylan! I wanted to point out that Firefox and IE7 & earlier only supported 2 downloads in parallel, as well as 8. ;-) The difference is they support 2 downloads in parallel for _persistent_ (HTTP/1.1 with Keep-Alive) connections, which is pretty much everyone these days. So for most of the world the IE8 change is a move from 2 to 6.

    There are some folks that still do HTTP/1.0, and there you’ll see 8 parallel downloads in Firefox and 4 (I believe) in IE7 & earlier. In fact, AOL downgrades to HTTP/1.0 to get the benefit of increasing their parallel downloads. Before downgrading your site to HTTP/1.0 I recommend doing extensive testing. I’m not convinced it’s the right move.

    I created a test page ( ) where you can test parallel downloads for HTTP/1.1.


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