RTP Payload for Timed Text Markup Language (TTML)
RFC 8759

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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
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   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