RTP Payload for Text Conversation
|The information below is for an old version of the document that is already published as an RFC
RFC Internet-Draft (avt WG)
(latest revision 2000-02-07)
Internet Engineering Task Force (IETF)
No shepherd assigned
RFC 2793 (Proposed Standard)
||Send notices to
Internet Engineering Task Force AVT WG
Internet Draft Gunnar Hellstrom
draft-ietf-avt-rtp-text-04.txt LM Ericsson
February, 6, 2000
Expires: August 6, 2000
RTP Payload for Text Conversation
STATUS OF THIS MEMO
This document is an Internet-Draft and it is in full conformance with
all provisions of section 10 of RFC2026. 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
The list of Internet-Draft Shadow Directories can be accessed at
Distribution of this document is unlimited.
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 .
Text conversation is used alone or in connection to other
conversational facilities such as video and voice, to form multimedia
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 [Page 1]
This memo defines a payload type for carrying text conversation
session contents in RTP packets. Text conversation session contents
are specified in ITU-T Recommendation T.140 . Text conversation is
used alone or in connection to other conversational facilities such
as video and voice, to form multimedia 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. Therefore, the 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
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.
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 
Hellstrom [Page 2]
2. Usage of RTP
When transport of T.140 text session data in RTP is desired, the
payload as described in this specification SHOULD be used.
A text conversation RTP packet as specified by this payload format
consists of an RTP header as defined in RFC 1889  followed
immediately by a block of T.140 data, defined here to be a
"T140block". There is no additional header specific to this
payload format. The T140block contains one or more T.140 code
elements as specified in . Most T.140 code elements are single
ISO 10646  characters, but some are multiple character
sequences. Each character is UTF-8 encoded  into one or more
The T140blocks MAY be transmitted redundantly according to the
payload format defined in RFC 2198 . 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
2.1 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 which use dynamic payload type number assignment, this
payload format is identified by the name "T140" (see section 6).
If redundancy is used per RFC 2198, the Payload Type MUST indicate
that payload format ("RED").
Sequence number: The Sequence Number MUST be increased by one for
each new transmitted packet. 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.
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. 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.
Hellstrom [Page 3]
2.2 Additional headers
There are no additional headers defined specific to this payload
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").
2.3 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, 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:
- 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.
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.
3. Recommended procedures
This section contains RECOMMENDED procedures for usage of the payload
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.
Hellstrom [Page 4]
3.1 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.
On reception, the RTP sequence number is compared with the sequence
number of the last correctly received packet. If they are
consecutive, the (only or primary) T140block is retrieved from the
3.2 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 one
redundant T140block in each packet. If there is a gap in
the RTP sequence numbers, 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. If there are
redundant T140blocks with sequence numbers matching those that are
missing, the redundant T140blocks may be substituted for the
Both for the case when redundancy is used and not used, missing data
SHOULD be marked. T.140 defines a missing data marker (Unicode
character 2607 "Lightning") which SHOULD be used as a marker to
indicate each missing T140block.
3.3 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 can be kept in a buffer to allow time for the
missing packet(s) to arrive. It is suggested that the waiting
time be limited to 0.5 seconds. For the case when redundancy is used
the waiting time SHOULD be extended to the number of redundancy
generations times the T.140 buffering timer if this product is known
to be greater than 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.
Hellstrom [Page 5]
3.4 Transmission during "silent periods" when redundancy is used.
When using the redundancy transmission scheme, and there is nothing
more to transmit from T.140, the latest T140block has a risk of
getting old before it is transmitted as redundant data. The result
is less useful protection against packet loss at the end of a text
input sequence. For cases where this should be avoided,
a zero-length primary T140block MAY be transmitted with the redundant
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 2.3.
Redundancy for the last T140block SHOULD 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.
Hellstrom [Page 6]
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) |
| 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 |
Figure: Examples of RTP text packets.
Hellstrom [Page 7]
5. 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.
6. MIME Media Type Registrations
This document defines a new RTP payload name and associated MIME
type, T140 (text/t140).
6.1 Registration of MIME media type text/t140
MIME media type name: text
MIME subtype name: t140
Required parameters: None
Optional parameters: None
Encoding considerations: T140 text can be transmitted
with RTP as specified in "draft-ietf-avt-rtp-text-04".
Security considerations: None
Interoperability considerations: None
Published specification: ITU-T T.140 Recommendation.
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:
Intended usage: COMMON
Hellstrom [Page 8]
Internet Draft Expires: August, 7, 2000
7. Author's Address
Ericsson Mobile Communication AB
SE-164 80 Stockholm
Tel: +46 708 204 288
Fax: +46 8 556 002 06
The author wants to thank Stephen Casner and Colin Perkins for
valuable support with reviews and advice on creation of this
document, and Michele Mizarro for verification of the usability of
the payload format for its intended purpose.
 ITU-T Recommendation T.140 (1998) - Text conversation protocol for
multimedia application, with amendment 1999.
 H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC 1889.
 C. Perkins, I. Kouvelas, V. Hardman, M. Handley, and J. Bolot,
"RTP payload for redundant audio data," RFC 2198, Internet
Engineering Task Force, Sept. 1997.
 S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
 ISO/IEC 10646-1: (1993), Universal Multiple Octet Coded Character
 F. Yergeau, "UTF-8, a transformation format of ISO 10646," RFC
2279, Internet Engineering Task Force, Jan. 1998.
10. Revision history.
Oct 6 1999. draft-01. Explanations in recommended procedures
Oct 22 1999. draft-02. Nomenclature, ordering of redundant packets,
send empty info to expire redundancy.
Jan 13 2000. draft-03. Clarification after working group last call,
Clarify that timestamp is not used for
Sequencing. Improvements in clarity.
Feb 6 2000. draft-04. Four "SHALL" changed to "MUST" because they
was the intention. Two typos corrected.
Hellstrom [Page 9]