Internet DRAFT - draft-hellstrom-avt-rfc2793bis
draft-hellstrom-avt-rfc2793bis
AVT Working Group
Internet Draft G. Hellstrom
<draft-hellstrom-avt-rfc2793bis-02.txt> Omnitor AB
Expires: April 2004
P. Jones
Cisco Systems, Inc.
October 2003
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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
[Note to RFC Editor: All references to RFC XXXX are to be replaced by
references to the RFC number of this memo, when published.]
Abstract
This memo describes how to carry 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.
This RTP payload description contains an optional possibility to
include redundant text from already transmitted packets in order to
reduce the risk of text loss caused by packet loss. The redundancy
coding follows RFC 2198.
Hellstrom Expires - April 2004 [Page 1]
RTP Payload for Text Conversation October 2003
Table of Contents
1. Introduction..................................................2
2. Conventions used in this document.............................3
3. Usage of RTP..................................................3
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"..........................................4
3.4 Use of Redundancy........................................5
3.5 Synchronization of Text with Other Media.................5
3.6 RTP packet header........................................5
3.7 Additional Headers.......................................6
3.8 T.140 Text Structure.....................................6
4. Recommended Procedure.........................................7
4.1 Recommended Basic Procedure..............................7
4.2 Recommended Procedure for Compensation for Lost Packets..8
4.3 Recommended Procedure for Compensation for Packets Out of
Order....................................................8
4.4 Transmission During "Silent Periods" when Redundancy is
Used.....................................................8
5. SDP Attribute for Flow Control................................9
6. Examples......................................................9
6.1 RTP Packetization Examples for the text/t140 format......9
6.2 RTP Packetization Examples for the audio/t140 format....11
6.3 SDP Examples............................................13
7. Security Considerations......................................14
8. MIME Media Type Registrations................................14
8.1 Registration of MIME Media Type text/t140...............14
8.2 Registration of MIME Media Type audio/t140..............15
8.3 Registration of MIME Media Type text/RED................16
9. Authors' Addresses...........................................17
10. Acknowledgements............................................18
11. Normative References........................................18
12. Informative References......................................18
13. Full Copyright Statement....................................18
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
Hellstrom Expires - April 2004 [Page 2]
RTP Payload for Text Conversation October 2003
conversation services. Text in text conversation sessions is sent 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). Therefore, it is generally
expected number of characters to transmit is low. 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 10 646-1 code with UTF-8 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 duplications, and with detection
and indication of losses. 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 channel load by the redundancy scheme is minimal.
This document updates and extends RFC 2793. The text is intended to
to clarify any ambiguities in RFC 2793, improve on the specific
implementation requirements learned through development experience,
give explicit usage examples and to introduce a means 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.
3. Usage of RTP
When transport of T.140 text session data in RTP is desired, the
payloads as described in this specification SHOULD be used.
Hellstrom Expires - April 2004 [Page 3]
RTP Payload for Text Conversation October 2003
3.1 Payload Format for Transmission of text/t140 Data
A text conversation RTP packet as specified by this 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 is no additional header specific to this
payload format.
This 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
most commonly use this format.
3.2 Payload Format for Transmission of audio/t140 Data
A text conversation RTP packet as specified by this payload format
consists of an RTP header as defined in RFC 3550 followed immediately
by a 16-bit "t140block counter" followed by a "T140block" (see
section 3.3). There is no additional header 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 may be
utilized to detect lost characters.
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
audio/t140 format, the T140block counter MUST always precede the
actual T140block, including redundant data transmissions.
The primary purpose for this 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 [8].
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. This implies
that each block MUST contain an integral number of UTF-8 encoded
characters regardless of the number of octets per character. It also
Hellstrom Expires - April 2004 [Page 4]
RTP Payload for Text Conversation October 2003
implies that 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 Synchronization of Text with Other Media
Usually, each medium in a session utilizes a separate RTP stream. In
that case, 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 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
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 text packets be properly
reproduced on the egress gateway before considering any subsequent
voice or 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.
3.6 RTP packet header
Each RTP packet starts with a fixed RTP header. The following fields
of the RTP fixed header are used 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 is identified by the name "T140" (see section 8).
If redundancy is used per RFC 2198, the Payload Type MUST indicate
that payload format ("RED").
Hellstrom Expires - April 2004 [Page 5]
RTP Payload for Text Conversation October 2003
Sequence number: The Sequence Number MUST be increased by one for
each new transmitted packet. 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. The clock frequency may be
specified for audio/t140 and is generally set to be 8000 Hz, as
that is most common for audio. 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
losses.
3.7 Additional Headers
There are no additional headers defined specific to this payload
format.
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.
3.8 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 increases the data rate.
Since packets are not generated at regular intervals and since the
audio/t140 format allows for other media to be interleaved, 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. While the audio/t140
Hellstrom Expires - April 2004 [Page 6]
RTP Payload for Text Conversation October 2003
payload format does not rely on the sequence numbers of packets to
identify missing data, the same rules apply. 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:
- 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
for a short moment. A maximum buffering time of 500 ms is specified.
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 correctly received packet. On
receipt of audio/t140 data, the T140block counter is compared with
the T140block counter of the last correctly received packet. If they
are consecutive, the (only or primary) T140block is retrieved from
the packet.
Hellstrom Expires - April 2004 [Page 7]
RTP Payload for Text Conversation October 2003
4.2 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) or T140block counters (audio/t140),
and redundant T140blocks are available in a subsequent packet, the
sequence numbers or T140block counters for the redundant T140blocks
should be inferred by counting backwards from the sequence number or
T140block counter in the RTP header for that packet. If there are
redundant T140blocks with sequence numbers matching those that are
missing, the redundant T140blocks may be substituted for the missing
T140blocks.
Both for the case when redundancy is used and not used, 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].
4.3 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.
4.4 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/t140payload
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
Hellstrom Expires - April 2004 [Page 8]
RTP Payload for Text Conversation October 2003
be so that sequence number inference for the redundant T140blocks
will be correct, as explained in section 3.8.
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.
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.
To control the character transmission rate, the "fmtp" attribute [7]
is used 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.
Devices in receipt of this parameter MUST adhere to the request to
apply by transmitting characters at a rate at or below the specified
<integer> value.
6. Examples
6.1 RTP Packetization Examples for the text/t140 format.
This is an example of a 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) |
Hellstrom Expires - April 2004 [Page 9]
RTP Payload for Text Conversation October 2003
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ T.140 encoded data +
| |
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is an example of an 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 |
+ +
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This 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
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hellstrom Expires - April 2004 [Page 10]
RTP Payload for Text Conversation October 2003
|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, this example 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 in order to
represent the empty 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 "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
Hellstrom Expires - April 2004 [Page 11]
RTP Payload for Text Conversation October 2003
This is an example of a 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 +
| |
+ +---------------+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is an example of an RTP packet with one redundant T140block
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, this example 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.
Hellstrom Expires - April 2004 [Page 12]
RTP Payload for Text Conversation October 2003
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.
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
Hellstrom Expires - April 2004 [Page 13]
RTP Payload for Text Conversation October 2003
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
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
7. Security Considerations
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. When
redundant data is included, the same security considerations as for
RFC 2198 apply. Additionally, all of the security considerations
from section 14 or RFC 3550 apply.
8. MIME Media Type Registrations
This document defines an RTP payload named "t140" and two associated
MIME types, "text/t140" and "audio/t140". Additionally, the MIME
type "text/RED" is defined to allow RFC 2198 to be used to carry
redundant text payloads.
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 character that may be received
per second.
Hellstrom Expires - April 2004 [Page 14]
RTP Payload for Text Conversation October 2003
Encoding considerations: T.140 text can be transmitted with RTP as
specified in RFC <TDB>.
Security considerations: None
Interoperability considerations: None
Published specification: ITU-T T.140 Recommendation.
RFC XXXX.
Applications which use this media type:
Text communication terminals and text conferencing tools.
Additional information: None
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 |
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.
Optional parameters:
cps: The maximum number of character that may be received
per second.
Encoding considerations: T.140 text can be transmitted with RTP as
specified in RFC XXXX.
Security considerations: None
Hellstrom Expires - April 2004 [Page 15]
RTP Payload for Text Conversation October 2003
Interoperability considerations: None
Published specification: ITU-T T.140 Recommendation.
RFC <TDB>.
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: None
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 |
8.3 Registration of MIME Media Type text/RED
MIME media type name: text
MIME subtype name: RED
Required parameters:
pt: a comma-separated list of RTP payload types. Because
comma is a special character, the list must be a quoted-string
(enclosed in double quotes). For static payload types, each
list element is simply the type number. For dynamic payload
types, each list element is a mapping of the dynamic payload
type number to an embedded MIME content-type specification for
the payload format corresponding to the dynamic payload type.
The format of the mapping is:
dynamic-payload-type "=" content-type
If the content-type string includes a comma, then the
content-type string MUST be a quoted-string. If the content-
type string does not include a comma, it MAY still be quoted.
Hellstrom Expires - April 2004 [Page 16]
RTP Payload for Text Conversation October 2003
Since it is part of the list which must itself be a quoted-
string, that means the quotation marks MUST be quoted with
backslash quoting as specified in RFC 2045. If the content-
type string itself contains a quoted-string, then the
requirement for backslash quoting is recursively applied. To
specify the text/RED payload format in SDP, the pt parameter
is mapped to an a=fmtp attribute by eliminating the parameter
name (pt) and changing the commas to slashes. For example,
'pt="101,102"' maps to 'a=fmtp:99 101/102'.
Optional parameters: ptime, maxptime
Encoding considerations:
This type is only defined for transfer via RTP [2].
Security considerations: None
Interoperability considerations: none
Published specification: RFC 2198
Applications which use this media type:
Text streaming and conferencing tools.
Additional information: 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 |
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.
Hellstrom Expires - April 2004 [Page 17]
RTP Payload for Text Conversation October 2003
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 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
2279, January 1998.
[7] Handley, M., Jacobson, V., "SDP: Session Description Protocol",
RFC 2327, April 1998.
12. Informative References
[8] Schulzrinne, H., Petrack, S., "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals", May 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
Hellstrom Expires - April 2004 [Page 18]
RTP Payload for Text Conversation October 2003
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
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 - April 2004 [Page 19]
RTP Payload for Text Conversation October 2003
Hellstrom Expires - April 2004 [Page 20]
