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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) -- Possible downref: Non-RFC (?) normative reference: ref. '4' ** Obsolete normative reference: RFC 4288 (ref. '9') (Obsoleted by RFC 6838) ** Obsolete normative reference: RFC 4566 (ref. '10') (Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 2326 (ref. '15') (Obsoleted by RFC 7826) Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Z. Fang 3 Internet-Draft Qualcomm Incorporated 4 Intended status: Standards Track January 20, 2013 5 Expires: July 24, 2013 7 RTP payload format for Enhanced Variable Rate Narrowband-Wideband Codec 8 (EVRC-NW) 9 draft-ietf-avt-rtp-evrc-nw-10 11 Abstract 13 This document specifies real-time transport protocol (RTP) payload 14 formats to be used for the Enhanced Variable Rate Narrowband-Wideband 15 Codec (EVRC-NW). Three media type registrations are included for 16 EVRC-NW RTP payload formats. In addition, a file format is specified 17 for transport of EVRC-NW speech data in storage mode applications 18 such as e-mail. 20 Status of this Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on July 24, 2013. 37 Copyright Notice 39 Copyright (c) 2013 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 4. EVRC-NW codec . . . . . . . . . . . . . . . . . . . . . . . . 6 58 5. RTP header usage . . . . . . . . . . . . . . . . . . . . . . . 7 59 6. Payload format . . . . . . . . . . . . . . . . . . . . . . . . 8 60 6.1. Encoding capability identification in EVRC-NW 61 interleaved/bundled format . . . . . . . . . . . . . . . . 8 62 7. Congestion Control Considerations . . . . . . . . . . . . . . 11 63 8. Storage format for the EVRC-NW Codec . . . . . . . . . . . . . 12 64 9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 13 65 9.1. Media Type Registrations . . . . . . . . . . . . . . . . . 13 66 9.1.1. Registration of Media Type audio/EVRCNW . . . . . . . 13 67 9.1.2. Registration of Media Type audio/EVRCNW0 . . . . . . . 15 68 9.1.3. Registration of Media Type audio/EVRCNW1 . . . . . . . 16 69 10. SDP mode attributes for EVRC-NW . . . . . . . . . . . . . . . 19 70 11. Mode Change Request/Response Considerations . . . . . . . . . 20 71 12. Mapping EVRC-NW media type parameters into SDP . . . . . . . . 22 72 13. Offer-Answer Model Considerations for EVRC-NW . . . . . . . . 23 73 14. Declarative SDP Considerations . . . . . . . . . . . . . . . . 25 74 15. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 75 16. Security Considerations . . . . . . . . . . . . . . . . . . . 29 76 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 77 17.1. Normative References . . . . . . . . . . . . . . . . . . . 30 78 17.2. Informative References . . . . . . . . . . . . . . . . . . 31 79 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32 81 1. Introduction 83 This document specifies the payload formats for packetization of 84 EVRC-NW encoded speech signals into the real-time transport protocol 85 (RTP). It defines support for the header-free, interleaved/bundled, 86 and compact bundle packet formats for the EVRC-NW codec as well as 87 discontinuous transmission (DTX) support for EVRC-NW encoded speech 88 transported via RTP. The EVRC-NW codec offers better speech quality 89 than the EVRC and EVRC-B codecs and better capacity than EVRC-WB 90 codec. EVRC-NW belongs to the EVRC family of codecs. 92 2. Conventions 94 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 95 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 96 document are to be interpreted as described in RFC 2119 [1]. 98 3. Background 100 EVRC-NW is an extension of both the EVRC-B [2] and EVRC-WB [3] speech 101 codecs developed in 3GPP2 with support for discontinuous transmission 102 (DTX). It provides enhanced voice quality and high spectral 103 efficiency. 105 The EVRC-NW codec operates on 20 ms frames, and the default sampling 106 rate is 16 kHz (wideband). Input and output at 8 kHz sampling rate 107 (narrowband) is also supported. The EVRC-NW codec can operate in 108 eight modes (0 to 7) defined in [4]. EVRC-NW modes 0, 1, and 7 are 109 interoperable with EVRC-WB. EVRC-NW modes 1 to 7 are interoperable 110 with EVRC-B. EVRC-NW modes 0 to 6 use the full set or a subset of 111 full rate, 1/2 rate, 1/4 rate and 1/8 rate frames. EVRC-NW mode 7 112 uses only 1/2 rate and 1/8 rate frames. By default, EVRC-NW supports 113 all narrowband modes (modes 1 to 7). The support of wideband mode 114 (mode 0) is optional. Mode change among modes 1 to 7 (or among modes 115 0 to 7 if the receiver supports wideband mode) results in codec 116 output bit-rate change but does not cause any decoding problems at 117 the receiver. EVRC-NW provides a standardized solution for 118 packetized voice applications that allow transitions between enhanced 119 quality and increased capacity. The most important service addressed 120 is IP telephony. Target devices can be IP phones or VoIP handsets, 121 media gateways, voice messaging servers, etc. 123 4. EVRC-NW codec 125 The EVRC-NW codec operates on 20 ms frames. It produces output 126 frames of one of the four different sizes: 171 bits (Rate 1), 80 bits 127 (Rate 1/2), 40 bits (Rate 1/4), or 16 bits (Rate 1/8). In addition, 128 there are two zero-bit codec frame types: blank (null) frames and 129 erasure frames. The default sampling rate is 16 kHz. Input and 130 output at 8 kHz sampling rate is also supported. 132 The frame type values and sizes of the associated codec data frames 133 are listed in the table below: 135 Value Rate Total codec data frame size in bytes (and in bits) 136 -------------------------------------------------------------------- 137 0 Blank (Null) 0 (0 bits) 138 1 1/8 2 (16 bits) 139 2 1/4 5 (40 bits) 140 3 1/2 10 (80 bits) 141 4 1 22 (171 bits; 5 bits padded at the end) 142 5 Erasure 0 (SHOULD NOT be transmitted by sender) 144 5. RTP header usage 146 The format of the RTP header is specified in RFC 3550 [5]. The 147 EVRC-NW payload formats (Section 6) use the fields of the RTP header 148 as specified in RFC 3550 [5]. 150 EVRC-NW has also the capability to operate with 8 kHz sampled input/ 151 output signals. The decoder does not require a priori knowledge 152 about the sampling rate of the original signal at the input of the 153 encoder. The decoder output can be at 8 kHz or 16 kHz regardless of 154 the sampling rate used at the encoder. Therefore, depending on the 155 implementation and the electroacoustic audio capabilities of the 156 devices, the input of the encoder and/or the output of the decoder 157 can be configured at 8 kHz; however, a 16 kHz RTP clock rate MUST 158 always be used. The RTP timestamp is increased by 320 for each 20 159 milliseconds. 161 The RTP header marker bit (M) SHALL be set to 1 if the first frame 162 carried in the packet contains a speech frame which is the first in a 163 talkspurt. For all other packets the marker bit SHALL be set to zero 164 (M=0). 166 6. Payload format 168 Three RTP packet formats are supported for the EVRC-NW codec - the 169 interleaved/bundled packet format, the header-free packet format, and 170 the compact bundled packet format. For all these formats, the 171 operational details and capabilities, such as ToC, interleaving, DTX, 172 and bundling, of EVRC-NW are exactly the same as those defined in 173 EVRC [6], EVRC-B [2] and EVRC-WB [3], except that 175 1. the mode change request field in the interleaved/bundled packet 176 format MUST be interpreted according to the definition of the 177 RATE_REDUC parameter as defined in EVRC-NW [4]. 179 2. the mode change request field in the interleaved/bundled packet 180 format SHOULD be honored by an EVRCNW encoding end point in an 181 one-to-one session with a dedicated EVRCNW decoding end point 182 such as in a two-party call or in a conference leg. 184 3. the reserved bit field in the first octet of the interleaved/ 185 bundled format has only one bit. Bit 1 of the first octet is an 186 EVRC-NW wideband/narrowband encoding capability identification 187 flag. 189 The media type audio/EVRCNW maps to the interleaved/bundled packet 190 format, audio/EVRCNW0 maps to the header-free packet format, and 191 audio/EVRCNW1 maps to the compact bundled packet format. 193 6.1. Encoding capability identification in EVRC-NW interleaved/bundled 194 format 196 The EVRC-NW interleaved/bundled format defines an encoding capability 197 identification flag, which is used to signal the local EVRC-NW 198 wideband/narrowband encoding capability at the time of construction 199 of an RTP packet to the far end of a communication session. This 200 capability identification flag allows the far end to use the MMM 201 field in its out-going (returning) EVRC-NW interleaved/bundled format 202 packets to request the desired EVRC-NW wideband or narrowband 203 encoding mode in accordance with the dynamic/instantaneous encoding 204 capability information. See RFC 3558 [6] for the definition of MMM 205 field. The following examples illustrate a few scenarios where the 206 encoding capability information is used: 208 o An end-to-end wideband communication is established first between 209 two communication end points using EVRC-NW interleaved/bundled 210 format. The called end point becomes wideband encoding incapable 211 during the call and makes the other end aware of this change using 212 the encoding capability identification flag. Based on the new 213 information the calling end point could change the MMM value in 214 its outgoing EVRC-NW packets from Mode-0 to Mode-4 to request 215 narrowband encoded traffic for bandwidth efficiency or from Mode-0 216 to Mode-1 for best perceptual quality. 218 o An end-to-end narrowband communication is established between an 219 EVRC-NW wideband encoding capable calling end point and an EVRC-NW 220 wideband encoding incapable called end point. The called end 221 point becomes EVRC-NW wideband encoding capable during the call 222 and makes the other end aware of this change using the encoding 223 capability identification flag. Based on the new information the 224 calling end point could change the MMM value in its outgoing 225 EVRC-NW packets from non-Mode-0 to Mode-0 to request wideband 226 traffic. 228 EVRC-NW interleaved/bundled format defines the encoding capability 229 identification flag in bit 1 of the first octet, as illustrated in 230 the figure below. The flag shall be set to zero (C=0) when the local 231 EVRC-NW encoder is capable of Mode-0 wideband encoding. The flag 232 shall be set to one (C=1) when the local EVRC-NW encoder is capable 233 of non-Mode-0 narrowband encoding only. See RFC 3558 [6] for 234 original definitions of other fields in the interleaved/bundled 235 format. 237 0 1 2 3 238 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 1 239 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 240 | RTP Header | 241 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 242 |R|C| LLL | NNN | MMM | Count | TOC | ... | TOC |padding| 243 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 244 | one or more codec data frames, one per TOC entry | 245 | .... | 246 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 248 Reserved (R): 1 bit 250 Reserved bit. MUST be set to zero by sender, SHOULD be ignored by 251 receiver. 253 Encoding capability identification (C): 1 bit 255 Must be set to zero by sender to indicate wideband encoding 256 capable or set to one to indicate narrowband encoding capable 257 only. 259 C = 0 : Mode-0 wideband encoding capable 261 = 1 : Mode-0 wideband encoding incapable, i.e. narrowband 262 encoding only. 264 7. Congestion Control Considerations 266 Congestion control for RTP is discussed in RFC 3550 [5], and in 267 applicable RTP profiles, e.g., RFC3551 [7]. This document does not 268 change those considerations. 270 Due to the header overhead, the number of frames encapsulated in each 271 RTP packet influences the overall bandwidth of the RTP stream. 272 Packing more frames in each RTP packet can reduce the number of 273 packets sent and hence the header overhead, at the expense of 274 increased delay and reduced error robustness. 276 8. Storage format for the EVRC-NW Codec 278 The storage format is used for storing EVRC-NW encoded speech frames, 279 e.g., as a file or e-mail attachment. 281 The file begins with a magic number to identify the vocoder that is 282 used. The magic number for EVRC-NW corresponds to the ASCII 283 character string "#!EVRCNW\n", i.e., "0x23 0x21 0x45 0x56 0x52 0x43 284 0x4E 0x57 0x0A". 286 The codec data frames are stored in consecutive order, with a single 287 ToC entry field, extended to one octet, prefixing each codec data 288 frame. The ToC field is extended to one octet by setting the four 289 most significant bits of the octet to zero. For example, a ToC value 290 of 4 (a full-rate frame) is stored as 0x04. The Value column in the 291 table in Section 4 provides the TOC values for corresponding frame 292 types. 294 Speech frames lost in transmission and non-received frames MUST be 295 stored as erasure frames (ToC value of 5) to maintain synchronization 296 with the original media. 298 9. IANA considerations 300 This document introduces a new EVRC-NW 'audio' media subtype. 302 9.1. Media Type Registrations 304 Following the guidelines in RFC 4855 [8] and RFC 4288 [9], this 305 section registers new 'audio' media subtypes for EVRC-NW. 307 9.1.1. Registration of Media Type audio/EVRCNW 309 Type name: audio 311 Subtype names: EVRCNW 313 Required parameters: None 315 Optional parameters: 317 These parameters apply to RTP transfer only. 319 mode-set-recv: A subset of EVRC-NW modes. Possible values are a 320 comma separated list of modes from the set {0,1,2,3,4,5,6,7} (see 321 Table 2.6.1.2-4 in 3GPP2 C.S0014-D). A decoder can use this 322 attribute to inform an encoder of its preference to operate in a 323 specified subset of modes. Absence of this parameter signals the 324 mode set {1,2,3,4,5,6,7}. 326 ptime: see RFC 4566 [10]. 328 maxptime: see RFC 4566. 330 maxinterleave: Maximum number for interleaving length (field LLL in 331 the Interleaving Octet)[0..7]. The interleaving lengths used in the 332 entire session MUST NOT exceed this maximum value. If not signaled, 333 the maxinterleave length MUST be 5. 335 silencesupp: see Section 6.1 in RFC 4788. 337 dtxmax: see Section 6.1 in RFC 4788. 339 dtxmin: see Section 6.1 in RFC 4788. 341 hangover: see Section 6.1 in RFC 4788. 343 Encoding considerations: 345 This media type is framed binary data (see RFC 4288, Section 4.8) and 346 is defined for transfer of EVRC-NW encoded data via RTP using the 347 interleaved/bundled packet format specified in RFC 3558 [6]. 349 Security considerations: See Section 16. 351 Interoperability considerations: None 353 Published specification: 355 The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer 356 method with the interleaved/bundled packet format via RTP is 357 specified in RFC 3558 [6]. See Section 6 of RFC XXXX for details for 358 EVRC-NW. [Note to the RFC editor: please replace XXXX with the RFC 359 number of this document.] 361 Applications that use this media type: 363 It is expected that many VoIP applications (as well as mobile 364 applications) will use this type. 366 Additional information: 368 The following applies to stored-file transfer methods: 370 Magic number: #!EVRCNW\n (see Section 8) 372 File extensions: enw, ENW 374 Macintosh file type code: None 376 Object identifier or OID: None 378 EVRC-NW speech frames may also be stored in the file format "3g2" 379 defined in 3GPP2 C.S0050-B, which is identified using the media types 380 "audio/3gpp2" or "video/3gpp2" registered by RFC 4393 [11]. 382 Person & email address to contact for further information: 384 Zheng Fang 386 Intended usage: COMMON 388 Restrictions on usage: 390 This media type can be used with the file format defined in Section 8 391 of RFC XXXX in contexts other than RTP. In context of transfers over 392 RTP, the RTP payload format specified in Section 4.1 of RFC 3558 [6] 393 is used for this media type. [Note to the RFC editor: please replace 394 XXXX with the RFC number of this document.] 396 Author: 398 Zheng Fang 400 Change controller: 402 IETF Payload working group delegated from the IESG. 404 9.1.2. Registration of Media Type audio/EVRCNW0 406 Type name: audio 408 Subtype names: EVRCNW0 410 Required parameters: None 412 Optional parameters: 414 These parameters apply to RTP transfer only. 416 mode-set-recv: A subset of EVRC-NW modes. Possible values are a 417 comma separated list of modes from the set {0,1,2,3,4,5,6,7} (see 418 Table 2.6.1.2-4 in 3GPP2 C.S0014-D). A decoder can use this 419 attribute to inform an encoder of its preference to operate in a 420 specified subset of modes. Absence of this parameter signals the 421 mode set {1,2,3,4,5,6,7}. 423 ptime: see RFC 4566. 425 silencesupp: see Section 6.1 in RFC 4788. 427 dtxmax: see Section 6.1 in RFC 4788. 429 dtxmin: see Section 6.1 in RFC 4788. 431 hangover: see Section 6.1 in RFC 4788. 433 Encoding considerations: 435 This media type is framed binary data (see RFC 4288, Section 4.8) and 436 is defined for transfer of EVRC-NW encoded data via RTP using the 437 header-free packet format specified in RFC 3558 [6]. 439 Security considerations: See Section 16. 441 Interoperability considerations: None 442 Published specification: 444 The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer 445 method with the header-free packet format via RTP is specified in RFC 446 3558 [6]. 448 Applications that use this media type: 450 It is expected that many VoIP applications (as well as mobile 451 applications) will use this type. 453 Additional information: None 455 Person & email address to contact for further information: 457 Zheng Fang 459 Intended usage: COMMON 461 Restrictions on usage: 463 This media type depends on RTP framing, and hence is only defined for 464 transfer via RTP [5], the RTP payload format specified in Section 4.2 465 of RFC 3558 [6] SHALL be used. This media type SHALL NOT be used for 466 storage or file transfer, instead audio/EVRCNW SHALL be used. 468 Author: 470 Zheng Fang 472 Change controller: 474 IETF Payload working group delegated from the IESG. 476 9.1.3. Registration of Media Type audio/EVRCNW1 478 Type name: audio 480 Subtype names: EVRCNW1 482 Required parameters: None 484 Optional parameters: 486 These parameters apply to RTP transfer only. 488 mode-set-recv: A subset of EVRC-NW modes. Possible values are a 489 comma separated list of modes from the set {0,1} (see Table 2.6.1.2-4 490 in 3GPP2 C.S0014-D). A decoder can use this attribute to inform an 491 encoder of its preference to operate in a specified subset of modes. 492 A value of 0 signals the support for wideband fixed rate (full or 493 half rate, depending on the value of 'fixedrate' parameter). A value 494 of 1 signals narrowband fixed rate (full or half rate, depending on 495 the value of 'fixedrate' parameter). Absence of this parameter 496 signals the mode 1. 498 ptime: see RFC 4566. 500 maxptime: see RFC 4566. 502 fixedrate: Indicates the EVRC-NW rate of the session while in single 503 rate operation. Valid values include: 0.5 and 1, where a value of 504 0.5 indicates the 1/2 rate while a value of 1 indicates the full 505 rate. If this parameter is not present, 1/2 rate is assumed. 507 silencesupp: see Section 6.1 in RFC 4788. 509 dtxmax: see Section 6.1 in RFC 4788. 511 dtxmin: see Section 6.1 in RFC 4788. 513 hangover: see Section 6.1 in RFC 4788. 515 Encoding considerations: 517 This media type is framed binary data (see RFC 4288, Section 4.8) and 518 is defined for transfer of EVRC-NW encoded data via RTP using the 519 compact bundled packet format specified in RFC 4788. 521 Security considerations: See Section 16 523 Interoperability considerations: None 525 Published specification: 527 The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D. The transfer 528 method with the compact bundled packet format via RTP is specified in 529 RFC 4788. 531 Applications that use this media type: 533 It is expected that many VoIP applications (as well as mobile 534 applications) will use this type. 536 Additional information: None 537 Person & email address to contact for further information: 539 Zheng Fang 541 Intended usage: COMMON 543 Restrictions on usage: 545 This media type depends on RTP framing, and hence is only defined for 546 transfer via RTP [5], the RTP payload format specified in Section 4 547 of RFC 4788 SHALL be used. This media type SHALL NOT be used for 548 storage or file transfer, instead audio/EVRCNW SHALL be used. 550 Author: 552 Zheng Fang 554 Change controller: 556 IETF Payload working group delegated from the IESG. 558 10. SDP mode attributes for EVRC-NW 560 'mode-set-recv' can be used by a decoder to inform an encoder of its 561 preference to operate in a specified subset of modes. Note that 562 indicating a preference implicitly indicates support for that 563 capability. If mode 0 is not preferred for media type EVRCNW0 or 564 EVRCNW1, then there is no indication that mode 0 is supported. 565 However absence of this parameter or absence of mode 0 in this 566 parameter for media type EVRCNW shall not preclude mode 0 support 567 during a call where mode 0 may be requested via the MMM field. 569 To inform the capability for wideband mode support, a decoder can 570 always decode all the narrowband modes (modes 1 to 7). Unless the 571 decoder indicates the support of mode 0 (i.e., preference) in this 572 parameter or in the MMM mode request field in interleaved/bundled 573 payload format, an encoder at the other side shall not operate in 574 mode 0. 576 To indicate a preference to operate in a subset of modes, a set has 577 been defined so that several modes can be expressed as a preference 578 in one attempt. For instance, the set {4,5,6,7} signals that the 579 receiver prefers the sender to operate in bandwidth-efficient 580 narrowband modes of EVRC-NW. 582 Note, during an active call session using the interleaved/bundled 583 packet format, the MMM mode request received from a communication 584 partner can contain a mode request different than the values in the 585 last mode-set-recv attribute. The partner's EVRC-NW wideband 586 decoding capability is determined by the latest mode-set-recv 587 attribute or MMM mode request field. For example, a mode request 588 with MMM=0 from a communication partner is an implicit indication of 589 the partner's EVRCNW wideband decoding capability and preference. An 590 EVRCNW wideband capable node receiving the request can operate in 591 wideband mode. A mode request with MMM=1, 2, ..., or 7 from a 592 communication partner is an implicit indication of the partner's 593 EVRCNW narrowband decoding preference. The encoder of an EVRCNW node 594 receiving the request shall honor the request and operate in 595 narrowband mode. 597 'sendmode' is used as a SDP mode attribute in EVRC [6], EVRC-B [2] 598 and EVRC-WB [3]. However it is deprecated in EVRC-NW. 600 11. Mode Change Request/Response Considerations 602 The interleaved/bundled packet format for the EVRC family of vocoders 603 supports a 3-bit field (MMM) that a communication node can use to 604 indicate its preferred compression mode to an opposite node. The 605 concept of the compression mode (also known as Capacity Operating 606 Point) was introduced to allow a controlled trade-off between voice 607 quality and channel capacity. The notion makes it possible to 608 exercise vocoders at the highest possible (average) bit-rate (hence, 609 highest voice quality) when the network is lightly loaded. 610 Conversely, once the network load increases the vocoders can be 611 requested to operate at lower average bit-rates so as to absorb the 612 additional network load without causing an undue increase in the 613 frame-erasure rates; the underlying premise is that while a higher 614 bit-rate improves the vocoder performance, it also increases the 615 network loading, risking a sharp decline in voice quality should the 616 frame-erasure rate be too high. By contrast, a lower bit-rate mode 617 of operation can result in accommodation of the additional network 618 load without causing unduly high frame-erasure rates, resulting in 619 better overall quality despite the inherently lower voice quality of 620 the lower bit-rate mode of the vocoder. 622 Accordingly, the MMM field should be used to request the far-end to 623 transmit compressed-speech using a mode that provides the best 624 balance between voice quality and capacity. However, in the case of 625 mobile-mobile calls, for example, there are two wireless sides 626 involved, each with a potentially different network load level and 627 hence a different preferred mode. In such cases, achieving optimal 628 end-to-end performance depends on coherent management of the 629 operative mode by the two sides. This requires that even if the 630 local node prefers a higher bit-rate vocoder mode, it should adjust 631 to a lower bit-rate mode if requested by the far end, in order to 632 avoid potentially high frame erasure rates due to heavy load at the 633 far end network. For similar reasons, in cases where a mode 634 requested by the far end should not be supported, it might still be 635 beneficial to consider switching to a supported vocoder mode 636 corresponding to a lower average bit-rate than requested. It is 637 recommended that the next lower average bit-rate supported vocoder 638 mode be used for encoding when a mode requested by the far end is not 639 supported. 641 A wideband-capable endpoint can use the information conveyed by the 642 C-bit of the RTP payload header to determine the optimal mode to 643 request of the far end. If the far end cannot provide Mode0 packets 644 (C-bit=1), then the choice of MMM can be based strictly on the local 645 network load. If the C-bit indicates remote end's Mode0 encoding 646 capability (C-bit=0), then even if the local network load is not 647 light, Mode0 can be requested knowing definitively that it will be 648 supported. This will permit operators to treat wideband-capable 649 mobiles preferentially, should they wish to adopt such policy. 651 12. Mapping EVRC-NW media type parameters into SDP 653 Information carried in the media type specification has a specific 654 mapping to fields in the Session Description Protocol (SDP) [10], 655 which is commonly used to describe RTP sessions. When SDP is used to 656 specify sessions employing EVRC-NW encoded speech, the mapping is as 657 follows. 659 o The media type ("audio") goes in SDP "m=" as the media name. 661 o The media subtype ("EVRCNW", "EVRCNW0" or "EVRCNW1") goes in SDP 662 "a=rtpmap" as the encoding name. 664 o The optional parameters 'ptime and 'maxptime' (for subtypes 665 EVRCNW, EVRCNW1) go in the SDP "a=ptime" and "a=maxptime" 666 attributes, respectively. 668 o Any remaining parameters (for subtypes EVRCNW, EVRCNW0 and 669 EVRCNW1) go in the SDP "a=fmtp" attribute by copying them from the 670 media type string as a semicolon separated list of parameter=value 671 pairs. 673 13. Offer-Answer Model Considerations for EVRC-NW 675 The following considerations apply when using the SDP offer-answer 676 procedures of RFC 3264 [12] to negotiate the use of EVRC-NW payload 677 in RTP: 679 o Since EVRC-NW is an extension of both EVRC-B and EVRC-WB, the 680 offerer SHOULD also announce EVRC-B and EVRC-WB support in its 681 "m=audio" lines, with EVRC-NW as the preferred codec. This will 682 allow interoperability with an answerer which supports only EVRC-B 683 and/or EVRC-WB. 685 Below is an example of such an offer: 687 m=audio 55954 RTP/AVP 98 99 100 688 a=rtpmap:98 EVRCNW0/16000 689 a=rtpmap:99 EVRCWB0/16000 690 a=rtpmap:100 EVRCB0/8000 691 a=fmtp:98 mode-set-recv=0,1,2,3,4,5,6 692 a=fmtp:99 mode-set-recv=0,4 693 a=fmtp:100 recvmode=0 695 If the answerer supports EVRC-NW then the answerer can keep the 696 payload type 98 in its answer and the conversation can be done using 697 EVRC-NW. Else, if the answerer supports only EVRC-WB and/or EVRC-B 698 then the answerer will leave only the payload type 99 and/or 100 699 respectively in its answer and the conversation will be done using 700 EVRC-WB and/or EVRC-B respectively. 702 An example answer for the above offer: 704 m=audio 55954 RTP/AVP 98 705 a=rtpmap:98 EVRCNW0/16000 706 a=fmtp:98 mode-set-recv=4 708 o 'mode-set-recv' is a uni-directional receive only parameter. 710 o An offerer can use 'mode-set-recv' to request that the remote 711 sender's encoder be limited to the list of modes signaled in 712 'mode-set-recv'. A remote sender MAY ignore 'mode-set-recv' 713 requests. However, a remote sender shall not assume the other 714 side can support mode 0, unless the offer includes mode 0 715 explicitly in 'mode-set-recv' or the remote sender receives mode 716 requests with MMM = 0 from the communication partner during an 717 active call using EVRC-NW interleaved/bundled format. 719 o The parameters 'maxptime' and 'ptime' will in most cases not 720 affect interoperability, however the setting of the parameters can 721 affect the performance of the application. The SDP offer-answer 722 handling of the 'ptime' parameter is described in RFC 3264 [12]. 723 The 'maxptime' parameter MUST be handled in the same way. 725 o For a sendonly stream, the 'mode-set-recv' parameter is not useful 726 and SHOULD NOT be used. 728 o When using EVRCNW1, the entire session MUST use the same fixed 729 rate and mode (0-Wideband or 1-Narrowband). 731 o For additional rules which MUST be followed while negotiating DTX 732 parameters, see Section 6.8 in RFC 4788 [2]. 734 o Any unknown parameter in an SDP offer MUST be ignored by the 735 receiver and MUST NOT be included in the SDP answer. 737 14. Declarative SDP Considerations 739 For declarative use of SDP in SAP [14] and RTSP [15], the following 740 considerations apply: 742 o Any 'maxptime' and 'ptime' values should be selected with care to 743 ensure that the session's participants can achieve reasonable 744 performance. 746 o The payload format configuration parameters are all declarative 747 and a participant MUST use the configuration(s) that is provided 748 for the session. More than one configuration MAY be provided if 749 necessary by declaring multiple RTP payload types, however the 750 number of types SHOULD be kept small. For declarative examples, 751 see Section 15. 753 o The usage of unidirectional receive-only parameters, such as 754 'mode-set-recv', should be excluded in any declarations, since 755 these parameters are meaningless in one-way streaming 756 applications. 758 15. Examples 760 Some example SDP session descriptions utilizing EVRC-NW encodings 761 follow. In these examples, long a=fmtp lines are folded to meet the 762 column width constraints of this document. The backslash ("\") at 763 the end of a line and the carriage return that follows it should be 764 ignored. Note that media subtype names are case-insensitive. 765 Parameter names are case-insensitive both in media types and in the 766 mapping to the SDP a=fmtp attribute. 768 Example usage of EVRCNW if wideband mode is supported: 770 m=audio 49120 RTP/AVP 97 98 99 771 a=rtpmap:97 EVRCNW/16000 772 a=rtpmap:98 EVRCWB/16000 773 a=rtpmap:99 EVRCB/8000 774 a=fmtp:97 mode-set-recv=0,1,2,3,4,5,6 775 a=fmtp:98 mode-set-recv=0,4 776 a=fmtp:99 recvmode=0 777 a=maxptime:120 779 Example usage of EVRCNW if wideband mode is not supported: 781 m=audio 49120 RTP/AVP 97 98 99 782 a=rtpmap:97 EVRCNW/16000 783 a=rtpmap:98 EVRCWB/16000 784 a=rtpmap:99 EVRCB/8000 785 a=fmtp:97 mode-set-recv=1,2,3,4,5,6 786 a=fmtp:98 mode-set-recv=4 787 a=fmtp:99 recvmode=0 788 a=maxptime:120 790 Example usage of EVRCNW0: 792 m=audio 49120 RTP/AVP 97 98 99 793 a=rtpmap:97 EVRCNW0/16000 794 a=rtpmap:98 EVRCWB0/16000 795 a=rtpmap:99 EVRCB0/8000 796 a=fmtp:97 mode-set-recv=0,1,2,3,4,5,6 797 a=fmtp:98 mode-set-recv=0,4 798 a=fmtp:99 recvmode=0 800 Example SDP answer from a media gateway requesting a terminal to 801 limit its encoder operation to EVRC-NW mode 4. 803 m=audio 49120 RTP/AVP 97 804 a=rtpmap:97 EVRCNW0/16000 805 a=fmtp:97 mode-set-recv=4 807 Example usage of EVRCNW1: 809 m=audio 49120 RTP/AVP 97 98 99 810 a=rtpmap:97 EVRCNW1/16000 811 a=rtpmap:98 EVRCWB1/16000 812 a=rtpmap:99 EVRCB1/8000 813 a=fmtp:97 fixedrate=0.5 814 a=fmtp:98 fixedrate=0.5 815 a=fmtp:99 fixedrate=0.5 816 a=maxptime:100 818 Example usage of EVRCNW with DTX with silencesupp=1: 820 m=audio 49120 RTP/AVP 97 98 99 821 a=rtpmap:97 EVRCNW/16000 822 a=rtpmap:98 EVRCWB/16000 823 a=rtpmap:99 EVRCB/8000 824 a=fmtp:97 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1; \ 825 mode-set-recv=0,1,2,3,4,5,6 826 a=fmtp:98 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1; \ 827 mode-set-recv=0,4 828 a=fmtp:99 recvmode=0 829 a=maxptime:120 831 Examples usage of EVRCNW with DTX with silencesupp=0: 833 m=audio 49120 RTP/AVP 97 98 99 834 a=rtpmap:97 EVRCNW/16000 835 a=rtpmap:98 EVRCWB/16000 836 a=rtpmap:99 EVRCB/8000 837 a=fmtp:97 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1; \ 838 mode-set-recv=0,1,2,3,4,5,6 839 a=fmtp:98 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1; \ 840 mode-set-recv=0,4 841 a=fmtp:99 recvmode=0 842 a=maxptime:120 844 Example offer answer exchange between EVRC-NW and legacy EVRC-B (RFC 845 4788): 847 Offer: 849 m=audio 55954 RTP/AVP 97 98 99 850 a=rtpmap:97 EVRCNW0/16000 851 a=rtpmap:98 EVRCWB0/16000 852 a=rtpmap:99 EVRCB0/8000 853 a=rtpmap:97 mode-set-recv=0,1,2,3,4,5,6 854 a=fmtp:98 mode-set-recv=0,4 855 a=fmtp:99 recvmode=0 857 Answer: 859 m=audio 55954 RTP/AVP 99 860 a=rtpmap:99 EVRCB0/8000 862 Example offer answer exchange between EVRC-NW and legacy EVRC-WB (RFC 863 5188): 865 Offer: 867 m=audio 55954 RTP/AVP 97 98 99 868 a=rtpmap:97 EVRCNW0/16000 869 a=rtpmap:98 EVRCWB0/16000 870 a=rtpmap:99 EVRCB0/8000 871 a=rtpmap:97 mode-set-recv=0,1,2,3,4,5,6 872 a=fmtp:98 mode-set-recv=0,4 873 a=fmtp:99 recvmode=0 875 Answer: 877 m=audio 55954 RTP/AVP 98 99 878 a=rtpmap:98 EVRCWB0/16000 880 16. Security Considerations 882 Since compression is applied to the payload formats end-to-end, and 883 the encodings do not exhibit significant non-uniformity, 884 implementations of this specification are subject to all the security 885 considerations specified in RFC 3558 [6]. Implementations using the 886 payload defined in this specification are subject to the security 887 considerations discussed in RFC 3558 [6], RFC 3550 [5], and any 888 appropriate profile (for example RFC 3551 [7]). Additional security 889 considerations are described in RFC 6562 [13]. 891 17. References 893 17.1. Normative References 895 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 896 Levels", BCP 14, RFC 2119, March 1997. 898 [2] Xie, Q. and R. Kapoor, "Enhancements to RTP Payload Formats for 899 EVRC Family Codecs", RFC 4788, January 2007. 901 [3] Desineni, H. and Q. Xie, "RTP Payload Format for the Enhanced 902 Variable Rate Wideband Codec (EVRC-WB) and the Media Subtype 903 Updates for EVRC-B Codec", RFC 5188, February 2008. 905 [4] "Enhanced Variable Rate Codec, Speech Service Options 3, 68, 906 70, and 73 for Wideband Spread Spectrum Digital Systems", 907 3GPP2 C.S0014-D v3.0, October 2010. 909 [5] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, 910 "RTP: A Transport Protocol for Real-Time Applications", STD 64, 911 RFC 3550, July 2003. 913 [6] Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs 914 (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558, 915 July 2003. 917 [7] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video 918 Conferences with Minimal Control", STD 65, RFC 3551, July 2003. 920 [8] Casner, S., "Media Type Registration of RTP Payload Formats", 921 RFC 4855, February 2007. 923 [9] Freed, N. and J. Klensin, "Media Type Specifications and 924 Registration Procedures", BCP 13, RFC 4288, December 2005. 926 [10] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 927 Description Protocol", RFC 4566, July 2006. 929 [11] Garudadri, H., "MIME Type Registrations for 3GPP2 Multimedia 930 Files", RFC 4393, March 2006. 932 [12] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 933 Session Description Protocol (SDP)", RFC 3264, June 2002. 935 [13] Perkins, C. and JM. Valin, "Guidelines for the Use of Variable 936 Bit Rate Audio with Secure RTP", RFC 6562, March 2012. 938 17.2. Informative References 940 [14] Handley, M., Perkins, C., and E. Whelan, "Session Announcement 941 Protocol", RFC 2974, October 2000. 943 [15] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming 944 Protocol (RTSP)", RFC 2326, April 1998. 946 Author's Address 948 Zheng Fang 949 Qualcomm Incorporated 950 5775 Morehouse Drive 951 San Diego, CA 92126 952 USA 954 Phone: +1 858 651 9484 955 Email: zfang@qualcomm.com 956 URI: http://www.qualcomm.com