Saturday, November 8, 2008

DNS Prefetching for Firefox

Recently I implemented patches to implement DNS prefetching in Firefox. I am primarily interested in their impact on Fennec (aka Mobile Firefox), but it looks like they will land first in Firefox 3.1 beta2. The, hopefully final, glitches are being shaken out of the patch now.

Google Chrome has a feature like this too.

DNS resolutions are always dominated by latency instead of bandwidth. Particularly on mobile networks the latencies are very high. That makes them perfect candidates for speculative pre-fetching. The advantage is in the latency improvement - instead of waiting for a hostname lookup when you click a link, do that lookup while you're reading the page the link is embedded in. Because the lookups are so small (generally one runt packet in and out) the cost of any wasted over optimistic lookups really doesn't impact the performance of browsing. Good payoff at low cost, the best of both worlds.

The basic benefit is simple: if you click on a link using a new hostname, you save a round trip time. On some networks this can be a substantial improvement (800ms or more) in responsiveness. Some describe this simply as "figuring out the IP address of every link before you click on it".

The Firefox implementation takes this approach one step further than just pre-resolving anchor href hostnames. It uses the prefetch logic on URLs that are being included in the current document. By this I mean that it uses the prefetch logic on things like images, css, and jscript that are being loaded right away, in addition to anchor links which might be clicked on at a slightly later time.

At first that seems non-sensical. How can you pre-fetch the DNS for something you are fetching right now? Where does the "pre" come in? The answer is not so much in the definition of "pre" as it is in the definition of "right now". Most HTTP User Agents, Firefox being no exception, limit their number of simultaneous connections and hosts. Typical pages embed quite a few objects and it is easy to run into these limits. When this happens the browser queues some of the requests. The Firefox pre-fetch DNS implementation allows those queued requests to overlap the high latency host resolution with whatever transfers might be going on without creating an excess level of parallelization.

While this is just a secondary benefit, it can be meaningful. For example, on the day I grabbed a snapshot of it required 23 unique DNS resolutions in order to render the base page. Most of these were in img URLs. When loading the page with the prefetch patches, even with a cold cache, 16 of them were either fully completed when needed for the first connection or at least already in progress. The result, measured on an EDGE network, was a 4% overall improvement in page load time. Not bad for something that does not reduce bandwidth consumption in any way.


Basically, it just works. You don't need to do anything. But there are a few configurables out there for both browsers and content providers.

First, as a browser you might want no part of this. Fair enough - its your browser. If you set the preference network.dns.disablePrefetch to true the prefetch code will never take effect, no matter what any other configuration is set to.

Furthermore, as a security measure, prefetching of embedded link hostnames is not done from documents loaded over https. If you want to allow it in that context too, just set the preference network.dns.disablePrefetchFromHTTPS to true.

Content providers have a couple neat tricks available too. These are meant to be compatible with Chrome.

For content to opt out of DNS pre-resolution it can be served with the x-dns-prefetch-control HTTP header set to off. The equivalent meta http-equiv element can be used instead of a response header too:
<meta http-equiv="x-dns-prefetch-control" content="off">

Setting content to on will reverse the effect. You can never turn pre-fetching on in a browser that has it disabled by preference, but you can undo the impact of a previous x-dns-prefetch control command. In this way, different content provider policies can apply to different portions of the document.

The last configuraton possibility allows the content provider to force the lookup of a particular hostname without providing an anchor using that name. This is done with the link tag:
<link rel="dns-prefetch" href="">

The href attribute can contain a full URL, or just a hostname. Hostname only attributes should preceed the hostname with two slashes:
<link rel="dns-prefetch" href="//">

Content providers might use the link notation in a site-wide home page in order to preload hostnames that are widely used throughout the site but perhaps not on the home page.