AVT Working Group G. Hellstrom
Internet Draft Omnitor AB
<draft-ietf-avt-rfc2793bis-03.txt>
Expires: September 2004 P. Jones
Cisco Systems, Inc.
March 2004
RTP Payload for Text Conversation
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
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[Notes to RFC Editor:
1. All references to RFC XXXX are to be replaced by references to
the RFC number of this memo, when published.
2. All references to RFC YYYY are to be replaced by references to
the document that registers the text/red MIME type.
Abstract
This memo describes how to carry real time text conversation session
contents in RTP packets. Text conversation session contents are
specified in ITU-T Recommendation T.140.
Two payload formats are described. One for transmitting text on a
separate RTP session dedicated for the transmission of text, and one
for transmitting audio and text data within one single RTP session.
This RTP payload description contains an optional possibility to
include redundant text from already transmitted packets in order to
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reduce the risk of text loss caused by packet loss. The redundancy
coding follows RFC 2198.
Table of Contents
1. Introduction..................................................3
2. Conventions used in this document.............................4
3. Usage of RTP..................................................4
3.1 Payload Format for Transmission of text/t140 Data........4
3.2 Payload Format for Transmission of audio/t140 Data.......4
3.3 The "T140block"..........................................5
3.4 Use of Redundancy........................................5
3.5 Use of Forward Error Correction..........................5
3.6 Synchronization of Text with Other Media.................5
3.7 RTP packet header........................................6
3.8 Structure of redundant data..............................7
3.9 T.140 Text Structure.....................................7
4. Recommended Procedure.........................................7
4.1 Recommended Basic Procedure..............................8
4.2 Detection of Lost Text Packets...........................8
4.3 Recommended Procedure for Compensation for Lost Packets..8
4.4 Recommended Procedure for Compensation for Packets Out of
Order....................................................9
4.5 Transmission During "Silent Periods" when Redundancy is
Used.....................................................9
5. SDP Attribute for Character Transmission Rate................10
6. Examples.....................................................11
6.1 RTP Packetization Examples for the text/t140 format.....11
6.2 RTP Packetization Examples for the audio/t140 format....13
6.3 SDP Examples............................................15
7. Security Considerations......................................16
7.1 Confidentiality.........................................16
7.2 Integrity...............................................16
7.3 Source authentication...................................16
8. IANA considerations..........................................16
8.1 Registration of MIME Media Type text/t140...............17
8.2 Registration of MIME Media Type audio/t140..............18
9. Authors' Addresses...........................................19
10. Acknowledgements............................................19
11. Normative References........................................19
12. Informative References......................................20
13. Intellectual Property Right Considerations..................20
14. Full Copyright Statement....................................20
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1. Introduction
This document defines two payload types for carrying text
conversation session contents in RTP packets. Text conversation
session contents are specified in ITU-T Recommendation T.140 [1].
Text conversation is used alone or in connection to other
conversational facilities such as video and voice, to form multimedia
conversation services. Text in multimedia conversation sessions is
sent character-by-character as soon as it is available, or with a
small delay for buffering.
The text is supposed to be entered by human users from a keyboard,
handwriting recognition, voice recognition or any other input method.
The rate of character entry is usually at a level of a few characters
per second or less, though text may be transmitted at a much higher
rate (e.g., automated systems or "copy and paste" operations may
produce a lot of text very rapidly). In general, though, only one or
a few new characters are expected to be transmitted with each packet.
T.140 specifies that text and other T.140 elements must be
transmitted in ISO 10646-1[5] code with UTF-8 [6] transformation.
That makes it easy to implement internationally useful applications
and to handle the text in modern information technology environments.
The payload of an RTP packet following this specification consists of
text encoded according to T.140 without any additional framing. A
common case will be a single ISO 10646 character, UTF-8 encoded.
T.140 requires the transport channel to provide characters without
duplication and in original order. Text conversation users expect
that text will be delivered with no or a low level of lost
information. If lost information can be indicated, the willingness to
accept loss is expected to be higher.
Therefore a mechanism based on RTP is specified here. It gives text
arrival in correct order, without duplication, and with detection and
indication of loss. It also includes an optional possibility to
repeat data for redundancy to lower the risk of loss. Since packet
overhead is usually much larger than the T.140 contents, the increase
in bandwidth with the use of redundancy is minimal.
By using RTP for text transmission in a multimedia conversation
application, uniform handling of text and other media can be achieved
in, as examples, conferencing systems, firewalls, and network
translation devices. This, in turn, eases the design and increases
the possibility for prompt and proper media delivery.
This document obsoletes RFC 2793 [11]. The text clarifies
ambiguities in RFC 2793, improves on the specific implementation
requirements learned through development experience, gives explicit
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usage examples, and introduces a method of transporting text
interleaved with voice within the same RTP session.
2. Conventions used in this document
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 [4].
3. Usage of RTP
Two payload formats for real-time text transmission with RTP are
described in this section, one for general text conversation use and
another for use between gateways.
3.1 Payload Format for Transmission of text/t140 Data
A text conversation RTP packet as specified by the text/t140 payload
format consists of an RTP header as defined in RFC 3550 [2] followed
immediately by a block of T.140 data, referred to as a "T140block"
(see section 3.3). There are no additional headers specific to this
payload format.
The text/t140 format is primarily used when text is transmitted on a
separate RTP session dedicated for the transmission of text and not
shared with other media, such as audio, DTMF, etc. IP textphone
devices and IP multimedia conversation devices and network elements
involved in communication with such devices most commonly use this
format.
3.2 Payload Format for Transmission of audio/t140 Data
A text conversation RTP packet as specified by the audio/t140 payload
format consists of an RTP header as defined in RFC 3550 followed
immediately by a 16-bit "t140block counter" (with the most
significant bit transmitted first) followed by a "T140block" (see
section 3.3). There are no additional headers specific to this
payload format.
The T140block counter MUST be initialized to zero the first time that
a packet containing a T140block is transmitted and MUST be
incremented by 1 each time that a new block is transmitted. Once the
counter reaches the value 0xFFFF, the counter is reset to 0 the next
time the counter is incremented. This T140block counter is used to
detect lost blocks and to avoid duplication of blocks.
For the purposes of readability, the remainder of this document only
refers to the T140block without making explicit reference to the
T140block counter. Readers should understand that when using the
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audio/t140 format, the T140block counter MUST always precede the
actual T140block, including redundant data transmissions.
The primary purpose for the audio/t140 payload specification is to
allow gateways that are interconnecting two PSTN networks to
interleave, through a single RTP session, audio and text data
received on the PSTN circuit. This is comparable to the way in which
DTMF is extracted and transmitted within an RTP session [10]. Note
that the audio/t140 format does not allow simultaneous audio and text
transmission, because the expectation is that at each moment, only
one payload type is selected for play-out.
3.3 The "T140block"
The T140block contains one or more T.140 code elements as specified
in [1]. Most T.140 code elements are single ISO 10646 [5]
characters, but some are multiple character sequences. Each
character is UTF-8 encoded [6] into one or more octets. Each block
MUST contain an integral number of UTF-8 encoded characters
regardless of the number of octets per character. Any composite
character sequence (CCS) SHOULD be placed within one block.
3.4 Use of Redundancy
The T140blocks MAY be transmitted redundantly according to the
payload format defined in RFC 2198 [3]. In that case, the RTP header
is followed by one or more redundant data block headers, the same
number of redundant data fields carrying T140blocks from previous
packets, and finally the new (primary) T140block for this packet.
3.5 Use of Forward Error Correction
When transmitting text via RTP it is possible to use other robustness
mechanisms, including Forward Error Correction (FEC) as described in
RFC 2733 [8]. In such applications, the same mechanisms MAY be
employed with text as with other media formats.
3.6 Synchronization of Text with Other Media
Usually, each medium in a session utilizes a separate RTP stream. As
such, if synchronization of the text and other media packets is
important, the streams MUST be associated when the sessions are
established and the streams MUST share the same reference clock
(refer to the description of the timestamp field as it relates to
synchronization in section 5.1 of RFC 3550). Association of RTP
streams is dependent on the particular session application and is
outside the scope of this document.
When audio/t140 is used, it is generally transmitted as interleaved
packets between voice packets or other kinds of audio packets. One
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should observe the RTP timestamps of the voice, text, or other audio
packets in order to reproduce the stream correctly when playing out
the audio. Note, also, that incoming text from a PSTN circuit might
be at a higher bit-rate than can be played out on an egress PSTN
circuit. As such, it is possible that, on the egress side, a gateway
may not complete the play out of the text packets before it is time
to play the next voice packet. Given that this application is
primarily for the benefit of deaf users utilizing PSTN textphone
devices, it is strongly RECOMMENDED that all received text packets be
properly reproduced on the egress gateway before considering any
subsequent other audio packets. If necessary, voice and other audio
packets should be discarded in order to properly reproduce the text
signals on the PSTN circuit, even if the text packets arrive late.
The users commonly use turn-taking indicators in the text stream, so
it can be expected that as long as text is transmitted, it is valid
text and should be given priority over voice.
3.7 RTP packet header
Each RTP packet starts with a fixed RTP header. The following fields
of the RTP fixed header are specified for T.140 text streams:
Payload Type (PT): The assignment of an RTP payload type is specific
to the RTP profile under which this payload format is used. For
profiles that use dynamic payload type number assignment, this
payload format can be identified by the MIME types "text/T140" and
"audio/T140" (see section 8). If redundancy is used per RFC 2198,
another payload type number needs to be provided for the redundancy
format. MIME types for identifying RFC 2198 are available in RFC
3555 and RFC YYYY.
Sequence number: The definition of sequence numbers is available in
RFC 3550 [2]. When transmitting text using the payload format for
text/t140, it is used for detection of packet loss and packets out
of order, and can be used in the process of retrieval of redundant
text, reordering of text and marking missing text. Character loss
is detected through the T140block counter when using the audio/t140
payload format.)
Timestamp: The RTP Timestamp encodes the approximate instance of
entry of the primary text in the packet. A clock frequency of 1000
Hz MUST be used for text/t140. For audio/T140, the clock frequency
MAY be set to any value, and SHOULD be set to the same value as for
any audio packets in the same RTP stream in order to avoid RTP
timestamp rate switching. The value SHOULD be set by out of band
mechanisms. Sequential packets MUST NOT use the same timestamp.
Since packets do not represent any constant duration, the timestamp
cannot be used to directly infer packet loss.
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M-bit: The M-bit has no defined meaning for t140 text streams and
MUST be set to 0.
3.8 Structure of redundant data.
When redundant transmission of the data according to RFC 2198 is
desired, the RTP header is followed by one or more redundant data
block headers, one for each redundant data block to be included.
Each of these headers provides the timestamp offset and length of the
corresponding data block plus a payload type number indicating this
payload format ("T140"). Redundant data older than 16383 divided by
the clock frequency MUST NOT be transmitted.
3.9 T.140 Text Structure
T.140 text is UTF-8 coded as specified in T.140 with no extra
framing. When using the format with redundant data, the transmitter
MAY select a number of T140block generations to retransmit in each
packet. A higher number introduces better protection against loss of
text but marginally increases the data rate.
The timestamp is not sufficient to identify a packet in the presence
of loss unless extra information is provided. Since sequence numbers
are not provided in the redundant header, some additional rules must
be followed to allow the redundant data corresponding to missing
primary data to be merged properly into the stream of primary data
T140blocks when using the text/t140 payload format. They are:
- Each redundant data block MUST contain the same data as a
T140block previously transmitted as primary data, and be
identified with a timestamp offset equating to the original
timestamp for that T140block.
- The redundant data MUST be placed in age order with most recent
redundant T140block last in the redundancy area.
- All T140blocks from the oldest desired generation up through the
generation immediately preceding the new (primary) T140block
MUST be included.
For the text/t140 payload format, these rules allow the sequence
numbers for the redundant T140blocks to be inferred by counting
backwards from the sequence number in the RTP header. The result
will be that all the text in the payload will be contiguous and in
order.
4. Recommended Procedure
This section contains RECOMMENDED procedures for usage of the payload
format. Based on the information in the received packets, the
receiver can:
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- reorder text received out of order.
- mark where text is missing because of packet loss.
- compensate for lost packets by using redundant data.
4.1 Recommended Basic Procedure
Packets are transmitted only when there is valid T.140 data to
transmit. The sequence number is used for sequencing of T.140 data.
T.140 specifies that T.140 data MAY be buffered before transmission
with a maximum buffering time of 500 ms. In order to keep the maximum
bit rate usage for text at a reasonable level, it is RECOMMENDED to
buffer T.140 data for transmission in 300 ms intervals. This time is
selected so that text users will still perceive a real time text
flow.
On reception of text/t140 data, the RTP sequence number is compared
with the sequence number of the last received packet. On receipt of
audio/t140 data, the T140block counter is compared with the T140block
counter of the last received packet. If they are consecutive, the
(only or primary) T140block is retrieved from the packet.
4.2 Detection of Lost Text Packets
Packet loss for text/t140 packets MAY be detected by observing gaps
in the sequence numbers of RTP packets received by the receiver.
With audio/t140, however, packets following a text packet might be
audio packets of a format other than audio/text, so the same rule
does not apply. Rather, receivers detect the loss of an audio/t140
packet by observing the value of the T140block counter in a
subsequent audio/t140 packet.
With both text/t140 and audio/t140, the loss of the last packet of a
sequence of packets cannot be detected until the next text packet is
transmitted.
4.3 Recommended Procedure for Compensation for Lost Packets
For reduction of data loss in case of packet loss, redundant data MAY
be included in the packets following to the procedures in RFC 2198.
If network conditions are not known, it is RECOMMENDED to use three
redundant T140blocks in each packet. If there is a gap in the RTP
sequence numbers for text/t140, and redundant T140blocks are
available in a subsequent packet, the sequence numbers for the
redundant T140blocks should be inferred by counting backwards from
the sequence number in the RTP header for that packet. Note that
redundant audio/t140 data contains the t140block counters preceding
the T140block, which simplifies the ordering by the receiver. If
there are redundant T140blocks with sequence numbers or t140block
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counters matching those that are missing, the redundant T140blocks
may be substituted for the missing T140blocks.
As an alternative (or in addition) to redundancy, Forward Error
Correction mechanisms MAY be used when transmitting text, as per RFC
2733 [8] or any other mechanism with the purpose of increasing the
reliability of text transmission.
There are also other mechanisms for increasing robustness of
transmission that MAY be applied.
Missing data SHOULD be marked by insertion of a missing text marker
in the received stream for each missing T140block, as specified in
ITU-T T.140 Addendum 1 [1].
4.4 Recommended Procedure for Compensation for Packets Out of Order
For protection against packets arriving out of order, the following
procedure MAY be implemented in the receiver. If analysis of a
received packet reveals a gap in the sequence and no redundant data
is available to fill that gap, the received packet SHOULD be kept in
a buffer to allow time for the missing packet(s) to arrive. It is
RECOMMENDED that the waiting time be limited to 0.5 seconds.
If a packet with a T140block belonging to the gap arrives before the
waiting time expires, this T140block is inserted into the gap and
then consecutive T140blocks from the leading edge of the gap may be
consumed. Any T140block which does not arrive before the time limit
expires should be treated as lost.
4.5 Transmission During "Silent Periods" when Redundancy is Used
When using the redundancy transmission scheme, and there is redundant
data, but no new T.140 data to transmit after the transmit buffering
interval described in section 4.1 has passed, a packet MUST be
transmitted containing a zero-length primary T140block and the
properly positioned redundant data. When using the audio/t140
payload format with an empty T140block, the T140block counter MUST
also be absent (as there is no actual T140block).
When using the text/t140 payload format, any zero-length T140blocks
that are sent as primary data MUST be included as redundant
T140blocks on subsequent packets just as normal text T140blocks would
be so that sequence number inference for the redundant T140blocks
will be correct, as explained in section 3.9.
When using the audio/t140 payload format, zero-length T140blocks sent
as primary data MUST NOT be included as redundant T140blocks, as it
would simply be a waste of bandwidth to send them.
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Redundancy for the last T140block MUST NOT be implemented by
repeatedly transmitting the same packet (with the same sequence
number) because this will cause the packet loss count, as reported in
RTCP, to decrement.
5. SDP Attribute for Character Transmission Rate
In some cases, it is necessary to limit the rate at which characters
are transmitted. While the "b=" SDP attribute could be used to limit
the rate of the RTP session, it may be that only the text stream in
an interleaved audio/text session needs special handling. For
example, when a PSTN gateway is interworking between an IP device
(not necessarily a textphone) and a PSTN textphone, it may be
necessary to limit the character rate from the IP device in order to
avoid throwing away characters at the PSTN gateway. At the same
time, no explicit bit rate restriction is necessarily applied to the
audio stream. Despite the fact that character rate may be limited in
a session, the instantaneous reception rate might be higher due to
jitter.
To control the character transmission rate, the MIME parameter "cps="
in the "fmtp" attribute [7] is defined (see section 8 ). It is used
in SDP with the following syntax:
a=fmtp:<format> cps=<integer>
The <format> field is populated with the payload type that is used
for text. The <integer> field contains an integer representing the
maximum number of characters that may be received per second.
Examples of use in SDP are found in section 6.3.
Devices in receipt of this parameter MUST adhere to the request by
transmitting characters at a rate at or below the specified <integer>
value. Note that this parameter was not defined in RFC 2793 [11].
Therefore implementations of the text/t140 format may be in use that
do not recognize and act according to this parameter. Receivers of
text/t140 SHALL therefore be designed so that they can handle
temporary reception of characters at a higher rate than this
parameter specifies, so that no malfunction is experienced for text
conversation with human input.
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6. Examples
6.1 RTP Packetization Examples for the text/t140 format.
Below is an example of a text/t140 RTP packet without redundancy.
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=0 |M| T140 PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp (1000Hz) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ T.140 encoded data +
| |
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Below is an example of a text/t140 RTP packet with one redundant
T140block.
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=0 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R" | "R" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | |
+-+-+-+-+-+-+-+-+ +
| |
+ "R" T.140 encoded redundant data +
| |
+ +---------------+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| "P" T.140 encoded primary data |
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Below is an example of an RTP packet with one redundant T140block
using text/t140 payload format. The primary data block is
empty, which is the case when transmitting a packet for the
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sole purpose of forcing the redundant data to be transmitted
in the absence of any new data.
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=0 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R" | "R" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | |
+-+-+-+-+-+-+-+-+ +
| |
+ "R" T.140 encoded redundant data +
| |
+ +---------------+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As a follow-on to the previous example, the example below shows
the next RTP packet in the sequence which does contain a real
T140block when using the text/t140 payload format. Note that the
empty block is present in the redundant transmissions of the
text/t140 payload format. This example shows 2 levels of
redundancy and one primary data block. The value of the "R2
block length" would be set to zero in order to
represent the empty T140block.
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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=0 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R1" | "R1" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R2" | "R2" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | |
+-+-+-+-+-+-+-+-+ +
| |
+ "R1" T.140 encoded redundant data +
| |
+ +---------------+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| "P" T.140 encoded primary data |
+ +
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.2 RTP Packetization Examples for the audio/t140 format
Below is an example of an audio/t140 RTP packet without
redundancy.
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=0 |M| T140 PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp (8000Hz) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T140block Counter | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
+ T.140 encoded data +
| |
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Below is an example of an RTP packet with one redundant T140block
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using audio/t140 payload format. The primary data block is
empty, which is the case when transmitting a packet for the
sole purpose of forcing the redundant data to be transmitted
in the absence of any new data. Note that since this is the
audio/t140 payload format, the redundant block of T.140 data is
immediately preceded with a T140block counter.
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=0 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R" | "R" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| T140 PT | T140block Counter | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ "R" T.140 encoded redundant data +
| |
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As a follow-on to the previous example, the example below shows
the next RTP packet in the sequence which does contain a new real
T140block when using the audio/t140 payload format. This
example has 2 levels of redundancy and one primary data block.
Since the previous primary block was empty, no redundant data
is included for that block. This is because when using the
audio/t140 payload format, any previously transmitted "empty"
T140blocks are NOT included as redundant data in subsequent
packets.
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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=0 |M| "RED" PT | sequence number of primary |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp of primary encoding "P" |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | timestamp offset of "R1" | "R1" block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| T140 PT | T140block Counter | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ "R1" T.140 encoded redundant data +
| |
+ +---------------+
| | T140block_ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter | "P" T.140 encoded primary data |
+-+-+-+-+-+-+-+-+ +
| |
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.3 SDP Examples
Below is an example of SDP describing RTP text transport on port
11000:
m=text 11000 RTP/AVP 98
a=rtpmap:98 t140/1000
Below is an example of SDP similar to the above example, but also
utilizing RFC 2198 to provide redundancy for the text packets:
m=text 11000 RTP/AVP 98 100
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98
Below is an example of SDP describing RTP text interleaved with G.711
audio packets within the same RTP session from port 7200 and at a
maximum text rate of 6 characters per second:
m=audio 7200 RTP/AVP 0 98
a=rtpmap:98 t140/8000
a=fmtp:98 cps=6
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Below is an example using RFC 2198 to provide redundancy to just the
text packets in an RTP session with interleaving text and G.711 at a
text rate no faster than 6 characters per second:
m=audio 7200 RTP/AVP 0 98 100
a=rtpmap:98 t140/8000
a=fmtp:98 cps=6
a=rtpmap:100 red/8000
a=fmtp:100 98/98
Note - While these examples utilize the RTP/AVP profile, it is not
intended to limit the scope of this memo to use with only that
profile. Rather, any appropriate profile may be used in conjunction
with this memo.
7. Security Considerations
All of the security considerations from section 14 of RFC 3550 apply.
7.1 Confidentiality
Since the intention of the described payload format is to carry text
in a text conversation, security measures in the form of encryption
are of importance. The amount of data in a text conversation session
is low and therefore any encryption method MAY be selected and
applied to T.140 session contents or to the whole RTP packets. SRTP
[9] provides a suitable method for ensuring confidentiality.
7.2 Integrity
It may be desirable to protect the text contents of an RTP stream
against manipulation. SRTP [9] provides methods for providing
integrity that MAY be applied.
7.3 Source authentication
Measures to make sure that the source of text is the intended one can
be accomplished by a combination of methods.
Text streams are usually used in a multimedia control environment.
Security measures for authentication are available and SHOULD be
applied in the registration and session establishment procedures, so
that the identity of the sender of the text stream is reliably
associated with the person or device setting up the session. Once
established, SRTP [9] mechanisms MAY be applied to ascertain that the
source is maintained the same during the session.
8. IANA considerations
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This document defines an RTP payload named "t140" and two associated
MIME types, "text/t140" and "audio/t140", to be registered by IANA.
8.1 Registration of MIME Media Type text/t140
MIME media type name: text
MIME subtype name: t140
Required parameters:
rate: The RTP timestamp clock rate, which is equal to the
sampling rate. The only valid value is 1000.
Optional parameters:
cps: The maximum number of characters that may be received
per second.
Encoding considerations: T.140 text can be transmitted with RTP as
specified in RFC XXXX.
Security considerations: See section 7 of RFC XXXX.
Interoperability considerations: This format is the same as
specified in RFC2793. For RFC2793 the "cps=" parameter was not
defined. Therefore there may be implementations that do not
consider this parameter. Receivers need to take that into account.
Published specification: ITU-T T.140 Recommendation.
RFC XXXX.
Applications which use this media type:
Text communication terminals and text conferencing tools.
Additional information: This type is only defined for transfer via
RTP.
Magic number(s): None
File extension(s): None
Macintosh File Type Code(s): None
Person & email address to contact for further information:
Gunnar Hellstrom
E-mail: gunnar.hellstrom@omnitor.se
Intended usage: COMMON
Author / Change controller:
Gunnar Hellstrom | IETF avt WG
gunnar.hellstrom@omnitor.se |
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8.2 Registration of MIME Media Type audio/t140
MIME media type name: audio
MIME subtype name: t140
Required parameters:
rate: The RTP timestamp clock rate, which is equal to the
sampling rate. This parameter SHOULD have the same value as for
any audio codec packets interleaved in the same RTP stream.
Optional parameters:
cps: The maximum number of characters that may be received
per second.
Encoding considerations: T.140 text can be transmitted with RTP as
specified in RFC XXXX.
Security considerations: See section 7 of RFC XXXX.
Interoperability considerations: None
Published specification: ITU-T T.140 Recommendation.
RFC XXXX.
Applications which use this media type:
Text communication systems and text conferencing tools that
transmit text associated with audio and within the same RTP
session as the audio, such as PSTN gateways that transmit
audio and text signals between two PSTN textphone users
over an IP network.
Additional information: This type is only defined for transfer
via RTP.
Magic number(s): None
File extension(s): None
Macintosh File Type Code(s): None
Person & email address to contact for further information:
Paul E. Jones
E-mail: paulej@packetizer.com
Intended usage: COMMON
Author / Change controller:
Paul E. Jones | IETF avt WG
paulej@packetizer.com |
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9. Authors' Addresses
Gunnar Hellstrom
Omnitor AB
Renathvagen 2
SE-121 37 Johanneshov
Sweden
Phone: +46 708 204 288 / +46 8 556 002 03
Fax: +46 8 556 002 06
E-mail: gunnar.hellstrom@omnitor.se
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
Phone: +1 919 392 6948
E-mail: paulej@packetizer.com
10. Acknowledgements
The authors want to thank Stephen Casner, Magnus Westerlund and Colin
Perkins for valuable support with reviews and advice on creation of
this document, to Mickey Nasiri at Ericsson Mobile Communication for
providing the development environment, and Michele Mizarro for
verification of the usability of the payload format for its intended
purpose.
11. Normative References
[1] ITU-T Recommendation T.140 (1998) - Text conversation protocol
for multimedia application, with amendment 1, (2000).
[2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, July 2003.
[3] Perkins, C., Kouvelas, I., Hardman, V., Handley, M. and J.
Bolot, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] ISO/IEC 10646-1: (1993), Universal Multiple Octet Coded
Character Set.
[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
3629, December 2003.
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[7] Handley, M., Jacobson, V., "SDP: Session Description Protocol",
RFC 2327, April 1998.
[8] Rosenberg, J., Schulzrinne, H., "An RTP Payload Format for
Generic Forward Error Correction", RFC 2733, December 1999.
[9] Baugher, McGrew, Carrara, Naslund, Norrman, The Secure Real-Time
Transport Protocol (SRTP), RFC 3711, March 2004.
12. Informative References
[10] Schulzrinne, H., Petrack, S., "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals", RFC 2833, May 2000.
[11] Hellstrom, G., "RTP Payload for text conversation.", RFC2793,
2000
13. Intellectual Property Right Considerations
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
14. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
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RTP Payload for Text Conversation March 2004
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Hellstrom Expires - September 2004 [Page 21]
