Intent to Implement and Ship: Link header support for the "prefetch" rel value

[Yoav Weiss]

Contact emails

yo...@yoav.ws / yoav....@akamai.com

Spec

https://tools.ietf.org/html/rfc5988

No TAG review for the feature, as it's a small extension of already shipped Link header support for other rel values.

Summary

The proposed change will add support for the "prefetch" rel value in Link headers, enabling easy addition of such resource hints and therefore easier hinting of resources needed for next navigation.

Is this feature supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?

Yes.

Debuggability

Resources prefetched by Link headers would be visible from the network panel in DevTools. I'm not sure if further debuggability support is required.

Interoperability and Compatibility Risk

Interoperability risk is low. Firefox supports the feature.

I haven't heard specific public signals from other vendors regarding "prefetch" Link header support, but I haven't heard objections from them when discussing Link header support in general (e.g. for "preload").

Compatibility risk is low, as the feature is already support in Firefox, and non-supporting browsers would simply not prefetch hinted resources, resulting in lack of optimization, but no breakage.

OWP launch tracking bug

Add crbug.com/658274

Entry on the feature dashboard

https://www.chromestatus.com/feature/5383221353119744

[Yoav Weiss]

On Sun, Jan 1, 2017 at 3:14 PM Yoav Weiss <yo...@yoav.ws> wrote:

Contact emails

yo...@yoav.ws / yoav....@akamai.com

Spec

https://tools.ietf.org/html/rfc5988

To clarify, the spec above defines Link headers in general. The "prefetch" rel value is defined in the Resource Hints spec. 

[Jochen Eisinger]

What referrer / origin headers will the prefetch use?

[Yoav Weiss]

On Mon, Jan 2, 2017 at 9:31 AM Jochen Eisinger <joc...@chromium.org> wrote:

What referrer / origin headers will the prefetch use?

It should use the Document's referrer/origin, similarly to rel=preload. I'm not sure that's currently well defined, tbh. 

[Jochen Eisinger]

for an actual navigation, only the main resource would be fetched with the current Document's referrer, so if you prefetch sub-resources, it'll be observably different from a regular navigation?

[Yoav Weiss]

Yes, if we're prefetching sub-resources, their referrer would be the prefetching document rather than the document that will eventually use them (which may be unknown at the time the resource is fetched). AFAIK, that's how `<link rel=prefetch>` works today. Triggering the prefetch from headers won't be any different.

[Jochen Eisinger]

Fair enough.

For <link> elements, the site can specify a referrer policy to use. It might make sense to have something similar for the headers. This is, however, unrelated to this feature at hand.

lgtm1 to implement & ship

[Rick Byers]

LGTM2

(filed one spec issue, but it's not blocking).

[Chris Harrelson]

LGTM3

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[Yoav Weiss]

Thanks! :)

LGTM3

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