RFC 4396
RTP Payload Format for 3rd Generation Partnership Project (3GPP) Timed Text
Network Working Group J. Rey Request for Comments: 4396 Y. Matsui Category: Standards Track Panasonic February 2006 RTP Payload Format for 3rd Generation Partnership Project (3GPP) Timed Text Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006).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 applications 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.
Table of Contents
1. Introduction ....................................................3 2. Motivation, Requirements, and Design Rationale ..................3 2.1. Motivation .................................................3 2.2. Basic Components of the 3GPP Timed Text Media Format .......4 2.3. Requirements ...............................................5 2.4. Limitations ................................................6 2.5. Design Rationale ...........................................7 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 .......................15 4.1.2. TYPE 1 Header ......................................17 4.1.3. TYPE 2 Header ......................................20 4.1.4. TYPE 3 Header ......................................23 4.1.5. TYPE 4 Header ......................................24 4.1.6. TYPE 5 Header ......................................25 4.2. Buffering of Sample Descriptions ..........................25 4.2.1. Dynamic SIDX Wraparound Mechanism ..................26 4.3. Finding Payload Header Values in 3GP Files ................28 4.4. Fragmentation of Timed Text Samples .......................31 4.5. Reassembling Text Samples at the Receiver .................33 4.6. On Aggregate Payloads .....................................35 4.7. Payload Examples ..........................................39 4.8. Relation to RFC 3640 ......................................43 4.9. Relation to RFC 2793 ......................................44 5. Resilient Transport ............................................45 6. Congestion Control .............................................46 7. Scene Description ..............................................47 7.1. Text Rendering Position and Composition ...................47 7.2. SMIL Usage ................................................48 7.3. Finding Layout Values in a 3GP File .......................48 8. 3GPP Timed Text Media Type .....................................49 9. SDP Usage ......................................................53 9.1. Mapping to SDP ............................................53 9.2. Parameter Usage in the SDP Offer/Answer Model .............53 9.2.1. Unicast Usage ......................................54 9.2.2. Multicast Usage ....................................57 9.3. Offer/Answer Examples .....................................58 9.4. Parameter Usage outside of Offer/Answer ...................60 10. IANA Considerations ...........................................60 11. Security Considerations .......................................60 12. References ....................................................61 12.1. Normative References .....................................61 12.2. Informative References ...................................61 13. Basics of the 3GP File Structure ..............................64 14. Acknowledgements ..............................................65
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 such as that for karaoke applications, scrolling text for newscasts, or hyperlinked text. These contents may or may not be synchronized with other media, such as audio or video. The purpose of this document 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 for 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 [22]. 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. Basics of the 3GP File Structure are in 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 downloading 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 a 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 know 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: 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, color,...), background color, horizontal and vertical justification, line width, scrolling, etc. 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. o The decoration: If some characters have different style, delay, blink, etc., 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, within each Text Sample, the decoration (i.e., Modifier Boxes) is 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 - positioning of the text box - text wrap indication
2.3. Requirements
Once the basic components are known, it is necessary to define which requirements the payload format shall 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 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 is an implementation design choice. 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 by both 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 also be
sent 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
means for coping with fragmentation and reassembly. Section
4.4 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, and 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.
The 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 8-
bit (unsigned) 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 16-bit (unsigned) integer, the sample
description shall not be conveyed. As in the case of the sample
size, no extension mechanism is provided.
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 document
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.
For finding 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 that the entity performing fragmentation
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. For
example:
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 consensus of the working group 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 are 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.
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], these terms refer 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 contains 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 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).
decoration or modifiers
These terms 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
Information that 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, thus building what is here called a
(transport) unit.
aggregation or aggregate packet
The payload of an aggregate (RTP) packet consists of several
(transport) units.
track or stream
3GP files contain audio/video and text tracks. This document
enables streaming of text tracks using RTP. Therefore, these
terms are used interchangeably 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.
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, it is 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 RTP Control Protocol (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 and 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
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. The first
contains an aggregate RTP payload with two complete text samples, and
the second contains one text sample fragment. After each unit header
is explained, detailed payload examples follow in Section 4.7.
+----------------------+ | | | 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 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 them
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.
In general, LEN may take the following values:
- TYPE = 1, LEN >= 8
- TYPE = 2, LEN > 9
- TYPE = 3, LEN > 6
- TYPE = 4, LEN > 6
- 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 usually be 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 Section 3, "Terminology", a text sample is composed of 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 also that 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: 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 of 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. Carrying
many sample descriptions out-of-band SHOULD be avoided, 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 wraparound 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.
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 such as 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. For example, 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 decoder needs the text string length in order to know
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 4.4, text strings MUST be
split at character boundaries for allowing the display of text
fragments. Therefore, 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 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 Section 3, "Terminology").
Regarding the text sample length: Timed text samples are not
generated at regular intervals, nor is there 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 closely 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*8 bits per second = 4.6 Kbps. 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 + (8-second text sample)
= 20 + 8 + 12 + 8 + (~6 chars/word * 5 word/s * 8 s * 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 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 4.6 Kbps,
these 244-byte packets can be sent redundantly up two times per
second:
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., the
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.
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 units 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.
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]. Senders MAY make use of TYPE 5 units. All receivers MUST implement support for TYPE 5 units, since it adds minimum complexity and may increase the robustness of the streaming session. The next section specifies how SIDX values are calculated.
(page 25 continued on part 2)
