idnits 2.17.1 draft-ietf-avtcore-multi-party-rtt-mix-10.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The abstract seems to contain references ([RFC4103]), which it shouldn't. Please replace those with straight textual mentions of the documents in question. == The 'Updates: ' line in the draft header should list only the _numbers_ of the RFCs which will be updated by this document (if approved); it should not include the word 'RFC' in the list. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Line 802 has weird spacing: '...example from ...' == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: A party not performing as a mixer MUST not include the CSRC list. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: BEL 0007 Bell Alert in session, provides for alerting during an active session. The display count SHOULD not be altered. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: INT ESC 0061 Interrupt (used to initiate mode negotiation procedure). The display count SHOULD not be altered. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: SGR 009B Ps 006D Select graphic rendition. Ps is rendition parameters specified in ISO 6429. The display count SHOULD not be altered. The SGR code SHOULD be stored for the current source. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: SOS 0098 Start of string, used as a general protocol element introducer, followed by a maximum 256 bytes string and the ST. The display count SHOULD not be altered. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: ST 009C String terminator, end of SOS string. The display count SHOULD not be altered. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'SHOULD not' in this paragraph: ESC 001B Escape - used in control strings. The display count SHOULD not be altered for the complete escape code. (Using the creation date from RFC4103, updated by this document, for RFC5378 checks: 2003-11-21) -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (18 November 2020) is 1252 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'Bob' is mentioned on line 1326, but not defined ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) ** Downref: Normative reference to an Informational RFC: RFC 8643 -- Possible downref: Non-RFC (?) normative reference: ref. 'T140' -- Possible downref: Non-RFC (?) normative reference: ref. 'T140ad1' Summary: 3 errors (**), 0 flaws (~~), 11 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVTCore G. Hellstrom 3 Internet-Draft Gunnar Hellstrom Accessible Communication 4 Updates: RFC 4103 (if approved) 18 November 2020 5 Intended status: Standards Track 6 Expires: 22 May 2021 8 RTP-mixer formatting of multi-party Real-time text 9 draft-ietf-avtcore-multi-party-rtt-mix-10 11 Abstract 13 Real-time text mixers for multi-party sessions need to identify the 14 source of each transmitted group of text so that the text can be 15 presented by endpoints in suitable grouping with other text from the 16 same source, while new text from other sources is also presented in 17 readable grouping as received interleaved in real-time. 19 Regional regulatory requirements specify provision of real-time text 20 in multi-party calls. RFC 4103 mixer implementations can use 21 traditional RTP functions for source identification, but the mixer 22 source switching performance is limited when using the default 23 transmission characteristics with redundancy. 25 Enhancements for RFC 4103 real-time text mixing is provided in this 26 document, suitable for a centralized conference model that enables 27 source identification and source switching. The intended use is for 28 real-time text mixers and multi-party-aware participant endpoints. 29 The specified mechanism build on the standard use of the CSRC list in 30 the RTP packet for source identification. The method makes use of 31 the same "text/t140" and "text/red" formats as for two-party 32 sessions. 34 A capability exchange is specified so that it can be verified that a 35 participant can handle the multi-party coded real-time text stream. 36 The capability is indicated by use of a media attribute "rtt-mixer". 38 The document updates RFC 4103[RFC4103] 40 A specifications of how a mixer can format text for the case when the 41 endpoint is not multi-party aware is also provided. 43 Status of This Memo 45 This Internet-Draft is submitted in full conformance with the 46 provisions of BCP 78 and BCP 79. 48 Internet-Drafts are working documents of the Internet Engineering 49 Task Force (IETF). Note that other groups may also distribute 50 working documents as Internet-Drafts. The list of current Internet- 51 Drafts is at https://datatracker.ietf.org/drafts/current/. 53 Internet-Drafts are draft documents valid for a maximum of six months 54 and may be updated, replaced, or obsoleted by other documents at any 55 time. It is inappropriate to use Internet-Drafts as reference 56 material or to cite them other than as "work in progress." 58 This Internet-Draft will expire on 22 May 2021. 60 Copyright Notice 62 Copyright (c) 2020 IETF Trust and the persons identified as the 63 document authors. All rights reserved. 65 This document is subject to BCP 78 and the IETF Trust's Legal 66 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 67 license-info) in effect on the date of publication of this document. 68 Please review these documents carefully, as they describe your rights 69 and restrictions with respect to this document. Code Components 70 extracted from this document must include Simplified BSD License text 71 as described in Section 4.e of the Trust Legal Provisions and are 72 provided without warranty as described in the Simplified BSD License. 74 Table of Contents 76 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 77 1.1. Selected solution and considered alternative . . . . . . 5 78 1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . 7 79 1.3. Intended application . . . . . . . . . . . . . . . . . . 8 80 2. Overview over the two specified solutions . . . . . . . . . . 8 81 2.1. Negotiated use of the RFC 4103 format for multi-party 82 transmission in a single RTP stream . . . . . . . . . . . 9 83 2.2. Mixing for multi-party unaware endpoints . . . . . . . . 9 84 3. Details for the multi-party aware mixing case . . . . . . . . 9 85 3.1. Offer/answer considerations . . . . . . . . . . . . . . . 9 86 3.2. Actions depending on capability negotiation result . . . 10 87 3.3. Use of fields in the RTP packets . . . . . . . . . . . . 10 88 3.4. Initial transmission of a BOM character . . . . . . . . . 11 89 3.5. Keep-alive . . . . . . . . . . . . . . . . . . . . . . . 11 90 3.6. Transmission interval . . . . . . . . . . . . . . . . . . 11 91 3.7. Only one source per packet . . . . . . . . . . . . . . . 11 92 3.8. Do not send received text to the originating source . . . 11 93 3.9. Clean incoming text . . . . . . . . . . . . . . . . . . . 11 94 3.10. Redundancy . . . . . . . . . . . . . . . . . . . . . . . 12 95 3.11. Source switching . . . . . . . . . . . . . . . . . . . . 12 96 3.12. Text placement in packets . . . . . . . . . . . . . . . . 12 97 3.13. Empty T140blocks . . . . . . . . . . . . . . . . . . . . 13 98 3.14. Creation of the redundancy . . . . . . . . . . . . . . . 13 99 3.15. Timer offset fields . . . . . . . . . . . . . . . . . . . 13 100 3.16. Other RTP header fields . . . . . . . . . . . . . . . . . 14 101 3.17. Pause in transmission . . . . . . . . . . . . . . . . . . 14 102 3.18. RTCP considerations . . . . . . . . . . . . . . . . . . . 14 103 3.19. Reception of multi-party contents . . . . . . . . . . . . 15 104 3.20. Performance considerations . . . . . . . . . . . . . . . 17 105 3.21. Security for session control and media . . . . . . . . . 17 106 3.22. SDP offer/answer examples . . . . . . . . . . . . . . . . 18 107 3.23. Packet sequence example from a source switch . . . . . . 19 108 3.24. Maximum character rate "CPS" . . . . . . . . . . . . . . 21 109 4. Presentation level considerations . . . . . . . . . . . . . . 21 110 4.1. Presentation by multi-party aware endpoints . . . . . . . 22 111 4.2. Multi-party mixing for multi-party unaware endpoints . . 24 112 5. Relation to Conference Control . . . . . . . . . . . . . . . 29 113 5.1. Use with SIP centralized conferencing framework . . . . . 30 114 5.2. Conference control . . . . . . . . . . . . . . . . . . . 30 115 6. Gateway Considerations . . . . . . . . . . . . . . . . . . . 30 116 6.1. Gateway considerations with Textphones (e.g. TTYs). . . 30 117 6.2. Gateway considerations with WebRTC. . . . . . . . . . . . 31 118 7. Updates to RFC 4103 . . . . . . . . . . . . . . . . . . . . . 31 119 8. Congestion considerations . . . . . . . . . . . . . . . . . . 31 120 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 121 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 122 10.1. Registration of the "rtt-mixer" sdp media attribute . . 32 123 11. Security Considerations . . . . . . . . . . . . . . . . . . . 33 124 12. Change history . . . . . . . . . . . . . . . . . . . . . . . 33 125 12.1. Changes included in 126 draft-ietf-avtcore-multi-party-rtt-mix-10 . . . . . . . 33 127 12.2. Changes included in 128 draft-ietf-avtcore-multi-party-rtt-mix-09 . . . . . . . 33 129 12.3. Changes included in 130 draft-ietf-avtcore-multi-party-rtt-mix-08 . . . . . . . 34 131 12.4. Changes included in 132 draft-ietf-avtcore-multi-party-rtt-mix-07 . . . . . . . 34 133 12.5. Changes included in 134 draft-ietf-avtcore-multi-party-rtt-mix-06 . . . . . . . 34 135 12.6. Changes included in 136 draft-ietf-avtcore-multi-party-rtt-mix-05 . . . . . . . 34 137 12.7. Changes included in 138 draft-ietf-avtcore-multi-party-rtt-mix-04 . . . . . . . 34 139 12.8. Changes included in 140 draft-ietf-avtcore-multi-party-rtt-mix-03 . . . . . . . 35 141 12.9. Changes included in 142 draft-ietf-avtcore-multi-party-rtt-mix-02 . . . . . . . 36 143 12.10. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 . . 36 144 12.11. Changes from 145 draft-hellstrom-avtcore-multi-party-rtt-source-03 to 146 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 36 147 12.12. Changes from 148 draft-hellstrom-avtcore-multi-party-rtt-source-02 to 149 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 36 150 12.13. Changes from 151 draft-hellstrom-avtcore-multi-party-rtt-source-01 to 152 -02 . . . . . . . . . . . . . . . . . . . . . . . . . . 37 153 12.14. Changes from 154 draft-hellstrom-avtcore-multi-party-rtt-source-00 to 155 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 38 156 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 157 13.1. Normative References . . . . . . . . . . . . . . . . . . 38 158 13.2. Informative References . . . . . . . . . . . . . . . . . 39 159 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 40 161 1. Introduction 163 RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for 164 transmission of real-time text (RTT) and the "text/t140" format. It 165 also specifies a redundancy format "text/red" for increased 166 robustness. RFC 4102 [RFC4102] registers the "text/red" format. 167 Regional regulatory requirements specify provision of real-time text 168 in multi-party calls. 170 Real-time text is usually provided together with audio and sometimes 171 with video in conversational sessions. 173 A requirement related to multi-party sessions from the presentation 174 level standard T.140 for real-time text is: "The display of text from 175 the members of the conversation should be arranged so that the text 176 from each participant is clearly readable, and its source and the 177 relative timing of entered text is visualized in the display." 179 Another requirement is that the mixing procedure must not introduce 180 delays in the text streams that are experienced disturbing the real- 181 time experience of the receiving users. 183 The redundancy scheme of RFC 4103 [RFC4103] enables efficient 184 transmission of redundant text in packets together with new text. 185 However the redundancy header format has no source indicators for the 186 redundant transmissions. The redundant parts in a packet must 187 therefore be from the same source as the new text. The recommended 188 transmission is one new and two redundant generations of text 189 (T140blocks) in each packet and the recommended transmission interval 190 for two-party use is 300 ms. 192 Real-time text mixers for multi-party sessions therefore need to 193 insert the source of each transmitted group of text from a conference 194 participant so that the text can be transmitted interleaved with text 195 groups from different sources in the rate they are created. This 196 enables the text groups to be presented by endpoints in suitable 197 grouping with other text from the same source. The presentation can 198 then be arranged so that text from different sources can be presented 199 in real-time and easily read while it is possible for a reading user 200 to also perceive approximately when the text was created in real time 201 by the different parties. The transmission and mixing is intended to 202 be done in a general way so that presentation can be arranged in a 203 layout decided by the endpoint. 205 There are existing implementations of RFC 4103 without the updates 206 from this document. These will not be able to receive and present 207 real-time text mixed for multi-party aware endpoints. 209 A negotiation mechanism is therefore needed for verification if the 210 parties are able to handle a multi-party coded stream and agreeing on 211 using that method. 213 A fall-back mixing procedure is also needed for cases when the 214 negotiation result indicates that a receiving endpoint is not capable 215 of handling the mixed format. This method is called the mixing 216 procedure for multi-party unaware endpoints. The fall-back method is 217 naturally not expected to meet all performance requirements placed on 218 the mixing procedure for multi-party aware endpoints. 220 The document updates RFC 4103[RFC4103] by introducing an attribute 221 for indicating capability for the multi-party mixing case and rules 222 for source indications and source switching. 224 1.1. Selected solution and considered alternative 226 A number of alternatives were considered when searching an efficient 227 and easily implemented multi-party method for real-time text. This 228 section explains a few of them briefly. 230 One RTP stream per source, sent in the same RTP session with 231 "text/red" format. 232 From some points of view, use of multiple RTP streams, one for 233 each source, sent in the same RTP session, called the RTP 234 translator model in [RFC3550], would be efficient, and use exactly 235 the same packet format as [RFC4103], the same payload type and a 236 simple SDP declaration. However, the RTP implementation in both 237 mixers and endpoints need to support multiple streams in the same 238 RTP session in order to use this mechanism. For best deployment 239 opportunity, it should be possible to upgrade existing endpoint 240 solutions to be multi-party aware with a reasonable effort. There 241 is currently a lack of support for multi-stream RTP in certain 242 implementation technologies. This fact made this solution not 243 selected for inclusion in this document. 245 The "text/red" format in RFC 4103 with shorter transmission 246 interval, and indicating source in CSRC. 247 The "text/red" format with "text/t140" payload in a single RTP 248 stream can be sent with 100 ms packet intervals instead of the 249 regular 300 ms. The source is indicated in the CSRC field. 250 Source switching can then be done every 100 ms while simultaneous 251 transmission occurs. With five participants sending text 252 simultaneously, the switching and transmission performance is 253 good. With more simultaneously sending participants, there will 254 be a noticable jerkiness in text presentation. The jerkiness will 255 be more expressed the more participants who send text 256 simultaneously. With ten sending participants, the jerkiness will 257 be about one second. Text sent from a source at the end of the 258 period its text is sent by the mixer will have close to zero extra 259 delay. Recent text will be presented with no or low delay. The 260 one second jerkiness will be noticable and slightly unpleasant, 261 but corresponds in time to what typing humans often cause by 262 hesitation or changing position while typing. A benefit of this 263 method is that no new packet format needs to be introduced and 264 implemented. Since simultaneous typing by more than two parties 265 is very rare, and in most applications also more than three 266 parties in a call is rare, this method can be used successfully 267 with good performance. Recovery of text in case of packet loss is 268 based on analysis of timestamps of received redundancy versus 269 earlier received text. Negotiation is based on a new sdp media 270 attribute "rtt-mixer". This method is selected to be the main one 271 specified in this document. 273 A new "text" media subtype with up to 15 sources in each packet. 274 The mechanism makes use of the RTP mixer model specified in 275 RFC3550[RFC3550]. Text from up to 15 sources can be included in 276 each packet. Packets are normally sent every 300 ms. The mean 277 delay will be 150 ms. The sources are indicated in strict order 278 in the CSRC list of the RTP packets. A new redundancy packet 279 format is specified. This method would result in good 280 performance, but would require standardisation and implementation 281 of new releases in the target technologies that would take more 282 time than desirable to complete. It was therefore not selected to 283 be included in this document. 285 The presentation planned by the mixer for multi-party unaware 286 endpoints. 287 It is desirable to have a method that does not require any 288 modifications in existing user devices implementing RFC 4103 for 289 RTT without explicit support of multi-party sessions. This is 290 possible by having the mixer insert a new line and a text 291 formatted source label before each switch of text source in the 292 stream. Switch of source can only be done in places in the text 293 where it does not disturb the perception of the contents. Text 294 from only one source can be presented in real time at a time. The 295 delay will therefore be varying. The method has also other 296 limitations, but is included in this document as a fallback 297 method. In calls where parties take turns properly by ending 298 their entries with a new line, the limitations will have limited 299 influence on the user experience. while only two parties send 300 text, these two will see the text in real time with no delay. 301 This method is specified as a fallback method in this document. 303 RTT transport in WebRTC 304 Transport of real-time text in the WebRTC technology is specified 305 to use the WebRTC data channel in 306 [I-D.ietf-mmusic-t140-usage-data-channel]. That spcification 307 contains a section briefly describing its use in multi-party 308 sessions. The focus of this document is RTP transport. 309 Therefore, even if the WebRTC transport provides good multi-party 310 performance, it is just mentioned in this document in relation to 311 providing gateways with multi-party capabilities between RTP and 312 WebRTC technologies. 314 1.2. Nomenclature 316 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 317 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 318 document are to be interpreted as described in [RFC2119]. 320 The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC, RTCP, RTP- 321 mixer, RTP-translator are explained in [RFC3550] 323 The term "T140block" is defined in RFC 4103 [RFC4103] to contain one 324 or more T.140 code elements. 326 "TTY" stands for a text telephone type used in North America. 328 "WebRTC" stands for web based communication specified by W3C and 329 IETF. 331 "DTLS-SRTP" stands for security specified in RFC 5764 [RFC5764]. 333 "multi-party aware" stands for an endpoint receiving real-time text 334 from multiple sources through a common conference mixer being able to 335 present the text in real-time separated by source and presented so 336 that a user can get an impression of the approximate relative timing 337 of text from different parties. 339 "multi-party unaware" stands for an endpoint not itself being able to 340 separate text from different sources when received through a common 341 conference mixer. 343 1.3. Intended application 345 The method for multi-party real-time text specified in this document 346 is primarily intended for use in transmission between mixers and 347 endpoints in centralised mixing configurations. It is also 348 applicable between mixers. An often mentioned application is for 349 emergency service calls with real-time text and voice, where a 350 calltaker want to make an attended handover of a call to another 351 agent, and stay observing the session. Multimedia conference 352 sessions with support for participants to contribute in text is 353 another application. Conferences with central support for speech-to- 354 text conversion is yet another mentioned application. 356 In all these applications, normally only one participant at a time 357 will send long text utterances. In some cases, one other participant 358 will occasionally contribute with a longer comment simultaneously. 359 That may also happen in some rare cases when text is interpreted to 360 text in another language in a conference. Apart from these cases, 361 other participants are only expected to contribute with very brief 362 utterings while others are sending text. 364 Text is supposed to be human generated, by some text input means, 365 such as typing on a keyboard or using speech-to-text technology. 366 Occasional small cut-and-paste operations may appear even if that is 367 not the initial purpose of real-time text. 369 The real-time characteristics of real-time text is essential for the 370 participants to be able to contribute to a conversation. If the text 371 is too much delayed from typing a letter to its presentation, then, 372 in some conference situations, the opportunity to comment will be 373 gone and someone else will grab the turn. A delay of more than one 374 second in such situations is an obstacle for good conversation. 376 2. Overview over the two specified solutions 378 This section contains a brief introduction of the two methods 379 specified in this document. 381 2.1. Negotiated use of the RFC 4103 format for multi-party transmission 382 in a single RTP stream 384 The main purpose of this document is to specify a method for true 385 multi-party real-time text mixing for multi-party aware endpoints. 386 The method use of the current format for real-time text in [RFC4103]. 387 It is an update of RFC 4103 by a clarification on one way to use it 388 in the multi-party situation. It is done by completing a negotiation 389 for this kind of multi-party capability and by indicating source in 390 the CSRC element in the RTP packets. Specific considerations are 391 made to be able to recover text after packet loss. 393 The detailed procedures for the multi-party aware case are specified 394 in Section 3 396 Please use [RFC4103] as reference when reading the specification. 398 2.2. Mixing for multi-party unaware endpoints 400 A method is also specified in this document for cases when the 401 endpoint participating in a multi-party call does not itself 402 implement any solution for multi-party handling of real-time text. 403 The solution requires the mixer to insert text dividers and readable 404 labels and only send text from one source at a time until a suitable 405 point appears for source change. This solution is a fallback method 406 with functional limitations that acts on the presentation level. 408 A party performing as a mixer, which has not negotiated the "rtt- 409 mixer" sdp media attribute, but negotiated a "text/red" or "text/ 410 t140" format in a session with a participant SHOULD, if nothing else 411 is specified for the application, format transmitted text to that 412 participant to be suitable to present on a multi-party unaware 413 endpoint as further specified in Section 4.2. 415 3. Details for the multi-party aware mixing case 417 3.1. Offer/answer considerations 419 RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a 420 redundancy format "text/red" for increased robustness of real-time 421 text transmission. This document updates RFC 4103[RFC4103] by 422 introducing a capability negotiation for handling multi-party real- 423 time text, a way to indicate the source of transmitted text, and 424 rules for efficient timing of the transmissions. 426 The capability negotiation is based on use of the sdp media attribute 427 "rtt-mixer". 429 Both parties shall indicate their capability in a session setup or 430 modification, and evaluate the capability of the counterpart. 432 The syntax is as follows: 433 "a=rtt-mixer" 435 3.2. Actions depending on capability negotiation result 437 A transmitting party SHALL send text according to the multi-party 438 format only when the negotiation for this method was successful and 439 when the CC field in the RTP packet is set to 1. In all other cases, 440 the packets SHALL be populated and interpreted as for a two-party 441 session. 443 A party which has negotiated the "rtt-mixer" sdp media attribute MUST 444 populate the CSRC-list and format the packets according to Section 3 445 if it acts as an rtp-mixer and sends multi-party text. 447 A party which has negotiated the "rtt-mixer" sdp media attribute MUST 448 interpret the contents of the "CC" field the CSRC-list and the 449 packets according to Section 3 in received rtp packets in the 450 corresponding RTP stream. 452 A party not performing as a mixer MUST not include the CSRC list. 454 3.3. Use of fields in the RTP packets 456 The CC field SHALL show the number of members in the CSRC list, which 457 SHALL be one (1) in transmissions from a mixer involved in a multi- 458 party session, and otherwise 0. 460 When transmitted from a mixer during a multi-party session, a CSRC 461 list SHALL be included in the packet. The single member in the CSRC- 462 list SHALL contain the SSRC of the source of the T140blocks in the 463 packet. When redundancy is used, the recommended level of redundancy 464 is to use one primary and two redundant generations of T140blocks. 465 In some cases, a primary or redundant T140block is empty, but is 466 still represented by a member in the redundancy header. 468 From other aspects, the contents of the RTP packets are equal to what 469 is specified in [RFC4103]. 471 3.4. Initial transmission of a BOM character 473 As soon as a participant is known to participate in a session with 474 another entity and being available for text reception, a Unicode BOM 475 character SHALL be sent to it by the other entity according to the 476 procedures in this section. If the transmitter is a mixer, then the 477 source of this character SHALL be indicated to be the mixer itself. 479 Note that the BOM character SHALL be transmitted with the same 480 redundancy procedures as any other text. 482 3.5. Keep-alive 484 After that, the transmitter SHALL send keep-alive traffic to the 485 receiver(s) at regular intervals when no other traffic has occurred 486 during that interval, if that is decided for the actual connection. 487 Recommendations for keep-alive can be found in [RFC6263]. 489 3.6. Transmission interval 491 A "text/red" transmitter in a mixer SHOULD send packets distributed 492 in time as long as there is something (new or redundant T140blocks) 493 to transmit. The maximum transmission interval SHOULD then be 330 494 ms. It is RECOMMENDED to send next packet to a receiver as soon as 495 new text to that receiver is available, as long as the time after the 496 latest sent packet to the same receiver is more than or equal to 100 497 ms, and also the maximum character rate to the receiver is not 498 exceeded. The intention is to keep the latency low and network load 499 limited while keeping a good protection against text loss in bursty 500 packet loss conditions. 502 3.7. Only one source per packet 504 New and redundant text from one source SHALL be transmitted in the 505 same packet if available for transmission at the same time. Text 506 from different sources MUST NOT be transmitted in the same packet. 508 3.8. Do not send received text to the originating source 510 Text received to a mixer from a participant SHOULD NOT be included in 511 transmission from the mixer to that participant. 513 3.9. Clean incoming text 515 A mixer SHALL handle reception, recovery of packet loss, deletion of 516 superfluous redundancy, marking of possible text loss and deletion of 517 'BOM' characters from each participant before queueing received text 518 for transmission to receiving participants. 520 3.10. Redundancy 522 A transmitting party using redundancy SHALL send redundant 523 repetitions of T140blocks already transmitted in earlier packets. 525 The number of redundant generations of T140blocks to include in 526 transmitted packets SHALL be deduced from the SDP negotiation. It 527 SHOULD be set to the minimum of the number declared by the two 528 parties negotiating a connection. It is RECOMMENDED to transmit one 529 original and two redundant generations of the T140blocks. 531 3.11. Source switching 533 When text from more than one source is available for transmission 534 from a mixer, the mixer SHALL let the sources take turns in having 535 their text transmitted. 537 The source with the oldest received text in the mixer SHOULD be next 538 in turn to get all its available unsent text transmitted. 540 3.12. Text placement in packets 542 The mixer SHOULD compose and transmit an RTP packet to a receiver 543 when one of the following conditions has occurred: 545 * 100 ms has passed since the latest transmission to that receiver, 546 and there is unsent text available for transmission. 548 * New text has arrived and more than 100 ms has passed since latest 549 transmission to that receiver. 551 * 330 ms has passed since already transmitted text was queued for 552 transmission as redundant text, and more than 100 ms has passed 553 since the latest transmission to that receiver, and the redundant 554 text is still not sent. 556 At time of transmission, the mixer SHALL populate the RTP packet with 557 all T140blocks queued for transmission originating from the source in 558 turn for transmission as long as this is not in conflict with the 559 allowed number of characters per second ("CPS") or the maximum packet 560 size. In this way, the latency of the latest received text is kept 561 low even in moments of simultaneous transmission from many sources. 563 The SSRC of the source shall be placed as the only member in the 564 CSRC-list. 566 Note: The CSRC-list in an RTP packet only includes the participant 567 who's text is included in text blocks. It is not the same as the 568 total list of participants in a conference. With audio and video 569 media, the CSRC-list would often contain all participants who are not 570 muted whereas text participants that don't type are completely silent 571 and thus are not represented in RTP packet CSRC-lists. 573 3.13. Empty T140blocks 575 If no unsent T140blocks were available for a source at the time of 576 populating a packet, but T140blocks are available which have not yet 577 been sent the full intended number of redundant transmissions, then 578 the primary T140block for that source is composed of an empty 579 T140block, and populated (without taking up any length) in a packet 580 for transmission. The corresponding SSRC SHALL be placed as usual in 581 its place in the CSRC-list. 583 The first packet in the session, the first after a source switch and 584 the first after a pause SHALL be poulated with the available 585 T140blocks for the source in turn to be sent as primary, and empty 586 T140blocks for the agreed number of redundancy generations. 588 3.14. Creation of the redundancy 590 The primary T140block from a source in the latest transmitted packet 591 is saved for populating the first redundant T140block for that source 592 in next transmission of text from that source. The first redundant 593 T140block for that source from the latest transmission is saved for 594 populating the second redundant T140block in next transmission of 595 text from that source. 597 Usually this is the level of redundancy used. If a higher number of 598 redundancy is negotiated, then the procedure SHALL be maintained 599 until all available redundant levels of T140blocks are placed in the 600 packet. If a receiver has negotiated a lower number of "text/red" 601 generations, then that level shall be the maximum used by the 602 transmitter. 604 The T140blocks saved for transmission as redundant data are assigned 605 a planned transmission time 330 ms after the current time. 607 3.15. Timer offset fields 609 The timestamp offset values are inserted in the redundancy header, 610 with the time offset from the RTP timestamp in the packet when the 611 corresponding T140block was sent as primary. 613 The timestamp offsets are expressed in the same clock tick units as 614 the RTP timestamp. 616 The timestamp offset values for empty T140blocks have no relevance 617 but SHOULD be assigned realistic values. 619 3.16. Other RTP header fields 621 The number of members in the CSRC list ( 0 or 1) shall be placed in 622 the "CC" header field. Only mixers place value 1 in the "CC" field. 623 A value of "0" indicates that the source is the transmitting device 624 itself and that the source is indicated by the SSRC field. This 625 value is used by endpoints, and by mixers sending data that it is 626 source of itself. 628 The current time SHALL be inserted in the timestamp. 630 The SSRC of the mixer for the RTT session SHALL be inserted in the 631 SSRC field of the RTP header. 633 The M-bit shall be handled as specified in [RFC4103]. 635 3.17. Pause in transmission 637 When there is no new T140block to transmit, and no redundant 638 T140block that has not been retransmitted the intended number of 639 times from any source, the transmission process can stop until either 640 new T140blocks arrive, or a keep-alive method calls for transmission 641 of keep-alive packets. 643 3.18. RTCP considerations 645 A mixer SHALL send RTCP reports with SDES, CNAME and NAME information 646 about the sources in the multi-party call. This makes it possible 647 for participants to compose a suitable label for text from each 648 source. 650 Integrity considerations SHALL be considered when composing these 651 fields. 653 3.19. Reception of multi-party contents 655 The "text/red" receiver included in an endpoint with presentation 656 functions will receive RTP packets in the single stream from the 657 mixer, and SHALL distribute the T140blocks for presentation in 658 presentation areas for each source. Other receiver roles, such as 659 gateways or chained mixers are also feasible, and requires 660 consideration if the stream shall just be forwarded, or distributed 661 based on the different sources. 663 3.19.1. Multi-party vs two-party use 665 If the "CC" field value of a received packet is 1, it indicates that 666 multi-party transmission is active, and the receiver MUST be prepared 667 to act on the source according to its role. If the CC value is 0, 668 the connection is point-to-point. 670 3.19.2. Level of redundancy 672 The used level of redundancy generations SHALL be evaluated from the 673 received packet contents. The number of generations (including the 674 primary) is equal to the number of members in the redundancy header. 676 3.19.3. Empty T140blocks 678 Empty T140blocks are included as fillers for unused primary or 679 redundancy levels in the packets. They just do not provide any 680 contents and do not contribute to the received streams. 682 3.19.4. Detection and indication of possible text loss 684 The RTP sequence numbers of the received packets SHALL be monitored 685 for gaps and packets out of order. If a sequence number gap appears 686 and still exists after some defined short time for jitter resolution, 687 the packets in the gap SHALL be regarded lost. 689 If it is known that only one source is active in the RTP session, 690 then it is likely that a gap equal to or larger than the agreed 691 number of redundancy generations causes text loss. In that case a 692 t140block SHALL be created with a marker for possible text loss 693 [T140ad1] and assigned to the source and inserted in the reception 694 buffer for that source. 696 If it is known that more than one source is active in the RTP 697 session, then it is not possible in general to evaluate if text was 698 lost when packets were lost. With two active sources and the 699 recommended number of redundancy generations (3), it can take a gap 700 of five lost packets until any text may be lost, but text loss can 701 also appear if three non-consecutive packets are lost when they 702 contained consecutive data from the same source. A simple method to 703 decide when there is risk for resulting text loss is to evaluate if 704 three or more packets were lost within one second. Then a t140block 705 SHALL be created with a marker for possible text loss [T140ad1] and 706 assigned to the SSRC of the transmitter as a general input from the 707 mixer. 709 Implementations MAY apply more refined methods for more reliable 710 detection of if text was lost or not. Any refined method SHOULD 711 rather falsely mark possible loss when there was no loss instead of 712 not marking possible loss when there was loss. 714 3.19.5. Extracting text and handling recovery 716 When applying the following procedures, the effects MUST be 717 considered of possible timestamp wrap around and the RTP session 718 possibly changing SSRC. 720 When a packet is received in an RTP session using the packetization 721 for multi-party aware endpoints, its T140blocks SHALL be extracted in 722 the following way. The description is adapted to the default 723 redundancy case using the original and two redundant generations. 725 When applying the following procedures, the effects MUST be 726 considered of possible timestamp wrap around and the RTP session 727 possibly changing SSRC. 729 The source SHALL be extracted from the CSRC-list if available, 730 otherwise from the SSRC. 732 If the received packet is the first packet received from the source, 733 then all T140blocks in the packet SHALL be retrieved and assigned to 734 a receive buffer for the source beginning with the second generation 735 redundancy, continuing with the first generation redundancy and 736 finally the primary. 738 Note that the normal case is that in the first packet, only the 739 primary data has contents. The redundant data has contents in the 740 first received packet from a source only after initial packet loss. 742 If the packet is not the first packet from a source, then if the 743 second generation redundant data is available, its timestamp SHALL be 744 created by subtracting its timestamp offset from the RTP timestamp. 745 If the resulting timestamp is later than the latest retrieved data 746 from the same source, then the redundant data SHALL be retrieved and 747 appended to the receive buffer. The process SHALL be continued in 748 the same way for the first generation redundant data. After that, 749 the primary data SHALL be retrieved from the packet and appended to 750 the receive buffer for the source. 752 3.19.6. Delete 'BOM' 754 Unicode character 'BOM' is used as a start indication and sometimes 755 used as a filler or keep alive by transmission implementations. 756 These SHALL be deleted after extraction from received packets. 758 3.20. Performance considerations 760 This solution has good performance for up to five participants 761 simultaneously sending text. At higher numbers of participants 762 simultaneously sending text, a jerkiness is visible in the 763 presentation of text. With ten participants simultaneously 764 transmitting text, the jerkiness is about one second. Even so, the 765 transmission of text catches up, so there is no resulting total delay 766 introduced. The solution is therefore suitable for emergency service 767 use, relay service use, and small or well-managed larger multimedia 768 conferences. Only in large unmanaged conferences with a high number 769 of participants there may on very rare occasions appear situations 770 when many participants happen to send text simultaneously, resulting 771 in unpleasantly long switching times. It should be noted that it is 772 only the number of users sending text within the same moment that 773 causes jerkiness, not the total number of users with RTT capability. 775 3.21. Security for session control and media 777 Security SHOULD be applied on both session control and media. In 778 applications where legacy endpoints without security may exist, a 779 negotiation SHOULD be performed to decide if security by encryption 780 will be applied. If no other security solution is mandated for the 781 application, then RFC 8643 OSRTP[RFC8643] SHOULD be applied to 782 negotiate SRTP media security with DTLS. Most SDP examples below are 783 for simplicity expressed without the security additions. The 784 principles (but not all details) for applying DTLS-SRTP security is 785 shown in a couple of the following examples. 787 3.22. SDP offer/answer examples 789 This sections shows some examples of SDP for session negotiation of 790 the real-time text media in SIP sessions. Audio is usually provided 791 in the same session, and sometimes also video. The examples only 792 show the part of importance for the real-time text media. 794 Offer example for "text/red" format and multi-party support: 796 m=text 11000 RTP/AVP 100 98 797 a=rtpmap:98 t140/1000 798 a=rtpmap:100 red/1000 799 a=fmtp:100 98/98/98 800 a=rtt-mixer 802 Answer example from a multi-party capable device 803 m=text 14000 RTP/AVP 100 98 804 a=rtpmap:98 t140/1000 805 a=rtpmap:100 red/1000 806 a=fmtp:100 98/98/98 807 a=rtt-mixer 809 Offer example for "text/red" format including multi-party 810 and security: 811 a=fingerprint: (fingerprint1) 812 m=text 11000 RTP/AVP 100 98 813 a=rtpmap:98 t140/1000 814 a=rtpmap:100 red/1000 815 a=fmtp:100 98/98/98 816 a=rtt-mixer 818 The "fingerprint" is sufficient to offer DTLS-SRTP, with the media 819 line still indicating RTP/AVP. 821 Note: For brevity, the entire value of the SDP fingerprint attribute 822 is not shown in this and the following example. 824 Answer example from a multi-party capable device with security 825 a=fingerprint: (fingerprint2) 826 m=text 16000 RTP/AVP 100 98 827 a=rtpmap:98 t140/1000 828 a=rtpmap:100 red/1000 829 a=fmtp:100 98/98/98 830 a=rtt-mixer 832 With the "fingerprint" the device acknowledges use of SRTP/DTLS. 834 Answer example from a multi-party unaware device that also 835 does not support security: 837 m=text 12000 RTP/AVP 100 98 838 a=rtpmap:98 t140/1000 839 a=rtpmap:100 red/1000 840 a=fmtp:100 98/98/98 842 3.23. Packet sequence example from a source switch 844 This example shows a symbolic flow of packets from a mixer including 845 loss and recovery. The sequence includes a source switch. A and B 846 are sources of RTT. P indicates primary data. R1 is first redundant 847 generation data and R2 is second redundant generation data. A1, B1, 848 A2 etc are text chunks (T140blocks) received from the respective 849 sources. X indicates dropped packet between the mixer and a 850 receiver. The session is assumed to use original and two redundant 851 generations of RTT. 853 |----------------| 854 |Seq no 101 | 855 |CC=1 | 856 |CSRC list A | 857 |R2: A1 | 858 |R1: A2 | 859 |P: A3 | 860 |----------------| 862 Assuming that earlier packets ( with text A1 and A2) were received in 863 sequence, text A3 is received from packet 101 and assigned to 864 reception area A. The mixer is now assumed to have received text 865 from source B and will send that text 100 ms after packet 101. 866 Transmission of A2 and A3 as redundancy is planned for 200 ms after 867 packet 101. 869 |----------------| 870 |Seq no 102 | 871 |CC=1 | 872 |CSRC list B | 873 |R2 Empty | 874 |R1: Empty | 875 |P: B1 | 876 |----------------| 877 B1 is retrieved from this packet. Redundant transmission of 878 B1 is planned 200 ms after packet 102. 880 X----------------| 881 X Seq no 103 | 882 X CC=1 | 883 X CSRC list A | 884 X R2: A2 | 885 X R1: A3 | 886 X P: Empty | 887 X----------------| 888 Packet 103 is assumed to be dropped in network problems. It 889 contains redundancy for A. Sending A3 as second level 890 redundancy is planned for 100 ms after packet 104. 892 X----------------| 893 X Seq no 104 | 894 X CC=1 | 895 X CSRC list B | 896 X R2: Empty | 897 X R1: B1 | 898 X P2: B2 | 899 X----------------| 900 Packet 104 contains text from B, assumed dropped in network 901 problems. The mixer has A3 redundancy to send and plans it 902 after 100 ms. 904 X----------------| 905 X Seq no 105 | 906 X CC=1 | 907 X CSRC list A | 908 X R2: A3 | 909 X R1: Empty | 910 X P: Empty | 911 X----------------| 912 Packet 105 is assumed to be dropped in network problems 913 B1 and B2 still needs to be transmitted as redundancy. 914 This is planned 320 ms after packet 105. 916 |----------------| 917 |Seq no 106 | 918 |CC=1 | 919 |CSRC list B | 920 | R2: B1 | 921 | R1: B2 | 922 | P: Empty | 923 |----------------| 925 Packet 106 is received 320 ms after packet 105. The latest received 926 sequence number was 102. 103,104,105 were lost. Three packets were 927 thus lost during less than one second. The simple rule for detection 928 of text loss then results in that an indicator for possible loss 929 (U'FFFD) [T140ad1], should be inserted generally for the mixer. 931 The second level redundancy in packet 106 is B1 and has timestamp 932 offset 620 ms. The timestamp of packet 106 minus 620 is the 933 timestamp of packet 101 which was received. So B1 does not need to 934 be retrieved. The first level redundancy in packet 106 is 420. The 935 timestamp of packet 106 minus 420 is later than the latest received 936 packet with source A. Therefore B2 is retrieved and assigned to the 937 input buffer for source B. No primary is available in packet 106 939 After this sequence, A3 and B1 and B2 have been received. In this 940 case no text was lost. 942 3.24. Maximum character rate "CPS" 944 The default maximum rate of reception of "text/t140" real-time text 945 is in RFC 4103 [RFC4103] specified to be 30 characters per second. 946 The value MAY be modified in the CPS parameter of the FMTP attribute 947 in the media section for the "text/t140" media. A mixer combining 948 real-time text from a number of sources may occasionally have a 949 higher combined flow of text coming from the sources. Endpoints 950 SHOULD therefore specify a suitable higher value for the CPS 951 parameter, corresponding to its real reception capability. A value 952 for "CPS" of 90 is the default for the "text/t140" stream in the 953 "text/red" format when multi-party real-time text is negotiated. See 954 RFC 4103 [RFC4103] for the format and use of the CPS parameter. The 955 same rules apply for the multi-party case except for the default 956 value. 958 4. Presentation level considerations 960 ITU-T T.140 [T140] provides the presentation level requirements for 961 the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for 962 erasure and other formatting functions and has the following general 963 statement for the presentation: 965 "The display of text from the members of the conversation should be 966 arranged so that the text from each participant is clearly readable, 967 and its source and the relative timing of entered text is visualized 968 in the display. Mechanisms for looking back in the contents from the 969 current session should be provided. The text should be displayed as 970 soon as it is received." 971 Strict application of T.140 [T140] is of essence for the 972 interoperability of real-time text implementations and to fulfill the 973 intention that the session participants have the same information of 974 the text contents of the conversation without necessarily having the 975 exact same layout of the conversation. 977 T.140 [T140] specifies a set of presentation control codes to include 978 in the stream. Some of them are optional. Implementations MUST be 979 able to ignore optional control codes that they do not support. 981 There is no strict "message" concept in real-time text. Line 982 Separator SHALL be used as a separator allowing a part of received 983 text to be grouped in presentation. The characters "CRLF" may be 984 used by other implementations as replacement for Line Separator. The 985 "CRLF" combination SHALL be erased by just one erasing action, just 986 as the Line Separator. Presentation functions are allowed to group 987 text for presentation in smaller groups than the line separators 988 imply and present such groups with source indication together with 989 text groups from other sources (see the following presentation 990 examples). Erasure has no specific limit by any delimiter in the 991 text stream. 993 4.1. Presentation by multi-party aware endpoints 995 A multi-party aware receiving party, presenting real-time text MUST 996 separate text from different sources and present them in separate 997 presentation fields. The receiving party MAY separate presentation 998 of parts of text from a source in readable groups based on other 999 criteria than line separator and merge these groups in the 1000 presentation area when it benefits the user to most easily find and 1001 read text from the different participants. The criteria MAY e.g. be 1002 a received comma, full stop, or other phrase delimiters, or a long 1003 pause. 1005 When text is received from multiple original sources, the 1006 presentation SHOULD provide a view where text is added in multiple 1007 presentation fields. 1009 If the presentation presents text from different sources in one 1010 common area, the presenting endpoint SHOULD insert text from the 1011 local user ended at suitable points merged with received text to 1012 indicate the relative timing for when the text groups were completed. 1013 In this presentation mode, the receiving endpoint SHALL present the 1014 source of the different groups of text. 1016 A view of a three-party RTT call in chat style is shown in this 1017 example . 1019 _________________________________________________ 1020 | |^| 1021 |[Alice] Hi, Alice here. |-| 1022 | | | 1023 |[Bob] Bob as well. | | 1024 | | | 1025 |[Eve] Hi, this is Eve, calling from Paris. | | 1026 | I thought you should be here. | | 1027 | | | 1028 |[Alice] I am coming on Thursday, my | | 1029 | performance is not until Friday morning.| | 1030 | | | 1031 |[Bob] And I on Wednesday evening. | | 1032 | | | 1033 |[Alice] Can we meet on Thursday evening? | | 1034 | | | 1035 |[Eve] Yes, definitely. How about 7pm. | | 1036 | at the entrance of the restaurant | | 1037 | Le Lion Blanc? | | 1038 |[Eve] we can have dinner and then take a walk |-| 1039 |______________________________________________|v| 1040 | But I need to be back to |^| 1041 | the hotel by 11 because I need |-| 1042 | | | 1043 | I wou |-| 1044 |______________________________________________|v| 1045 | of course, I underst | 1046 |________________________________________________| 1048 Figure 3: Example of a three-party RTT call presented in chat style 1049 seen at participant 'Alice's endpoint. 1051 Other presentation styles than the chat style may be arranged. 1053 This figure shows how a coordinated column view MAY be presented. 1055 _____________________________________________________________________ 1056 | Bob | Eve | Alice | 1057 |____________________|______________________|_______________________| 1058 | | |I will arrive by TGV. | 1059 |My flight is to Orly| |Convenient to the main | 1060 | |Hi all, can we plan |station. | 1061 | |for the seminar? | | 1062 |Eve, will you do | | | 1063 |your presentation on| | | 1064 |Friday? |Yes, Friday at 10. | | 1065 |Fine, wo | |We need to meet befo | 1066 |___________________________________________________________________| 1067 Figure 4: An example of a coordinated column-view of a three-party 1068 session with entries ordered vertically in approximate time-order. 1070 4.2. Multi-party mixing for multi-party unaware endpoints 1072 When the mixer has indicated multi-party capability by the "rtt- 1073 mixer" sdp attribute in an SDP negotiation, but the multi-party 1074 capability negotiation fails with an endpoint, then the agreed "text/ 1075 red" or "text/t140" format SHALL be used and the mixer SHOULD compose 1076 a best-effort presentation of multi-party real-time text in one 1077 stream intended to be presented by an endpoint with no multi-party 1078 awareness. 1080 This presentation format has functional limitations and SHOULD be 1081 used only to enable participation in multi-party calls by legacy 1082 deployed endpoints implementing only RFC 4103 without any multi-party 1083 extensions specified in this document. 1085 The principles and procedures below do not specify any new protocol 1086 elements. They are instead composed from the information in ITU-T 1087 T.140 [T140] and an ambition to provide a best effort presentation on 1088 an endpoint which has functions only for two-party calls. 1090 The mixer mixing for multi-party unaware endpoints SHALL compose a 1091 simulated limited multi-party RTT view suitable for presentation in 1092 one presentation area. The mixer SHALL group text in suitable groups 1093 and prepare for presentation of them by inserting a new line between 1094 them if the transmitted text did not already end with a new line. A 1095 presentable label SHOULD be composed and sent for the source 1096 initially in the session and after each source switch. With this 1097 procedure the time for source switching is depending on the actions 1098 of the users. In order to expedite source switch, a user can for 1099 example end its turn with a new line. 1101 4.2.1. Actions by the mixer at reception from the call participants 1103 When text is received by the mixer from the different participants, 1104 the mixer SHALL recover text from redundancy if any packets are lost. 1105 The mark for lost text [T140ad1] SHOULD be inserted in the stream if 1106 unrecoverable loss appears. Any Unicode "BOM" characters, possibly 1107 used for keep-alive shall be deleted. The time of creation of text 1108 (retrieved from the RTP timestamp) SHALL be stored together with the 1109 received text from each source in queues for transmission to the 1110 recipients. 1112 4.2.2. Actions by the mixer for transmission to the recipients 1114 The following procedure SHOULD be applied for each recipient of 1115 multi-part text from the mixer. 1117 The text for transmission SHOULD be formatted by the mixer for each 1118 receiving user for presentation in one single presentation area. 1119 Text received from a participant SHOULD NOT be included in 1120 transmission to that participant. When there is text available for 1121 transmission from the mixer to a receiving party from more than one 1122 participant, the mixer SHOULD switch between transmission of text 1123 from the different sources at suitable points in the transmitted 1124 stream. 1126 When switching source, the mixer SHOULD insert a line separator if 1127 the already transmitted text did not end with a new line (line 1128 separator or CRLF). A label SHOULD be composed from information in 1129 the CNAME and NAME fields in RTCP reports from the participant to 1130 have its text transmitted, or from other session information for that 1131 user. The label SHOULD be delimited by suitable characters (e.g. '[ 1132 ]') and transmitted. The CSRC SHOULD indicate the selected source. 1133 Then text from that selected participant SHOULD be transmitted until 1134 a new suitable point for switching source is reached. 1136 Integrity considerations SHALL be taken when composing the label. 1138 Seeking a suitable point for switching source SHOULD be done when 1139 there is older text waiting for transmission from any party than the 1140 age of the last transmitted text. Suitable points for switching are: 1142 * A completed phrase ended by comma 1144 * A completed sentence 1146 * A new line (line separator or CRLF) 1148 * A long pause (e.g. > 10 seconds) in received text from the 1149 currently transmitted source 1151 * If text from one participant has been transmitted with text from 1152 other sources waiting for transmission for a long time (e.g. > 1 1153 minute) and none of the other suitable points for switching has 1154 occurred, a source switch MAY be forced by the mixer at next word 1155 delimiter, and also if even a word delimiter does not occur within 1156 a time (e.g. 15 seconds) after the scan for word delimiter 1157 started. 1159 When switching source, the source which has the oldest text in queue 1160 SHOULD be selected to be transmitted. A character display count 1161 SHOULD be maintained for the currently transmitted source, starting 1162 at zero after the label is transmitted for the currently transmitted 1163 source. 1165 The status SHOULD be maintained for the latest control code for 1166 Select Graphic Rendition (SGR) from each source. If there is an SGR 1167 code stored as the status for the current source before the source 1168 switch is done, a reset of SGR shall be sent by the sequence SGR 0 1169 [009B 0000 006D] after the new line and before the new label during a 1170 source switch. See SGR below for an explanation. This transmission 1171 does not influence the display count. 1173 If there is an SGR code stored for the new source after the source 1174 switch, that SGR code SHOULD be transmitted to the recipient before 1175 the label. This transmission does not influence the display count. 1177 4.2.3. Actions on transmission of text 1179 Text from a source sent to the recipient SHOULD increase the display 1180 count by one per transmitted character. 1182 4.2.4. Actions on transmission of control codes 1184 The following control codes specified by T.140 require specific 1185 actions. They SHOULD cause specific considerations in the mixer. 1186 Note that the codes presented here are expressed in UCS-16, while 1187 transmission is made in UTF-8 transform of these codes. 1189 BEL 0007 Bell Alert in session, provides for alerting during an 1190 active session. The display count SHOULD not be altered. 1192 NEW LINE 2028 Line separator. Check and perform a source switch if 1193 appropriate. Increase display count by 1. 1195 CR LF 000D 000A A supported, but not preferred way of requesting a 1196 new line. Check and perform a source switch if appropriate. 1197 Increase display count by 1. 1199 INT ESC 0061 Interrupt (used to initiate mode negotiation 1200 procedure). The display count SHOULD not be altered. 1202 SGR 009B Ps 006D Select graphic rendition. Ps is rendition 1203 parameters specified in ISO 6429. The display count SHOULD not be 1204 altered. The SGR code SHOULD be stored for the current source. 1206 SOS 0098 Start of string, used as a general protocol element 1207 introducer, followed by a maximum 256 bytes string and the ST. 1208 The display count SHOULD not be altered. 1210 ST 009C String terminator, end of SOS string. The display count 1211 SHOULD not be altered. 1213 ESC 001B Escape - used in control strings. The display count SHOULD 1214 not be altered for the complete escape code. 1216 Byte order mark "BOM" (U+FEFF) "Zero width, no break space", used 1217 for synchronization and keep-alive. SHOULD be deleted from 1218 incoming streams. Shall be sent first after session establishment 1219 to the recipient. The display count shall not be altered. 1221 Missing text mark (U+FFFD) "Replacement character", represented as a 1222 question mark in a rhombus, or if that is not feasible, replaced 1223 by an apostrophe ', marks place in stream of possible text loss. 1224 SHOULD be inserted by the reception procedure in case of 1225 unrecoverable loss of packets. The display count SHOULD be 1226 increased by one when sent as for any other character. 1228 SGR If a control code for selecting graphic rendition (SGR), other 1229 than reset of the graphic rendition (SGR 0) is sent to a 1230 recipient, that control code shall also be stored as status for 1231 the source in the storage for SGR status. If a reset graphic 1232 rendition (SGR 0) originated from a source is sent, then the SGR 1233 status storage for that source shall be cleared. The display 1234 count shall not be increased. 1236 BS (U+0008) Back Space, intended to erase the last entered character 1237 by a source. Erasure by backspace cannot always be performed as 1238 the erasing party intended. If an erasing action erases all text 1239 up to the end of the leading label after a source switch, then the 1240 mixer must not transmit more backspaces. Instead it is 1241 RECOMMENDED that a letter "X" is inserted in the text stream for 1242 each backspace as an indication of the intent to erase more. A 1243 new line is usually coded by a Line Separator, but the character 1244 combination "CRLF" MAY be used instead. Erasure of a new line is 1245 in both cases done by just one erasing action (Backspace). If the 1246 display count has a positive value it is decreased by one when the 1247 BS is sent. If the display count is at zero, it is not altered. 1249 4.2.5. Packet transmission 1251 A mixer transmitting to a multi-party unaware terminal SHOULD send 1252 primary data only from one source per packet. The SSRC SHOULD be the 1253 SSRC of the mixer. The CSRC list SHOULD contain one member and be 1254 the SSRC of the source of the primary data. 1256 4.2.6. Functional limitations 1258 When a multi-party unaware endpoint presents a conversation in one 1259 display area in a chat style, it inserts source indications for 1260 remote text and local user text as they are merged in completed text 1261 groups. When an endpoint using this layout receives and presents 1262 text mixed for multi-party unaware endpoints, there will be two 1263 levels of source indicators for the received text; one generated by 1264 the mixer and inserted in a label after each source switch, and 1265 another generated by the receiving endpoint and inserted after each 1266 switch between local and remote source in the presentation area. 1267 This will waste display space and look inconsistent to the reader. 1269 New text can be presented only from one source at a time. Switch of 1270 source to be presented takes place at suitable places in the text, 1271 such as end of phrase, end of sentence, line separator and 1272 inactivity. Therefore the time to switch to present waiting text 1273 from other sources may become long and will vary and depend on the 1274 actions of the currently presented source. 1276 Erasure can only be done up to the latest source switch. If a user 1277 tries to erase more text, the erasing actions will be presented as 1278 letter X after the label. 1280 Text loss because of network errors may hit the label between entries 1281 from different parties, causing risk for misunderstanding from which 1282 source a piece of text is. 1284 These facts makes it strongly RECOMMENDED to implement multi-party 1285 awareness in RTT endpoints. The use of the mixing method for multi- 1286 party-unaware endpoints should be left for use with endpoints which 1287 are impossible to upgrade to become multi-party aware. 1289 4.2.7. Example views of presentation on multi-party unaware endpoints 1291 The following pictures are examples of the view on a participant's 1292 display for the multi-party-unaware case. 1294 _________________________________________________ 1295 | Conference | Alice | 1296 |________________________|_________________________| 1297 | |I will arrive by TGV. | 1298 |[Bob]:My flight is to |Convenient to the main | 1299 |Orly. |station. | 1300 |[Eve]:Hi all, can we | | 1301 |plan for the seminar. | | 1302 | | | 1303 |[Bob]:Eve, will you do | | 1304 |your presentation on | | 1305 |Friday? | | 1306 |[Eve]:Yes, Friday at 10.| | 1307 |[Bob]: Fine, wo |We need to meet befo | 1308 |________________________|_________________________| 1310 Figure 5: Alice who has a conference-unaware client is receiving the 1311 multi-party real-time text in a single-stream. This figure shows how 1312 a coordinated column view MAY be presented on Alice's device. 1314 _________________________________________________ 1315 | |^| 1316 |[Alice] Hi, Alice here. |-| 1317 | | | 1318 |[mix][Bob] Bob as well. | | 1319 | | | 1320 |[Eve] Hi, this is Eve, calling from Paris | | 1321 | I thought you should be here. | | 1322 | | | 1323 |[Alice] I am coming on Thursday, my | | 1324 | performance is not until Friday morning.| | 1325 | | | 1326 |[mix][Bob] And I on Wednesday evening. | | 1327 | | | 1328 |[Eve] we can have dinner and then walk | | 1329 | | | 1330 |[Eve] But I need to be back to | | 1331 | the hotel by 11 because I need | | 1332 | |-| 1333 |______________________________________________|v| 1334 | of course, I underst | 1335 |________________________________________________| 1337 Figure 6: An example of a view of the multi-party unaware 1338 presentation in chat style. Alice is the local user. 1340 5. Relation to Conference Control 1341 5.1. Use with SIP centralized conferencing framework 1343 The SIP conferencing framework, mainly specified in RFC 1344 4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable 1345 for coordinating sessions including multi-party RTT. The RTT stream 1346 between the mixer and a participant is one and the same during the 1347 conference. Participants get announced by notifications when 1348 participants are joining or leaving, and further user information may 1349 be provided. The SSRC of the text to expect from joined users MAY be 1350 included in a notification. The notifications MAY be used both for 1351 security purposes and for translation to a label for presentation to 1352 other users. 1354 5.2. Conference control 1356 In managed conferences, control of the real-time text media SHOULD be 1357 provided in the same way as other for media, e.g. for muting and 1358 unmuting by the direction attributes in SDP [RFC4566]. 1360 Note that floor control functions may be of value for RTT users as 1361 well as for users of other media in a conference. 1363 6. Gateway Considerations 1365 6.1. Gateway considerations with Textphones (e.g. TTYs). 1367 Multi-party RTT sessions may involve gateways of different kinds. 1368 Gateways involved in setting up sessions SHALL correctly reflect the 1369 multi-party capability or unawareness of the combination of the 1370 gateway and the remote endpoint beyond the gateway. 1372 One case that may occur is a gateway to PSTN for communication with 1373 textphones (e.g. TTYs). Textphones are limited devices with no 1374 multi-party awareness, and it SHOULD therefore be suitable for the 1375 gateway to not indicate multi-party awareness for that case. Another 1376 solution is that the gateway indicates multi-party capability towards 1377 the mixer, and includes the multi-party mixer function for multi- 1378 party unaware endpoints itself. This solution makes it possible to 1379 make adaptations for the functional limitations of the textphone 1380 (TTY). 1382 More information on gateways to textphones (TTYs) is found in RFC 1383 5194[RFC5194] 1385 6.2. Gateway considerations with WebRTC. 1387 Gateway operation to real-time text in WebRTC may also be required. 1388 In WebRTC, RTT is specified in 1389 [I-D.ietf-mmusic-t140-usage-data-channel]. 1391 A multi-party bridge may have functionality for communicating by RTT 1392 both in RTP streams with RTT and WebRTC t140 data channels. Other 1393 configurations may consist of a multi-party bridge with either 1394 technology for RTT transport and a separate gateway for conversion of 1395 the text communication streams between RTP and t140 data channel. 1397 In WebRTC, it is assumed that for a multi-party session, one t140 1398 data channel is established for each source from a gateway or bridge 1399 to each participant. Each participant also has a data channel with 1400 two-way connection with the gateway or bridge. 1402 The t140 channel used both ways is for text from the WebRTC user and 1403 from the bridge or gateway itself to the WebRTC user. The label 1404 parameter of this t140 channel is used as NAME field in RTCP to 1405 participants on the RTP side. The other t140 channels are only for 1406 text from other participants to the WebRTC user. 1408 When a new participant has entered the session with RTP transport of 1409 rtt, a new t140 channel SHOULD be established to WebRTC users with 1410 the label parameter composed from the NAME field in RTCP on the RTP 1411 side. 1413 When a new participant has entered the multi-party session with RTT 1414 transport in a WebRTC t140 data channel, the new participant SHOULD 1415 be announced by a notification to RTP users. The label parameter 1416 from the WebRTC side SHOULD be used as the NAME RTCP field on the RTP 1417 side, or other available session information. 1419 7. Updates to RFC 4103 1421 This document updates RFC 4103[RFC4103] by introducing an sdp media 1422 attribute "rtt-mixer" for negotiation of multi-party mixing 1423 capability with the [RFC4103] format, and by specifying the rules for 1424 packets when multi-party capability is negotiated and in use. 1426 8. Congestion considerations 1428 The congestion considerations and recommended actions from RFC 4103 1429 [RFC4103] are valid also in multi-party situations. 1431 The first action in case of congestion SHOULD be to temporarily 1432 increase the transmission interval up to two seconds. 1434 If the unlikely situation appears that more than 20 participants in a 1435 conference send text simultaneously, it will take more than 7 seconds 1436 between presentation of text from each of these participants. More 1437 time than that can cause confusion in the session. It is therefore 1438 RECOMMENDED that the mixer discards such text in excess inserts a 1439 general indication of possible text loss [T140ad1] in the session. 1440 If the main text contributor is indicated in any way, the mixer MAY 1441 avoid deleting text from that participant. 1443 9. Acknowledgements 1445 James Hamlin for format and performance aspects. 1447 10. IANA Considerations 1449 10.1. Registration of the "rtt-mixer" sdp media attribute 1451 [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the 1452 RFC number of this document.] 1454 IANA is asked to register the new sdp attribute "rtt-mixer". 1456 Contact name: IESG 1458 Contact email: iesg@ietf.org 1460 Attribute name: rtt-mixer 1462 Attribute semantics: See RFCXXXX Section 3.1 1464 Attribute value: none 1466 Usage level: media 1468 Purpose: Indicate support by mixer and endpoint of multi-party 1469 mixing for real-time text transmission, using a common RTP-stream 1470 for transmission of text from a number of sources mixed with one 1471 source at a time and the source indicated in a single CSRC-list 1472 member. 1474 Charset Dependent: no 1476 O/A procedure: See RFCXXXX Section 3.1 1478 Mux Category: normal 1480 Reference: RFCXXXX 1482 11. Security Considerations 1484 The RTP-mixer model requires the mixer to be allowed to decrypt, pack 1485 and encrypt secured text from the conference participants. Therefore 1486 the mixer needs to be trusted. This is similar to the situation for 1487 central mixers of audio and video. 1489 The requirement to transfer information about the user in RTCP 1490 reports in SDES, CNAME and NAME fields, and in conference 1491 notifications, for creation of labels may have privacy concerns as 1492 already stated in RFC 3550 [RFC3550], and may be restricted of 1493 privacy reasons. The receiving user will then get a more symbolic 1494 label for the source. 1496 Participants with malicious intentions may appear and e.g. disturb 1497 the multi-party session by a continuous flow of text, or masquerading 1498 as text from other participants. Counteractions should be to require 1499 secure signaling, media and authentication, and to provide higher 1500 level conference functions e.g. for blocking and expelling 1501 participants. 1503 12. Change history 1505 12.1. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-10 1507 The packet composition was modified for interleaving packets from 1508 different sources. 1510 The packet reception was modified for the new interleaving method. 1512 The packet sequence examples was adjusted for the new interleaving 1513 method. 1515 Modifications according to responses to Brian Rosen of 2020-11-03 1517 12.2. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-09 1519 Changed name on the SDP media attribute to "rtt-mixer" 1521 Restructure of section 2 for balance between aware and unaware cases. 1523 Moved conference control to own section. 1525 Improved clarification of recovery and loss in the packet sequence 1526 example. 1528 A number of editorial corrections and improvements. 1530 12.3. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-08 1532 Deleted the method requiring a new packet format "text/rex" because 1533 of the longer standardization and implementation period it needs. 1535 Focus on use of RFC 4103 text/red format with shorter transmission 1536 interval, and source indicated in CSRC. 1538 12.4. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-07 1540 Added a method based on the "text/red" format and single source per 1541 packet, negotiated by the "rtt-mixer" sdp attribute. 1543 Added reasoning and recommendation about indication of loss. 1545 The highest number of sources in one packet is 15, not 16. Changed. 1547 Added in information on update to RFC 4103 that RFC 4103 explicitly 1548 allows addition of FEC method. The redundancy is a kind of forward 1549 error correction.. 1551 12.5. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-06 1553 Improved definitions list format. 1555 The format of the media subtype parameters is made to match the 1556 requirements. 1558 The mapping of media subtype parameters to sdp is included. 1560 The CPS parameter belongs to the t140 subtype and does not need to be 1561 registered here. 1563 12.6. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-05 1565 nomenclature and editorial improvements 1567 "this document" used consistently to refer to this document. 1569 12.7. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-04 1571 'Redundancy header' renamed to 'data header'. 1573 More clarifications added. 1575 Language and figure number corrections. 1577 12.8. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-03 1579 Mention possible need to mute and raise hands as for other media. 1580 ---done ---- 1582 Make sure that use in two-party calls is also possible and explained. 1583 - may need more wording - 1585 Clarify the RTT is often used together with other media. --done-- 1587 Tell that text mixing is N-1. A users own text is not received in 1588 the mix. -done- 1590 In 3. correct the interval to: A "text/rex" transmitter SHOULD send 1591 packets distributed in time as long as there is something (new or 1592 redundant T140blocks) to transmit. The maximum transmission interval 1593 SHOULD then be 300 ms. It is RECOMMENDED to send a packet to a 1594 receiver as soon as new text to that receiver is available, as long 1595 as the time after the latest sent packet to the same receiver is more 1596 than 150 ms, and also the maximum character rate to the receiver is 1597 not exceeded. The intention is to keep the latency low while keeping 1598 a good protection against text loss in bursty packet loss conditions. 1599 -done- 1601 In 1.3 say that the format is used both ways. -done- 1603 In 13.1 change presentation area to presentation field so that reader 1604 does not think it shall be totally separated. -done- 1606 In Performance and intro, tell the performance in number of 1607 simultaneous sending users and introduced delay 16, 150 vs 1608 requirements 5 vs 500. -done -- 1610 Clarify redundancy level per connection. -done- 1612 Timestamp also for the last data header. To make it possible for all 1613 text to have time offset as for transmission from the source. Make 1614 that header equal to the others. -done- 1616 Mixer always use the CSRC list, even for its own BOM. -done- 1618 Combine all talk about transmission interval (300 ms vs when text has 1619 arrived) in section 3 in one paragraph or close to each other. -done- 1621 Documents the goal of good performance with low delay for 5 1622 simultaneous typers in the introduction. -done- 1623 Describe better that only primary text shall be sent on to receivers. 1624 Redundancy and loss must be resolved by the mixer. -done- 1626 12.9. Changes included in draft-ietf-avtcore-multi-party-rtt-mix-02 1628 SDP and better description and visibility of security by OSRTP RFC 1629 8634 needed. 1631 The description of gatewaying to WebRTC extended. 1633 The description of the data header in the packet is improved. 1635 12.10. Changes to draft-ietf-avtcore-multi-party-rtt-mix-01 1637 2,5,6 More efficient format "text/rex" introduced and attribute 1638 a=rtt-mix deleted. 1640 3. Brief about use of OSRTP for security included- More needed. 1642 4. Brief motivation for the solution and why not rtp-translator is 1643 used added to intro. 1645 7. More limitations for the multi-party unaware mixing method 1646 inserted. 1648 8. Updates to RFC 4102 and 4103 more clearly expressed. 1650 9. Gateway to WebRTC started. More needed. 1652 12.11. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 1653 to draft-ietf-avtcore-multi-party-rtt-mix-00 1655 Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00 1657 Replaced CDATA in IANA registration table with better coding. 1659 Converted to xml2rfc version 3. 1661 12.12. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 1662 to -03 1664 Changed company and e-mail of the author. 1666 Changed title to "RTP-mixer formatting of multi-party Real-time text" 1667 to better match contents. 1669 Check and modification where needed of use of RFC 2119 words SHALL 1670 etc. 1672 More about the CC value in sections on transmitters and receivers so 1673 that 1-to-1 sessions do not use the mixer format. 1675 Enhanced section on presentation for multi-party-unaware endpoints 1677 A paragraph recommending CPS=150 inserted in the performance section. 1679 12.13. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 1680 to -02 1682 In Abstract and 1. Introduction: Introduced wording about regulatory 1683 requirements. 1685 In section 5: The transmission interval is decreased to 100 ms when 1686 there is text from more than one source to transmit. 1688 In section 11 about SDP negotiation, a SHOULD-requirement is 1689 introduced that the mixer should make a mix for multi-party unaware 1690 endpoints if the negotiation is not successful. And a reference to a 1691 later chapter about it. 1693 The presentation considerations chapter 14 is extended with more 1694 information about presentation on multi-party aware endpoints, and a 1695 new section on the multi-party unaware mixing with low functionality 1696 but SHOULD a be implemented in mixers. Presentation examples are 1697 added. 1699 A short chapter 15 on gateway considerations is introduced. 1701 Clarification about the text/t140 format included in chapter 10. 1703 This sentence added to the chapter 10 about use without redundancy. 1704 "The text/red format SHOULD be used unless some other protection 1705 against packet loss is utilized, for example a reliable network or 1706 transport." 1708 Note about deviation from RFC 2198 added in chapter 4. 1710 In chapter 9. "Use with SIP centralized conferencing framework" the 1711 following note is inserted: Note: The CSRC-list in an RTP packet only 1712 includes participants who's text is included in one or more text 1713 blocks. It is not the same as the list of participants in a 1714 conference. With audio and video media, the CSRC-list would often 1715 contain all participants who are not muted whereas text participants 1716 that don't type are completely silent and so don't show up in RTP 1717 packet CSRC-lists. 1719 12.14. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 1720 to -01 1722 Editorial cleanup. 1724 Changed capability indication from fmtp-parameter to SDP attribute 1725 "rtt-mix". 1727 Swapped order of redundancy elements in the example to match reality. 1729 Increased the SDP negotiation section 1731 13. References 1733 13.1. Normative References 1735 [I-D.ietf-mmusic-t140-usage-data-channel] 1736 Holmberg, C. and G. Hellstrom, "T.140 Real-time Text 1737 Conversation over WebRTC Data Channels", Work in Progress, 1738 Internet-Draft, draft-ietf-mmusic-t140-usage-data-channel- 1739 14, 10 April 2020, . 1742 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1743 Requirement Levels", BCP 14, RFC 2119, 1744 DOI 10.17487/RFC2119, March 1997, 1745 . 1747 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1748 Jacobson, "RTP: A Transport Protocol for Real-Time 1749 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1750 July 2003, . 1752 [RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type", 1753 RFC 4102, DOI 10.17487/RFC4102, June 2005, 1754 . 1756 [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text 1757 Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005, 1758 . 1760 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1761 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 1762 July 2006, . 1764 [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer 1765 Security (DTLS) Extension to Establish Keys for the Secure 1766 Real-time Transport Protocol (SRTP)", RFC 5764, 1767 DOI 10.17487/RFC5764, May 2010, 1768 . 1770 [RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for 1771 Keeping Alive the NAT Mappings Associated with RTP / RTP 1772 Control Protocol (RTCP) Flows", RFC 6263, 1773 DOI 10.17487/RFC6263, June 2011, 1774 . 1776 [RFC8643] Johnston, A., Aboba, B., Hutton, A., Jesske, R., and T. 1777 Stach, "An Opportunistic Approach for Secure Real-time 1778 Transport Protocol (OSRTP)", RFC 8643, 1779 DOI 10.17487/RFC8643, August 2019, 1780 . 1782 [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for 1783 multimedia application text conversation", February 1998, 1784 . 1786 [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), 1787 Protocol for multimedia application text conversation", 1788 February 2000, 1789 . 1791 13.2. Informative References 1793 [RFC4353] Rosenberg, J., "A Framework for Conferencing with the 1794 Session Initiation Protocol (SIP)", RFC 4353, 1795 DOI 10.17487/RFC4353, February 2006, 1796 . 1798 [RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A 1799 Session Initiation Protocol (SIP) Event Package for 1800 Conference State", RFC 4575, DOI 10.17487/RFC4575, August 1801 2006, . 1803 [RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol 1804 (SIP) Call Control - Conferencing for User Agents", 1805 BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006, 1806 . 1808 [RFC5194] van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real- 1809 Time Text over IP Using the Session Initiation Protocol 1810 (SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008, 1811 . 1813 Author's Address 1815 Gunnar Hellstrom 1816 Gunnar Hellstrom Accessible Communication 1817 SE-13670 Vendelso 1818 Sweden 1820 Email: gunnar.hellstrom@ghaccess.se