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RTP Payload for TTML Timed Text
draft-sandford-payload-rtp-ttml-00

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author James Sandford
Last updated 2019-01-17
Replaced by draft-ietf-payload-rtp-ttml, RFC 8759
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draft-sandford-payload-rtp-ttml-00
A/V Transport Payloads Workgroup                             J. Sandford
Internet-Draft                          British Broadcasting Corporation
Intended status: Informational                          January 17, 2019
Expires: July 21, 2019

                    RTP Payload for TTML Timed Text
                   draft-sandford-payload-rtp-ttml-00

Abstract

   This memo describes a Real-time Transport Protocol (RTP) payload
   format for TTML, an XML based timed text format for live and file
   based workflows from W3C.  This payload format is specifically
   targeted at live workflows using TTML.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 21, 2019.

Copyright Notice

   Copyright (c) 2019 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
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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions, Definitions, and Abbreviations . . . . . . . . .   2
   3.  Media Format Description  . . . . . . . . . . . . . . . . . .   3
     3.1.  Relation to Other Text Payload Types  . . . . . . . . . .   3
   4.  Payload Format  . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Payload Data  . . . . . . . . . . . . . . . . . . . . . .   4
       4.2.1.  TTML Profile for RTP Carriage . . . . . . . . . . . .   5
   5.  Payload Examples  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Congestion Control Considerations . . . . . . . . . . . . . .   6
   7.  Payload Format Parameters . . . . . . . . . . . . . . . . . .   6
     7.1.  Clock Rate  . . . . . . . . . . . . . . . . . . . . . . .   6
     7.2.  Mapping to SDP  . . . . . . . . . . . . . . . . . . . . .   7
       7.2.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  RFC Editor Considerations  . . . . . . . . . . . . .   9
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   TTML (Timed Text Markup Language)[TTML] is a media type for
   describing timed text such as closed captions (also known as
   subtitles) in television workflows or broadcasts as XML.  This
   document specifies how TTML should be mapped into an RTP stream in
   live workflows including, but not restricted to, those described in
   the television broadcast oriented EBU-TT Part 3[TECH3370]
   specification.

2.  Conventions, Definitions, and Abbreviations

   Unless otherwise stated, the term "document" is used in this draft to
   refer to the TTML document being transmitted in the payload of the
   RTP packet(s).

   Where the term "word" is used in this draft, it is to refer to byte
   aligned or 32-bit aligned words of data in a computing sense and not
   to refer to linguistic words that might appear in the transported
   text.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

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   document are to be interpreted as described in BCP14 [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.  RFC 4103 for Text Conversation
   [RFC4103] is intended for low data rate conversation with its own
   session management and minimal formatting capabilities.  RFC 4734
   Events for Modem, Fax, and Text Telephony Signals [RFC4734] deals in
   large parts with the control signalling of facsimile and other
   systems.  RFC 4396 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.

4.  Payload Format

   In addition to the required RTP headers, the payload contains a
   section for the TTML document being transmitted (User Data Words),
   and a field for the Length of that data.  Each RTP payload contains
   one or part of one TTML document.

   A representation of the payload format for TTML is Figure 1.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|X| CC    |M|    PT       |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Synchronization Source (SSRC) Identifier            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved            |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       User Data Words...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: RTP Payload Format for TTML

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4.1.  RTP Header Usage

   RTP packet header fields SHALL be interpreted as per RFC 3550
   [RFC3550], with the following specifics:

   Marker Bit (M): 1 bit
      The Marker Bit is set to "1" to indicate the last packet of a
      document.  Otherwise set to "0".  Note: The first packet might
      also be the last.

   Timestamp: 32 bits
      The RTP Timestamp encodes the time of the text in the packet.  The
      clock frequency used is dependent on the application and is
      specified in the media type rate parameter as per Section 7.1.
      Documents spread across multiple packets MUST use the same
      timestamp but different consecutive Sequence Numbers.  Sequential
      documents MUST NOT use the same timestamp.  Because packets do not
      represent any constant duration, the timestamp cannot be used to
      directly infer packet loss.

   Reserved: 16 bits
      These bits are reserved for future use and MUST be set to 0x0.

   Length: 16 bits
      The length of User Data Words in bytes.

   User Data Words: integer number of words whose length is defined by
   the character encoding
      User Data Words contains the text of the whole document being
      transmitted or a part of the document being transmitted.
      Documents using character encodings where characters are not
      represented by a single byte MUST be serialized in big endian
      order, a.k.a. network byte order.  When the document spans more
      than one RTP packet, the entire document is obtained by
      concatenating User Data Words from each contributing packet in
      ascending order of Sequence Number.

4.2.  Payload Data

   Documents carried in User Data Words are encoded in accordance with
   one of the defined TTML profiles specified in its registry [TTML].
   These profiles specify the document structure used, systems models,
   timing, and other considerations.

   Additionally, documents carried over RTP MUST conform to the
   following profile.

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4.2.1.  TTML Profile for RTP Carriage

   This section defines constraints on the content and processing of the
   TTML payload for RTP carriage.

4.2.1.1.  Payload content restrictions

   Multiple TTML subtitle streams MUST NOT be interleaved in a single
   RTP stream.

   The TTML document instance MUST use the "media" value of the
   "ttp:timeBase" parameter attribute on the root element.

   This is equivalent to the following TTML2 content profile definition
   document:

   <?xml version="1.0" encoding="UTF-8"?>
   <profile xmlns="http://www.w3.org/ns/ttml#parameter"
       xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
       xmlns:tt="http://www.w3.org/ns/ttml"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.w3.org/ns/ttml#parameter"
       type="content" combine="mostRestrictive">
       <features xml:base="http://www.w3.org/ns/ttml/feature/">
           <tt:metadata>
               <ttm:desc>
                   This document is a minimal TTML2 content profile
                   definition document intended to express the minimal
                   requirements to apply when carrying TTML over RTP.
               </ttm:desc>
           </tt:metadata>
           <feature value="required">#timeBase-media</feature>
           <feature value="prohibited">#timeBase-smpte</feature>
           <feature value="prohibited">#timeBase-clock</feature>
       </features>
   </profile>

4.2.1.2.  Payload processing requirements

   If the TTML document payload is assessed to be invalid then it MUST
   be discarded.  When processing a valid document, the following
   requirements apply.

   The epoch E relative to which computed media times are offset MUST be
   set to the RTP timestamp in the header of the RTP packet in which the
   TTML document instance is carried.

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   When processing a sequence of TTML documents each delivered in the
   same RTP stream, exactly zero or one document SHALL be considered
   active at each moment in the RTP time line.

   Each TTML document becomes active at E.  In the event that a document
   D_(n-1) with E_(n-1) is active, and document D_(n) is delivered with
   E_(n) where E_(n-1) < E_(n), processing of D_(n-1) MUST be stopped at
   E_(n) and processing of D_(n) MUST begin.

   When all defined content within a document has ended, i.e. the active
   intermediate synchronic document contains no content, then processing
   of the document MAY be stopped.

4.2.1.2.1.  EBU-TT Live considerations

   EBU-TT Live is a profile of TTML intended to support live
   contribution of TTML documents as a stream independently of the
   carriage mechanism.  When EBU-TT Live documents are carried in an RTP
   stream, or when the TTML documents being transferred over RTP use
   EBU-TT Live semantics, the following considerations apply:

   E is considered to be the Availability Time as defined by EBU-TT
   Live.  It is an error if two documents are delivered such that
   E_(n-1) < E_(n) and the "ebuttp:sequenceNumber" of E_(n-1) is greater
   than the "ebuttp:sequenceNumber" of E_(n).  Every EBU-TT Live
   document in a single RTP stream MUST have a
   "ebuttp:sequenceIdentifier" with the same value.

5.  Payload Examples

   *TODO*: Provide an example, valid payload.

6.  Congestion Control Considerations

   The general congestion control considerations for transporting RTP
   data apply; see RTP [RFC3550].

7.  Payload Format Parameters

   This RTP payload format is identified using the existing application/
   ttml+xml media type.

7.1.  Clock Rate

   The default clock rate for TTML over RTP is 1000Hz.  The clock rate
   SHOULD be included in any advertisements of the RTP stream where
   possible.  This parameter has not been added to the media type
   definition as it is not applicable to TTML usage other than within

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   RTP streams.  In other contexts, timing is defined within the TTML
   document.

7.2.  Mapping to SDP

   The mapping of the above defined payload format media type and its
   parameters SHALL be done according to Section 3 of RFC 4855
   [RFC4855].

   o  The type name "application" goes in SDP "m=" as the media name.

   o  The media subtype "ttml+xml" goes in SDP "a=rtpmap" as the
      encoding name,

   o  The clock rate also goes in "a=rtpmap" as the clock rate.

   Additional format specific parameters as described in the media type
   specification SHALL be included in the SDP file in "a=fmtp" as a
   semicolon separated list of "parameter=value" pairs as described in
   [RFC4855].

7.2.1.  Examples

   A sample SDP mapping is as follows:

   m=application 30000 RTP/AVP 112
   a=rtpmap:112 ttml+xml/90000
   a=fmtp:112 charset=utf-8

   In this example, a dynamic payload type 112 is used.  The 90 kHz RTP
   timestamp rate is specified in the "a=rtpmap" line after the subtype.

8.  IANA Considerations

   This document makes use of the media type application/ttml+xml.  The
   media types registry should be updated to make reference to this
   document for the application/ttml+xml media type.

9.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550] and any applicable RTP profile, e.g.  AVP
   [RFC3551].

   To avoid potential buffer overflow attacks, receivers should take
   care to validate that the User Data Words in the RTP payload are of
   the appropriate length (using the Length field).

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10.  References

10.1.  Normative References

   [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/info/rfc2119>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
              <https://www.rfc-editor.org/info/rfc4103>.

   [RFC4396]  Rey, J. and Y. Matsui, "RTP Payload Format for 3rd
              Generation Partnership Project (3GPP) Timed Text",
              RFC 4396, DOI 10.17487/RFC4396, February 2006,
              <https://www.rfc-editor.org/info/rfc4396>.

   [RFC4734]  Schulzrinne, H. and T. Taylor, "Definition of Events for
              Modem, Fax, and Text Telephony Signals", RFC 4734,
              DOI 10.17487/RFC4734, December 2006,
              <https://www.rfc-editor.org/info/rfc4734>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <https://www.rfc-editor.org/info/rfc4855>.

   [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/info/rfc8174>.

   [TECH3370]
              "TECH 3370 - EBU-TT PART 3: LIVE CONTRIBUTION", May 2017.

   [TTML]     "TTML Media Type Definition and Profile Registry", January
              2017.

10.2.  Informative References

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <https://www.rfc-editor.org/info/rfc3551>.

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Appendix A.  RFC Editor Considerations

   *TODO* To be filled

Appendix B.  Acknowledgements

   *TODO*

Author's Address

   James Sandford
   British Broadcasting Corporation
   Dock House, MediaCityUK
   Salford
   United Kingdom

   Phone: +44 30304 09549
   Email: james.sandford@bbc.co.uk

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