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rfc:rfc5188

Network Working Group H. Desineni Request for Comments: 5188 Qualcomm Updates: 4788 Q. Xie Category: Standards Track Motorola

                                                         February 2008
                      RTP Payload Format for
        the Enhanced Variable Rate Wideband Codec (EVRC-WB)
           and the Media Subtype Updates for EVRC-B Codec

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document specifies Real-time Transport Protocol (RTP) payload
 formats to be used for the Enhanced Variable Rate Wideband Codec
 (EVRC-WB) and updates the media type registrations for EVRC-B codec.
 Several media type registrations are included for EVRC-WB RTP payload
 formats.  In addition, a file format is specified for transport of
 EVRC-WB speech data in storage mode applications such as email.

Desineni & Xie Standards Track [Page 1] RFC 5188 EVRC-WB RTP Payload Format February 2008

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
 3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
 4.  EVRC-WB Codec  . . . . . . . . . . . . . . . . . . . . . . . .  3
 5.  RTP Header Usage . . . . . . . . . . . . . . . . . . . . . . .  4
 6.  Payload Format . . . . . . . . . . . . . . . . . . . . . . . .  4
 7.  Congestion Control Considerations  . . . . . . . . . . . . . .  5
 8.  Storage Format for the EVRC-WB Codec . . . . . . . . . . . . .  5
 9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   9.1.  Media Type Registrations . . . . . . . . . . . . . . . . .  5
     9.1.1.  Registration of Media Type audio/EVRCWB  . . . . . . .  6
     9.1.2.  Registration of Media Type audio/EVRCWB0 . . . . . . .  8
     9.1.3.  Registration of Media Type audio/EVRCWB1 . . . . . . .  9
     9.1.4.  Updated Registration of Media Type audio/EVRCB . . . . 11
     9.1.5.  Updated Registration of Media Type audio/EVRCB0  . . . 13
 10. SDP Mode Attributes for EVRC-WB and EVRC-B . . . . . . . . . . 15
 11. EVRC-B Interoperability with Legacy Implementations (RFC 4788) 15
 12. Mapping EVRC-WB Media Type Parameters into SDP . . . . . . . . 16
 13. Mapping EVRC-B Media Type Parameters into SDP  . . . . . . . . 16
 14. Offer-Answer Model Considerations for EVRC-WB  . . . . . . . . 16
 15. Offer-Answer Model Considerations for EVRC-B . . . . . . . . . 18
 16. Declarative SDP Considerations . . . . . . . . . . . . . . . . 18
 17. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
 18. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
 19. Changes to RFC 4788  . . . . . . . . . . . . . . . . . . . . . 22
 20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   20.1. Normative References . . . . . . . . . . . . . . . . . . . 22
   20.2. Informative References . . . . . . . . . . . . . . . . . . 23

Desineni & Xie Standards Track [Page 2] RFC 5188 EVRC-WB RTP Payload Format February 2008

1. Introduction

 This document specifies the payload formats for packetization of
 EVRC-WB encoded speech signals into the Real-time Transport Protocol
 (RTP).  It defines support for the header-free, interleaved/bundled,
 and compact bundle packet formats for the EVRC-WB codec as well as
 discontinuous transmission (DTX) support for EVRC-WB encoded speech
 transported via RTP.  The EVRC-WB codec offers better speech quality
 than the EVRC and EVRC-B codecs.  EVRC-WB belongs to the EVRC family
 of codecs.  This document also updates the media type registrations
 for the EVRC-B codec.

2. Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [1].

3. Background

 EVRC-WB is a wideband extension of the EVRC-B [4] speech codec
 developed in the Third Generation Partnership Project 2 (3GPP2) with
 support for discontinuous transmission (DTX).  It provides enhanced
 (wideband) voice quality.
 The EVRC-WB codec operates on 20-ms frames, and the default sampling
 rate is 16 kHz.  Input and output at an 8-kHz sampling rate are also
 supported.  The EVRC-WB codec can operate in three modes (0, 4, and
 7) defined in [5].  EVRC-WB modes 4 and 7 are interoperable with
 EVRC-B.  EVRC-WB mode 4 uses full-rate, 1/2-rate, and 1/8-rate
 frames.  EVRC-WB mode 7 uses only 1/2 rate and 1/8 rate frames.  Mode
 change results in codec output bit-rate change but do not cause any
 decoding problems at the receiver.  For successful decoding, the
 decoder does not need to know the encoder's current mode of
 operation.  EVRC-WB provides a standardized solution for packetized
 voice applications that allow transitions between narrowband and
 wideband telephony.  The most important service addressed is IP
 telephony.  Target devices can be IP phones or Voice over IP (VoIP)
 handsets, media gateways, voice messaging servers, etc.

4. EVRC-WB Codec

 The EVRC-WB codec operates on 20-ms frames.  It produces output
 frames of one of the three different sizes: 171 bits, 80 bits, or 16
 bits.  In addition, there are two zero-bit codec frame types: blank
 (null) frames and erasure frames.  The default sampling rate is 16
 kHz.  Input and output at an 8-kHz sampling rate are also supported.

Desineni & Xie Standards Track [Page 3] RFC 5188 EVRC-WB RTP Payload Format February 2008

 The frame type values and sizes of the associated codec data frames
 are listed in the table below:
  Value   Rate      Total codec data frame size in bytes (and in bits)
  --------------------------------------------------------------------
      0     Blank      0     (0 bit)
      1     1/8        2    (16 bits)
      2     1/4        5    (40 bits)
      3     1/2       10    (80 bits)
      4     1         22    (171 bits; 5 bits padded at the end)
      5     Erasure    0    (SHOULD NOT be transmitted by sender)

5. RTP Header Usage

 The format of the RTP header is specified in RFC 3550 [6].  The
 EVRC-WB payload formats (Section 6) use the fields of the RTP header
 in a manner consistent with RFC 3550 [6].
 EVRC-WB also has the capability to operate with 8-kHz sampled input/
 output signals.  The decoder does not require a priori knowledge
 about the sampling rate of the original signal at the input of the
 encoder.  The decoder output can be at 8 kHz or 16 kHz regardless of
 the sampling rate used at the encoder.  Therefore, depending on the
 implementation and the electro acoustic audio capabilities of the
 devices, the input of the encoder and/or the output of the decoder
 can be configured at 8 kHz; however, a 16-kHz RTP clock rate MUST
 always be used.  The RTP timestamp is increased by 320 for each 20
 milliseconds.
 The RTP header marker bit (M) SHALL be set to 1 if the first frame
 carried in the packet contains a speech frame that is the first in a
 talkspurt.  For all other packets, the marker bit SHALL be set to
 zero (M=0).

6. Payload Format

 Three RTP packet formats are supported for the EVRC-WB codec -- the
 interleaved/bundled packet format, the header-free packet format, and
 the compact bundled packet format.  For all these formats, the
 operational details and capabilities, such as Table of Contents
 (ToC), interleaving, DTX, and bundling, of EVRC-WB are exactly the
 same as those of EVRC-B, as defined in [3], except that the mode
 change request field in the ToC MUST be interpreted according to the
 definition of the RATE_REDUC parameter as defined in EVRC-WB [5].
 The media type audio/EVRCWB maps to the interleaved/bundled packet
 format, audio/EVRCWB0 maps to the header-free packet format, and
 audio/EVRCWB1 maps to the compact bundled packet format.

Desineni & Xie Standards Track [Page 4] RFC 5188 EVRC-WB RTP Payload Format February 2008

7. Congestion Control Considerations

 Congestion control for RTP SHALL be used in accordance with RFC 3550
 [6], and with any applicable RTP profile, e.g., RFC 3551 [11].
 Due to the header overhead, the number of frames encapsulated in each
 RTP packet influences the overall bandwidth of the RTP stream.
 Packing more frames in each RTP packet can reduce the number of
 packets sent and hence the header overhead, at the expense of
 increased delay and reduced error robustness.

8. Storage Format for the EVRC-WB Codec

 The storage format is used for storing EVRC-WB encoded speech frames,
 e.g., as a file or email attachment.
 The file begins with a magic number to identify the vocoder that is
 used.  The magic number for EVRC-WB corresponds to the ASCII
 character string "#!EVCWB\n", i.e., "0x23 0x21 0x45 0x56 0x43 0x57
 0x42 0x0A".
 The codec data frames are stored in consecutive order, with a single
 ToC entry field, extended to one octet, prefixing each codec data
 frame.  The ToC field is extended to one octet by setting the four
 most significant bits of the octet to zero.  For example, a ToC value
 of 4 (a full-rate frame) is stored as 0x04.  See Section 4 for the
 mapping from frame type to ToC value.
 Speech frames lost in transmission and non-received frames MUST be
 stored as erasure frames (ToC value of 5) to maintain synchronization
 with the original media.

9. IANA Considerations

 This document updates the audio/EVRCB and audio/EVRCB0 media types
 defined in RFC 4788 [3] and adds new EVRC-WB 'audio' media subtypes.

9.1. Media Type Registrations

 Following the guidelines in RFC 4855 [9] and RFC 4288 [10], this
 section registers new 'audio' media subtypes for EVRC-WB and updates
 the audio/EVRCB and audio/EVRCB0 media type registrations contained
 in RFC 4788 [3].

Desineni & Xie Standards Track [Page 5] RFC 5188 EVRC-WB RTP Payload Format February 2008

9.1.1. Registration of Media Type audio/EVRCWB

 Type name: audio
 Subtype name: EVRCWB
 Required parameters: None
 Optional parameters:
 These parameters apply to RTP transfer only.
 mode-set-recv: A subset of EVRC-WB modes.  Possible values are a
 comma-separated list of modes from the set {0,4,7} (see Table
 2.5.1.2-1 in 3GPP2 C.S0014-C).  A decoder can use this attribute to
 inform an encoder of its preference to operate in a specified subset
 of modes.  Absence of this parameter signals the mode set {0,4,7}.
 sendmode: A mode of the EVRC-WB codec.  An encoder can use this to
 signal its current mode of operation.  Possible values are 0,4,7 (see
 Table 2.5.1.2-1 in 3GPP2 C.S0014-C).  Absence of this parameter
 signals mode 0.
 ptime: See RFC 4566.
 maxptime: See RFC 4566.
 maxinterleave: Maximum number for interleaving length (field LLL in
 the Interleaving Octet)[0..7].  The interleaving lengths used in the
 entire session MUST NOT exceed this maximum value.  If not signaled,
 the maxinterleave length MUST be 5.
 silencesupp: See Section 6.1 in RFC 4788.
 dtxmax: See Section 6.1 in RFC 4788.
 dtxmin: See Section 6.1 in RFC 4788.
 hangover: See Section 6.1 in RFC 4788.
 Encoding considerations:
 This media type is framed binary data (see RFC 4288, Section 4.8) and
 is defined for transfer of EVRC-WB encoded data via RTP using the
 interleaved/bundled packet format specified in RFC 3558.
 Security considerations: See Section 18 of RFC 5188.

Desineni & Xie Standards Track [Page 6] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Interoperability considerations: None
 Published specification:
 The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C.  The transfer
 method with the interleaved/bundled packet format via RTP is
 specified in RFC 3558 and RFC 5188.
 3GPP2 C.S0050-B, 3GPP2 File Formats for Multimedia Services.
 3GPP2 specifications are publicly accessible at http://www.3gpp2.org
 Applications that use this media type:
 It is expected that many VoIP applications (as well as mobile
 applications) will use this type.
 Additional information:
 The following applies to stored-file transfer methods:
 Magic number: #!EVCWB\n (see Section 8 of RFC 5188)
 File extensions: evw, EVW
 Macintosh file type code: None
 Object identifier or OID: None
 EVRC-WB speech frames may also be stored in the file format "3g2"
 defined in 3GPP2 C.S0050-B, which is identified using the media types
 "audio/3gpp2" or "video/3gpp2" registered by RFC 4393.
 Person & email address to contact for further information:
 Harikishan Desineni <hd@qualcomm.com>
 Intended usage: COMMON
 Restrictions on usage:
 When this media type is used in the context of transfer over RTP, the
 RTP payload format specified in Section 4.1 of RFC 3558 SHALL be
 used.  In all other contexts, the file format defined in Section 8 of
 RFC 5188 SHALL be used.

Desineni & Xie Standards Track [Page 7] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Author:
 Harikishan Desineni
 Change controller:
 IETF Audio/Video Transport working group delegated from the IESG.

9.1.2. Registration of Media Type audio/EVRCWB0

 Type name: audio
 Subtype name: EVRCWB0
 Required parameters: None
 Optional parameters:
 These parameters apply to RTP transfer only.
 mode-set-recv: A subset of EVRC-WB modes.  Possible values are a
 comma-separated list of modes from the set {0,4,7} (see Table
 2.5.1.2-1 in 3GPP2 C.S0014-C).  A decoder can use this attribute to
 inform an encoder of its preference to operate in a specified subset
 of modes.  Absence of this parameter signals the mode set {0,4,7}.
 sendmode: A mode of the EVRC-WB codec.  An encoder can use this to
 signal its current mode of operation.  Possible values are 0,4,7 (see
 Table 2.5.1.2-1 in 3GPP2 C.S0014-C).  Absence of this parameter
 signals mode 0.
 ptime: See RFC 4566.
 silencesupp: See Section 6.1 in RFC 4788.
 dtxmax: See Section 6.1 in RFC 4788.
 dtxmin: See Section 6.1 in RFC 4788.
 hangover: See Section 6.1 in RFC 4788.
 Encoding considerations:
 This media type is framed binary data (see RFC 4288, Section 4.8) and
 is defined for transfer of EVRC-WB encoded data via RTP using the
 header-free packet format specified in RFC 3558.
 Security considerations: See Section 18 of RFC 5188.

Desineni & Xie Standards Track [Page 8] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Interoperability considerations: None
 Published specification:
 The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C.  The transfer
 method with the header-free packet format via RTP is specified in RFC
 3558 and RFC 5188.
 3GPP2 C.S0050-B, 3GPP2 File Formats for Multimedia Services.
 3GPP2 specifications are publicly accessible at http://www.3gpp2.org
 Applications that use this media type:
 It is expected that many VoIP applications (as well as mobile
 applications) will use this type.
 Additional information: None
 Person & email address to contact for further information:
 Harikishan Desineni <hd@qualcomm.com>
 Intended usage: COMMON
 Restrictions on usage:
 This media type depends on RTP framing and hence is only defined for
 transfer via RTP [6]; the RTP payload format specified in Section 4.2
 of RFC 3558 SHALL be used.  This media type SHALL NOT be used for
 storage or file transfer using the file format defined in Section 8
 of RFC 5188; instead, audio/EVRCWB SHALL be used.
 Author:
 Harikishan Desineni
 Change controller:
 IETF Audio/Video Transport working group delegated from the IESG.

9.1.3. Registration of Media Type audio/EVRCWB1

 Type name: audio
 Subtype name: EVRCWB1
 Required parameters: None

Desineni & Xie Standards Track [Page 9] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Optional parameters:
 These parameters apply to RTP transfer only.
 mode-set-recv: A subset of EVRC-WB modes.  Possible values are a
 comma-separated list of modes from the set {0,4,7} (see Table
 2.5.1.2-1 in 3GPP2 C.S0014-C).  A decoder can use this attribute to
 inform an encoder of its preference to operate in a specified subset
 of modes.  A value of 0 signals the support for wideband fixed rate
 (full or half rate, depending on the value of the 'fixedrate'
 parameter).  A value of 4 signals narrowband fixed full rate.  A
 value of 7 signals narrowband fixed half rate.  Absence of this
 parameter signals mode 0.
 sendmode: A mode of the EVRC-WB codec.  An encoder can use this to
 signal its current mode of operation.  Possible values are 0,4,7 (see
 Table 2.5.1.2-1 in 3GPP2 C.S0014-C). 'sendmode' with value 0 signals
 wideband fixed-rate operation (full or half rate, depending on the
 value of the 'fixedrate' parameter). 'sendmode' with value 4 signals
 narrowband fixed full-rate operation. 'sendmode' with value 7 signals
 narrowband fixed half-rate operation.  The 'fixedrate' parameter MUST
 NOT be present when the 'sendmode' value is 4 or 7.  Absence of this
 parameter signals mode 0.
 ptime: See RFC 4566.
 maxptime: See RFC 4566.
 fixedrate: Indicates the EVRC-WB rate of the session while in single-
 rate operation.  Valid values include 0.5 and 1, where a value of 0.5
 indicates the 1/2 rate while a value of 1 indicates the full rate.
 If this parameter is not present, 1/2 rate is assumed.
 silencesupp: See Section 6.1 in RFC 4788.
 dtxmax: See Section 6.1 in RFC 4788.
 dtxmin: See Section 6.1 in RFC 4788.
 hangover: See Section 6.1 in RFC 4788.
 Encoding considerations:
 This media type is framed binary data (see RFC 4288, Section 4.8) and
 is defined for transfer of EVRC-WB encoded data via RTP using the
 compact bundle packet format specified in RFC 4788.
 Security considerations: See Section 18 of RFC 5188.

Desineni & Xie Standards Track [Page 10] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Interoperability considerations: None
 Published specification:
 The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C.  The transfer
 method with the compact bundled packet format via RTP is specified in
 RFC 4788 and RFC 5188.
 3GPP2 C.S0050-B, 3GPP2 File Formats for Multimedia Services.
 3GPP2 specifications are publicly accessible at http://www.3gpp2.org
 Applications that use this media type:
 It is expected that many VoIP applications (as well as mobile
 applications) will use this type.
 Additional information: None
 Person & email address to contact for further information:
 Harikishan Desineni <hd@qualcomm.com>
 Intended usage: COMMON
 Restrictions on usage:
 This media type depends on RTP framing and hence is only defined for
 transfer via RTP [6]; the RTP payload format specified in Section 4
 of RFC 4788 SHALL be used.  This media type SHALL NOT be used for
 storage or file transfer using the file format defined in Section 8
 of RFC 5188; instead, audio/EVRCWB SHALL be used.
 Author:
 Harikishan Desineni
 Change controller:
 IETF Audio/Video Transport working group delegated from the IESG.

9.1.4. Updated Registration of Media Type audio/EVRCB

 Type name: audio
 Subtype name: EVRCB
 Required parameters: None

Desineni & Xie Standards Track [Page 11] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Optional parameters:
 These parameters apply to RTP transfer only.
 recvmode: A mode of the EVRC-B codec.  A decoder can use this
 attribute to inform an encoder of its preference to operate in a
 specified mode.  Possible values are 0..7 (see the encoder operating
 point column in Table 2-6 of 3GPP2 C.S0014-B).
 sendmode: A mode of the EVRC-B codec.  An encoder can use this to
 signal its current mode of operation.  Possible values are 0..7 (see
 encoder operating point column in Table 2-6 of 3GPP2 C.S0014-B).
 ptime: See RFC 4566.
 maxptime: See RFC 4566.
 maxinterleave: Maximum number for interleaving length (field LLL in
 the Interleaving Octet).  The interleaving lengths used in the entire
 session MUST NOT exceed this maximum value.  If not signaled, the
 maxinterleave length MUST be 5.
 silencesupp: See Section 6.1 of RFC 4788 for a definition.  If this
 parameter is not present, the default value 1 MUST be assumed.
 dtxmax: See Section 6.1 of RFC 4788.
 dtxmin: See Section 6.1 of RFC 4788.
 hangover: See Section 6.1 of RFC 4788.
 Encoding considerations:
 This media type is framed binary data (see RFC 4288, Section 4.8) and
 is defined for transfer of EVRC-B encoded data via RTP using the
 interleaved/bundled packet format specified in RFC 3558.
 Security considerations: See Section 9 of RFC 4788.
 Interoperability considerations: None
 Published specification:
 The EVRC-B vocoder is specified in 3GPP2 C.S0014-B.  The transfer
 method with the interleaved/bundled packet format via RTP is
 specified in RFC 3558, RFC 4788, and RFC 5188.

Desineni & Xie Standards Track [Page 12] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Applications that use this media type:
 It is expected that many VoIP applications (as well as mobile
 applications) will use this type.
 Additional information: The following information applies for the
 storage format only.
 Magic number: #!EVRC-B\n (see Section 5 of RFC 4788)
 File extensions: evb, EVB
 Macintosh file type code: None
 Object identifier or OID: None
 Person & email address to contact for further information:
 Harikishan Desineni <hd@qualcomm.com>
 Intended usage: COMMON
 Restrictions on usage:
 When this media type is used in the context of transfer over RTP, the
 RTP payload format specified in Section 4.1 of RFC 3558 SHALL be
 used.  In all other contexts, the file format defined in Section 5 of
 RFC 4788 SHALL be used.
 Author:
 Qiaobing Xie / Harikishan Desineni
 Change controller:
 IETF Audio/Video Transport working group delegated from the IESG.

9.1.5. Updated Registration of Media Type audio/EVRCB0

 Type name: audio
 Subtype name: EVRCB0
 Required parameters: None
 Optional parameters:
 These parameters apply to RTP transfer only.

Desineni & Xie Standards Track [Page 13] RFC 5188 EVRC-WB RTP Payload Format February 2008

 recvmode: A mode of the EVRC-B codec.  A decoder can use this
 attribute to inform an encoder of its preference to operate in a
 specified mode.  Possible values are 0..7 (see the encoder operating
 point column in Table 2-6 of 3GPP2 C.S0014-B).
 sendmode: A mode of the EVRC-B codec.  An encoder can use this to
 signal its current mode of operation.  Possible values are 0..7 (see
 the encoder operating point column in Table 2-6 of 3GPP2 C.S0014-B).
 silencesupp: See Section 6.1 of RFC 4788 for a definition.  If this
 parameter is not present, the default value 1 MUST be assumed.
 dtxmax: see Section 6.1 of RFC 4788.
 dtxmin: see Section 6.1 of RFC 4788.
 hangover: see Section 6.1 of RFC 4788.
 Encoding considerations:
 This media type is framed binary data (see RFC 4288, Section 4.8) and
 is defined for transfer of EVRC-B encoded data via RTP using the
 header-free packet format specified in RFC 3558.
 Security considerations: See Section 9 of RFC 4788.
 Interoperability considerations: None
 Published specification:
 The EVRC-B vocoder is specified in 3GPP2 C.S0014-B.  The transfer
 method with the header-free packet format via RTP is specified in RFC
 3558, RFC 4788, and RFC 5188.
 Applications that use this media type:
 It is expected that many VoIP applications (as well as mobile
 applications) will use this type.
 Additional information: None
 Person & email address to contact for further information:
 Harikishan Desineni <hd@qualcomm.com>
 Intended usage: COMMON

Desineni & Xie Standards Track [Page 14] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Restrictions on usage:
 When this media type is used in the context of transfer over RTP, the
 RTP payload format specified in Section 4.2 of RFC 3558 SHALL be
 used.
 This media type depends on RTP framing and hence is only defined for
 transfer via RTP [6]; the RTP payload format specified in Section 4.2
 of RFC 3558 SHALL be used.  This media type SHALL NOT be used for
 storage or file transfer using the file format defined in Section 5
 of RFC 4788; instead, audio/EVRCB SHALL be used.
 Author:
 Qiaobing Xie / Harikishan Desineni
 Change controller:
 IETF Audio/Video Transport working group delegated from the IESG.

10. SDP Mode Attributes for EVRC-WB and EVRC-B

 'sendmode' can be used by a sender (EVRC-WB or EVRC-B) to announce
 its encoder's current mode of operation.  A sender can change its
 mode anytime, and this does not cause any decoding problems at the
 receiver.
 'recvmode' is defined for use with EVRC-B.  A decoder can use this
 attribute to inform an encoder of its preference to operate in a
 specified mode.  The receiver will continue to decode properly even
 if the sender does not operate in the preferred mode.
 'mode-set-recv' is defined for use with EVRC-WB.  A decoder can use
 this attribute to inform an encoder of its preference to operate in a
 specified subset of modes.  The receiver will continue to decode
 properly even if the sender does not operate in one of the preferred
 modes.  A set has been defined so that several modes can be expressed
 as a preference in one attempt.  For instance, the set {4,7} signals
 that the receiver prefers the sender to operate in narrowband modes
 of EVRC-WB.

11. EVRC-B Interoperability with Legacy Implementations (RFC 4788)

 This document adds new optional parameters "recvmode" and "sendmode"
 to the original EVRC-B media types "audio/EVRCB" and "audio/EVRCB0"
 defined in RFC 4788 [3].  Existing RFC 4788 [3] implementations will
 not send these parameters in the Session Description Protocol (SDP)
 and will ignore them if they are received.  This will allow

Desineni & Xie Standards Track [Page 15] RFC 5188 EVRC-WB RTP Payload Format February 2008

 interoperability between RFC 4788 [3] and RFC 5188 implementations of
 EVRC-B.  For an example offer-and-answer exchange, see Section 17.

12. Mapping EVRC-WB Media Type Parameters into SDP

 Information carried in the media type specification has a specific
 mapping to fields in the Session Description Protocol (SDP) [8],
 which is commonly used to describe RTP sessions.  When SDP is used to
 specify sessions employing EVRC-WB encoded speech, the mapping is as
 follows.
 o  The media type ("audio") goes in SDP "m=" as the media name.
 o  The media subtype ("EVRCWB", "EVRCWB0", or "EVRCWB1") goes in SDP
    "a=rtpmap" as the encoding name.
 o  The optional parameters 'ptime' and 'maxptime' (for subtypes
    EVRCWB, EVRCWB1) go in the SDP "a=ptime" and "a=maxptime"
    attributes, respectively.
 o  Any remaining parameters (for subtypes EVRCWB, EVRCWB0, and
    EVRCWB1) go in the SDP "a=fmtp" attribute by copying them from the
    media type string as a semicolon-separated list of parameter=value
    pairs.

13. Mapping EVRC-B Media Type Parameters into SDP

 The new optional parameters 'recvmode' and 'sendmode' (for 'audio'
 subtypes EVRCB and EVRCB0) go in the SDP "a=fmtp" attribute by
 copying them directly from the media type string.
 For all other media type parameters, the specification in Section 6.7
 of RFC 4788 [3] still applies.

14. Offer-Answer Model Considerations for EVRC-WB

 The following considerations apply when using the SDP offer-answer
 procedures of RFC 3264 [7] to negotiate the use of EVRC-WB payload in
 RTP:
 o  Since EVRC-WB is an extension of EVRC-B, the offerer SHOULD
    announce EVRC-B support in its "m=audio" line, with EVRC-WB as the
    preferred codec.  This will allow interoperability with an
    answerer that supports only EVRC-B.

Desineni & Xie Standards Track [Page 16] RFC 5188 EVRC-WB RTP Payload Format February 2008

 Below is an example of such an offer:
        m=audio 55954 RTP/AVP 98 99
        a=rtpmap:98 EVRCWB0/16000
        a=rtpmap:99 EVRCB0/8000
        a=fmtp:98 mode-set-recv=0,4;sendmode=0
        a=fmtp:99 recvmode=0 sendmode=4
 If the answerer supports EVRC-WB, then the answerer can keep the
 payload type 98 in its answer and the conversation can be done using
 EVRC-WB.  Else, if the answerer supports only EVRC-B, then the
 answerer will leave only the payload type 99 in its answer and the
 conversation will be done using EVRC-B.
 An example answer for the above offer is the following:
        m=audio 55954 RTP/AVP 98
        a=rtpmap:98 EVRCWB0/16000
        a=fmtp:98 mode-set-recv=4;sendmode=4
 o  'mode-set-recv' is a unidirectional receive-only parameter.
 o  'sendmode' is a unidirectional send-only parameter.
 o  Using 'sendmode', a sender can signal its current mode of
    operation.  Note that a receiver may receive RTP media well before
    the arrival of SDP with a (first-time, or updated) 'sendmode'
    parameter.
 o  An offerer can use 'mode-set-recv' to request that the remote
    sender's encoder be limited to the list of modes signaled in
    'mode-set-recv'.  A remote sender MAY ignore 'mode-set-recv'
    requests.
 o  The parameters 'maxptime' and 'ptime' will in most cases not
    affect interoperability; however, the setting of the parameters
    can affect the performance of the application.  The SDP offer-
    answer handling of the 'ptime' parameter is described in RFC 3264
    [7].  The 'maxptime' parameter MUST be handled in the same way.
 o  For a sendonly stream, the 'mode-set-recv' parameter is not useful
    and SHOULD NOT be used.
 o  For a recvonly stream, the 'sendmode' parameter is not useful and
    SHOULD NOT be used.
 o  When using EVRCWB1, the entire session MUST use the same fixed
    rate and mode (0-Wideband or 4,7-Narrowband).

Desineni & Xie Standards Track [Page 17] RFC 5188 EVRC-WB RTP Payload Format February 2008

 o  For additional rules that MUST be followed while negotiating DTX
    parameters, see Section 6.8 in [3].
 o  Any unknown parameter in an SDP offer MUST be ignored by the
    receiver and MUST NOT be included in the SDP answer.

15. Offer-Answer Model Considerations for EVRC-B

 See Section 6.8 of [3] for offer-answer usage of EVRC-B.  The
 following are several additional considerations for EVRC-B.
 o  'recvmode' is a unidirectional receive-only parameter.
 o  'sendmode' is a unidirectional send-only parameter.
 o  Using 'recvmode', a receiver can signal the remote sender to
    operate its encoder in the specified mode.  A remote sender MAY
    ignore 'recvmode' requests.
 o  Using 'sendmode', a sender can signal its current mode of
    operation.  Note that a receiver may receive RTP media well before
    the arrival of SDP with a (first-time, or updated) 'sendmode'
    parameter.
 o  For a sendonly stream, the 'recvmode' parameter is not useful and
    SHOULD NOT be used.
 o  For a recvonly stream, the 'sendmode' parameter is not useful and
    SHOULD NOT be used.

16. Declarative SDP Considerations

 For declarative use of SDP in the Session Announcement Protocol (SAP)
 [12] and the Real Time Streaming Protocol (RTSP) [13], the following
 considerations apply:
 o  Any 'maxptime' and 'ptime' values should be selected with care to
    ensure that the session's participants can achieve reasonable
    performance.
 o  The payload format configuration parameters are all declarative,
    and a participant MUST use the configuration(s) that is provided
    for the session.  More than one configuration may be provided if
    necessary by declaring multiple RTP payload types; however, the
    number of types should be kept small.  For declarative examples,
    see Section 17.

Desineni & Xie Standards Track [Page 18] RFC 5188 EVRC-WB RTP Payload Format February 2008

17. Examples

 Some example SDP session descriptions utilizing EVRC-WB and EVRC-B
 encodings follow.  In these examples, long a=fmtp lines are folded to
 meet the column width constraints of this document.  The backslash
 ("\") at the end of a line and the carriage return that follows it
 should be ignored.  Note that media subtype names are case-
 insensitive.  Parameter names are case-insensitive both in media
 types and in the mapping to the SDP a=fmtp attribute.
   Example usage of EVRCWB:
      m=audio 49120 RTP/AVP 97 98
      a=rtpmap:97 EVRCWB/16000
      a=rtpmap:98 EVRCB0/8000
      a=fmtp:97 mode-set-recv=0,4;sendmode=0
      a=fmtp:98 recvmode=0 sendmode=0
      a=maxptime:120
   Example usage of EVRCWB0:
      m=audio 49120 RTP/AVP 97 98
      a=rtpmap:97 EVRCWB0/16000
      a=rtpmap:98 EVRCB0/8000
      a=fmtp:97 mode-set-recv=0,4;sendmode=0
      a=fmtp:98 recvmode=0 sendmode=0
   Example SDP answer from a media gateway requesting a terminal to
   limit its encoder operation to EVRC-WB mode 4:
      m=audio 49120 RTP/AVP 97
      a=rtpmap:97 EVRCWB0/16000
      a=fmtp:97 mode-set-recv=4;sendmode=4
   Example usage of EVRCWB1:
     m=audio 49120 RTP/AVP 97 98
     a=rtpmap:97 EVRCWB1/16000
     a=fmtp:97 mode-set-recv=4;sendmode=4
     a=maxptime:100

Desineni & Xie Standards Track [Page 19] RFC 5188 EVRC-WB RTP Payload Format February 2008

   Example usage of EVRCWB with DTX with silencesupp=1:
      m=audio 49120 RTP/AVP 97 98
      a=rtpmap:97 EVRCWB/16000
      a=rtpmap:98 EVRCB0/8000
      a=fmtp:97 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1 \
      mode-set-recv=0,4; sendmode=0
      a=fmtp:98 recvmode=0 sendmode=0
      a=maxptime:120
   Example usage of EVRCWB with DTX with silencesupp=0:
      m=audio 49120 RTP/AVP 97 98
      a=rtpmap:97 EVRCWB/16000
      a=rtpmap:98 EVRCB0/8000
      a=fmtp:97 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1 \
      mode-set-recv=0,4;sendmode=0
      a=fmtp:98 recvmode=0 sendmode=0
      a=maxptime:120
   Example usage of EVRCB:
      m=audio 49120 RTP/AVP 97
      a=rtpmap:97 EVRCB/8000
      a=fmtp:97 recvmode=0 sendmode=4
      a=maxptime:120
   Example usage of EVRCB0:
      m=audio 49120 RTP/AVP 97
      a=rtpmap:97 EVRCB0/8000
      a=fmtp:97 recvmode=0 sendmode=4
   Example offer-answer exchange between EVRC-WB and
   legacy EVRC-B (RFC 4788):
    Offer:
      m=audio 55954 RTP/AVP 98 99
      a=rtpmap:98 EVRCWB0/16000
      a=rtpmap:99 EVRCB0/8000
      a=fmtp:98 mode-set-recv=0,4;sendmode=0
      a=fmtp:99 recvmode=0 sendmode=0
    Answer:
      m=audio 55954 RTP/AVP 99
      a=rtpmap:99 EVRCB0/8000

Desineni & Xie Standards Track [Page 20] RFC 5188 EVRC-WB RTP Payload Format February 2008

   Example offer-answer exchange between EVRC-WB and
   updated EVRC-B (RFC 5188):
    Offer:
      m=audio 55954 RTP/AVP 98 99
      a=rtpmap:98 EVRCWB0/16000
      a=rtpmap:99 EVRCB0/8000
      a=fmtp:98 mode-set-recv=0,4; sendmode=0
      a=fmtp:99 recvmode=0 sendmode=0
    Answer:
      m=audio 55954 RTP/AVP 99
      a=rtpmap:99 EVRCB0/8000
      a=fmtp:99 recvmode=0 sendmode=4
   In the above example, note that the answerer has chosen
   to send in mode 4 even though the offerer was willing to
   receive in mode 0. 'recvmode' is a receiver's preference,
   but the sender can send in a different mode.
   Example offer-answer exchanges for interoperability between
   legacy (RFC 4788) and updated EVRC-B (RFC 5188) implementations:
      Offer from an offerer that supports updated EVRC-B (RFC 5188)
      implementation:
        m=audio 55954 RTP/AVP 99
        a=rtpmap:99 EVRCB0/8000
        a=fmtp:99 recvmode=0 sendmode=4
      Answer from an answerer that supports only
      legacy EVRC-B (RFC 4788) implementation:
        m=audio 55954 RTP/AVP 99
        a=rtpmap:99 EVRCB0/8000
      Offer from an offerer that supports only
      legacy EVRC-B (RFC 4788) implementation:
        m=audio 55954 RTP/AVP  99
        a=rtpmap:99 EVRCB0/8000

Desineni & Xie Standards Track [Page 21] RFC 5188 EVRC-WB RTP Payload Format February 2008

      Answer from an answerer that supports updated
      EVRC-B (RFC 5188) implementation:
        m=audio 55954 RTP/AVP 99
        a=rtpmap:99 EVRCB0/8000
        a=fmtp:99 recvmode=0 sendmode=4

18. Security Considerations

 Since compression is applied to the payload formats end-to-end, and
 the encodings do not exhibit significant non-uniformity,
 implementations of this specification are subject to all the security
 considerations specified in RFC 3558 [2].  Implementations using the
 payload defined in this specification are subject to the security
 considerations discussed in RFC 3558 [2], RFC 3550 [6], and any
 appropriate profile (for example, RFC 3551 [11]).

19. Changes to RFC 4788

 This document updates RFC 4788 [3], and the updates are summarized
 below:
 o  Added new media type attribute "sendmode" to media subtypes EVRCB
    and EVRCB0.  This attribute can be used to signal the EVRC-B
    encoder's current mode of operation.
 o  Added new media type attribute "recvmode" to media subtypes EVRCB
    and EVRCB0.  This attribute can be used to signal the EVRC-B
    decoder's preferred operating mode to a remote sender.

20. References

20.1. Normative References

 [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [2]   Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs
       (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558,
       July 2003.
 [3]   Xie, Q. and R. Kapoor, "Enhancements to RTP Payload Formats for
       EVRC Family Codecs", RFC 4788, January 2007.
 [4]   "Enhanced Variable Rate Codec, Speech Service Option 3 and 68
       for Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014-B
       v1.0 , May 2006.

Desineni & Xie Standards Track [Page 22] RFC 5188 EVRC-WB RTP Payload Format February 2008

 [5]   "Enhanced Variable Rate Codec, Speech Service Option 3,68 and
       70 for Wideband Spread Spectrum Digital Systems", 3GPP2
       C.S0014-C v1.0 , October 2006.
 [6]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", STD 64,
       RFC 3550, March 1997.
 [7]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
       Session Description Protocol (SDP)", RFC 3264, June 2002.
 [8]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
       Description Protocol", RFC 4566, July 2006.
 [9]   Casner, S., "Media Type Specifications and Registration
       Procedures", RFC 4855, February 2007.
 [10]  Freed, N. and J. Klensin, "Media Type Specifications and
       Registration Procedures", BCP 13, RFC 4288, December 2005.

20.2. Informative References

 [11]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
       Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
 [12]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
       Protocol", RFC 2974, October 2000.
 [13]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
       Protocol (RTSP)", RFC 2326, April 1998.

Desineni & Xie Standards Track [Page 23] RFC 5188 EVRC-WB RTP Payload Format February 2008

Authors' Addresses

 Harikishan Desineni
 Qualcomm
 5775 Morehouse Drive
 San Diego, CA  92126
 USA
 Phone: +1 858 845 8996
 EMail: hd@qualcomm.com
 URI:   http://www.qualcomm.com
 Qiaobing Xie
 Motorola
 1501 W. Shure Drive, 2-F9
 Arlington Heights, IL  60004
 USA
 Phone: +1-847-372-8481
 EMail: Qiaobing.Xie@Gmail.com
 URI:   http://www.motorola.com

Desineni & Xie Standards Track [Page 24] RFC 5188 EVRC-WB RTP Payload Format February 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
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Desineni & Xie Standards Track [Page 25]

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