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Hellstrom 3 Internet-Draft Omnitor 4 Updates: RFC 4102, RFC 4103 (if March 26, 2020 5 approved) 6 Intended status: Standards Track 7 Expires: September 27, 2020 9 Indicating source of multi-party Real-time text 10 draft-hellstrom-avtcore-multi-party-rtt-source-02 12 Abstract 14 Real-time text mixers for multi-party sessions need to identify the 15 source of each transmitted group of text so that the text can be 16 presented by endpoints in suitable grouping with other text from the 17 same source. 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 with redundancy. 25 An enhancement for RFC 4103 real-time text mixing is provided in the 26 present specification, suitable for a centralized conference model 27 that enables source identification and efficient source switching. 28 The intended use is for real-time text mixers and multi-party-aware 29 participant endpoints. The mechanism builds on use of the CSRC list 30 in the RTP packet. 32 A capability exchange is specified so that it can be verified that a 33 participant can handle the multi-party coded real-time text stream. 34 The capability is indicated by an sdp media attribute "rtt-mix". 36 A brief description about how a mixer can format text for the case 37 when the endpoint is not multi-party aware is also provided. 39 Status of This Memo 41 This Internet-Draft is submitted in full conformance with the 42 provisions of BCP 78 and BCP 79. 44 Internet-Drafts are working documents of the Internet Engineering 45 Task Force (IETF). Note that other groups may also distribute 46 working documents as Internet-Drafts. The list of current Internet- 47 Drafts is at https://datatracker.ietf.org/drafts/current/. 49 Internet-Drafts are draft documents valid for a maximum of six months 50 and may be updated, replaced, or obsoleted by other documents at any 51 time. It is inappropriate to use Internet-Drafts as reference 52 material or to cite them other than as "work in progress." 54 This Internet-Draft will expire on September 27, 2020. 56 Copyright Notice 58 Copyright (c) 2020 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents 63 (https://trustee.ietf.org/license-info) in effect on the date of 64 publication of this document. Please review these documents 65 carefully, as they describe your rights and restrictions with respect 66 to this document. Code Components extracted from this document must 67 include Simplified BSD License text as described in Section 4.e of 68 the Trust Legal Provisions and are provided without warranty as 69 described in the Simplified BSD License. 71 Table of Contents 73 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 74 2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . 4 75 3. Intended application . . . . . . . . . . . . . . . . . . . . 4 76 4. Use of fields in the RTP packets . . . . . . . . . . . . . . 4 77 5. Actions at transmission by a mixer . . . . . . . . . . . . . 5 78 6. Actions at reception . . . . . . . . . . . . . . . . . . . . 6 79 7. RTCP considerations . . . . . . . . . . . . . . . . . . . . . 7 80 8. Chained operation . . . . . . . . . . . . . . . . . . . . . . 7 81 9. Usage without redundancy . . . . . . . . . . . . . . . . . . 8 82 10. Use with SIP centralized conferencing framework . . . . . . . 8 83 11. SDP Capability negotiation . . . . . . . . . . . . . . . . . 8 84 12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9 85 13. Performance considerations . . . . . . . . . . . . . . . . . 11 86 14. Presentation level considerations . . . . . . . . . . . . . . 12 87 14.1. Presentation by multi-party aware endpoints . . . . . . 12 88 14.2. Multi-party mixing for multi-party unaware endpoints . . 14 89 15. Gateway Considerations . . . . . . . . . . . . . . . . . . . 17 90 16. Congestion considerations . . . . . . . . . . . . . . . . . . 17 91 17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 92 18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 93 19. Security Considerations . . . . . . . . . . . . . . . . . . . 17 94 20. Change history . . . . . . . . . . . . . . . . . . . . . . . 18 95 20.1. Changes from version -00 to -01 . . . . . . . . . . . . 18 96 20.2. Changes from version -01 to -02 . . . . . . . . . . . . 18 98 21. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 99 21.1. Normative References . . . . . . . . . . . . . . . . . . 19 100 21.2. Informative References . . . . . . . . . . . . . . . . . 20 101 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 103 1. Introduction 105 RFC 4103[RFC4103] specifies use of RFC 3550 RTP [RFC3550] for 106 transmission of real-time text (RTT) and the "text/t140" format. It 107 also specifies a redundancy format "text/red" for increased 108 robustness. RFC 4102 [RFC4102] registers the "text/red" format. 109 Regional regulatory requirements specify provision of real-time text 110 in multi-party calls. 112 The redundancy scheme enables efficient transmission of redundant 113 text in packets together with new text. However the redundant header 114 format has no source indicators for the redundant transmissions. An 115 assumption has had to be made that the redundant parts in a packet 116 are from the same source as the new text. The recommended 117 transmission is one new and two redundant generations of text 118 (T140blocks) in each packet and the recommended transmission interval 119 is 300 ms. 121 A mixer, selecting between text input from different sources and 122 transmitting it in a common stream needs to make sure that the 123 receiver can assign the received text to the proper sources for 124 presentation. Therefore, without any extra rule for source 125 identification, the mixer needs to stop sending new text from that 126 source and then make sure that all text so far has been sent with all 127 intended redundancy levels (usually two) before switching source. 128 That causes the very long time of one second to switch between 129 transmission of text from one source to text from another source. 130 Both the total throughput and the switching performance in the mixer 131 is too low for most applications. 133 A more efficient source identification scheme requires that each 134 redundant T140block has its source individually preserved. The 135 present specification introduces a source indicator by specific rules 136 for populating the CSRC-list in the RTP-packet. 138 A negotiation mechanism is also introduced for verification that the 139 receiver is able to handle the multi-party coded stream. 141 A fall-back mixing procedure is briefly specified for cases when the 142 negotiation fails. 144 2. Nomenclature 146 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 147 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 148 document are to be interpreted as described in [RFC2119]. 150 The terms SDES, CNAME, NAME, SSRC, CSRC, CSRC list, CC are explained 151 in [RFC3550] 153 The term "T140block" is defined in RFC 4103 [RFC4103] to contain one 154 or more T.140 code elements. 156 3. Intended application 158 The scheme for identification of source of redundant transmissions is 159 intended for transmission from entities taking the mixer role in 160 centralised mixing configurations for RTT. It is intended for 161 reception by both participants and mixers. 163 4. Use of fields in the RTP packets 165 RFC 4103[RFC4103] specifies use of RFC 3550 RTP[RFC3550], and a 166 redundancy format "text/red" for increased robustness. This 167 specification updates RFC 4102[RFC4102] and RFC 4103[RFC4103] by 168 introducing a rule for populating and using the CSRC-list in the RTP 169 packet in order to enhance the performance in multi-party RTT 170 sessions. 172 The first member in the CSRC-list shall contain the SSRC of the 173 source of the primary T140block in the packet. The second and 174 further members in the CSRC-list shall contain the SSRC of the source 175 of the first, second, etc redundant generation T140blocks included in 176 the packet. ( the recommended level of redundancy is to use one 177 primary and two redundant generations of T140blocks.) In some cases, 178 a primary or redundant T140block is empty, but is still represented 179 by a member in the redundancy header. For such cases, the 180 corresponding CSRC-list member MUST also be included. 182 The CC field shall show the number of members in the CSRC list. 184 Note: This specification departs from section 4 of RFC 2198 [RFC2198] 185 which associates the whole of the CSRC-list with the primary data and 186 assumes that the same list applies to reconstructed redundant data. 187 In this specification a text block is associated with exactly one 188 CSRC list member as described above. Also RFC 2198 [RFC2198] 189 anticipates infrequent change to CSRCs; implementers should be aware 190 that the order of the CSRC-list according to this specification will 191 rotate during transitions between transmission from the mixer of text 192 originated by different participants. 194 The picture below shows a typical RTP packet with multi-party RTT 195 contents and coding according to the present specification. 197 0 1 2 3 198 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 1 199 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 200 |V=2|P|X| CC=3 |M| "RED" PT | sequence number of primary | 201 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 202 | timestamp of primary encoding "P" | 203 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 204 | synchronization source (SSRC) identifier | 205 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 206 | CSRC list member 1 = SSRC of source of "P" | 207 | CSRC list member 2 = SSRC of source of "R1" | 208 | CSRC list member 3 = SSRC of source of "R2" | 209 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 210 |1| T140 PT | timestamp offset of "R2" | "R2" block length | 211 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 212 |1| T140 PT | timestamp offset of "R1" | "R1" block length | 213 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 214 |0| T140 PT | "R2" T.140 encoded redundant data | 215 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+ 216 | | "R1" T.140 encoded redundant data | | 217 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+ 218 | "P" T.140 encoded primary data | 219 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 220 Figure 1: text/red packet with sources indicated in the CSRC-list. 222 5. Actions at transmission by a mixer 224 A text/red transmitter is usually sending packets at a regular 225 transmission interval as long as there is something (new or redundant 226 T140blocks) to transmit. The default transmission interval for 227 point-to-point operation is 300 ms. 229 For multi-party operation, the transmission interval should be 230 decreased to 100 ms for periods when there is text available for 231 transmission from more than one source. The mixer has its own SSRC, 232 and its own RTP sequence number series. At time of transmission, the 233 RTP packet SHALL be populated with a T140block combined from all 234 T140blocks queued for transmission originating from next source in 235 turn for getting its text transmitted (as long as this is not in 236 conflict with the allowed number of characters per second).This 237 T140block shall be placed in the primary area of the packet. The 238 SSRC of its source shall be placed as the first member in the CSRC- 239 list. The current time is inserted in the timestamp. 241 If no unsent T140blocks were available at this time, but T140blocks 242 are available which have not yet been sent the full intended number 243 of redundant transmissions, then the primary T140block is composed of 244 an empty T140block, and populated (without taking up any length) in a 245 packet for transmission. The SSRC of the mixer is included in the 246 first place of the CSRC-list. 248 The primary T140block, in the latest transmission is used to populate 249 the first redundant T140block, and its source is placed as the second 250 member of the CSRC-list. The first redundant T140block from the 251 latest transmission is now placed as the second redundant T140block, 252 and the corresponding CSRC placed in its place in the CSRC-list. 254 Usually this is the level of redundancy used. If a higher number of 255 redundancy is used, then the procedure is maintained until all 256 available redundant levels of T140blocks and their sources are placed 257 in the packet. If a receiver has negotiated a lower number of text/ 258 red generations, then that level shall be the maximum used by the 259 transmitter. 261 The timer offset values are inserted in the redundancy header, with 262 the time offset from when the corresponding T140block was sent as 263 original. 265 The number of members in the CSRC list shall be placed in the "CC" 266 header field. 268 When there is no new T140block to transmit, and no redundant 269 T140block that has not been retransmitted the intended number of 270 times, the transmission process can stop until either new T140blocks 271 arrive, or a keep-alive method calls for transmission of keep-alive 272 packets. 274 6. Actions at reception 276 The enhanced "text/red" receiver included in an endpoint with 277 presentation functions will receive RTP packets in the single stream 278 from the mixer, and shall distribute the T140blocks for presentation 279 in presentation areas for each source. Other tasks for receivers, 280 such as gateways or chained mixers are also feasible, and requires 281 consideration if the stream shall just be forwarded, or a 282 distribution based on different sources is needed. 284 The CC field of the received packets indicate the used level of 285 redundancy for the current packet. 287 The RTP sequence numbers of the received packets are monitored for 288 gaps or packets out of order. 290 As long as the sequence is correct, each packet is unpacked in order. 291 The T140blocks are extracted from the primary area, and the 292 corresponding SSRC is extracted from the first position in the CSRC 293 list and used for assigning the new T140block to the correct 294 presentation area (or correspondingly). 296 If a sequence number gap appears and is still there after some 297 defined time for jitter resolution, T140data needs to be recovered 298 from redundant data. If the gap is wider than the number of 299 generations of redundant T140blocks in the packet, then a t140block 300 is created with a marker for text loss [T140ad1] and assigned to the 301 SSRC of the transmitter as a general input from the mixer. 303 Then, the T140blocks in the received packet are retrieved beginning 304 with the highest redundant generation, grouping them with the 305 corresponding SSRC from the CSRC-list and assigning them to the 306 presentation areas per source. Finally the primary T140block is 307 retrieved from the packet and similarly its source retrieved from the 308 first position in the CSRC-list, and then assigned to the 309 corresponding presentation handling for that source. 311 If the sequence number gap was equal to or less than the number of 312 redundancy generations in the received packet, no missing text marker 313 shall be inserted, and instead the T140blocks and their SSRCs fully 314 recovered from the redundancy information and the CSRC-list in the 315 way indicated above. 317 Unicode character BOM is sometimes used as a filler or keep alive by 318 transmission implementations. These should be deleted on reception. 320 Note that empty T140blocks are sometimes included in the packets. 321 They just do not provide any contents. 323 7. RTCP considerations 325 A mixer should send RTCP reports with SDES, CNAME and NAME 326 information about the sources in the conference. This makes it 327 possible for participants to compose a suitable label for text from 328 each source. 330 8. Chained operation 332 By strictly applying the rules for CSRC-list population by all 333 conforming devices, mixers can be arranged in chains. 335 9. Usage without redundancy 337 The CSRC list member should be used as source indicator also for 338 cases when the text/t140 format is used. That may be the case when 339 robustness in transmission is provided by some other means than by 340 redundancy and the text/red format. All aspects of this memo applies 341 except the redundant generations in transmission. 343 For the use case without redundancy using the text/t140 format, the 344 CC field in the RTP packet shall have the value 1, and the CSRC list 345 contain one member. 347 The text/red format SHOULD be used unless some other protection 348 against packet loss is utilized, for example a reliable network or 349 transport. 351 10. Use with SIP centralized conferencing framework 353 The SIP conferencing framework, mainly specified in RFC 354 4353[RFC4353], RFC 4579[RFC4579] and RFC 4575[RFC4575] is suitable 355 for coordinating sessions including multi-party RTT. The RTT stream 356 between the mixer and a participant is one and the same during the 357 conference. Participants get announced by notifications when 358 participants are joining or leaving, and further user information may 359 be provided. The SSRC of the text to expect from joined users can be 360 included in a notification. This can be used both for security 361 purposes and for translation to a label for presentation to other 362 users. 364 Note: The CSRC-list in an RTP packet only includes participants who's 365 text is included in one or more text blocks. It is not the same as 366 the list of participants in a conference. With audio and video 367 media, the CSRC-list would often contain all participants who are not 368 muted whereas text participants that don't type are completely silent 369 and thus are not represented in RTP packet CSRC-lists. 371 11. SDP Capability negotiation 373 There are RTT implementations which implement RFC 4103 [RFC4103] but 374 not the present specification. Sending mixed text according to the 375 present specification to a device implementing only RFC 4103 376 [RFC4103] would lead to unreadable presented text. Therefore, in 377 order to negotiate RTT mixing capability according to the present 378 specification, all devices supporting the present specification shall 379 include an sdp media attribute "rtt-mix" in the sdp, indicating this 380 capability in offers and answers. Multi-party streams using the 381 coding of the present specification intended for multi-party aware 382 endpoints must not be sent to devices which have not indicated the 383 "rtt-mix" capability. 385 Implementations not understanding this parameter MUST ignore it 386 according to common SDP rules. 388 The sdp media attribute defined here, is named "rtt-mix". It has no 389 parameters. It is intended to be used in "text" media descriptions 390 with "text/red" and "text/t140" formats. It indicates capability to 391 use source indications in the CSRC list according to this 392 specification. It also indicates ability to receive RTT packets with 393 an interval of 100 ms. 395 Syntax: 397 a=rtt-mix 399 The attribute is used in offer/answer procedures in a declarative 400 way. Both parties express their capability to use sources in the 401 CSRC list as specified in the present specification. 403 A party who has expressed the "rtt-mix" capability MUST populate the 404 CSRC-list according to the present specification if it acts as an 405 rtp-mixer and sends to a party who has expressed the "rtt-mix" 406 capability. 408 A party who has expressed the "rtt-mix" capability MUST interpret the 409 contents of the CSRC-list according to the present specification in 410 received rtp packets from parties who have expressed "rtt-mix" 411 capability . 413 A party MUST NOT transmit packets with the multi-party format 414 according to the present specification to a party who has not 415 expressed "rtt-mix" capability. 417 A party performing as a mixer, which has expressed the "rtt-mix" 418 capability, but not received "rtt-mix" capability indication in a 419 session with a participant SHOULD, if nothing else is specified for 420 the application, format transmitted text to that participant to be 421 suitable to present on a multi-party unaware endpoint as further 422 specified in section Section 14.2. 424 12. Examples 426 This example shows a symbolic flow of packets from a mixer with loss 427 and recovery. A, B and C are sources of RTT. M is the mixer. P 428 indicates primary data. R1 is first redundant generation data and R2 429 is second redundant generation data. A1, B1, A2 etc are text chunks 430 (T140blocks) received from the respective sources. X indicates 431 dropped packet between the mixer and a receiver. 433 |----------------| 434 |Seq no 1 | 435 |CSRC list A,M,B | 436 |R2 B99 | 437 |R1: Empty | 438 |P: A1 | 439 |----------------| 441 Assuming that earlier packets were received in sequence, text A1 is 442 received from packet 1 and assigned to reception area A. 444 |----------------| 445 |Seq no 2 | 446 |CC=3 | 447 |CSRC list C,A,M | 448 |R2 Empty | 449 |R1: A1 | 450 |P: C1 | 451 |----------------| 452 Text C1 is received from packet 2 and assigned to reception area C. 454 X----------------| 455 X Seq no 3 | 456 X CC=3 | 457 X CSRC list A,C,A| 458 X R2: A1 | 459 X R1: C1 | 460 X P: A2 | 461 X----------------| 462 Packet 3 is assumed to be dropped in network problems 464 X----------------| 465 X Seq no 4 | 466 X CC=3 | 467 X CSRC list B,A,C| 468 X R2: C1 | 469 X R1: A2 | 470 X P: B1 | 471 X----------------| 472 Packet 4 is assumed to be dropped in network problems 473 X----------------| 474 X Seq no 5 | 475 X CC=3 | 476 X CSRC list A,B,A| 477 X R2: A2 | 478 X R1: B1 | 479 X P: A3 | 480 X----------------| 481 Packet 5 is assumed to be dropped in network problems 483 |----------------| 484 |Seq no 6 | 485 |CC=3 | 486 |CSRC list C,A,B | 487 |R2: B1 | 488 |R1: A3 | 489 |P: C2 | 490 |----------------| 491 Packet 6 is received. The latest received sequence number before 6 was 2. 492 Recovery is therefore tried for 3,4,5. 493 But there is no coverage for seq no 3. A missing text mark (U'FFFD) 494 is created and appended to the mixer reception area. 495 For seqno 4, text B1 is recovered and appended to reception area B. 496 For seqno 5, text A3 is recovered and appended to reception area A. 497 Primary text C2 is received and appended to reception area C. 499 With only one or two packets lost, there would not be any need to 500 create a missing text marker, and all text would be recovered. 502 It will be a design decision how to present the missing text markers 503 assigned to the mixer as a source. 505 13. Performance considerations 507 This specification allows new text from one source per packet. 508 Packets are transmitted with timed intervals. The default 509 transmission interval is 300 ms for RFC 4103[RFC4103], and is 510 suitable for transmission from single sources. However when more 511 sources contribute to the flow, a shorter transmission interval may 512 be applicable. The transmission interval is therefore recommended to 513 be 100 ms for mixers when there is text from more than one source 514 available for transmission. This interval provides for smooth flow 515 of text with acceptable latency from at least 5 sources 516 simultaneously. 518 14. Presentation level considerations 520 ITU-T T.140 [T140] provides the presentation level requirements for 521 the RFC 4103 [RFC4103] transport. T.140 [T140] has functions for 522 erasure and other formatting functions and has the following general 523 statement for the presentation: 525 "The display of text from the members of the conversation should be 526 arranged so that the text from each participant is clearly readable, 527 and its source and the relative timing of entered text is visualized 528 in the display. Mechanisms for looking back in the contents from the 529 current session should be provided. The text should be displayed as 530 soon as it is received." 532 Strict application of T.140 [T140] is of essence for the 533 interoperability of real-time text implementations and to fulfill the 534 intention that the session participants have the same information of 535 the text contents of the conversation without necessarily having the 536 exact same layout of the conversation. This also includes the 537 ability to ignore optional presentation control codes not supported 538 by a receiving application. 540 T.140 [T140] specifies a set of presentation control codes to include 541 in the stream. Some of them are optional. It is essential that 542 implementations are able to ignore 544 There is no strict "message" concept in real-time text. Line 545 separator is used as a separator allowing a part of received text to 546 be grouped in presentation. The characters CR LF are allowed as 547 replacement for Line Separator. The CR LF combination is erased by 548 just one erasing action. Presentation functions are allowed to group 549 text for presentation in smaller groups than the line separators 550 imply and present such groups with source indication together with 551 text groups from other sources (see the following presentation 552 examples). Erasure has no specific limit by any delimiter in the 553 text stream. 555 14.1. Presentation by multi-party aware endpoints 557 A multi-party aware receiving party, presenting real-time text MUST 558 separate text from different sources and present them in separate 559 presentation areas. The receiving party may separate presentation of 560 parts of text from a source in readable groups based on other 561 criteria than line separator and merge these groups in the 562 presentation area when it benefits the user to most easily find and 563 read text from the different participants. The criteria may e.g. be 564 a received comma, full stop, or other phrase delimiters, or a long 565 pause. 567 When text is received from multiple original sources, the 568 presentation provides a view where text is added in multiple places 569 simultaneously. 571 If the presentation presents text from different sources in one 572 common area, the endpoint SHOULD insert text from the local user 573 ended at suitable points merged with received text to indicate the 574 relative timing for when the text groups were completed. In this 575 presentation mode, the receiving endpoint SHALL present the source of 576 the different groups of text. 578 A view of a three-party RTT call in chat style is shown in this 579 example . 581 _________________________________________________ 582 | | | 583 |[Alice] Hi, Alice here. | | 584 | | | 585 |[Bob] Bob as well. | | 586 | | | 587 |[Eve] Hi, this is Eve, calling from Paris. | | 588 | I thought you should be here. | | 589 | | | 590 |[Alice] I am coming on Thursday, my | | 591 | performance is not until Friday morning.| | 592 | | | 593 |[Bob] And I on Wednesday evening. | | 594 | | | 595 |[Alice] Can we meet on Thursday evening? | | 596 | | | 597 |[Eve] Yes, definitely. How about 7pm. | | 598 | at the entrance of the restaurant | | 599 | Le Lion Blanc? | | 600 |[Eve] we can have dinner and then take a walk | | 601 | | | 602 | But I need to be back to | | 603 | the hotel by 11 because I need | | 604 | |-| 605 | I wou |-| 606 |______________________________________________|v| 607 | of course, I underst | 608 |________________________________________________| 610 Figure 2: Example of a three-party RTT call presented in chat style. 612 Figure 1: Example of a three-party call with chat style. 614 Other presentation styles than the chat style may be arranged. 616 This figure shows how a coordinated column view MAY be presented. 618 _____________________________________________________________________ 619 | Bob | Eve | Alice | 620 |____________________|______________________|_______________________| 621 | | |I will arrive by TGV. | 622 |My flight is to Orly| |Convenient to the main | 623 | |Hi all, can we plan |station. | 624 | |for the seminar? | | 625 |Eve, will you do | | | 626 |your presentation on| | | 627 |Friday? |Yes, Friday at 10. | | 628 |Fine, wo | |We need to meet befo | 629 |___________________________________________________________________| 631 Figure 3: An example of a coordinated column-view of a three-party 632 session with entries ordered vertically in approximate time-order. 634 14.2. Multi-party mixing for multi-party unaware endpoints 636 When the mixer has indicated multi-party capability in an sdp 637 negotiation, but the multi-party capability negotiation fails with an 638 endpoint, then the mixer SHOULD compose a best-effort presentation of 639 multi-party real-time text in one stream intended to be presented by 640 an endpoint with no multi-party awareness. 642 This presentation format has severe functional limitations and should 643 be used only to enable participation in multi-party calls by legacy 644 deployed endpoints. 646 The text should be formatted by the mixer for presentation in one 647 single presentation area. When there is text available for 648 transmission from the mixer to the participant for more than one 649 participant, the mixer shall switch between transmission of text from 650 different sources at suitable points in the transmitted stream. When 651 switching source, the mixer shall insert a line separator and a label 652 composed from information in the CNAME and NAME fields in RTCP 653 reports from the participant to have its text transmitted. The label 654 SHALL be delimited by suitable characters. Then text from that 655 selected participant shall be transmitted until a new suitable point 656 for switching source is reached. 658 Suitable points for switching are: a completed phrase ended by comma, 659 a completed sentence, a line separator or other valid code for new 660 line. A long pause (e.g. > 10 seconds) in received text from the 661 currently transmitted source is also a suitable switching point. If 662 text from one participant has been transmitted with text from other 663 sources waiting for transmission for a long time (e.g. > 1 minute) 664 and none of the suitable points for switching has occurred, a source 665 switch MAY be forced by the mixer at next word separator. When 666 switching source, the text that has been waiting longest should be 667 selected to be transmitted. 669 Erasure by backspace can not always be performed as the erasing party 670 intended. If an erasing action erases all text up to the leading 671 label after a source switch, then the mixer must not transmit more 672 backspaces. Instead it is recommended that a letter "X" is inserted 673 in the text stream for each backspace, as an indication of the intent 674 to erase more. A new line is usually coded by a Line Separator, but 675 the character combination CR LF may be used instead. Erasure of a 676 new line is in both cases done by just one erasing action 677 (Backspace). 679 A mixer transmitting to a multi-party unaware terminal shall send 680 primary data only from one source per packet. The SSRC shall be the 681 SSRC of the mixer. The CSRC list SHOULD contain one member and be 682 the SSRC of the source of the primary data. 684 When a multi-party unaware endpoint presents a conversation in one 685 display area in a chat style, it inserts source indications for 686 remote text and local user text as they are merged in completed text 687 groups. When an endpoint using this layout receives and presents 688 text mixed for multi-party unaware endpoints, there will be two 689 levels of source indicators for the received text; one generated by 690 the mixer and inserted in a label after each source switch, and 691 another generated by the receiving endpoint and inserted after each 692 switch between local and remote source in the presentation area. 693 This will waste display space and look inconsistent to the reader. 695 This fact, combined with the slowness in source switching and the 696 limited support of erasure makes it strongly RECOMMENDED to implement 697 multi-party awareness in RTT endpoints. The use of the multi-party- 698 unaware mixing method should be left for use with endpoints which are 699 impossible to upgrade to become multi-party aware. 701 The following pictures are examples of the view on a participant's 702 display for the multi-party-unaware case. 704 _________________________________________________ 705 | Conference | Alice | 706 |________________________|_________________________| 707 | |I will arrive by TGV. | 708 |[Bob]:My flight is to |Convenient to the main | 709 |Orly. |station. | 710 |[Eve]:Hi all, can we | | 711 |plan for the seminar. | | 712 | | | 713 |[Bob]:Eve, will you do | | 714 |your presentation on | | 715 |Friday? | | 716 |[Eve]:Yes, Friday at 10.| | 717 |[Bob]: Fine, wo |We need to meet befo | 718 |________________________|_________________________| 720 Figure 4: Alice who has a conference-unaware client is receiving the 721 multi-party real-time text in a single-stream. This figure shows how 722 a coordinated column view MAY be presented on Alice's device. 724 _________________________________________________ 725 | |^| 726 |[Alice] Hi, Alice here. | | 727 | | | 728 |[mix][Bob] Bob as well. | | 729 | | | 730 |[Eve] Hi, this is Eve, calling from Paris | | 731 | I thought you should be here. | | 732 | | | 733 |[Alice] I am coming on Thursday, my | | 734 | performance is not until Friday morning.| | 735 | | | 736 |[mix][Bob] And I on Wednesday evening. | | 737 | | | 738 |[Eve] we can have dinner and then walk | | 739 | | | 740 |[Eve] But I need to be back to | | 741 | the hotel by 11 because I need |-| 742 | |-| 743 |______________________________________________|v| 744 | of course, I underst | 745 |________________________________________________| 747 Figure 5: An example of a view of the multi-party unaware 748 presentation in chat style. Alice is the local user. 750 15. Gateway Considerations 752 Multi-party RTT sessions may involve gateways of different kinds. It 753 is essential that gateways involved in setting up sessions correctly 754 reflect the multi-party capability or unawareness of the combination 755 of the gateway and the remote endpoint beyond the gateway. 757 One case that may occur is a gateway to PSTN for communication with 758 textphones (e.g. TTYs). Textphones are limited devices with no 759 multi-party awareness, and it would therefore be suitable for the 760 gateway to not indicate multi-party awareness for that case. 762 16. Congestion considerations 764 The congestion considerations and recommended actions from RFC 4103 765 [RFC4103] are valid also in multi-party situations. 767 17. Acknowledgements 769 18. IANA Considerations 771 [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the 772 RFC number of this document.] 774 IANA is asked to register the new sdp attribute "rtt-mix". 776 +---------------------+------------------------------------------+ 777 | Contact name: | IESG | 778 | Contact email: | iesg@ietf.org | 779 | Attribute name: | rtt-mix | 780 | Attribute syntax | a=rtt-mix | 781 | Attribute semantics | See RFCXXXX section 11. | 782 | Attribute value | - | 783 | Usage level: | media | 784 | Purpose: | Indicate support for the rtp-mixer format| 785 | | for real-time text transmission | 786 | O/A procedure | Declarative | 787 | Mux Category | normal | 788 | Reference: | RFCXXXX | 789 +---------------------+------------------------------------------+ 791 19. Security Considerations 793 The RTP-mixer model requires the mixer to be allowed to decrypt, pack 794 and encrypt secured text from the conference participants. Therefore 795 the mixer needs to be trusted. This is similar to the situation for 796 central mixers of audio and video. 798 The requirement to transfer information about the user in RTCP 799 reports in SDES, CNAME and NAME fields for creation of labels may 800 have privacy concerns as already stated in RFC 3550 [RFC3550], and 801 may be restricted of privacy reasons. The receiving user will then 802 get a more symbolic label for the source. 804 20. Change history 806 20.1. Changes from version -00 to -01 808 Editorial cleanup. 810 Changed capability indication from fmtp-parameter to sdp attribute 811 "rtt-mix". 813 Swapped order of redundancy elements in the example to match reality. 815 Increased the SDP negotiation section 817 20.2. Changes from version -01 to -02 819 In Abstract and 1. Introduction: Introduced wording about regulatory 820 requirements. 822 In section 5: The transmission interval is decreased to 100 ms when 823 there is text from more than one source to transmit. 825 In section 11 about sdp negotiation, a SHOULD-requirement is 826 introduced that the mixer should make a mix for multi-party unaware 827 endpoints if the negotiation is not successful. And a reference to a 828 later chapter about it. 830 The presentation considerations chapter 14 is extended with more 831 information about presentation on multi-party aware endpoints, and a 832 new section on the multi-party unaware mixing with low functionality 833 but SHOULD a be implemented in mixers. Presentation examples are 834 added. 836 A short chapter 15 on gateway considerations is introduced. 838 Clarification about the text/t140 format included in chapter 10. 840 This sentence added to the chapter 10 about use without redundancy. 841 "The text/red format SHOULD be used unless some other protection 842 against packet loss is utilized, for example a reliable network or 843 transport." 845 Note about deviation from RFC 2198 added in chapter 4. 847 In chapter 9. "Use with SIP centralized conferencing framework" the 848 following note is inserted: Note: The CSRC-list in an RTP packet only 849 includes participants who's text is included in one or more text 850 blocks. It is not the same as the list of participants in a 851 conference. With audio and video media, the CSRC-list would often 852 contain all participants who are not muted whereas text participants 853 that don't type are completely silent and so don't show up in RTP 854 packet CSRC-lists. 856 21. References 858 21.1. Normative References 860 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 861 Requirement Levels", BCP 14, RFC 2119, 862 DOI 10.17487/RFC2119, March 1997, 863 . 865 [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., 866 Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- 867 Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, 868 DOI 10.17487/RFC2198, September 1997, 869 . 871 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 872 Jacobson, "RTP: A Transport Protocol for Real-Time 873 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 874 July 2003, . 876 [RFC4102] Jones, P., "Registration of the text/red MIME Sub-Type", 877 RFC 4102, DOI 10.17487/RFC4102, June 2005, 878 . 880 [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text 881 Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005, 882 . 884 [T140] ITU-T, "Recommendation ITU-T T.140 (02/1998), Protocol for 885 multimedia application text conversation", February 1998. 887 [T140ad1] ITU-T, "Recommendation ITU-T.140 Addendum 1 - (02/2000), 888 Protocol for multimedia application text conversation", 889 February 2000. 891 21.2. Informative References 893 [RFC4353] Rosenberg, J., "A Framework for Conferencing with the 894 Session Initiation Protocol (SIP)", RFC 4353, 895 DOI 10.17487/RFC4353, February 2006, 896 . 898 [RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A 899 Session Initiation Protocol (SIP) Event Package for 900 Conference State", RFC 4575, DOI 10.17487/RFC4575, August 901 2006, . 903 [RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol 904 (SIP) Call Control - Conferencing for User Agents", 905 BCP 119, RFC 4579, DOI 10.17487/RFC4579, August 2006, 906 . 908 Author's Address 910 Gunnar Hellstrom 911 Omnitor 912 Esplanaden 30 913 Vendelso 13670 914 Sweden 916 Phone: +46708204288 917 Email: gunnar.hellstrom@omnitor.se