]> Akamai

wilaw@akamai.com

Cisco

snandaku@cisco.com

Applications and Real-Time Media Over QUIC MoQ MSF WebVTT 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 Status information for this document may be found at . Discussion of this document takes place on the Media Over QUIC Working Group mailing list (), which is archived at . Subscribe at .

Introduction This document defines how WebVTT content is packaged for delivery over 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 () provide a mechanism for delivering time-synchronized metadata alongside media content. This specification defines a JSON packaging format that maps WebVTT cues to MSF event timeline records, preserving WebVTT semantics while enabling synchronized delivery over MOQT.

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 when, and only when, they appear in all capitals, as shown here.

Cue Structure Each WebVTT cue is mapped to an MSF event timeline record (). 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

The text field preserves WebVTT cue text tags. Times are in milliseconds, consistent with MSF media time conventions (). 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.

Settings Object The optional settings object maps WebVTT cue settings to JSON properties, preserving the WebVTT syntax verbatim:

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"

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 (). 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.

Region Properties Each region object in the regions array maps WebVTT region settings:

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"

Example initialization record:

Event Timeline Track Structure Following , 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.

Example A complete event timeline payload with two cues: 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.

Catalog Declaration A WebVTT caption track is declared in the MSF catalog () with the following fields:

• packaging: MUST be eventtimeline ()
• eventType: MUST be urn:msf:captions:webvtt ()
• depends: MUST list the video track(s) this caption track accompanies ()
• mimeType: MUST be application/json ()
• role: SHOULD be caption or subtitle as appropriate ()
• lang: SHOULD specify the caption language per RFC 5646 ()

Example:

Security Considerations TODO

IANA Considerations This document requests registration of the urn:msf:captions:webvtt event type in the "MSF Event Timeline Types" registry established by :

Event Type	Description	Specification
urn:msf:captions:webvtt	WebVTT caption cues	this document

Normative References Akamai Cisco This document specifies the MOQT Streaming Format, designed to operate on Media Over QUIC Transport. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. W3C In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments TODO