Media Over QUIC W. Law Internet-Draft Akamai Intended status: Standards Track S. Nandakumar Expires: 7 January 2027 Cisco 6 July 2026 WebVTT Packaging for MOQT Streaming Format draft-wilaw-moq-webvtt-msf-00 Abstract This document specifies the JSON packaging format for delivering WebVTT caption and subtitle content as event timeline tracks within the MOQT Streaming Format (MSF). About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-wilaw-moq-webvtt-msf/. Discussion of this document takes place on the Media Over QUIC Working Group mailing list (mailto:moq@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/moq/. Subscribe at https://www.ietf.org/mailman/listinfo/moq/. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 7 January 2027. Law & Nandakumar Expires 7 January 2027 [Page 1] Internet-Draft WebVTT Packaging for MOQT Streaming Form July 2026 Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Cue Structure . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Settings Object . . . . . . . . . . . . . . . . . . . . . 3 4. Regions and Styles . . . . . . . . . . . . . . . . . . . . . 4 4.1. Region Properties . . . . . . . . . . . . . . . . . . . . 4 5. Event Timeline Track Structure . . . . . . . . . . . . . . . 5 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Catalog Declaration . . . . . . . . . . . . . . . . . . . . . 6 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 10. Normative References . . . . . . . . . . . . . . . . . . . . 7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction This document defines how WebVTT [WEBVTT] content is packaged for delivery over MSF [MSF] event timelines using the urn:msf:captions:webvtt event type. WebVTT is a widely-used format for delivering captions and subtitles on the web. MSF event timeline tracks ([MSF], Section 11) provide a mechanism for delivering time-synchronized metadata alongside media content. This specification defines a JSON [JSON] packaging format that maps WebVTT cues to MSF event timeline records, preserving WebVTT semantics while enabling synchronized delivery over MOQT. Law & Nandakumar Expires 7 January 2027 [Page 2] Internet-Draft WebVTT Packaging for MOQT Streaming Form July 2026 2. Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Cue Structure Each WebVTT cue is mapped to an MSF event timeline record ([MSF], Section 11.1). The record's m (media time) index indicates when the cue becomes active, and the data field contains a JSON object with the following fields: +==========+========+==========+================================+ | Field | Type | Required | Description | +==========+========+==========+================================+ | start | Number | Yes | Cue start time in milliseconds | +----------+--------+----------+--------------------------------+ | end | Number | Yes | Cue end time in milliseconds | +----------+--------+----------+--------------------------------+ | id | String | No | Cue identifier | +----------+--------+----------+--------------------------------+ | text | String | Yes | Cue payload (may contain tags) | +----------+--------+----------+--------------------------------+ | settings | Object | No | Positioning settings | +----------+--------+----------+--------------------------------+ | region | String | No | Region identifier | +----------+--------+----------+--------------------------------+ Table 1 The text field preserves WebVTT cue text tags. Times are in milliseconds, consistent with MSF media time conventions ([MSF], Section 10.1). The m index value MUST equal the start value in the corresponding data object. WebVTT NOTE blocks and the file header description are discarded during packaging. 3.1. Settings Object The optional settings object maps WebVTT cue settings to JSON properties, preserving the WebVTT syntax verbatim: Law & Nandakumar Expires 7 January 2027 [Page 3] Internet-Draft WebVTT Packaging for MOQT Streaming Form July 2026 +==========+========+===============================================+ | Field | Type | Description | +==========+========+===============================================+ | vertical | String | Writing direction: "rl" or "lr" | +----------+--------+-----------------------------------------------+ | line | String | Line position (e.g., "0", "-1", | | | | "50%,center") | +----------+--------+-----------------------------------------------+ | position | String | Text position (e.g., "50%", | | | | "10%,line-left") | +----------+--------+-----------------------------------------------+ | size | String | Cue box size percentage (e.g., | | | | "80%") | +----------+--------+-----------------------------------------------+ | align | String | Text alignment: "start", "center", | | | | "end", "left", or "right" | +----------+--------+-----------------------------------------------+ Table 2 4. Regions and Styles WebVTT region definitions and CSS style blocks are delivered as an initialization record at media time 0, analogous to codec initialization data ([MSF], Section 5.3.11). Receivers MUST process this record before rendering any cues. An initialization record is distinguished from cue records by the presence of regions and/or styles keys in the data object. 4.1. Region Properties Each region object in the regions array maps WebVTT region settings: Law & Nandakumar Expires 7 January 2027 [Page 4] Internet-Draft WebVTT Packaging for MOQT Streaming Form July 2026 +================+========+==========+==========================+ | Field | Type | Required | Description | +================+========+==========+==========================+ | id | String | Yes | Region identifier | +----------------+--------+----------+--------------------------+ | width | String | No | Width as percentage of | | | | | video (default "100%") | +----------------+--------+----------+--------------------------+ | lines | Number | No | Number of visible lines | | | | | (default 3) | +----------------+--------+----------+--------------------------+ | regionanchor | String | No | X,Y anchor within region | | | | | (default "0%,100%") | +----------------+--------+----------+--------------------------+ | viewportanchor | String | No | X,Y anchor on viewport | | | | | (default "0%,100%") | +----------------+--------+----------+--------------------------+ | scroll | String | No | Scroll mode; only valid | | | | | value is "up" | +----------------+--------+----------+--------------------------+ Table 3 Example initialization record: { "m": 0, "data": { "regions": [ {"id": "bottom", "width": "100%", "lines": 2, "regionanchor": "50%,100%", "viewportanchor": "50%,90%"} ], "styles": "::cue { background: rgba(0,0,0,0.8); }" } } 5. Event Timeline Track Structure Following [MSF], Section 11.3, a WebVTT event timeline track uses independent and incremental objects within each MOQT Group: * The first Object in each Group MUST be independent, containing all accumulated cue records up to that point (enabling mid-stream join). * Subsequent Objects within the same Group MAY contain only new cue records. Law & Nandakumar Expires 7 January 2027 [Page 5] Internet-Draft WebVTT Packaging for MOQT Streaming Form July 2026 6. Example A complete event timeline payload with two cues: [ { "m": 0, "data": { "start": 0, "end": 2500, "text": "Welcome to the show." } }, { "m": 2500, "data": { "start": 2500, "end": 5000, "text": "Hello everyone!", "settings": {"align": "center"} } } ] The m index is required by MSF for timeline synchronization and subscriber join; the start and end values within data make each cue self-contained for rendering. 7. Catalog Declaration A WebVTT caption track is declared in the MSF catalog ([MSF], Section 5.3) with the following fields: * packaging: MUST be eventtimeline ([MSF], Section 5.3.3) * eventType: MUST be urn:msf:captions:webvtt ([MSF], Section 5.3.4) * depends: MUST list the video track(s) this caption track accompanies ([MSF], Section 5.3.14) * mimeType: MUST be application/json ([MSF], Section 5.3.17) * role: SHOULD be caption or subtitle as appropriate ([MSF], Section 5.3.6) * lang: SHOULD specify the caption language per RFC 5646 ([MSF], Section 5.3.24) Law & Nandakumar Expires 7 January 2027 [Page 6] Internet-Draft WebVTT Packaging for MOQT Streaming Form July 2026 Example: { "name": "captions-en", "packaging": "eventtimeline", "eventType": "urn:msf:captions:webvtt", "mimeType": "application/json", "depends": ["video"], "role": "caption", "lang": "en", "isLive": true } 8. Security Considerations TODO 9. IANA Considerations This document requests registration of the urn:msf:captions:webvtt event type in the "MSF Event Timeline Types" registry established by [MSF], Section 14.2: +=========================+=====================+===============+ | Event Type | Description | Specification | +=========================+=====================+===============+ | urn:msf:captions:webvtt | WebVTT caption cues | this document | +-------------------------+---------------------+---------------+ Table 4 10. Normative References [JSON] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, . [MSF] Law, W. and S. Nandakumar, "MOQT Streaming Format", Work in Progress, Internet-Draft, draft-ietf-moq-msf-01, 2 June 2026, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Law & Nandakumar Expires 7 January 2027 [Page 7] Internet-Draft WebVTT Packaging for MOQT Streaming Form July 2026 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [WEBVTT] W3C, "WebVTT: The Web Video Text Tracks Format", April 2019, . Acknowledgments TODO Authors' Addresses Will Law Akamai Email: wilaw@akamai.com Suhas Nandakumar Cisco Email: snandaku@cisco.com Law & Nandakumar Expires 7 January 2027 [Page 8]