LGTM3
On 10/28/26 5:21 a.m., Chris Harrelson wrote:
LGTM2
On Tue, Oct 27, 2026 at 7:05 PM Alex Russell
<[email protected]> wrote:
LGTM1; we've had intermittent feedback over the years about how
WebTransport wasn't well suited to latency-sensitive work, and
it's great to see these additions.
On Thursday, October 22, 2026 at 3:11:29 PM UTC-7 Monica Chintala
wrote:
FYI, this feature depends on WebTransport send groups and
stream prioritization - Chrome Platform Status
<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fchromestatus.com%2Ffeature%2F5235261159112704&data=05%7C02%7Cmonicach%40microsoft.com%7C9e0b9f4149a541fafafe08df307d863d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C639282987879917270%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=fMplyzw4jssGTpxdGq0wgG5zQ2s6HhhTQI6b1Zm6cps%3D&reserved=0>,
which is currently in progress for I2S. I'm working on getting
the necessary approvals now so we're ready to proceed, while
ensuring this feature ships only after the dependent feature
has shipped.
On Thursday, October 22, 2026 at 2:24:42 PM UTC-7 Chromestatus
wrote:
*Contact emails*
[email protected], [email protected]
*Explainer*
https://github.com/w3c/webtransport/blob/main/explainer.md#5-sending-and-receiving-datagrams
*Specification*
https://www.w3.org/TR/webtransport/#datagram-writable
*Summary*
Adds the modern WebTransport Datagram sending API.
WebTransportDatagramDuplexStream.createWritable() creates
independent WebTransportDatagramsWritable streams. Each
writable has mutable sendGroup and sendOrder attributes
for expressing relative priority among queued WebTransport
data in the same send group. Chromium currently applies
this ordering among Datagram writables. The implementation
also reports the negotiated maximum outgoing Datagram
payload size and discards oversized writes as required by
the WebTransport specification.
*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*
The original WebTransport Datagram API exposes only a
single writable stream, preventing applications from
independently scheduling different classes of
latency-sensitive messages. The newer API allows
applications such as games, remote-interaction systems,
and real-time media applications to create multiple
Datagram writables and prioritize urgent data over less
time-sensitive data while preserving FIFO ordering within
each writable. Implementing this API also resolves
WebTransport Datagram interoperability failures included
in Interop 2026.
*Initial public proposal*
https://github.com/w3c/webtransport/issues/451
*TAG review*
This is an incremental API addition within the existing
WebTransport specification and does not introduce a new
architectural primitive. A separate feature-level TAG
review is not planned.
*TAG review status*
Not applicable
*Goals for experimentation*
None
*Risks*
*Interoperability and Compatibility*
Interoperability risk Medium. The API is defined in the
WebTransport Candidate Recommendation and covered by WPT.
Implementations may nevertheless differ in scheduling
boundaries, particularly priority interaction between
streams and Datagrams. Chromium currently implements
ordering among Datagram writables; unified
stream-and-Datagram arbitration may require additional
implementation work and specification clarification.
Compatibility risk Low. The API adds createWritable() and
WebTransportDatagramsWritable without removing
datagrams.writable. Existing applications continue to
work. The negotiated maxDatagramSize and oversized-write
behavior are intentional conformance improvements but
could expose applications that assumed a fixed outgoing
Datagram limit.
/Gecko/:
Shipped/Shipping
(https://bugzilla.mozilla.org/show_bug.cgi?id=2007174) Firefox
implemented and related wpt tests are passing
/WebKit/:
Shipped/Shipping (https://github.com/WebKit/WebKit/pull/53733)
WebKit
implemented and related wpt tests are passing
/Web developers/:
Positive
(https://github.com/web-platform-tests/interop/issues/1121) The
Interop 2026 WebTransport proposal received 28 👍
reactions and explicitly requested coverage of
prioritization features and Datagram interoperability.
Developers also reported active WebTransport use and
significant cross-browser differences in Datagram behavior.
/Other signals/: Consensus to add this feature was
achieved at the W3C TPAC conference in November 2025:
https://www.w3.org/wiki/WebTransport/Meetings2023#2025_W3C_TPAC_Meeting_-_Kobe,_Japan
*Ergonomics*
Low to medium. Multiple writable streams and mutable
sendGroup and sendOrder attributes provide useful
scheduling control but introduce concepts developers must
understand. Higher sendOrder values take precedence within
a send group, so badly chosen priorities can starve
lower-priority data or produce unexpected ordering. FIFO
behavior within each writable and standardized defaults
preserve straightforward behavior for applications that do
not need advanced prioritization.
*Activation*
Low. This is an additive API behind the
WebTransportDatagramsWritable runtime feature. It does not
remove or replace the existing datagrams.writable API.
Enabling the feature also makes maxDatagramSize reflect
the negotiated outgoing payload limit and discards writes
that exceed that limit, as required by the specification.
The main activation risk is inconsistent priority
scheduling across browsers, particularly around the
boundary between streams and Datagrams. Rollout should
follow WPT and interoperability validation.
*Security*
Low. The API does not grant new network access, expose
additional data, or weaken WebTransport's secure-context
requirements. The primary implementation concern is
renderer-driven resource exhaustion through excessive
writable registrations or queued Datagram payloads.
Chromium mitigates this through network-service validation
and limits on writable registrations, queued Datagram
counts, retained bytes, expiration, and scheduling batch
sizes. Invalid renderer-provided identifiers and priority
updates are rejected.
*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 API implemented in
cross-platform Blink and network-service code. It does not
remove or change the availability of the existing
datagrams.writable API and requires no WebView-specific
integration. Behavior follows the existing availability of
WebTransport in WebView.
*Debuggability*
Chrome DevTools does not currently provide
feature-specific inspection for Datagram writable queues
or priorities. Existing WebTransport diagnostics, NetLog,
network-service logging, and QUIC logging can be used to
diagnose connection and scheduling 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/idlharness.https.sub.any.html?label=experimental&label=master&aligned
<https://wpt.fyi/results/webtransport/idlharness.https.sub.any.html?label=experimental&label=master&aligned>
https://wpt.fyi/results/webtransport/datagrams.https.any.html?label=experimental&label=master&aligned
<https://wpt.fyi/results/webtransport/datagrams.https.any.html?label=experimental&label=master&aligned>
https://wpt.fyi/results/webtransport/datagrams-writable.https.any.html?label=experimental&label=master&aligned
<https://wpt.fyi/results/webtransport/datagrams-writable.https.any.html?label=experimental&label=master&aligned>
https://wpt.fyi/results/webtransport/sendgroup.https.any.html?label=experimental&label=master&aligned
<https://wpt.fyi/results/webtransport/sendgroup.https.any.html?label=experimental&label=master&aligned>
*Flag name on about://flags*
enable-experimental-web-platform-features
*Finch feature name*
WebTransportDatagramsWritable
*Rollout plan*
Will ship enabled for all users
*Requires code in //chrome?*
False
*Tracking bug*
https://issues.chromium.org/issues/547572475
*Availability expectation*
The feature is expected to be available across major
browsers as part of the W3C WebTransport specification.
Firefox implements the Datagram writable and
prioritization APIs, and WebKit has landed an
implementation that is currently in development.
*Adoption expectation*
Adoption is expected primarily among latency-sensitive
WebTransport applications, including games,
remote-interaction systems, and real-time media.
Applications that need multiple independently prioritized
Datagram queues are expected to begin adopting the API
within 12 months of broad browser availability.
*Adoption plan*
Adoption is expected to happen organically through
WebTransport documentation, examples, and Interop 2026
visibility. Existing applications do not need to migrate
because the existing datagrams.writable API remains available.
*Estimated milestones*
Shipping on desktop 159
DevTrial on desktop 155
Shipping on Android 159
DevTrial on Android 155
Shipping on WebView 159
*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).
No API-shape changes are currently anticipated. The
WebTransport Working Group may further clarify scheduling
boundaries and priority arbitration between streams and
Datagrams in the same send group. Such clarification is
expected to affect scheduling behavior only, not the
exposed API.
*Link to entry on the Chrome Platform Status*
https://chromestatus.com/feature/5183354944225280?gate=6105683064913920
*Links to previous Intent discussions*
Intent to Prototype:
https://groups.google.com/a/chromium.org/d/msgid/blink-dev/6a920d86.e8a51b51.6abb3.0299.GAE%40google.com
This intent message was generated by Chrome Platform
Status <https://chromestatus.com>.
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