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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. 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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (7 May 2020) is 1444 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 910, but not defined -- Possible downref: Non-RFC (?) normative reference: ref. 'T140' -- Possible downref: Non-RFC (?) normative reference: ref. 'T140ad1' Summary: 0 errors (**), 0 flaws (~~), 9 warnings (==), 6 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 4102, RFC 4103 (if approved) 7 May 2020 5 Intended status: Standards Track 6 Expires: 8 November 2020 8 RTP-mixer formatting of multi-party Real-time text 9 draft-ietf-avtcore-multi-party-rtt-mix-00 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. 18 Regional regulatory requirements specify provision of real-time text 19 in multi-party calls. RFC 4103 mixer implementations can use 20 traditional RTP functions for source identification, but the mixer 21 source switching performance is limited when using the default 22 transmission with redundancy. 24 An enhancement for RFC 4103 real-time text mixing is provided in the 25 present specification, suitable for a centralized conference model 26 that enables source identification and efficient source switching. 27 The intended use is for real-time text mixers and multi-party-aware 28 participant endpoints. The mechanism builds on use of the CSRC list 29 in the RTP packet. 31 A capability exchange is specified so that it can be verified that a 32 participant can handle the multi-party coded real-time text stream. 33 The capability is indicated by an sdp media attribute "rtt-mix". 35 A brief description about how a mixer can format text for the case 36 when the endpoint is not multi-party aware is also provided. 38 Status of This Memo 40 This Internet-Draft is submitted in full conformance with the 41 provisions of BCP 78 and BCP 79. 43 Internet-Drafts are working documents of the Internet Engineering 44 Task Force (IETF). Note that other groups may also distribute 45 working documents as Internet-Drafts. The list of current Internet- 46 Drafts is at https://datatracker.ietf.org/drafts/current/. 48 Internet-Drafts are draft documents valid for a maximum of six months 49 and may be updated, replaced, or obsoleted by other documents at any 50 time. It is inappropriate to use Internet-Drafts as reference 51 material or to cite them other than as "work in progress." 53 This Internet-Draft will expire on 8 November 2020. 55 Copyright Notice 57 Copyright (c) 2020 IETF Trust and the persons identified as the 58 document authors. All rights reserved. 60 This document is subject to BCP 78 and the IETF Trust's Legal 61 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 62 license-info) in effect on the date of publication of this document. 63 Please review these documents carefully, as they describe your rights 64 and restrictions with respect to this document. Code Components 65 extracted from this document must include Simplified BSD License text 66 as described in Section 4.e of the Trust Legal Provisions and are 67 provided without warranty as described in the Simplified BSD License. 69 Table of Contents 71 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 72 2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . 4 73 3. Intended application . . . . . . . . . . . . . . . . . . . . 4 74 4. Use of fields in the RTP packets . . . . . . . . . . . . . . 4 75 5. Actions at transmission by a mixer . . . . . . . . . . . . . 5 76 6. Actions at reception . . . . . . . . . . . . . . . . . . . . 7 77 7. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 8 78 8. Chained operation . . . . . . . . . . . . . . . . . . . . . . 8 79 9. Usage without redundancy . . . . . . . . . . . . . . . . . . 8 80 10. Use with SIP centralized conferencing framework . . . . . . . 8 81 11. SDP Capability negotiation . . . . . . . . . . . . . . . . . 9 82 12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10 83 13. Performance considerations . . . . . . . . . . . . . . . . . 12 84 14. Presentation level considerations . . . . . . . . . . . . . . 12 85 14.1. Presentation by multi-party aware endpoints . . . . . . 13 86 14.2. Multi-party mixing for multi-party unaware endpoints . . 15 87 15. Gateway Considerations . . . . . . . . . . . . . . . . . . . 20 88 16. Congestion considerations . . . . . . . . . . . . . . . . . . 21 89 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21 90 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 91 19. Security Considerations . . . . . . . . . . . . . . . . . . . 21 92 20. Change history . . . . . . . . . . . . . . . . . . . . . . . 22 93 20.1. Changes from 94 draft-hellstrom-avtcore-multi-party-rtt-source-03 to 95 draft-ietf-avtcore-multi-party-rtt-mix-00 . . . . . . . 22 97 20.2. Changes from 98 draft-hellstrom-avtcore-multi-party-rtt-source-02 to 99 -03 . . . . . . . . . . . . . . . . . . . . . . . . . . 22 100 20.3. Changes from 101 draft-hellstrom-avtcore-multi-party-rtt-source-01 to 102 -02 . . . . . . . . . . . . . . . . . . . . . . . . . . 22 103 20.4. Changes from 104 draft-hellstrom-avtcore-multi-party-rtt-source-00 to 105 -01 . . . . . . . . . . . . . . . . . . . . . . . . . . 23 106 21. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 107 21.1. Normative References . . . . . . . . . . . . . . . . . . 23 108 21.2. Informative References . . . . . . . . . . . . . . . . . 24 109 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24 111 1. Introduction 113 RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for 114 transmission of real-time text (RTT) and the "text/t140" format. It 115 also specifies a redundancy format "text/red" for increased 116 robustness. RFC 4102 [RFC4102] registers the "text/red" format. 117 Regional regulatory requirements specify provision of real-time text 118 in multi-party calls. 120 The redundancy scheme enables efficient transmission of redundant 121 text in packets together with new text. However the redundant header 122 format has no source indicators for the redundant transmissions. An 123 assumption has had to be made that the redundant parts in a packet 124 are from the same source as the new text. The recommended 125 transmission is one new and two redundant generations of text 126 (T140blocks) in each packet and the recommended transmission interval 127 is 300 ms. 129 A mixer, selecting between text input from different sources and 130 transmitting it in a common stream needs to make sure that the 131 receiver can assign the received text to the proper sources for 132 presentation. Therefore, without any extra rule for source 133 identification, the mixer needs to stop sending new text from that 134 source and then make sure that all text so far has been sent with all 135 intended redundancy levels (usually two) before switching source. 136 That causes the very long time of one second to switch between 137 transmission of text from one source to text from another source. 138 Both the total throughput and the switching performance in the mixer 139 is too low for most applications. 141 A more efficient source identification scheme requires that each 142 redundant T140block has its source individually preserved. The 143 present specification introduces a source indicator by specific rules 144 for populating the CSRC-list in the RTP-packet. 146 A negotiation mechanism is also introduced for verification that the 147 receiver is able to handle the multi-party coded stream. 149 A fall-back mixing procedure is specified for cases when the 150 negotiation fails. 152 2. Nomenclature 154 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 155 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 156 document are to be interpreted as described in [RFC2119]. 158 The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC are explained 159 in [RFC3550] 161 The term "T140block" is defined in RFC 4103 [RFC4103] to contain one 162 or more T.140 code elements. 164 3. Intended application 166 The scheme for identification of source of redundant transmissions is 167 intended for transmission from entities taking the mixer role in 168 centralised mixing configurations for RTT. It is intended for 169 reception by both endpoints and mixers. 171 4. Use of fields in the RTP packets 173 RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a 174 redundancy format "text/red" for increased robustness. This 175 specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by 176 introducing a rule for populating and using the CSRC-list in the RTP 177 packet in order to enhance the performance in multi-party RTT 178 sessions. 180 When transmitted from a mixer, the first member in the CSRC-list 181 SHALL contain the SSRC of the source of the primary T140block in the 182 packet. The second and further members in the CSRC-list SHALL 183 contain the SSRC of the source of the first, second, etc redundant 184 generations of T140blocks included in the packet. ( the recommended 185 level of redundancy is to use one primary and two redundant 186 generations of T140blocks.) In some cases, a primary or redundant 187 T140block is empty, but is still represented by a member in the 188 redundancy header. For such cases, the corresponding CSRC-list 189 member MUST also be included. 191 The CC field SHALL show the number of members in the CSRC list. 193 Note: This specification departs from section 4 of RFC 2198 [RFC2198] 194 which associates the whole of the CSRC-list with the primary data and 195 assumes that the same list applies to reconstructed redundant data. 196 In the present specification a T140block is associated with exactly 197 one CSRC list member as described above. Also RFC 2198 [RFC2198] 198 anticipates infrequent change to CSRCs; implementers should be aware 199 that the order of the CSRC-list according to this specification will 200 vary during transitions between transmission from the mixer of text 201 originated by different participants. 203 The picture below shows a typical RTP packet with multi-party RTT 204 contents and coding according to the present specification. 206 0 1 2 3 207 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 208 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 209 |V=2|P|X| CC=3 |M| "RED" PT | sequence number of primary | 210 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 211 | timestamp of primary encoding "P" | 212 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 213 | synchronization source (SSRC) identifier | 214 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 215 | CSRC list member 1 = SSRC of source of "P" | 216 | CSRC list member 2 = SSRC of source of "R1" | 217 | CSRC list member 3 = SSRC of source of "R2" | 218 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 219 |1| T140 PT | timestamp offset of "R2" | "R2" block length | 220 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 221 |1| T140 PT | timestamp offset of "R1" | "R1" block length | 222 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 223 |0| T140 PT | "R2" T.140 encoded redundant data | 224 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+ 225 | | "R1" T.140 encoded redundant data | | 226 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+ 227 | "P" T.140 encoded primary data | 228 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 229 Figure 1: text/red packet with sources indicated in the CSRC-list. 231 5. Actions at transmission by a mixer 233 A "text/red" transmitter is usually sending packets at a regular 234 transmission interval as long as there is something (new or redundant 235 T140blocks) to transmit. The default transmission interval for 236 point-to-point operation is 300 ms. 238 For multi-party operation, the transmission interval from mixers 239 SHOULD be decreased to 100 ms for periods when there is text 240 available for transmission from more than two sources. It is also 241 allowed for the mixer to send a packet as soon as text has been 242 received from a source as long as the maximum number of characters 243 per second indicated by the recipient is not exceeded, and also the 244 number of packets sent per second to a recipient is kept under a 245 specified number. This number SHALL be 10 if no other limit is 246 applied for the application. The mixer has its own SSRC, and its own 247 RTP sequence number series. At time of transmission, the mixer SHALL 248 populate the RTP packet with a T140block combined from all T140blocks 249 queued for transmission originating from next source in turn for 250 getting its text transmitted as long as this is not in conflict with 251 the allowed number of characters per second. This T140block shall be 252 placed in the primary area of the packet. The SSRC of its source 253 shall be placed as the first member in the CSRC-list. The current 254 time SHALL be inserted in the timestamp. 256 If no unsent T140blocks were available at this time, but T140blocks 257 are available which have not yet been sent the full intended number 258 of redundant transmissions, then the primary T140block is composed of 259 an empty T140block, and populated (without taking up any length) in a 260 packet for transmission. The SSRC of the mixer is included in the 261 first place of the CSRC-list. 263 The primary T140block, in the latest transmission is used to populate 264 the first redundant T140block, and its source is placed as the second 265 member of the CSRC-list. The first redundant T140block from the 266 latest transmission is placed as the second redundant T140block, and 267 the corresponding CSRC placed in its place in the CSRC-list. 269 Usually this is the level of redundancy used. If a higher number of 270 redundancy is used, then the procedure is maintained until all 271 available redundant levels of T140blocks and their sources are placed 272 in the packet. If a receiver has negotiated a lower number of text/ 273 red generations, then that level shall be the maximum used by the 274 transmitter. 276 The timer offset values are inserted in the redundancy header, with 277 the time offset from when the corresponding T140block was sent as 278 original. 280 The number of members in the CSRC list shall be placed in the "CC" 281 header field. Only mixers place values >0 in the "CC" field. 283 When there is no new T140block to transmit, and no redundant 284 T140block that has not been retransmitted the intended number of 285 times, the transmission process can stop until either new T140blocks 286 arrive, or a keep-alive method calls for transmission of keep-alive 287 packets. 289 6. Actions at reception 291 The enhanced "text/red" receiver included in an endpoint with 292 presentation functions will receive RTP packets in the single stream 293 from the mixer, and shall distribute the T140blocks for presentation 294 in presentation areas for each source. Other tasks for receivers, 295 such as gateways or chained mixers are also feasible, and requires 296 consideration if the stream shall just be forwarded, or a 297 distribution based on different sources is needed. 299 If the "CC" field value of a received packet is >1, it indicates the 300 used level of redundancy for the current packet and that the enhanced 301 packet format is used. If the CC field value is 0 or 1, it indicates 302 that the enhanced format is not used. If the CC value is 1, the CSRC 303 indicates the source of primary data. 305 The RTP sequence numbers of the received packets SHALL be monitored 306 for gaps and packets out of order. 308 As long as the sequence is correct, each packet SHALL be unpacked in 309 order. The T140blocks SHALL be extracted from the primary area, and 310 the corresponding SSRC SHALL be extracted from the first position in 311 the CSRC list and used for assigning the new T140block to the correct 312 presentation area (or correspondingly). 314 If a sequence number gap appears and is still there after some 315 defined time for jitter resolution, T140data SHALL be recovered from 316 redundant data. If the gap is wider than the number of generations 317 of redundant T140blocks in the packet, then a t140block SHALL be 318 created with a marker for text loss [T140ad1] and assigned to the 319 SSRC of the transmitter as a general input from the mixer. 321 Then, the T140blocks in the received packet SHALL be retrieved 322 beginning with the highest redundant generation, grouping them with 323 the corresponding SSRC from the CSRC-list and assigning them to the 324 presentation areas per source. Finally the primary T140block SHALL 325 be retrieved from the packet and similarly its source retrieved from 326 the first position in the CSRC-list, and then assigned to the 327 corresponding presentation area for that source. 329 If the sequence number gap was equal to or less than the number of 330 redundancy generations in the received packet, a missing text marker 331 SHALL NOT be inserted, and instead the T140blocks and their SSRCs 332 fully recovered from the redundancy information and the CSRC-list in 333 the way indicated above. 335 Unicode character BOM is sometimes used as a filler or keep alive by 336 transmission implementations. These SHALL be deleted on reception. 338 Note that empty T140blocks are sometimes included in the packets. 339 They just do not provide any contents. 341 7. RTCP considerations 343 A mixer SHALL send RTCP reports with SDES, CNAME and NAME information 344 about the sources in the multi-party call. This makes it possible 345 for participants to compose a suitable label for text from each 346 source. 348 8. Chained operation 350 By strictly applying the rules for CSRC-list population by all 351 conforming devices, mixers MAY be arranged in chains. 353 9. Usage without redundancy 355 The CSRC list member SHALL be used as source indicator also for cases 356 when the "text/t140" format is used from a mixer. That MAY be the 357 case when robustness in transmission is provided by some other means 358 than by redundancy and the "text/red" format. All aspects of this 359 memo SHALL be applied except the redundant generations in 360 transmission. 362 For the use case without redundancy using the "text/t140" format, the 363 "CC" field in the RTP packet shall have the value 1, and the CSRC 364 list SHALL contain one member when sent from a mixer. 366 The "text/red" format SHOULD be used unless some other protection 367 against packet loss is utilized, for example a reliable network or 368 transport. 370 10. Use with SIP centralized conferencing framework 372 The SIP conferencing framework, mainly specified in RFC 373 4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable 374 for coordinating sessions including multi-party RTT. The RTT stream 375 between the mixer and a participant is one and the same during the 376 conference. Participants get announced by notifications when 377 participants are joining or leaving, and further user information may 378 be provided. The SSRC of the text to expect from joined users can be 379 included in a notification. This can be used both for security 380 purposes and for translation to a label for presentation to other 381 users. 383 Note: The CSRC-list in an RTP packet only includes participants who's 384 text is included in one or more text blocks. It is not the same as 385 the list of participants in a conference. With audio and video 386 media, the CSRC-list would often contain all participants who are not 387 muted whereas text participants that don't type are completely silent 388 and thus are not represented in RTP packet CSRC-lists. 390 11. SDP Capability negotiation 392 There are RTT implementations which implement RFC 4103 [RFC4103] but 393 not the present specification for real-time awareness. Sending mixed 394 text according to the present specification to a device implementing 395 only RFC 4103 [RFC4103] would lead to unreadable presented text. 396 Therefore, in order to negotiate RTT mixing capability according to 397 the present specification, all devices supporting the present 398 specification for multi-party aware participants SHALL include an sdp 399 media attribute "rtt-mix" in the sdp, indicating this capability in 400 offers and answers. Multi-party streams using the coding of the 401 present specification intended for multi-party aware endpoints MUST 402 NOT be sent to devices which have not indicated the "rtt-mix" 403 capability. 405 Implementations not understanding this parameter MUST ignore it 406 according to common SDP rules. 408 The sdp media attribute defined here, is named "rtt-mix". It has no 409 parameters. It is intended to be used in "text" media descriptions 410 with "text/red" and "text/t140" formats. It indicates capability to 411 use source indications in the CSRC list according to the present 412 specification. It also indicates ability to receive 10 real-time 413 text packets per second. 415 Syntax: 417 a=rtt-mix 419 The attribute is used in offer/answer procedures in a declarative 420 way. Both parties express their capability to use sources in the 421 CSRC list as specified in the present specification. 423 A party who has expressed the "rtt-mix" capability MUST populate the 424 CSRC-list according to the present specification if it acts as an 425 rtp-mixer and sends to a party who has expressed the "rtt-mix" 426 capability. 428 A party who has expressed the "rtt-mix" capability MUST interpret the 429 contents of the CSRC-list according to the present specification in 430 received rtp packets from parties who have expressed "rtt-mix" 431 capability . 433 A party MUST NOT transmit packets with the format for multi-party 434 aware participants according to the present specification to a party 435 who has not expressed "rtt-mix" capability. 437 A party performing as a mixer, which has expressed the "rtt-mix" 438 capability, but not received "rtt-mix" capability indication in a 439 session with a participant SHOULD, if nothing else is specified for 440 the application, format transmitted text to that participant to be 441 suitable to present on a multi-party unaware endpoint as further 442 specified in section Section 14.2. 444 12. Examples 446 This example shows a symbolic flow of packets from a mixer with loss 447 and recovery. A, B and C are sources of RTT. M is the mixer. P 448 indicates primary data. R1 is first redundant generation data and R2 449 is second redundant generation data. A1, B1, A2 etc are text chunks 450 (T140blocks) received from the respective sources. X indicates 451 dropped packet between the mixer and a receiver. 453 |----------------| 454 |Seq no 1 | 455 |CSRC list A,M,B | 456 |R2 B99 | 457 |R1: Empty | 458 |P: A1 | 459 |----------------| 461 Assuming that earlier packets were received in sequence, text A1 is 462 received from packet 1 and assigned to reception area A. 464 |----------------| 465 |Seq no 2 | 466 |CC=3 | 467 |CSRC list C,A,M | 468 |R2 Empty | 469 |R1: A1 | 470 |P: C1 | 471 |----------------| 472 Text C1 is received from packet 2 and assigned to reception area C. 474 X----------------| 475 X Seq no 3 | 476 X CC=3 | 477 X CSRC list A,C,A| 478 X R2: A1 | 479 X R1: C1 | 480 X P: A2 | 481 X----------------| 482 Packet 3 is assumed to be dropped in network problems 484 X----------------| 485 X Seq no 4 | 486 X CC=3 | 487 X CSRC list B,A,C| 488 X R2: C1 | 489 X R1: A2 | 490 X P: B1 | 491 X----------------| 492 Packet 4 is assumed to be dropped in network problems 494 X----------------| 495 X Seq no 5 | 496 X CC=3 | 497 X CSRC list A,B,A| 498 X R2: A2 | 499 X R1: B1 | 500 X P: A3 | 501 X----------------| 502 Packet 5 is assumed to be dropped in network problems 504 |----------------| 505 |Seq no 6 | 506 |CC=3 | 507 |CSRC list C,A,B | 508 |R2: B1 | 509 |R1: A3 | 510 |P: C2 | 511 |----------------| 512 Packet 6 is received. The latest received sequence number was 2. 513 Recovery is therefore tried for 3,4,5. But there is no coverage 514 for seq no 3. A missing text mark (U'FFFD) is created and 515 appended to the mixer reception area. 516 For seqno 4, text B1 is recovered and appended to reception area B. 517 For seqno 5, text A3 is recovered and appended to reception area A. 518 Primary text C2 is received and appended to reception area C. 520 With only one or two packets lost, there would not be any need to 521 create a missing text marker, and all text would be recovered. 523 It will be a design decision how to present the missing text markers 524 assigned to the mixer as a source. 526 13. Performance considerations 528 This specification allows new text from one source per packet. 529 Packets SHOULD be transmitted with timed intervals. The default 530 transmission interval is 300 ms for RFC 4103[RFC4103], which is 531 suitable for transmission from single sources. However when more 532 sources contribute to the flow, a shorter transmission interval MAY 533 be applied. The transmission interval is therefore RECOMMENDED to be 534 100 ms for mixers when there is text from more than two sources 535 available for transmission to the same recipient.It is also allowed 536 for the mixer to send a packet as soon as text has been received from 537 a source as long as the maximum number of characters per second 538 indicated by the recipient is not exceeded, and also the number of 539 packets sent per second to a recipient is kept under a number. In 540 order to achieve good performance, a receiver for multi-party calls 541 SHOULD declare a sufficient CPS value in SDP for the number of 542 allowable characters per second. These characteristics provide for 543 smooth flow of text with acceptable latency from at least 5 sources 544 simultaneously. 546 The default maximum rate of reception of real-time text is in RFC 547 4103 [RFC4103] specified to be 30 characters per second. The value 548 MAY be modified in the CPS parameter of the FMTP attribute in the 549 media section for RFC 4103. A mixer combining real-time text from a 550 number of sources may have a higher combined flow of text coming from 551 the sources. Endpoints SHOULD therefore specify a suitable higher 552 value for the CPS parameter, corresponding to its real reception 553 capability. A value for CPS of 150 is RECOMMENDED because it would 554 be suitable for reception in most multi-party sessions, and a 555 realistic value for the capacity in most implementation environments. 556 See RFC 4103 [RFC4103] for the format and use of the CPS parameter. 558 14. Presentation level considerations 560 ITU-T T.140 [T140] provides the presentation level requirements for 561 the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for 562 erasure and other formatting functions and has the following general 563 statement for the presentation: 565 "The display of text from the members of the conversation should be 566 arranged so that the text from each participant is clearly readable, 567 and its source and the relative timing of entered text is visualized 568 in the display. Mechanisms for looking back in the contents from the 569 current session should be provided. The text should be displayed as 570 soon as it is received." 571 Strict application of T.140 [T140] is of essence for the 572 interoperability of real-time text implementations and to fulfill the 573 intention that the session participants have the same information of 574 the text contents of the conversation without necessarily having the 575 exact same layout of the conversation. This also includes the 576 ability to ignore optional presentation control codes not supported 577 by a receiving application. 579 T.140 [T140] specifies a set of presentation control codes to include 580 in the stream. Some of them are optional. Implementations MUST be 581 able to ignore optional control codes that they do not support. 583 There is no strict "message" concept in real-time text. Line 584 Separator SHALL be used as a separator allowing a part of received 585 text to be grouped in presentation. The characters "CRLF" may be 586 used by other implementations as replacement for Line Separator. The 587 "CRLF" combination SHALL be erased by just one erasing action, just 588 as the Line Separator. Presentation functions are allowed to group 589 text for presentation in smaller groups than the line separators 590 imply and present such groups with source indication together with 591 text groups from other sources (see the following presentation 592 examples). Erasure has no specific limit by any delimiter in the 593 text stream. 595 14.1. Presentation by multi-party aware endpoints 597 A multi-party aware receiving party, presenting real-time text MUST 598 separate text from different sources and present them in separate 599 presentation areas. The receiving party MAY separate presentation of 600 parts of text from a source in readable groups based on other 601 criteria than line separator and merge these groups in the 602 presentation area when it benefits the user to most easily find and 603 read text from the different participants. The criteria MAY e.g. be 604 a received comma, full stop, or other phrase delimiters, or a long 605 pause. 607 When text is received from multiple original sources, the 608 presentation SHOULD provide a view where text is added in multiple 609 places simultaneously. 611 If the presentation presents text from different sources in one 612 common area, the presenting endpoint SHOULD insert text from the 613 local user ended at suitable points merged with received text to 614 indicate the relative timing for when the text groups were completed. 615 In this presentation mode, the receiving endpoint SHALL present the 616 source of the different groups of text. 618 A view of a three-party RTT call in chat style is shown in this 619 example . 621 _________________________________________________ 622 | | | 623 |[Alice] Hi, Alice here. | | 624 | | | 625 |[Bob] Bob as well. | | 626 | | | 627 |[Eve] Hi, this is Eve, calling from Paris. | | 628 | I thought you should be here. | | 629 | | | 630 |[Alice] I am coming on Thursday, my | | 631 | performance is not until Friday morning.| | 632 | | | 633 |[Bob] And I on Wednesday evening. | | 634 | | | 635 |[Alice] Can we meet on Thursday evening? | | 636 | | | 637 |[Eve] Yes, definitely. How about 7pm. | | 638 | at the entrance of the restaurant | | 639 | Le Lion Blanc? | | 640 |[Eve] we can have dinner and then take a walk | | 641 | | | 642 | But I need to be back to | | 643 | the hotel by 11 because I need | | 644 | |-| 645 | I wou |-| 646 |______________________________________________|v| 647 | of course, I underst | 648 |________________________________________________| 650 Figure 1: Example of a three-party call with chat style. 652 Figure 2: Example of a three-party RTT call presented in chat style. 654 Other presentation styles than the chat style may be arranged. 656 This figure shows how a coordinated column view MAY be presented. 658 _____________________________________________________________________ 659 | Bob | Eve | Alice | 660 |____________________|______________________|_______________________| 661 | | |I will arrive by TGV. | 662 |My flight is to Orly| |Convenient to the main | 663 | |Hi all, can we plan |station. | 664 | |for the seminar? | | 665 |Eve, will you do | | | 666 |your presentation on| | | 667 |Friday? |Yes, Friday at 10. | | 668 |Fine, wo | |We need to meet befo | 669 |___________________________________________________________________| 671 Figure 3: An example of a coordinated column-view of a three-party 672 session with entries ordered vertically in approximate time-order. 674 14.2. Multi-party mixing for multi-party unaware endpoints 676 When the mixer has indicated multi-party capability in an sdp 677 negotiation, but the multi-party capability negotiation fails with an 678 endpoint, then the mixer SHOULD compose a best-effort presentation of 679 multi-party real-time text in one stream intended to be presented by 680 an endpoint with no multi-party awareness. 682 This presentation format has functional limitations and SHOULD be 683 used only to enable participation in multi-party calls by legacy 684 deployed endpoints. 686 The principles and procedures below do not specify any new protocol 687 elements or behaviors. They are instead composed from the 688 information in ITU-T T.140 [T140] and an ambition to provide a best 689 effort presentation on an endpoint which has functions only for two- 690 party calls. 692 The mixer mixing for multi-party unaware endpoints SHALL compose a 693 simulated limited multi-party RTT view suitable for presentation in 694 one presentation area. The mixer SHALL group text in suitable groups 695 and prepare for presentation of them by inserting a new line betwwen 696 them if the transmitted text did not already end with a new line. A 697 presentable label SHOULD be composed and sent for the source 698 initially in the session and after each source switch. With this 699 procedure the time for source switching is depending on the actions 700 of the users. In order to expedite source switch, a user can for 701 example end its turn with a new line. 703 14.2.1. Actions by the mixer at reception from the call participants 705 When text is received by the mixer from the different participants, 706 the mixer SHALL recover text from redundancy if any packets are lost. 707 The mark for lost text [T140ad1] SHOULD be inserted in the stream if 708 unrecoverable loss appears. Any Unicode BOM characters, possibly 709 used for keep-alive shall be deleted. The time of arrival of text 710 SHALL be stored together with the received text from each source in a 711 queue for transmission to the recipients. 713 14.2.2. Actions by the mixer for transmission to the recipients 715 The following procedure SHOULD be applied for each recipient of 716 multi-part text from the mixer. 718 The text for transmission SHOULD be formatted by the mixer for each 719 receiving user for presentation in one single presentation area. 720 Text received from a participant SHOULD NOT be included in 721 transmission to that participant. When there is text available for 722 transmission from the mixer to a receiving party from more than one 723 participant, the mixer SHOULD switch between transmission of text 724 from the different sources at suitable points in the transmitted 725 stream. 727 When switching source, the mixer SHOULD insert a line separator if 728 the already transmitted text did not end with a new line (line 729 separator or CRLF). A label SHOULD be composed from information in 730 the CNAME and NAME fields in RTCP reports from the participant to 731 have its text transmitted, or from other session information for that 732 user. The label SHOULD be delimited by suitable characters (e.g. '[ 733 ]') and transmitted. The CSRC SHOULD indicate the selected source. 734 Then text from that selected participant SHOULD be transmitted until 735 a new suitable point for switching source is reached. 737 Seeking a suitable point for switching source SHOULD be done when 738 there is older text waiting for transmission from any party than the 739 age of the last transmitted text. Suitable points for switching are: 741 * A completed phrase ended by comma 743 * A completed sentence 745 * A new line (line separator or CRLF) 747 * A long pause (e.g. > 10 seconds) in received text from the 748 currently transmitted source 750 * If text from one participant has been transmitted with text from 751 other sources waiting for transmission for a long time (e.g. > 1 752 minute) and none of the suitable points for switching has 753 occurred, a source switch MAY be forced by the mixer at next word 754 delimiter, and also if even a word delimiter does not occur within 755 a time (e.g. 15 seconds) after the scan for word delimiter 756 started. 758 When switching source, the source which has the oldest text in queue 759 SHOULD be selected to be transmitted. A character display count 760 SHOULD be maintained for the currently transmitted source, starting 761 at zero after the label is transmitted for the currently transmitted 762 source. 764 There SHOULD be a storage for the latest control code for Select 765 Graphic Rendition (SGR) from each source. If there is an SGR code 766 stored for the current source before the source switch is done, a 767 reset of SGR shall be sent by the sequence SGR 0 [009B 0000 006D] 768 after the new line and before the new label during a source switch. 769 See SGR below for an explanation. This transmission does not 770 influence the display count. If there is an SGR code stored for the 771 new source after the source switch, that SGR code SHOULD be 772 transmitted to the recipient before the label. This transmission 773 does not influence the display count. 775 14.2.3. Actions on transmission of text 777 Text from a source sent to the recipient SHOULD increase the display 778 count by one per transmitted character. 780 14.2.4. Actions on transmission of control codes 782 The following control codes specified by T.140 require specific 783 actions. They SHOULD cause specific considerations in the mixer. 784 Note that the codes presented here are expressed in UCS-16, while 785 transmission is made in UTF-8 transform of these codes. 787 BEL 0007 Bell Alert in session, provides for alerting during an 788 active session. The display count SHOULD not be altered. 790 NEW LINE 2028 Line separator. Check and perform a source switch if 791 appropriate. Increase display count by 1. 793 CR LF 000D 000A A supported, but not preferred way of requesting a 794 new line. Check and perform a source switch if appropriate. 795 Increase display count by 1. 797 INT ESC 0061 Interrupt (used to initiate mode negotiation 798 procedure). The display count SHOULD not be altered. 800 SGR 009B Ps 006D Select graphic rendition. Ps is rendition 801 parameters specified in ISO 6429. The display count SHOULD not be 802 altered. The SGR code SHOULD be stored for the current source. 804 SOS 0098 Start of string, used as a general protocol element 805 introducer, followed by a maximum 256 bytes string and the ST. 806 The display count SHOULD not be altered. 808 ST 009C String terminator, end of SOS string. The display count 809 SHOULD not be altered. 811 ESC 001B Escape - used in control strings. The display count SHOULD 812 not be altered for the complete escape code. 814 Byte order mark FEFF Zero width, no break space, used for 815 synchronization and keep-alive. SHOULD be deleted from incoming 816 streams. Shall be sent first after session establishment to the 817 recipient. The display count shall not be altered. 819 Missing text mark FFFD Replacement character, marks place in stream 820 of possible text loss. SHOULD be inserted by the reception 821 procedure in case of unrecoverable loss of packets. The display 822 count SHOULD be increased by one when sent as for any other 823 character. 825 SGR If a control code for selecting graphic rendition (SGR), other 826 than reset of the graphic rendition (SGR 0) is sent to a 827 recipient, that control code shall also be stored for the source 828 in the storage for SGR. If a reset graphic rendition (SGR 0) 829 originated from a source is sent, then the SGR storage for that 830 source shall be cleared. The display count shall not be 831 increased. 833 BS 0008 Back Space, intended to erase the last entered character by 834 a source. Erasure by backspace cannot always be performed as the 835 erasing party intended. If an erasing action erases all text up 836 to the end of the leading label after a source switch, then the 837 mixer must not transmit more backspaces. Instead it is 838 RECOMMENDED that a letter "X" is inserted in the text stream for 839 each backspace as an indication of the intent to erase more. A 840 new line is usually coded by a Line Separator, but the character 841 combination "CRLF" MAY be used instead. Erasure of a new line is 842 in both cases done by just one erasing action (Backspace). If the 843 display count has a positive value it is decreased by one when the 844 BS is sent. If the display count is at zero, it is not altered. 846 14.2.5. Packet transmission 848 A mixer transmitting to a multi-party unaware terminal SHOULD send 849 primary data only from one source per packet. The SSRC SHOULD be the 850 SSRC of the mixer. The CSRC list SHOULD contain one member and be 851 the SSRC of the source of the primary data. 853 14.2.6. Functional limitations 855 When a multi-party unaware endpoint presents a conversation in one 856 display area in a chat style, it inserts source indications for 857 remote text and local user text as they are merged in completed text 858 groups. When an endpoint using this layout receives and presents 859 text mixed for multi-party unaware endpoints, there will be two 860 levels of source indicators for the received text; one generated by 861 the mixer and inserted in a label after each source switch, and 862 another generated by the receiving endpoint and inserted after each 863 switch between local and remote source in the presentation area. 864 This will waste display space and look inconsistent to the reader. 866 This fact, combined with the slowness in source switching and the 867 limited support of erasure makes it strongly RECOMMENDED to implement 868 multi-party awareness in RTT endpoints. The use of the mixing method 869 for multi-party-unaware endpoints should be left for use with 870 endpoints which are impossible to upgrade to become multi-party 871 aware. 873 14.2.7. Example views of presentation on multi-party unaware endpoints 875 The following pictures are examples of the view on a participant's 876 display for the multi-party-unaware case. 878 _________________________________________________ 879 | Conference | Alice | 880 |________________________|_________________________| 881 | |I will arrive by TGV. | 882 |[Bob]:My flight is to |Convenient to the main | 883 |Orly. |station. | 884 |[Eve]:Hi all, can we | | 885 |plan for the seminar. | | 886 | | | 887 |[Bob]:Eve, will you do | | 888 |your presentation on | | 889 |Friday? | | 890 |[Eve]:Yes, Friday at 10.| | 891 |[Bob]: Fine, wo |We need to meet befo | 892 |________________________|_________________________| 894 Figure 4: Alice who has a conference-unaware client is receiving the 895 multi-party real-time text in a single-stream. This figure shows how 896 a coordinated column view MAY be presented on Alice's device. 898 _________________________________________________ 899 | |^| 900 |[Alice] Hi, Alice here. | | 901 | | | 902 |[mix][Bob] Bob as well. | | 903 | | | 904 |[Eve] Hi, this is Eve, calling from Paris | | 905 | I thought you should be here. | | 906 | | | 907 |[Alice] I am coming on Thursday, my | | 908 | performance is not until Friday morning.| | 909 | | | 910 |[mix][Bob] And I on Wednesday evening. | | 911 | | | 912 |[Eve] we can have dinner and then walk | | 913 | | | 914 |[Eve] But I need to be back to | | 915 | the hotel by 11 because I need |-| 916 | |-| 917 |______________________________________________|v| 918 | of course, I underst | 919 |________________________________________________| 921 Figure 5: An example of a view of the multi-party unaware 922 presentation in chat style. Alice is the local user. 924 15. Gateway Considerations 926 Multi-party RTT sessions may involve gateways of different kinds. 927 Gateways involved in setting up sessions SHALL correctly reflect the 928 multi-party capability or unawareness of the combination of the 929 gateway and the remote endpoint beyond the gateway. 931 One case that may occur is a gateway to PSTN for communication with 932 textphones (e.g. TTYs). Textphones are limited devices with no 933 multi-party awareness, and it SHOULD therefore be suitable for the 934 gateway to not indicate multi-party awareness for that case. Another 935 solution is that the gateway indicates multi-party capability towards 936 the mixer, and includes the multi-party mixer function for multi- 937 party unaware endpoints itself. This solution makes it possible to 938 make adaptations for the functional limitations of the textphone 939 (TTY). 941 16. Congestion considerations 943 The congestion considerations and recommended actions from RFC 4103 944 [RFC4103] are valid also in multi-party situations. 946 17. Acknowledgements 948 18. IANA Considerations 950 [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the 951 RFC number of this document.] 953 IANA is asked to register the new sdp attribute "rtt-mix". 955 +---------------------+----------------------------------------+ 956 | Contact name: | IESG | 957 +---------------------+----------------------------------------+ 958 | Contact email: | iesg@ietf.org | 959 +---------------------+----------------------------------------+ 960 | Attribute name: | rtt-mix | 961 +---------------------+----------------------------------------+ 962 | Attribute syntax | a=rtt-mix | 963 +---------------------+----------------------------------------+ 964 | Attribute semantics | See RFCXXXX Section 11 | 965 +---------------------+----------------------------------------+ 966 | Attribute value | - | 967 +---------------------+----------------------------------------+ 968 | Usage level: | media | 969 +---------------------+----------------------------------------+ 970 | Purpose: | Indicate support for the rtp-mixer | 971 | | format for real-time text transmission | 972 +---------------------+----------------------------------------+ 973 | O/A procedure | Declarative | 974 +---------------------+----------------------------------------+ 975 | Mux Category | normal | 976 +---------------------+----------------------------------------+ 977 | Reference: | RFCXXXX | 978 +---------------------+----------------------------------------+ 980 Table 1 982 19. Security Considerations 984 The RTP-mixer model requires the mixer to be allowed to decrypt, pack 985 and encrypt secured text from the conference participants. Therefore 986 the mixer needs to be trusted. This is similar to the situation for 987 central mixers of audio and video. 989 The requirement to transfer information about the user in RTCP 990 reports in SDES, CNAME and NAME fields for creation of labels may 991 have privacy concerns as already stated in RFC 3550 [RFC3550], and 992 may be restricted of privacy reasons. The receiving user will then 993 get a more symbolic label for the source. 995 20. Change history 997 20.1. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-03 to 998 draft-ietf-avtcore-multi-party-rtt-mix-00 1000 Changed file name to draft-ietf-avtcore-multi-party-rtt-mix-00 1002 Replaced CDATA in IANA registration table with better coding. 1004 Converted to xml2rfc version 3. 1006 20.2. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-02 to 1007 -03 1009 Changed company and e-mail of the author. 1011 Changed title to "RTP-mixer formatting of multi-party Real-time text" 1012 to better match contents. 1014 Check and modification where needed of use of RFC 2119 words SHALL 1015 etc. 1017 More about the CC value in sections on transmitters and receivers so 1018 that 1-to-1 sessions do not use the mixer format. 1020 Enhanced section on presentation for multi-party-unaware endpoints 1022 A paragraph recommending CPS=150 inserted in the performance section. 1024 20.3. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-01 to 1025 -02 1027 In Abstract and 1. Introduction: Introduced wording about regulatory 1028 requirements. 1030 In section 5: The transmission interval is decreased to 100 ms when 1031 there is text from more than one source to transmit. 1033 In section 11 about sdp negotiation, a SHOULD-requirement is 1034 introduced that the mixer should make a mix for multi-party unaware 1035 endpoints if the negotiation is not successful. And a reference to a 1036 later chapter about it. 1038 The presentation considerations chapter 14 is extended with more 1039 information about presentation on multi-party aware endpoints, and a 1040 new section on the multi-party unaware mixing with low functionality 1041 but SHOULD a be implemented in mixers. Presentation examples are 1042 added. 1044 A short chapter 15 on gateway considerations is introduced. 1046 Clarification about the text/t140 format included in chapter 10. 1048 This sentence added to the chapter 10 about use without redundancy. 1049 "The text/red format SHOULD be used unless some other protection 1050 against packet loss is utilized, for example a reliable network or 1051 transport." 1053 Note about deviation from RFC 2198 added in chapter 4. 1055 In chapter 9. "Use with SIP centralized conferencing framework" the 1056 following note is inserted: Note: The CSRC-list in an RTP packet only 1057 includes participants who's text is included in one or more text 1058 blocks. It is not the same as the list of participants in a 1059 conference. With audio and video media, the CSRC-list would often 1060 contain all participants who are not muted whereas text participants 1061 that don't type are completely silent and so don't show up in RTP 1062 packet CSRC-lists. 1064 20.4. Changes from draft-hellstrom-avtcore-multi-party-rtt-source-00 to 1065 -01 1067 Editorial cleanup. 1069 Changed capability indication from fmtp-parameter to sdp attribute 1070 "rtt-mix". 1072 Swapped order of redundancy elements in the example to match reality. 1074 Increased the SDP negotiation section 1076 21. References 1078 21.1. Normative References 1080 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1081 Requirement Levels", BCP 14, RFC 2119, 1082 DOI 10.17487/RFC2119, March 1997, 1083 . 1085 [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., 1086 Handley, M., Bolot, J.C., Vega-Garcia, A., and S. Fosse- 1087 Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, 1088 DOI 10.17487/RFC2198, September 1997, 1089 . 1091 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 1092 Jacobson, "RTP: A Transport Protocol for Real-Time 1093 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 1094 July 2003, . 1096 [RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type", 1097 RFC 4102, DOI 10.17487/RFC4102, June 2005, 1098 . 1100 [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text 1101 Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005, 1102 . 1104 [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for 1105 multimedia application text conversation", February 1998, 1106 . 1108 [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), 1109 Protocol for multimedia application text conversation", 1110 February 2000, 1111 . 1113 21.2. Informative References 1115 [RFC4353] Rosenberg, J., "A Framework for Conferencing with the 1116 Session Initiation Protocol (SIP)", RFC 4353, 1117 DOI 10.17487/RFC4353, February 2006, 1118 . 1120 [RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A 1121 Session Initiation Protocol (SIP) Event Package for 1122 Conference State", RFC 4575, DOI 10.17487/RFC4575, August 1123 2006, . 1125 [RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol 1126 (SIP) Call Control - Conferencing for User Agents", 1127 BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006, 1128 . 1130 Author's Address 1131 Gunnar Hellstrom 1132 Gunnar Hellstrom Accessible Communication 1133 Esplanaden 30 1134 SE-13670 Vendelso 1135 Sweden 1137 Email: gunnar.hellstrom@ghaccess.se