WebVTT Packaging for MOQT Streaming Format

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Table of Contents

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.¶

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:¶

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.¶

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.¶

{
    "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.¶

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": "<v Alice>Hello everyone!</v>",
            "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)¶

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:¶

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, <https://www.rfc-editor.org/rfc/rfc8259>.

[MSF]

Law, W. and S. Nandakumar, "MOQT Streaming Format", Work in Progress, Internet-Draft, draft-ietf-moq-msf-01, 2 June 2026, <https://datatracker.ietf.org/doc/html/draft-ietf-moq-msf-01>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[WEBVTT]

W3C, "WebVTT: The Web Video Text Tracks Format", April 2019, <https://www.w3.org/TR/webvtt1/>.

Acknowledgments

TODO¶

Authors' Addresses