RTP Payload for Text Conversation
RFC 2793

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 Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

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.

1. Introduction

   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 [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
   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.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4]

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 [2] 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 [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

Hellstrom                   Standards Track                     [Page 2]
RFC 2793           RTP Payload for Text Conversation            May 2000