RTP Payload Format for 3rd Generation Partnership Project (3GPP) Timed Text
draft-ietf-avt-rtp-3gpp-timed-text-15
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4396.
|
|
|---|---|---|---|
| Authors | Yoshinori Matsui , Jose Rey | ||
| Last updated | 2020-01-21 (Latest revision 2005-06-12) | ||
| Replaces | draft-rey-avt-3gpp-timed-text | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4396 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Allison J. Mankin | ||
| Send notices to | csp@csperkins.org, magnus.westerlund@ericsson.com |
draft-ietf-avt-rtp-3gpp-timed-text-15
Internet Draft J. Rey
draft-ietf-avt-rtp-3gpp-timed-text-15.txt Y. Matsui
Panasonic
Expires: December 13, 2005 June 13, 2005
RTP Payload Format for 3GPP Timed Text
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Abstract
This document specifies an RTP payload format for the transmission of
3GPP (3rd Generation Partnership Project) timed text. 3GPP timed
text is a time-lined decorated text media format with defined storage
in a 3GP file. Timed Text can be synchronized with audio/video
contents and used in application such as captioning, titling and
multimedia presentations. In the following sections the problems of
streaming timed text are addressed and a payload format for streaming
3GPP timed text over RTP is specified.
IETF draft - Expires December 13, 2005 [Page 1]
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Table of Contents
1. Introduction....................................................4
2. Motivation, Requirements and Design Rationale...................4
2.1. Motivation...................................................4
2.2. Basic Components of the 3GPP Timed Text Media Format.........4
2.3. Requirements.................................................5
2.4. Limitations..................................................7
2.5. Design Rationale.............................................8
3. Terminology....................................................10
4. RTP Payload Format for 3GPP Timed Text.........................12
4.1. Payload Header Definitions..................................13
4.1.1. Common Payload Header Fields.............................14
4.1.2. TYPE 1 Header............................................16
4.1.3. TYPE 2 Header............................................19
4.1.4. TYPE 3 Header............................................22
4.1.5. TYPE 4 Header............................................23
4.1.6. TYPE 5 Header............................................23
4.2. Buffering of Sample Descriptions............................24
4.2.1. Dynamic SIDX wrap-around mechanism.......................24
4.3. Finding payload header values in 3GP files..................26
4.4. Fragmentation of Timed Text Samples.........................29
4.5. Reassembling Text Samples at the Receiver...................30
4.6. On Aggregate Payloads.......................................32
4.7. Payload Examples............................................36
4.8. Relation to RFC 3640........................................40
4.9. Relation to RFC 2793........................................41
5. Resilient Transport............................................41
6. Congestion control.............................................42
7. Scene Description..............................................43
7.1. Text Rendering Position and Composition.....................43
7.2. SMIL usage..................................................44
7.3. Finding layout values in a 3GP file.........................44
8. 3GPP Timed Text Media Type.....................................44
9. SDP usage......................................................48
9.1. Mapping to SDP..............................................48
9.2. Parameter Usage in the SDP Offer/Answer Model...............48
9.2.1. Unicast Usage............................................49
9.2.2. Multicast Usage..........................................51
9.3. Offer/Answer Examples.......................................52
9.4. Parameter Usage outside of Offer/Answer.....................54
10. IANA Considerations...........................................54
11. Security considerations.......................................54
12. References....................................................55
12.1. Normative References.......................................55
12.2. Informative References.....................................55
13. Annexes.......................................................57
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13.1. Basics of the 3GP File Structure...........................57
14. Acknowledgements..............................................58
15. Authors' Addresses............................................58
16. IPR Notices...................................................59
17. Full Copyright Statement......................................59
[Note to the RFC Editor:
- Please replace "RFCXXXX" with the RFC designation of this document
when published,
- Please substitute "draft-ietf-..." references with the
corresponding RFC number if available at the time of publication]
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1. Introduction
3GPP timed text is a media format for time-lined decorated text
specified in the 3GPP Technical Specification TS 26.245 "Transparent
end-to-end packet switched streaming service (PSS); Timed Text Format
(Release 6)" [1]. Besides plain text, the 3GPP timed text format
allows the creation of decorated text like for karaoke applications,
scrolling text for newscasts or hyperlinked text. These contents may
or may not be synchronized with other media, like audio or video.
The purpose of this draft is to provide a means to stream 3GPP timed
text contents using RTP [3]. This includes the streaming of timed
text being read out of a (3GP) file as well as the streaming of timed
text generated in real-time, a.k.a. live streaming.
Section 2 contains the motivation of this document, an overview of
the media format, the requirements and the design rationale. Section
3 defines the terminology used. Section 4 specifies the payload
headers, the fragmentation and re-assembly rules for text samples,
the rules for payload aggregation and the relations of this document
to RFC 3640 [12] and RFC 2793 [24]. Section 5 specifies some simple
schemes for resilient transport and gives pointers to other possible
mechanisms. Section 6 addresses congestion control. Section 7
specifies scene description. Section 8 defines the media type.
Section 9 specifies SDP for unicast and multicast sessions, including
usage in the Offer / Answer model [13]. Sections 10 and 11 address
IANA and security considerations. Section 12 lists references.
Annexes are included as Section 13.
2. Motivation, Requirements and Design Rationale
2.1. Motivation
The 3GPP timed text format was developed for use in the services
specified in the 3GPP Transparent End-to-end Packet-switched
Streaming Services (3GPP PSS) specification [16].
As of today, PSS allows to download 3GPP timed text contents stored
in 3GP files. However, due to the lack of a RTP payload format, it
is not possible to stream 3GPP timed text contents over RTP.
This document specifies such payload format.
2.2. Basic Components of the 3GPP Timed Text Media Format
Before going into the details of the design, it is necessary to have
knowledge about how the media format is constructed. We can identify
four differentiated functional components: layout information,
default formatting, text strings and decoration. In the following we
shortly explain these and match them to their designations in a 3GP
file:
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o Initial spatial layout information related to the text
strings: these are the height and width of the text region
where text is displayed, the position of the text region in
the display and the layer or proximity of the text to the
user. In 3GP files, this information is contained in the
Track Header Box (3GP file designations are capitalized for
clarity).
o Default settings for formatting and positioning of text:
style (font, size, colour,...), background colour, horizontal
and vertical justification, line width, scrolling, etcetera.
For 3GP files, this corresponds to the Sample Descriptions.
o The actual text strings: encoded characters using either UTF-
8 [18] or UTF-16 [19] encoding and,
o The decoration: if some characters have different style,
delay, blink, etcetera... this needs to be indicated. The
decoration is only present in the text samples if it is
actually needed. Otherwise, the default settings as above
apply. In 3GP files text strings and decoration inside the
Text Samples, i.e. Modifier Boxes are appended to the text
strings, if needed. At the time of writing this payload
format the following modifiers are specified in the 3GPP
timed text media format specification [1]:
- text highlight,
- highlight color,
- blinking text,
- karaoke feature,
- hyperlink,
- text delay,
- text style and,
- positioning of the text box and,
- text wrap indication.
2.3. Requirements
Once the basic components are known, it is necessary to define which
requirements shall the payload format fulfill:
1. It shall enable both live streaming and streaming from a 3GP
file.
Informative note: for the purpose of this document, the
term live streaming refers to those scenarios where the
timed text stream is sent from a live encoder. Upon
reception the content may or may not be stored in a 3GP
file. Typically, in live streaming applications, the
sender encapsulates the timed text content in RTP
packets following the guidelines given in this document.
At the receiving side, a buffer is used to cancel the
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network delay and delay jitter. If receiver and sender
support packet loss resilience mechanisms (see Section
5) it may also be possible to recover from packet
losses. Note that how sender and receiver actually
manage and dimension the buffers are implementation
design choices.
2. Furthermore, it shall be possible for an RTP receiver using this
payload format, and capable of storing in 3GP format, to obtain
all necessary information from the RTP packets for storing the
received text contents according to the 3GP file format. This
file may or may not be the same as the original file.
Informative note: the 3GP file format itself is based on
the ISO Base Media File Format recommendation [2].
Section 13.1 gives some insight into the 3GP file
structure. Further, Sections 4.3 and 7.3 specify where
the information needed for filling in payload headers is
found in a 3GP file. For live streaming, appropriate
values complying with the format and units described in
[1] shall be used. Where needed, clarifications on
appropriate values are given in this document.
3. It shall enable efficient and resilient transport of timed text
contents over RTP. In particular:
a. Enable the transmission of the sample descriptions both by
out-of-band and in-band means. Sample descriptions are
important information, which potentially apply to several
text samples. These default formatting settings are
typically transmitted out-of-band (reliably) once at the
initialization phase. If additional sample descriptions
are needed in the course of a session, these may be sent
also out-of-band or in-band. In-band transmission,
although unreliable, may be more appropriate for sending
sample descriptions if these should be sent frequently, as
opposed to establishing an additional communication channel
for SDP, for example. It is also useful in cases where an
out-of-band channel may not be available and for live
streaming, where contents are not known a priori. Thus,
the payload format shall enable out-of-band and in-band
transmission of sample descriptions. Section 4.1.6
specifies a payload header for transmitting sample
descriptions in-band. Section 9 specifies how sample
descriptions are mapped to SDP.
b. Enable the fragmentation of a text sample into several RTP
packets in order to cover a wide range of applications and
network environments. In general, fragmentation should be
a rare event given the low bit rates and relatively small
text sample sizes. However, the 3GPP Timed Text media
format does allow for larger text samples. Therefore, the
payload format shall take this into account and provide a
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means for coping with fragmentation and reassembly.
Section 4.3 deals with fragmentation.
c. Enable the aggregation of units into an RTP packet for
making the transport more efficient. In a mobile
communication environment a typical text sample size is
around 100-200 bytes. If the available bit rate and the
packet size allow it, units should be aggregated into one
RTP packet. Section 4.6 deals with aggregation.
d. Enable the use of resilient transport mechanisms, such as
repetition, retransmission [11] and FEC [7] (see Section
5.) For a more general discussion, refer to RFC 2354 [8],
which discusses available mechanisms for stream repair.
2.4. Limitations
The payload headers have been optimized in size for RTP. Instead
of using 32-bit (S)LEN, SDUR, SIDX header fields which would carry
many unused bits much of the time, it has been a design choice to
reduce the size of these fields. As a consequence, this payload
format has reduced maximum values with respect to sizes and
durations of (text) samples and sample descriptions. These maximum
values differ from those allowed in 3GP files, where they are
expressed using 32-bit (unsigned) integers. In some cases
extension mechanisms are provided to deal with larger values.
However, it is noted that the values used here should be enough for
the streaming applications targeted.
Following limitations apply:
1. The maximum size of text samples carried in RTP packets is
restricted to be a 16-bit (unsigned) integer (this includes the
text strings and modifiers). This means a maximum size for the
unit would be about 64 Kbytes. No extension mechanism is
provided.
2. The sample description index values are restricted to be an
(unsigned) 8-bit integer. An extension mechanism is given in
Section 4.3.
3. The text sample duration is restricted to be a 24-bit (unsigned)
integer. This yields a maximum duration at a timestamp
clockrate of 1000 Hz of about 4.6 hours. Nevertheless, an
extension mechanism is provided in Section 4.3.
4. Sample descriptions are also restricted in size: if the size
cannot be expressed as a (unsigned) 16-bit integer, the sample
description shall not be conveyed. As in the case of the sample
size, no extension mechanism is provided.
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5. A further limitation concerns the UTF-16 encodings supported:
only transport of text strings following big endian byte order
is supported. See Section 4.1.1 for details.
2.5. Design Rationale
The following design choices were made:
1. 'Unit' approach: the payload formats specified in this draft
follow a simple scheme: a 3-byte common header (Common Payload
Header) followed by a specific header for each text sample
(fragment) type. Following these headers, the text sample
contents are placed (Section 4.1.1 and following). This
structure is called a 'unit'.
The following units have been devised to comply with the
requirements mentioned in Section 2.3:
a. A TYPE 1 unit that contains one complete text sample,
b. A TYPE 2 unit that contains a complete text string or a
fragment thereof,
c. A TYPE 3 unit that contains the complete modifiers or only
the first fragment thereof,
d. A TYPE 4 unit that contains one modifier fragment other
than the first and,
e. A TYPE 5 unit that contains one sample description.
This 'unit' approach was motivated by the following reasons:
1. Allows a simple classification of the text samples and
text sample fragments that can be conveyed by the
payload format.
2. Enables easy interoperability with RFC 3640 [12].
During the development of this payload format, interest
was shown from MPEG-4 standardization participants in
developing a common payload structure for the transport
of 3GPP Timed Text. While interoperability is not
strictly necessary for this payload format to work, it
has been pursued in this payload format. Section 4.8
explains how this is done.
2. Character count is not implemented. This payload format does
detect lost text samples fragments but it does not enable an RTP
receiver to find out the exact number of text characters lost.
In fact, the fragment size included in the payload headers does
not help in finding the number of lost characters, because the
UTF-8/UTF-16 [18][19] encodings used yield a variable number of
bytes per character.
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For finding out the exact number of lost characters, an
additional field reflecting the character count (and possibly
the character offset) upon fragmentation would be required.
This would additionally require the entity performing
fragmentation to count the characters included in each text
fragment.
One benefit of having a character count would be that the
display application would be able to replace missing characters
through some other character representing character loss, e.g.:
If we take the "Some text is lost now" and assume the loss
of a packet containing the text in the middle, this could
be displayed (with a character count):
"Some ############now"
As opposed to:
"Some #now"
Which is what this payload format enables ("#" indicates a
missing character or packet, respectively).
However, it is the opinion of the authors that for applications
such as subtitling applications and multimedia presentations
that use this payload format, such partial error correction is
not worth the cost of including two additional fields, namely
character count and character offset. Instead, it is
recommended that some more overhead be invested to provide full
error correction by protecting the less text sample fragments
using the measures outlined in Section 5.
3. Fragment re-assembly: in order to re-assemble the text samples,
offset information is needed. Instead of a character or byte
offset, a single byte, TOTAL/THIS, is used. These two values
indicate the total number and current index of fragments of a
text sample. This is simpler than having a character offset
field in each fragment. Details in Section 4.1.3.
4. A length field, LEN, is present in the common header fields.
While the length in the RTP payload format is not needed by most
RTP applications (typically lower layers, like UDP, provide this
information) it does ease interoperability with RFC 3640. This
is because the Access Units (AUs) used for carriage of data in
RFC 3640 must include a length indication. Details in Section
4.8.
5. The header fields in the specific payload headers (TYPE headers
in Sections 4.1.2 to 4.1.6) have been arranged for easy
processing on 32-bit machines. For this reason the fields SIDX
and SDUR are swapped in TYPE 1 unit, compared to the other
units.
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3. Terminology
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 RFC 2119 [5].
Furthermore, the following terms are used and have specific meaning
within the context of this document:
text sample or whole text sample
In the 3GPP Timed Text media format [1] this term refers to a
unit of timed text data as contained in the source (3GP) file.
This includes the text string byte count, possibly a Byte Order
Mark, the text string and any modifiers that may follow. Its
equivalent in audio/video would be a frame.
In this document, however, a text sample comprises only text
strings followed by zero or more modifiers. This definition of
text sample excludes the 16-bit text string byte count and the
16-bit Byte Order Mark (BOM) present in 3GP file text samples
(see Section 4.3 and Figure 9). The 16-bit BOM is not
transported in RTP as explained in Section 4.1.1.
text strings:
text strings is the term used to denote the actual text
characters encoded either as UTF-8 or UTF-16. When using this
payload format, the text string does not contain any byte order
mark (BOM). See Figure 9 for details.
fragment or text sample fragment:
a fraction of a text sample. A fragment may contain either text
strings or modifier (decoration) contents, but not both at the
same time.
sample contents:
general term to identify timed text data transported when using
this payload format. Sample contents may be one or several text
samples, sample descriptions and sample fragments (note that, as
per Section 4.6, there is only one case in which more than one
fragment may be included in a payload).
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decoration/modifiers:
the terms "decoration" and "modifiers" are used interchangeably
throughout the document to denote the contents of the text
sample that modify the default text formatting. Modifiers may,
for example, specify different font size for a particular
sequence of characters or define karaoke timing for the sample.
sample description:
this term is used to denote information which is potentially
shared by more than one text sample. In a 3GP file a sample
description is stored in a place where it can be shared. It
contains setup and default information such as scrolling
direction, text box position, delay value, default font,
background color, etc.
units or transport units:
the payload headers specified in this document encapsulate text
samples, fragments thereof and sample descriptions by placing a
common header and specific payload header (Sections 4.1.1 to
4.1.6) before them and so building what is here called a
(transport) unit.
aggregation / aggregate packet
The payload of an aggregate (RTP) packet consists of several
(transport) units.
track / stream
3GP files contain audio/video and text tracks. This document
enables to stream text tracks using RTP. Therefore both terms
are exchanged in this document in the context of 3GP files.
Media Header Box / Track Header Box / ...
the 3GP file format makes use of these structures defined in the
ISO Base File Format [2]. When referring to these in this
document, initials are capitalized for clarity.
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4. RTP Payload Format for 3GPP Timed Text
The format of an RTP packet containing 3GPP timed text is shown
below:
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 |
/+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |U| R | TYPE| LEN | :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
U| : (variable header fields depending on TYPE :
N| : :
I< +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
T| | |
| : SAMPLE CONTENTS :
| | +-+-+-+-+-+-+-+-+
| | |
\+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. 3GPP Timed Text RTP Packet Format.
Marker bit (M): the marker bit SHALL be set to 1 if the RTP packet
includes one or more whole text samples or the last fragment of a
text sample; otherwise set to zero (0).
Timestamp: the timestamp MUST indicate the sampling instant of the
earliest (or only) unit contained in the RTP packet. The initial
value SHOULD be randomly determined, as specified in RTP [3].
The timestamp value should provide enough timing resolution for
expressing the duration of text samples, for synchronizing text
with other media and for performing RTCP measurements such as
the interarrival delay jitter or the RTCP Packet Receipt Times
Report Block (Section 4.3 of RFC 3611 [20]). This is compliant
to RTP, section 5.1:
"The resolution of the clock MUST be sufficient for the
desired synchronization accuracy and for measuring packet
arrival jitter (one tick per video frame is typically not
sufficient)"
The above observation applies to both timed text tracks included
in a 3GP file as well as live streaming sessions. In the case
of a 3GP timed text track, the timestamp clockrate is the value
of the "timescale" parameter in the Media Header Box for that
text track. Each track in a 3GP file MAY have its own clockrate
as specified in the Media Header Box. Likewise, live streaming
applications SHALL use an appropriate timestamp clockrate. A
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default value of 1000 Hz is RECOMMENDED. Other timestamp
clockrates MAY be used. In this case, the typical behavior here
is to match the 3GPP timed text clockrate to that used by an
associated audio or video stream.
In an aggregate payload, units MUST be placed in play-out order,
i.e. earliest first in the payload. If TYPE 1 units are
aggregated, the timestamp of the subsequent units MUST be
obtained by adding the timed text sample duration of previous
samples to the RTP timestamp value. There are two exceptions to
this rule: TYPE 5 units and an aggregate payload containing two
fragments of the same text sample. The details of the timestamp
calculation are given in Section 4.6.
Finally, timestamp clockrates MUST be signaled by out-of-band
means at session setup, e.g., using the media type "rate"
parameter in SDP. See Section 9 for details.
Payload Type (PT): the payload type is set dynamically and sent by
out-of-band means.
The usage of the remaining RTP header fields, namely V, P, X, CC, SN
and SSRC, follows the rules of RTP and the profile in use.
4.1. Payload Header Definitions
The (transport) units specified in this document consist of a set of
common fields (U, R, TYPE, LEN), followed by specific header fields
(TYPES 1-5) and text sample contents. See Figure 1 and Figure 2.
In Figure 2 two example RTP packets are depicted. Thereby, the first
one contains an aggregate RTP payload with two complete text samples
and the second one contains one text sample fragment. After each
unit header is explained, detailed payload examples follow in Section
4.7.
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+----------------------+
| |
| RTP Header |
| |
---------+----------------------+
| | |
| |COMMON + TYPE 1 Header|
| ........................
UNIT 1 - | |
| | Text Sample |
| | |
|-------\........................
-------/| |
| |COMMON + TYPE 1 Header|
| ........................
UNIT 2 - | |
| | Text Sample |
| | |
| | |
---------+----------------------+
+----------------------+
| |
| RTP Header |
| |
---------+----------------------+
| | COMMON + TYPE 2 |
| | (or 3 or 4) Hdr |
| ........................
UNIT 3 - | |
| | Text Sample Fragment |
| | |
| | |
---------+----------------------+
Figure 2. Example RTP packets.
4.1.1. Common Payload Header Fields
The fields common to all payload headers have the following format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE | LEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. Common payload header fields.
Where:
o U (1 bit) "UTF Transformation flag": this is used to inform RTP
receivers whether UTF-8 (U=0) or UTF-16 (U=1) was used to encode
the text string. UTF-16 text strings transported by this payload
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format MUST be serialized in big endian order, a.k.a. network byte
order.
Informative note: timed text clients complying with the 3GPP
Timed Text format [1] are only required to understand the big
endian serialization. Thus, in order to ease interoperability,
the reverse serialization (little endian) is not supported by
this payload format.
For the payload formats defined in this document, the U bit is
only used in TYPE 1 and TYPE 2 headers. Senders MUST set the U
bit to zero in TYPE 3, TYPE 4 and TYPE 5 headers. Consequently,
receivers MUST ignore the U bit in TYPE 3, TYPE 4 and TYPE 5
headers.
o R (4 bits) "Reserved bits": for future extensions. This field
MUST be set to zero (0x0) and MUST be ignored by receivers.
o TYPE (3 bits) "Type Field": this field specifies which specific
header fields follow. The following TYPE values are defined:
- TYPE 1, for a whole text sample
- TYPE 2, for a text string fragment (without modifiers)
- TYPE 3, for a whole modifier box or the first fragment of a
modifier box
- TYPE 4, for a modifier fragment other than first.
- TYPE 5, for a sample description. Exactly one header per
sample description.
- TYPE 0, 6 and 7 are reserved for future extensions. Note that
future extensions are possible, e.g., a unit that explicitly
signals the number of characters present in a fragment (see
Section 2.5). In order to guarantee backwards-compatibility,
it SHALL be possible that older clients ignore (newer) units
they do not understand, without invalidating the timestamp
calculation mechanisms or otherwise preventing from decoding
the other units.
o Finally, the LEN (16 bits) "Length Field": indicates the size (in
bytes) of this header field and all the fields following, i.e. the
LEN field followed by the unit payload: text strings and modifiers
(if any). This definition only excludes the initial U/R/TYPE byte
of the common header. The LEN field follows network byte order.
The way in which LEN is obtained when streaming out of a 3GP file
depends on the particular unit type. This is explained for each
unit in the sections below.
For live streaming, both sample length and the LEN value for the
current fragment MUST be calculated during the sampling process or
during fragmentation.
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In general, LEN may take the following values:
- TYPE = 1, LEN >= 8,
- TYPE = 2, LEN > 9,
- TYPE = 3, LEN > 6,
- TYPE = 4, LEN > 6 and,
- TYPE = 5, LEN > 3.
Receivers MUST discard units that do not comply with these values.
However, the RTP header fields and the rest of the units in the
payload (if any) are still useful, as guaranteed by the
requirement for future extensions above.
In the following subsections the different payload headers for the
values of TYPE are specified.
4.1.2. TYPE 1 Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE | LEN (always >=8) | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | TLEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLEN |
+-+-+-+-+-+-+-+-+
Figure 4. TYPE 1 Header Format.
This header type is used to transport whole text samples. This unit
should be the most common case, i.e. the text sample should be
usually small enough to be transported in one unit without having to
separate text strings from modifiers. In an aggregate (RTP packet)
payload containing several text samples, every sample is preceded by
its own TYPE 1 header (see Figure 12).
Informative note: as indicated in the Terminology Section, a
text sample is composed by the text strings followed by the
modifiers (if any). This is also how text samples are stored in
3GP files. The separation of a text sample into text strings
and modifiers is only needed for large samples (or small
available IP MTU sizes, see Section 4.4) and it is accomplished
with TYPE 2 and TYPE 3 headers, as explained in the Sections
below.
Note that also empty text samples are considered whole text samples,
although they do not contain sample contents. Empty text samples may
be used to clear the display or to put an end to samples of unknown
duration, for example. Units without sample contents SHALL have a
LEN field value of 8 (0x0008).
The fields above have the following meaning:
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o U, R and TYPE as defined in Section 4.1.1.
o LEN, in this case, represents the length of the (complete) text
sample plus eight (8) bytes of headers. For finding the length if
the text sample in the Sample Size Box of 3GP files, see Section
4.3.
o SIDX (8 bits) "Text Sample Entry Index": this is an index used to
identify the sample descriptions.
The SIDX field is used to find the sample description
corresponding to the unit's payload. There are two types of SIDX
values: static and dynamic.
Static SIDX values are used to identify sample descriptions that
MUST be sent out-of-band and MUST remain active during the whole
session. A static SIDX value is unequivocally linked to one
particular sample description during the whole session. It SHOULD
be avoided that many sample descriptions are carried
out-of-band, since these may become large and, ultimately,
transport is not the goal of the out-of-band channel. Thus, this
feature is RECOMMENDED for transporting those sample descriptions
that provide a set of minimum default format settings. Static
SIDX values MUST fall in the (closed) interval [129,254].
Dynamic SIDX values are used for sample descriptions sent in-band.
Sample descriptions MAY be sent in-band for several reasons:
because they are generated in real time, for transport resiliency
or both. A dynamic SIDX value is unequivocally linked to one
particular sample description during the period in which this is
active in the session and it SHALL NOT be modified during that
period. This period MAY be smaller than or equal to the session
duration. This period is not known a priori. A maximum of 64
dynamic simultaneously active SIDX values is allowed at any
moment. Dynamic SIDX values MUST fall in the closed interval
[0,127]. This should be enough for both, recorded content and
live streaming applications. Nevertheless, a wrap-around
mechanism is provided in Section 4.2.1 to handle streaming
sessions where more than 64 SIDX values might be needed. Servers
MAY make use of dynamic sample descriptions. Clients MUST be able
to receive and interpret dynamic sample descriptions.
Finally, SIDX values 128 and 255 are reserved for future use.
o SDUR (24 bits) "Text Sample Duration": indicates the sample
duration in RTP timestamp units of the text sample. For this
field, a length of 3 bytes is preferred to 2 bytes. This is
because, for a typical clockrate of 1000 Hz, 16 bits would allow
for a maximum duration of just 65 seconds, which might be too
short for some streams. On the other hand, 24 bits at 1000 Hz
allow for a maximum duration of about 4.6 hours, while for 90 KHz,
this value is about 3 minutes. These values should be enough for
streaming applications. However, if a larger duration is needed,
the extension mechanism specified in Section 4.3 SHALL be used.
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Apart from defining the time period during which the text is
displayed, the duration field is also used to find the timestamp
of subsequent units within the aggregate RTP packet payload (if
any). This is explained in Section 4.6.
Text samples have generally a known duration at the time of
transmission. However, in some cases like live streaming, the
time for which a text piece shall be presented might not be known
a priori. Thus, the value zero SDUR=0 (0x000000) is reserved to
signal unknown duration. The amount of time that a sample of
unknown duration is presented is determined by the timestamp of
the next sample that shall be displayed at the receiver: text
samples of unknown duration SHALL be displayed until the next text
sample becomes active, as indicated by its timestamp.
The next example illustrates how units of unknown duration MUST be
presented. If no text sample following is available, it is an
implementation issue what should be displayed. E.g. a server
could send an empty sample to clear the text box.
Example: imagine you are in an airport watching the latest news
report while you wait for your plane. Airports are loud, so the
news report is transcribed in the lower area of the screen.
This area displays two lines of text: the headlines and the
words spoken by the news speaker. As usual, the headlines are
shown for a longer time than the rest. This time is, in
principle, unknown to the stream server, which is streaming
live. A headline is just replaced when the next headline is
received.
However, upon storing a text sample with SDUR=0 in a 3GP file, the
SDUR value MUST be changed to the effective duration of the text
sample, which MUST be always greater than zero (note that the ISO
file format [2] explicitly forbids a sample duration of zero).
The effective duration MUST be calculated as the timestamp
difference between the current sample (with unknown duration) and
the next text sample that is displayed.
Note that samples of unknown duration SHALL NOT use features,
which require knowledge of the duration of the sample up front.
Such features are scrolling and karaoke in [1]. This also applies
for future extensions of the Timed Text format. Furthermore, only
sample descriptions (TYPE 5 units) MAY follow units of unknown
duration in the same aggregate payload. Otherwise, it would not
be possible to calculate the timestamp of these other units.
For text contents stored in 3GP files, see Section 4.3 for details
on how to extract the duration value. For live streaming, live
encoders SHALL assign appropriate values and units according to
[1] and later releases.
o TLEN (16 bits), "Text String Length", is a byte-count of the text
string. The text string length is needed by the decoder to know
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where the modifiers in the payload start. TLEN is not present in
text string fragments (TYPE 2) since it can be deductively
calculated from the LEN values of each fragment.
The TLEN value is obtained from the text samples as contained in
3GP files. Refer to Section 4.3. For live content, the TLEN MUST
be obtained during the sampling process.
o Finally, the actual text sample is placed after the TLEN field.
As defined in Section 3, a text sample consists of a string of
characters encoded using either UTF-8 or UTF-16, followed by zero
or more modifiers. Note also, that no BOM and no byte count are
included in the strings carried in the payload (as opposed to text
samples stored in 3GP files [1]).
4.1.3. TYPE 2 Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE | LEN( always >9) | TOTAL | THIS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SLEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5. TYPE 2 Header Format.
This header type is used to transport either a whole text string or a
fragment of it. TYPE 2 units SHALL NOT contain modifiers. In
detail:
o U, R and TYPE as defined in Section 4.1.1.
o SIDX and SDUR as defined in Section 4.1.2.
Note that the U, SIDX and SDUR fields are meaningful since
partial text strings can also be displayed.
o The LEN field (16 bits) indicates the length of the text string
fragment plus nine (9) bytes of headers. Its value is calculated
upon fragmentation. LEN MUST always be greater than nine (0x0009).
Otherwise, the unit MUST be discarded.
According to the guidelines in Section 4.3, text strings MUST be
split at character boundaries for allowing the display of text
fragments. Hence, a text fragment MUST contain at least one
character in either UTF-8 or UTF-16. Actually, this is just a
formalism since by observing the guidelines, much larger fragments
should be created.
Note also, that TYPE 2 units do not contain an explicit text
string length, TLEN (see TYPE 1). This is because TYPE 2 units do
not contain any modifiers after the text string. If needed, the
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length of the received string can be obtained using the LEN values
of the TYPE 2 units.
o The SLEN field (16 bits) indicates the size (in bytes) of the
original (whole) text sample to which this fragment belongs. This
length comprises the text string plus any modifier boxes present
(and includes neither the byte order mark nor the text string
length as mentioned in the Terminology Section).
Regarding the text sample length: timed text samples are neither
generated at regular intervals nor there is a default sample size.
If 3GP files are streamed, the length of the text samples is
calculated beforehand and included in the track itself, while for
live encoding it is the real time encoder that SHALL choose an
appropriate size for each text sample. In this case, the amount
of text 'captured' in a sample depends on the text source and the
particular application (see examples below). Samples may, e.g.,
be tailored to match the packet MTU as close as possible or to
provide a given redundancy for the available bit rate. The
encoding application MUST also take into account the delay
constraints of the real-time session and assess whether FEC,
retransmission or other similar techniques are reasonable options
for stream repair.
The following examples shall illustrate how a real-time encoder
may choose its settings to adapt to the scenario constraints.
Example: imagine a newscast scenario, where the spoken news
is transcribed and synchronized with the image and voice of
the reporter. We assume that the news speaker talks at an
average speed of 5 words per second with an average word
length of 5 characters plus one space per word, i.e. 30
characters per second. We assume an available IP MTU of 576
bytes and an available bitrate of 576*8bits per
second=4.6Kbps. We assume each character can be encoded
using 2-bytes in UTF-16. In this scenario, several
constraints may apply, for example: available IP MTU,
available bandwidth, allowable delay and required redundancy.
If the target were to minimize the packet overhead, a text
sample covering 8 seconds of text would be closest to the IP
MTU: IP/UDP/RTP/TYPE1 Header + (8s text sample)=20+8+12+8+(~6
chars/word * 5 word/s * 8s *2 chars/word)= 528 bytes < 576
bytes. For other scenarios, like lossy networks, it may
happen that just one packet per sample is too low of a
redundancy. In this case, a choice could be that the encoder
'collects' text every second, thus yielding text samples
(TYPE 1 units) of 68 bytes, TYPE 1 header included. We can,
e.g., include three contiguous text samples in one RTP
payload: the current and last two text samples (see below).
This accounts to a total IP packet size of 20+8+12+3*(8+60)=
244 bytes. Now, with the same available bitrate of 4.6Kbps,
these 244-byte packets can be sent redundantly up two times
per second:
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RTP payload (1,2,3)(1,2,3) (2,3,4)(2,3,4) (3,4,5)(3,4,5) ...
Time: <----1s------> <----1s------> <-----1s-----> ...
This means that each text sample is sent at least six times,
which should provide enough redundancy. Although not as
bandwidth efficient (488*8 < 528*8 < 576*8 bps) as the
previous packetization, this option increases the stream
redundancy while still meeting the delay and bandwidth
constraints.
Another example would be a user sending timed text from a
type-in area in the display. In this case, the text sample
is created as soon as the user clicks the 'send' button.
Depending on the packet length, fragmentation may be needed.
In a video conferencing application, text is synchronized
with audio and video. Thus, the text samples shall be
displayed long enough to be read by a human, shall fit in the
video screen and shall 'capture' the audio contents rendered
during the time the corresponding video and audio is
rendered.
For stored content, see Section 4.3 for details on how to find the
SLEN value in a 3GP file. For live content, the SLEN MUST be
obtained during the sampling process.
Finally, note that clients MAY use SLEN to buffer space for the
remaining fragments of a text sample.
o The fields TOTAL (4 bits) and THIS (4 bits) indicate the total
number of fragments in which the original text sample (i.e. text
string and its modifiers) has been fragmented and which order
occupies the current fragment in that sequence, respectively.
Note that the sequence number alone cannot replace the
functionality of the THIS field, since packets (and fragments) may
be repeated, e.g., as in repeated transmission (see Section 5).
Thus, an indication for "fragment offset" is needed.
The usual "byte offset" field is not used here for two reasons: a)
it would take one more byte and b) it does not provide any
information on the character offset. UTF-8/UTF-16 text strings
have, in general, a variable character length ranging from 1 to 6
bytes. Therefore, the TOTAL/THIS solution is preferred. It could
also be argued that the LEN and SLEN fields be used for this
purpose, but while they would provide information about the
completeness of the text sample, they do not specify the order of
the fragments.
In all cases (TYPEs 2, 3 and 4), if the value of THIS is greater
than TOTAL or if TOTAL equals zero (0x0), the fragment SHALL be
discarded.
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o Finally, the sample contents following the SLEN field consist of a
fragment of the UTF-8/UTF-16 character string; no modifiers
follow.
4.1.4. TYPE 3 Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE | LEN( always >6) |TOTAL | THIS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6. TYPE 3 Header Format.
This header type is used to transport either the entire modifier
contents present in a text sample or just the first fragment of them.
This depends on whether the modifier boxes fit in the current RTP
payload.
If a text sample containing modifiers is fragmented this header MUST
be used to transport the first fragment or, if possible, the complete
modifiers.
In detail:
o The U, R and TYPE fields are defined as in Section 4.1.1.
o LEN indicates the length of the modifier contents. Its value is
obtained upon fragmentation. Additionally, the LEN field MUST be
greater than six (0x0006). Otherwise, the unit MUST be discarded.
o The TOTAL/THIS field has the same meaning as for TYPE 2.
For TYPE 3 unit containing the last (trailing) modifier fragment,
the value of TOTAL MUST be equal to that of THIS (TOTAL=THIS). In
addition, TOTAL=THIS MUST be greater than one, because the total
number of fragments of a text sample is logically always larger
than one.
Otherwise, if TOTAL is different from THIS in a TYPE 3 unit, this
means that the unit contains the first fragment of the modifiers.
o The SDUR has the same definition for TYPE 1. Since the fragments
are always transported in own RTP packets, this field is only
needed to know how long this fragment is valid. This may, e.g.,
be used to determine how long it should be kept in the display
buffer.
Note that the SLEN and SIDX fields are not present in TYPE 3 unit
headers. This is because: a) these fragments do not contain text
strings and b) these types of fragments are applied over text string
fragments, which already contain this information.
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4.1.5. TYPE 4 Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE | LEN( always >6) |TOTAL | THIS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7. TYPE 4 Header Format.
This header type is placed before modifier fragments, other than the
first one.
The U, R and TYPE fields are used as per Section 4.1.1.
LEN indicates as for TYPE 3 the length of the modifier contents and
SHALL also be obtained upon fragmentation. The LEN field MUST be
greater than six (0x0006). Otherwise, the unit MUST be discarded.
TOTAL/THIS is used as in TYPE 2.
The SDUR field is defined as in TYPE 1. The reasoning behind the
absence of SLEN and SIDX is the same as in TYPE 3 units.
4.1.6. TYPE 5 Header
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE | LEN( always >3) | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8. TYPE 5 Header Format.
This header type is used to transport (dynamic) sample descriptions.
Every sample description MUST have its own TYPE 5 header.
The U, R and TYPE fields are used as per Section 4.1.1.
The LEN field indicates the length of the sample description, plus
three units accounting for the SIDX and LEN field itself. Thus, this
field MUST be greater than three (0x0003). Otherwise, the unit MUST
be discarded.
If the sample is streamed from a 3GP file, the length of the sample
description contents (i.e. what comes after SIDX in the unit itself)
is obtained from the file (see Section 4.3).
The SIDX field contains a dynamic SIDX value assigned to the sample
description carried as sample content of this unit. As only dynamic
sample descriptions are carried using TYPE 5, the possible SIDX
values are in the (closed) interval [0,127].
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Senders MAY make use of TYPE 5 units. All receivers MUST implement
support for TYPE 5 units, since it adds minimum complexity and it may
increase the robustness of the streaming session.
The next section specifies how SIDX values are calculated.
4.2. Buffering of Sample Descriptions
The buffering of sample descriptions is a matter of the client's
timed text codec implementation. In order to work properly, this
payload format requires that:
o Static sample descriptions MUST be buffered at the client, at
least, for the duration of the session.
o If dynamic sample descriptions are used, their buffering and
update of the SIDX values MUST follow the mechanism described in
the next section.
4.2.1. Dynamic SIDX wrap-around mechanism
The use of dynamic sample descriptions by senders is OPTIONAL.
However, if used, senders MUST implement this mechanism. Receivers
MUST always implement it.
Dynamic SIDX values remain active either during the entire duration
of the session (if used just once) or in different intervals of it
(if used once or more).
Note: in the following SIDX means dynamic SIDX.
For choosing the wrap-around mechanism, the following rationale was
used: there are 128 dynamic SIDX values possible, [0..127]. If one
chooses to allow a maximum of 127 to be used as dynamic SIDXs, then
any reordered packet with a new sample description would make the
mechanism fail. E.g., if the last packet received is SIDX=5, then
all 127 values except SIDX=6 would be "active". Now, if a reordered
packet arrives with a new description, SIDX=9, it will be mistakenly
discarded, because the SIDX=9 is, at that moment, marked as "active"
and active sample descriptions shall not be re-written. Therefore,
a "guard interval" is introduced. This guard interval reduces the
number of active SIDXs at any point in time to 64. Although most
timed text applications will probably need less than 64 sample
descriptions during a session (in total), a wrap-around mechanism to
handle the need for more is described here.
Thereby, a sliding window of 64 active SIDX values is used. Values
within the window are "active"; all others are marked "inactive". An
SIDX value becomes active if at least one sample description
identified by that SIDX has been received. Since sample descriptions
MAY be sent redundantly, it is possible that a client receives a
given SIDX several times. However, active sample descriptions SHALL
NOT be overwritten: the receiver SHALL ignore redundant sample
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descriptions and it MUST use the already cached copy. The "guard
interval" of (64) inactive values ensures that always the correct
association SIDX <-> sample description is used.
Informative note: as for the "guard interval" value itself, 64
as 128/2 was considered simple enough while still meeting the
expected maximum number of sample descriptions. Besides that,
there's no other motivation for choosing 64 or a different
value.
The following algorithm is used to buffer dynamic sample descriptions
maintain the dynamic SIDX values:
Let X be the last SIDX received that updated the range of active
sample descriptions. Let Y be a value within the allowed range for
dynamic SIDX: [0,127], and different from X. Let Z be the SIDX of the
last received sample description. Then:
1. Initialize all dynamic SIDX values as inactive. For stored
contents, read the sample description index in the Sample to
Chunk box ("stsc") for that sample. For live streaming, the
first value MAY be zero or any other value in the interval
above. Go to step 2.
2. First in-band sample description with SIDX=Z is received and
stored, Set X=Z. Go to step 3.
3. Any SIDX within the interval [X+1 modulo(128), X+64 modulo(128)]
is marked as inactive and any corresponding sample description
is deleted. Any SIDX within the interval [X+65 modulo(128), X]
is set active. Go to step 4 (wait state).
4. Wait for next sample description. Once the client is
initialized, the interval of active SIDX values MUST change
whenever a sample description with an SIDX value in the inactive
set is received. I.e., upon reception of a sample description
with SIDX=Z do:
a. If Z is in the (closed) interval [X+1 modulo(128), X+64
modulo(128)] then set X=Z, store the sample description and
go to step 3.
b. Else Z must be in the interval [X+65 modulo(128), X], thus:
i. If SIDX=Z is not stored, then store the sample
description. Go to beginning of step 4 (wait state).
ii. Else go to the beginning of step 4 (wait state).
Informative note: it is allowed to send any value of SIDX=X in
the interval [0,127]. E.g., if [64..127] is the current active
set and SIDX=0 is sent a new sample description is defined (0)
and an old one deleted (64), thus [65..127] and [0] are active.
Similarly, one could now send SIDX=64, thus inverting the active
and inactive sets.
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Example,
if X=4, any SIDX in the interval [5,68] is inactive. Active
SIDX values are in the complementary interval [69,127] plus
[0,4]. E.g., if the client receives a SIDX=6, then the active
interval is now different: [0,6] plus [71,127]. If the received
SIDX is in the current active interval no change SHALL be
applied.
4.3. Finding payload header values in 3GP files
For the purpose of streaming timed text contents, some values in the
boxes contained in a 3GP file are mapped to fields of this payload
header. This section explains where to find those values.
Additionally, for the duration and sample description indexes,
extension mechanisms are provided. All senders MUST implement the
extension mechanisms described herein.
If the file is streamed out of a 3GP file, thee following guidelines
SHALL be followed.
Note: all fields in the objects (boxes) of a 3GP file are found
in network byte order.
Information obtained from the Sample Table Box (stbl):
o Sample Descriptions and Sample Description length: the
Sample Description box (stsd, inside the stbl) contains the
sample descriptions. For timed text media, each element of
stsd is a timed text sample entry (type "tx3g").
The (unsigned) 32 bits of the "size" field in the stsd box
represent the length (in bytes) of the sample description, as
carried in TYPE 5 units. On the other hand, the LEN field of
TYPE 5 units is restricted to 16 bits. Therefore if the
value of "size" is greater than (2^16-1-3)[bytes], then the
sample description SHALL NOT be streamed with this payload
format. There is no extension mechanism defined in this
case, since fragmentation of sample descriptions is not
defined (sample descriptions are typically up to some 200
bytes in size). Note: the three (3) accounts for the TYPE 5
header fields included in the LEN value.
o SDUR from the Decoding Time to Sample Box (stts). The
(unsigned) 32 bits of the "sample delta" field are used for
calculating SDUR. However, since SDUR field is only 3 bytes
long, then text samples with duration values larger than
(2^24-1)/(timestamp clockrate)[seconds] cannot be streamed
directly. The solution is simple: copies of the
corresponding text sample SHALL be sent. Thereby, the
timestamp and duration values SHALL be adjusted so that a
continuous display is guaranteed as it just one sample would
have been sent. I.e., a sample with timestamp TS and
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duration SDUR can be sent as two samples having timestamps
TS1 and TS2 and durations SDUR1 and SDUR2, such that TS1=TS,
TS2=TS1+SDUR1 and SDUR=SDUR1+SDUR2.
o Text sample length from the Sample Size Box (stsz). The
(unsigned) 32 bits of the "sample size" or "entry size" (one
of them, depending on whether the sample size is fixed or
variable) indicate the length (in bytes) of the 3GP text
sample. For obtaining the length of the (actual) streamed
text sample, the lengths of the text string byte count (2
bytes) and, in case of UTF-16 strings, the length the BOM
(also 2 bytes) SHALL be deducted. This is illustrated in
Figure 9.
Text Sample according to 3GPP TS 26.245
TEXT SAMPLE (length=stsz)
.--------------------------------------------------.
/ \
TEXT STRING (length=TBC)
.------------------------------------.
/ \
TBC BOM MODIFIERS
+---+---+----------------------------------+-----------+
||
|| TBC BOM -> TLEN field
|| +---+---+ U bit
||
\/
Text Sample according to this Payload Format
TEXT SAMPLE (length=SLEN w/o TBC,BOM)
.--------------------------------------------.
/ \
TEXT STRING (length=TLEN)
.--------------------------------.
/ \
TEXT STRING MODIFIERS
+----------------------------------+-----------+
KEY:
TBC= Text string Byte Count
BOM= Byte Order Mark
Figure 9. Text sample composition.
Moreover, since the LEN field in TYPE 1 unit header is 16-bit
long, then larger text sample sizes than (2^16-1-8) [bytes]
SHALL NOT be streamed. Also in this case, there is no
extension mechanism defined. This is because this maximum is
considered enough for the targeted streaming applications.
(Note: the eight (8) accounts for the TYPE 1 header fields
included in the LEN value).
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o SIDX from the Sample to Chunk Box (stsc): the stsc Box is
used to find samples and their corresponding sample
descriptions. These are referenced by the "sample
description index", a (unsigned) 32-bit integer. If possible,
these indices may be directly mapped to the SIDX field.
However, there are several cases where this may not be
possible:
a) The total number of indices used is greater than the
number of indices available, i. e., if the static sample
descriptions are more than 127 or the dynamic ones are
more than 64 or,
b) The original SIDX value ranges do not fit in the
allowed ranges for static (129-254) or dynamic (0-127)
values.
Therefore, when assigning SIDX values to the sample
descriptions, the following guidelines are provided:
o Static sample descriptions can simply be assigned
consecutive values within the range 129-254 (closed
interval). This range should be well enough for static
sample descriptions.
o As for dynamic sample descriptions:
a) Streams that use less than 64 dynamic sample
descriptions SHOULD use consecutive values for SIDX
anywhere in the range 0-127 (closed interval).
b) For streams with more than 64 sample descriptions,
the SIDX values MUST be assigned in usage order, and if
any sample description shall be used after it has been
set inactive, it will need to be re-sent and assigned a
new SIDX value (according to the algorithm in
Section4.2.1).
Information obtained from the Media Data Box:
o Text strings, TLEN, U bit and modifiers from the Media Data
Box (mdat). Text strings, 16-bit text string byte count,
Byte Order Mark (BOM, indicating UTF encoding) and modifier
boxes can be found here.
For TYPE 1 units, the value of TLEN is extracted from the
text string byte count that precedes the text string in the
text sample, as stored in the 3GP file. If UTF-16 encoding
is used, two (2) more bytes have to be deducted from this
byte count beforehand, in order to exclude the BOM. See
Figure 9.
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4.4. Fragmentation of Timed Text Samples
This section explains why text samples may have to be fragmented and
discusses some of the possible approaches to do it. A solution is
proposed together with rules and recommendations for fragmenting and
transporting text samples.
3GPP Timed Text applications are expected to operate at low bitrates.
This fact, added to the small size of timed text samples (typically
one or two hundred bytes) makes fragmentation of text samples a rare
event. Samples should usually fit into the MTU size of the used
network path.
Nevertheless, some text strings (e.g. ending roll in a movie) and
some modifier boxes (i.e. for hyperlinks, for karaoke or for styles)
may become large. This may also apply for future modifier boxes. In
such cases, the first option to consider is whether it is possible to
adjust the encoding (e.g. the size of sample) in such a way that
fragmentation is avoided. If so, this is preferred to fragmentation
and SHOULD be done.
Otherwise, if this is not possible or other constraints avoid it,
fragmentation MAY be used and the basic guidelines given in this
document MUST be followed:
o It is RECOMMENDED that text samples are fragmented as seldom as
possible, i.e. the least possible number of fragments is created
out of a text sample.
o If there is some bitrate and free space in the payload available,
sample descriptions (if at hand) SHOULD be aggregated.
o Text strings MUST split at character boundaries, see TYPE 2
header. Otherwise, it is not possible to display the text
contents of a fragment if a previous fragment was lost. As a
consequence, text string fragmentation requires knowledge of the
UTF-8/UTF-16 encoding formats to determine character boundaries.
o Unlike text strings, the modifier boxes are NOT REQUIRED to split
at meaningful boundaries. However, it is RECOMMENDED to do so
whenever possible. This decreases the effects of packet loss.
This payload format does not ensure that partially received
modifiers be applied to text strings. If only part of the
modifiers is received, it is an application issue how to deal with
these, i.e. whether to use them or not.
Informative note: ensuring that partially received modifiers can
be applied to text strings in all cases (for all modifier types
and for all fragment loss constellations) would place additional
requirements on the payload format. In particular this would
require that: a) senders understand the semantics of the
modifier boxes and b) specific fragment headers for each of the
modifier boxes are defined, in addition to the payload formats
defined below. Understanding the modifiers semantics means
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knowing, e.g., where does each modifier start and end, which
text fragments are affected, which modifiers may or may not be
split or what the fields indicate. This is necessary for being
able to split the modifiers in such a way that each fragment can
be applied independent of previous packet losses. This would
require a more intelligent fragmentation entity and more complex
headers. Given the low probability of fragmentation and the
desire to keep the requirements low, it does not seem reasonable
to specify such modifier box specific headers.
o Modifier and text string fragments SHOULD be protected against
packet losses, i.e. using FEC [7], retransmission [11], repetition
(Section 5) or an equivalent technique. This minimizes the
effects of packet loss.
o An additional requirement when fragmenting text samples is that
the start of the modifiers MUST be indicated using the payload
header defined for that purpose, i.e. a TYPE 3 unit MUST be used
(see Section 4.1.4). This enables a receiver to detect the start
of the modifiers as long as there are not two or more consecutive
packet losses.
o Finally, sample descriptions SHALL NOT be fragmented, because they
contain important information that may affect several text
samples.
4.5. Reassembling Text Samples at the Receiver
The payload headers defined in this document allow reassembling
fragmented text samples. For this purpose, the standard RTP
timestamp, the duration field (SDUR) and the fields TOTAL/THIS in the
payload headers are used.
Units that belong to the same text sample MUST have the same
timestamp. TYPE 5 units do not comply with this rule since they are
not part of any particular text sample.
The process for collecting the different fragments (units) of a text
sample is as follows:
1. Search for units having the same timestamp value, i.e., units
that belong to the same text sample or sample descriptions that
shall become available at that time instant. If several units
of the same sample are repeated, only one of them SHALL be used.
Repeated units are those that have the same timestamp and the
same values for TOTAL/THIS.
Note that, as mentioned in Section 4.1.1, the receiver
SHALL ignore units with unrecognized TYPE value.
However, the RTP header fields and the rest of the units
(if any) in the payload are still useful.
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2. Check within this set whether any of the units from the text
sample is missing. This is done using the TOTAL and THIS
fields; the TOTAL field indicates how many fragments were
created out of the text sample and the THIS field indicates the
position of this fragment in the text sample. As result of this
operation two outcomes are possible:
a. No fragment is missing. Then the THIS field SHALL be used
to order the fragments and reassemble the text sample
before forwarding it to the decoding application. Special
care SHALL be taken when reassembling the text string as
indicated in bullet 4 below.
b. One or more fragments are missing: check whether this
fragment belongs to the text string or to the modifiers:
TYPE 2 units identify text string fragments, TYPE 3 and 4
modifier fragments:
i. If the fragment or fragments missing belong to the
text string and the modifiers were received complete,
then the received text characters may, at least, be
displayed as plain text. Some modifiers may only be
applied as long as it is possible to identify the
character numbers, e.g. if only last text string
fragment is lost. This is the case for modifiers
defining specific font styles ('styl'), highlighted
characters ('hlit'), karaoke feature ('krok)' and
blinking characters ('blnk'). Other modifiers such as
'dlay' or 'tbox' can be applied without the knowledge
of the character number. It is an application issue
to decide whether to use apply the modifiers or not.
ii. If the fragment missing belongs to the modifiers and
the text strings were received complete, then the
incomplete modifiers may be used. The text string
SHOULD at least be displayed as plain text. As
mentioned in Section 4.3 modifiers may split without
observing meaningful boundaries. Hence, it may not
always be possible to make use of partially received
modifiers. However, to avoid this, it is RECOMMENDED
that the modifiers do split at meaningful boundaries.
iii. A third possibility is that it is not possible to
discern whether modifiers or text strings were
received complete. E.g. if the TYPE 3 unit of a
sample plus the following or preceding packet is lost,
there is no way for the RTP receiver to know if one if
both packets lost belong to the modifiers or there is
also some text strings. Repetition, FEC,
retransmission or other protection mechanisms as per
section 4.6 are RECOMMENDED to avoid this situation.
iv. Finally, if it is sure that neither text strings nor
modifiers were received complete, then the text
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strings and the modifiers may be rendered partially or
may be discarded. This is an application choice.
3. Sample descriptions can be directly associated with the
reassembled text samples, via the sample description index
(SIDX).
4. Reassembling of text strings: since the text strings transported
in RTP packets MUST NOT include any byte order mark (BOM), the
receiver MUST prepend it to the reassembled UTF-16 string before
handling it to the timed text decoder (see Figure 9). The value
of the BOM is 0xFEFF because only big endian serialization of
UTF-16 strings is supported by this payload format.
4.6. On Aggregate Payloads
Units SHOULD be aggregated to avoid overhead, whenever possible. The
aggregate payloads MUST comply with one of the following ordered
configurations:
1. Zero or more sample descriptions (TYPE 5) followed by zero or more
whole text samples (TYPE 1 units). At least one unit of either
type MUST be present.
2. Zero or more sample descriptions followed by zero or one modifier
fragment, either TYPE 3 or TYPE 4. At least one unit MUST be
present.
3. Zero or more sample descriptions followed by zero or one text
string fragment (TYPE 2) followed by zero or one TYPE 3 unit. If
a TYPE 2 unit and a TYPE 3 unit are present, then they MUST belong
to the same text sample. At least one unit MUST be present.
Some observations:
o Different aggregates than the ones listed above SHALL NOT be used.
o Sample descriptions MUST be placed in the aggregate payload before
the occurrence of any non-TYPE 5 units.
o Correct reception of TYPE 5 units is important since their
contents may be referenced by several other units in the stream.
Receivers are unable to use text samples until their corresponding
sample description is received. Accordingly, a sender SHOULD send
multiple copies of a sample description to ensure reliability (see
section 5). Receivers MAY use payload specific feedback messages
[21] to tell a sender that they have received a particular sample
description.
o Regarding timestamp calculation: in general, the rules for
calculating the timestamp of units in an aggregate payload depend
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on the type of unit. Based on the possible constellations for
aggregate payloads as above we have:
o Sample descriptions MUST receive the RTP timestamp of the
packet in which they are included.
Note that for TYPE 5 units, the timestamp actually does not
represent the instant when they are played out, but instead
the instant at which they become available for use.
o For the first configuration: the first TYPE 1 unit receives
the RTP timestamp. The timestamp of any subsequent TYPE 1
unit MUST be obtained by adding sample duration and
timestamp, both of the preceding TYPE 1 unit.
o For the second and third configuration, all units, TYPE 2,
3 and 4, MUST receive the RTP timestamp.
Refer to detailed examples on the timestamp calculation
below.
o As per configuration 3 above, a payload MAY contain several
fragments of one (and only one) text sample. If so, then exactly
one TYPE 2 unit followed by exactly one TYPE 3 unit are allowed in
the same payload. This is in line with RFC 3640 [12], Section
2.4, which explicitly disallows combining fragments of different
samples in the same RTP payload. Note that, in this special case,
no timestamp calculation is needed. I. e., the RTP timestamp of
both units is equal to the timestamp in the packet's RTP header.
o Finally, note that the use of empty text samples allows for
aggregating non-consecutive TYPE 1 units in the same payload. Two
text samples, with timestamps TS1 and TS3 and durations SDUR1 and
SDUR3, are not consecutive if it holds TS1+SDUR1 < TS3. A
solution for this is to include an empty TYPE 1 unit with duration
SDUR2 between them, such that TS2+SDUR2 = TS1+SDUR1+SDUR2 = TS3.
Some examples of aggregate payloads are illustrated in Figure 10
(Note: the figure is not scaled.)
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N/A TS1 TS2 TS3
+------+-----+------+-----+
|TYPE5 |TYPE1|TYPE1 |TYPE1|
+------+-----+------+-----+
N/A sdur1 sdur2 sdur3
N/A TS4
+-----+-------+
|TYPE5| TYPE 1| a)
+-----+-------+
N/A sdur4
TS4 TS4 TS4
+--------------+ +--------------+
| TYPE2 | |TYPE2 |TYPE 3 | b)
+--------------+ +--------------+
sdur4 sdur4 sdur4
TS4 TS4
+--------------+ +--------------+
| TYPE2| TYPE 3| | TYPE4 | c)
+--------------+ +--------------+
sdur4 sdur4 sdur4
|----------PAYLOAD 1------| |--PAYLOAD 2---| |--PAYLOAD 3---|
rtpts1 rtpts2 rtpts3
KEY:
TSx means Text Sample x,
rtptsy represents the standard RTP timestamp for PAYLOAD y
sdurz the duration of unit z
N/A means not applicable
Figure 10. Example aggregate payloads.
In Figure 10 four text samples (TS1 through TS4) are sent using three
RTP packets. These configurations have been chosen to show how the 5
TYPE headers are used. Additionally, three different possibilities
for the last text sample, TS4, are depicted: a), b) and c).
In Figure 11, option b) from Figure 10 is chosen to illustrate how
the timestamp for each unit is found
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N/A TS1 TS2 TS3 TS4 TS4 TS4
+------+-----+------+-----+ +--------------+ +--------------+
|TYPE5 |TYPE1|TYPE1 |TYPE1| | TYPE2 | |TYPE2 |TYPE 3 |
+------+-----+------+-----+ +--------------+ +--------------+
N/A sdur1 sdur2 sdur3 sdur4 sdur4 sdur4
(#1) (#2) (#3) (#4) (#5) (#6) (#7)
|----------PAYLOAD 1------| |--PAYLOAD 2---| |--PAYLOAD 3---|
rtpts1 rtpts2 rtpts3
Figure 11. Selected payloads from Figure 10.
Assuming TSx means Text Sample x, rtptsy represents the standard RTP
timestamp for PAYLOAD y and sdurz the duration of unit z, the
timestamp for unit #z, ts(#z), can be found as the sum of rtptsy and
the cumulative sum of the durations of preceding units in that
payload (except in the case of PAYLOAD 3 as per rule 3 above). Thus,
we have:
1. for the units in the first aggregate payload, PAYLOAD 1:
ts(#1)= rtpts1,
ts(#2)= rtpts1,
ts(#3)= rtpts1 + sdur1,
ts(#4)= rtpts1 + sdur1 + sdur2,
Note that the TYPE 5 and the first TYPE 1 unit have both the
RTP timestamp.
2. for PAYLOAD 2:
ts(#5)= rtpts2,
3. for PAYLOAD 3:
ts(#6)= ts(#7)= rtpsts2= rtpts3
According to configuration 3 above, the TYPE2 and the TYPE 3
units shall belong to the same sample. Hence rtpts3 must be
equal to rtpts2. For the same reason, the value of SDUR is
not be used to calculate the timestamp of the next unit.
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4.7. Payload Examples
Some example of payloads using the defined headers are shown below:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE1| LEN (always >=8) | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | TLEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLEN | |
+---------------+ |
| text string (no.bytes=TLEN) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| modifiers (no.bytes=LEN - 8 - TLEN) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE1| LEN (always >=8) | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | TLEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLEN | |
+---------------+ |
| text string (no.bytes=TLEN) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| modifiers (no.bytes=LEN - 8 - TLEN) |
| +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12. A payload carrying two TYPE 1 units.
In Figure 12 an RTP packet carrying two TYPE 1 units is depicted. It
can be seen how the length fields LEN and TLEN can be used to find
the start of the next unit (LEN), find the start of the modifiers
(TLEN) and find the length of the modifiers (LEN-TLEN).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE5| LEN( always >3) | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| sample description (no.bytes=LEN - 3) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE1| LEN (always >=8) | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | TLEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLEN | |
+-+-+-+-+-+-+-+-+ |
| text string fragment (no.bytes=TLEN) |
| |
| |
| +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13. An RTP packet carrying a TYPE 5 and a TYPE 1 unit.
In Figure 13, a sample description and a TYPE 1 unit are aggregated.
The TYPE 1 unit happens to contain only text strings and is small so
that an additional the TYPE 5 unit is included for taking advantage
of the available bits in the packet.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE2| LEN( always >9) |TOTAL=4|THIS=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SLEN | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| text string fragment (no.bytes=LEN - 9) |
| |
: :
: :
| +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14. Payload with first text string fragment of a sample.
In Figure 14, Figure 15 and Figure 16 a text sample is split into
three RTP packets. In the first one, the text string is big and
takes the whole packet length. In the second packet in Figure 15,
the only possibility for carrying two fragments of the same text
sample is represented (see configuration 3 in Section 4.6). The last
packet showed carries the last modifier fragment, a TYPE 4.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE2| LEN( always >9) |TOTAL=4|THIS=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | SIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SLEN | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| text string fragment (no.bytes=LEN - 9) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE3| LEN( always >6) |TOTAL=4|THIS=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| modifiers (no.bytes=LEN - 6) |
| +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15. An RTP packet carrying a TYPE2 unit and a TYPE 3 unit.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U| R |TYPE4| LEN( always >6) |TOTAL=4|THIS=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SDUR | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| modifiers (no.bytes=LEN - 6) |
| +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16. An RTP packet carrying last modifiers fragment (TYPE 4).
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4.8. Relation to RFC 3640
RFC 3640 defines a payload format for the transport of any
non-multiplexed MPEG-4 elementary stream. One of the various MPEG-4
elementary streams types are MPEG-4 timed text streams, specified in
MPEG-4 part 17 [28], also known as ISO/IEC 14496-17. MPEG-4 timed
text streams are capable of carrying 3GPP timed text data, as
specified in 3GPP TS 26.245 [1].
MPEG-4 timed text streams are intentionally constructed so as to
guarantee interoperability between RFC 3640 and this payload format.
This means that the construction of the RTP packets carrying timed
text is the same. I.e., the MPEG-4 timed text elementary stream as
per ISO/IEC 14496-17 is identical to the (aggregate) payloads
constructed using this payload format.
Figure 11 illustrates the process of constructing an RTP packet
containing timed text. As it can be seen in the partition block, the
(transport) units used in this payload format are identical to the
Timed Text Units (TTUs) defined in ISO/IEC 14496-17. Likewise, the
rules for payload aggregation as per Section 4.6 are identical to the
ones defined in ISO/IEC 14496-17 and compliant with RFC 3640. As a
result, an RTP packet that uses this payload format is identical to
and RTP packet using RFC 3640 conveying TTUs according to ISO/IEC
14496-17. In particular, MPEG-4 Part 17 specifies that when using
RFC 3640 for transporting timed text streams, the "streamType"
parameter value is set to 0x0D and the value of the
"objectTypeIndication" in "config" takes the value 0x08.
+--------------------------------------+
Text samples | +--------------+ +--------------+ |
as per 3GPP | |Text Sample 1 | |Text Sample N | |
TS 26245 | +--------------+ +--------------+ |
+--------------------------------------+
\/
+-------------------------------------------------------------------+
| Partition Text Samples into units. TTU[i]= TYPE i units. |
| |
|[U R TYPE LEN][{TOTAL,THIS}SIDX{SDUR}{TLEN}{SLEN}][SampleContents] |
|{..} means present if applicable, [..] means always present |
+-------------------------------------------------------------------+
\/ \/
+-------------------------------------------------------------------+
| Aggregation (if possible) |
+-------------------------------------------------------------------+
\/ \/
+-------------------------------------------------------------------+
| RTP Entity adds and fills RTP header and Sends RTP packet, where |
| RTP packets according to this Payload Format = |
|= RTP packets carrying MPEG-4 Timed Text ES over RFC3640 |
+-------------------------------------------------------------------+
Figure 11. Relation to RFC 3640.
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Note: the use of RFC 3640 for transport of ISO/IEC 14496-17 data does
not require any new SDP parameters or any new mode definition.
4.9. Relation to RFC 2793
The RFC 2793 [24] and its revision [25] specify a protocol for
enabling text conversation. Typical applications of this payload
format are text communication terminals and text conferencing tools.
Text session contents are specified in ITU-T Recommendation T.140
[26]. T.140 text is UTF-8 coded as specified in T.140 [26] with no
extra framing. The T140block contains one or more T.140 code
elements as specified in T.140. Code elements are control sequences
such as "New Line", "Interrupt", "String Terminator" or "Start of
String". Most T.140 code elements are single ISO 10646 [27]
characters, but some are multiple character sequences. Each character
is UTF-8 encoded [18] into one or more octets.
This payload format may also be used for conversational applications
(even for instant messaging). However, this is not the main target
of it. The differentiating feature of 3GPP Timed Text media format
is that it allows text decoration. This is especially useful in
multimedia presentations, karaoke, commercial banners, news tickers,
karaoke, clickable text strings and captions. T.140 text contents
used in RFC 2793 do not allow the use of text decoration.
Furthermore, the conversational text RTP payload format recommends a
method to include redundant text from already transmitted packets in
order to reduce the risk of text loss caused by packet loss. Thereby
payloads would include a redundant copy of the last payload sent.
This payload format does not describe such method, but this is also
applicable here. As explained in Section 5 packet redundancy SHOULD
be use, whenever possible. The aggregation guidelines in Section 4.6
allow redundant payloads.
5. Resilient Transport
Apart from the basic fragmentation guidelines described in the
section above, the simplest option for packet loss resilient
transport is packet repetition. Such mechanism may consist of a
strict window-based repetition mechanism or, simply, a repetition
mechanism in a wider sense, where new and old packets are mixed, for
example.
A server MAY decide to use repetition as a measure for packet loss
resilience. Thereby, a server MAY send the same RTP payloads or just
some of the units from the payloads.
As for the case of complete payloads, single repeated units MUST
match exactly the same units sent in the first transmission, i.e. if
fragmentation is needed, it SHALL be performed only once for each
text sample Only then, a receiver can use the already received and
the repeated units to reconstruct the original text samples. Since
the RTP timestamp is used to group together the fragments of a
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sample, care must taken to preserve the timing of units when
constructing new RTP packets.
E.g. if a text sample was originally sent as a single
non-fragmented text sample (one TYPE 1 unit), a repetition of
that sample MUST be sent also as a single non-fragmented text
sample in one unit. Likewise, if the original text sample was
fragmented and spread over several RTP packets, say a total of 3
units, then the repeated fragments SHALL also have the same byte
boundaries and use the same unit headers and bytes per fragment.
With repetition, repeated units resolve to the same timestamp as
their originals. Where redundant units are available, only one of
them SHALL be used.
Regarding the RTP header fields:
o if the whole RTP payload is repeated, all payload-specific fields
in the RTP header (the M, TS and PT fields) MUST keep their
original values except the sequence number that MUST be
incremented to comply with RTP (the fields TOTAL/THIS enable to
re-assemble fragments with different sequence numbers).
o in packets containing single repeated units, the general rules in
Section 3 for assigning values to the RTP header fields apply.
Particularly relevant here is to keep the value of the RTP
timestamp to preserve the timing of the units.
Apart from repetition other mechanisms such as FEC [7],
retransmission [11] or similar techniques could be used to cope with
packet losses.
6. Congestion control
Congestion control for RTP SHALL be implemented in accordance with
RTP [3], and the applicable RTP profile, e.g. RTP/AVP [17].
When using this payload format, mainly two factors may affect the
congestion control:
o The use of (unit) aggregation may make the payload format more
bandwidth efficient, by avoiding header overhead and thus reducing
the used bitrate.
o The use of resilient transport mechanisms: although timed text
applications typically operate at low bitrates, the increase due to
resilient transport shall be considered for congestion control
mechanisms. This applies to all mechanisms but especially to less
efficient ones like repetition.
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7. Scene Description
7.1. Text Rendering Position and Composition
In order to set up a timed text session, regardless of the stream
being stored in a 3GP file or streamed live, some initial layout
information is needed by the communicating peers.
+-------------------------------------------+
| <-> tx | +-------------+
| +-------------------------------+ |<---|Display Area |
| ^ | | | +-------------+
| : | | |
| :ty| | | +-------------+
| : | |<---------|Video track |
| : | | | +-------------+
| : | | |
| : | | |
| : | | |
| v | | |
| - | x-------------------------+ | | +-------------+
|h ^ | | |<-----------|Text Track |
|e : +---|-------------------------|-+ | +-------------+
|i : | +---------------------+ | |
|g : | | | | | +-------------+
|h : | | |<------------ |Text Box |
|t v | +---------------------+ | | +-------------+
| - +-------------------------+ |
+-------------------------------------------+
<........................>
w i d t h
Figure 17. Illustration of text rendering position and composition
The parameters used for negotiating the position and size of the text
track in the display area are shown in Figure 17. These are the
"width" and "height" of the text track, its translation values, "tx"
and "ty", and its "layer" or proximity to the user.
At the same time, the sender of the stream needs to know the
receiver's capabilities. In this case, the maximum allowable values
for the text track height and width: "max-h" and "max-w", for the
stream the receiver shall display.
This layout information MUST be conveyed in a reliable form previous
to the start of the session, e.g. during session announcement or in
an Offer/Answer (O/A) exchange. An example of a reliable transport
may be the out-of-band channel used for SDP. Sections 8 and 9
provide details on the mapping of these parameters to SDP
descriptions and their usage in O/A.
For stored content, the layout values expressing stream properties
MUST be obtained from the Track Header Box. See Section 7.3.
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For live streaming appropriate values as negotiated during session
set-up shall be used.
7.2. SMIL usage
The attributes contained in the Track Header Boxes of a 3GP file only
specify the spatial relationship of the tracks within the given 3GP
file.
If multiple 3GP files are sent, they require spatial synchronization.
For example, for a text and video stream, the positions of the text
and video tracks in Figure 17 shall be determined. For such purpose,
SMIL [9] MAY be used.
SMIL assigns regions in the display to each of those files and places
the tracks within those regions. Generally, in SMIL, the position of
one track (or stream) is expressed relative to another track. This
is different to the 3GP file, where the upper left corner is the
reference for all translation offsets. Hence, only if the position
in SMIL is relative to the video track origin, then this translation
offset has the same value as (tx, ty) in the 3GP file.
Note also that the original track header information is used for each
track only within its region, as assigned by SMIL. Therefore, even
if SMIL scene description is used, the track header information
pieces SHOULD be sent anyway as they represent the intrinsic media
properties. See 3GPP SMIL Language Profile in [29] for details.
7.3. Finding layout values in a 3GP file
In a 3GP file, within the Track Header Box (tkhd):
o tx, ty: these values specify the translation offset of the
(text) track relative to the upper left corner of the video
track, if present. They are the second but last and third
but last values in the unity matrix; values are fixed-point
16.16 values, restricted to be (signed) integers (i.e., the
lower 16 bits of each value shall be all zeros). Therefore,
only the first 16 bits are used for obtaining the value of
the media type parameters.
o width, height: they have the same name in the tkhd box. All
(unsigned) 32 bits are meaningful.
o layer: all (signed) 16 bits are used.
8. 3GPP Timed Text Media Type
The media subtype for the 3GPP Timed Text codec is allocated from the
standards tree. The top-level media type under which this payload
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format is registered is 'video'. This registration is done using the
template defined in [31] and following RFC 3555 [30].
The receiver MUST ignore any unrecognized parameter.
Media type: video
Media subtype: 3gpp-tt
Required parameters
rate:
Refer to Section 3 in RFCXXXX.
sver:
The parameter "sver" contains a list of supported
backwards-compatible versions of the timed text format
specification (3GPP TS 26.245) that the sender accepts
to receive (and which are the same that it would be
willing to send). The first value is the value
preferred to receive (or preferred to send). The first
value MAY be followed by a comma-separated list of
versions that SHOULD be used as alternatives. The order
is meaningful, being first the most preferred and last
the least preferred. Each entry has the format
Zi(xi*256+yi), where "Zi" is the number of the Release,
"xi" and "yi" are taken from the 3GPP specification
version, i.e. vZi.xi.yi. For example, for 3GPP TS
26.245 v6.0.0, Zi(xi*256+yi)=6(0), the version value is
"60". (Note that "60" is the concatenation of the
values Zi=6 and (xi*256+yi)=0 and not its product.)
If no "sver" value is available, for example, when
streaming out of a 3GP file, the default value "60",
corresponding to the 3GPP Release 6 version of 3GPP TS
26.245, SHALL be used.
Optional parameters:
tx:
This parameter indicates the horizontal translation
offset in pixels of the text track with respect to the
origin of the video track. This value is the decimal
representation of a 16-bit signed integer. Refer to TS
3GPP 26.245 for an illustration of this parameter.
ty:
This parameter indicates the vertical translation offset
in pixels of the text track with respect to the origin
of the video track. This value is the decimal
representation of a 16-bit signed integer. Refer to TS
3GPP 26.245 for an illustration of this parameter.
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layer:
This parameter indicates the proximity of the text track
to the viewer. More negative values mean closer to the
viewer. This parameter has no units. This value is the
decimal representation of a 16-bit signed integer.
tx3g:
This parameter MUST be used for conveying sample
descriptions out-of-band. It contains a comma-separated
list of base64-encoded entries. The entries of this
list that MAY follow any particular order and the list
SHALL NOT be empty. Each entry is the result of running
base64 encoding over the concatenation of the (static)
SIDX value as 8-bit unsigned integer and the (static)
sample description for that SIDX, in this order. The
format of a sample description entry can be found in
3GPP TS 26.245 Release 6 and later releases. All
servers and clients MUST understand this parameter and
MUST be capable of using the sample description(s)
contained in it. Please refer to RFC 3548 for details
on the base64 encoding.
width:
This parameter indicates the width in pixels of the text
track or area of the text being sent. This value is the
decimal representation of a 32-bit unsigned integer.
Refer to TS 3GPP 26.245 for an illustration of this
parameter.
height:
This parameter indicates the height in pixels of the
text track being sent. This value is the decimal
representation of a 32-bit unsigned integer. Refer to
TS 3GPP 26.245 for an illustration of this parameter.
max-w:
This parameter indicates display capabilities. This is
the maximum "width" value that the sender of this
parameter supports. This value is the decimal
representation of a 32-bit unsigned integer.
max-h:
This parameter indicates display capabilities. This is
the maximum "height" value that the sender of this
parameter supports. This value is the decimal
representation of a 32-bit unsigned integer.
Encoding considerations:
This media type is framed (see section 4.8 in [31]) and
partially contains binary data.
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Restrictions on usage:
This media type depends on RTP framing, and hence is only
defined for transfer via RTP [3]. Transport within other framing
protocols is not defined at this time.
Security considerations:
Please refer to Section 11 of RFCXXXX.
Interoperability considerations:
The 3GPP Timed Text media format and its file storage is
specified in Release 6 of 3GPP TS 26.245 "Transparent end-to-end
packet switched streaming service (PSS); Timed Text Format
(Release 6)". Note also that 3GPP may in future Releases
specify extensions or updates to the timed text media format in
a backwards-compatible way, e. g. new modifier boxes or
extensions to the sample descriptions. The payload format
defined in RFCXXXX allows for such extensions. For future 3GPP
Releases of the Timed Text Format, the parameter "sver" is used
to identify the exact specification used.
The defined storage format for 3GPP Timed Text format is the
3GPP File Format (3GP) [32]. 3GP files may be transferred using
the media type video/3gpp as registered by RFC 3839 [33]. The
3GPP File Format is a container file that may contain, e.g.,
audio and video which may be synchronized with the
3GPP Timed Text.
Published specification: RFC XXXX
Applications which use this media type:
Multimedia streaming applications.
Additional information:
the 3GPP Timed Text media format is specified in 3GPP TS 26.245
"Transparent end-to-end packet switched streaming service (PSS);
Timed Text Format (Release 6)". This document and future
extensions to the 3GPP Timed Text format are publicly available
at http://www.3gpp.org.
Magic number(s): None.
File extension(s): None.
Macintosh File Type Code(s): None.
Person & email address to contact for further information:
Jose Rey, jose.rey@eu.panasonic.com
Yoshinori Matsui, matsui.yoshinori@jp.panasonic.com
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Audio/Video Transport Working Group.
Intended usage: COMMON
Authors:
Jose Rey
Yoshinori Matsui
Change controller:
IETF Audio/Video Transport Working Group delegated from the
IESG.
9. SDP usage
9.1. Mapping to SDP
The information carried in the media type specification has a
specific mapping to fields in SDP [4]. If SDP is used to specify
sessions using this payload format, the mapping is done as follows:
o The media type ("video") goes in the SDP "m=" as the media name.
m=video <port number> RTP/<RTP profile> <dynamic payload type>
o The media subtype ("3gpp-tt") and the timestamp clockrate "rate"
(the RECOMMENDED 1000 Hz or other value) go in SDP "a=rtpmap" line
as the encoding name and rate, respectively:
a=rtpmap:<payload type> 3gpp-tt/1000
o The REQUIRED parameter "sver" goes in the SDP "a=fmtp" attribute
by copying it directly from the media type string as a semicolon
separated parameter=value pair.
o The OPTIONAL parameters "tx", "ty", "layer", "tx3g", "width",
"height", "max-w" and "max-h" go in the SDP "a=fmtp" attribute by
copying them directly from the media type string as a semicolon
separated list of parameter=value(s) pairs:
a=fmtp:<dynamic payload type> <parameter
name>=<value>[,<value>][; <parameter name>=<value>]
o Any unknown parameter to the device that uses the SDP SHALL be
ignored. E.g. parameters added in media format later
specifications MAY be copied into the SDP and SHALL be ignored
by receivers that do not understand them.
9.2. Parameter Usage in the SDP Offer/Answer Model
In this section the meaning of the SDP parameters defined in this
document within the Offer/Answer [13] context is explained.
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In unicast, sender and receiver typically negotiate the streams, i.e.
which codecs and parameter values are used in the session. This is
also possible in multicast to a lesser extend.
Additionally, the meaning of the parameters MAY vary depending on
which direction it used. In the following sections, a
"<directionality> offer" means an offer that contains a stream set to
<directionality>. <directionality> may take the values sendrecv,
sendonly and recvonly. Similar considerations apply for answers.
E.g. an answer to sendonly offer is a recvonly answer.
9.2.1. Unicast Usage
The following types of parameters are used in this payload format:
1. Declarative parameters: offerer and answerer declare the values
they will use for the incoming (sendrecv/recvonly) or outgoing
(sendonly) stream. Offerer and answerer MAY use different
values.
a. "tx", "ty" and "layer": these are parameters describing
where the received text track is placed. Depending on the
directionality:
i. MUST appear in all sendrecv offers and answers and in
all recvonly offers and answers (thus applying to the
incoming stream). In the case of sendrecv offers and
answers and in recvonly offers, these values SHOULD be
used by the sender of the stream unless it has a
particular preference, in which case, it MUST make
sure that these different values do not corrupt the
presentation. For recvonly answers, the answerer MAY
accept the proposed values for the incoming stream (in
a sendonly offer, see bullet below) or respond with
different ones. The offerer MUST use the returned
values.
ii. MAY appear in sendonly offers and MUST appear in
sendonly answers. In sendonly offers they specify the
values that the offerer proposes for sending (see
example in Section 9.3). In sendonly answers these
values SHOULD be copied from the corresponding
recvonly offer upon accepting the stream, unless a
particular preference by the receiver if the stream
exists, as explained in the previous bullet.
2. Parameters describing the display capabilities, "max-h" and
"max-w", which indicate the maximum dimensions of the text track
(text display area) for the incoming stream "tx" and "ty" values
(see Figure 17). "max-h" and "max-w" MUST be included in all
offers and answers where "tx" and "ty" refer to the incoming
stream, thus excluding sendonly offers and answers (see example
in Section 9.3), where they SHALL NOT be present.
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3. Parameters describing the sent stream properties, i.e. the
sender of the stream decides upon the values of these:
a. "width" and "height", specify the text track dimensions.
They SHALL ALWAYS be present in sendrecv and sendonly
offers and answers. For recvonly answers, the answerer
MUST include the offered parameter values (if any) verbatim
in the answer upon accepting the stream.
b. "tx3g" contains static sample descriptions. It MAY only be
present in sendrecv and sendonly offers and answers. This
parameter applies to the stream that offerers or answerers
send.
4. Negotiable parameters, which MUST be agreed on. This is the
case of "sver". This parameter MUST be present in every offer
and answer. The answerer SHALL choose one supported value from
the offerer's list or else it MUST remove the stream or reject
the session.
5. Symmetric parameters: "rate", timestamp clockrate, belongs to
this class. Symmetric parameters MUST be echoed verbatim in the
answer. Otherwise the stream MUST be removed or the session
rejected.
The following Table 1 summarises all options:
+..---------------------------+----------+----------+----------+
| ``--..__ Directionality/ | sendrecv | recvonly | sendonly |
+ Type of ``--..__ O or A +----------+----------+----------+
| Parameter ``--..__ | O/A | O/A | O/A |
+--------------+------------``+----------+----------+----------+
| Declarative |tx, ty, layer | M/M | M/M | m/M |
| | | | | |
+--------------+--------------+----------+----------+----------+
| Display |max-h, max-w | M/M | M/M | -/- |
| Capabilities | | | | |
+--------------+--------------+----------+----------+----------+
| Stream |height, width | M/M | -/(M) | M/M |
| properties |tx3g | m/m | -/- | m/m |
| | | | | |
+--------------+--------------+----------+----------+----------+
| Negotiable |sver | M/M | M/M | M/M |
| | | | | |
+--------------+--------------+----------+----------+----------+
| Symmetric |rate | M/M | M/M | M/M |
+--------------+--------------+----------+----------+----------+
Table 1. Parameter usage in Unicast Offer / Answer.
Key:
o M means MUST be present
o m means MAY be present (such as proposed values)
o (M) or (m) means MUST or MAY, if applicable
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o a hyphen ("-") means the parameter MUST NOT be present.
Other observations regarding parameter usage:
o Translation and transparency values: in sendonly offers "tx",
"ty" and "layer" indicate proposed values. This is useful for
visually composed sessions where the different streams occupy
different parts of the display, e.g., a video stream and the
captions. These are just suggested values because it is the
peer rendering the text that ultimately decides where to place
the text track.
o Text track (area) dimensions, "height" and "width": in the case
of sendonly offers, an answerer accepting the offer MUST be
prepared to render the stream using these values. If any of
these conditions are not met, the stream MUST be removed or the
session rejected.
o Display capabilities, "max-h" and "max-w": an answerer sending a
stream SHALL ensure that the "height" and "width" values in the
answer are compatible with the offerer's signalled capabilities.
o Version handling via "sver": the idea is that offerer and
answerer communicate using the same version. This is achieved
by letting the answerer choose from a list of supported
versions, "sver". For recvonly streams, the first value in the
list is the preferred version to receive. Consequently, for
sendonly (and sendrecv) streams the first value is the one
preferred for sending (and receiving). The answerer MUST choose
one value and return it in the answer. Upon receiving the
answer, the offerer SHALL be prepared to send (sendonly and
sendrecv) and receive (recvonly and sendrecv) a stream using
that version. If none of the versions in the list is supported
the stream MUST be removed or the session rejected. Note that,
if alternative non-compatible versions are offered, then this
SHALL be done using different payload types.
9.2.2. Multicast Usage
In multicast the parameter usage is similar to the unicast case,
except in the following cases:
o the parameters "tx", "ty" and "layer" in multicast offers only
have meaning for sendrecv and recvonly streams. In order for all
clients to have the same vision of the session, they MUST be used
symmetrically.
o for "height", "width" and the "tx3g" (for sendrecv and sendonly),
multicast offers specify which values of these parameters the
participants MUST use for sending. Thus, if the stream is
accepted, the answerer MUST also here include them verbatim in the
answer (also "tx3g", if present).
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o The capability parameters, "max-h" and "max-w", SHALL NOT be used
in multicast. If the offered text track should change in size, a
new offer SHALL be used instead.
o Regarding version handling:
In the case of multicast offers, an answerer MAY accept a
multicast offer as long as one of the versions listed in the
"sver" is supported. Therefore, if the stream is accepted, the
answerer MUST choose its preferred version but, unlike in unicast,
the offerer SHALL NOT change the offered stream to this chosen
version because there may be other session participants that do
support the newer extensions. Consequently, different session
participants may end up using different backwards-compatible media
format versions. It is RECOMMENDED that the multicast offer
contains a limited number of versions, in order for all
participants to have the same view of the session. This is a
responsibility of the session creator. If none of the offered
versions is supported, the stream SHALL be removed or the session
rejected. Also in this case, if alternative non-compatible
versions are offered, then this SHALL be done using different
payload types.
9.3. Offer/Answer Examples
In these unicast O/A examples the long lines are wrapped around.
Static sample descriptions are shortened for clarity.
For sendrecv :
O -> A
m=video <port> RTP/AVP 98
a=rtpmap:98 3gpp-tt/1000
a=fmtp:98 tx=100; ty=100; layer=0; height=80; width=100; max-h=120;
max-w=160; sver=6256,60; tx3g=81...
a=sendrecv
A -> O
m=video <port> RTP/AVP 98..
a=rtpmap:98 3gpp-tt/1000
a=fmtp:98 tx=100; ty=95; layer=0; height=90; width=100; max-h=100;
max-w=160; sver=60; tx3g=82...
a=sendrecv
In this example the offerer is telling the answerer where it will
place the received stream and what is the maximum height and width
allowable for the stream that it will receive. Also, it tells the
answerer the dimensions of the text track for the stream sent and
which sample description it shall use. It offers two versions, 6256
and 60. The answerer responds with an equivalent set of parameters
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for the stream it receives. In this case the answerer's "max-h" and
"max-w" are compatible with the offerer's "height" and "width".
Otherwise, the answerer would have to remove this stream and the
offerer would have to issue a new offer taking the answerer's
capabilities into account. This is possible only if multiple payload
types are present in the initial offer so that at least one of them
matches the answerer's capabilities as expressed by "max-h" and
"max-w" in the negative answer. Note also that the answerer's text
box dimensions fit within the maximum values signalled in the offer.
Finally, the answerer chooses to use version 60 of the timed text
format.
For recvonly:
Offerer -> Answerer
m=video <port> RTP/AVP 98
a=rtpmap:98 3gpp-tt/1000
a=fmtp:98 tx=100; ty=100; layer=0; max-h=120; max-w=160; sver=6256,60
a=recvonly
A -> O
m=video <port> RTP/AVP 98..
a=rtpmap:98 3gpp-tt/1000
a=fmtp:98 tx=100; ty=100; layer=0; height=90; width=100; sver=60;
tx3g=82...
a=sendonly
In this case, the offer is different from the previous case: it does
not include the stream properties: "height", "width" and "tx3g". The
answerer copies the "tx", "ty" and "layer" values, thus acknowledging
these. "max-h" and "max-w" are not present in the answer because the
"tx" and "ty" (and "layer") in this special case do not apply to the
received, but to the sent stream. Also, if offerer and answerer had
very different displays sizes, it would not be possible to express
the answerer's capabilities. In the example above and for an
answerer with a 50x50 display, the translation values are already out
of range.
For sendonly:
O -> A
m=video <port> RTP/AVP 98
a=rtpmap:98 3gpp-tt/1000
a=fmtp:98 tx=100; ty=100; layer=0; height=80; width=100;
sver=6256,60; tx3g=81...
a=sendonly
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A -> O
m=video <port> RTP/AVP 98..
a=rtpmap:98 3gpp-tt/1000
a=fmtp:98 tx=100; ty=100; layer=0; height=80; width=100; max-h=100;
max-w=160; sver=60
a=recvonly
Note that "max-h" and "max-w" are not present in the offer. Also,
with this answer, the answerer would accept the offer as is (thus
echoing "tx", "ty", "height", "width" and "layer") and additionally
inform the offerer about its capabilities: "max-h" and "max-w".
Another possible answer for this case would be:
A -> O
m=video <port> RTP/AVP 98..
a=rtpmap:98 3gpp-tt/1000
a=fmtp:98 tx=120; ty=105; layer=0; max-h=95; max-w=150; sver=60
a=recvonly
In this case the answerer does not accept the values offered. The
offerer MUST use these values or else remove the stream.
9.4. Parameter Usage outside of Offer/Answer
SDP may also be employed outside of the Offer/Answer context, for
instance for multimedia sessions that are announced through the
Session Announcement Protocol (SAP) [14], or streamed through the
Real Time Streaming Protocol (RTSP) [15].
In this case, the receiver of a session description is required to
support the parameters and given values for the streams or else it
MUST reject the session. It is the responsibility of the sender (or
creator) of the session descriptions to define the session parameters
so that the probability of unsuccessful session setup is minimized.
This is out of the scope of this document.
10. IANA Considerations
IANA is requested to register the media subtype name "3gpp-tt" for
the media type "video" as specified in Section 8 of this document.
11. Security considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [3] and any applicable RTP profile, e.g. AVP [17].
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In particular, an attacker may invalidate the current set of active
sample descriptions at the client by means of repeating a packet with
an old sample description, i.e. replay attack. This would mean that
the display of the text would be corrupted, if displayed at all.
Another form of attack may consist in sending redundant fragments,
whose boundaries do not match the exact boundaries of the originals
(as indicated by LEN) or fragments that carry different sample
lengths (SLEN). This may cause a decoder to crash.
These types of attack may easily be avoided by using source
authentication and integrity protection.
Additionally, peers in a timed text session may desire to retain
privacy in their communication, i.e. confidentiality.
This payload format does not provide any mechanisms for achieving
these. Confidentiality, integrity protection and authentication have
to be solved by a mechanism external to this payload format, e.g.,
SRTP [10].
12. References
12.1. Normative References
[1] Transparent end-to-end packet switched streaming service (PSS);
Timed Text Format (Release 6), TS 26.245 v 6.0.0, June 2004.
[2] ISO/IEC 14496-12:2004 Information technology - Coding of
audio-visual objects - Part 12: ISO base media file format.
[3] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, "RTP: A
Transport Protocol for Real-Time Applications", STD 64, RFC 3550,
July 2003.
[4] M. Handley, V. Jacobson, "SDP: Session Description Protocol",
RFC 2327, April 1998.
[5] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," BCP 14, RFC 2119, IETF, March 1997.
[6] S. Josefsson (Ed.), "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
12.2. Informative References
[7] J. Rosenberg, H. Schulzrinne, "An RTP Payload Format for Generic
Forward Error Correction", RFC 2733, December 1999.
[8] C. Perkins, O. Hodson, "Options for Repair of Streaming Media",
RFC 2354, June 1998.
[9] W3C, "Synchronised Multimedia Integration Language (SMIL 2.0)",
August, 2001.
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[10] M. Baugher, D. A. McGrew, D. Oran, R. Blom, E. Carrara, M.
Naslund, K. Norrman, "The Secure Real-Time Transport Protocol",
RFC 3711, March 2004.
[11] J. Rey et al., "RTP Retransmission Payload Format",
draft-ietf-avt-rtp-retransmission-11.txt, work in progress, March
2005.
[12] Van der Meer et al., "RTP Payload Format for Transport of MPEG-4
Elementary Streams ", RFC 3640, November 2003.
[13] J. Rosenberg., H. Schulzrinne, " An Offer/Answer Model with the
Session Description Protocol (SDP)", RFC 3264, June 2002.
[14] M. Handley, et al. "Session Announcement Protocol", RFC 2974,
October 2000.
[15] H. Schulzrinne, et al.,"Real Time Streaming Protocol (RTSP)",
RFC 2326, April 1998.
[16] Transparent end-to-end packet switched streaming service (PSS);
Protocols and codecs (Release 6), TS 26.234 v 6.1.0, September
2004.
[17] H. Schulzrinne, S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[18] F. Yergeau, "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, October 1996.
[19] P. Hoffman, F. Yergeau, "UTF-16, an encoding of ISO 10646", RFC
2781, February 2000.
[20] Friedman, et al., "RTP Control Protocol Extended Reports (RTCP
XR)", RFC 3611, November 2003.
[21] Ott, et al., "Extended RTP Profile for RTCP-based Feedback
(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-11.txt, work in
progress, August 2004.
[22] IETF RFC 3267: "Real-Time Transport Protocol (RTP) Payload
Format and File Storage Format for the Adaptive Multi-Rate (AMR)
Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs", Sjoberg J. et
al., June 2002.
[23] IETF RFC 3016: "RTP Payload Format for MPEG-4 Audio/Visual
Streams", Kikuchi Y. et al., November 2000.
[24] G. Hellstrom, "RTP Payload for Text Conversation", RFC 2793, May
2000.
[25] G. Hellstrom, P. Jones, "RTP Payload for Text Conversation",
draft-ietf-avt-rfc2793bis-09.txt, Work In Progress, August 2004.
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[26] ITU-T Recommendation T.140 (1998) - Text conversation protocol
for multimedia application, with amendment 1, (2000).
[27] ISO/IEC 10646-1: (1993), Universal Multiple Octet Coded
Character Set.
[28] ISO/IEC FCD 14496-17 Information technology - Coding of
audio-visual objects - Part 17: Streaming text format, Work in
progress, June 2004.
[29] Transparent end-to-end Packet-switched Streaming Service (PSS);
3GPP SMIL language profile, (Release 6), TS 26.246 v 6.0.0, June
2004.
[30] Casner, S. and P. Hoschka, "MIME Type Registration of RTP
Payload Formats", RFC 3555, July 2003.
[31] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", draft-freed-media-type-reg-04, April
2005.
[32] Transparent end-to-end packet switched streaming service (PSS);
3GPP file format (3GP) (Release 6), TS 26.244 V6.3. March 2005.
[33] Castagno, R. and D. Singer, "MIME Type Registrations for 3rd
Generation Partnership Project (3GPP) Multimedia files", RFC 3839,
July 2004.
13. Annexes
13.1. Basics of the 3GP File Structure
This section provides a coarse overview of the 3GP file structure,
which follows the ISO Base Media file Format [2].
Each 3GP file consists of "Boxes". In general, a 3GP file contains
the File Type Box (ftyp), the Movie Box (moov), and the Media Data
Box (mdat). The File Type Box identifies the type and properties of
the 3GP file itself. The Movie Box and the Media Data Box, serving
as containers, include own boxes for each media. Boxes start with a
header, which indicates both size and type (these fields are called
namely "size" and "type"). Additionally, each box type may include a
number of boxes.
In the following, only those boxes are mentioned, which are useful
for the purposes of this payload format.
The Movie Box (moov) contains one or more Track Boxes (trak), which
include information about each track. A Track Box contains, among
others, the Track Header Box (tkhd), the Media Header Box (mdhd) and
the Media Information Box (minf).
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The Track Header Box specifies the characteristics of a single track,
where a track is, in this case, the streamed text during a session.
Exactly one Track Header Box is present for a track. It contains
information about the track, such as the spatial layout (width and
height), the video transformation matrix and the layer number. Since
these pieces of information are essential and static, i.e. constant
for the duration of the session, they must be sent prior to the
transmission of any text samples.
The Media Header Box contains the "timescale" or number of time units
that pass in one second, i.e. cycles per second or Hertz. The Media
Information Box includes the Sample Table Box (stbl) which contains
all the time and data indexing of the media samples in a track.
Using this box, it is possible to locate samples in time, determine
their type, their size, container, and offset into that container.
Inside the Sample Table Box we can find the Sample Description Box
(stsd, for finding sample descriptions), the Decoding Time to Sample
Box (stts, for finding sample duration), the Sample Size Box (stsz)
and the Sample to Chunk Box (stsc, for finding the sample description
index).
Finally, the Media Data Box contains the media data itself. In timed
text tracks this box contains text samples. Its equivalent to audio
and video is audio and video frames, respectively. The text sample
consists of the text length, the text string, and one or several
Modifier Boxes. The text length is the size of the text in bytes.
The text string is plain text to render. The Modifier Box is
information to render in addition to the text such as colour, font,
etc.
14. Acknowledgements
The authors would like to thank Dave Singer, Jan van der Meer, Magnus
Westerlund and Colin Perkins for their comments and suggestions to
this document.
The authors would also like to thank Markus Gebhard for the free and
publicly available JavE ASCII Editor (used for the ASCII drawings in
this document) and Henrik Levkowetz for the Idnits web service.
15. Authors' Addresses
Jose Rey jose.rey@eu.panasonic.com
Panasonic R&D Center Germany GmbH
Monzastr. 4c
D-63225 Langen, Germany
Phone: +49-6103-766-134
Fax: +49-6103-766-166
Yoshinori Matsui matsui.yoshinori@jp.panasonic.com
Matsushita Electric Industrial Co., LTD.
1006 Kadoma
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Kadoma-shi, Osaka, Japan
Phone: +81 6 6900 9689
Fax: +81 6 6900 9699
16. IPR Notices
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on the procedures with respect to rights in RFC documents can be
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17. Full Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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