Add support for compression dictionary transport #1854

pmeenan · 2025-08-25T20:43:06Z

Add processing steps for handling HTTP Compression Dictionary Transport content encoding and dictionary negotiation (RFC pending publication).

This adds a processing layer between the HTTP cache and network fetch that handles most of the dictionary-based content encoding (including matching dictionaries to outgoing requests).

Additionally, it adds processing above the HTTP cache for storing the dictionaries for future use and defines the "compression-dictionary" initiator and destination (the matching HTML spec update is in-process).

Fix #1739, #1839

At least two implementers are interested (and none opposed):
- Chrome
- Firefox
- Safari
Tests are written and can be reviewed and commented upon at:
- https://github.com/web-platform-tests/wpt/tree/master/fetch/compression-dictionary
- PR for removal of "tentative": Removed tentative from compression dictionary tests web-platform-tests/wpt#54532
Implementation bugs are filed:
- Chromium: https://crbug.com/1413922
- Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1882979
- WebKit: https://bugs.webkit.org/show_bug.cgi?id=295249
- Deno (not for CORS changes): Add support for compression dictionary transport denoland/deno#30533
MDN issue is filed: https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Compression_dictionary_transport
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pmeenan · 2025-09-03T19:05:21Z

The RFC is pending publication so this will have to wait until that happens (should be any day now). RFC number has been assigned and final edits are complete: https://www.rfc-editor.org/auth48/rfc9842

…mber

pmeenan · 2025-11-11T16:47:15Z

RFC has published now so this should be ready to go (just rebased it).

pmeenan · 2025-11-13T16:07:15Z

From the TPAC 2025 discussions, I added support for opaque responses to use compression dictionary when the Cross-Origin-Resource-Policy response header is set to cross-origin.

annevk · 2025-11-18T13:11:27Z

fetch.bs

+    "HTTP-COMPRESSION-DICTIONARIES": {
+        "aliasOf": "RFC9842"
+    },


We can just reference the RFC number directly. No need for an alias.

annevk · 2025-11-18T13:11:53Z

fetch.bs

 <p>A <a for=/>request</a> has an associated
 <dfn export for=request id=concept-request-initiator>initiator</dfn>, which is
 the empty string,
+"<code>compression-dictionary</code>",


Why do we need a distinct initiator if we also have a distinct destination?

annevk · 2025-11-18T13:14:10Z

fetch.bs

+
+ <li><p>If <var>key</var> is null, then return null.
+
+ <li><p>Return the unique compression-dictionary cache associated with <var>key</var>. [[!HTTP-COMPRESSION-DICTIONARIES]]


Are there PRs against Storage and Clear-Site-Data about clearing this cache? It would be good if those were linked from the commit message.

annevk · 2025-11-18T13:14:54Z

fetch.bs


+ <li>
+  <p>If <var>response</var>'s <a for=response>header list</a>
+  <a for="header list">contains</a> `<code>Use-As-Dictionary</code>`, then:


Suggested change

<a for="header list">contains</a> `<code>Use-As-Dictionary</code>`, then:

<a for="header list">contains</a> `<code>Use-As-Dictionary</code>`:

annevk · 2025-11-18T13:15:42Z

fetch.bs

+   <li><p>If <var>dictionaryValue</var> is null or <var>dictionaryValue</var>["<code>match</code>"]
+   does not <a for=map>exist</a>, then return <var>response</var>.
+
+   <li><p>Let <var>pattern</var> be the result of creating a URL pattern from


"creating a URL pattern" needs some kind of link to a definition.

annevk · 2025-11-18T13:18:07Z

fetch.bs

+   <li><p>Let <var>corpPolicy</var> be the result of <a for="header list">getting</a>
+   `<a http-header><code>Cross-Origin-Resource-Policy</code></a>` from <var>response</var>'s
+   <a for=response>header list</a>.
+
+   <li><p>If <var>request</var>'s <a for=request>response tainting</a> is "<code>opaque</code>" and
+   <var>corpPolicy</var> is not `<code>cross-origin</code>`, then return <var>response</var>.


This isn't good enough. We want corpPolicy to be a good value across redirects. We should have tests for that as well.

annevk · 2025-11-18T13:18:38Z

fetch.bs

+   <li><p>If <var>request</var>'s <a for=request>response tainting</a> is "<code>opaque</code>" and
+   <var>corpPolicy</var> is not `<code>cross-origin</code>`, then return <var>response</var>.
+
+   <li><p>Let <var>expirationTime</var> be the time at which the <var>response</var> becomes stale.


This needs more links to definitions.

annevk · 2025-11-18T13:19:27Z

fetch.bs

+   <li><p>If <var>expirationTime</var> is not in the future, then return <var>response</var>.
+
+   <li><p>Store <var>response</var> in <var>compressionDictionaryCache</var> with its associated
+   <var>dictionaryValue</var> and <var>expirationTime</var>.


Shouldn't we also store pattern somewhere?

annevk · 2025-11-18T13:21:04Z

fetch.bs

+ <li><p>If the user agent is configured to block cookies for <var>request</var>, then return the
+ result of running <a>HTTP-network fetch</a> given <var>fetchParams</var>,
+ <var>includeCredentials</var>, and <var>forceNewConnection</var>.


I don't understand this. The dictionary is partitioned so how is it different from the HTTP cache?

annevk · 2025-11-18T13:22:30Z

fetch.bs

+ <li><p><a for="header list">Combine</a> (`<code>Accept-Encoding</code>`, `<code>dcb</code>`)
+ in <var>request</var>'s <a for=request>header list</a>.
+
+ <li><p><a for="header list">Combine</a> (`<code>Accept-Encoding</code>`, `<code>dcz</code>`)
+ in <var>request</var>'s <a for=request>header list</a>.


Are we doing this even if web developers set these headers? That seems rather sketchy.

This was referenced Aug 26, 2025

Removed tentative from compression dictionary tests web-platform-tests/wpt#54532

Open

Add support for compression dictionary transport denoland/deno#30533

Open

domenic mentioned this pull request Sep 4, 2025

Added support for link rel=compression-dictionary whatwg/html#11620

Open

6 tasks

pmeenan force-pushed the dictionaries branch from 8decfba to e089eab Compare November 11, 2025 15:22

pmeenan added 6 commits November 11, 2025 10:23

Added compression-dictionary destination

1d64408

Added processing of the use-as-dictionary response header

cd5a16d

Added support for dictionary matching

7d86734

Added "dcb" and "dcz" decoding

606c33f

Fixed build errors

bb72fc2

Switched compression dictionary reference to use the (pending) RFC nu…

8bc5398

…mber

pmeenan force-pushed the dictionaries branch from e089eab to 8bc5398 Compare November 11, 2025 15:24

pmeenan requested a review from annevk November 11, 2025 16:47

Add no-cors dictionary support with Cross-Origin-Resource-Policy opt-in

defe29f

annevk reviewed Nov 18, 2025

View reviewed changes


		<li><p>If <var>key</var> is null, then return null.

		<li><p>Return the unique compression-dictionary cache associated with <var>key</var>. [[!HTTP-COMPRESSION-DICTIONARIES]]

	<a for="header list">contains</a> `<code>Use-As-Dictionary</code>`, then:
	<a for="header list">contains</a> `<code>Use-As-Dictionary</code>`:

Add support for compression dictionary transport #1854

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