Intent to Extend Origin Trial: Compression Dictionary Transport
Tsuyoshi Horo
Contact emails
ho...@chromium.org, pme...@chromium.org, yoav...@chromium.org, kenji...@chromium.org
Explainer
https://github.com/WICG/compression-dictionary-transport
Specification
https://datatracker.ietf.org/doc/draft-ietf-httpbis-compression-dictionary/
Design docs
https://docs.google.com/document/d/1IcRHLv-e9boECgPA5J4t8NDv9FPHDGgn0C12kfBgANg/edit
https://github.com/WICG/compression-dictionary-transport
https://datatracker.ietf.org/doc/draft-ietf-httpbis-compression-dictionary/
Summary
An Origin Trial for Compression Dictionary Transport was scheduled to start in Chrome 117 and end in Chrome 122. But due to a critical issue, we could not start the Origin Trial until Chrome 119. The design of the feature has also evolved during the origin trial and RFC process. We’d like to continue the Origin Trial to get more feedback on the updated feature.
Blink component
TAG review
https://github.com/w3ctag/design-reviews/issues/877
TAG review status
Closed
Risks
Interoperability and Compatibility
Interoperability and Compatibility risk are low. This feature introduces a new compression method for transporting resources over HTTP. Web sites can know the browser support for the new feature by checking `document.createElement('link').relList.supports('dictionary')`. Also web servers can know the browser support by checking the `Accept-Encoding` request header and the new `Use-As-Dictionary` request header.
This feature is an opt-in feature. And the dictionary storage is isolated using the top level site and the frame origin as the key. That means, if there is no dictionary registered for the site, the behavior of Chrome will not change while browsing the site. Also this feature is only usable within a secure-context so this feature will not increase the risk of having network proxies meddle with the content’s encoding. For enterprises that have deployed HTTPS-intercepting proxies that do not properly handle unknown encodings there is an enterprise policy exposed to disable the feature.
Gecko: Positive (https://github.com/mozilla/standards-positions/issues/771)
WebKit: No signal (https://github.com/WebKit/standards-positions/issues/160)
Web developers: Positive
Other signals:
Ergonomics
To reduce memory usage in network services, dictionary metadata is stored in a database on disk. And to avoid performance degradation for normal requests that do not use a dictionary, the reading of this metadata is designed not to block network requests. In other words, if the reading of metadata from the database is not completed before the request header is ready to be sent to the server, the dictionary may not be used even if it is already registered in the database.
Activation
To adopt this feature, web developers need to make changes in their web servers or build processes for static resources. Currently there is no major server software which supports compression dictionaries. Some CDNs have shared interest in supporting shared dictionary compression (e.g. publicly mentioned in a blog post by Cloudflare).
Security
Chrome registers the response as a dictionary only when the response is CORS-readable from the document origin. Also we use a registered dictionary to decompress the response only when the response is CORS-readable from the document origin. Additionally, the dictionary and the compressed resource are required to be from the same origin as each other. So this should not introduce any new attack vector of information leaks.
The dictionaries are partitioned with the storage cache and are cleared whenever cookies or cache is cleared to ensure that the dictionaries can not be abused as a tracking vector.
WebView application risks
Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?
No
Goals for experimentation
We would like to collect feedback on the updated API design of Compression Dictionary Transport feature. Also, we would like to continue some experiments using this feature to measure its performance impact.
Ongoing technical constraints
None
Debuggability
We have introduced chrome://net-internals/#sharedDictionary. Using it, web developers can manage the registered dictionaries. Also web developers can check the related HTTP request and response headers (Use-As-Dictionary, Sec-Available-Dictionary, Accept-Encoding, Content-Encoding).
Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?
Yes
Is this feature fully tested by web-platform-tests?
No. We will rewrite some browser_tests to WPT.
Flag name on chrome://flags
chrome://flags/#enable-compression-dictionary-transport-backend chrome://flags/#enable-compression-dictionary-transport
Finch feature name
CompressionDictionaryTransportBackend CompressionDictionaryTransport
Requires code in //chrome?
True
Tracking bug
Launch bug
https://launch.corp.google.com/launch/4266286
Estimated milestones
Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/5124977788977152
Links to previous Intent discussions
Intent to prototype: https://groups.google.com/a/chromium.org/g/blink-dev/c/-qYpLo9DTjw/m/JX6kbUOtBQAJ
Intent to experiment: https://groups.google.com/a/chromium.org/g/blink-dev/c/NgH-BeYO72E/m/oup5DpbxAAAJ
Tsuyoshi Horo
Domenic Denicola
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