WebVTT Packaging for MOQT Streaming Format
draft-wilaw-moq-webvtt-msf-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Will Law , Suhas Nandakumar | ||
| Last updated | 2026-07-06 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
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draft-wilaw-moq-webvtt-msf-00
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/.
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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
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Please review these documents carefully, as they describe your rights
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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.
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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:
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+==========+========+===============================================+
| 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:
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+================+========+==========+==========================+
| 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.
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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)
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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,
<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>.
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[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
Will Law
Akamai
Email: wilaw@akamai.com
Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
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