| RFC 8865 | T.140 Data Channel | January 2021 |
| Holmberg & Hellström | Standards Track | [Page] |
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.¶
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 athttps://www.rfc-editor.org/info/rfc8865.¶
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.¶
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"; seeSection 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, andSection 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, and6 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.¶
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.¶
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 | SeeSection 4.1 |
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.¶
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.¶
An offerer and answererMUST, 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 answererMUST include the 'subprotocol' attribute parameter, with a "t140" parameter value, in the 'dcmap' attribute.¶
The offerer and answererMAY 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 answererMUST 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 answererMUST reject the offer. If either of those attribute parameters is received in an answer, the offererMUST NOT accept the answer. Instead, the answererMUST 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, itMUST 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"¶
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 inSection 6.7 of [RFC8864].¶
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 valueMUST 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 answererMAY 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 answererMUST 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, itSHOULD 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.¶
'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].¶
'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.¶
If the offerer wishes to both send and receive text on a T.140 data channel, itSHOULD 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, itMUST 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, itMUST 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, itMUST 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, itMUST be prepared to receive T.140 data as soon as the state of the T.140 data channel allows it.¶
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 answererSHOULD 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 answererMUST 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 answererMUST 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 answererMUST 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, itMUST be prepared to receive data as soon as the state of the T.140 data channel allows transmission of data.¶
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 offererMUST 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 isRECOMMENDED 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.¶
If an endpoint wishes to modify a previously negotiated text direction in an ongoing session, itMUST initiate an offer that indicates the new direction, following the rules inSection 4.2.3.1. If the answerer accepts the offer, it follows the procedures inSection 4.2.3.2.¶
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 eoAnswer: 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 recvonlyAnswer: 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¶
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.¶
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.¶
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.¶
BufferingMAY 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 isRECOMMENDED 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.¶
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 isRECOMMENDED 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 endpointMUST NOT retransmit T140blocks. Similarly, a receiverSHOULD indicate to the user that a channel has been reestablished and text might have been lost. ThisMAY 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.¶
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.¶
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:¶
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-partyconnections. At the time of writing this specification, a multi-party solutionfor RTP-based T.140 transport had not yet been specified. Once such a solutionis specified, it might have an impact on the above interworking guidance and considerations.¶
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 answerersMUST NOT include the attributes directly in the "m=" section associated with the "application/webrtc-datachannel" media type. Instead, the attributesMUST be associated with individual data channels, using the SDP 'dcsa' attribute. A specification that defines a subprotocol that uses the attributesMUST 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.¶
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.¶
Per this document, the subprotocol identifier "t140" has been added to the "WebSocket Subprotocol Name Registry" as follows:¶
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 inSection 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:¶
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 inSection 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:¶
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:¶
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 inSection 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:¶
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:¶
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:¶
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:¶
This document is based on an earlier Internet-Draft edited byKeith Drage,Juergen Stoetzer-Bradler, andAlbrecht Schwarz.¶
Thomas Belling provided useful comments on the initial (pre‑submission) version of the current document.Paul Kyzivat andBernard Aboba provided comments on the document.¶