RTP Payload for Timed Text Markup Language (TTML)
RFC 8759
Internet Engineering Task Force (IETF) J. Sandford
Request for Comments: 8759 British Broadcasting Corporation
Category: Standards Track March 2020
ISSN: 2070-1721
RTP Payload for Timed Text Markup Language (TTML)
Abstract
This memo describes a Real-time Transport Protocol (RTP) payload
format for Timed Text Markup Language (TTML), an XML-based timed text
format from W3C. This payload format is specifically targeted at
streaming workflows using TTML.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8759.
Copyright Notice
Copyright (c) 2020 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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Conventions and Definitions
3. Media Format Description
3.1. Relation to Other Text Payload Types
3.2. TTML2
4. Payload Format
4.1. RTP Header Usage
4.2. Payload Data
5. Payload Content Restrictions
6. Payload Processing Requirements
6.1. TTML Processor Profile
6.1.1. Feature Extension Designation
6.1.2. Processor Profile Document
6.1.3. Processor Profile Signalling
7. Payload Examples
8. Fragmentation of TTML Documents
9. Protection against Loss of Data
10. Congestion Control Considerations
11. Payload Format Parameters
11.1. Clock Rate
11.2. Session Description Protocol (SDP) Considerations
11.2.1. Examples
12. IANA Considerations
13. Security Considerations
14. Normative References
15. Informative References
Acknowledgements
Author's Address
1. Introduction
TTML (Timed Text Markup Language) [TTML2] is a media type for
describing timed text, such as closed captions and subtitles in
television workflows or broadcasts, as XML. This document specifies
how TTML should be mapped into an RTP stream in streaming workflows,
including (but not restricted to) those described in the television-
broadcast-oriented European Broadcasting Union Timed Text (EBU-TT)
Part 3 [TECH3370] specification. This document does not define a
media type for TTML but makes use of the existing application/
ttml+xml media type [TTML-MTPR].
2. Conventions and Definitions
Unless otherwise stated, the term "document" refers to the TTML
document being transmitted in the payload of the RTP packet(s).
The term "word" refers to a data word aligned to a specified number
of bits in a computing sense and not to linguistic words that might
appear in the transported text.
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. Media Format Description
3.1. Relation to Other Text Payload Types
Prior payload types for text are not suited to the carriage of closed
captions in television workflows. "RTP Payload for Text
Conversation" [RFC4103] is intended for low data rate conversation
with its own session management and minimal formatting capabilities.
"Definition of Events for Modem, Fax, and Text Telephony Signals"
[RFC4734] deals in large parts with the control signalling of
facsimile and other systems. "RTP Payload Format for 3rd Generation
Partnership Project (3GPP) Timed Text" [RFC4396] describes the
carriage of a timed text format with much more restricted formatting
capabilities than TTML. The lack of an existing format for TTML or
generic XML has necessitated the creation of this payload format.
3.2. TTML2
TTML2 (Timed Text Markup Language, Version 2) [TTML2] is an XML-based
markup language for describing textual information with associated
Show full document text