    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.

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    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

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    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.

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       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.

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    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].

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    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.

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    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.

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    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.

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    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

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       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.

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    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

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       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.

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       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.

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       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

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    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

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    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.

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       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).

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       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.

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    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

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    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

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    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

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    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.

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    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
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