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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the RFC 3978 Section 5.4 Copyright Line does not match the current year == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: When redundant transmission of the data according to RFC 2198 is desired, the RTP header is followed by one or more redundant data block headers, one for each redundant data block to be included. Each of these headers provides the timestamp offset and length of the corresponding data block plus a payload type number indicating this payload format ("T140"). Redundant data older than 16383 divided by the clock frequency MUST not be transmitted. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (October 2003) is 7493 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 section? '1' on line 870 looks like a reference -- Missing reference section? '2' on line 873 looks like a reference -- Missing reference section? '8' on line 895 looks like a reference -- Missing reference section? '5' on line 884 looks like a reference -- Missing reference section? '6' on line 887 looks like a reference -- Missing reference section? '3' on line 877 looks like a reference -- Missing reference section? '7' on line 890 looks like a reference -- Missing reference section? '4' on line 881 looks like a reference Summary: 8 errors (**), 0 flaws (~~), 3 warnings (==), 10 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVT Working Group 3 Internet Draft G. Hellstrom 4 Omnitor AB 5 Expires: April 2004 6 P. Jones 7 Cisco Systems, Inc. 8 October 2003 10 RTP Payload for Text Conversation 12 Status of this Memo 14 This document is an Internet-Draft and is in full conformance with 15 all provisions of Section 10 of RFC 2026. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 [Note to RFC Editor: All references to RFC XXXX are to be replaced by 33 references to the RFC number of this memo, when published.] 35 Abstract 37 This memo describes how to carry text conversation session contents 38 in RTP packets. Text conversation session contents are specified in 39 ITU-T Recommendation T.140 [1]. 41 Text conversation is used alone or in connection to other 42 conversational facilities such as video and voice, to form multimedia 43 conversation services. 45 This RTP payload description contains an optional possibility to 46 include redundant text from already transmitted packets in order to 47 reduce the risk of text loss caused by packet loss. The redundancy 48 coding follows RFC 2198. 50 RTP Payload for Text Conversation October 2003 52 Table of Contents 53 1. Introduction..................................................2 54 2. Conventions used in this document.............................3 55 3. Usage of RTP..................................................3 56 3.1 Payload Format for Transmission of text/t140 Data........4 57 3.2 Payload Format for Transmission of audio/t140 Data.......4 58 3.3 The "T140block"..........................................4 59 3.4 Use of Redundancy........................................5 60 3.5 Synchronization of Text with Other Media.................5 61 3.6 RTP packet header........................................5 62 3.7 Additional Headers.......................................6 63 3.8 T.140 Text Structure.....................................6 64 4. Recommended Procedure.........................................7 65 4.1 Recommended Basic Procedure..............................7 66 4.2 Recommended Procedure for Compensation for Lost Packets..8 67 4.3 Recommended Procedure for Compensation for Packets Out of 68 Order....................................................8 69 4.4 Transmission During "Silent Periods" when Redundancy is 70 Used.....................................................8 71 5. SDP Attribute for Flow Control................................9 72 6. Examples......................................................9 73 6.1 RTP Packetization Examples for the text/t140 format......9 74 6.2 RTP Packetization Examples for the audio/t140 format....11 75 6.3 SDP Examples............................................13 76 7. Security Considerations......................................14 77 8. MIME Media Type Registrations................................14 78 8.1 Registration of MIME Media Type text/t140...............14 79 8.2 Registration of MIME Media Type audio/t140..............15 80 8.3 Registration of MIME Media Type text/RED................16 81 9. Authors' Addresses...........................................17 82 10. Acknowledgements............................................18 83 11. Normative References........................................18 84 12. Informative References......................................18 85 13. Full Copyright Statement....................................18 87 1. Introduction 89 This document defines two payload types for carrying text 90 conversation session contents in RTP packets. Text conversation 91 session contents are specified in ITU-T Recommendation T.140 [1]. 92 Text conversation is used alone or in connection to other 93 conversational facilities such as video and voice, to form multimedia 94 RTP Payload for Text Conversation October 2003 96 conversation services. Text in text conversation sessions is sent as 97 soon as it is available, or with a small delay for buffering. 99 The text is supposed to be entered by human users from a keyboard, 100 handwriting recognition, voice recognition or any other input method. 101 The rate of character entry is usually at a level of a few characters 102 per second or less, though text may be transmitted at a much higher 103 rate (e.g., automated systems or "copy and paste" operations may 104 produce a lot of text very rapidly). Therefore, it is generally 105 expected number of characters to transmit is low. Only one or a few 106 new characters are expected to be transmitted with each packet. 108 T.140 specifies that text and other T.140 elements MUST be 109 transmitted in ISO 10 646-1 code with UTF-8 transformation. That 110 makes it easy to implement internationally useful applications, and 111 to handle the text in modern information technology environments. 112 The payload of an RTP packet following this specification consists of 113 text encoded according to T.140 without any additional framing. A 114 common case will be a single ISO 10646 character, UTF-8 encoded. 116 T.140 requires the transport channel to provide characters without 117 duplication and in original order. Text conversation users expect 118 that text will be delivered with no or a low level of lost 119 information. If lost information can be indicated, the willingness to 120 accept loss is expected to be higher. 122 Therefore a mechanism based on RTP is specified here. It gives text 123 arrival in correct order, without duplications, and with detection 124 and indication of losses. It also includes an optional possibility 125 to repeat data for redundancy to lower the risk of loss. Since packet 126 overhead is usually much larger than the T.140 contents, the increase 127 in channel load by the redundancy scheme is minimal. 129 This document updates and extends RFC 2793. The text is intended to 130 to clarify any ambiguities in RFC 2793, improve on the specific 131 implementation requirements learned through development experience, 132 give explicit usage examples and to introduce a means of transporting 133 text interleaved with voice within the same RTP session. 135 2. Conventions used in this document 137 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 138 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 139 document are to be interpreted as described in RFC 2119. 141 3. Usage of RTP 143 When transport of T.140 text session data in RTP is desired, the 144 payloads as described in this specification SHOULD be used. 146 RTP Payload for Text Conversation October 2003 148 3.1 Payload Format for Transmission of text/t140 Data 150 A text conversation RTP packet as specified by this payload format 151 consists of an RTP header as defined in RFC 3550 [2] followed 152 immediately by a block of T.140 data, referred to as a "T140block" 153 (see section 3.3). There is no additional header specific to this 154 payload format. 156 This format is primarily used when text is transmitted on a separate 157 RTP session dedicated for the transmission of text and not shared 158 with other media, such as audio, DTMF, etc. IP textphone devices 159 most commonly use this format. 161 3.2 Payload Format for Transmission of audio/t140 Data 163 A text conversation RTP packet as specified by this payload format 164 consists of an RTP header as defined in RFC 3550 followed immediately 165 by a 16-bit "t140block counter" followed by a "T140block" (see 166 section 3.3). There is no additional header specific to this payload 167 format. 169 The T140block counter MUST be initialized to zero the first time that 170 a packet containing a T140block is transmitted and MUST be 171 incremented by 1 each time that a new block is transmitted. Once the 172 counter reaches the value 0xFFFF, the counter is reset to 0 the next 173 time the counter is incremented. This T140block counter may be 174 utilized to detect lost characters. 176 For the purposes of readability, the remainder of this document only 177 refers to the T140block without making explicit reference to the 178 T140block counter. Readers should understand that when using the 179 audio/t140 format, the T140block counter MUST always precede the 180 actual T140block, including redundant data transmissions. 182 The primary purpose for this payload specification is to allow 183 gateways that are interconnecting two PSTN networks to interleave, 184 through a single RTP session, audio and text data received on the 185 PSTN circuit. This is comparable to the way in which DTMF is 186 extracted and transmitted within an RTP session [8]. 188 3.3 The "T140block" 190 The T140block contains one or more T.140 code elements as specified 191 in [1]. Most T.140 code elements are single ISO 10646 [5] 192 characters, but some are multiple character sequences. Each 193 character is UTF-8 encoded [6] into one or more octets. This implies 194 that each block MUST contain an integral number of UTF-8 encoded 195 characters regardless of the number of octets per character. It also 196 RTP Payload for Text Conversation October 2003 198 implies that any composite character sequence (CCS) SHOULD be placed 199 within one block. 201 3.4 Use of Redundancy 203 The T140blocks MAY be transmitted redundantly according to the 204 payload format defined in RFC 2198 [3]. In that case, the RTP header 205 is followed by one or more redundant data block headers, the same 206 number of redundant data fields carrying T140blocks from previous 207 packets, and finally the new (primary) T140block for this packet. 209 3.5 Synchronization of Text with Other Media 211 Usually, each medium in a session utilizes a separate RTP stream. In 212 that case, if synchronization of the text and other media packets is 213 important, the streams MUST be associated when the sessions are 214 established and the streams MUST share the reference clock (refer to 215 the description of the timestamp field as it relates to 216 synchronization in section 5.1 of RFC 3550). Association of RTP 217 streams is dependent on the particular session application and is 218 outside the scope of this document. 220 When audio/t140 is used, it is generally transmitted as interleaved 221 packets between voice packets or other kinds of audio packets. One 222 should observe the RTP timestamps of the voice, text, or other audio 223 packets in order to reproduce the stream correctly when playing out 224 the audio. Note, also, that incoming text from a PSTN circuit might 225 be at a higher bit-rate than can be played out on an egress PSTN 226 circuit. As such, it is possible that, on the egress side, a gateway 227 may not complete the play out of the text packets before it is time 228 to play the next voice packet. Given that this application is 229 primarily for the benefit of deaf users utilizing PSTN textphone 230 devices, it is strongly RECOMMENDED that all text packets be properly 231 reproduced on the egress gateway before considering any subsequent 232 voice or other audio packets. If necessary, voice and other audio 233 packets should be discarded in order to properly reproduce the text 234 signals on the PSTN circuit. 236 3.6 RTP packet header 238 Each RTP packet starts with a fixed RTP header. The following fields 239 of the RTP fixed header are used for T.140 text streams: 241 Payload Type (PT): The assignment of an RTP payload type is specific 242 to the RTP profile under which this payload format is used. For 243 profiles that use dynamic payload type number assignment, this 244 payload format is identified by the name "T140" (see section 8). 245 If redundancy is used per RFC 2198, the Payload Type MUST indicate 246 that payload format ("RED"). 248 RTP Payload for Text Conversation October 2003 250 Sequence number: The Sequence Number MUST be increased by one for 251 each new transmitted packet. When transmitting text using the 252 payload format for text/t140, it is used for detection of packet 253 loss and packets out of order, and can be used in the process of 254 retrieval of redundant text, reordering of text and marking missing 255 text. (Character loss is detected through the T140block counter 256 when using the audio/t140 payload format.) 258 Timestamp: The RTP Timestamp encodes the approximate instance of 259 entry of the primary text in the packet. A clock frequency of 1000 260 Hz MUST be used for text/t140. The clock frequency may be 261 specified for audio/t140 and is generally set to be 8000 Hz, as 262 that is most common for audio. Sequential packets MUST NOT use the 263 same timestamp. Since packets do not represent any constant 264 duration, the timestamp cannot be used to directly infer packet 265 losses. 267 3.7 Additional Headers 269 There are no additional headers defined specific to this payload 270 format. 272 When redundant transmission of the data according to RFC 2198 is 273 desired, the RTP header is followed by one or more redundant data 274 block headers, one for each redundant data block to be included. 275 Each of these headers provides the timestamp offset and length of the 276 corresponding data block plus a payload type number indicating this 277 payload format ("T140"). Redundant data older than 16383 divided by 278 the clock frequency MUST not be transmitted. 280 3.8 T.140 Text Structure 282 T.140 text is UTF-8 coded as specified in T.140 with no extra 283 framing. When using the format with redundant data, the transmitter 284 MAY select a number of T140block generations to retransmit in each 285 packet. A higher number introduces better protection against loss of 286 text but increases the data rate. 288 Since packets are not generated at regular intervals and since the 289 audio/t140 format allows for other media to be interleaved, the 290 timestamp is not sufficient to identify a packet in the presence of 291 loss unless extra information is provided. Since sequence numbers are 292 not provided in the redundant header, some additional rules must be 293 followed to allow the redundant data corresponding to missing primary 294 data to be merged properly into the stream of primary data T140blocks 295 when using the text/t140 payload format. While the audio/t140 296 RTP Payload for Text Conversation October 2003 298 payload format does not rely on the sequence numbers of packets to 299 identify missing data, the same rules apply. They are: 301 - Each redundant data block MUST contain the same data as a 302 T140block previously transmitted as primary data, and be 303 identified with a timestamp offset equating to the original 304 timestamp for that T140block. 305 - The redundant data MUST be placed in age order with most recent 306 redundant T140block last in the redundancy area. 307 - All T140blocks from the oldest desired generation up through the 308 generation immediately preceding the new (primary) T140block 309 MUST be included. 311 For the text/t140 payload format, these rules allow the sequence 312 numbers for the redundant T140blocks to be inferred by counting 313 backwards from the sequence number in the RTP header. The result 314 will be that all the text in the payload will be contiguous and in 315 order. 317 4. Recommended Procedure 319 This section contains RECOMMENDED procedures for usage of the payload 320 format. Based on the information in the received packets, the 321 receiver can: 323 - reorder text received out of order. 324 - mark where text is missing because of packet loss. 325 - compensate for lost packets by using redundant data. 327 4.1 Recommended Basic Procedure 329 Packets are transmitted only when there is valid T.140 data to 330 transmit. The sequence number is used for sequencing of T.140 data. 332 T.140 specifies that T.140 data MAY be buffered before transmission 333 for a short moment. A maximum buffering time of 500 ms is specified. 334 In order to keep the maximum bit rate usage for text at a reasonable 335 level, it is RECOMMENDED to buffer T.140 data for transmission in 300 336 ms intervals. This time is selected so that text users will still 337 perceive a real time text flow. 339 On reception of text/t140 data, the RTP sequence number is compared 340 with the sequence number of the last correctly received packet. On 341 receipt of audio/t140 data, the T140block counter is compared with 342 the T140block counter of the last correctly received packet. If they 343 are consecutive, the (only or primary) T140block is retrieved from 344 the packet. 346 RTP Payload for Text Conversation October 2003 348 4.2 Recommended Procedure for Compensation for Lost Packets 350 For reduction of data loss in case of packet loss, redundant data MAY 351 be included in the packets following to the procedures in RFC 2198. 352 If network conditions are not known, it is RECOMMENDED to use three 353 redundant T140blocks in each packet. If there is a gap in the RTP 354 sequence numbers (for text/t140) or T140block counters (audio/t140), 355 and redundant T140blocks are available in a subsequent packet, the 356 sequence numbers or T140block counters for the redundant T140blocks 357 should be inferred by counting backwards from the sequence number or 358 T140block counter in the RTP header for that packet. If there are 359 redundant T140blocks with sequence numbers matching those that are 360 missing, the redundant T140blocks may be substituted for the missing 361 T140blocks. 363 Both for the case when redundancy is used and not used, missing data 364 SHOULD be marked by insertion of a missing text marker in the 365 received stream for each missing T140block, as specified in ITU-T 366 T.140 Addendum 1 [1]. 368 4.3 Recommended Procedure for Compensation for Packets Out of Order 370 For protection against packets arriving out of order, the following 371 procedure MAY be implemented in the receiver. If analysis of a 372 received packet reveals a gap in the sequence and no redundant data 373 is available to fill that gap, the received packet SHOULD be kept in 374 a buffer to allow time for the missing packet(s) to arrive. It is 375 RECOMMENDED that the waiting time be limited to 0.5 seconds. 377 If a packet with a T140block belonging to the gap arrives before the 378 waiting time expires, this T140block is inserted into the gap and 379 then consecutive T140blocks from the leading edge of the gap may be 380 consumed. Any T140block which does not arrive before the time limit 381 expires should be treated as lost. 383 4.4 Transmission During "Silent Periods" when Redundancy is Used 385 When using the redundancy transmission scheme, and there is redundant 386 data, but no new T.140 data to transmit after the transmit buffering 387 interval described in section 4.1 has passed, a packet MUST be 388 transmitted containing a zero-length primary T140block and the 389 properly positioned redundant data. When using the audio/t140payload 390 format with an empty T140block, the T140block counter MUST also be 391 absent (as there is no actual T140block). 393 When using the text/t140 payload format, any zero-length T140blocks 394 that are sent as primary data MUST be included as redundant 395 T140blocks on subsequent packets just as normal text T140blocks would 396 RTP Payload for Text Conversation October 2003 398 be so that sequence number inference for the redundant T140blocks 399 will be correct, as explained in section 3.8. 401 When using the audio/t140 payload format, zero-length T140blocks sent 402 as primary data MUST NOT be included as redundant T140blocks, as it 403 would simply be a waste of bandwidth to send them. 405 Redundancy for the last T140block MUST NOT be implemented by 406 repeatedly transmitting the same packet (with the same sequence 407 number) because this will cause the packet loss count, as reported in 408 RTCP, to decrement. 410 5. SDP Attribute for Character Transmission Rate 412 In some cases, it is necessary to limit the rate at which characters 413 are transmitted. While the "b=" SDP attribute could be used to limit 414 the rate of the RTP session, it may be that only the text stream in 415 an interleaved audio/text session needs special handling. For 416 example, when a PSTN gateway is interworking between an IP device 417 (not necessarily a textphone) and a PSTN textphone, it may be 418 necessary to limit the character rate from the IP device in order to 419 avoid throwing away characters at the PSTN gateway. At the same 420 time, no explicit bit rate restriction is necessarily applied to the 421 audio stream. 423 To control the character transmission rate, the "fmtp" attribute [7] 424 is used with the following syntax: 426 a=fmtp: cps= 428 The field is populated with the payload type that is used 429 for text. The field contains an integer representing the 430 maximum number of characters that may be received per second. 432 Devices in receipt of this parameter MUST adhere to the request to 433 apply by transmitting characters at a rate at or below the specified 434 value. 436 6. Examples 438 6.1 RTP Packetization Examples for the text/t140 format. 440 This is an example of a T140 RTP packet without redundancy. 441 0 1 2 3 442 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 1 443 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 444 |V=2|P|X| CC=0 |M| T140 PT | sequence number | 445 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 446 | timestamp (1000Hz) | 447 RTP Payload for Text Conversation October 2003 449 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 450 | synchronization source (SSRC) identifier | 451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 452 + T.140 encoded data + 453 | | 454 + +---------------+ 455 | | 456 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 458 This is an example of an RTP packet with one redundant T140block. 459 0 1 2 3 460 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 1 461 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 462 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 463 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 464 | timestamp of primary encoding "P" | 465 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 466 | synchronization source (SSRC) identifier | 467 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 468 |1| T140 PT | timestamp offset of "R" | "R" block length | 469 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 470 |0| T140 PT | | 471 +-+-+-+-+-+-+-+-+ + 472 | | 473 + "R" T.140 encoded redundant data + 474 | | 475 + +---------------+ 476 | | | 477 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 478 | "P" T.140 encoded primary data | 479 + + 480 + +---------------+ 481 | | 482 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 484 This is an example of an RTP packet with one redundant T140block 485 using text/t140 payload format. The primary data block is 486 empty, which is the case when transmitting a packet for the 487 sole purpose of forcing the redundant data to be transmitted 488 in the absence of any new data. 489 0 1 2 3 490 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 1 491 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 492 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 493 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 494 | timestamp of primary encoding "P" | 495 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 496 | synchronization source (SSRC) identifier | 497 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 498 RTP Payload for Text Conversation October 2003 500 |1| T140 PT | timestamp offset of "R" | "R" block length | 501 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 502 |0| T140 PT | | 503 +-+-+-+-+-+-+-+-+ + 504 | | 505 + "R" T.140 encoded redundant data + 506 | | 507 + +---------------+ 508 | | 509 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 511 As a follow-on to the previous example, this example shows the 512 next RTP packet in the sequence which does contain a real 513 T140block when using the text/t140 payload format. Note that the 514 empty block is present in the redundant transmissions of the 515 text/t140 payload format. This example shows 2 levels of 516 redundancy and one primary data block. The value of the "R2 517 block length" would be set to zero in order to in order to 518 represent the empty T140block. 519 0 1 2 3 520 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 1 521 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 522 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 523 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 524 | timestamp of primary encoding "P" | 525 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 526 | synchronization source (SSRC) identifier | 527 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 528 |1| T140 PT | timestamp offset of "R1" | "R1" block length | 529 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 530 |1| T140 PT | timestamp offset of "R2" | "R2" block length | 531 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 532 |0| T140 PT | | 533 +-+-+-+-+-+-+-+-+ + 534 | | 535 + "R1" T.140 encoded redundant data + 536 | | 537 + +---------------+ 538 | | | 539 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 540 | "P" T.140 encoded primary data | 541 + + 542 + +---------------+ 543 | | 544 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 546 6.2 RTP Packetization Examples for the audio/t140 format 547 RTP Payload for Text Conversation October 2003 549 This is an example of a T140 RTP packet without redundancy. 550 0 1 2 3 551 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 1 552 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 553 |V=2|P|X| CC=0 |M| T140 PT | sequence number | 554 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 555 | timestamp (8000Hz) | 556 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 557 | synchronization source (SSRC) identifier | 558 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 559 | T140block Counter | | 560 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 561 + T.140 encoded data + 562 | | 563 + +---------------+ 564 | | 565 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 567 This is an example of an RTP packet with one redundant T140block 568 using audio/t140 payload format. The primary data block is 569 empty, which is the case when transmitting a packet for the 570 sole purpose of forcing the redundant data to be transmitted 571 in the absence of any new data. Note that since this is the 572 audio/t140 payload format, the redundant block of T.140 data 573 is immediately preceded with a T140block Counter. 574 0 1 2 3 575 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 1 576 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 577 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 578 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 579 | timestamp of primary encoding "P" | 580 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 581 | synchronization source (SSRC) identifier | 582 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 583 |1| T140 PT | timestamp offset of "R" | "R" block length | 584 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 585 |0| T140 PT | T140block Counter | | 586 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 587 | | 588 + "R" T.140 encoded redundant data + 589 | | 590 + +---------------+ 591 | | 592 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 594 As a follow-on to the previous example, this example shows the 595 next RTP packet in the sequence which does contain a new real 596 T140block when using the audio/t140 payload format. This 597 example has 2 levels of redundancy and one primary data block. 599 RTP Payload for Text Conversation October 2003 601 Since the previous primary block was empty, no redundant data 602 is included for that block. This is because when using the 603 audio/t140 payload format, any previously transmitted "empty" 604 T140blocks are NOT included as redundant data in subsequent 605 packets. 606 0 1 2 3 607 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 1 608 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 609 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 610 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 611 | timestamp of primary encoding "P" | 612 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 613 | synchronization source (SSRC) identifier | 614 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 615 |1| T140 PT | timestamp offset of "R1" | "R1" block length | 616 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 617 |1| T140 PT | T140block Counter | | 618 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 619 | | 620 + "R1" T.140 encoded redundant data + 621 | | 622 + +---------------+ 623 | | T140block_ | 624 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 625 | Counter | "P" T.140 encoded primary data | 626 +-+-+-+-+-+-+-+-+ + 627 | | 628 + +---------------+ 629 | | 630 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 632 6.3 SDP Examples 634 Below is an example of SDP describing RTP text transport on port 635 11000: 637 m=text 11000 RTP/AVP 98 638 a=rtpmap:98 t140/1000 640 Below is an example of SDP similar to the above example, but also 641 utilizing RFC 2198 to provide redundancy for the text packets: 643 m=text 11000 RTP/AVP 98 100 644 a=rtpmap:98 t140/1000 645 a=rtpmap:100 red/1000 646 a=fmtp:100 98/98 647 RTP Payload for Text Conversation October 2003 649 Below is an example of SDP describing RTP text interleaved with G.711 650 audio packets within the same RTP session from port 7200 and at a 651 maximum text rate of 6 characters per second: 653 m=audio 7200 RTP/AVP 0 98 654 a=rtpmap:98 t140/8000 655 a=fmtp:98 cps=6 657 Below is an example using RFC 2198 to provide redundancy to just the 658 text packets in an RTP session with interleaving text and G.711 at a 659 text rate no faster than 6 characters per second: 661 m=audio 7200 RTP/AVP 0 98 100 662 a=rtpmap:98 t140/8000 663 a=fmtp:98 cps=6 664 a=rtpmap:100 red/8000 665 a=fmtp:100 98/98 667 7. Security Considerations 669 Since the intention of the described payload format is to carry text 670 in a text conversation, security measures in the form of encryption 671 are of importance. The amount of data in a text conversation session 672 is low and therefore any encryption method MAY be selected and 673 applied to T.140 session contents or to the whole RTP packets. When 674 redundant data is included, the same security considerations as for 675 RFC 2198 apply. Additionally, all of the security considerations 676 from section 14 or RFC 3550 apply. 678 8. MIME Media Type Registrations 680 This document defines an RTP payload named "t140" and two associated 681 MIME types, "text/t140" and "audio/t140". Additionally, the MIME 682 type "text/RED" is defined to allow RFC 2198 to be used to carry 683 redundant text payloads. 685 8.1 Registration of MIME Media Type text/t140 687 MIME media type name: text 689 MIME subtype name: t140 691 Required parameters: 692 rate: The RTP timestamp clock rate, which is equal to the 693 sampling rate. The only valid value is 1000. 695 Optional parameters: 696 cps: The maximum number of character that may be received 697 per second. 699 RTP Payload for Text Conversation October 2003 701 Encoding considerations: T.140 text can be transmitted with RTP as 702 specified in RFC . 704 Security considerations: None 706 Interoperability considerations: None 708 Published specification: ITU-T T.140 Recommendation. 709 RFC XXXX. 711 Applications which use this media type: 712 Text communication terminals and text conferencing tools. 714 Additional information: None 716 Magic number(s): None 717 File extension(s): None 718 Macintosh File Type Code(s): None 720 Person & email address to contact for further information: 721 Gunnar Hellstrom 722 E-mail: gunnar.hellstrom@omnitor.se 724 Intended usage: COMMON 726 Author / Change controller: 727 Gunnar Hellstrom | IETF avt WG 728 gunnar.hellstrom@omnitor.se | 730 8.2 Registration of MIME Media Type audio/t140 732 MIME media type name: audio 734 MIME subtype name: t140 736 Required parameters: 737 rate: The RTP timestamp clock rate, which is equal to the 738 sampling rate. 740 Optional parameters: 741 cps: The maximum number of character that may be received 742 per second. 744 Encoding considerations: T.140 text can be transmitted with RTP as 745 specified in RFC XXXX. 747 Security considerations: None 748 RTP Payload for Text Conversation October 2003 750 Interoperability considerations: None 752 Published specification: ITU-T T.140 Recommendation. 753 RFC . 755 Applications which use this media type: 756 Text communication systems and text conferencing tools that 757 transmit text associated with audio and within the same RTP 758 session as the audio, such as PSTN gateways that transmit 759 audio and text signals between two PSTN textphone users 760 over an IP network. 762 Additional information: None 764 Magic number(s): None 765 File extension(s): None 766 Macintosh File Type Code(s): None 768 Person & email address to contact for further information: 769 Paul E. Jones 770 E-mail: paulej@packetizer.com 772 Intended usage: COMMON 774 Author / Change controller: 775 Paul E. Jones | IETF avt WG 776 paulej@packetizer.com | 778 8.3 Registration of MIME Media Type text/RED 780 MIME media type name: text 782 MIME subtype name: RED 784 Required parameters: 785 pt: a comma-separated list of RTP payload types. Because 786 comma is a special character, the list must be a quoted-string 787 (enclosed in double quotes). For static payload types, each 788 list element is simply the type number. For dynamic payload 789 types, each list element is a mapping of the dynamic payload 790 type number to an embedded MIME content-type specification for 791 the payload format corresponding to the dynamic payload type. 792 The format of the mapping is: 794 dynamic-payload-type "=" content-type 796 If the content-type string includes a comma, then the 797 content-type string MUST be a quoted-string. If the content- 798 type string does not include a comma, it MAY still be quoted. 800 RTP Payload for Text Conversation October 2003 802 Since it is part of the list which must itself be a quoted- 803 string, that means the quotation marks MUST be quoted with 804 backslash quoting as specified in RFC 2045. If the content- 805 type string itself contains a quoted-string, then the 806 requirement for backslash quoting is recursively applied. To 807 specify the text/RED payload format in SDP, the pt parameter 808 is mapped to an a=fmtp attribute by eliminating the parameter 809 name (pt) and changing the commas to slashes. For example, 810 'pt="101,102"' maps to 'a=fmtp:99 101/102'. 812 Optional parameters: ptime, maxptime 814 Encoding considerations: 815 This type is only defined for transfer via RTP [2]. 817 Security considerations: None 819 Interoperability considerations: none 821 Published specification: RFC 2198 823 Applications which use this media type: 824 Text streaming and conferencing tools. 826 Additional information: none 828 Person & email address to contact for further information: 829 Paul E. Jones 830 E-mail: paulej@packetizer.com 832 Intended usage: COMMON 834 Author / Change controller: 835 Paul E. Jones | IETF avt WG 836 paulej@packetizer.com | 838 9. Authors' Addresses 840 Gunnar Hellstrom 841 Omnitor AB 842 Renathvagen 2 843 SE-121 37 Johanneshov 844 Sweden 845 Phone: +46 708 204 288 / +46 8 556 002 03 846 Fax: +46 8 556 002 06 847 E-mail: gunnar.hellstrom@omnitor.se 849 Paul E. Jones 850 Cisco Systems, Inc. 852 RTP Payload for Text Conversation October 2003 854 7025 Kit Creek Rd. 855 Research Triangle Park, NC 27709 856 Phone: +1 919 392 6948 857 E-mail: paulej@packetizer.com 859 10. Acknowledgements 861 The authors want to thank Stephen Casner and Colin Perkins for 862 valuable support with reviews and advice on creation of this 863 document, to Mickey Nasiri at Ericsson Mobile Communication for 864 providing the development environment, and Michele Mizarro for 865 verification of the usability of the payload format for its intended 866 purpose. 868 11. Normative References 870 [1] ITU-T Recommendation T.140 (1998) - Text conversation protocol 871 for multimedia application, with amendment 1, (2000). 873 [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, 874 "RTP: A Transport Protocol for Real-Time Applications", RFC 875 3550, July 2003. 877 [3] Perkins, C., Kouvelas, I., Hardman, V., Handley, M. and J. 878 Bolot, "RTP Payload for Redundant Audio Data", RFC 2198, 879 September 1997. 881 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement 882 Levels", BCP 14, RFC 2119, March 1997. 884 [5] ISO/IEC 10646-1: (1993), Universal Multiple Octet Coded 885 Character Set. 887 [6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 888 2279, January 1998. 890 [7] Handley, M., Jacobson, V., "SDP: Session Description Protocol", 891 RFC 2327, April 1998. 893 12. Informative References 895 [8] Schulzrinne, H., Petrack, S., "RTP Payload for DTMF Digits, 896 Telephony Tones and Telephony Signals", May 2000. 898 13. Intellectual Property Right Considerations 900 The IETF takes no position regarding the validity or scope of any 901 intellectual property or other rights that might be claimed to 902 pertain to the implementation or use of the technology described in 903 RTP Payload for Text Conversation October 2003 905 this document or the extent to which any license under such rights 906 might or might not be available; neither does it represent that it 907 has made any effort to identify any such rights. Information on the 908 IETF's procedures with respect to rights in standards-track and 909 standards-related documentation can be found in BCP-11. 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