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Versions: (draft-sandford-payload-rtp-ttml) 00 01 02 03 04 05

A/V Transport Payloads Workgroup                             J. Sandford
Internet-Draft                          British Broadcasting Corporation
Intended status: Standards Track                         29 October 2019
Expires: 1 May 2020


                    RTP Payload for TTML Timed Text
                     draft-ietf-payload-rtp-ttml-05

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 1 May 2020.

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 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.





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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
     3.2.  TTML2 . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Payload Format  . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Payload Data  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Payload content restrictions  . . . . . . . . . . . . . . . .   5
   6.  Payload processing requirements . . . . . . . . . . . . . . .   6
     6.1.  TTML Processor profile  . . . . . . . . . . . . . . . . .   7
       6.1.1.  Feature extension designation . . . . . . . . . . . .   7
       6.1.2.  Processor profile document  . . . . . . . . . . . . .   7
       6.1.3.  Processor profile signalling  . . . . . . . . . . . .   8
   7.  Payload Examples  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Fragmentation of TTML Documents . . . . . . . . . . . . . . .  11
   9.  Protection Against Loss of Data . . . . . . . . . . . . . . .  11
   10. Congestion Control Considerations . . . . . . . . . . . . . .  12
   11. Payload Format Parameters . . . . . . . . . . . . . . . . . .  12
     11.1.  Clock Rate . . . . . . . . . . . . . . . . . . . . . . .  12
     11.2.  Mapping to SDP . . . . . . . . . . . . . . . . . . . . .  12
       11.2.1.  Examples . . . . . . . . . . . . . . . . . . . . . .  13
     11.3.  Offer/Answer Considerations  . . . . . . . . . . . . . .  13
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  15
   16. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Appendix A.  RFC Editor Considerations  . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

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 live workflows
   including, but not restricted to, those described in the television
   broadcast oriented 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, Definitions, and Abbreviations

   Unless otherwise stated, the term "document" refers to the TTML
   document being transmitted in the payload of the RTP packet(s).



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   The term "word" refers to a data word aligned to a specified number
   of bits 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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this 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.

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.








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       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 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 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 at receive.

   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  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.  Where a document will not fit




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      within the 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.


















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      <?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:XXXX#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 each 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



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   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:XXXX#rtp-relative-media-time

   The namespace "urn:ietf:rfc:XXXX" 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".


















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      <?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:XXXX#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 XXXX.
                  </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:XXXX">
              <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



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   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 [TTML2] Section 6.

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.





































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      <?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







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8.  Fragmentation of TTML Documents

   Many of the use cases for TTML are low bit-rate with RTP packets
   expected to fit within the MTU.  However, some documents may exceed
   the 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 dependant 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 allow document loss to be within acceptable limits
   in all anticipated load conditions.  Where such guarantees cannot be
   provided, implementations MUST use a mechanism to protect against
   packet loss.  Potential mechanisms include FEC [RFC2733],
   retransmission [RFC4588], duplication [ST2022-7], or an equivalent
   technique.




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10.  Congestion Control Considerations

   Congestion control for RTP SHALL be used in accordance with
   [RFC3550], and with any applicable RTP profile: e.g., [RFC3551].
   Circuit Breakers [RFC8083] is an update to RTP [RFC3550] that 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 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
   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.  Mapping to SDP

   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



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   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=im1t

                       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 IMSC 1 Text profile.

11.3.  Offer/Answer Considerations

   All parameters are declarative.

12.  IANA Considerations

   No IANA action.

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.





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   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.  RFC 4732 [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 RFC 7303 [RFC7303], XML Media
   Types, Section 10, 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 TTML 2 Appendix P [TTML2].

14.  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
   formats this payload format is intended to carry.




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15.  Normative References

   [IANA]     IANA, "IANA - Media Types - Application", February 2019,
              <https://www.iana.org/assignments/media-types/media-
              types.xhtml#application>.

   [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, "TECH 3370 - EBU-TT PART 3:
              LIVE CONTRIBUTION", May 2017,
              <https://tech.ebu.ch/publications/tech3370>.

   [TTML-MTPR]
              W3C - Timed Text Working Group, "TTML Media Type




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              Definition and Profile Registry", January 2017,
              <https://www.w3.org/TR/ttml-profile-registry/>.

   [TTML2]    W3C - Timed Text Working Group, "Timed Text Markup
              Language 2 (TTML2)", November 2018,
              <https://www.w3.org/TR/ttml2/>.

16.  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>.

   [RFC2733]  Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
              for Generic Forward Error Correction", RFC 2733,
              DOI 10.17487/RFC2733, December 1999,
              <https://www.rfc-editor.org/info/rfc2733>.

   [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>.



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   [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>.

   [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, "ST 2022-7:2019 - Seamless Protection Switching of
              SMPTE ST 2022 IP Datagrams", November 2019,
              <https://ieeexplore.ieee.org/document/8716822>.

Appendix A.  RFC Editor Considerations

   Note to RFC Editor: This section may be removed after carrying out
   all the instructions of this section.

   The namespace "urn:ietf:rfc:XXXX" is to be replaced with the
   namespace for this document once it has received an RFC number.

   "RFC XXXX" in Figure 3 is to be replaced with the RFC number for this
   document.

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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