Network Working Group G. Hellstrom
Request for Comments: 2793 Omnitor AB
Category: Standards Track May 2000
RTP Payload for Text Conversation
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2000). All Rights Reserved.
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.
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.
Hellstrom Standards Track [Page 1]RFC 2793 RTP Payload for Text Conversation May 2000
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 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.
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 
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 octets. This implies
that each block MUST contain an integral number of UTF-8 encoded
Hellstrom Standards Track [Page 2]RFC 2793 RTP Payload for Text Conversation May 2000