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

Document Type RFC - Proposed Standard (January 2021; No errata)
Updates RFC 8373
Authors Christer Holmberg  , Gunnar Hellstrom 
Last updated 2021-01-19
Replaces draft-holmberg-mmusic-t140-usage-data-channel
Stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication (wg milestone: Mar 2020 - Submit T.140 Real-ti... )
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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],
   the generic T.140 and gateway considerations defined in Sections 3,
   5, and 6 of this document can also be applied when a T.140 data
   channel is established using another mechanism (e.g., the mechanism
   defined in [RFC8832]).  Section 5 of [RFC8864] defines the mapping
   between the SDP 'dcmap' attribute parameters and the protocol
   parameters used in [RFC8832].

   This document updates [RFC8373] by defining how the SDP 'hlang-send'
   and 'hlang-recv' attributes are used for the "application/webrtc-
   datachannel" media type.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  WebRTC Data Channel Considerations

   The following WebRTC data channel property values [RFC8831] apply to
   a T.140 data channel:

              +--------------------------+-----------------+
              | Subprotocol Identifier   | t140            |
              +--------------------------+-----------------+
              | Transmission reliability | reliable        |
              +--------------------------+-----------------+
              | Transmission order       | in-order        |
              +--------------------------+-----------------+
              | Label                    | See Section 4.1 |
              +--------------------------+-----------------+

                                 Table 1

      |  NOTE: T.140 requires the transport channel to provide
      |  transmission of real-time text without duplication and in
      |  original order.  Therefore, T.140 does not specify reliable and
      |  ordered transmission of T.140 data on the application layer.
      |  Instead, when RTP-based transport is used, the RTP sequence
      |  number is used to detect packet loss and out-of-order packets,
      |  and a redundancy mechanism is used to achieve reliable delivery
      |  of T.140 data.  By using the WebRTC data channel's reliable and
      |  in-order transmission features [RFC8831] for the T.140 data
      |  channel, there is no need for a redundancy mechanism or a
      |  mechanism to detect data loss and out-of-order delivery at the
      |  application level.  The latency characteristics of the T.140
      |  data channel are also regarded as sufficient to meet the
      |  application requirements of T.140.

4.  SDP Considerations

   The generic SDP considerations, including the SDP offer/answer
   procedures [RFC3264], for negotiating a WebRTC data channel are
   defined in [RFC8864].  This section, and its subsections, define the
   SDP considerations that are specific to a T.140 data channel,
   identified by the 'subprotocol' attribute parameter, with a "t140"
   parameter value, in the 'dcmap' attribute.

4.1.  Use of the 'dcmap' Attribute

   An offerer and answerer MUST, in each offer and answer, include an
   SDP 'dcmap' attribute [RFC8864] in the SDP media description
   ("m=" section) [RFC4566] describing the Stream Control Transmission
   Protocol (SCTP) association [RFC4960] used to realize the T.140 data
   channel.

   The offerer and answerer MUST include the 'subprotocol' attribute
   parameter, with a "t140" parameter value, in the 'dcmap' attribute.

   The offerer and answerer MAY include the 'priority' attribute
   parameter and the 'label' attribute parameter in the 'dcmap'
   attribute value, as specified in [RFC8864].

      |  NOTE: As specified in [RFC8831], when a data channel is
      |  negotiated using the mechanism defined in [RFC8832], the
      |  'label' attribute parameter value has to be the same in both
      |  directions.  That rule also applies to data channels negotiated
      |  using the mechanism defined in this document.

   The offerer and answerer MUST NOT include the 'max-retr' or 'max-
   time' attribute parameter in the 'dcmap' attribute.  If either of
   those attribute parameters is received in an offer, the answerer MUST
   reject the offer.  If either of those attribute parameters is
   received in an answer, the offerer MUST NOT accept the answer.
   Instead, the answerer MUST take appropriate actions, e.g., by sending
   a new offer without a T.140 data channel or by terminating the
   session.

   If the 'ordered' attribute parameter is included in the 'dcmap'
   attribute, it MUST be assigned the value 'true'.

   Below is an example of the 'dcmap' attribute for a T.140 data channel
   with stream id=3 and without any label:

   a=dcmap:3 subprotocol="t140"

4.2.  Use of the 'dcsa' Attribute

   An offerer and answerer can, in each offer and answer, include one or
   more SDP 'dcsa' attributes [RFC8864] in the "m=" section describing
   the SCTP association used to realize the T.140 data channel.

   If an offerer or answerer receives a 'dcsa' attribute that contains
   an SDP attribute whose usage has not been defined for a T.140 data
   channel, the offerer or answerer should ignore the 'dcsa' attribute,
   following the rules in Section 6.7 of [RFC8864].

4.2.1.  Maximum Character Transmission Rate

   A 'dcsa' attribute can contain the SDP 'fmtp' attribute, which is
   used to indicate a maximum character transmission rate [RFC4103].
   The 'cps' attribute parameter is used to indicate the maximum
   character transmission rate that the endpoint that includes the
   attribute is able to receive, and the value is used as a mean value
   in characters per second over any 10-second interval.

   If the 'fmtp' attribute is included, the 'format' attribute parameter
   value MUST be set to 't140'.

   If no 'fmtp' attribute with a 'cps' attribute parameter is included,
   the default value of 30 applies [RFC4103].

   The offerer and answerer MAY modify the 'cps' attribute parameter
   value in subsequent offers and answers.

   This document does not define any other usage of the 'fmtp' attribute
   for a T.140 channel.  If an offerer or answerer receives a 'dcsa'
   attribute that contains an 'fmtp' attribute that is not set according
   to the procedure above, the offerer or answerer MUST ignore the
   'dcsa' attribute.

      |  NOTE: The 'cps' attribute parameter is especially useful when a
      |  T.140 data channel endpoint is acting as a gateway (Section 6)
      |  and is interworking with a T.140 transport mechanism that has
      |  restrictions on how many characters can be sent per second.

   If an endpoint receives text at a higher rate than it can handle,
   e.g., because the sending endpoint does not support the 'cps'
   attribute parameter, it SHOULD either (1) indicate to the sending
   endpoint that it is not willing to receive more text, using the
   direction attributes (Section 4.2.3) or (2) use a flow-control
   mechanism to reduce the rate.  However, in certain applications,
   e.g., emergency services, it is important to regain human interaction
   as soon as possible, and it might therefore be more appropriate to
   simply discard the received overflow, insert a mark for loss
   [T140ad1], and continue to process the received text as soon as
   possible.

      |  NOTE: At the time of writing this specification, the
      |  standardized API for WebRTC data channels does not support flow
      |  control.  Should such support be available at some point, a
      |  receiving endpoint might use it in order to slow down the rate
      |  of text received from the sending endpoint.

4.2.2.  Real-Time Text Conversation Languages

   'dcsa' attributes can contain the SDP 'hlang-send' and 'hlang-recv'
   attributes [RFC8373] to negotiate the language to be used for the
   real-time text conversation.

   For a T.140 data channel, the modality is "written" [RFC8373].

4.2.3.  Real-Time Text Direction

   'dcsa' attributes can contain the SDP 'sendonly', 'recvonly',
   'sendrecv', and 'inactive' attributes [RFC4566] to negotiate the
   direction in which text can be transmitted in a real-time text
   conversation.

      |  NOTE: A WebRTC data channel is always bidirectional.  The usage
      |  of the 'dcsa' attribute only affects the direction in which
      |  implementations are allowed to transmit text on a T.140 data
      |  channel.

   The offer/answer rules for the direction attributes are based on the
   rules for unicast streams defined in [RFC3264], as described below.
   Note that the rules only apply to the direction attributes.

   Session-level direction attributes [RFC4566] have no impact on a
   T.140 data channel.

4.2.3.1.  Generating an Offer

   If the offerer wishes to both send and receive text on a T.140 data
   channel, it SHOULD mark the data channel as sendrecv with a
   'sendrecv' attribute inside a 'dcsa' attribute.  If the offerer does
   not explicitly mark the data channel, an implicit 'sendrecv'
   attribute inside a 'dcsa' attribute is applied by default.

   If the offerer wishes to only send text on a T.140 data channel, it
   MUST mark the data channel as sendonly with a 'sendonly' attribute
   inside a 'dcsa' attribute.

   If the offerer wishes to only receive text on a T.140 data channel,
   it MUST mark the data channel as recvonly with a 'recvonly' attribute
   inside a 'dcsa' attribute.

   If the offerer wishes to neither send nor receive text on a T.140
   data channel, it MUST mark the data channel as inactive with an
   'inactive' attribute inside a 'dcsa' attribute.

   If the offerer has marked a data channel as sendrecv (or if the
   offerer did not explicitly mark the data channel) or recvonly, it
   MUST be prepared to receive T.140 data as soon as the state of the
   T.140 data channel allows it.

4.2.3.2.  Generating an Answer

   When the answerer accepts an offer and marks the direction of the
   text in the corresponding answer, the direction is based on the
   marking (or the lack of explicit marking) in the offer.

   If the offerer either explicitly marked the data channel as sendrecv
   or did not mark the data channel, the answerer SHOULD mark the data
   channel as sendrecv, sendonly, recvonly, or inactive with a
   'sendrecv', 'sendonly', 'recvonly', or 'inactive' attribute,
   respectively, inside a 'dcsa' attribute.  If the answerer does not
   explicitly mark the data channel, an implicit 'sendrecv' attribute
   inside a 'dcsa' attribute is applied by default.

   If the offerer marked the data channel as sendonly, the answerer MUST
   mark the data channel as recvonly or inactive with a 'recvonly' or
   'inactive' attribute, respectively, inside a 'dcsa' attribute.

   If the offerer marked the data channel as recvonly, the answerer MUST
   mark the data channel as sendonly or inactive with a 'sendonly' or
   'inactive' attribute, respectively, inside a 'dcsa' attribute.

   If the offerer marked the data channel as inactive, the answerer MUST
   mark the data channel as inactive with an 'inactive' attribute inside
   a 'dcsa' attribute.

   If the answerer has marked a data channel as sendrecv or recvonly, it
   MUST be prepared to receive data as soon as the state of the T.140
   data channel allows transmission of data.

4.2.3.3.  Offerer Receiving an Answer

   When the offerer receives an answer to the offer and the answerer has
   marked a data channel as sendrecv (or the answerer did not mark the
   data channel) or recvonly in the answer, the offerer can start
   sending T.140 data as soon as the state of the T.140 data channel
   allows it.  If the answerer has marked the data channel as inactive
   or sendonly, the offerer MUST NOT send any T.140 data.

   If the answerer has not marked the direction of a T.140 data channel
   in accordance with the procedures above, it is RECOMMENDED that the
   offerer not process that scenario as an error situation but rather
   assume that the answerer might both send and receive T.140 data on
   the data channel.

4.2.3.4.  Modifying the Text Direction

   If an endpoint wishes to modify a previously negotiated text
   direction in an ongoing session, it MUST initiate an offer that
   indicates the new direction, following the rules in Section 4.2.3.1.
   If the answerer accepts the offer, it follows the procedures in
   Section 4.2.3.2.

4.3.  Examples

   Below is an example of an "m=" section of an offer for a T.140 data
   channel offering real-time text conversation in Spanish and
   Esperanto, and an "m=" section in the associated answer accepting
   Esperanto.  The maximum character transmission rate is set to 20.  As
   the offerer and answerer have not explicitly indicated the real-time
   text direction, the default direction "sendrecv" applies.

   Offer:

      m=application 911 UDP/DTLS/SCTP webrtc-datachannel
      c=IN IP6 2001:db8::3
      a=max-message-size:1000
      a=sctp-port 5000
      a=setup:actpass
      a=dcmap:2 label="ACME customer service";subprotocol="t140"
      a=dcsa:2 fmtp:t140 cps=20
      a=dcsa:2 hlang-send:es eo
      a=dcsa:2 hlang-recv:es eo

   Answer:

      m=application 2004 UDP/DTLS/SCTP webrtc-datachannel
      c=IN IP6 2001:db8::1
      a=max-message-size:1000
      a=sctp-port 6000
      a=setup:passive
      a=dcmap:2 label="ACME customer service";subprotocol="t140"
      a=dcsa:2 fmtp:t140 cps=20
      a=dcsa:2 hlang-send:eo
      a=dcsa:2 hlang-recv:eo

   Below is an example of an "m=" section of an offer for a T.140 data
   channel where the offerer wishes to only receive real-time text, and
   an "m=" section in the associated answer indicating that the answerer
   will only send real-time text.  No maximum character transmission
   rate is indicated.  No preference for the language to be used for the
   real-time text conversation is indicated.

   Offer:

      m=application 1400 UDP/DTLS/SCTP webrtc-datachannel
      c=IN IP6 2001:db8::3
      a=max-message-size:1000
      a=sctp-port 5000
      a=setup:actpass
      a=dcmap:2 label="ACME customer service";subprotocol="t140"
      a=dcsa:2 recvonly

   Answer:

      m=application 2400 UDP/DTLS/SCTP webrtc-datachannel
      c=IN IP6 2001:db8::1
      a=max-message-size:1000
      a=sctp-port 6000
      a=setup:passive
      a=dcmap:2 label="ACME customer service";subprotocol="t140"
      a=dcsa:2 sendonly

5.  T.140 Considerations

5.1.  Session-Layer Functions

   Section 6.1 of [T140] describes the generic T.140 session control
   functions at a high level, in a manner that is independent of the
   signaling protocol.  The list below describes how the functions are
   realized when using a T.140 data channel.

   Prepare session:  An endpoint can indicate its support of T.140 data
      channels using signaling-specific means (e.g., using SIP OPTIONS
      [RFC3261]) or by indicating the support in an offer or answer
      (Section 4).

   Initiate session:  An offer is used to request the establishment of a
      T.140 data channel (Section 4).

   Accept session:  An answer is used to accept a request to establish a
      T.140 data channel (Section 4).

   Deny session:  An answer is used to reject a request to establish a
      T.140 data channel, using the generic procedures for rejecting a
      data channel [RFC8864].

   Disconnect session:  An offer or answer is used to disable a
      previously established T.140 data channel, using the generic
      procedures for closing a data channel [RFC8864].

   Data:  Data is sent on an established T.140 data channel
      (Section 5.2).

5.2.  Data Encoding and Sending

   T.140 text is encoded and framed as T140blocks [RFC4103].

   Each T140block is sent on the SCTP stream [RFC4960] used to realize
   the T.140 data channel using standard T.140 transmission procedures
   [T140].  One or more T140blocks can be sent in a single SCTP user
   message [RFC4960].  Unlike RTP-based transport for real-time text
   [RFC4103], T.140 data channels do not use redundant transmission of
   text; this is because the T.140 data channel achieves robust
   transmission by using the "reliable" mode of the data channel.

   Data-sending procedures conform to [T140].

   See Section 8 of [T140] for coding details.

      |  NOTE: The T.140 coding details contain information on optional
      |  control codes for controlling the presentation; these control
      |  codes may not be supported by the presentation level of the
      |  receiving application.  The receiving application is expected
      |  to handle reception of such T.140 control codes appropriately
      |  (e.g., ignore and skip them) even if their effect on the
      |  presentation is not supported.

5.3.  Data Buffering

   As described in [T140], buffering can be used to reduce overhead,
   with the maximum assigned transmission interval of T140blocks from
   the buffer being 500 ms as long as there is text to send.

   Buffering MAY also be used for staying within the maximum character
   transmission rate (Section 4.2).

   An implementation needs to take the user requirements for smooth flow
   and low latency in real-time text conversation into consideration
   when assigning a transmission interval.  It is RECOMMENDED to use the
   default transmission interval of 300 ms [RFC4103], for T.140 data
   channels.  Implementers might also use lower values for specific
   applications requiring low latency, taking the increased overhead
   into consideration.

5.4.  Loss of T140blocks

   In the case of network failure or congestion, T.140 data channels
   might fail and get torn down.  If this happens but the session is
   sustained, it is RECOMMENDED that implementations try to reestablish
   the T.140 data channels.  As a T.140 data channel does not provide a
   mechanism for the receiver to identify retransmitted T140blocks after
   channel reestablishment, the sending endpoint MUST NOT retransmit
   T140blocks.  Similarly, a receiver SHOULD indicate to the user that a
   channel has been reestablished and text might have been lost.  This
   MAY be done by inserting the missing text markers [T140ad1] or in any
   other way evident to the user.

      |  NOTE: If the SCTP association [RFC4960] used to realize the
      |  T.140 data channel fails and gets torn down, it needs to be
      |  reestablished before the T.140 data channel can be
      |  reestablished.  After the T.140 data channel is reestablished,
      |  the procedures defined in this section apply, regardless of
      |  whether only the T.140 data channel or the whole SCTP
      |  association got torn down.

5.5.  Multi-party Considerations

   If an implementation needs to support multi-party scenarios, the
   implementation needs to support multiple simultaneous T.140 data
   channels, one for each remote party.  At the time of writing this
   document, this is true even in scenarios where each participant
   communicates via a centralized conference server.  This is because,
   unlike RTP media, WebRTC data channels and the T.140 protocol do not
   support the indication of the source of T.140 data.  The 'label'
   attribute parameter in the SDP 'dcmap' attribute (Section 4.1) can be
   used by the offerer to provide additional information about each
   T.140 data channel and help implementations to distinguish between
   them.

      |  NOTE: Future extensions to T.140 or the T140block might permit
      |  the indication of the source of T.140 data, in which case it
      |  might be possible to use a single T.140 data channel to
      |  transport data from multiple remote sources.  The usage of a
      |  single T.140 data channel, without any protocol extensions,
      |  would require the conference server to only forward real-time
      |  text from one source at any given time and, for example,
      |  include human-readable text labels in the real-time text stream
      |  that indicate the source whenever the conference server
      |  switches the source.  This would allow the receiver to present
      |  real-time text from different sources separately.  The
      |  procedures for such a mechanism are outside the scope of this
      |  document.

6.  Gateway Considerations

   A number of real-time text transports and protocols have been defined
   for both packet-switched and circuit-switched networks.  Many are
   based on the ITU-T T.140 protocol at the application and presentation
   levels [T140].  At the time of writing this document, some mechanisms
   are no longer used, as the technologies they use have been obsoleted,
   while others are still in use.

   When performing interworking between T.140 data channels and real-
   time text in other transports and protocols, a number of factors need
   to be considered.  At the time of writing this document, the most
   common IP-based real-time text transport is the RTP-based mechanism
   defined in [RFC4103].  While this document does not define a complete
   interworking solution, the list below provides some guidance and
   considerations to take into account when designing a gateway for
   interworking between T.140 data channels and RTP-based T.140
   transport:

   *  For each T.140 data channel, there is an RTP stream for real-time
      text [RFC4103].  Redundancy is by default declared and used on the
      RTP stream.  There is no redundancy on the T.140 data channel, but
      the reliable property [RFC8864] is set on it.

   *  During a normal text flow, T140blocks received from one network
      are forwarded towards the other network.  Keepalive traffic is
      handled by lower layers on the T.140 data channel.  A gateway
      might have to extract keepalives from incoming RTP streams and MAY
      generate keepalives on outgoing RTP streams.

   *  If the gateway detects or suspects loss of data on the RTP stream
      and the lost data has not been retrieved using a redundancy
      mechanism, the gateway SHOULD insert the T.140 missing text marker
      [T140ad1] in the data sent on the outgoing T.140 data channel.

   *  If the gateway detects that the T.140 data channel has failed and
      got torn down, once the data channel has been reestablished the
      gateway SHOULD insert the T.140 missing text marker [T140ad1] in
      the data sent on the outgoing RTP stream if it detects or suspects
      that data sent by the remote T.140 data channel endpoint was lost.

   *  If the gateway detects that the T.140 data channel has failed and
      got torn down, once the data channel has been reestablished the
      gateway SHOULD insert the T.140 missing text marker [T140ad1] in
      the data sent on the outgoing T.140 data channel if it detects or
      suspects that data sent or to be sent on the T.140 data channel
      was lost during the failure.

   *  The gateway MUST indicate the same text transmission direction
      (Section 4.2.3) on the T.140 data channel and the RTP stream.

      |  NOTE: In order for the gateway to insert a missing text marker
      |  or perform other actions that require that the gateway have
      |  access to the T.140 data, the T.140 data cannot be encrypted
      |  end to end between the T.140 data channel endpoint and the RTP
      |  endpoint.  At the time of writing this document, no mechanism
      |  to provide such end-to-end encryption is defined.

      |  NOTE: The guidance and considerations above are for two-party
      |  connections.  At the time of writing this specification, a
      |  multi-party solution for RTP-based T.140 transport had not yet
      |  been specified.  Once such a solution is specified, it might
      |  have an impact on the above interworking guidance and
      |  considerations.

7.  Update to RFC 8373

   This document updates [RFC8373] by defining how the SDP 'hlang-send'
   and 'hlang-recv' attributes are used for the "application/webrtc-
   datachannel" media type.

   SDP offerers and answerers MUST NOT include the attributes directly
   in the "m=" section associated with the "application/webrtc-
   datachannel" media type.  Instead, the attributes MUST be associated
   with individual data channels, using the SDP 'dcsa' attribute.  A
   specification that defines a subprotocol that uses the attributes
   MUST specify the modality for that subprotocol, or how to retrieve
   the modality if the subprotocol supports multiple modalities.  The
   subprotocol is indicated using the SDP 'dcmap' attribute.

8.  Security Considerations

   The generic WebRTC security considerations are defined in [RFC8826]
   and [RFC8827].

   The generic security considerations for WebRTC data channels are
   defined in [RFC8831].  As data channels are always encrypted by
   design, the T.140 data channels will also be encrypted.

   The generic security considerations for negotiating data channels
   using the SDP offer/answer mechanism are defined in [RFC8864].  There
   are no additional security considerations specific to T.140 data
   channels.

   When performing interworking between T.140 data channels and RTP-
   based T.140 transport [RFC4103], in order for a gateway to insert a
   missing text marker or perform other actions that require that the
   gateway have access to the T.140 data, the T.140 data cannot be
   encrypted end to end between the T.140 data channel endpoint and the
   RTP endpoint.

9.  IANA Considerations

9.1.  Subprotocol Identifier "t140"

   Per this document, the subprotocol identifier "t140" has been added
   to the "WebSocket Subprotocol Name Registry" as follows:

   Subprotocol Identifier:  t140

   Subprotocol Common Name:  ITU-T T.140 Real-Time Text

   Subprotocol Definition:  RFC 8865

   Reference:  RFC 8865

9.2.  SDP 'fmtp' Attribute

   This document defines the usage of the SDP 'fmtp' attribute, if this
   attribute is included in an SDP 'dcsa' attribute associated with a
   T.140 real-time text session over a WebRTC data channel.  The usage
   is defined in Section 4.2.1.

   The usage level "dcsa (t140)" has been added to the registration of
   the SDP 'fmtp' attribute in the "Session Description Protocol (SDP)
   Parameters" registry as follows:

   Contact name:  IESG

   Contact email:  iesg@ietf.org

   Attribute name:  fmtp

   Usage level:  dcsa (t140)

   Purpose:  Indicate format parameters for a T.140 data channel, such
      as maximum character transmission rates.

   Reference:  RFC 8865

9.3.  SDP Language Attributes

   This document modifies the usage of the SDP 'hlang-send' and 'hlang-
   recv' attributes, if these attributes are included in SDP 'dcsa'
   attributes associated with a T.140 data channel.  The modified usage
   is described in Section 4.2.2.

   The usage level "dcsa (t140)" has been added to the registration of
   the SDP 'hlang-send' attribute in the "Session Description Protocol
   (SDP) Parameters" registry as follows:

   Contact name:  IESG

   Contact email:  iesg@ietf.org

   Attribute name:  hlang-send

   Usage level:  dcsa (t140)

   Purpose:  Negotiate the language to be used on a T.140 data channel.

   Reference:  RFC 8865

   The usage level "dcsa (t140)" has been added to the registration of
   the SDP 'hlang-recv' attribute in the "Session Description Protocol
   (SDP) Parameters" registry as follows:

   Contact name:  IESG

   Contact email:  iesg@ietf.org

   Attribute name:  hlang-recv

   Usage level:  dcsa (t140)

   Purpose:  Negotiate the language to be used on a T.140 data channel.

   Reference:  RFC 8865

9.4.  SDP Media Direction Attributes

   This document modifies the usage of the SDP 'sendonly', 'recvonly',
   'sendrecv', and 'inactive' attributes, if these attributes are
   included in SDP 'dcsa' attributes associated with a T.140 data
   channel.  The modified usage is described in Section 4.2.3.

   The usage level "dcsa (t140)" has been added to the registration of
   the SDP 'sendonly' attribute in the "Session Description Protocol
   (SDP) Parameters" registry as follows:

   Contact name:  IESG

   Contact email:  iesg@ietf.org

   Attribute name:  sendonly

   Usage level:  dcsa (t140)

   Purpose:  Negotiate the direction in which real-time text can be sent
      on a T.140 data channel.

   Reference:  RFC 8865

   The usage level "dcsa (t140)" has been added to the registration of
   the SDP 'recvonly' attribute in the "Session Description Protocol
   (SDP) Parameters" registry as follows:

   Contact name:  IESG

   Contact email:  iesg@ietf.org

   Attribute name:  recvonly

   Usage level:  dcsa (t140)

   Purpose:  Negotiate the direction in which real-time text can be sent
      on a T.140 data channel.

   Reference:  RFC 8865

   The usage level "dcsa (t140)" has been added to the registration of
   the SDP 'sendrecv' attribute in the "Session Description Protocol
   (SDP) Parameters" registry as follows:

   Contact name:  IESG

   Contact email:  iesg@ietf.org

   Attribute name:  sendrecv

   Usage level:  dcsa (t140)

   Purpose:  Negotiate the direction in which real-time text can be sent
      on a T.140 data channel.

   Reference:  RFC 8865

   The usage level "dcsa (t140)" has been added to the registration of
   the SDP 'inactive' attribute in the "Session Description Protocol
   (SDP) Parameters" registry as follows:

   Contact name:  IESG

   Contact email:  iesg@ietf.org

   Attribute name:  inactive

   Usage level:  dcsa (t140)

   Purpose:  Negotiate the direction in which real-time text can be sent
      on a T.140 data channel.

   Reference:  RFC 8865

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
              <https://www.rfc-editor.org/info/rfc4103>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8373]  Gellens, R., "Negotiating Human Language in Real-Time
              Communications", RFC 8373, DOI 10.17487/RFC8373, May 2018,
              <https://www.rfc-editor.org/info/rfc8373>.

   [RFC8826]  Rescorla, E., "Security Considerations for WebRTC",
              RFC 8826, DOI 10.17487/RFC8826, January 2021,
              <https://www.rfc-editor.org/info/rfc8826>.

   [RFC8827]  Rescorla, E., "WebRTC Security Architecture", RFC 8827,
              DOI 10.17487/RFC8827, January 2021,
              <https://www.rfc-editor.org/info/rfc8827>.

   [RFC8831]  Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
              Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021,
              <https://www.rfc-editor.org/info/rfc8831>.

   [RFC8864]  Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
              Even, Ed., "Negotiation Data Channels Using the Session
              Description Protocol (SDP)", RFC 8864,
              DOI 10.17487/RFC8864, January 2021,
              <https://www.rfc-editor.org/info/rfc8864>.

   [T140]     ITU-T, "Protocol for multimedia application text
              conversation", Recommendation ITU-T T.140, February 1998,
              <https://www.itu.int/rec/T-REC-T.140-199802-I/en>.

   [T140ad1]  ITU-T, "Recommendation ITU-T.140 Addendum 1 (02/2000),
              Protocol for multimedia application text conversation",
              February 2000,
              <https://www.itu.int/rec/T-REC-T.140-200002-I!Add1/en>.

10.2.  Informative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC8832]  Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data Channel
              Establishment Protocol", RFC 8832, DOI 10.17487/RFC8832,
              January 2021, <https://www.rfc-editor.org/info/rfc8832>.

Acknowledgements

   This document is based on an earlier Internet-Draft edited by Keith
   Drage, Juergen Stoetzer-Bradler, and Albrecht Schwarz.

   Thomas Belling provided useful comments on the initial
   (pre-submission) version of the current document.  Paul Kyzivat and
   Bernard Aboba provided comments on the document.

Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   FI-02420 Jorvas
   Finland

   Email: christer.holmberg@ericsson.com

   Gunnar Hellström
   Gunnar Hellström Accessible Communication
   Esplanaden 30
   SE-136 70 Vendelsö
   Sweden

   Email: gunnar.hellstrom@ghaccess.se