Re: [Web Timing] Getting root timings to recommendation

On Fri, 24 Sep 2010 00:44:58 +0200, Zhiheng Wang <>  

> On Wed, Sep 22, 2010 at 1:47 AM, Sigbjørn Vik <> wrote:
>> In all cases, there might be an ongoing DNS request for the domain in
>> question at the time when the fetch process is actually started.
>      Thanks for clarifying. Some of the prefetching cases here are done  
> by a
> previous navigation so the current one should act like getting
> the cached version, especially when the prefetch is done before the
> navigation start.
>      On the other hand, what a browser does if there is already an  
> ongoing
> DNS request while it needs to lookup the same domain again?
> Would it go ahead with a new request or would it just wait? What if the
> prefetch is initiated by third-party plug-ins?
> I saw this question before but not sure about the answer to it.

This is browser and implementation dependent, while the spec needs to  
cater for any implementation.

In most cases DNS prefetch will be something the browser doesn't even  
remember. The browser asks the OS to look up a domain, and then forgets  
about it. Some time later, the OS will have cached the domain. In those  
cases, when the browser starts looking up a domain for real, the answer  
might already be cached in the OS, or the OS might keep track of already  
having a pending request for that domain, and not send a second one. In  
other cases, the browser might keep track of the DNS requests itself.  
Plugins can request resources through the browser, in which case the  
fetching should be very similar as if the browser did it itself (but the  
exact answer is (browser) implementation dependent), or plugins can  
request resources on their own, in which case the browser knows nothing  
about it (and the exact answer is (plugin) implementation dependent).

Sigbjørn Vik
Quality Assurance
Opera Software

Received on Friday, 24 September 2010 06:55:38 UTC