T.140 Real-Time Text Conversation over WebRTC Data Channels
RFC 8865

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Internet Engineering Task Force (IETF)                       C. Holmberg
Request for Comments: 8865                                      Ericsson
Updates: 8373                                               G. Hellström
Category: Standards Track      Gunnar Hellström Accessible Communication
ISSN: 2070-1721                                             January 2021

      T.140 Real-Time Text Conversation over WebRTC Data Channels

Abstract

   This document specifies how a Web Real-Time Communication (WebRTC)
   data channel can be used as a transport mechanism for real-time text
   using the ITU-T Protocol for multimedia application text conversation
   (Recommendation ITU-T T.140) and how the Session Description Protocol
   (SDP) offer/answer mechanism can be used to negotiate such a data
   channel, referred to as a T.140 data channel.  This document updates
   RFC 8373 to specify its use with WebRTC data channels.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8865.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions
   3.  WebRTC Data Channel Considerations
   4.  SDP Considerations
     4.1.  Use of the 'dcmap' Attribute
     4.2.  Use of the 'dcsa' Attribute
       4.2.1.  Maximum Character Transmission Rate
       4.2.2.  Real-Time Text Conversation Languages
       4.2.3.  Real-Time Text Direction
     4.3.  Examples
   5.  T.140 Considerations
     5.1.  Session-Layer Functions
     5.2.  Data Encoding and Sending
     5.3.  Data Buffering
     5.4.  Loss of T140blocks
     5.5.  Multi-party Considerations
   6.  Gateway Considerations
   7.  Update to RFC 8373
   8.  Security Considerations
   9.  IANA Considerations
     9.1.  Subprotocol Identifier "t140"
     9.2.  SDP 'fmtp' Attribute
     9.3.  SDP Language Attributes
     9.4.  SDP Media Direction Attributes
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The ITU-T Protocol for multimedia application text conversation
   (Recommendation ITU-T T.140) [T140] defines a protocol for text
   conversation, also known as real-time text.  The transport used for
   IP networks is the "RTP Payload for Text Conversation" mechanism
   [RFC4103], based on the Real-time Transport Protocol (RTP) [RFC3550].

   This document specifies how a Web Real-Time Communication (WebRTC)
   data channel [RFC8831] can be used as a transport mechanism for T.140
   and how the Session Description Protocol (SDP) offer/answer mechanism
   for data channels [RFC8864] can be used to negotiate such a data
   channel.

   In this document, a T.140 data channel refers to a WebRTC data
   channel for which the instantiated subprotocol is "t140" and where
   the channel is negotiated using the SDP offer/answer mechanism
   [RFC8864].

      |  NOTE: The decision to transport real-time text using a WebRTC
      |  data channel instead of using RTP-based transport [RFC4103] is
      |  motivated by use case "U-C 5: Real-time text chat during an
      |  audio and/or video call with an individual or with multiple
      |  people in a conference"; see Section 3.2 of [RFC8831].

   The brief notation "T.140" is used as a name for the text
   conversation protocol according to [T140].

   Real-time text is intended to be entered by human users via 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.

   Section 3 defines the generic data channel properties for a T.140
   data channel, and Section 4 defines how they are conveyed in an SDP
   'dcmap' attribute.  While this document defines how to negotiate a
   T.140 data channel using the SDP offer/answer mechanism [RFC8864],