LGTM1
On 10/15/26 5:49 p.m., Chromestatus wrote:
*Contact emails*
[email protected]
*Explainer*
https://github.com/w3c/webtransport/blob/main/explainer.md#11-handling-session-draining
*Specification*
https://w3c.github.io/webtransport/#dom-webtransport-draining
*Design docs*
https://github.com/w3c/webtransport/blob/main/explainer.md#11-handling-session-draining
*Summary*
WebTransport.draining is a promise that resolves when the server
indicates that the WebTransport session will be gracefully retired.
Applications can use this signal to establish a replacement session
while existing streams and the current session remain usable.
*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*
A WebTransport server may need to retire a session for maintenance,
load balancing, deployment, or connection-lifetime management. Without
a draining signal, an application learns about this only when the
session closes, which can cause an avoidable interruption while a
replacement connection is established. The draining promise gives the
application advance notice without changing the lifetime or usability
of the current session.
*Initial public proposal*
/No information provided/
*TAG review*
This is a small incremental addition to an existing feature which is
already standardized.
*TAG review status*
Not applicable
*Goals for experimentation*
None
*Risks*
*Interoperability and Compatibility*
Compatibility risk is low because this adds a new read-only promise
attribute to WebTransport and does not change existing behavior. Code
that does not read the attribute is unaffected. The promise resolves
only after a server-side draining signal and does not close the
session or alter existing stream semantics. Interoperability risk is
low. The attribute is available in Firefox 155 and Safari 26.4.
Applications supporting older browsers should check for the attribute
before using it.
/Gecko/:
Shipped/Shipping (https://bugzilla.mozilla.org/show_bug.cgi?id=2007160) WebTransport.draining shipped
in Firefox 155 -
https://chromium.googlesource.com/external/github.com/mdn/browser-compat-data/%2B/HEAD/api/WebTransport.json#804
/WebKit/:
Shipped/Shipping (https://bugs.webkit.org/show_bug.cgi?id=302762) WebTransport.draining
is available in Safari 26.4 -
https://chromium.googlesource.com/external/github.com/mdn/browser-compat-data/%2B/HEAD/api/WebTransport.json#811
/Web developers/:
Positive (https://github.com/web-platform-tests/interop/issues/1121) The
Interop 2026 WebTransport proposal received 28 positive reactions.
Developers also reported active WebTransport use and significant
cross-engine differences.
/Other signals/:
*Ergonomics*
The API is a stable Promise<undefined> attribute, consistent with the
existing WebTransport.ready and WebTransport.closed lifecycle
promises. It can be observed before or after the connection becomes
ready. The promise resolves at most once.
*Activation*
No special activation is required. A WebTransport server triggers the
promise by sending the protocol-defined draining signal. Sites that do
not use the new attribute are unaffected.
*Security*
This does not grant a new capability or expose application data. It
exposes a server-originated lifecycle signal for an already
established, secure-context WebTransport session. The information is
controlled by the connected server and is closely related to
connection state already observable by the application.
*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 read-only attribute and does not modify
existing WebTransport behavior.
*Debuggability*
The promise and its state can be inspected from JavaScript. Existing
DevTools Network tooling continues to expose the WebTransport
connection and its streams. No new DevTools UI is required for the
initial launch.
*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/draining.https.any.html?label=experimental&label=master&aligned
<https://wpt.fyi/results/webtransport/draining.https.any.html?label=experimental&label=master&aligned>
*Flag name on about://flags*
enable-experimental-web-platform-features
*Finch feature name*
WebTransportDraining
*Rollout plan*
Will ship enabled for all users
*Requires code in //chrome?*
False
*Tracking bug*
https://issues.chromium.org/issues/564353368
*Availability expectation*
WebTransport is available across the major browser engines, and
draining is part of the W3C WebTransport API. The feature is expected
to remain available across browsers as implementations converge on the
current specification.
*Adoption expectation*
Adoption is expected among applications that maintain long-lived
WebTransport sessions and need to replace sessions gracefully during
server maintenance, deployment, load balancing, or connection-lifetime
management. Broader adoption will follow overall WebTransport usage.
*Adoption plan*
No dedicated outreach campaign is planned. Adoption is expected to
occur organically through WebTransport documentation and client and
server library support.
*Non-OSS dependencies*
Does the feature depend on any code or APIs outside the Chromium open
source repository and its open-source dependencies to function?
No.
*Estimated milestones*
Shipping on desktop 158
DevTrial on desktop 153
Shipping on Android 158
DevTrial on Android 153
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 known for the draining promise semantics.
*Link to entry on the Chrome Platform Status*
https://chromestatus.com/feature/6427766428925952?gate=4865096399585280
This intent message was generated by Chrome Platform Status
<https://chromestatus.com>.
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