On 10/14/26 6:45 p.m., Chromestatus wrote:
*Contact emails*
[email protected]
*Explainer*
https://github.com/w3c/webtransport/blob/main/explainer.md
*Specification*
https://www.w3.org/TR/webtransport/#dom-webtransportoptions-congestioncontrol
*Summary*
Adds the congestionControl constructor option and read-only attribute
to WebTransport. Applications can request "default", "throughput", or
"low-latency" behavior, allowing the user agent to select suitable
congestion-control behavior for each session.
*Blink component*
Blink>Network>WebTransport
<https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3ENetwork%3EWebTransport%22>
*Web Feature ID*
webtransport <https://webstatus.dev/features/webtransport>
*Motivation*
WebTransport applications have different performance requirements.
Interactive applications prioritize low latency, while bulk transfers
prioritize throughput. This hint lets applications express that
preference without selecting or controlling a specific
congestion-control algorithm.
*Initial public proposal*
/No information provided/
*TAG review*
Not applicable as a separate review. This is an incremental
implementation of an API already defined in the W3C WebTransport
specification and covered by the broader WebTransport TAG review:
https://github.com/w3ctag/design-reviews/issues/1212
*TAG review status*
Not applicable
*Goals for experimentation*
None
*Risks*
*Interoperability and Compatibility*
Low. The API is additive, standardized, and already implemented by
Firefox and Safari. Implementations may choose different underlying
algorithms while preserving the standardized hint and fallback behavior
/Gecko/:
Shipped/Shipping (https://github.com/mozilla/standards-positions/issues/167)
/WebKit/:
Shipped/Shipping (https://github.com/WebKit/standards-positions/issues/18)
Are they shipping congestion control, or just WebTransport?
/Web developers/:
Positive (https://github.com/web-platform-tests/interop/issues/1121) The
Interop 2026 WebTransport proposal received 28 👍 reactions.
Developers also reported active WebTransport use and significant
cross-engine differences.
/Other signals/:
*Ergonomics*
Low. The three values are straightforward, but developers must
understand that the option is a hint and may resolve to "default" when
the requested behavior is unavailable.
*Activation*
Low. This is an additive constructor option and attribute. The value
is a preference rather than a guarantee, and unsupported preferences
may fall back to "default".
*Security*
Low. The API grants no new network access and does not expose
congestion-window or network-state information. Applications provide
only a high-level preference; the browser retains control of the
selected algorithm.
*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?
Low. This is an additive WebTransport option and attribute implemented
in shared Blink and network-service code. Existing WebView
applications are unaffected, and applications using the option must
treat it as a preference that may fall back to "default".
*Debuggability*
The requested value is readable through transport.congestionControl .
Existing DevTools networking, NetLog, and QUIC logging can diagnose
connection behavior
*Will this feature be supported on all six Blink platforms (Windows,
Mac, Linux, ChromeOS, Android, and Android WebView)?*
Yes
*Is this feature fully tested by web-platform-tests
<https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?*
Yes
https://wpt.fyi/results/webtransport/congestion-control.https.any.html?label=experimental&label=master&aligned
<https://wpt.fyi/results/webtransport/congestion-control.https.any.html?label=experimental&label=master&aligned>
*Flag name on about://flags*
chrome://flags/#enable-experimental-web-platform-features
*Finch feature name*
WebTransportCongestionControl
*Rollout plan*
Will ship enabled for all users
*Requires code in //chrome?*
False
*Tracking bug*
https://issues.chromium.org/issues/501268547
*Measurement*
Validate API exposure and connection behavior through WPT, Chromium
tests, and developer feedback from latency-sensitive and
high-throughput WebTransport applications.
*Availability expectation*
Already supported by Firefox and Safari as part of the W3C
WebTransport specification. Chromium support is implemented behind an
experimental runtime feature.
*Adoption expectation*
Expected to be adopted by games, interactive media, remote-interaction
applications, and bulk-transfer services that need to express latency
or throughput preferences.
*Adoption plan*
Adoption is expected to happen organically through existing
WebTransport applications and documentation.
*Estimated milestones*
Shipping on desktop 158
DevTrial on desktop 150
Shipping on Android 158
DevTrial on Android 150
Shipping on WebView 158
*Anticipated spec changes*
Open questions about a feature may be a source of future web compat or
interop issues. Please list open issues (e.g. links to known github
issues in the project for the feature specification) whose resolution
may introduce web compat/interop risk (e.g., changing to naming or
structure of the API in a non-backward-compatible way).
None. The option and attribute are already defined by the W3C
WebTransport specification.
*Link to entry on the Chrome Platform Status*
https://chromestatus.com/feature/5180947573112832?gate=5529995941380096
This intent message was generated by Chrome Platform Status
<https://chromestatus.com>.
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