]> Akamai

wilaw@akamai.com

Cisco

snandaku@cisco.com

Applications and Real-Time Media Over QUIC MoQ MSF IMSC1 TTML This document specifies the packaging format for delivering IMSC1 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 IMSC1 is a constrained TTML profile for subtitles and captions. This document defines how IMSC1 content is packaged as an MSF Event Timeline track (Section 8 of ) using the urn:msf:captions:imsc1 event type.

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.

Event Timeline Structure Each MOQT Object in an IMSC1 track contains a JSON array of Event Timeline records as defined in Section 8.1 of . Each record is a JSON object with two fields:

• m: Media presentation timestamp in milliseconds.
• data: Payload object whose structure is defined by this specification.

The publisher MUST include the initialization entry as the first record (with m set to 0) in the first MOQT Object of each MOQT Group, per the independence requirement in Section 8.3 of . Two packaging modes are defined:

• Cue mode (): JSON representation of individual cues, suitable for text-based subtitle/caption content.
• Document mode (): Full IMSC1 XML representation, supporting embedded images, complex layouts, and animation.

Cue Mode Cue mode maps IMSC1 styling, region definitions, and individual cues to JSON objects.

Initialization Entry The initialization entry carries the IMSC1 profile identifier and reusable style and region definitions.

Field	Type	Required	Description
profile	String	Yes	IMSC1 profile URI
styles	Object	No	Named style definitions
regions	Object	No	Named region definitions

The data field MUST contain an init object with the fields above:

Cue Entry Each cue entry maps an IMSC1 <p> element to JSON. The m field MUST equal the start value.

Field	Type	Required	Description
start	Number	Yes	Start time in milliseconds
end	Number	Yes	End time in milliseconds
region	String	No	Region identifier from init
style	String	No	Style identifier from init
content	Array	Yes	Content elements

Each content element MUST be one of:

• Text span: Object with a REQUIRED text field (String) and an OPTIONAL style field (String) referencing a named style. Inline style overrides MAY be specified as additional fields (e.g., fontStyle, color).
• Line break: Object with the single field br set to true.

Example:

Document Mode Document mode preserves the full IMSC1 XML representation for content requiring embedded SMPTE-TT images, complex animation, or styling features beyond cue mode. The m field MUST equal the start value.

Field	Type	Required	Description
start	Number	Yes	Start time in milliseconds
end	Number	Yes	End time in milliseconds
format	String	Yes	"isd" or "fragment"
encoding	String	No	"base64" if Base64 encoded
xml	String	Yes	IMSC1 XML content

When encoding is absent, the xml field contains plain UTF-8 XML. When set to "base64", the xml field is encoded per Section 4 of . Example: ..." } } ] ]]>

Catalog Declaration IMSC1 tracks MUST be declared in the MSF Catalog (Section 5 of ) with the following track object fields:

Field	MSF Section	Requirement
namespace	5.2.2	Per
name	5.2.3	Per
packaging	5.2.4	MUST be eventtimeline
eventType	5.2.5	MUST be urn:msf:captions:imsc1
mimeType	5.2.19	MUST be application/json
lang	5.2.32	BCP 47 language tag
depends	5.2.14	MUST reference associated media track(s)

This specification defines one additional field:

• imsc1Mode (String): MUST be present. Value MUST be cue or document.

Example:

Security Considerations TODO

IANA Considerations This document requests registration of the following event type in the "MSF Event Timeline Types" registry established by .

Event Type	Reference
urn:msf:captions:imsc1	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. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] 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