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RTP Payload for TTML Timed Text

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8759.
Author James Sandford
Last updated 2019-10-17 (Latest revision 2019-10-08)
Replaces draft-sandford-payload-rtp-ttml
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestone
Mar 2020
Submit RTP Payload Format for TTML Timed Text for Proposed Standard
Document shepherd Roni Even
Shepherd write-up Show Last changed 2019-09-24
IESG IESG state Became RFC 8759 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES. Needs 7 more YES or NO OBJECTION positions to pass.
Responsible AD Barry Leiba
Send notices to Roni Even <>
IANA IANA review state IANA OK - No Actions Needed
A/V Transport Payloads Workgroup                             J. Sandford
Internet-Draft                          British Broadcasting Corporation
Intended status: Standards Track                         October 7, 2019
Expires: April 9, 2020

                    RTP Payload for TTML Timed Text


   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

   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 April 9, 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
   ( 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
       4.2.1.  TTML Profile for RTP Carriage . . . . . . . . . . . .   5
   5.  Payload Examples  . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Congestion Control Considerations . . . . . . . . . . . . . .  10
   7.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  11
     7.1.  Clock Rate  . . . . . . . . . . . . . . . . . . . . . . .  11
     7.2.  Mapping to SDP  . . . . . . . . . . . . . . . . . . . . .  11
       7.2.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  RFC Editor Considerations  . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   TTML (Timed Text Markup Language)[TTML2] 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.  This document does not define a media type for TTML
   but makes use of the existing application/ttml+xml media type

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).

   The term "word" refers 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.

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 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: 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

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      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.  Note that the length of each
      data word depends on the character encoding used: for UTF-8 a word
      is 8 bits, while it is 16 bits for UTF-16.

4.2.  Payload Data

   Documents carried in User Data Words are encoded in accordance with
   one 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.

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

4.2.1.  TTML Profile for RTP Carriage

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

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   <?xml version="1.0" encoding="UTF-8"?>
   <profile xmlns=""
       <features xml:base="">
                   This document is a minimal TTML2 content profile
                   definition document intended to express the minimal
                   requirements to apply when carrying TTML over RTP.
           <feature value="required">#timeBase-media</feature>
           <feature value="prohibited">#timeBase-smpte</feature>
           <feature value="prohibited">#timeBase-clock</feature>
   </profile>  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.

   Each TTML document becomes active at the 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.

   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
   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.

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   This specification defines the following TTML feature extension

   o  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  Processor profile document

   The required syntax and semantics declared in the following minimal
   TTML2 processor profile 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=""
       <features xml:base="">
                   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.
           <feature value="required">#timeBase-media</feature>
           <feature value="optional">#profile-full-version-2</feature>
       <extensions xml:base="urn:ietf:rfc:XXXX">
           <extension restricts="#timeBase-media" value="required">

   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.  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

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   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.

5.  Payload Examples

   The following 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"
         <ttm:title>Timed Text TTML Example</ttm:title>
         <ttm:copyright>The Authors (c) 2006</ttm:copyright>
         <!-- s1 specifies default color, font, and text alignment -->
         <style xml:id="s1"
         <region xml:id="subtitleArea"
           tts:extent="78% 11%"
           tts:padding="1% 5%"
     <body region="subtitleArea">
         <p xml:id="subtitle1" dur="5.0s" style="s1">
           How truly delightful!

6.  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]

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   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.

7.  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

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

   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).  It may be appropriate to use the same Synchronization
   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

7.2.  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].

   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].  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

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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;codecs=im1t

   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.

8.  IANA Considerations

   No IANA action.

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 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.  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

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   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].

10.  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.

11.  References

11.1.  Normative References

   [IANA]     IANA, "IANA - Media Types - Application", February 2019,

   [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-

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   [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, <>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,

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

   [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-

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

              European Broadcasting Union, "TECH 3370 - EBU-TT PART 3:
              LIVE CONTRIBUTION", May 2017,

              W3C - Timed Text Working Group, "TTML Media Type
              Definition and Profile Registry", January 2017,

   [TTML2]    W3C - Timed Text Working Group, "Timed Text Markup
              Language 2 (TTML2)", November 2018,

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11.2.  Informative References

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

   [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-

   [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,

   [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,

   [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-

   [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-

   [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-

   [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, <>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,

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   [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, <>.

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.

Author's Address

   James Sandford
   British Broadcasting Corporation
   Dock House, MediaCityUK
   United Kingdom

   Phone: +44 30304 09549

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