LGTM3
/Daniel
On 2026-10-20 01:36, Alex Russell wrote:
LGTM2
On Thursday, October 15, 2026 at 5:47:21 PM UTC-7 Mike Taylor wrote:
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
<https://github.com/w3c/webtransport/blob/main/explainer.md#11-handling-session-draining>
*Specification*
https://w3c.github.io/webtransport/#dom-webtransport-draining
<https://w3c.github.io/webtransport/#dom-webtransport-draining>
*Design docs*
https://github.com/w3c/webtransport/blob/main/explainer.md#11-handling-session-draining
<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
<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
<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
<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
<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
<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
<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
<https://chromestatus.com/feature/6427766428925952?gate=4865096399585280>
This intent message was generated by Chrome Platform Status
<https://chromestatus.com>.
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