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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 Web Feature ID 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? Yes 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-writable.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 (eg links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (eg, 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. -- You received this message because you are subscribed to the Google Groups "blink-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/6ada7f0d.aa78ed10.2a46b3.01af.GAE%40google.com.
