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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (August 2004) is 7187 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) == Unused Reference: '12' is defined on line 841, but no explicit reference was found in the text == Unused Reference: '15' is defined on line 851, but no explicit reference was found in the text -- Possible downref: Non-RFC (?) normative reference: ref. '1' -- Possible downref: Non-RFC (?) normative reference: ref. '5' ** Obsolete normative reference: RFC 2327 (ref. '7') (Obsoleted by RFC 4566) ** Obsolete normative reference: RFC 2733 (ref. '8') (Obsoleted by RFC 5109) -- Obsolete informational reference (is this intentional?): RFC 2833 (ref. '15') (Obsoleted by RFC 4733, RFC 4734) -- Obsolete informational reference (is this intentional?): RFC 2793 (ref. '16') (Obsoleted by RFC 4103) Summary: 8 errors (**), 0 flaws (~~), 5 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group G. Hellstrom 3 Internet Draft Omnitor AB 4 P. Jones 5 Expires: February 2005 Cisco Systems, Inc. 6 August 2004 8 RTP Payload for Text Conversation 10 Status of this Memo 12 By submitting this Internet-Draft, we certify that any applicable 13 patent or other IPR claims of which we are aware have been 14 disclosed, and any of which we become aware will be disclosed, in 15 accordance with RFC 3668 (BCP 79). 17 By submitting this Internet-Draft, we accept the provisions of 18 Section 3 of RFC 3667 (BCP 78). 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six 26 months and may be updated, replaced, or obsoleted by other 27 documents at any time. It is inappropriate to use Internet-Drafts 28 as reference material or cite them other than as "work in 29 progress". 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html 37 This document is a submission of the IETF AVT WG. Comments should 38 be directed to the AVT WG mailing list, avt@ietf.org. 40 Abstract 42 This memo describes how to carry real time text conversation 43 session contents in RTP packets. Text conversation session contents 44 are specified in ITU-T Recommendation T.140. 46 One payload format is described for transmitting text on a separate 47 RTP session dedicated for the transmission of text. 49 This RTP payload description recommends a method to include 50 redundant text from already transmitted packets in order to reduce 51 the risk of text loss caused by packet loss. 53 Table of Contents 55 1. Introduction...................................................3 56 2. Conventions used in this document..............................4 57 3. Usage of RTP...................................................4 58 3.1 Motivations and rationale..................................4 59 3.2 Payload Format for Transmission of text/t140 Data..........4 60 3.3 The "T140block"............................................4 61 3.4 Synchronization of Text with Other Media...................5 62 3.5 RTP packet header..........................................5 63 4. Protection against loss of data................................6 64 4.1 Payload Format when using Redundancy.......................6 65 4.2 Using redundancy with the text/t140 format.................6 66 5. Recommended Procedure..........................................7 67 5.1 Recommended Basic Procedure................................7 68 5.2 Transmission before and after "Idle Periods"...............8 69 5.3 Detection of Lost Text Packets.............................8 70 5.4 Compensation for Packets Out of Order......................9 71 6. Parameter for Character Transmission Rate......................9 72 7. Examples......................................................10 73 7.1 RTP Packetization Examples for the text/t140 format.......10 74 7.2 SDP Examples..............................................12 75 8. Security Considerations.......................................12 76 8.1 Confidentiality...........................................13 77 8.2 Integrity.................................................13 78 8.3 Source authentication.....................................13 79 9. Congestion Considerations.....................................13 80 10. IANA considerations..........................................15 81 10.1 Registration of MIME Media Type text/t140................15 82 10.2 SDP mapping of MIME parameters...........................16 83 10.3 Offer/Answer Consideration...............................16 84 11. Authors' Addresses...........................................16 85 12. Acknowledgements.............................................17 86 13. Normative References.........................................17 87 14. Informative References.......................................18 88 15. Intellectual Property Statement..............................18 89 16. Copyright Statement..........................................18 91 [Notes to RFC Editor: 92 1. All references to RFC XXXX are to be replaced by references to 93 the RFC number of this memo, when published. 94 2. All references to RFC YYYY [9] are to be replaced by references 95 to the document that registers the text/red MIME type.] 97 1. Introduction 99 This document defines a payload type for carrying text conversation 100 session contents in RTP [2] packets. Text conversation session 101 contents are specified in ITU-T Recommendation T.140 [1]. Text 102 conversation is used alone or in connection to other conversational 103 facilities such as video and voice, to form multimedia conversation 104 services. Text in multimedia conversation sessions is sent 105 character-by-character as soon as it is available, or with a small 106 delay for buffering. 108 The text is intended to be entered by human users from a keyboard, 109 handwriting recognition, voice recognition or any other input 110 method. The rate of character entry is usually at a level of a few 111 characters per second or less. In general, only one or a few new 112 characters are expected to be transmitted with each packet. Small 113 blocks of text may be prepared by the user and pasted into the user 114 interface for transmission during the conversation, occasionally 115 causing packets to carry more payload. 117 T.140 specifies that text and other T.140 elements must be 118 transmitted in ISO 10646-1[5] code with UTF-8 [6] transformation. 119 That makes it easy to implement internationally useful applications 120 and to handle the text in modern information technology 121 environments. The payload of an RTP packet following this 122 specification consists of text encoded according to T.140 without 123 any additional framing. A common case will be a single ISO 10646 124 character, UTF-8 encoded. 126 T.140 requires the transport channel to provide characters without 127 duplication and in original order. Text conversation users expect 128 that text will be delivered with no or a low level of lost 129 information. 131 Therefore a mechanism based on RTP is specified here. It gives text 132 arrival in correct order, without duplication, and with detection 133 and indication of loss. It also includes an optional possibility to 134 repeat data for redundancy to lower the risk of loss. Since packet 135 overhead is usually much larger than the T.140 contents, the 136 increase in bandwidth with the use of redundancy is minimal. 138 By using RTP for text transmission in a multimedia conversation 139 application, uniform handling of text and other media can be 140 achieved in, as examples, conferencing systems, firewalls, and 141 network translation devices. This, in turn, eases the design and 142 increases the possibility for prompt and proper media delivery. 144 This document obsoletes RFC 2793 [16]. The text clarifies 145 ambiguities in RFC 2793, improves on the specific implementation 146 requirements learned through development experience and gives 147 explicit usage examples. 149 2. Conventions used in this document 151 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 152 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in 153 this document are to be interpreted as described in RFC 2119 [4]. 155 3. Usage of RTP 157 The payload format for real-time text transmission with RTP [2] 158 described in this memo is intended for general text conversation 159 use and called text/t140 after its MIME registration. 161 3.1 Motivations and rationale 163 The text/t140 format is intended to be used for text transmitted on 164 a separate RTP session dedicated for the transmission of text and 165 not shared with other media. 167 The text/t140 format MAY be used for any non-gateway application as 168 well as in gateways. It MAY be used simultaneously with other media 169 streams, transmitted as a separate RTP session, as required in real 170 time multimedia applications. 172 The text/t140 format specified in this memo is compatible with its 173 earlier definition in RFC2793. It is just refined, with the main 174 intention to minimize interoperability problems and encourage good 175 reliability and functionality. 177 By specifying text transmission as a text medium, many good effects 178 are gained. Routing, device selection, invocation of transcoding, 179 selection of quality of service parameters and other high and low 180 level functions are depending on each medium being explicitly 181 specified. 183 3.2 Payload Format for Transmission of text/t140 Data 185 A text/t140 conversation RTP payload format consists of one and 186 only one block of T.140 data, referred to as a "T140block" (see 187 section 3.3). There are no additional headers specific to this 188 payload format. The fields in the RTP header are set as defined in 189 section 3.5. 191 3.3 The "T140block" 193 T.140 text is UTF-8 coded as specified in T.140 with no extra 194 framing. The T140block contains one or more T.140 code elements as 195 specified in [1]. Most T.140 code elements are single ISO 10646 196 [5] characters, but some are multiple character sequences. Each 197 character is UTF-8 encoded [6] into one or more octets. Each block 198 MUST contain an integral number of UTF-8 encoded characters 199 regardless of the number of octets per character. Any composite 200 character sequence (CCS) SHOULD be placed within one block. 202 3.4 Synchronization of Text with Other Media 204 Usually, each medium in a session utilizes a separate RTP stream. 205 As such, if synchronization of the text and other media packets is 206 important, the streams MUST be associated when the sessions are 207 established and the streams MUST share the same reference clock 208 (refer to the description of the timestamp field as it relates to 209 synchronization in section 5.1 of RFC 3550 [2]). Association of 210 RTP streams can be done through the CNAME field of RTCP SDES 211 function. It is dependent on the particular application and is 212 outside the scope of this document. 214 3.5 RTP packet header 216 Each RTP packet starts with a fixed RTP header. The following 217 fields of the RTP fixed header are specified for T.140 text 218 streams: 220 Payload Type (PT): The assignment of an RTP payload type is 221 specific to the RTP profile under which this payload format is 222 used. For profiles that use dynamic payload type number 223 assignment, this payload format can be identified by the MIME 224 type "text/t140" (see section 10). If redundancy is used per RFC 225 2198, another payload type number needs to be provided for the 226 redundancy format. The MIME type for identifying RFC 2198 is 227 available in RFC YYYY [9]. 229 Sequence number: The definition of sequence numbers is available in 230 RFC 3550 [2]. When transmitting text using the payload format for 231 text/t140, it is used for detection of packet loss and packets 232 out of order, and can be used in the process of retrieval of 233 redundant text, reordering of text and marking missing text. 235 Timestamp: The RTP Timestamp encodes the approximate instance of 236 entry of the primary text in the packet. A clock frequency of 237 1000 Hz MUST be used. Sequential packets MUST NOT use the same 238 timestamp. Since packets do not represent any constant duration, 239 the timestamp cannot be used to directly infer packet loss. 241 M-bit: The M-bit MUST be included. The first packet in a session, 242 and the first packet after an idle period, SHOULD be 243 distinguished by setting the marker bit in the RTP data header to 244 one. The marker bit in all other packets MUST be set to zero. 245 The reception of the marker bit MAY be used for refined methods 246 for detection of loss. 248 4. Protection against loss of data 250 Consideration must be devoted to keeping loss of text caused by 251 packet loss within acceptable limits. (See ITU-T F.703 [17]) 253 The default method that MUST be used when no other method is 254 explicitly selected is redundancy in accordance with RFC 2198 [3]. 255 When this method is used, the original text and two redundant 256 generations SHOULD be transmitted if the application or end-to-end 257 conditions do not call for other levels of redundancy to be used. 259 Forward Error Correction mechanisms as per RFC 2733 [8] or any 260 other mechanism with the purpose of increasing the reliability of 261 text transmission MAY be used as an alternative or complement to 262 redundancy. Text data MAY be sent without additional protection if 263 end-to-end network conditions allow the text quality requirements 264 specified in ITU-T F.703 [17] to be met in all anticipated load 265 conditions. 267 4.1 Payload Format when using Redundancy 269 When using the payload format with redundant data, the transmitter 270 may select a number of T140block generations to retransmit in each 271 packet. A higher number introduces better protection against loss 272 of text but marginally increases the data rate. 274 The RTP header is followed by one or more redundant data block 275 headers, one for each redundant data block to be included. Each of 276 these headers provides the timestamp offset and length of the 277 corresponding data block plus a payload type number indicating the 278 payload format text/t140. 280 After the redundant data block headers follows the redundant data 281 fields carrying T140blocks from previous packets, and finally the 282 new (primary) T140block for this packet. 284 Redundant data that would need a timestamp offset higher than 16383 285 due to its age at transmission MUST NOT be included in transmitted 286 packets. 288 4.2 Using redundancy with the text/t140 format. 290 Since text is transmitted only when there is text to transmit, the 291 timestamp is not used to identify a lost packet. Rather, missing 292 sequence numbers are used to detect lost text packets at reception. 293 Also, since sequence numbers are not provided in the redundant 294 header, some additional rules must be followed to allow the 295 redundant data corresponding to missing primary data to be merged 296 properly into the stream of primary data T140blocks. 297 They are: 299 - Each redundant data block MUST contain the same data as a 300 T140block previously transmitted as primary data. 301 - The redundant data MUST be placed in age order with most 302 recent redundant T140block last in the redundancy area. 303 - All T140blocks from the oldest desired generation up through 304 the generation immediately preceding the new (primary) 305 T140block MUST be included. 307 These rules allow the sequence numbers for the redundant T140blocks 308 to be inferred by counting backwards from the sequence number in 309 the RTP header. The result will be that all the text in the 310 payload will be contiguous and in order. 312 If there is a gap in the received RTP sequence numbers, and 313 redundant T140blocks are available in a subsequent packet, the 314 sequence numbers for the redundant T140blocks should be inferred by 315 counting backwards from the sequence number in the RTP header for 316 that packet. If there are redundant T140blocks with sequence 317 numbers matching those that are missing, the redundant T140blocks 318 may be substituted for the missing T140blocks. 320 5. Recommended Procedure 322 This section contains RECOMMENDED procedures for usage of the 323 payload format. Based on the information in the received packets, 324 the receiver can: 326 - reorder text received out of order. 327 - mark where text is missing because of packet loss. 328 - compensate for lost packets by using redundant data. 330 5.1 Recommended Basic Procedure 332 Packets are transmitted when there is valid T.140 data to transmit. 334 T.140 specifies that T.140 data MAY be buffered for transmission 335 with a maximum buffering time of 500 ms. A buffering time of 300 ms 336 is RECOMMENDED, when the application or end-to-end network 337 conditions are not known to require another value. 339 If no new data is available for a longer period than the buffering 340 time, the transmission process is in an idle period. 342 When new text is available for transmission after an idle period, 343 it is RECOMMENDED to send it as soon as possible. After this 344 transmission, it is RECOMMENDED to buffer T.140 data in buffering 345 time intervals, until next idle period. This is done in order to 346 keep the maximum bit rate usage for text at a reasonable level. The 347 buffering time MUST be selected so that text users will perceive a 348 real time text flow. 350 5.2 Transmission before and after "Idle Periods". 352 When valid T.140 data has been sent and no new T.140 data is 353 available for transmission after the selected buffering time, an 354 empty T140block SHOULD be transmitted. This situation is regarded 355 to be the beginning of an idle period. The procedure is recommended 356 in order to more rapidly detect potentially missing text before an 357 idle period. 359 An empty T140block contains no data. 361 When redundancy is used, transmission continues with a packet at 362 every transmission timer expiration and insertion of an empty 363 T.140block as primary, until the last non-empty T140block has been 364 transmitted as primary and as redundant data with all intended 365 generations of redundancy. The last packet before an idle period 366 will contain only one non-empty T140block as redundant data, while 367 the remainder of the redundancy packet will contain empty 368 T140blocks. 370 Any empty T140block that is sent as primary data MUST be included 371 as redundant T140blocks in subsequent packets just as normal text 372 T140blocks would be, unless the empty T140block is too old to be 373 transmitted. This is done so that sequence number inference for the 374 redundant T140blocks will be correct, as explained in section 4.2. 376 After an idle period, the transmitter SHOULD set the M-bit to one 377 in the first packet with new text. 379 5.3 Detection of Lost Text Packets 381 Packet loss for text/t140 packets MAY be detected by observing gaps 382 in the sequence numbers of RTP packets received by the receiver. 384 With text/t140 the loss of packets is usually detected by 385 comparison of the sequence of RTP packets as they arrive. Any 386 discrepancy MAY be used to indicate loss. The highest RTP sequence 387 number received may also be compared with that in RTCP reports, as 388 an additional check for loss of the last packet before an idle 389 period. 391 Missing data SHOULD be marked by insertion of a missing text marker 392 in the received stream for each missing T140block, as specified in 393 ITU-T T.140 Addendum 1 [1]. 395 Since empty T140blocks are transmitted in the beginning of an idle 396 period, there is a slight risk of falsely marking loss of text, 397 when only an empty T140block was lost and when using text/t140. 398 Procedures based on detection of the packet with the M-bit set to 399 one MAY be used to reduce the risk for introducing false markers of 400 loss. 402 If redundancy is used with the text/t140 format, and a packet is 403 received with fewer redundancy levels than normally in the session, 404 it SHOULD be treated as if one empty T140block has been received 405 for each excluded level in the received packet. This is because the 406 only occasion when a T140block is excluded from transmission is 407 when it is an empty T140block that has become too old to be 408 transmitted. 410 If two successive packets have the same number of redundant 411 generations, it SHOULD be treated as the general redundancy level 412 for the session. Change of the general redundancy level SHOULD only 413 be done after an idle period. 415 The text/t140 format relies on use of the sequence number in the 416 RTP packet header for detection of loss and is therefore not 417 suitable for an application where it needs to be alternating with 418 other payloads in the same RTP stream. It would be complicated and 419 unreliable to try to detect loss of data at the edges of the shifts 420 between t140 text and other stream contents. It is therefore 421 RECOMMENDED to be the only payload type in the RTP stream. 423 5.4 Compensation for Packets Out of Order 425 For protection against packets arriving out of order, the following 426 procedure MAY be implemented in the receiver. If analysis of a 427 received packet reveals a gap in the sequence and no redundant data 428 is available to fill that gap, the received packet SHOULD be kept 429 in a buffer to allow time for the missing packet(s) to arrive. It 430 is RECOMMENDED that the waiting time be limited to 1 second. 432 If a packet with a T140block belonging to the gap arrives before 433 the waiting time expires, this T140block is inserted into the gap 434 and then consecutive T140blocks from the leading edge of the gap 435 may be consumed. Any T140block which does not arrive before the 436 time limit expires should be treated as lost and a missing text 437 marker inserted ( see section 5.3 ). 439 6. Parameter for Character Transmission Rate 441 In some cases, it is necessary to limit the rate at which 442 characters are transmitted. For example, when a PSTN gateway is 443 interworking between an IP device and a PSTN textphone, it may be 444 necessary to limit the character rate from the IP device in order 445 to avoid throwing away characters in case of buffer overflow at the 446 PSTN gateway. 448 To control the character transmission rate, the MIME parameter 449 "cps" in the "fmtp" attribute [7] is defined (see section 10 ). It 450 is used in SDP with the following syntax: 452 a=fmtp: cps= 454 The field is populated with the payload type that is used 455 for text. The field contains an integer representing the 456 maximum number of characters that may be received per second. The 457 value shall be used as a mean value over any 10 second interval. 458 The default value is 30. 460 Examples of use in SDP are found in section 7.2. 462 In receipt of this parameter, devices MUST adhere to the request by 463 transmitting characters at a rate at or below the specified 464 value. Note that this parameter was not defined in 465 RFC 2793 [16]. Therefore implementations of the text/t140 format 466 may be in use that do not recognize and act according to this 467 parameter. Receivers of text/t140 SHALL therefore be designed so 468 that they can handle temporary reception of characters at a higher 469 rate than this parameter specifies, so that malfunction because of 470 buffer overflow is avoided for text conversation with human input. 472 7. Examples 474 7.1 RTP Packetization Examples for the text/t140 format. 476 Below is an example of a text/t140 RTP packet without redundancy. 477 0 1 2 3 478 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 1 479 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 480 |V=2|P|X| CC=0 |M| T140 PT | sequence number | 481 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 482 | timestamp (1000Hz) | 483 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 484 | synchronization source (SSRC) identifier | 485 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 486 | T.140 encoded data | 487 + +---------------+ 488 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 490 Below is an example of a text/t140 RTP packet with one redundant 491 T140block. 493 0 1 2 3 494 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 1 495 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 496 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 497 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 498 | timestamp of primary encoding "P" | 499 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 500 | synchronization source (SSRC) identifier | 501 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 502 |1| T140 PT | timestamp offset of "R" | "R" block length | 503 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 504 |0| T140 PT | "R" T.140 encoded redundant data | 505 +-+-+-+-+-+-+-+-+ +---------------+ 506 + | | 507 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+ 508 | "P" T.140 encoded primary data | 509 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 511 Below is an example of an RTP packet with one redundant T140block 512 using text/t140 payload format. The primary data block is 513 empty, which is the case when transmitting a packet for the 514 sole purpose of forcing the redundant data to be transmitted 515 in the absence of any new data. 517 0 1 2 3 518 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 1 519 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 520 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 521 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 522 | timestamp of primary encoding "P" | 523 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 524 | synchronization source (SSRC) identifier | 525 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 526 |1| T140 PT | timestamp offset of "R" | "R" block length | 527 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 528 |0| T140 PT | "R" T.140 encoded redundant data | 529 +-+-+-+-+-+-+-+-+ +---------------+ 530 | | 531 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 533 As a follow-on to the previous example, the example below shows 534 the next RTP packet in the sequence which does contain a real 535 T140block when using the text/t140 payload format. Note that the 536 empty block is present in the redundant transmissions of the 537 text/t140 payload format. This example shows 2 levels of 538 redundancy and one primary data block. The value of the "R2 539 block length" would be set to zero in order to 540 represent the empty T140block. 542 0 1 2 3 543 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 1 544 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 545 |V=2|P|X| CC=0 |M| "RED" PT | sequence number of primary | 546 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 547 | timestamp of primary encoding "P" | 548 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 549 | synchronization source (SSRC) identifier | 550 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 551 |1| T140 PT | timestamp offset of "R2" | "R2" block length | 552 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 553 |1| T140 PT | timestamp offset of "R1" | "R1" block length | 554 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 555 |0| T140 PT | "R1" T.140 encoded redundant data | 556 +-+-+-+-+-+-+-+-+ +---------------+ 557 | | | 558 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+ 559 | "P" T.140 encoded primary data | 560 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 562 7.2 SDP Examples 564 Below is an example of SDP describing RTP text transport on port 565 11000: 567 m=text 11000 RTP/AVP 98 568 a=rtpmap:98 t140/1000 570 Below is an example of SDP similar to the above example, but also 571 utilizing RFC 2198 to provide the recommended two levels of 572 redundancy for the text packets: 574 m=text 11000 RTP/AVP 98 100 575 a=rtpmap:98 t140/1000 576 a=rtpmap:100 red/1000 577 a=fmtp:100 98/98/98 579 Note - While these examples utilize the RTP/AVP profile, it is not 580 intended to limit the scope of this memo to use with only that 581 profile. Rather, any appropriate profile may be used in 582 conjunction with this memo. 584 8. Security Considerations 586 All of the security considerations from section 14 of RFC 3550 [2] 587 apply. 589 8.1 Confidentiality 591 Since the intention of the described payload format is to carry 592 text in a text conversation, security measures in the form of 593 encryption are of importance. The amount of data in a text 594 conversation session is low and therefore any encryption method MAY 595 be selected and applied to T.140 session contents or to the whole 596 RTP packets. SRTP [14] provides a suitable method for ensuring 597 confidentiality. 599 8.2 Integrity 601 It may be desirable to protect the text contents of an RTP stream 602 against manipulation. SRTP [14] provides methods for providing 603 integrity that MAY be applied. 605 8.3 Source authentication 607 Measures to make sure that the source of text is the intended one 608 can be accomplished by a combination of methods. 610 Text streams are usually used in a multimedia control environment. 611 Security measures for authentication are available and SHOULD be 612 applied in the registration and session establishment procedures, 613 so that the identity of the sender of the text stream is reliably 614 associated with the person or device setting up the session. Once 615 established, SRTP [14] mechanisms MAY be applied to ascertain that 616 the source is maintained the same during the session. 618 9. Congestion Considerations 620 The congestion considerations from section 10 of RFC 3550 [2], 621 section 6 of RFC 2198 [3] and any used profile, e.g. the section 622 about congestion in chapter 2 of RFC 3551 [11] apply with the 623 following application specific considerations. 625 Automated systems MUST NOT use this format to send large amounts of 626 text at a rate significantly above that which a human user could 627 enter. 629 Even if the network load from users of text conversation is usually 630 very low, for best-effort networks an application MUST monitor the 631 packet loss rate and take appropriate actions to reduce its sending 632 rate if this application sends at higher rate than what TCP would 633 achieve over the same path. The reason is that this application, 634 due to its recommended usage of two or more redundancy levels, is 635 very robust against packet loss. At the same time, due to the low 636 bit-rate of text conversations, if one considers the discussion in 637 RFC 3714 [13], this application will experience very high packet 638 loss rates before it needs to perform any reduction in the sending 639 rate. 641 If the application needs to reduce its sending rate, it SHOULD NOT 642 reduce the number of redundancy levels below the default amount 643 specified in section 4. Instead, the following actions are 644 RECOMMENDED in order of priority: 646 - Increase the shortest time between transmissions described in 647 section 5.1 from the recommended 300 ms to 500 ms that is the 648 highest value allowable according to T.140. 650 - Limit the maximum rate of characters transmitted. 652 - Increase the shortest time between transmissions to a higher 653 value, not higher than 5 seconds. This will cause unpleasant 654 delays in transmission, beyond what is allowed according to 655 T.140, but text will still be conveyed in the session with some 656 usability. 658 - Exclude participants from the session. 660 Please note that if the reduction in bit-rate achieved through the 661 above measures are not sufficient, the only remaining action is to 662 terminate the session. 664 As guidance, some load figures are provided here as examples based 665 on use of IPv4, including the load from IP, UDP and RTP headers 666 without compression. 668 -Experience tells that a common mean character transmission rate 669 during a complete PSTN text telephony session in reality is around 670 2 characters per second. 672 -A maximum performance of 20 characters per second is enough even 673 for voice to text applications. 675 -With the (unusually high) load of 20 characters per second, in a 676 language that make use of three octets UTF-8 characters, two 677 redundant levels and 300 ms between transmissions, the maximum load 678 of this application is 3300 bits/s. 680 -When the restrictions mentioned above are applied, limiting 681 transmission to 10 characters per second, using 5 s between 682 transmissions, the maximum load of this application in a language 683 that uses one octet per UTF-8 character is 300 bits/s. 685 Note also, that this payload can be used in a congested situation 686 as a last resort to maintain some contact when audio and video 687 media need to be stopped. The availability of one low bit-rate 688 stream for text in such adverse situations may be crucial for 689 maintaining some communication in a critical situation. 691 10. IANA considerations 693 This document defines one RTP payload format named "t140" and an 694 associated MIME type "text/t140", to be registered by IANA. 696 10.1 Registration of MIME Media Type text/t140 698 MIME media type name: text 700 MIME subtype name: t140 702 Required parameters: 703 rate: The RTP timestamp clock rate, which is equal to the 704 sampling rate. The only valid value is 1000. 706 Optional parameters: 707 cps: The maximum number of characters that may be received 708 per second. The deafult value is 30. 710 Encoding considerations: T.140 text can be transmitted with RTP 711 as specified in RFC XXXX. 713 Security considerations: See section 8 of RFC XXXX. 715 Interoperability considerations: This format is the same as 716 specified in RFC2793. For RFC2793 the "cps=" parameter was not 717 defined. Therefore there may be implementations that do not 718 consider this parameter. Receivers need to take that into 719 account. 721 Published specification: ITU-T T.140 Recommendation. 722 RFC XXXX. 724 Applications which use this media type: 725 Text communication terminals and text conferencing tools. 727 Additional information: This type is only defined for transfer 728 via RTP. 730 Magic number(s): None 731 File extension(s): None 732 Macintosh File Type Code(s): None 734 Person & email address to contact for further information: 735 Gunnar Hellstrom 736 E-mail: gunnar.hellstrom@omnitor.se 738 Intended usage: COMMON 740 Author / Change controller: 741 Gunnar Hellstrom | IETF avt WG 742 gunnar.hellstrom@omnitor.se | 744 10.2 SDP mapping of MIME parameters 746 The information carried in the MIME media type specification has a 747 specific mapping to fields in the Session Description Protocol 748 (SDP) [7], which is commonly used to describe RTP sessions. When 749 SDP is used to specify sessions employing the text/t140 format, the 750 mapping is as follows: 752 - The MIME type ("text" ) goes in SDP "m=" as the media name. 754 - The MIME subtype (payload format name) goes in SDP "a=rtpmap" 755 as the encoding name. The RTP clock rate in "a=rtpmap" MUST be 756 1000 for text/t140. 758 - The parameter "cps" goes in SDP "a=fmtp" attribute. 760 - When the payload type is used with redundancy according to 761 RFC 2198, the level of redundancy is shown by the number of 762 elements in the slash-separated payload type list in the 763 "fmtp" parameter of the redundancy declaration as defined in 764 RFC YYYY [9] and RFC 2198 [3]. 766 10.3 Offer/Answer Consideration 768 In order to achieve interoperability within the framework of the 769 offer/answer model [10], the following consideration should be 770 made: 772 - The "cps" parameter is declarative. Both sides may provide a 773 value, which is independent of the other side. 775 11. Authors' Addresses 777 Gunnar Hellstrom 778 Omnitor AB 779 Renathvagen 2 780 SE-121 37 Johanneshov 781 Sweden 782 Phone: +46 708 204 288 / +46 8 556 002 03 783 Fax: +46 8 556 002 06 784 E-mail: gunnar.hellstrom@omnitor.se 786 Paul E. Jones 787 Cisco Systems, Inc. 788 7025 Kit Creek Rd. 789 Research Triangle Park, NC 27709 790 USA 791 Phone: +1 919 392 6948 792 E-mail: paulej@packetizer.com 794 12. Acknowledgements 796 The authors want to thank Stephen Casner, Magnus Westerlund and 797 Colin Perkins for valuable support with reviews and advice on 798 creation of this document, to Mickey Nasiri at Ericsson Mobile 799 Communication for providing the development environment, Michele 800 Mizarro for verification of the usability of the payload format for 801 its intended purpose, and Andreas Piirimets for editing support and 802 validation. 804 13. Normative References 806 [1] ITU-T Recommendation T.140 (1998) - Text conversation protocol 807 for multimedia application, with amendment 1, (2000). 809 [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, 810 "RTP: A Transport Protocol for Real-Time Applications", RFC 811 3550, July 2003. 813 [3] Perkins, C., Kouvelas, I., Hardman, V., Handley, M. and J. 814 Bolot, "RTP Payload for Redundant Audio Data", RFC 2198, 815 September 1997. 817 [4] Bradner, S., "Key words for use in RFCs to Indicate 818 Requirement Levels", BCP 14, RFC 2119, March 1997. 820 [5] ISO/IEC 10646-1: (1993), Universal Multiple Octet Coded 821 Character Set. 823 [6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", 824 RFC 3629, December 2003. 826 [7] Handley, M., Jacobson, V., "SDP: Session Description 827 Protocol", RFC 2327, April 1998. 829 [8] Rosenberg, J., Schulzrinne, H., "An RTP Payload Format for 830 Generic Forward Error Correction", RFC 2733, December 1999. 832 [9] Jones, P. , "Registration of the text/red MIME Sub-Type", 833 draft-ietf-avt-text-red, RFC YYYY, 2004. 835 [10] Rosenberg, J., Schulzrinne, H., "An Offer/Answer Model with 836 the Session Description Protocol (SDP)", RFC 3264, June 2002. 838 [11] Schultzrinne, J., Perkins, C., "RTP Profile for Audio and 839 Video Conference with Minimal Control", RFC 3551, July 2003. 841 [12] Postel, J.,"Internet Protocol", RFC 791, 1981. 843 14. Informative References 845 [13] Floyd, S., Kempf, J., IAB Concerns Regarding Congestion 846 Control for Voice Traffic in the Internet, RFC 3714,March 2004 848 [14] Baugher, McGrew, Carrara, Naslund, Norrman, The Secure Real- 849 Time Transport Protocol (SRTP), RFC 3711, March 2004. 851 [15] Schulzrinne, H., Petrack, S., "RTP Payload for DTMF Digits, 852 Telephony Tones and Telephony Signals", RFC 2833, May 2000. 854 [16] Hellstrom, G., "RTP Payload for text conversation.", RFC2793, 855 2000 857 [17] ITU-T Recommendation F.703, Multimedia Conversational 858 Services, Nov 2000. 860 15. 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