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2	A/V Transport Payloads Workgroup                             J. Sandford
3	Internet-Draft                          British Broadcasting Corporation
4	Intended status: Standards Track                         October 7, 2019
5	Expires: April 9, 2020

7	                    RTP Payload for TTML Timed Text
8	                     draft-ietf-payload-rtp-ttml-03

10	Abstract

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

17	Status of This Memo

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

22	   Internet-Drafts are working documents of the Internet Engineering
23	   Task Force (IETF).  Note that other groups may also distribute
24	   working documents as Internet-Drafts.  The list of current Internet-
25	   Drafts is at http://datatracker.ietf.org/drafts/current/.

27	   Internet-Drafts are draft documents valid for a maximum of six months
28	   and may be updated, replaced, or obsoleted by other documents at any
29	   time.  It is inappropriate to use Internet-Drafts as reference
30	   material or to cite them other than as "work in progress."

32	   This Internet-Draft will expire on April 9, 2020.

34	Copyright Notice

36	   Copyright (c) 2019 IETF Trust and the persons identified as the
37	   document authors.  All rights reserved.

39	   This document is subject to BCP 78 and the IETF Trust's Legal
40	   Provisions Relating to IETF Documents
41	   (http://trustee.ietf.org/license-info) in effect on the date of
42	   publication of this document.  Please review these documents
43	   carefully, as they describe your rights and restrictions with respect
44	   to this document.  Code Components extracted from this document must
45	   include Simplified BSD License text as described in Section 4.e of
46	   the Trust Legal Provisions and are provided without warranty as
47	   described in the Simplified BSD License.

49	Table of Contents

51	   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
52	   2.  Conventions, Definitions, and Abbreviations . . . . . . . . .   2
53	   3.  Media Format Description  . . . . . . . . . . . . . . . . . .   3
54	     3.1.  Relation to Other Text Payload Types  . . . . . . . . . .   3
55	     3.2.  TTML2 . . . . . . . . . . . . . . . . . . . . . . . . . .   3
56	   4.  Payload Format  . . . . . . . . . . . . . . . . . . . . . . .   3
57	     4.1.  RTP Header Usage  . . . . . . . . . . . . . . . . . . . .   4
58	     4.2.  Payload Data  . . . . . . . . . . . . . . . . . . . . . .   5
59	       4.2.1.  TTML Profile for RTP Carriage . . . . . . . . . . . .   5
60	   5.  Payload Examples  . . . . . . . . . . . . . . . . . . . . . .   9
61	   6.  Congestion Control Considerations . . . . . . . . . . . . . .  10
62	   7.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  11
63	     7.1.  Clock Rate  . . . . . . . . . . . . . . . . . . . . . . .  11
64	     7.2.  Mapping to SDP  . . . . . . . . . . . . . . . . . . . . .  11
65	       7.2.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .  12
66	   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
67	   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
68	   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
69	   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
70	     11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
71	     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
72	   Appendix A.  RFC Editor Considerations  . . . . . . . . . . . . .  16
73	   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

75	1.  Introduction

77	   TTML (Timed Text Markup Language)[TTML2] is a media type for
78	   describing timed text such as closed captions (also known as
79	   subtitles) in television workflows or broadcasts as XML.  This
80	   document specifies how TTML should be mapped into an RTP stream in
81	   live workflows including, but not restricted to, those described in
82	   the television broadcast oriented EBU-TT Part 3[TECH3370]
83	   specification.  This document does not define a media type for TTML
84	   but makes use of the existing application/ttml+xml media type
85	   [TTML-MTPR].

87	2.  Conventions, Definitions, and Abbreviations

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

92	   The term "word" refers to byte aligned or 32-bit aligned words of
93	   data in a computing sense and not to refer to linguistic words that
94	   might appear in the transported text.

96	   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
97	   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
98	   "OPTIONAL" in this document are to be interpreted as described in
99	   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
100	   capitals, as shown here.

102	3.  Media Format Description

104	3.1.  Relation to Other Text Payload Types

106	   Prior payload types for text are not suited to the carriage of closed
107	   captions in Television Workflows.  RFC 4103 for Text Conversation
108	   [RFC4103] is intended for low data rate conversation with its own
109	   session management and minimal formatting capabilities.  RFC 4734
110	   Events for Modem, Fax, and Text Telephony Signals [RFC4734] deals in
111	   large parts with the control signalling of facsimile and other
112	   systems.  RFC 4396 for 3rd Generation Partnership Project (3GPP)
113	   Timed Text [RFC4396] describes the carriage of a timed text format
114	   with much more restricted formatting capabilities than TTML.  The
115	   lack of an existing format for TTML or generic XML has necessitated
116	   the creation of this payload format.

118	3.2.  TTML2

120	   TTML2 (Timed Text Markup Language, Version 2)[TTML2] is an XML-based
121	   markup language for describing textual information with associated
122	   timing metadata.  One of its primary use cases is the description of
123	   subtitles and closed captions.  A number of profiles exist that adapt
124	   TTML2 for use in specific contexts [TTML-MTPR].  These include both
125	   file based and streaming workflows.

127	4.  Payload Format

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

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

136	    0                   1                   2                   3
137	    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
138	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
139	   |V=2|P|X| CC    |M|    PT       |        Sequence Number        |
140	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
141	   |                           Timestamp                           |
142	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
143	   |           Synchronization Source (SSRC) Identifier            |
144	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
145	   |           Reserved            |             Length            |
146	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
147	   |                       User Data Words...
148	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

150	   Figure 1: RTP Payload Format for TTML

152	4.1.  RTP Header Usage

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

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

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

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

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

178	   User Data Words: The length of User Data Words MUST match the value
179	   specified in the Length field
180	      User Data Words contains the text of the whole document being
181	      transmitted or a part of the document being transmitted.
182	      Documents using character encodings where characters are not
183	      represented by a single byte MUST be serialized in big endian
184	      order, a.k.a. network byte order.  When the document spans more
185	      than one RTP packet, the entire document is obtained by
186	      concatenating User Data Words from each contributing packet in
187	      ascending order of Sequence Number.  Note that the length of each
188	      data word depends on the character encoding used: for UTF-8 a word
189	      is 8 bits, while it is 16 bits for UTF-16.

191	4.2.  Payload Data

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

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

201	4.2.1.  TTML Profile for RTP Carriage

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

206	4.2.1.1.  Payload content restrictions

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

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

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

217	   <?xml version="1.0" encoding="UTF-8"?>
218	   <profile xmlns="http://www.w3.org/ns/ttml#parameter"
219	       xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
220	       xmlns:tt="http://www.w3.org/ns/ttml"
221	       type="content"
222	       designator="urn:ietf:rfc:XXXX#content"
223	       combine="mostRestrictive">
224	       <features xml:base="http://www.w3.org/ns/ttml/feature/">
225	           <tt:metadata>
226	               <ttm:desc>
227	                   This document is a minimal TTML2 content profile
228	                   definition document intended to express the minimal
229	                   requirements to apply when carrying TTML over RTP.
230	               </ttm:desc>
231	           </tt:metadata>
232	           <feature value="required">#timeBase-media</feature>
233	           <feature value="prohibited">#timeBase-smpte</feature>
234	           <feature value="prohibited">#timeBase-clock</feature>
235	       </features>
236	   </profile>

238	4.2.1.2.  Payload processing requirements

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

244	   Each TTML document becomes active at the epoch E.  E MUST be set to
245	   the RTP Timestamp in the header of the RTP packet carrying the TTML
246	   document.  Computed TTML media times are offset relative to E.

248	   When processing a sequence of TTML documents each delivered in the
249	   same RTP stream, exactly zero or one document SHALL be considered
250	   active at each moment in the RTP time line.  In the event that a
251	   document D_(n-1) with E_(n-1) is active, and document D_(n) is
252	   delivered with E_(n) where E_(n-1) < E_(n), processing of D_(n-1)
253	   MUST be stopped at E_(n) and processing of D_(n) MUST begin.

255	   When all defined content within a document has ended then processing
256	   of the document MAY be stopped.  This can be tested by constructing
257	   the intermediate synchronic document sequence from the document, as
258	   defined by TTML2.  If the last intermediate synchronic document in
259	   the sequence is both active and contains no region elements, then all
260	   defined content within the document has ended.

262	4.2.1.2.1.  TTML Processor profile

264	4.2.1.2.1.1.  Feature extension designation

266	   This specification defines the following TTML feature extension
267	   designation:

269	   o  urn:ietf:rfc:XXXX#rtp-relative-media-time

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

273	   A TTML content processor supports the "#rtp-relative-media-time"
274	   feature extension if it processes media times in accordance with the
275	   payload processing requirements specified in this document, i.e. that
276	   the epoch E is set to the time equivalent to the RTP Timestamp as
277	   detailed above in Section 4.2.1.2.

279	4.2.1.2.1.2.  Processor profile document

281	   The required syntax and semantics declared in the following minimal
282	   TTML2 processor profile MUST be supported by the receiver, as
283	   signified by those "feature" or "extension" elements whose "value"
284	   attribute is set to "required":

286	   <?xml version="1.0" encoding="UTF-8"?>
287	   <profile xmlns="http://www.w3.org/ns/ttml#parameter"
288	       xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
289	       xmlns:tt="http://www.w3.org/ns/ttml"
290	       type="processor"
291	       designator="urn:ietf:rfc:XXXX#processor"
292	       combine="mostRestrictive">
293	       <features xml:base="http://www.w3.org/ns/ttml/feature/">
294	           <tt:metadata>
295	               <ttm:desc>
296	                   This document is a minimal TTML2 processor profile
297	                   definition document intended to express the minimal
298	                   requirements of a TTML processor able to process TTML
299	                   delivered over RTP according to RFC XXXX.
300	               </ttm:desc>
301	           </tt:metadata>
302	           <feature value="required">#timeBase-media</feature>
303	           <feature value="optional">#profile-full-version-2</feature>
304	       </features>
305	       <extensions xml:base="urn:ietf:rfc:XXXX">
306	           <extension restricts="#timeBase-media" value="required">
307	               #rtp-relative-media-time
308	           </extension>
309	       </extensions>
310	   </profile>

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

316	4.2.1.2.1.3.  Processor profile signalling

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

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

338	5.  Payload Examples

340	   The following is an example of a valid TTML document that may be
341	   carried using the payload format described in this document:

343	   <?xml version="1.0" encoding="UTF-8"?>
344	   <tt xml:lang="en"
345	    xmlns="http://www.w3.org/ns/ttml"
346	    xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
347	    xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
348	    xmlns:tts="http://www.w3.org/ns/ttml#styling"
349	    ttp:timeBase="media"
350	    >
351	     <head>
352	       <metadata>
353	         <ttm:title>Timed Text TTML Example</ttm:title>
354	         <ttm:copyright>The Authors (c) 2006</ttm:copyright>
355	       </metadata>
356	       <styling>
357	         <!-- s1 specifies default color, font, and text alignment -->
358	         <style xml:id="s1"
359	           tts:color="white"
360	           tts:fontFamily="proportionalSansSerif"
361	           tts:fontSize="100%"
362	           tts:textAlign="center"
363	         />
364	       </styling>
365	       <layout>
366	         <region xml:id="subtitleArea"
367	           style="s1"
368	           tts:extent="78% 11%"
369	           tts:padding="1% 5%"
370	           tts:backgroundColor="black"
371	           tts:displayAlign="after"
372	         />
373	       </layout>
374	     </head>
375	     <body region="subtitleArea">
376	       <div>
377	         <p xml:id="subtitle1" dur="5.0s" style="s1">
378	           How truly delightful!
379	         </p>
380	       </div>
381	     </body>
382	   </tt>

384	6.  Congestion Control Considerations

386	   Congestion control for RTP SHALL be used in accordance with
387	   [RFC3550], and with any applicable RTP profile: e.g., [RFC3551].
388	   Circuit Breakers [RFC8083] is an update to RTP [RFC3550] that defines
389	   criteria for when one is required to stop sending RTP Packet Streams.
390	   Applications implementing this standard MUST comply with [RFC8083]
391	   with particular attention paid to Section 4.4 on Media Usability.
392	   [RFC8085] provides additional information on the best practices for
393	   applying congestion control to UDP streams.

395	7.  Payload Format Parameters

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

401	7.1.  Clock Rate

403	   The default clock rate for TTML over RTP is 1000Hz.  The clock rate
404	   SHOULD be included in any advertisements of the RTP stream where
405	   possible.  This parameter has not been added to the media type
406	   definition as it is not applicable to TTML usage other than within
407	   RTP streams.  In other contexts, timing is defined within the TTML
408	   document.

410	   When choosing a clock rate, implementers should consider what other
411	   media their TTML streams may be used in conjunction with (e.g. video
412	   or audio).  It may be appropriate to use the same Synchronization
413	   Source and Clock Rate as the related media.  As TTML streams may be
414	   aperiodic, implementers should also consider the frequency range over
415	   which they expect packets to be sent and the temporal resolution
416	   required.

418	7.2.  Mapping to SDP

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

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

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

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

430	   Additional format specific parameters as described in the media type
431	   specification SHALL be included in the SDP file in "a=fmtp" as a
432	   semicolon separated list of "parameter=value" pairs as described in
433	   [RFC4855].  The "codecs" parameter MUST be included in the "a=fmtp"
434	   line of the SDP file.  Specific requirements for the "codecs"
435	   parameter are included in Section 4.2.1.2.1.3.

437	7.2.1.  Examples

439	   A sample SDP mapping is as follows:

441	   m=application 30000 RTP/AVP 112
442	   a=rtpmap:112 ttml+xml/90000
443	   a=fmtp:112 charset=utf-8;codecs=im1t

445	   In this example, a dynamic payload type 112 is used.  The 90 kHz RTP
446	   timestamp rate is specified in the "a=rtpmap" line after the subtype.
447	   The codecs parameter defined in the "a=fmtp" line indicates that the
448	   TTML data conforms to IMSC 1 Text profile.

450	8.  IANA Considerations

452	   No IANA action.

454	9.  Security Considerations

456	   RTP packets using the payload format defined in this specification
457	   are subject to the security considerations discussed in the RTP
458	   specification [RFC3550] , and in any applicable RTP profile such as
459	   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
460	   SAVPF [RFC5124].  However, as "Securing the RTP Protocol Framework:
461	   Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202]
462	   discusses, it is not an RTP payload format's responsibility to
463	   discuss or mandate what solutions are used to meet the basic security
464	   goals like confidentiality, integrity, and source authenticity for
465	   RTP in general.  This responsibility lays on anyone using RTP in an
466	   application.  They can find guidance on available security mechanisms
467	   and important considerations in "Options for Securing RTP Sessions"
468	   [RFC7201].  Applications SHOULD use one or more appropriate strong
469	   security mechanisms.  The rest of this Security Considerations
470	   section discusses the security impacting properties of the payload
471	   format itself.

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

477	   This payload format places no specific restrictions on the size of
478	   TTML documents that may be transmitted.  As such, malicious
479	   implementations could be used to perform denial-of-service (DoS)
480	   attacks.  RFC 4732 [RFC4732] provides more information on DoS attacks
481	   and describes some mitigation strategies.  Implementers should take
482	   into consideration that the size and frequency of documents
483	   transmitted using this format may vary over time.  As such, sender
484	   implementations should avoid producing streams that exhibit DoS-like
485	   behaviour and receivers should avoid false identification of a
486	   legitimate stream as malicious.

488	   As with other XML types and as noted in RFC 7303 [RFC7303], XML Media
489	   Types, Section 10, repeated expansion of maliciously constructed XML
490	   entities can be used to consume large amounts of memory, which may
491	   cause XML processors in constrained environments to fail.

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

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

509	   Further information related to privacy and security at a document
510	   level can be found in TTML 2 Appendix P [TTML2].

512	10.  Acknowledgements

514	   Thanks to Nigel Megitt, James Gruessing, Robert Wadge, Andrew Bonney,
515	   James Weaver, John Fletcher, Frans De jong, and Willem Vermost for
516	   their valuable feedback throughout the development of this document.
517	   Thanks to the W3C Timed Text Working Group and EBU Timed Text working
518	   group for their substantial efforts in developing the timed text
519	   formats this payload format is intended to carry.

521	11.  References

523	11.1.  Normative References

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

529	   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
530	              Requirement Levels", BCP 14, RFC 2119,
531	              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
532	              editor.org/info/rfc2119>.

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

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

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

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

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

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

560	   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
561	              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
562	              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

564	   [TECH3370]
565	              European Broadcasting Union, "TECH 3370 - EBU-TT PART 3:
566	              LIVE CONTRIBUTION", May 2017,
567	              <https://tech.ebu.ch/publications/tech3370>.

569	   [TTML-MTPR]
570	              W3C - Timed Text Working Group, "TTML Media Type
571	              Definition and Profile Registry", January 2017,
572	              <https://www.w3.org/TR/ttml-profile-registry/>.

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

578	11.2.  Informative References

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

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

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

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

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

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

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

615	   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
616	              Real-time Transport Control Protocol (RTCP)-Based Feedback
617	              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
618	              2008, <https://www.rfc-editor.org/info/rfc5124>.

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

624	   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
625	              Framework: Why RTP Does Not Mandate a Single Media
626	              Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
627	              2014, <https://www.rfc-editor.org/info/rfc7202>.

629	Appendix A.  RFC Editor Considerations

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

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

637	Author's Address

639	   James Sandford
640	   British Broadcasting Corporation
641	   Dock House, MediaCityUK
642	   Salford
643	   United Kingdom

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