WebTransport congestion control

Category: Network / Connectivity
Type: Chromium catches up
Status: Proposed (Chrome Proposed)
Intent stage: None

Breaking change: No
Enterprise impact: None
Chromium flag: chrome://flags/#enable-experimental-web-platform-features
Spec maturity: Published standard
Web Platform Tests: Yes

WPT details: https://wpt.fyi/results/webtransport/congestion-control.https.any.html?label=experimental&label=master&aligned

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.

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.

Standards & signals

Specification: https://www.w3.org/TR/webtransport/#dom-webtransportoptions-congestioncontrol
Firefox: Neutral — Firefox exposes the API and currently falls back to "default" for non-default requests, as permitted by the specification.
Safari: Neutral — WebKit exposes the requested value, but its current open-source QUIC configuration does not use the congestion-control option.
Web developers: Positive — The Interop 2026 WebTransport proposal received 28 👍 reactions. Developers also reported active WebTransport use and significant cross-engine differences.
Tracking bug: https://issues.chromium.org/issues/501268547

Ecosystem

Web Platform Tests: https://wpt.fyi/results/?q=webtransport%2Fcongestion-control
Chrome Platform Showcase: https://chrome-platform-showcase.paulkinlan-ea.deno.net/features
GenDN reference: https://gendn.paulkinlan-ea.deno.net/features
chrome.dev demos search: https://www.google.com/search?q=site%3Achrome.dev+WebTransport%20congestion%20control
MDN Web Docs: https://developer.mozilla.org/en-US/search?q=WebTransport%20congestion%20control
Baseline status: https://webstatus.dev/?q=WebTransport%20congestion%20control

Explainers: https://github.com/w3c/webtransport/blob/main/explainer.md

View on chromestatus.com