This document provides an overview of the current implementation status of the Compression Dictionary Transport feature in Chrome and instructions on how to enable it.
Starting from version 117, Chrome experimentally supports Compression Dictionary Transport feature. This feature adds support for using designated previous responses, as an external dictionary for Brotli- or Zstandard-compressed HTTP responses.
This API can be enabled by chrome://flags or via Origin Trial.
If you want to try Compression Dictionary Transport feature, you have to enable both chrome://flags/#enable-compression-dictionary-transport and chrome://flags/#enable-compression-dictionary-transport-backend. The details about each flag are described here.
Origin Trial token can be obtained from the Origin Trial console. You can enable this feature by setting Origin-Trials HTTP response header in the page's HTML,
Origin-Trial: YOUR_ORIGIN_TRIAL_TOKEN
or adding a meta tag in the <header> element.
<meta http-equiv="origin-trial" content="YOUR_ORIGIN_TRIAL_TOKEN">
This feature supports third-party Origin Trial. If you are serving third-party scripts, you can enable this feature by adding a meta tag.
(() => { const meta = document.createElement('meta'); meta.httpEquiv = 'origin-trial'; meta.content = 'YOUR_THIRD_PARTY_ORIGIN_TRIAL_TOKEN'; document.head.appendChild(meta); })()
Currently third-party Origin Trial doesn't support Origin-Trial HTTP response header. See this doc for more details.
The origin trial token works only when the backend of this feature is enabled. Currently, the backend is gradually being rolled out. So the backend may not be enabled in your Chrome yet. If you want to try this feature using an Origin Trial token, please enable the backend at chrome://flags/#enable-compression-dictionary-transport-backend.
When the backend is enabled, Chrome will check the dictionary database while fetching resources. This may negatively affect Chrome's loading performance. Therefore, we are conducting experiments to ensure that this does not cause regressions before rolling it out to all users.
TODO(crbug.com/40255884): When we enable the backend for all Chrome, remove this section.
If you want to check whether the Compression Dictionary Transport feature is enabled or not, you can try the following code:
document.createElement('link').relList.supports('compression-dictionary')
If the code above returns true, the Compression Dictionary Transport feature is enabled.
Note: Until M126, dictionary was used instead of compression-dictionary.
When Chrome receives a HTTP response with use-as-dictionary: <options> header, and if the the response type is not opaque, Chrome registers the response as a dictionary.
Chrome supports following options (see the explainer and the spec draft for more details):
raw.raw. So if this field is set but it is not raw, Chrome doesn't register the response as a dictionary. This logic of checking type was introduced at M118.Note: These options fields are expected to change when we officially launch this feature, depending on the outcome of the spec discussion.
The lifetime of registered dictionary is calculated from the response's freshness.
While running the Origin Trial experiment, the max expiration time is limited to 30 days. This limitation can be removed by enabling chrome://flags/#enable-compression-dictionary-transport.
Chrome fetches a dedicated dictionary when it detects <link rel="compression-dictionary" href="DICTIONARY_URL"> in the page, or Link: <DICTIONARY_URL>; rel="compression-dictionary" HTTP response header is set in the response of the page's HTML file.
Note: Until M126, dictionary was used instead of compression-dictionary.
While fetching a HTTP request, Chrome match the request against the available dictionary match URL patterns.
If a dictionary is available for the request, Chrome will add dcb and dczin the Accept-Encoding request header, as well as a Available-Dictionary: <SHA-256> header with the hash of the dictionary.
Chrome 117.0.5857.0 introduced support for Shared Brotli, and Chrome 118.0.5952.0 adds support for Shared Zstandard.
Shared Zstandard can be enabled/disabled from chrome://flags/#enable-shared-zstd.
Until M126, br-d and zstd-d encoding was used. After M127, dcb and dcz encodings are used. See this spec change for more details about this change of the encoding.
From Chrome 121, Chrome may not use stored shared dictionares when the connection is using HTTP/1 for non-localhost requests. From Chrome 125, Chrome may not use stored shared dictionares when the connection is using HTTP/2 for non-localhost requests. Also Chrome may not use shared dictionares when the HTTPS connection's certificate is not rooted by a well known root CA (eg: using a user installed root certificate). This is for an investigation for an issue that some network appliances are interfering with HTTPS traffic by inspecting encrypted responses but failing to properly decode the shared dictionary encoded content.
If you want to use shared dictionaries with HTTP/1, please enable chrome://flags/#enable-compression-dictionary-transport-allow-http1. If you want to use shared dictionaries with HTTP/2, please enable chrome://flags/#enable-compression-dictionary-transport-allow-http2. Also if you want to use shared dictionaries over the HTTPS connection which certificate is not rooted by a well known root CA, please disable chrome://flags/#enable-compression-dictionary-transport-require-known-root-cert.
Developers can manage the registered dictionaries in chrome://net-internals/#sharedDictionary.