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

Document Type RFC - Proposed Standard (March 2020; No errata)
Last updated 2020-03-18
Replaces draft-sandford-payload-rtp-ttml
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Send notices to Roni Even <roni.even@huawei.com>
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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
   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
   timing metadata.  One of its primary use cases is the description of
   subtitles and closed captions.  A number of profiles exist that adapt
   TTML2 for use in specific contexts [TTML-MTPR].  These include both
   file-based and streaming workflows.

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

4.1.  RTP Header Usage

   RTP packet header fields SHALL be interpreted, as per [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 epoch of the TTML document in User
      Data Words.  Further detail on its usage may be found in
      Section 6.  The clock frequency used is dependent on the
      application and is specified in the media type rate parameter, as
      per Section 11.1.  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 and
      ignored upon reception.

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

   User Data Words: The length of User Data Words MUST match the
   value specified in the Length field
      The User Data Words section 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 serialised in big-
      endian order, a.k.a., network byte order.  Where a document will
      not fit within the Path MTU, it may be fragmented across multiple
      packets.  Further detail on fragmentation may be found in
      Section 8.

4.2.  Payload Data

   TTML documents define a series of changes to text over time.  TTML
   documents carried in User Data Words are encoded in accordance with
   one or more of the defined TTML profiles specified in the TTML
   registry [TTML-MTPR].  These profiles specify the document structure
   used, systems models, timing, and other considerations.  TTML
   profiles may restrict the complexity of the changes, and operational
   requirements may limit the maximum duration of TTML documents by a
   deployment configuration.  Both of these cases are out of scope of
   this document.

   Documents carried over RTP MUST conform to the following profile, in
   addition to any others used.

5.  Payload Content Restrictions

   This section defines constraints on the content of TTML documents
   carried over RTP.

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

   The TTML document instance's root "tt" element in the
   "http://www.w3.org/ns/ttml" namespace MUST include a "timeBase"
   attribute in the "http://www.w3.org/ns/ttml#parameter" namespace
   containing the value "media".

   This is equivalent to the TTML2 content profile definition document
   in Figure 2.

   <?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"
       type="content"
       designator="urn:ietf:rfc:8759#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>

      Figure 2: TTML2 Content Profile Definition for Documents Carried
                                  over RTP

6.  Payload Processing Requirements

   This section defines constraints on the processing of the TTML
   documents carried over RTP.

   If a TTML document is assessed to be invalid, then it MUST be
   discarded.  This includes empty documents, i.e., those of zero
   length.  When processing a valid document, the following requirements
   apply.

   Each TTML document becomes active at its epoch E.  E MUST be set to
   the RTP Timestamp in the header of the RTP packet carrying the TTML
   document.  Computed TTML media times are offset relative to E, in
   accordance with Section I.2 of [TTML2].

   When processing a sequence of TTML documents, where each is delivered
   in the same RTP stream, exactly zero or one document SHALL be
   considered active at each moment in the RTP time line.  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, then processing
   of the document MAY be stopped.  This can be tested by constructing
   the intermediate synchronic document sequence from the document, as
   defined by [TTML2].  If the last intermediate synchronic document in
   the sequence is both active and contains no region elements, then all
   defined content within the document has ended.

   As described above, the RTP Timestamp does not specify the exact
   timing of the media in this payload format.  Additionally, documents
   may be fragmented across multiple packets.  This renders the RTCP
   jitter calculation unusable.

6.1.  TTML Processor Profile

6.1.1.  Feature Extension Designation

   This specification defines the following TTML feature extension
   designation:

      "urn:ietf:rfc:8759#rtp-relative-media-time"

   The namespace "urn:ietf:rfc:8759" is as defined by [RFC2648].

   A TTML content processor supports the "#rtp-relative-media-time"
   feature extension if it processes media times in accordance with the
   payload processing requirements specified in this document, i.e.,
   that the epoch E is set to the time equivalent to the RTP Timestamp,
   as detailed above in Section 6.

6.1.2.  Processor Profile Document

   The required syntax and semantics declared in the minimal TTML2
   processor profile in Figure 3 MUST be supported by the receiver, as
   signified by those "feature" or "extension" elements whose "value"
   attribute is set to "required".

   <?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"
       type="processor"
       designator="urn:ietf:rfc:8759#processor"
       combine="mostRestrictive">
       <features xml:base="http://www.w3.org/ns/ttml/feature/">
           <tt:metadata>
               <ttm:desc>
                   This document is a minimal TTML2 processor profile
                   definition document intended to express the
                   minimal requirements of a TTML processor able to
                   process TTML delivered over RTP according to
                   RFC 8759.
               </ttm:desc>
           </tt:metadata>
           <feature value="required">#timeBase-media</feature>
           <feature value="optional">
               #profile-full-version-2
           </feature>
       </features>
       <extensions xml:base="urn:ietf:rfc:8759">
           <extension restricts="#timeBase-media" value="required">
               #rtp-relative-media-time
           </extension>
       </extensions>
   </profile>

        Figure 3: TTML2 Processor Profile Definition for Processing
                         Documents Carried over RTP

   Note that this requirement does not imply that the receiver needs to
   support either TTML1 or TTML2 profile processing, i.e., the TTML2
   "#profile-full-version-2" feature or any of its dependent features.

6.1.3.  Processor Profile Signalling

   The "codecs" media type parameter MUST specify at least one processor
   profile.  Short codes for TTML profiles are registered at
   [TTML-MTPR].  The processor profiles specified in "codecs" MUST be
   compatible with the processor profile specified in this document.
   Where multiple options exist in "codecs" for possible processor
   profile combinations (i.e., separated by "|" operator), every
   permitted option MUST be compatible with the processor profile
   specified in this document.  Where processor profiles (other than the
   one specified in this document) are advertised in the "codecs"
   parameter, the requirements of the processor profile specified in
   this document MAY be signalled, additionally using the "+" operator
   with its registered short code.

   A processor profile (X) is compatible with the processor profile
   specified here (P) if X includes all the features and extensions in P
   (identified by their character content) and the "value" attribute of
   each is, at least, as restrictive as the "value" attribute of the
   feature or extension in P that has the same character content.  The
   term "restrictive" here is as defined in Section 6 of [TTML2].

7.  Payload Examples

   Figure 4 is an example of a valid TTML document that may be carried
   using the payload format described in this document.

   <?xml version="1.0" encoding="UTF-8"?>
   <tt xml:lang="en"
    xmlns="http://www.w3.org/ns/ttml"
    xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
    xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
    xmlns:tts="http://www.w3.org/ns/ttml#styling"
    ttp:timeBase="media"
    >
     <head>
       <metadata>
         <ttm:title>Timed Text TTML Example</ttm:title>
         <ttm:copyright>The Authors (c) 2006</ttm:copyright>
       </metadata>
       <styling>
         <!--
           s1 specifies default color, font, and text alignment
         -->
         <style xml:id="s1"
           tts:color="white"
           tts:fontFamily="proportionalSansSerif"
           tts:fontSize="100%"
           tts:textAlign="center"
         />
       </styling>
       <layout>
         <region xml:id="subtitleArea"
           style="s1"
           tts:extent="78% 11%"
           tts:padding="1% 5%"
           tts:backgroundColor="black"
           tts:displayAlign="after"
         />
       </layout>
     </head>
     <body region="subtitleArea">
       <div>
         <p xml:id="subtitle1" dur="5.0s" style="s1">
           How truly delightful!
         </p>
       </div>
     </body>
   </tt>

                      Figure 4: Example TTML Document

8.  Fragmentation of TTML Documents

   Many of the use cases for TTML are low bit-rate with RTP packets
   expected to fit within the Path MTU.  However, some documents may
   exceed the Path MTU.  In these cases, they may be split between
   multiple packets.  Where fragmentation is used, the following
   guidelines MUST be followed:

   *  It is RECOMMENDED that documents be fragmented as seldom as
      possible, i.e., the least possible number of fragments is created
      out of a document.

   *  Text strings MUST split at character boundaries.  This enables
      decoding of partial documents.  As a consequence, document
      fragmentation requires knowledge of the UTF-8/UTF-16 encoding
      formats to determine character boundaries.

   *  Document fragments SHOULD be protected against packet losses.
      More information can be found in Section 9.

   When a document spans more than one RTP packet, the entire document
   is obtained by concatenating User Data Words from each consecutive
   contributing packet in ascending order of Sequence Number.

   As described in Section 6, only zero or one TTML document may be
   active at any point in time.  As such, there MUST only be one
   document transmitted for a given RTP Timestamp.  Furthermore, as
   stated in Section 4.1, the marker bit MUST be set for a packet
   containing the last fragment of a document.  A packet following one
   where the marker bit is set contains the first fragment of a new
   document.  The first fragment might also be the last.

9.  Protection against Loss of Data

   Consideration must be devoted to keeping loss of documents due to
   packet loss within acceptable limits.  What is deemed acceptable
   limits is dependent on the TTML profile(s) used and use case, among
   other things.  As such, specific limits are outside the scope of this
   document.

   Documents MAY be sent without additional protection if end-to-end
   network conditions guarantee that document loss will be within
   acceptable limits under all anticipated load conditions.  Where such
   guarantees cannot be provided, implementations MUST use a mechanism
   to protect against packet loss.  Potential mechanisms include Forward
   Error Correction (FEC) [RFC5109], retransmission [RFC4588],
   duplication [ST2022-7], or an equivalent technique.

10.  Congestion Control Considerations

   Congestion control for RTP SHALL be used in accordance with [RFC3550]
   and with any applicable RTP profile, e.g., [RFC3551].  "Multimedia
   Congestion Control: Circuit Breakers for Unicast RTP Sessions"
   [RFC8083] is an update to "RTP: A Transport Protocol for Real-time
   Applications" [RFC3550], which defines criteria for when one is
   required to stop sending RTP packet streams.  Applications
   implementing this standard MUST comply with [RFC8083], with
   particular attention paid to Section 4.4 on Media Usability.
   [RFC8085] provides additional information on the best practices for
   applying congestion control to UDP streams.

11.  Payload Format Parameters

   This RTP payload format is identified using the existing application/
   ttml+xml media type as registered with IANA [IANA] and defined in
   [TTML-MTPR].

11.1.  Clock Rate

   The default clock rate for TTML over RTP is 1000 Hz.  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
   RTP streams.  In other contexts, timing is defined within the TTML
   document.

   When choosing a clock rate, implementers should consider what other
   media their TTML streams may be used in conjunction with (e.g., video
   or audio).  In these situations, it is RECOMMENDED that streams use
   the same clock source and clock rate as the related media.  As TTML
   streams may be aperiodic, implementers should also consider the
   frequency range over which they expect packets to be sent and the
   temporal resolution required.

11.2.  Session Description Protocol (SDP) Considerations

   The mapping of the application/ttml+xml media type and its parameters
   [TTML-MTPR] SHALL be done according to Section 3 of [RFC4855].

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

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

   *  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].  The "codecs" parameter MUST be included in the "a=fmtp"
   line of the SDP file.  Specific requirements for the "codecs"
   parameter are included in Section 6.1.3.

11.2.1.  Examples

   A sample SDP mapping is presented in Figure 5.

   m=application 30000 RTP/AVP 112
   a=rtpmap:112 ttml+xml/90000
   a=fmtp:112 charset=utf-8;codecs=im2t

                       Figure 5: Example SDP Mapping

   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.
   The codecs parameter defined in the "a=fmtp" line indicates that the
   TTML data conforms to Internet Media and Captions (IMSC) 1.1 Text
   profile [TTML-IMSC1.1].

12.  IANA Considerations

   This document has no IANA actions.

13.  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 in any applicable RTP profile, such as
   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
   SAVPF [RFC5124].  However, as "Securing the RTP Protocol Framework:
   Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202]
   discusses, it is not an RTP payload format's responsibility to
   discuss or mandate what solutions are used to meet the basic security
   goals (like confidentiality, integrity, and source authenticity) for
   RTP in general.  This responsibility lays on anyone using RTP in an
   application.  They can find guidance on available security mechanisms
   and important considerations in "Options for Securing RTP Sessions"
   [RFC7201].  Applications SHOULD use one or more appropriate strong
   security mechanisms.  The rest of this Security Considerations
   section discusses the security impacting properties of the payload
   format itself.

   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).

   This payload format places no specific restrictions on the size of
   TTML documents that may be transmitted.  As such, malicious
   implementations could be used to perform denial-of-service (DoS)
   attacks.  [RFC4732] provides more information on DoS attacks and
   describes some mitigation strategies.  Implementers should take into
   consideration that the size and frequency of documents transmitted
   using this format may vary over time.  As such, sender
   implementations should avoid producing streams that exhibit DoS-like
   behaviour, and receivers should avoid false identification of a
   legitimate stream as malicious.

   As with other XML types and as noted in Section 10 of "XML Media
   Types" [RFC7303], repeated expansion of maliciously constructed XML
   entities can be used to consume large amounts of memory, which may
   cause XML processors in constrained environments to fail.

   In addition, because of the extensibility features for TTML and of
   XML in general, it is possible that "application/ttml+xml" may
   describe content that has security implications beyond those
   described here.  However, TTML does not provide for any sort of
   active or executable content, and if the processor follows only the
   normative semantics of the published specification, this content will
   be outside TTML namespaces and may be ignored.  Only in the case
   where the processor recognizes and processes the additional content
   or where further processing of that content is dispatched to other
   processors would security issues potentially arise.  And in that
   case, they would fall outside the domain of this RTP payload format
   and the application/ttml+xml registration document.

   Although not prohibited, there are no expectations that XML
   signatures or encryption would normally be employed.

   Further information related to privacy and security at a document
   level can be found in Appendix P of [TTML2].

14.  Normative References

   [IANA]     IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [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>.

   [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>.

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,
              <https://www.rfc-editor.org/info/rfc7303>.

   [RFC8083]  Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", RFC 8083,
              DOI 10.17487/RFC8083, March 2017,
              <https://www.rfc-editor.org/info/rfc8083>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [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] European Broadcasting Union, "EBU-TT, Part 3, Live
              Subtitling Applications: System Model and Content Profile
              for Authoring and Contribution", EBU-TT Part 3, Tech 3370,
              May 2017, <https://tech.ebu.ch/publications/tech3370>.

   [TTML-MTPR]
              Adams, G., Ed. and M. Dolan, Ed., "TTML Media Type
              Definition and Profile Registry", W3C Working Group Note,
              April 2019,
              <https://www.w3.org/TR/ttml-profile-registry/>.

   [TTML2]    Adams, G., Ed. and C. Concolato, Ed., "Timed Text Markup
              Language 2 (TTML2)", W3C Recommendation REC-
              ttml2-20181108, November 2018,
              <https://www.w3.org/TR/ttml2/>.

15.  Informative References

   [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
              DOI 10.17487/RFC2648, August 1999,
              <https://www.rfc-editor.org/info/rfc2648>.

   [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>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [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>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <https://www.rfc-editor.org/info/rfc4585>.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <https://www.rfc-editor.org/info/rfc4588>.

   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
              Denial-of-Service Considerations", RFC 4732,
              DOI 10.17487/RFC4732, December 2006,
              <https://www.rfc-editor.org/info/rfc4732>.

   [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>.

   [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, DOI 10.17487/RFC5109, December
              2007, <https://www.rfc-editor.org/info/rfc5109>.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <https://www.rfc-editor.org/info/rfc5124>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <https://www.rfc-editor.org/info/rfc7201>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
              2014, <https://www.rfc-editor.org/info/rfc7202>.

   [ST2022-7] SMPTE, "Seamless Protection Switching of RTP Datagrams",
              ST 2022-7:2019, DOI 10.5594/SMPTE.ST2022-7.2019, May 2019,
              <https://ieeexplore.ieee.org/document/8716822>.

   [TTML-IMSC1.1]
              Lemieux, P., Ed., "TTML Profiles for Internet Media
              Subtitles and Captions 1.1", W3C Recommendation REC-ttml-
              imsc1.1-20181108, November 2018,
              <https://www.w3.org/TR/ttml-imsc1.1/>.

Acknowledgements

   Thanks to Nigel Megitt, James Gruessing, Robert Wadge, Andrew Bonney,
   James Weaver, John Fletcher, Frans de Jong, and Willem Vermost for
   their valuable feedback throughout the development of this document.
   Thanks to the W3C Timed Text Working Group and EBU Timed Text Working
   Group for their substantial efforts in developing the timed text
   format this payload format is intended to carry.

Author's Address

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

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