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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4960 (Obsoleted by RFC 9260) -- Possible downref: Non-RFC (?) normative reference: ref. 'T140' -- Possible downref: Non-RFC (?) normative reference: ref. 'T140ad1' Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 MMUSIC Working Group C. Holmberg 3 Internet-Draft Ericsson 4 Updates: 8373 (if approved) December 11, 2019 5 Intended status: Standards Track 6 Expires: June 13, 2020 8 T.140 Real-time Text Conversation over WebRTC Data Channels 9 draft-ietf-mmusic-t140-usage-data-channel-10 11 Abstract 13 This document specifies how a WebRTC data channel can be used as a 14 transport mechanism for Real-time text using the ITU-T Protocol for 15 multimedia application text conversation (Recommendation ITU-T 16 T.140), and how the SDP offer/answer mechanism can be used to 17 negotiate such data channel, referred to as T.140 data channel. The 18 document updates RFC 8373. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on June 13, 2020. 37 Copyright Notice 39 Copyright (c) 2019 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 3. WebRTC Data Channel Considerations . . . . . . . . . . . . . 3 57 4. SDP Considerations . . . . . . . . . . . . . . . . . . . . . 4 58 4.1. Use of dcmap Attribute . . . . . . . . . . . . . . . . . 4 59 4.2. Use of dcsa Attribute . . . . . . . . . . . . . . . . . . 5 60 4.2.1. Maximum Character Transmission Rate . . . . . . . . . 5 61 4.2.2. Real-time Text Conversation Languages . . . . . . . . 6 62 4.2.3. Real-time Text Direction . . . . . . . . . . . . . . 6 63 4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8 64 5. T.140 Considerations . . . . . . . . . . . . . . . . . . . . 10 65 5.1. Session Layer Functions . . . . . . . . . . . . . . . . . 10 66 5.2. Data Encoding and Sending . . . . . . . . . . . . . . . . 11 67 5.3. Data Buffering . . . . . . . . . . . . . . . . . . . . . 11 68 5.4. Loss of T140blocks . . . . . . . . . . . . . . . . . . . 11 69 5.5. Multi-party Considerations . . . . . . . . . . . . . . . 12 70 6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 12 71 7. Update to RFC 8373 . . . . . . . . . . . . . . . . . . . . . 13 72 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 73 9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 14 74 9.1. Subprotocol Identifier t140 . . . . . . . . . . . . . . . 14 75 9.2. SDP fmtp Attribute . . . . . . . . . . . . . . . . . . . 14 76 9.3. SDP Language Attributes . . . . . . . . . . . . . . . . . 15 77 9.4. SDP Media Direction Attributes . . . . . . . . . . . . . 16 78 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 79 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 80 11.1. Normative References . . . . . . . . . . . . . . . . . . 17 81 11.2. Informative References . . . . . . . . . . . . . . . . . 18 82 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 84 1. Introduction 86 The ITU-T Protocol for multimedia application text conversation 87 (Recommendation ITU-T T.140) [T140] defines a protocol for text 88 conversation, also known as real-time text. The transport used for 89 IP networks is the "RTP Payload for Text Conversation" [RFC4103] 90 mechanism, based on the Real-time Transport Protocol (RTP) [RFC3550]. 92 This document specifies how a WebRTC data channel 93 [I-D.ietf-rtcweb-data-channel] can be used as a transport mechanism 94 for T.140, and how the SDP offer/answer mechanism 95 [I-D.ietf-mmusic-data-channel-sdpneg] can be used to negotiate such 96 data channel. 98 In this document, a T.140 data channel refers to a WebRTC data 99 channel for which the instantiated sub-protocol is "t140", and where 100 the channel is negotiated using the SDP-based external negotiation 101 method [I-D.ietf-mmusic-data-channel-sdpneg]. 103 NOTE: The decision to transport real-time text using a WebRTC data 104 channel, instead of using RTP based transport [RFC4103], is motivated 105 by use-case "U-C 5: Real-time text chat during an audio and/or video 106 call with an individual or with multiple people in a conference", see 107 Section 3.2 of [I-D.ietf-rtcweb-data-channel]. 109 The brief notation "T.140" is used as a name for the text 110 conversation protocol according to [T140]. 112 Section 3 defines the generic data channel properties for a T.140 113 data channel, and Section 4 defines how they are conveyed in an SDP 114 dcmap attribute. While this document defines how to establish a 115 T.140 data channel using the SDP-based external negotiation method 116 [I-D.ietf-mmusic-data-channel-sdpneg], the generic T.140 and gateway 117 considerations defined in Section 3, Section 5 and Section 6 of this 118 document can also be applied when a T.140 data channel is established 119 using another mechanism (e.g., the mechanism defined in 120 [I-D.ietf-rtcweb-data-protocol]). Section 5 of 121 [I-D.ietf-mmusic-data-channel-sdpneg] defines the mapping between the 122 SDP dcmap attribute parameters and the protocol parameters used in 123 [I-D.ietf-rtcweb-data-protocol]. 125 This document is based on an earlier Internet draft edited by Keith 126 Drage, Juergen Stoetzer-Bradler and Albrecht Schwarz. 128 2. Conventions 130 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 131 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 132 "OPTIONAL" in this document are to be interpreted as described in BCP 133 14 [RFC2119] [RFC8174] when, and only when, they appear in all 134 capitals, as shown here. 136 3. WebRTC Data Channel Considerations 138 The following WebRTC data channel property values 139 [I-D.ietf-rtcweb-data-channel] apply to a T.140 data channel: 141 +--------------------------+-------------------------------+ 142 | Subprotocol Identifier | t140 | 143 | Transmission reliability | reliable | 144 | Transmission order | in-order | 145 | Label | See Section 4.1 and Section 6 | 146 +--------------------------+-------------------------------+ 148 NOTE: T.140 requires the transport channel to provide transmission of 149 real-time text without duplication and in original order. Therefore, 150 T.140 does not specify reliable and ordered transmission of T.140 151 data on the application layer. Instead, when RTP based transport is 152 used, the RTP sequence number is used to detect packet loss and out- 153 of-order packets, and a redundancy mechanism is used to achieve 154 reliable delivery of T.140 data. By using the WebRTC data channel 155 reliable and in-order transmission features 156 [I-D.ietf-rtcweb-data-channel] for the T.140 data channel, there is 157 no need for a redundancy mechanism or a mechanism to detect data loss 158 and out-of-order delivery at the application level. The latency 159 characteristics of the T.140 data channel is also regarded to be 160 sufficient to meet the application requirements of T.140. 162 4. SDP Considerations 164 The generic SDP considerations, including the SDP Offer/Answer 165 procedures, for negotiating a WebRTC data channel are defined in 166 [I-D.ietf-mmusic-data-channel-sdpneg]. This section defines the SDP 167 considerations that are specific to a T.140 data channel. 169 4.1. Use of dcmap Attribute 171 An offerer and answerer MUST, in each offer and answer, include an 172 SDP 'dcmap' attribute [I-D.ietf-mmusic-data-channel-sdpneg] in the 173 SDP media description (m= section) [I-D.ietf-mmusic-rfc4566bis] 174 describing the SCTP association [RFC4960] used to realize the T.140 175 data channel. 177 The offerer and answerer MUST include the subprotocol attribute 178 parameter, with a "t140" parameter value, in the 'dcmap' attribute 179 value. 181 The offerer and answerer MAY include the priority attribute parameter 182 and the label attribute parameter in the 'dcmap' attribute value. If 183 the offerer includes a label attribute parameter, the answerer MUST 184 NOT change the value in the answer. 186 NOTE: As specified in [I-D.ietf-rtcweb-data-channel], when a data 187 channel is negotiated using the mechanism defined in 188 [I-D.ietf-rtcweb-data-protocol], the label attribute parameter value 189 has to be the same in both directions. That rule also applies to 190 data channels negotiated using the mechanism defined in this 191 document. 193 The offerer and answerer MUST NOT include the max-retr and max-time 194 attribute parameters in the 'dcmap' attribute. 196 If the ordered attribute parameter is included in the 'dcmap' 197 attribute, it MUST be assigned the value 'true'. 199 Below is an example of the 'dcmap' attribute for a T.140 data channel 200 with stream id=3 and without any label: 202 a=dcmap:3 subprotocol="t140" 204 4.2. Use of dcsa Attribute 206 An offerer and answerer MAY, in each offer and answer, include one or 207 more SDP 'dcsa' attributes [I-D.ietf-mmusic-data-channel-sdpneg] in 208 the m= section describing the SCTP association used to realize the 209 T.140 data channel. 211 If an offerer or answerer receives a 'dcsa' attribute that contains 212 an SDP attribute which usage has not been defined for a T.140 data 213 channel, the offerer or answerer should ignore the 'dcsa' attribute, 214 following the rules in Section 6.7 of 215 [I-D.ietf-mmusic-data-channel-sdpneg]. 217 4.2.1. Maximum Character Transmission Rate 219 A 'dcsa' attribute can contain the SDP 'fmtp' attribute used to 220 indicate a maximum character transmission rate [RFC4103]. The 'cps' 221 attribute parameter is used to indicate the maximum character 222 transmission rate that the endpoint that includes the attribute is 223 able to receive, and the value is used as a mean value in characters 224 per second over any 10-second interval. 226 If the 'fmtp' attribute is included, the 'format' attribute parameter 227 MUST be set to "-". 229 If the 'fmtp' attribute is not included, the default value of 30 230 applies [RFC4103]. 232 The offerer and answerer MAY modify the 'cps' attribute parameter 233 value in subsequent offers and answers. 235 This document does not define any other usage of the 'fmtp' attribute 236 for a T.140 channel. If an offerer or answerer receives a 'dcsa' 237 attribute that contains an 'fmtp' attribute that is not according to 238 the procedure above, the offerer or answerer MUST ignore the 'dcsa' 239 attribute. 241 NOTE: The 'cps' attribute parameter is especially useful when a T.140 242 data channel endpoint is acting as a gateway (Section 6) and is 243 interworking with a T.140 transport mechanism that have restrictions 244 on how many characters can be sent per second. 246 4.2.2. Real-time Text Conversation Languages 248 'dcsa' attributes can contain the SDP 'hlang-send' and 'hlang-recv' 249 attributes [RFC8373] to negotiate the language to be used for the 250 real-time text conversation. 252 For a T.140 data channel, the modality is "written" [RFC8373]. 254 4.2.3. Real-time Text Direction 256 'dcsa' attributes can contain the SDP 'sendonly', 'recvonly', 257 'sendrecv' and 'inactive' attributes [I-D.ietf-mmusic-rfc4566bis] to 258 negotiate the direction in which text can be transmitted in a real- 259 time text conversation. 261 NOTE: A WebRTC data channel is always bi-directional. The usage of 262 the 'dcsa' attribute only affects the direction in which 263 implementations are allowed to transmit text on a T.140 data channel. 265 The offer/answer rules for the direction attributes are based on the 266 rules for unicast streams defined in [RFC3264], as described below. 267 Note that the rules only apply to the direction attributes. 269 Session level direction attributes [I-D.ietf-mmusic-rfc4566bis] have 270 no impact on a T.140 data channel. 272 4.2.3.1. Generating an Offer 274 If the offerer wishes to both send and receive text on a T.140 data 275 channel, it SHOULD mark the data channel as sendrecv with a 276 'sendrecv' attribute inside a 'dcsa' attribute. If the offerer does 277 not explicitly mark the data channel, an implicit 'sendrecv' 278 attribute inside a 'dcsa' attribute is applied by default. 280 If the offerer wishes to only send text on a T.140 data channel, it 281 MUST mark the data channel as sendonly with a 'sendonly' attribute 282 inside a 'dcsa' attribute. 284 If the offerer wishes to only receive text on a T.140 data channel, 285 it MUST mark the data channel as recvonly with a 'recvonly' attribute 286 inside a 'dcsa' attribute. 288 If the offerer wishes to neither send nor receive text on a T.140 289 data channel, it MUST mark the data channel as inactive with an 290 'inactive' attribute inside a 'dcsa' attribute. 292 If the offerer has marked a data channel as sendrecv (or if the 293 offerer did not explicitly mark the data channel) or recvonly, it 294 MUST be prepared to receive T.140 data as soon as the state of the 295 T.140 data channel allows it. 297 4.2.3.2. Generating an Answer 299 When the answerer accepts an offer, and marks the direction of the 300 text in the corresponding answer, the direction is based on the 301 marking (or the lack of explicit marking) in the offer. 303 If the offerer explicitly marked the data channel as sendrecv, or if 304 the offerer did not mark the data channel, the answerer SHOULD mark 305 the data channel as sendrecv, sendonly, recvonly or inactive with a 306 'sendrecv', 'sendonly', 'recvonly' or 'inactive' attribute 307 respectively inside a 'dcsa' attribute. If the answerer does not 308 explicitly mark the data channel, an implicit 'sendrecv' attribute 309 inside a 'dcsa' attribute is applied by default. 311 If the offerer marked the data channel as sendonly, the answerer MUST 312 mark the data channel as recvonly or inactive with a 'recvonly' or 313 'inactive' attribute respectively inside a 'dcsa' attribute. 315 If the offerer marked the data channel as recvonly, the answerer MUST 316 mark the data channel as sendonly or inactive with a 'sendonly' or 317 'inactive' attribute respectively inside a 'dcsa' attribute. 319 If the offerer marked the data channel as inactive, the answerer MUST 320 mark the data channel as inactive with an 'inactive' attribute inside 321 a 'dcsa' attribute. 323 If the answerer has marked a data channel as sendrecv or recvonly, it 324 MUST be prepared to receive data as soon as the state of the T.140 325 data channel allows transmission of data. 327 4.2.3.3. Offerer Receiving an Answer 329 When the offerer receives an answer to the offer and the answerer has 330 marked a data channel as sendrecv (or the answerer did not mark the 331 data channel) or recvonly in the answer, the offerer can start 332 sending T.140 data as soon as the state of the T.140 data channel 333 allows it. If the answerer has marked the data channel as inactive 334 or sendonly, the offerer MUST NOT send any T.140 data. 336 If the answerer has not marked the direction of a T.140 data channel 337 in accordance with the procedures above, it is RECOMMENDED that the 338 offerer does not process that as an error situation, but rather 339 assume that the answerer might both send and receive T.140 data on 340 the data channel. 342 4.2.3.4. Modify Text Direction 344 If an endpoint wishes to modify a previously negotiated text 345 direction in an ongoing session, it MUST initiate an offer that 346 indicates the new direction, following the rules in Section 4.2.3.1. 347 If the answerer accepts the offer it follows the procedures in 348 Section 4.2.3.2. 350 4.3. Examples 352 Below is an example of an m= section of an offer for a T.140 data 353 channel offering real-time text conversation in Spanish and 354 Esperanto, and an m= section in the associated answer accepting 355 Esperanto. The maximum character transmission rate is set to 20. As 356 the offerer and answerer have not explicitly indicated the real-time 357 text direction, the default direction "sendrecv" applies. 359 Offer: 361 m=application 911 UDP/DTLS/SCTP webrtc-datachannel 362 c=IN IP6 2001:db8::3 363 a=max-message-size:1000 364 a=sctp-port 5000 365 a=dcmap:2 label="ACME customer service";subprotocol="t140" 366 a=dcsa:2 fmtp:- cps=20 367 a=dcsa:2 hlang-send:es eo 368 a=dcsa:2 hlang-recv:es eo 370 Answer: 372 m=application 2004 UDP/DTLS/SCTP webrtc-datachannel 373 c=IN IP6 2001:db8::1 374 a=max-message-size:1000 375 a=sctp-port 6000 376 a=dcmap:2 label="ACME customer service";subprotocol="t140" 377 a=dcsa:2 fmtp:- cps=20 378 a=dcsa:2 hlang-send:eo 379 a=dcsa:2 hlang-recv:eo 381 Below is an example of an m= section of an offer for a T.140 data 382 channel where the offerer wishes to only receive real-time text, and 383 an m= section in the associated answer indicating that the answerer 384 will only send real-time text. No maximum character transmission 385 rate is indicated. No preference for the language to be used for the 386 real-time text conversation is indicated. 388 Offer: 390 m=application 1400 UDP/DTLS/SCTP webrtc-datachannel 391 c=IN IP6 2001:db8::3 392 a=max-message-size:1000 393 a=sctp-port 5000 394 a=dcmap:2 label="ACME customer service";subprotocol="t140" 395 a=dcsa:2 recvonly 397 Answer: 399 m=application 2400 UDP/DTLS/SCTP webrtc-datachannel 400 c=IN IP6 2001:db8::1 401 a=max-message-size:1000 402 a=sctp-port 6000 403 a=dcmap:2 label="ACME customer service";subprotocol="t140" 404 a=dcsa:2 sendonly 406 5. T.140 Considerations 408 5.1. Session Layer Functions 410 Section 6.1 of [T140] describes the generic T.140 session control 411 functions at a high-level in a signalling protocol independent 412 manner. The list below describes how the functions are realized when 413 using a T.140 data channel. 415 o Prepare session: An endpoint can indicate its support of T.140 416 data channels using signalling specific means (e.g., using SIP 417 OPTIONS [RFC3261]), or by indicating the support in an offer or 418 answer (Section 4) 419 o Initiate session: An offer used to request the establishment of a 420 T.140 data channel (Section 4) 421 o Accept session: An answer used to accept a request to establish a 422 T.140 data channel (Section 4) 423 o Deny session: An answer used to reject a request to establish a 424 T.140 data channel, using the generic procedures for rejecting a 425 data channel [I-D.ietf-mmusic-data-channel-sdpneg] 426 o Disconnect session: An offer or answer used to disable a 427 previously established T.140 data channel, using the generic 428 procedures for closing a data channel 429 [I-D.ietf-mmusic-data-channel-sdpneg] 430 o Data: Data sent on an established T.140 data channel (Section 5.2) 432 5.2. Data Encoding and Sending 434 T.140 text is encoded and framed as T140blocks [RFC4103]. 436 Each T140block is sent on the SCTP stream [RFC4960] used to realize 437 the T.140 data channel using standard T.140 transmission procedures 438 [T140]. One or more T140blocks can be sent in a single SCTP user 439 message [RFC4960]. Unlike RTP based transport for real-time text 440 [RFC4103], T.140 data channels do not use redundant transmission of 441 text. The reason for this is that the T.140 data channel achieves 442 robust transmission by using the "reliable" mode of the data channel. 444 Data sending and reporting procedures conform to [T140]. 446 See Section 8 of [T140] for coding details. 448 NOTE: The T.140 coding details contain information on optional 449 control codes for controlling the presentation which may not be 450 supported by the presentation level of the receiving application. 451 The receiving application is expected to handle reception of such 452 T.140 control codes appropriately (e.g. ignore and skip them) even if 453 their effect on the presentation is not supported. 455 5.3. Data Buffering 457 As described in [T140], buffering can be used to reduce overhead, 458 with the maximum buffering time being 500 ms. It can also be used 459 for staying within the maximum character transmission rate 460 (Section 4.2), if such has been provided by the peer. 462 An implementation needs to take the user requirements for smooth flow 463 and low latency in real-time text conversation into consideration 464 when assigning a buffer time. It is RECOMMENDED to use the default 465 transmission interval of 300 milliseconds [RFC4103], or lower, for 466 T.140 data channels. 468 5.4. Loss of T140blocks 470 In case of network failure or congestion, T.140 data channels might 471 fail and get torn down. If this happens but the session sustains, it 472 is RECOMMENDED that implementations tries to reestablish the T.140 473 data channels. If reestablishment of the T.140 data channel is 474 successful, an implementation MUST evaluate if any T140blocks were 475 lost. Retransmission of already successfully transmitted T140blocks 476 MUST be avoided, and missing text markers [T140ad1] SHOULD be 477 inserted in the received data stream where loss is detected or 478 suspected. 480 NOTE: If the SCTP association [RFC4960] used to realize the T.140 481 data channel fails and gets torn down, it needs to be re-established 482 before the T.140 data channel can be reestablished. The procedure 483 after the reestablishment of the T.140 data channel defined in this 484 section apply no matter if only the T.140 data channel, or the whole 485 SCTP association, got torn down. 487 5.5. Multi-party Considerations 489 If an implementation needs to support multi-party scenarios, the 490 implementation needs to support multiple simultaneous T.140 data 491 channels, one for each remote party. At the time of writing this 492 document, this is true even in scenarios where each participant 493 communicates via a centralized conference server. The reason is 494 that, unlike RTP media, WebRTC data channels and the T.140 protocol 495 do not support the indication of the source of T.140 data. The SDP 496 'dcmap' attribute label attribute parameter (Section 4.1) can be used 497 by the offerer to provide additional information about each T.140 498 data channel, and help implementations to distinguish between them. 500 NOTE: Future extensions to T.140, or to the T140block, might allow 501 indicating the source of T.140 data, in which case it might be 502 possible to use a single T.140 data channel to transport data from 503 multiple remote sources. The usage of a single T.140 data channel, 504 without any protocol extensions, would require the conference server 505 to only forward real-time text from one source at any given time, and 506 e.g., include human readable text labels in the real-time text stream 507 that indicates the source whenever the conference server switches the 508 source. This would allow the receiver to present real-time text from 509 different sources separated. The procedures of such mechanism is 510 outside the scope of this document. 512 6. Gateway Considerations 514 A number of real-time text transports and protocols have been defined 515 for both packet-switched and circuit-switched networks. Many are 516 based on the ITU-T T.140 protocol on application and presentation 517 level [T140]. At the time of writing this document, some mechanisms 518 are no longer used, as the technologies they use have been obsoleted, 519 while others are still in use. 521 When performing interworking between T.140 data channels and real- 522 time text in other transports and protocols, a number of factors need 523 to be considered. At the time of writing this document, the most 524 common IP-based real-time text transport is the RTP based mechanism 525 defined in [RFC4103]. While this document does not define a complete 526 interworking solution, this list below provides some guidance and 527 considerations to take into account when designing a gateway for 528 interworking between T.140 data channels and RTP-based T.140 529 transport: 531 o For each T.140 data channel there is an RTP stream for real-time 532 text [RFC4103]. Redundancy is by default declared and used on the 533 RTP stream. There is no redundancy on the T.140 data channel, but 534 the reliable property [I-D.ietf-mmusic-data-channel-sdpneg] is set 535 on it. 536 o During a normal text flow, T140blocks received from one network 537 are forwarded towards the other network. Keep-alive traffic is 538 implicit on the T.140 data channel. A gateway might have to 539 extract keep-alives from incoming RTP streams, and MAY generate 540 keep-alives on outgoing RTP streams. 541 o If the gateway detects or suspects loss of data on the RTP stream, 542 and the lost data has not been retrieved using a redundancy 543 mechanism, the gateway SHOULD insert the T.140 missing text marker 544 [T140ad1] in the data sent on the outgoing T.140 data channel. 545 o If the gateway detects that the T.140 data channel has failed and 546 got torn down, once the data channel has been reestablished the 547 gateway SHOULD insert the T.140 missing text marker [T140ad1] in 548 the data sent on the outgoing RTP stream if it detects or suspects 549 that data sent by the remote T.140 data channel endpoint was lost. 550 o If the gateway detects that the T.140 data channel has failed and 551 got torn down, once the data channel has been reestablished the 552 gateway SHOULD insert the T.140 missing text marker [T140ad1] in 553 the data sent on the outgoing T.140 data channel if it detects or 554 suspects that data sent or to be sent on the T.140 data channel 555 was lost during the failure. 556 o The gateway MUST indicate the same text transmission direction 557 (Section 4.2.3) on the T.140 data channel and the RTP stream. 559 NOTE: In order for the gateway to insert a missing text marker, or to 560 perform other actions that require that the gateway has access to the 561 T.140 data, the T.140 data cannot be encrypted end-to-end between the 562 T.140 data channel endpoint and the RTP endpoint. At the time of 563 writing this document, no mechanism to provide such end-to-end 564 encryption is defined. 566 7. Update to RFC 8373 568 This document updates RFC 8373 [RFC8373], by defining how the SDP 569 hlang-send and hlang-recv attributes are used for the "application/ 570 webrtc-datachannel" media type. 572 SDP offerers and answerers MUST NOT include the attributes directly 573 in the m= section associated with the 'application/webrtc- 574 datachannel' media type. Instead, the attributes MUST be associated 575 with individual data channels, using the SDP 'dcsa' attribute. A 576 specification that defines a subprotocol that uses the attributes 577 MUST specify the modality for that subprotocol, or how to retrieve 578 the modality if the subprotocol supports multiple modalities. The 579 subprotocol is indicated using the SDP 'dcmap' attribute. 581 8. Security Considerations 583 The generic security considerations for WebRTC data channels are 584 defined in [I-D.ietf-rtcweb-data-channel]. As data channels are 585 always encrypted by design, the T.140 data channels will also be 586 encrypted. 588 The generic security considerations for the SDP-based external 589 negotiation method are defined in 590 [I-D.ietf-mmusic-data-channel-sdpneg]. There are no additional T.140 591 data channel specific security considerations. 593 9. IANA considerations 595 [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the 596 RFC number of this document.] 598 9.1. Subprotocol Identifier t140 600 This document adds the subprotocol identifier "t140" to the 601 "WebSocket Subprotocol Name Registry" as follows: 603 +--------------------------+-------------+ 604 | Subprotocol Identifier: | t140 | 605 | Subprotocol Common Name: | ITU-T T.140 | 606 | Subprotocol Definition: | RFCXXXX | 607 | Reference: | RFCXXXX | 608 +--------------------------+-------------+ 610 9.2. SDP fmtp Attribute 612 This document defines the usage of the SDP 'fmtp' attribute, if this 613 attribute is included in an SDP 'dcsa' attribute and associated with 614 an T.140 real-time text session over a WebRTC data channel. The 615 usage is defined in Section 4.2.1. 617 The usage level "dcsa(t140)" is added to the IANA registration of the 618 SDP 'fmtp' attribute as follows: 620 +-----------------------+-------------------------------------------+ 621 | Contact name: | IESG | 622 | Contact email: | iesg@ietf.org | 623 | Attribute name: | fmtp | 624 | Usage level: | dcsa(t140) | 625 | Purpose: | Indicate the maximum transmission rate | 626 | | that an endpoint is willing to receive on | 627 | | a T.140 data channel. | 628 | Reference: | RFCXXXX | 629 +-----------------------+-------------------------------------------+ 631 9.3. SDP Language Attributes 633 This document modifies the usage of the SDP 'hlang-send' and 'hlang- 634 recv' attributes, if these attributes are included in SDP 'dcsa' 635 attributes associated with an T.140 data channel. The modified usage 636 is described in Section 4.2.2. 638 The usage level "dcsa(t140)" is added to the IANA registration of the 639 SDP 'hlang-send' attribute as follows: 641 +-----------------------+-------------------------------------------+ 642 | Contact name: | IESG | 643 | Contact email: | iesg@ietf.org | 644 | Attribute name: | hlang-send | 645 | Usage level: | dcsa(t140) | 646 | Purpose: | Negotiate the language to be used on a | 647 | | T.140 data channel. | 648 | Reference: | RFCXXXX | 649 +-----------------------+-------------------------------------------+ 651 The usage level "dcsa(t140)" is added to the IANA registration of the 652 SDP 'hlang-recv' attribute as follows: 654 +-----------------------+-------------------------------------------+ 655 | Contact name: | IESG | 656 | Contact email: | iesg@ietf.org | 657 | Attribute name: | hlang-recv | 658 | Usage level: | dcsa(t140) | 659 | Purpose: | Negotiate the language to be used on a | 660 | | T.140 data channel. | 661 | Reference: | RFCXXXX | 662 +-----------------------+-------------------------------------------+ 664 9.4. SDP Media Direction Attributes 666 This document modifies the usage of the SDP 'sendonly', 'recvonly', 667 'sendrecv' and 'inactive' attributes, if these attributes are 668 included in SDP 'dcsa' attributes associated T.140 data channel. The 669 modified usage is described in Section 4.2.3. 671 The usage level "dcsa(t140)" is added to the IANA registration of the 672 SDP 'sendonly' attribute as follows: 674 +-----------------------+-------------------------------------------+ 675 | Contact name: | IESG | 676 | Contact email: | iesg@ietf.org | 677 | Attribute name: | sendonly | 678 | Usage level: | dcsa(t140) | 679 | Purpose: | Negotiate the direction in which real- | 680 | | time text can be sent on a T.140 data | 681 | | channel. | 682 | Reference: | RFCXXXX | 683 +-----------------------+-------------------------------------------+ 685 The usage level "dcsa(t140)" is added to the IANA registration of the 686 SDP 'recvonly' attribute as follows: 688 +-----------------------+-------------------------------------------+ 689 | Contact name: | IESG | 690 | Contact email: | iesg@ietf.org | 691 | Attribute name: | recvonly | 692 | Usage level: | dcsa(t140) | 693 | Purpose: | Negotiate the direction in which real- | 694 | | time text can be sent on a T.140 data | 695 | | channel. | 696 | Reference: | RFCXXXX | 697 +-----------------------+-------------------------------------------+ 699 The usage level "dcsa(t140)" is added to the IANA registration of the 700 SDP 'sendrecv' attribute as follows: 702 +-----------------------+-------------------------------------------+ 703 | Contact name: | IESG | 704 | Contact email: | iesg@ietf.org | 705 | Attribute name: | sendrecv | 706 | Usage level: | dcsa(t140) | 707 | Purpose: | Negotiate the direction in which real- | 708 | | time text can be sent on a T.140 data | 709 | | channel. | 710 | Reference: | RFCXXXX | 711 +-----------------------+-------------------------------------------+ 712 The usage level "dcsa(t140)" is added to the IANA registration of the 713 SDP 'inactive' attribute as follows: 715 +-----------------------+-------------------------------------------+ 716 | Contact name: | IESG | 717 | Contact email: | iesg@ietf.org | 718 | Attribute name: | inactive | 719 | Usage level: | dcsa(t140) | 720 | Purpose: | Negotiate the direction in which real- | 721 | | time text can be sent on a T.140 data | 722 | | channel. | 723 | Reference: | RFCXXXX | 724 +-----------------------+-------------------------------------------+ 726 10. Acknowledgements 728 This document is based on an earlier Internet draft edited by Keith 729 Drage, Juergen Stoetzer-Bradler and Albrecht Schwarz. 731 Thomas Belling provided useful comments on the initial (pre- 732 submission) version of the draft. Gunnar Hellstrom provided comments 733 and text on the draft. Paul Kyzivat and Bernard Aboba provided 734 comments on the draft. 736 11. References 738 11.1. Normative References 740 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 741 Requirement Levels", BCP 14, RFC 2119, 742 DOI 10.17487/RFC2119, March 1997, . 745 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 746 with Session Description Protocol (SDP)", RFC 3264, 747 DOI 10.17487/RFC3264, June 2002, . 750 [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text 751 Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005, 752 . 754 [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", 755 RFC 4960, DOI 10.17487/RFC4960, September 2007, 756 . 758 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 759 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 760 May 2017, . 762 [RFC8373] Gellens, R., "Negotiating Human Language in Real-Time 763 Communications", RFC 8373, DOI 10.17487/RFC8373, May 2018, 764 . 766 [I-D.ietf-mmusic-rfc4566bis] 767 Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP: 768 Session Description Protocol", draft-ietf-mmusic- 769 rfc4566bis-37 (work in progress), August 2019. 771 [I-D.ietf-rtcweb-data-channel] 772 Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data 773 Channels", draft-ietf-rtcweb-data-channel-13 (work in 774 progress), January 2015. 776 [I-D.ietf-mmusic-data-channel-sdpneg] 777 Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R. 778 Even, "SDP-based Data Channel Negotiation", draft-ietf- 779 mmusic-data-channel-sdpneg-28 (work in progress), May 780 2019. 782 [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for 783 multimedia application text conversation", February 1998. 785 [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), 786 Protocol for multimedia application text conversation", 787 February 2000. 789 11.2. Informative References 791 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 792 A., Peterson, J., Sparks, R., Handley, M., and E. 793 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 794 DOI 10.17487/RFC3261, June 2002, . 797 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 798 Jacobson, "RTP: A Transport Protocol for Real-Time 799 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 800 July 2003, . 802 [I-D.ietf-rtcweb-data-protocol] 803 Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel 804 Establishment Protocol", draft-ietf-rtcweb-data- 805 protocol-09 (work in progress), January 2015. 807 Author's Address 809 Christer Holmberg 810 Ericsson 811 Hirsalantie 11 812 Jorvas 02420 813 Finland 815 Email: christer.holmberg@ericsson.com