MMUSIC Working Group C. Holmberg Internet-Draft Ericsson Updates: 8373 (if approved) G. Hellstrom Intended status: Standards TrackGunnar Hellstrom Accessible Communicatio Expires: October 12, 2020 April 10, 2020 T.140 Real-time Text Conversation over WebRTC Data Channels draft-ietf-mmusic-t140-usage-data-channel-14 Abstract This document specifies how a 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 SDP offer/answer mechanism can be used to negotiate such data channel, referred to as T.140 data channel. This document updates RFC 8373 to specify its use with WebRTC data channels. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 12, 2020. Copyright Notice Copyright (c) 2020 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 (http://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 Holmberg & Hellstrom Expires October 12, 2020 [Page 1] Internet-Draft T.140 Data Channel April 2020 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 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. WebRTC Data Channel Considerations . . . . . . . . . . . . . 4 4. SDP Considerations . . . . . . . . . . . . . . . . . . . . . 4 4.1. Use of dcmap Attribute . . . . . . . . . . . . . . . . . 4 4.2. Use of dcsa Attribute . . . . . . . . . . . . . . . . . . 5 4.2.1. Maximum Character Transmission Rate . . . . . . . . . 5 4.2.2. Real-time Text Conversation Languages . . . . . . . . 6 4.2.3. Real-time Text Direction . . . . . . . . . . . . . . 7 4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 9 5. T.140 Considerations . . . . . . . . . . . . . . . . . . . . 10 5.1. Session Layer Functions . . . . . . . . . . . . . . . . . 10 5.2. Data Encoding and Sending . . . . . . . . . . . . . . . . 11 5.3. Data Buffering . . . . . . . . . . . . . . . . . . . . . 11 5.4. Loss of T140blocks . . . . . . . . . . . . . . . . . . . 11 5.5. Multi-party Considerations . . . . . . . . . . . . . . . 12 6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 12 7. Update to RFC 8373 . . . . . . . . . . . . . . . . . . . . . 13 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 14 9.1. Subprotocol Identifier t140 . . . . . . . . . . . . . . . 14 9.2. SDP fmtp Attribute . . . . . . . . . . . . . . . . . . . 15 9.3. SDP Language Attributes . . . . . . . . . . . . . . . . . 15 9.4. SDP Media Direction Attributes . . . . . . . . . . . . . 16 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 11.1. Normative References . . . . . . . . . . . . . . . . . . 17 11.2. Informative References . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 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" [RFC4103] mechanism, based on the Real-time Transport Protocol (RTP) [RFC3550]. This document specifies how a WebRTC data channel [I-D.ietf-rtcweb-data-channel] can be used as a transport mechanism for T.140, and how the SDP offer/answer mechanism for data channels Holmberg & Hellstrom Expires October 12, 2020 [Page 2] Internet-Draft T.140 Data Channel April 2020 [I-D.ietf-mmusic-data-channel-sdpneg] 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 sub-protocol is "t140", and where the channel is negotiated using the SDP-based external negotiation method [I-D.ietf-mmusic-data-channel-sdpneg]. 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 [I-D.ietf-rtcweb-data-channel]. 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 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. 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 establish a T.140 data channel using the SDP-based external negotiation method [I-D.ietf-mmusic-data-channel-sdpneg], the generic T.140 and gateway considerations defined in Section 3, Section 5 and Section 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 [I-D.ietf-rtcweb-data-protocol]). Section 5 of [I-D.ietf-mmusic-data-channel-sdpneg] defines the mapping between the SDP dcmap attribute parameters and the protocol parameters used in [I-D.ietf-rtcweb-data-protocol]. 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. Holmberg & Hellstrom Expires October 12, 2020 [Page 3] Internet-Draft T.140 Data Channel April 2020 3. WebRTC Data Channel Considerations The following WebRTC data channel property values [I-D.ietf-rtcweb-data-channel] apply to a T.140 data channel: +--------------------------+-------------------------------+ | Subprotocol Identifier | t140 | | Transmission reliability | reliable | | Transmission order | in-order | | Label | See Section 4.1 and Section 6 | +--------------------------+-------------------------------+ 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 reliable and in-order transmission features [I-D.ietf-rtcweb-data-channel] 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 is also regarded to be 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 [I-D.ietf-mmusic-data-channel-sdpneg]. This section, and its subsections, define the SDP considerations that are specific to a T.140 data channel, identified by an SDP 'dcmap' attribute [I-D.ietf-mmusic-data-channel-sdpneg] with a "t140" attribute parameter value. 4.1. Use of dcmap Attribute An offerer and answerer MUST, in each offer and answer, include an SDP 'dcmap' attribute [I-D.ietf-mmusic-data-channel-sdpneg] in the SDP media description (m= section) [I-D.ietf-mmusic-rfc4566bis] describing the 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 value. Holmberg & Hellstrom Expires October 12, 2020 [Page 4] Internet-Draft T.140 Data Channel April 2020 The offerer and answerer MAY include the priority attribute parameter and the label attribute parameter in the 'dcmap' attribute value, as specified in [I-D.ietf-mmusic-data-channel-sdpneg]. NOTE: As specified in [I-D.ietf-rtcweb-data-channel], when a data channel is negotiated using the mechanism defined in [I-D.ietf-rtcweb-data-protocol], 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 the max- time attribute parameters 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 dcsa Attribute An offerer and answerer can, in each offer and answer, include one or more SDP 'dcsa' attributes [I-D.ietf-mmusic-data-channel-sdpneg] 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 which 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 [I-D.ietf-mmusic-data-channel-sdpneg]. 4.2.1. Maximum Character Transmission Rate A 'dcsa' attribute can contain the SDP 'fmtp' attribute 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 Holmberg & Hellstrom Expires October 12, 2020 [Page 5] Internet-Draft T.140 Data Channel April 2020 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 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 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 have 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 indicate to the sending endpoint that it is not willing to receive more text, using the direction attributes (Section 4.2.3), or 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 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]. Holmberg & Hellstrom Expires October 12, 2020 [Page 6] Internet-Draft T.140 Data Channel April 2020 4.2.3. Real-time Text Direction 'dcsa' attributes can contain the SDP 'sendonly', 'recvonly', 'sendrecv' and 'inactive' attributes [I-D.ietf-mmusic-rfc4566bis] to negotiate the direction in which text can be transmitted in a real- time text conversation. NOTE: A WebRTC data channel is always bi-directional. 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 [I-D.ietf-mmusic-rfc4566bis] 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. Holmberg & Hellstrom Expires October 12, 2020 [Page 7] Internet-Draft T.140 Data Channel April 2020 If the offerer explicitly marked the data channel as sendrecv, or if the offerer 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 does not process that 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. Modify 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. Holmberg & Hellstrom Expires October 12, 2020 [Page 8] Internet-Draft T.140 Data Channel April 2020 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. Holmberg & Hellstrom Expires October 12, 2020 [Page 9] Internet-Draft T.140 Data Channel April 2020 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 signalling protocol independent manner. The list below describes how the functions are realized when using a T.140 data channel. o Prepare session: An endpoint can indicate its support of T.140 data channels using signalling specific means (e.g., using SIP OPTIONS [RFC3261]), or by indicating the support in an offer or answer (Section 4) o Initiate session: An offer used to request the establishment of a T.140 data channel (Section 4) o Accept session: An answer used to accept a request to establish a T.140 data channel (Section 4) o Deny session: An answer used to reject a request to establish a T.140 data channel, using the generic procedures for rejecting a data channel [I-D.ietf-mmusic-data-channel-sdpneg] o Disconnect session: An offer or answer used to disable a previously established T.140 data channel, using the generic procedures for closing a data channel [I-D.ietf-mmusic-data-channel-sdpneg] o Data: Data sent on an established T.140 data channel (Section 5.2) Holmberg & Hellstrom Expires October 12, 2020 [Page 10] Internet-Draft T.140 Data Channel April 2020 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. The reason for this is that 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 which 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 milliseconds [RFC4103], for T.140 data channels. Implementers might also use lower values for specific applications requiring low latency, taking the increased overhead in consideration. 5.4. Loss of T140blocks In case of network failure or congestion, T.140 data channels might fail and get torn down. If this happens but the session sustains, 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. Holmberg & Hellstrom Expires October 12, 2020 [Page 11] Internet-Draft T.140 Data Channel April 2020 Similarly, a receiver SHOULD indicate to the user that there has been a channel reestablishment, and that 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 re-established before the T.140 data channel can be reestablished. The procedures after the reestablishment of the T.140 data channel defined in this section apply no matter if 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. The reason is that, unlike RTP media, WebRTC data channels and the T.140 protocol do not support the indication of the source of T.140 data. The SDP 'dcmap' attribute label attribute parameter (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 to the T140block, might allow indicating 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 e.g., 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 of such 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 on application and presentation level [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 Holmberg & Hellstrom Expires October 12, 2020 [Page 12] Internet-Draft T.140 Data Channel April 2020 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, this 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: o 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 [I-D.ietf-mmusic-data-channel-sdpneg] is set on it. o During a normal text flow, T140blocks received from one network are forwarded towards the other network. Keep-alive traffic is handled by lower layers on the T.140 data channel. A gateway might have to extract keep-alives from incoming RTP streams, and MAY generate keep-alives on outgoing RTP streams. o 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. o 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. o 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. o 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 to perform other actions that require that the gateway has 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. 7. Update to RFC 8373 This document updates RFC 8373 [RFC8373], by defining how the SDP hlang-send and hlang-recv attributes are used for the "application/ webrtc-datachannel" media type. Holmberg & Hellstrom Expires October 12, 2020 [Page 13] Internet-Draft T.140 Data Channel April 2020 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 [I-D.ietf-rtcweb-security-arch] and [I-D.ietf-rtcweb-security]. The generic security considerations for WebRTC data channels are defined in [I-D.ietf-rtcweb-data-channel]. As data channels are always encrypted by design, the T.140 data channels will also be encrypted. The generic security considerations for the SDP-based external negotiation method are defined in [I-D.ietf-mmusic-data-channel-sdpneg]. There are no additional T.140 data channel specific security considerations. When performing interworking between T.140 data channels and real- time text and the RTP based mechanism defined in [RFC4103], in order for a gateway to insert a missing text marker, or to perform other actions that require that the gateway has 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 [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the RFC number of this document.] 9.1. Subprotocol Identifier t140 This document adds the subprotocol identifier "t140" to the "WebSocket Subprotocol Name Registry" as follows: +--------------------------+----------------------------+ | Subprotocol Identifier: | t140 | | Subprotocol Common Name: | ITU-T T.140 Real-Time Text | | Subprotocol Definition: | RFCXXXX | | Reference: | RFCXXXX | +--------------------------+----------------------------+ Holmberg & Hellstrom Expires October 12, 2020 [Page 14] Internet-Draft T.140 Data Channel April 2020 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 and associated with an 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)" is 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: | RFCXXXX | +-----------------------+-------------------------------------------+ 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 an T.140 data channel. The modified usage is described in Section 4.2.2. The usage level "dcsa(t140)" is added to the registration of the SDP 'hland-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: | RFCXXXX | +-----------------------+-------------------------------------------+ The usage level "dcsa(t140)" is added to the registration of the SDP 'hland-recv' attribute in the Session Description Protocol (SDP) Parameters registry as follows: Holmberg & Hellstrom Expires October 12, 2020 [Page 15] Internet-Draft T.140 Data Channel April 2020 +-----------------------+-------------------------------------------+ | 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: | RFCXXXX | +-----------------------+-------------------------------------------+ 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 T.140 data channel. The modified usage is described in Section 4.2.3. The usage level "dcsa(t140)" is 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: | RFCXXXX | +-----------------------+-------------------------------------------+ The usage level "dcsa(t140)" is 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: | RFCXXXX | +-----------------------+-------------------------------------------+ Holmberg & Hellstrom Expires October 12, 2020 [Page 16] Internet-Draft T.140 Data Channel April 2020 The usage level "dcsa(t140)" is 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: | RFCXXXX | +-----------------------+-------------------------------------------+ The usage level "dcsa(t140)" is 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: | RFCXXXX | +-----------------------+-------------------------------------------+ 10. 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 draft. Paul Kyzivat and Bernard Aboba provided comments on the draft. 11. References 11.1. 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, . Holmberg & Hellstrom Expires October 12, 2020 [Page 17] Internet-Draft T.140 Data Channel April 2020 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002, . [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005, . [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8373] Gellens, R., "Negotiating Human Language in Real-Time Communications", RFC 8373, DOI 10.17487/RFC8373, May 2018, . [I-D.ietf-mmusic-rfc4566bis] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP: Session Description Protocol", draft-ietf-mmusic- rfc4566bis-37 (work in progress), August 2019. [I-D.ietf-rtcweb-data-channel] Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channels", draft-ietf-rtcweb-data-channel-13 (work in progress), January 2015. [I-D.ietf-mmusic-data-channel-sdpneg] Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R. Even, "SDP-based Data Channel Negotiation", draft-ietf- mmusic-data-channel-sdpneg-28 (work in progress), May 2019. [I-D.ietf-rtcweb-security-arch] Rescorla, E., "WebRTC Security Architecture", draft-ietf- rtcweb-security-arch-20 (work in progress), July 2019. [I-D.ietf-rtcweb-security] Rescorla, E., "Security Considerations for WebRTC", draft- ietf-rtcweb-security-12 (work in progress), July 2019. [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for multimedia application text conversation", February 1998. Holmberg & Hellstrom Expires October 12, 2020 [Page 18] Internet-Draft T.140 Data Channel April 2020 [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), Protocol for multimedia application text conversation", February 2000. 11.2. 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, . [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, . [I-D.ietf-rtcweb-data-protocol] Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel Establishment Protocol", draft-ietf-rtcweb-data- protocol-09 (work in progress), January 2015. Authors' Addresses Christer Holmberg Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: christer.holmberg@ericsson.com Gunnar Hellstrom Gunnar Hellstrom Accessible Communication Esplanaden 30 Vendelso 136 70 Sweden Email: gunnar.hellstrom@ghaccess.se Holmberg & Hellstrom Expires October 12, 2020 [Page 19]