draft-ietf-payload-rtp-ttml-02.txt   rfc8759.txt 
A/V Transport Payloads Workgroup J. Sandford Internet Engineering Task Force (IETF) J. Sandford
Internet-Draft British Broadcasting Corporation Request for Comments: 8759 British Broadcasting Corporation
Intended status: Standards Track June 11, 2019 Category: Standards Track March 2020
Expires: December 13, 2019 ISSN: 2070-1721
RTP Payload for TTML Timed Text RTP Payload for Timed Text Markup Language (TTML)
draft-ietf-payload-rtp-ttml-02
Abstract Abstract
This memo describes a Real-time Transport Protocol (RTP) payload This memo describes a Real-time Transport Protocol (RTP) payload
format for TTML, an XML based timed text format for live and file format for Timed Text Markup Language (TTML), an XML-based timed text
based workflows from W3C. This payload format is specifically format from W3C. This payload format is specifically targeted at
targeted at live workflows using TTML. streaming workflows using TTML.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79.
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Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on December 13, 2019. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8759.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
2. Conventions, Definitions, and Abbreviations . . . . . . . . . 2 2. Conventions and Definitions
3. Media Format Description . . . . . . . . . . . . . . . . . . 3 3. Media Format Description
3.1. Relation to Other Text Payload Types . . . . . . . . . . 3 3.1. Relation to Other Text Payload Types
3.2. TTML2 . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. TTML2
4. Payload Format . . . . . . . . . . . . . . . . . . . . . . . 3 4. Payload Format
4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 4 4.1. RTP Header Usage
4.2. Payload Data . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Payload Data
4.2.1. TTML Profile for RTP Carriage . . . . . . . . . . . . 5 5. Payload Content Restrictions
5. Payload Examples . . . . . . . . . . . . . . . . . . . . . . 9 6. Payload Processing Requirements
6. Congestion Control Considerations . . . . . . . . . . . . . . 10 6.1. TTML Processor Profile
7. Payload Format Parameters . . . . . . . . . . . . . . . . . . 11 6.1.1. Feature Extension Designation
7.1. Clock Rate . . . . . . . . . . . . . . . . . . . . . . . 11 6.1.2. Processor Profile Document
7.2. Mapping to SDP . . . . . . . . . . . . . . . . . . . . . 11 6.1.3. Processor Profile Signalling
7.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . 12 7. Payload Examples
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. Fragmentation of TTML Documents
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. Protection against Loss of Data
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 10. Congestion Control Considerations
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 11. Payload Format Parameters
11.1. Normative References . . . . . . . . . . . . . . . . . . 13 11.1. Clock Rate
11.2. Informative References . . . . . . . . . . . . . . . . . 14 11.2. Session Description Protocol (SDP) Considerations
Appendix A. RFC Editor Considerations . . . . . . . . . . . . . 16 11.2.1. Examples
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 12. IANA Considerations
13. Security Considerations
14. Normative References
15. Informative References
Acknowledgements
Author's Address
1. Introduction 1. Introduction
TTML (Timed Text Markup Language)[TTML2] is a media type for TTML (Timed Text Markup Language) [TTML2] is a media type for
describing timed text such as closed captions (also known as describing timed text, such as closed captions and subtitles in
subtitles) in television workflows or broadcasts as XML. This television workflows or broadcasts, as XML. This document specifies
document specifies how TTML should be mapped into an RTP stream in how TTML should be mapped into an RTP stream in streaming workflows,
live workflows including, but not restricted to, those described in including (but not restricted to) those described in the television-
the television broadcast oriented EBU-TT Part 3[TECH3370] broadcast-oriented European Broadcasting Union Timed Text (EBU-TT)
specification. This document does not define a media type for TTML Part 3 [TECH3370] specification. This document does not define a
but makes use of the existing application/ttml+xml media type media type for TTML but makes use of the existing application/
[TTML-MTPR]. ttml+xml media type [TTML-MTPR].
2. Conventions, Definitions, and Abbreviations 2. Conventions and Definitions
Unless otherwise stated, the term "document" is used in this draft to Unless otherwise stated, the term "document" refers to the TTML
refer to the TTML document being transmitted in the payload of the document being transmitted in the payload of the RTP packet(s).
RTP packet(s).
Where the term "word" is used in this draft, it is to refer to byte The term "word" refers to a data word aligned to a specified number
aligned or 32-bit aligned words of data in a computing sense and not of bits in a computing sense and not to linguistic words that might
to refer to linguistic words that might appear in the transported appear in the transported text.
text.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in BCP14 [RFC2119] "OPTIONAL" in this document are to be interpreted as described in
[RFC8174] when, and only when, they appear in all capitals, as shown BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
here. capitals, as shown here.
3. Media Format Description 3. Media Format Description
3.1. Relation to Other Text Payload Types 3.1. Relation to Other Text Payload Types
Prior payload types for text are not suited to the carriage of closed Prior payload types for text are not suited to the carriage of closed
captions in Television Workflows. RFC 4103 for Text Conversation captions in television workflows. "RTP Payload for Text
[RFC4103] is intended for low data rate conversation with its own Conversation" [RFC4103] is intended for low data rate conversation
session management and minimal formatting capabilities. RFC 4734 with its own session management and minimal formatting capabilities.
Events for Modem, Fax, and Text Telephony Signals [RFC4734] deals in "Definition of Events for Modem, Fax, and Text Telephony Signals"
large parts with the control signalling of facsimile and other [RFC4734] deals in large parts with the control signalling of
systems. RFC 4396 for 3rd Generation Partnership Project (3GPP) facsimile and other systems. "RTP Payload Format for 3rd Generation
Timed Text [RFC4396] describes the carriage of a timed text format Partnership Project (3GPP) Timed Text" [RFC4396] describes the
with much more restricted formatting capabilities than TTML. The carriage of a timed text format with much more restricted formatting
lack of an existing format for TTML or generic XML has necessitated capabilities than TTML. The lack of an existing format for TTML or
the creation of this payload format. generic XML has necessitated the creation of this payload format.
3.2. TTML2 3.2. TTML2
TTML2 (Timed Text Markup Language, Version 2)[TTML2] is an XML-based TTML2 (Timed Text Markup Language, Version 2) [TTML2] is an XML-based
markup language for describing textual information with associated markup language for describing textual information with associated
timing metadata. One of its primary use cases is the description of timing metadata. One of its primary use cases is the description of
subtitles and closed captions. A number of profiles exist that adapt subtitles and closed captions. A number of profiles exist that adapt
TTML2 for use in specific contexts [TTML-MTPR]. These include both TTML2 for use in specific contexts [TTML-MTPR]. These include both
file based and streaming workflows. file-based and streaming workflows.
4. Payload Format 4. Payload Format
In addition to the required RTP headers, the payload contains a In addition to the required RTP headers, the payload contains a
section for the TTML document being transmitted (User Data Words), section for the TTML document being transmitted (User Data Words) and
and a field for the Length of that data. Each RTP payload contains a field for the length of that data. Each RTP payload contains one
one or part of one TTML document. or part of one TTML document.
A representation of the payload format for TTML is Figure 1. A representation of the payload format for TTML is Figure 1.
0 1 2 3 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 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 | |V=2|P|X| CC |M| PT | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp | | Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Source (SSRC) Identifier | | Synchronization Source (SSRC) Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Length | | Reserved | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User Data Words... | User Data Words...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP Payload Format for TTML Figure 1: RTP Payload Format for TTML
4.1. RTP Header Usage 4.1. RTP Header Usage
RTP packet header fields SHALL be interpreted as per RFC 3550 RTP packet header fields SHALL be interpreted, as per [RFC3550], with
[RFC3550], with the following specifics: the following specifics:
Marker Bit (M): 1 bit Marker Bit (M): 1 bit
The Marker Bit is set to "1" to indicate the last packet of a The marker bit is set to "1" to indicate the last packet of a
document. Otherwise set to "0". Note: The first packet might document. Otherwise, set to "0". Note: The first packet might
also be the last. also be the last.
Timestamp: 32 bits Timestamp: 32 bits
The RTP Timestamp encodes the time of the text in the packet. The The RTP Timestamp encodes the epoch of the TTML document in User
clock frequency used is dependent on the application and is Data Words. Further detail on its usage may be found in
specified in the media type rate parameter as per Section 7.1. Section 6. The clock frequency used is dependent on the
Documents spread across multiple packets MUST use the same application and is specified in the media type rate parameter, as
timestamp but different consecutive Sequence Numbers. Sequential per Section 11.1. Documents spread across multiple packets MUST
documents MUST NOT use the same timestamp. Because packets do not use the same timestamp but different consecutive Sequence Numbers.
represent any constant duration, the timestamp cannot be used to Sequential documents MUST NOT use the same timestamp. Because
directly infer packet loss. packets do not represent any constant duration, the timestamp
cannot be used to directly infer packet loss.
Reserved: 16 bits Reserved: 16 bits
These bits are reserved for future use and MUST be set to 0x0. These bits are reserved for future use and MUST be set to 0x0 and
ignored upon reception.
Length: 16 bits Length: 16 bits
The length of User Data Words in bytes. The length of User Data Words in bytes.
User Data Words: integer number of data words User Data Words: The length of User Data Words MUST match the
User Data Words contains the text of the whole document being value specified in the Length field
transmitted or a part of the document being transmitted. The User Data Words section contains the text of the whole
Documents using character encodings where characters are not document being transmitted or a part of the document being
represented by a single byte MUST be serialized in big endian transmitted. Documents using character encodings where characters
order, a.k.a. network byte order. When the document spans more are not represented by a single byte MUST be serialised in big-
than one RTP packet, the entire document is obtained by endian order, a.k.a., network byte order. Where a document will
concatenating User Data Words from each contributing packet in not fit within the Path MTU, it may be fragmented across multiple
ascending order of Sequence Number. Note that the length of data packets. Further detail on fragmentation may be found in
words will depend on the character encoding used (e.g. 8 bit words Section 8.
for UTF-8, 16 bit words for UTF-16).
4.2. Payload Data 4.2. Payload Data
Documents carried in User Data Words are encoded in accordance with TTML documents define a series of changes to text over time. TTML
one of the defined TTML profiles specified in its registry documents carried in User Data Words are encoded in accordance with
[TTML-MTPR]. These profiles specify the document structure used, one or more of the defined TTML profiles specified in the TTML
systems models, timing, and other considerations. registry [TTML-MTPR]. These profiles specify the document structure
used, systems models, timing, and other considerations. TTML
Additionally, documents carried over RTP MUST conform to the profiles may restrict the complexity of the changes, and operational
following profile. requirements may limit the maximum duration of TTML documents by a
deployment configuration. Both of these cases are out of scope of
this document.
4.2.1. TTML Profile for RTP Carriage Documents carried over RTP MUST conform to the following profile, in
addition to any others used.
This section defines constraints on the content and processing of the 5. Payload Content Restrictions
TTML payload for RTP carriage.
4.2.1.1. 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 Multiple TTML subtitle streams MUST NOT be interleaved in a single
RTP stream. RTP stream.
The TTML document instance MUST use the "media" value of the The TTML document instance's root "tt" element in the
"ttp:timeBase" parameter attribute on the root element. "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 following TTML2 content profile definition This is equivalent to the TTML2 content profile definition document
document: in Figure 2.
<?xml version="1.0" encoding="UTF-8"?> <?xml version="1.0" encoding="UTF-8"?>
<profile xmlns="http://www.w3.org/ns/ttml#parameter" <profile xmlns="http://www.w3.org/ns/ttml#parameter"
xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
xmlns:tt="http://www.w3.org/ns/ttml" xmlns:tt="http://www.w3.org/ns/ttml"
type="content" type="content"
designator="urn:ietf:rfc:XXXX#content" designator="urn:ietf:rfc:8759#content"
combine="mostRestrictive"> combine="mostRestrictive">
<features xml:base="http://www.w3.org/ns/ttml/feature/"> <features xml:base="http://www.w3.org/ns/ttml/feature/">
<tt:metadata> <tt:metadata>
<ttm:desc> <ttm:desc>
This document is a minimal TTML2 content profile This document is a minimal TTML2 content profile
definition document intended to express the minimal definition document intended to express the
requirements to apply when carrying TTML over RTP. minimal requirements to apply when carrying TTML
over RTP.
</ttm:desc> </ttm:desc>
</tt:metadata> </tt:metadata>
<feature value="required">#timeBase-media</feature> <feature value="required">#timeBase-media</feature>
<feature value="prohibited">#timeBase-smpte</feature> <feature value="prohibited">#timeBase-smpte</feature>
<feature value="prohibited">#timeBase-clock</feature> <feature value="prohibited">#timeBase-clock</feature>
</features> </features>
</profile> </profile>
4.2.1.2. Payload processing requirements Figure 2: TTML2 Content Profile Definition for Documents Carried
over RTP
If the TTML document payload is assessed to be invalid then it MUST 6. Payload Processing Requirements
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 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 the RTP Timestamp in the header of the RTP packet carrying the TTML
document. Computed TTML media times are offset relative to E. 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 When processing a sequence of TTML documents, where each is delivered
same RTP stream, exactly zero or one document SHALL be considered in the same RTP stream, exactly zero or one document SHALL be
active at each moment in the RTP time line. In the event that a considered active at each moment in the RTP time line. In the event
document D_(n-1) with E_(n-1) is active, and document D_(n) is 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) 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. MUST be stopped at E_(n) and processing of D_(n) MUST begin.
When all defined content within a document has ended then processing When all defined content within a document has ended, then processing
of the document MAY be stopped. This can be tested by constructing of the document MAY be stopped. This can be tested by constructing
the intermediate synchronic document sequence from the document, as the intermediate synchronic document sequence from the document, as
defined by TTML2. If the last intermediate synchronic document in defined by [TTML2]. If the last intermediate synchronic document in
the sequence is both active and contains no region elements, then all the sequence is both active and contains no region elements, then all
defined content within the document has ended. defined content within the document has ended.
4.2.1.2.1. TTML Processor profile 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.
4.2.1.2.1.1. Feature extension designation 6.1. TTML Processor Profile
6.1.1. Feature Extension Designation
This specification defines the following TTML feature extension This specification defines the following TTML feature extension
designation: designation:
o urn:ietf:rfc:XXXX#rtp-relative-media-time "urn:ietf:rfc:8759#rtp-relative-media-time"
The namespace "urn:ietf:rfc:XXXX" is as defined by [RFC2648]. The namespace "urn:ietf:rfc:8759" is as defined by [RFC2648].
A TTML content processor supports the "#rtp-relative-media-time" A TTML content processor supports the "#rtp-relative-media-time"
feature extension if it processes media times in accordance with the feature extension if it processes media times in accordance with the
payload processing requirements specified in this document, i.e. that payload processing requirements specified in this document, i.e.,
the epoch E is set to the time equivalent to the RTP Timestamp as that the epoch E is set to the time equivalent to the RTP Timestamp,
detailed above in Section 4.2.1.2. as detailed above in Section 6.
4.2.1.2.1.2. Processor profile document 6.1.2. Processor Profile Document
The required syntax and semantics declared in the following minimal The required syntax and semantics declared in the minimal TTML2
TTML2 processor profile MUST be supported by the receiver, as processor profile in Figure 3 MUST be supported by the receiver, as
signified by those "feature" or "extension" elements whose "value" signified by those "feature" or "extension" elements whose "value"
attribute is set to "required": attribute is set to "required".
<?xml version="1.0" encoding="UTF-8"?> <?xml version="1.0" encoding="UTF-8"?>
<profile xmlns="http://www.w3.org/ns/ttml#parameter" <profile xmlns="http://www.w3.org/ns/ttml#parameter"
xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
xmlns:tt="http://www.w3.org/ns/ttml" xmlns:tt="http://www.w3.org/ns/ttml"
type="processor" type="processor"
designator="urn:ietf:rfc:XXXX#processor" designator="urn:ietf:rfc:8759#processor"
combine="mostRestrictive"> combine="mostRestrictive">
<features xml:base="http://www.w3.org/ns/ttml/feature/"> <features xml:base="http://www.w3.org/ns/ttml/feature/">
<tt:metadata> <tt:metadata>
<ttm:desc> <ttm:desc>
This document is a minimal TTML2 processor profile This document is a minimal TTML2 processor profile
definition document intended to express the minimal definition document intended to express the
requirements of a TTML processor able to process TTML minimal requirements of a TTML processor able to
delivered over RTP according to RFC XXXX. process TTML delivered over RTP according to
RFC 8759.
</ttm:desc> </ttm:desc>
</tt:metadata> </tt:metadata>
<feature value="required">#timeBase-media</feature> <feature value="required">#timeBase-media</feature>
<feature value="optional">#profile-full-version-2</feature> <feature value="optional">
#profile-full-version-2
</feature>
</features> </features>
<extensions xml:base="urn:ietf:rfc:XXXX"> <extensions xml:base="urn:ietf:rfc:8759">
<extension restricts="#timeBase-media" value="required"> <extension restricts="#timeBase-media" value="required">
#rtp-relative-media-time #rtp-relative-media-time
</extension> </extension>
</extensions> </extensions>
</profile> </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 Note that this requirement does not imply that the receiver needs to
support either TTML1 or TTML2 profile processing, i.e. the TTML2 support either TTML1 or TTML2 profile processing, i.e., the TTML2
"#profile-full-version-2" feature or any of its dependent features. "#profile-full-version-2" feature or any of its dependent features.
4.2.1.2.1.3. Processor profile signalling 6.1.3. Processor Profile Signalling
The "codecs" media type parameter MUST specify at least one processor The "codecs" media type parameter MUST specify at least one processor
profile. Short codes for TTML profiles are registered at profile. Short codes for TTML profiles are registered at
[TTML-MTPR]. The processor profiles specified in "codecs" MUST be [TTML-MTPR]. The processor profiles specified in "codecs" MUST be
compatible with the processor profile specified in this document. compatible with the processor profile specified in this document.
Where multiple options exist in "codecs" for possible processor Where multiple options exist in "codecs" for possible processor
profile combinations (i.e. separated by "|" operator), every profile combinations (i.e., separated by "|" operator), every
permitted option MUST be compatible with the processor profile permitted option MUST be compatible with the processor profile
specified in this document. Where processor profiles other than the specified in this document. Where processor profiles (other than the
one specified in this document are advertised in the "codecs" one specified in this document) are advertised in the "codecs"
parameter, the requirements of the processor profile specified in parameter, the requirements of the processor profile specified in
this document MAY be signalled additionally using the "+" operator this document MAY be signalled, additionally using the "+" operator
with its registered short code. with its registered short code.
A processor profile (X) is compatible with the processor profile in A processor profile (X) is compatible with the processor profile
this document (P) if X includes all the features and extensions in P, specified here (P) if X includes all the features and extensions in P
identified by their character content, and the "value" attribute of (identified by their character content) and the "value" attribute of
each is at least as restrictive as the "value" attribute of the each is, at least, as restrictive as the "value" attribute of the
feature or extension in P that has the same character content. The feature or extension in P that has the same character content. The
term "restrictive" here is as defined in [TTML2] Section 6. term "restrictive" here is as defined in Section 6 of [TTML2].
5. Payload Examples 7. Payload Examples
The following is an example of a valid TTML document that may be Figure 4 is an example of a valid TTML document that may be carried
carried using the payload format described in this document: using the payload format described in this document.
<?xml version="1.0" encoding="UTF-8"?> <?xml version="1.0" encoding="UTF-8"?>
<tt xml:lang="en" <tt xml:lang="en"
xmlns="http://www.w3.org/ns/ttml" xmlns="http://www.w3.org/ns/ttml"
xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttm="http://www.w3.org/ns/ttml#metadata"
xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
xmlns:tts="http://www.w3.org/ns/ttml#styling" xmlns:tts="http://www.w3.org/ns/ttml#styling"
ttp:timeBase="media" ttp:timeBase="media"
> >
<head> <head>
<metadata> <metadata>
<ttm:title>Timed Text TTML Example</ttm:title> <ttm:title>Timed Text TTML Example</ttm:title>
<ttm:copyright>The Authors (c) 2006</ttm:copyright> <ttm:copyright>The Authors (c) 2006</ttm:copyright>
</metadata> </metadata>
<styling> <styling>
<!-- s1 specifies default color, font, and text alignment --> <!--
s1 specifies default color, font, and text alignment
-->
<style xml:id="s1" <style xml:id="s1"
tts:color="white" tts:color="white"
tts:fontFamily="proportionalSansSerif" tts:fontFamily="proportionalSansSerif"
tts:fontSize="100%" tts:fontSize="100%"
tts:textAlign="center" tts:textAlign="center"
/> />
</styling> </styling>
<layout> <layout>
<region xml:id="subtitleArea" <region xml:id="subtitleArea"
style="s1" style="s1"
skipping to change at page 10, line 46 skipping to change at line 412
</head> </head>
<body region="subtitleArea"> <body region="subtitleArea">
<div> <div>
<p xml:id="subtitle1" dur="5.0s" style="s1"> <p xml:id="subtitle1" dur="5.0s" style="s1">
How truly delightful! How truly delightful!
</p> </p>
</div> </div>
</body> </body>
</tt> </tt>
6. Congestion Control Considerations Figure 4: Example TTML Document
Congestion control for RTP SHALL be used in accordance with RFC 3550 8. Fragmentation of TTML Documents
[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.
Circuit Breakers [RFC8083] is an update to RTP [RFC3550] that defines Many of the use cases for TTML are low bit-rate with RTP packets
criteria for when one is required to stop sending RTP Packet Streams expected to fit within the Path MTU. However, some documents may
and applications implementing this standard MUST comply with it. RFC exceed the Path MTU. In these cases, they may be split between
8085 [RFC8085] provides additional information on the best practices multiple packets. Where fragmentation is used, the following
for applying congestion control to UDP streams. guidelines MUST be followed:
7. Payload Format Parameters * It is RECOMMENDED that documents be fragmented as seldom as
possible, i.e., the least possible number of fragments is created
out of a document.
* Text strings MUST split at character boundaries. This enables
decoding of partial documents. As a consequence, document
fragmentation requires knowledge of the UTF-8/UTF-16 encoding
formats to determine character boundaries.
* Document fragments SHOULD be protected against packet losses.
More information can be found in Section 9.
When a document spans more than one RTP packet, the entire document
is obtained by concatenating User Data Words from each consecutive
contributing packet in ascending order of Sequence Number.
As described in Section 6, only zero or one TTML document may be
active at any point in time. As such, there MUST only be one
document transmitted for a given RTP Timestamp. Furthermore, as
stated in Section 4.1, the marker bit MUST be set for a packet
containing the last fragment of a document. A packet following one
where the marker bit is set contains the first fragment of a new
document. The first fragment might also be the last.
9. Protection against Loss of Data
Consideration must be devoted to keeping loss of documents due to
packet loss within acceptable limits. What is deemed acceptable
limits is dependent on the TTML profile(s) used and use case, among
other things. As such, specific limits are outside the scope of this
document.
Documents MAY be sent without additional protection if end-to-end
network conditions guarantee that document loss will be within
acceptable limits under all anticipated load conditions. Where such
guarantees cannot be provided, implementations MUST use a mechanism
to protect against packet loss. Potential mechanisms include Forward
Error Correction (FEC) [RFC5109], retransmission [RFC4588],
duplication [ST2022-7], or an equivalent technique.
10. Congestion Control Considerations
Congestion control for RTP SHALL be used in accordance with [RFC3550]
and with any applicable RTP profile, e.g., [RFC3551]. "Multimedia
Congestion Control: Circuit Breakers for Unicast RTP Sessions"
[RFC8083] is an update to "RTP: A Transport Protocol for Real-time
Applications" [RFC3550], which defines criteria for when one is
required to stop sending RTP packet streams. Applications
implementing this standard MUST comply with [RFC8083], with
particular attention paid to Section 4.4 on Media Usability.
[RFC8085] provides additional information on the best practices for
applying congestion control to UDP streams.
11. Payload Format Parameters
This RTP payload format is identified using the existing application/ This RTP payload format is identified using the existing application/
ttml+xml media type as registered with IANA [IANA] and defined in ttml+xml media type as registered with IANA [IANA] and defined in
[TTML-MTPR]. [TTML-MTPR].
7.1. Clock Rate 11.1. Clock Rate
The default clock rate for TTML over RTP is 1000Hz. The clock rate The default clock rate for TTML over RTP is 1000 Hz. The clock rate
SHOULD be included in any advertisements of the RTP stream where SHOULD be included in any advertisements of the RTP stream where
possible. This parameter has not been added to the media type possible. This parameter has not been added to the media type
definition as it is not applicable to TTML usage other than within definition as it is not applicable to TTML usage other than within
RTP streams. In other contexts, timing is defined within the TTML RTP streams. In other contexts, timing is defined within the TTML
document. document.
When choosing a clock rate, implementers should consider what other When choosing a clock rate, implementers should consider what other
media their TTML streams may be used in conjunction with (e.g. video media their TTML streams may be used in conjunction with (e.g., video
or audio). It may be appropriate to use the same Synchronization or audio). In these situations, it is RECOMMENDED that streams use
Source and Clock Rate as the related media. As TTML streams may be the same clock source and clock rate as the related media. As TTML
aperiodic, implementers should also consider the frequency range over streams may be aperiodic, implementers should also consider the
which they expect packets to be sent and the temporal resolution frequency range over which they expect packets to be sent and the
required. temporal resolution required.
7.2. Mapping to SDP 11.2. Session Description Protocol (SDP) Considerations
The mapping of the application/ttml+xml media type and its parameters The mapping of the application/ttml+xml media type and its parameters
[TTML-MTPR] SHALL be done according to Section 3 of RFC 4855 [TTML-MTPR] SHALL be done according to Section 3 of [RFC4855].
[RFC4855].
o The type name "application" goes in SDP "m=" as the media name. * 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 * The media subtype "ttml+xml" goes in SDP "a=rtpmap" as the
encoding name, encoding name.
o The clock rate also goes in "a=rtpmap" as the clock rate. * The clock rate also goes in "a=rtpmap" as the clock rate.
Additional format specific parameters as described in the media type Additional format-specific parameters, as described in the media type
specification SHALL be included in the SDP file in "a=fmtp" as a specification, SHALL be included in the SDP file in "a=fmtp" as a
semicolon separated list of "parameter=value" pairs as described in semicolon-separated list of "parameter=value" pairs, as described in
[RFC4855]. The "codecs" parameter MUST be included in the SDP file. [RFC4855]. The "codecs" parameter MUST be included in the "a=fmtp"
Specific requirements for the "codecs" parameter are included in line of the SDP file. Specific requirements for the "codecs"
Section 4.2.1.2.1.3. parameter are included in Section 6.1.3.
7.2.1. Examples 11.2.1. Examples
A sample SDP mapping is as follows: A sample SDP mapping is presented in Figure 5.
m=application 30000 RTP/AVP 112 m=application 30000 RTP/AVP 112
a=rtpmap:112 ttml+xml/90000 a=rtpmap:112 ttml+xml/90000
a=fmtp:112 charset=utf-8;codecs=im1t a=fmtp:112 charset=utf-8;codecs=im2t
Figure 5: Example SDP Mapping
In this example, a dynamic payload type 112 is used. The 90 kHz RTP 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. timestamp rate is specified in the "a=rtpmap" line after the subtype.
The codecs parameter defined in the "a=fmtp" line indicates that the The codecs parameter defined in the "a=fmtp" line indicates that the
TTML data conforms to IMSC 1 Text profile. TTML data conforms to Internet Media and Captions (IMSC) 1.1 Text
profile [TTML-IMSC1.1].
8. IANA Considerations 12. IANA Considerations
No IANA action. This document has no IANA actions.
9. Security Considerations 13. Security Considerations
RTP packets using the payload format defined in this specification RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP are subject to the security considerations discussed in the RTP
specification [RFC3550] , and in any applicable RTP profile such as specification [RFC3550] and in any applicable RTP profile, such as
RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/ RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/
SAVPF [RFC5124]. However, as "Securing the RTP Protocol Framework: SAVPF [RFC5124]. However, as "Securing the RTP Protocol Framework:
Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202] Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202]
discusses, it is not an RTP payload format's responsibility to discusses, it is not an RTP payload format's responsibility to
discuss or mandate what solutions are used to meet the basic security discuss or mandate what solutions are used to meet the basic security
goals like confidentiality, integrity, and source authenticity for goals (like confidentiality, integrity, and source authenticity) for
RTP in general. This responsibility lays on anyone using RTP in an RTP in general. This responsibility lays on anyone using RTP in an
application. They can find guidance on available security mechanisms application. They can find guidance on available security mechanisms
and important considerations in "Options for Securing RTP Sessions" and important considerations in "Options for Securing RTP Sessions"
[RFC7201]. Applications SHOULD use one or more appropriate strong [RFC7201]. Applications SHOULD use one or more appropriate strong
security mechanisms. The rest of this Security Considerations security mechanisms. The rest of this Security Considerations
section discusses the security impacting properties of the payload section discusses the security impacting properties of the payload
format itself. format itself.
To avoid potential buffer overflow attacks, receivers should take To avoid potential buffer overflow attacks, receivers should take
care to validate that the User Data Words in the RTP payload are of care to validate that the User Data Words in the RTP payload are of
the appropriate length (using the Length field). the appropriate length (using the Length field).
This payload format places no specific restrictions on the size of This payload format places no specific restrictions on the size of
TTML documents that may be transmitted. As such, malicious TTML documents that may be transmitted. As such, malicious
implementations could be used to perform denial-of-service (DoS) implementations could be used to perform denial-of-service (DoS)
attacks. RFC 4732 [RFC4732] provides more information on DoS attacks attacks. [RFC4732] provides more information on DoS attacks and
and describes some mitigation strategies. Implementers should take describes some mitigation strategies. Implementers should take into
into consideration that the size and frequency of documents consideration that the size and frequency of documents transmitted
transmitted using this format may vary over time. As such, sender using this format may vary over time. As such, sender
implementations should avoid producing streams that exhibit DoS-like implementations should avoid producing streams that exhibit DoS-like
behaviour and receivers should avoid false identification of a behaviour, and receivers should avoid false identification of a
legitimate stream as malicious. legitimate stream as malicious.
As with other XML types and as noted in RFC 7303 [RFC7303], XML Media As with other XML types and as noted in Section 10 of "XML Media
Types, Section 10, repeated expansion of maliciously constructed XML Types" [RFC7303], repeated expansion of maliciously constructed XML
entities can be used to consume large amounts of memory, which may entities can be used to consume large amounts of memory, which may
cause XML processors in constrained environments to fail. cause XML processors in constrained environments to fail.
In addition, because of the extensibility features for TTML and of In addition, because of the extensibility features for TTML and of
XML in general, it is possible that "application/ttml+xml" may XML in general, it is possible that "application/ttml+xml" may
describe content that has security implications beyond those describe content that has security implications beyond those
described here. However, TTML does not provide for any sort of described here. However, TTML does not provide for any sort of
active or executable content, and if the processor follows only the active or executable content, and if the processor follows only the
normative semantics of the published specification, this content will normative semantics of the published specification, this content will
be outside TTML namespaces and may be ignored. Only in the case be outside TTML namespaces and may be ignored. Only in the case
where the processor recognizes and processes the additional content, where the processor recognizes and processes the additional content
or where further processing of that content is dispatched to other or where further processing of that content is dispatched to other
processors, would security issues potentially arise. And in that processors would security issues potentially arise. And in that
case, they would fall outside the domain of this RTP payload format case, they would fall outside the domain of this RTP payload format
and the application/ttml+xml registration document. and the application/ttml+xml registration document.
Although not prohibited, there are no expectations that XML Although not prohibited, there are no expectations that XML
signatures or encryption would normally be employed. signatures or encryption would normally be employed.
Further information related to privacy and security at a document Further information related to privacy and security at a document
level can be found in TTML 2 Appendix P [TTML2]. level can be found in Appendix P of [TTML2].
10. Acknowledgements
Thanks to Nigel Megitt, James Gruessing, Robert Wadge, Andrew Bonney,
James Weaver, John Fletcher, Frans De jong, and Villem Vermost for
their valuable feedback throughout the development of this draft.
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 14. Normative References
[IANA] "IANA - Media Types - Application", February 2019, [IANA] IANA, "Media Types",
<https://www.iana.org/assignments/media-types/ <https://www.iana.org/assignments/media-types>.
media-types.xhtml#application>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>. July 2003, <https://www.rfc-editor.org/info/rfc3550>.
skipping to change at page 14, line 35 skipping to change at line 635
<https://www.rfc-editor.org/info/rfc8083>. <https://www.rfc-editor.org/info/rfc8083>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>. March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[TECH3370] [TECH3370] European Broadcasting Union, "EBU-TT, Part 3, Live
"TECH 3370 - EBU-TT PART 3: LIVE CONTRIBUTION", May 2017, Subtitling Applications: System Model and Content Profile
<https://tech.ebu.ch/publications/tech3370>. for Authoring and Contribution", EBU-TT Part 3, Tech 3370,
May 2017, <https://tech.ebu.ch/publications/tech3370>.
[TTML-MTPR] [TTML-MTPR]
"TTML Media Type Definition and Profile Registry", January Adams, G., Ed. and M. Dolan, Ed., "TTML Media Type
2017, <https://www.w3.org/TR/ttml-profile-registry/>. Definition and Profile Registry", W3C Working Group Note,
April 2019,
<https://www.w3.org/TR/ttml-profile-registry/>.
[TTML2] "Timed Text Markup Language 2 (TTML2)", November 2018, [TTML2] Adams, G., Ed. and C. Concolato, Ed., "Timed Text Markup
Language 2 (TTML2)", W3C Recommendation REC-
ttml2-20181108, November 2018,
<https://www.w3.org/TR/ttml2/>. <https://www.w3.org/TR/ttml2/>.
11.2. Informative References 15. Informative References
[RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648, [RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
DOI 10.17487/RFC2648, August 1999, DOI 10.17487/RFC2648, August 1999,
<https://www.rfc-editor.org/info/rfc2648>. <https://www.rfc-editor.org/info/rfc2648>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551, Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003, DOI 10.17487/RFC3551, July 2003,
<https://www.rfc-editor.org/info/rfc3551>. <https://www.rfc-editor.org/info/rfc3551>.
skipping to change at page 15, line 26 skipping to change at line 678
Generation Partnership Project (3GPP) Timed Text", Generation Partnership Project (3GPP) Timed Text",
RFC 4396, DOI 10.17487/RFC4396, February 2006, RFC 4396, DOI 10.17487/RFC4396, February 2006,
<https://www.rfc-editor.org/info/rfc4396>. <https://www.rfc-editor.org/info/rfc4396>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control "Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006, DOI 10.17487/RFC4585, July 2006,
<https://www.rfc-editor.org/info/rfc4585>. <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 [RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732, Denial-of-Service Considerations", RFC 4732,
DOI 10.17487/RFC4732, December 2006, DOI 10.17487/RFC4732, December 2006,
<https://www.rfc-editor.org/info/rfc4732>. <https://www.rfc-editor.org/info/rfc4732>.
[RFC4734] Schulzrinne, H. and T. Taylor, "Definition of Events for [RFC4734] Schulzrinne, H. and T. Taylor, "Definition of Events for
Modem, Fax, and Text Telephony Signals", RFC 4734, Modem, Fax, and Text Telephony Signals", RFC 4734,
DOI 10.17487/RFC4734, December 2006, DOI 10.17487/RFC4734, December 2006,
<https://www.rfc-editor.org/info/rfc4734>. <https://www.rfc-editor.org/info/rfc4734>.
[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <https://www.rfc-editor.org/info/rfc5109>.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
2008, <https://www.rfc-editor.org/info/rfc5124>. 2008, <https://www.rfc-editor.org/info/rfc5124>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014, Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<https://www.rfc-editor.org/info/rfc7201>. <https://www.rfc-editor.org/info/rfc7201>.
[RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP [RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP
Framework: Why RTP Does Not Mandate a Single Media Framework: Why RTP Does Not Mandate a Single Media
Security Solution", RFC 7202, DOI 10.17487/RFC7202, April Security Solution", RFC 7202, DOI 10.17487/RFC7202, April
2014, <https://www.rfc-editor.org/info/rfc7202>. 2014, <https://www.rfc-editor.org/info/rfc7202>.
Appendix A. RFC Editor Considerations [ST2022-7] SMPTE, "Seamless Protection Switching of RTP Datagrams",
ST 2022-7:2019, DOI 10.5594/SMPTE.ST2022-7.2019, May 2019,
<https://ieeexplore.ieee.org/document/8716822>.
Note to RFC Editor: This section may be removed after carrying out [TTML-IMSC1.1]
all the instructions of this section. Lemieux, P., Ed., "TTML Profiles for Internet Media
Subtitles and Captions 1.1", W3C Recommendation REC-ttml-
imsc1.1-20181108, November 2018,
<https://www.w3.org/TR/ttml-imsc1.1/>.
The namespace "urn:ietf:rfc:XXXX" is to be replaced with the Acknowledgements
namespace for this document once it has received an RFC number.
Thanks to Nigel Megitt, James Gruessing, Robert Wadge, Andrew Bonney,
James Weaver, John Fletcher, Frans de Jong, and Willem Vermost for
their valuable feedback throughout the development of this document.
Thanks to the W3C Timed Text Working Group and EBU Timed Text Working
Group for their substantial efforts in developing the timed text
format this payload format is intended to carry.
Author's Address Author's Address
James Sandford James Sandford
British Broadcasting Corporation British Broadcasting Corporation
Dock House, MediaCityUK Dock House, MediaCityUK
Salford Salford
United Kingdom United Kingdom
Phone: +44 30304 09549 Phone: +44 30304 09549
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