Return a `content-encoding` header for resource timing and more #1796

guohuideng2024 · 2024-12-13T22:25:26Z

The major change is to add content-encoding to response header list. This PR also adds description on how content-encoding is determined. (content negotiation)

The purpose is to pass such value to resource timing. Further details are available at
w3c/resource-timing#381.

Note: Per discussion at 12/05/2024 webPerWG call (https://docs.google.com/document/d/1mpFDrAWuV6IgvJ1KiL9sgIlcboC5uArtF8r_oqS1Sco/edit?tab=t.0#heading=h.af6v74wysf4m), we decided to allow arbitrary "content-encoding" value at "fetch". We only filter such value at client side, before passing the value to resource timing.

Related PR to modify resource timing specification:
w3c/resource-timing#411

At least two implementers are interested (and none opposed):
Likely, Discussed and agreed at Feb 29, 2024 W3C WebPerf call, Chromium already receiving content-encoding through fetch. But not sure about other browsers.
Tests are written and can be reviewed and commented upon at:
[to be updated with a new link] https://chromium-review.googlesource.com/c/chromium/src/+/5958411
Implementation bugs are filed:
Chromium: https://issues.chromium.org/issues/327941462
Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1886107
WebKit: https://bugs.webkit.org/show_bug.cgi?id=271632
Deno (not for CORS changes): …
MDN issue is filed:
New PerformanceResourceTiming.contentEncoding field mdn/content#32823
The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

Bug: w3c/resource-timing#381

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And some corrections.

fetch.bs

noamr · 2025-01-07T13:45:40Z

fetch.bs

+     <li><p>Let <var>accept-coding</var> be the result of <a for="header list">getting</a>
+       `<code>Accept-Encoding</code>` from <var>request</var>'s <a for=request>header list</a>.
+        If <var>accept-encoding</var> is not null and the server selects one of the encoding options,
+        let <var>coding</var> be the selected encoding option; otherwise, i.e., no encoding is used,


You can't have "let" in the middle of a phrase, see style in the rest of the document.

Fixed. I see why the "let" in the middle of the sentence is a bad idea. However, I didn't find the style guide in this document. I found this page: https://github.com/guohuideng2024/fetch/tree/main instead, so I think the style guide may have been moved? I wonder if there is more style guide that I can study. Thanks!

annevk · 2025-01-07T14:33:55Z

Thanks for taking the time to pick this up. However, it doesn't seem like this addresses all the issues with #1742? I recommend studying the feedback on that PR.

guohuideng2024 · 2025-01-14T21:55:15Z

Thanks for taking the time to pick this up. However, it doesn't seem like this addresses all the issues with #1742? I recommend studying the feedback on that PR.

Hi Anne! I think I should have put up some background information here.

You mentioned in Pass in Content-Encoding to resource-timing #1742 that the spec must define how the value is determined. This PR is trying to do that. The value is a result of the "content negotiation" (determine what encoding should be used) so I tried to add that into the existing text.
Note that Pass in Content-Encoding to resource-timing #1742 is a change similar to one for a previously added field contentType. But contentEncoding is very different, it's not an extracted MIME type, but a result of "content negotiation". So, this PR should be very different from Pass in Content-Encoding to resource-timing #1742
We originally thought that the filtering should happen at the "fetch" stage. But in the last web perf meeting Patrick brought up that the returned contentEncoding can be a proprietary value and that value is needed by service worker. So, the unfiltered value must be kept by the browser and the filtering should happen right before reported to resourceTiming.

https://docs.google.com/document/d/1mpFDrAWuV6IgvJ1KiL9sgIlcboC5uArtF8r_oqS1Sco/edit?tab=t.0#heading=h.af6v74wysf4m

Therefore, in this fetch doc I didn't mention filtering. I mentioned "filtering" in the resourceTiming spec:
w3c/resource-timing#411
And I am going to add more details about the filtering there.

Does this sound right to you? I am new to fetch and I may have missed a lot of things here. Thanks for your patience and guidance.
Guohui

This CL introduce a contentEncoding field to Performance resource timing object. This field is behind a feature flag. PR to resource timing specification: w3c/resource-timing#411 PR to fetch specification: whatwg/fetch#1796 Bug: 327941462 Change-Id: I70cad190fe658fb3dbf8b401ff8393bc1d0782f0

This CL introduce a contentEncoding field to Performance resource timing object. This field is behind a feature flag. PR to resource timing specification: w3c/resource-timing#411 PR to fetch specification: whatwg/fetch#1796 Bug: 327941462 Change-Id: I70cad190fe658fb3dbf8b401ff8393bc1d0782f0 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6098321 Commit-Queue: Guohui Deng <guohuideng@microsoft.com> Reviewed-by: Noam Rosenthal <nrosenthal@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org> Reviewed-by: Yoav Weiss (@Shopify) <yoavweiss@chromium.org> Cr-Commit-Position: refs/heads/main@{#1407331}

annevk · 2025-01-17T13:04:39Z

fetch.bs

+   <var>value</var>.
+
+   <li><p>Otherwise, if <var>value</var> is not <var>candidateValue</var>, return failure.
+  </ol>


So the idea here is that duplicate values are okay as long as they are fully identical? So gzip and GZIP is not okay?

annevk · 2025-01-17T13:05:52Z

fetch.bs

+  </ol>
+
+ <li><p>If <var>candidateValue</var> is the empty string or has a <a for=/>code point</a> that is
+ not an <a for=/>ASCII digit</a>, then return null.


Why would it only contain digits? Typical values for Content-Encoding are gzip or deflate as I understand it.

annevk · 2025-01-17T13:06:52Z

fetch.bs

+        If <var>accept-encoding</var> is not null and the server selects one of the encoding options,
+        set <var>coding</var> to the selected encoding option; otherwise, i.e., no encoding is used,
+        set <var>coding</var> to <a href=https://httpwg.org/specs/rfc9110.html#field.accept-encoding>
+        <code>"identity"</code></a>.


The indentation here doesn't follow the style guide.

How would we know what the server selects here? This doesn't make much sense to me.

annevk · 2025-01-17T13:08:14Z

fetch.bs

-     <a for="data: URL struct">body</a> <a for="byte sequence">as a body</a>.
+     <var>mimeType</var>), (`<code>Content-Encoding</code>`, <var>coding</var>) », and
+     <a for=response>body</a> is <var>dataURLStruct</var>'s <a for="data: URL struct">
+     body</a> <a for="byte sequence">as a body</a>.


This also doesn't follow the formatting guidelines. Also, do we really want to return Content-Encoding for data: URLs? Why?

guohuideng2024 added 3 commits December 11, 2024 15:38

Add Content Encoding response header to fetch.

2c939cb

Add more about content-encoding

dc2b573

And some corrections.

Remove a trailing space.

7647a50

guohuideng2024 mentioned this pull request Dec 13, 2024

Expose a filtered content-encoding value to resource timing w3c/resource-timing#411

Open

content-encoding always available

f7bdfa9

guohuideng2024 marked this pull request as ready for review December 17, 2024 19:37

noamr mentioned this pull request Jan 2, 2025

Provide clear guidelines for what Timing-Allow-Origin should expose w3c/webappsec#664

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noamr reviewed Jan 7, 2025

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Fix indentation and the "let" in the middle of sentence.

94ecd4c

chromium-wpt-export-bot mentioned this pull request Jan 16, 2025

Expose contentEncoding in PerformanceResourceTiming web-platform-tests/wpt#50115

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annevk reviewed Jan 17, 2025

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Return a `content-encoding` header for resource timing and more #1796

Return a `content-encoding` header for resource timing and more #1796

guohuideng2024 commented Dec 13, 2024 •

edited by pr-preview bot

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annevk Jan 17, 2025

Return a content-encoding header for resource timing and more #1796

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Return a `content-encoding` header for resource timing and more #1796

Return a `content-encoding` header for resource timing and more #1796

guohuideng2024 commented Dec 13, 2024 •

edited by pr-preview bot

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