RTP Payload for TTML Timed Text
draft-sandford-payload-rtp-ttml-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Author | James Sandford | ||
| Last updated | 2019-02-04 (Latest revision 2019-01-24) | ||
| Replaced by | draft-ietf-payload-rtp-ttml, RFC 8759 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-sandford-payload-rtp-ttml-02
A/V Transport Payloads Workgroup J. Sandford
Internet-Draft British Broadcasting Corporation
Intended status: Informational February 4, 2019
Expires: August 8, 2019
RTP Payload for TTML Timed Text
draft-sandford-payload-rtp-ttml-02
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 August 8, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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
4. Payload Format . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 4
4.2. Payload Data . . . . . . . . . . . . . . . . . . . . . . 4
4.2.1. TTML Profile for RTP Carriage . . . . . . . . . . . . 5
5. Payload Examples . . . . . . . . . . . . . . . . . . . . . . 8
6. Congestion Control Considerations . . . . . . . . . . . . . . 9
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 10
7.1. Media Type Definition . . . . . . . . . . . . . . . . . . 10
7.2. Clock Rate . . . . . . . . . . . . . . . . . . . . . . . 14
7.3. Mapping to SDP . . . . . . . . . . . . . . . . . . . . . 14
7.3.1. Examples . . . . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. RFC Editor Considerations . . . . . . . . . . . . . 18
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
TTML (Timed Text Markup Language)[TTML] is a media type for
describing timed text such as closed captions (also known as
subtitles) in television workflows or broadcasts as XML. This
document specifies how TTML should be mapped into an RTP stream in
live workflows including, but not restricted to, those described in
the television broadcast oriented EBU-TT Part 3[TECH3370]
specification.
2. Conventions, Definitions, and Abbreviations
Unless otherwise stated, the term "document" is used in this draft to
refer to the TTML document being transmitted in the payload of the
RTP packet(s).
Where the term "word" is used in this draft, it is to refer to byte
aligned or 32-bit aligned words of data in a computing sense and not
to refer to linguistic words that might appear in the transported
text.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD 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.
4. Payload Format
In addition to the required RTP headers, the payload contains a
section for the TTML document being transmitted (User Data Words),
and a field for the Length of that data. Each RTP payload contains
one or part of one TTML document.
A representation of the payload format for TTML is Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Source (SSRC) Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User Data Words...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP Payload Format for TTML
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4.1. RTP Header Usage
RTP packet header fields SHALL be interpreted as per RFC 3550
[RFC3550], with the following specifics:
Marker Bit (M): 1 bit
The Marker Bit is set to "1" to indicate the last packet of a
document. Otherwise set to "0". Note: The first packet might
also be the last.
Timestamp: 32 bits
The RTP Timestamp encodes the time of the text in the packet. The
clock frequency used is dependent on the application and is
specified in the media type rate parameter as per Section 7.2.
Documents spread across multiple packets MUST use the same
timestamp but different consecutive Sequence Numbers. Sequential
documents MUST NOT use the same timestamp. Because packets do not
represent any constant duration, the timestamp cannot be used to
directly infer packet loss.
Reserved: 16 bits
These bits are reserved for future use and MUST be set to 0x0.
Length: 16 bits
The length of User Data Words in bytes.
User Data Words: integer number of words whose length is defined by
the character encoding
User Data Words contains the text of the whole document being
transmitted or a part of the document being transmitted.
Documents using character encodings where characters are not
represented by a single byte MUST be serialized in big endian
order, a.k.a. network byte order. When the document spans more
than one RTP packet, the entire document is obtained by
concatenating User Data Words from each contributing packet in
ascending order of Sequence Number.
4.2. Payload Data
Documents carried in User Data Words are encoded in accordance with
one of the defined TTML profiles specified in its registry [TTML].
These profiles specify the document structure used, systems models,
timing, and other considerations.
Additionally, documents carried over RTP MUST conform to the
following profile.
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4.2.1. TTML Profile for RTP Carriage
This section defines constraints on the content and processing of the
TTML payload for RTP carriage.
4.2.1.1. Payload content restrictions
Multiple TTML subtitle streams MUST NOT be interleaved in a single
RTP stream.
The TTML document instance MUST use the "media" value of the
"ttp:timeBase" parameter attribute on the root element.
This is equivalent to the following TTML2 content profile definition
document:
<?xml version="1.0" encoding="UTF-8"?>
<profile xmlns="http://www.w3.org/ns/ttml#parameter"
xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
xmlns:tt="http://www.w3.org/ns/ttml"
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>
4.2.1.2. Payload processing requirements
If the TTML document payload is assessed to be invalid then it MUST
be discarded. When processing a valid document, the following
requirements apply.
The epoch E relative to which computed TTML media times are offset
MUST be set to the RTP Timestamp in the header of the RTP packet in
which the TTML document instance is carried.
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When processing a sequence of TTML documents each delivered in the
same RTP stream, exactly zero or one document SHALL be considered
active at each moment in the RTP time line.
Each TTML document becomes active at E. In the event that a document
D_(n-1) with E_(n-1) is active, and document D_(n) is delivered with
E_(n) where E_(n-1) < E_(n), processing of D_(n-1) MUST be stopped at
E_(n) and processing of D_(n) MUST begin.
When all defined content within a document has ended, i.e. the active
intermediate synchronic document contains no content, then processing
of the document MAY be stopped.
4.2.1.2.1. TTML Processor profile
4.2.1.2.1.1. Feature extension designation
This specification defines the following TTML feature extension
designation:
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 4.2.1.2.
4.2.1.2.1.2. 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="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>
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.
4.2.1.2.1.3. Processor profile signalling
If signalling this processor profile in the "codecs" parameter of the
media type, the registered short code for the processor profile
SHOULD be combined with each other applicable processor profile using
the "+" operator.
Note that short codes for TTML profiles are registered at [TTML].
4.2.1.2.2. EBU-TT Live considerations
EBU-TT Live is a profile of TTML intended to support live
contribution of TTML documents as a stream independently of the
carriage mechanism. When EBU-TT Live documents are carried in an RTP
stream, or when the TTML documents being transferred over RTP use
EBU-TT Live semantics, the following considerations apply:
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E is considered to be the Availability Time as defined by EBU-TT
Live. It is an error if two documents are delivered such that
E_(n-1) < E_(n) and the "ebuttp:sequenceNumber" of E_(n-1) is greater
than the "ebuttp:sequenceNumber" of E_(n). Every EBU-TT Live
document in a single RTP stream MUST have a
"ebuttp:sequenceIdentifier" with the same value.
5. Payload Examples
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"
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>
6. Congestion Control Considerations
Congestion control for RTP SHALL be used in accordance with RFC 3550
[RFC3550], and with any applicable RTP profile: e.g., RFC 3551
[RFC3551]. An additional requirement if best-effort service is being
used is users of this payload format MUST monitor packet loss to
ensure that the packet loss rate is within acceptable parameters.
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Circuit Breakers [RFC8083] is an update to RTP [RFC3550] that defines
criteria for when one is required to stop sending RTP Packet Streams
and applications implementing this standard MUST comply with it. RFC
8085 [RFC8083] 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. The existing Media Type definition for
application/ttml+xml has been included below, taken from [TTML].
7.1. Media Type Definition
Type name:
application
Subtype name:
ttml+xml
Required parameters:
None.
Optional parameters:
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charset:
If specified, the charset parameter must match the XML
encoding declaration, or if absent, the actual encoding. See also
Encoding Considerations below.
profile:
The document profile of a TTMLDocument Instance may be
specified using an optional profile parameter, which, if specified,
the value of which must adhere to the syntax and semantics of
ttp:profile parameter defined by TTML 1.0 Second Edition, Section
6.2.8 ttp:profile of the published specification.
codecs:
The optional codecs parameter provides a short form version
of the profile parameter with multiple-profile combinatorial
capability. If a short (4-character) form of a profile is
registered in the TTML Profile Registry, it is recommended that
this codecs parameter be used and not the profile parameter. The
nominal value of this parameter is a single 4 character code from
the registry.
Additionally, applications using the entries in the registry are
encouraged to adopt the following combination syntax:
Employ two combination operators, '+' (AND) and '|' (OR), which may
be used to specify, respectively, that multiple processor profiles
apply (simultaneously) or that any processor profile of a list of
profiles may apply individually. If both operators are used in a
codecs value, then the '+' operator has precedence.
The example: "A+B|C+D|E" states that a TTML processor that
implements any one of A+B or C+D or E processor profiles satisfies,
at first order, the requirements to fetch and begin
decode/processing of a TTML document, where X+Y means that both X
and Y processor profiles must be supported, and X|Y means that
either X or Y processor profile must be supported.
For more information about processor profile combination, see TTML2
Profile Combination.
Encoding considerations:
Same for application/xml, except constrained to either UTF-8 or
UTF-16. See IETF RFC 7303, XML Media Types, Section 3.2. For the
purpose of filling out the IANA Application for Media Type
(http://www.iana.org/cgi-bin/mediatypes.pl), the value binary
applies.
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Security considerations:
As with other XML types and as noted in IETF RFC 7303, 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
registration document.
Although not prohibited, there are no expectations that XML
signatures or encryption would normally be employed.
Interoperability considerations:
The published specification describes processing semantics that
dictate behavior that must be followed when dealing with, among
other things, unrecognized elements and attributes, both in TTML
namespaces and in other namespaces.
Because TTML is extensible, conformant "application/ttml+xml"
processors may expect (and enforce) that content received is
well-formed XML, but it cannot be guaranteed that the content is
valid to a particular DTD or Schema or that the processor will
recognize all of the elements and attributes in the document.
Published specification:
This media type registration is extracted from the TTML Profile
Registry.
Applications that use this media type:
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TTML is used in the television industry for the purpose of
authoring, transcoding and exchanging timed text information and
for delivering captions, subtitles, and other metadata for
television material repurposed for the Web or, more generally,
the Internet.
There is partial and full support of TTML in components used by
several Web browsers plugins, and in a number of caption authoring
tools.
Additional information:
Magic number(s):
File extension(s):
.ttml
Macintosh file type code(s):
"TTML"
Fragment identifiers:
For documents labeled as application/ttml+xml, the fragment
identifier notation is intended to be used with xml:id attributes,
as described in section 7.2.1 of the Timed Text Markup Language 1
(TTML1) specification.
Person & email address to contact for further information:
Timed Text Working Group (public-tt@w3.org)
Intended usage:
COMMON
Restrictions on usage:
None
Author:
The published specification is a work product of the World Wide Web
Consortium's Timed Text (TT) Working Group.
Change controller:
The W3C has change control over this specification.
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7.2. 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). 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
required.
7.3. Mapping to SDP
The mapping of the above defined payload format media type and its
parameters SHALL be done according to Section 3 of RFC 4855
[RFC4855].
o The type name "application" goes in SDP "m=" as the media name.
o The media subtype "ttml+xml" goes in SDP "a=rtpmap" as the
encoding name,
o The clock rate also goes in "a=rtpmap" as the clock rate.
Additional format specific parameters as described in the media type
specification SHALL be included in the SDP file in "a=fmtp" as a
semicolon separated list of "parameter=value" pairs as described in
[RFC4855].
7.3.1. 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=fmpt" line indicates that the
TTML data conforms to IMSC 1 Text profile.
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8. IANA Considerations
This document makes use of the media type application/ttml+xml. The
media types registry SHOULD be updated to make reference to this
document for the application/ttml+xml media type.
9. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550] , and 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
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
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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. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
<https://www.rfc-editor.org/info/rfc4103>.
[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>.
[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>.
[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>.
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[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[TECH3370]
"TECH 3370 - EBU-TT PART 3: LIVE CONTRIBUTION", May 2017.
[TTML] "TTML Media Type Definition and Profile Registry", January
2017.
[TTML2] "Timed Text Markup Language 2 (TTML2)", November 2018.
10.2. Informative References
[RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
DOI 10.17487/RFC2648, August 1999,
<https://www.rfc-editor.org/info/rfc2648>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<https://www.rfc-editor.org/info/rfc3551>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC4396] Rey, J. and Y. Matsui, "RTP Payload Format for 3rd
Generation Partnership Project (3GPP) Timed Text",
RFC 4396, DOI 10.17487/RFC4396, February 2006,
<https://www.rfc-editor.org/info/rfc4396>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/info/rfc4585>.
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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>.
[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>.
Appendix A. RFC Editor Considerations
*TODO* To be filled
Appendix B. Acknowledgements
*TODO*
Author's Address
James Sandford
British Broadcasting Corporation
Dock House, MediaCityUK
Salford
United Kingdom
Phone: +44 30304 09549
Email: james.sandford@bbc.co.uk
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