Network Working Group                                        H. Desineni
Internet-Draft                                                  Qualcomm
Updates: 4788 (if approved)                                       Q. Xie
Expires: July 15, 2007                                          Motorola
                                                        January 11, 2007


 RTP payload format for EVRC-WB codec and MIME updates for EVRC-B codec
                   draft-ietf-avt-rtp-evrc-wb-00.txt

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Copyright Notice

   Copyright (C) The Internet Society (2007).

Abstract

   This document specifies real-time transport protocol (RTP) payload
   formats to be used for the EVRC wideband codec (EVRC-WB) and updates
   the MIME 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 e-mail.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  EVRC-WB codec  . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  RTP header usage . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Payload format . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Congestion Control Considerations  . . . . . . . . . . . . . .  9
   8.  Storage format for EVRC-WB Codec . . . . . . . . . . . . . . . 10
   9.  Media type definitions . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Registration of Media Type EVRCWB  . . . . . . . . . . . . 11
     9.2.  Registration of Media Type EVRCWB0 . . . . . . . . . . . . 13
     9.3.  Registration of Media Type EVRCWB1 . . . . . . . . . . . . 15
     9.4.  Updated Registration of Media Type EVRCB . . . . . . . . . 17
     9.5.  Updated Registration of Media Type EVRCB0  . . . . . . . . 19
     9.6.  Updated Registration of Media Type EVRCB1  . . . . . . . . 20
   10. EVRC-B RFC XXXX Interoperability with legacy EVRC-B (RFC
       4788) implementations  . . . . . . . . . . . . . . . . . . . . 23
   11. Mapping MIME Parameters into SDP . . . . . . . . . . . . . . . 24
   12. Offer-Answer Model Considerations for EVRC-WB  . . . . . . . . 25
   13. Offer-Answer Model Considerations for EVRC-B . . . . . . . . . 27
   14. Declarative SDP Considerations . . . . . . . . . . . . . . . . 28
   15. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
   17. Security Considerations  . . . . . . . . . . . . . . . . . . . 34
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     18.1. Normative References . . . . . . . . . . . . . . . . . . . 35
     18.2. Informative References . . . . . . . . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
   Intellectual Property and Copyright Statements . . . . . . . . . . 38




















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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.EVRC-WB codec offers better speech quality than
   EVRC and EVRC-B codecs.  EVRC-WB belongs to the EVRC family of
   codecs.This document also updates the MIME registrations for EVRC-B
   codec.








































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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].














































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3.  Background

   EVRC-WB is a wideband extension of EVRC-B [4] speech codec developed
   in 3GPP2 with support for discontinuous transmission (DTX).  It
   provides enhanced (wideband) voice quality.

   EVRC-WB codec operates on 20 ms frames, and the default sampling rate
   is 16 kHz.  Input and output at 8 kHz sampling rate is also
   supported.  EVRC-WB modes are defined in [5].  EVRC-WB mode 4 is
   interoperable with EVRC-B.  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 VoIP
   handsets, media gateways, voice messaging servers, etc.





































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4.  EVRC-WB codec

   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: null frames and
   erasure frames.  The default sampling rate is 16 kHz.  Input and
   output at 8 kHz sampling rate is also supported.

   The frame type values and size of the associated codec data frame are
   listed in the table below:

    Value   Rate      Total codec data frame size (in octets)
    --------------------------------------------------------------------
        0     Blank      0    (0 bit)
        1     1/8        2    (16 bits)
        2     1/4        5    (40 bits; not valid for EVRC-WB)
        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)
































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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.

   EVRC-WB has also 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 8kHz or 16kHz regardless of
   the sampling rate used at the encoder.  Therefore, depending on the
   implementation and the audio electro acoustic 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 assignment of an RTP payload type for the payload formats
   described in Section 6 is outside the scope of the document, and will
   not be specified here.  It is expected that the RTP profile under
   which this payload format is being used will assign a payload type
   for this codec or specify that the payload type is to be bound
   dynamically (see Section 11).




























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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 ToC, interleaving, DTX,
   and bundling, of EVRC-WB are exactly the same as those of EVRC-B, as
   defined in [4], 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].









































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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 [13].

   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.









































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8.  Storage format for EVRC-WB Codec

   The storage format is used for storing EVRC-WB encoded speech frames,
   e.g., as a file or e-mail 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 to maintain synchronization with the
   original media.































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9.  Media type definitions

9.1.  Registration of Media Type EVRCWB

   Type name: audio

   Subtype names: EVRCWB

   Required parameters: none

   Optional parameters:

   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 XX [5]).
   A decoder can use this to signal an encoder to limit its modes to the
   specified subset.  Absence of this parameter signals a set of all
   EVRC-WB modes.

   sendmode: A mode of EVRC-WB codec.  Encoder can use this to signal
   its current mode of operation.  Possible values are a comma separated
   list of modes from the set: 0,4,7 (see Table XX [5]).

   ptime: see RFC 4566 [10].

   maxptime: The maximum amount of media which can be encapsulated in
   each packet, expressed as time in milliseconds.  The time MUST be
   calculated as the sum of the time the media present in the packet
   represents.  The time SHOULD be a multiple of the duration of a
   single codec data frame (20 msec).  If not signaled, the default
   maxptime value MUST be 200 milliseconds.

   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 in [2]

   dtxmax: see Section 6.1 in [2]

   dtxmin: see Section 6.1 in [2]

   hangover: see Section 6.1 in [2]

   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



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   Interleaved/Bundled packet format specified in RFC 3558 [14].

   Security considerations: See Section 17 of RFC XXXX.

   Interoperability considerations: none

   Published specification:

   The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C [5].  Transfer
   method with Interleaved/Bundled packet format via RTP is specified in
   RFC 3558 and RFC XXXX.

   3GPP2 C.S0050-A, 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 information applies for storage format only.

   Magic number: #!EVCWB\n (see Section 8 of RFC XXXX)

   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-A, which is identified using the media types
   "audio/3gpp2" or "video/3gpp2" registered by RFC4393.

   Person & email address to contact for further information:

   Harikishan Desineni <hd@qualcomm.com>

   Intended usage: COMMON

   Restrictions on usage:

   This media type may be used with RTP framing (RFC 3550 [6]) and as a
   storage format.  When used with RTP, the procedures in Section 6 MUST
   be followed.  In all other contexts, the storage format defined in



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   Section 8 MAY be used.

   Author:

   Harikishan Desineni

   Change controller:

   IETF Audio/Video Transport working group delegated from the IESG.

9.2.  Registration of Media Type EVRCWB0

   Type name: audio

   Subtype names: EVRCWB0

   Required parameters:

   Optional parameters:

   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 XX [5]).
   A decoder can use this to signal an encoder to limit its modes to the
   specified subset.  Absence of this parameter signals a set of all
   EVRC-WB modes.

   sendmode: A mode of EVRC-WB codec.  Encoder can use this to signal
   its current mode of operation.  Possible values are a comma separated
   list of modes from the set: 0,4,7 (see Table XX [5]).

   ptime: see RFC 4566 [10].

   silencesupp: see Section 6.1 in [2]

   dtxmax: see Section 6.1 in [2]

   dtxmin: see Section 6.1 in [2]

   hangover: see Section 6.1 in [2]

   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 17 of RFC XXXX.




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   Interoperability considerations: none

   Published specification:

   The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C [5].  Transfer
   method with header free packet format via RTP is specified in RFC
   3558 and RFC XXXX.

   3GPP2 C.S0050-A, 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 information applies for storage format only.

   Magic number: #!EVCWB\n (see Section 8 of RFC XXXX)

   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-A, which is identified using the media types
   "audio/3gpp2" or "video/3gpp2" registered by RFC4393.

   Person & email address to contact for further information:

   Harikishan Desineni <hd@qualcomm.com>

   Intended usage: COMMON

   Restrictions on usage:

   This media type may be used with RTP framing (RFC 3550) and as a
   storage format.  When used with RTP, the procedures in Section 6 MUST
   be followed.  In all other contexts, the storage format defined in
   Section 8 MAY be used.

   Author:




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   Harikishan Desineni

   Change controller:

   IETF Audio/Video Transport working group delegated from the IESG.

9.3.  Registration of Media Type EVRCWB1

   Type name: audio

   Subtype names: EVRCWB1

   Required parameters:

   Optional parameters:

   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 XX [5]).
   A decoder can use this to signal an encoder to limit its modes to the
   specified subset.  Absence of this parameter signals a set of all
   EVRC-WB modes.

   sendmode: A mode of EVRC-WB codec.  Encoder can use this to signal
   its current mode of operation.  Possible values are a comma separated
   list of modes from the set: 0,4,7 (see Table XX [5]).

   ptime: see RFC 4566 [10].

   maxptime: The maximum amount of media which can be encapsulated in
   each packet, expressed as time in milliseconds.  The time MUST be
   calculated as the sum of the time the media present in the packet
   represents.  The time SHOULD be an integer multiple of the duration
   of a single codec data frame (20 msec).  If not signaled, the default
   maxptime value MUST be 200 milliseconds.

   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 [2]

   dtxmax: see Section 6.1 in [2]

   dtxmin: see Section 6.1 in [2]

   hangover: see Section 6.1 in [2]




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   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 17 of RFC XXXX.

   Interoperability considerations: none

   Published specification:

   The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C.  Transfer method
   with compact bundled packet format via RTP is specified in RFC 4788
   and RFC XXXX.

   3GPP2 C.S0050-A, 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 information applies for storage format only.

   Magic number: #!EVCWB\n (see Section 8 of RFC XXXX)

   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-A, 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:



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   This media type may be used with RTP framing (RFC 3550 [6]) and as a
   storage format.  When used with RTP, the procedures in Section 6 MUST
   be followed.  In all other contexts, the storage format defined in
   Section 8 MAY be used.

   Author:

   Harikishan Desineni

   Change controller:

   IETF Audio/Video Transport working group delegated from the IESG.

9.4.  Updated Registration of Media Type EVRCB

   Type name: audio

   Subtype names: EVRCB

   Required parameters: none

   Optional parameters:

   recvmode: A mode of EVRC-B codec.  A decoder can use this to signal
   an encoder to operate in the specified mode.  Possible values are a
   comma separated list of modes from the set: 0..7 (see encoder
   operating point column in Table 2-6 [4]).

   sendmode: A mode of EVRC-B codec.  An encoder can use this to signal
   its current mode of operation.  Possible values are a comma separated
   list of modes from the set: 0..7 (see encoder operating point column
   in Table 2-6 [4]).

   ptime: see RFC 4566

   maxptime: The maximum amount of media which can be encapsulated in
   each packet, expressed as time in milliseconds.  The time MUST be
   calculated as the sum of the time the media present in the packet
   represents.  The time SHOULD be a multiple of the duration of a
   single codec data frame (20 msec).  If not signaled, the default
   maxptime value MUST be 200 milliseconds.

   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 for definition.  If this parameter is



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   not present, the default value 1 MUST be assumed.

   dtxmax: see Section 6.1 of [2]

   dtxmin: see Section 6.1 of [2]

   hangover: see Section 6.1 of [2]

   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.  Transfer method
   with Interleaved/Bundled packet format via RTP is specified in RFC
   3558,RFC XXXX and RFC 4788.

   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 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:

   This media type depends on RTP framing, and hence is only defined for
   transfer via RTP (RFC 3550).  Transfer within other framing protocols
   is not defined at this time.

   Author:




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   Qiaobing Xie / Harikishan Desineni

   Change controller:

   IETF Audio/Video Transport working group delegated from the IESG.

9.5.  Updated Registration of Media Type EVRCB0

   Type name: audio

   Subtype names: EVRCB0

   Required parameters: none

   Optional parameters:

   recvmode: A mode of EVRC-B codec.  A decoder can use this to signal
   an encoder to operate in the specified mode.  Possible values are a
   comma separated list of modes from the set: 0..7 (see encoder
   operating point column in Table 2-6 [4]).

   sendmode: A mode of EVRC-B codec.  An encoder can use this to signal
   its current mode of operation.  Possible values are a comma separated
   list of modes from the set: 0..7 (see encoder operating point column
   in Table 2-6 [4]).

   silencesupp: see Section 6.1 for definition.  If this parameter is
   not present, the default value 1 MUST be assumed.

   dtxmax: see Section 6.1 of [2]

   dtxmin: see Section 6.1 of [2]

   hangover: see Section 6.1 of [2]

   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.  Transfer method



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   with Header-Free packet format via RTP is specified in RFC 3558, RFC
   4788 and RFC XXXX.

   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 (RFC 3550).  Transfer within other framing protocols
   is not defined at this time.

   Author:

   Qiaobing Xie / Harikishan Desineni

   Change controller:

   IETF Audio/Video Transport working group delegated from the IESG.

9.6.  Updated Registration of Media Type EVRCB1

   Type name: audio

   Subtype names: EVRCB1

   Required parameters: none

   Optional parameters:

   recvmode: A mode of EVRC-B codec.  A decoder can use this to signal
   an encoder to operate in the specified mode.  Possible values are a
   comma separated list of modes from the set: 0..7 (see encoder
   operating point column in Table 2-6 [4]).

   sendmode: A mode of EVRC-B codec.  An encoder can use this to signal
   its current mode of operation.  Possible values are a comma separated
   list of modes from the set: 0..7 (see encoder operating point column



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   in Table 2-6 [4]).

   silencesupp: see Section 6.1 for definition.  If this parameter is
   not present, the default value 1 MUST be assumed.

   dtxmax: see Section 6.1 of [2]

   dtxmin: see Section 6.1 of [2]

   hangover: see Section 6.1 of [2]

   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
   compact bundle packet format specified in RFC 4788.

   Security considerations: See Section 9 of RFC 4788.

   Interoperability considerations: none

   Published specification:

   The EVRC-B vocoder is specified in 3GPP2 C.S0014-B.  Transfer method
   with compact bundle packet format via RTP is specified in RFC 4788
   and RFC XXXX.

   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 (RFC 3550).  Transfer within other framing protocols
   is not defined at this time.

   Author:




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   Qiaobing Xie / Harikishan Desineni

   Change controller:

   IETF Audio/Video Transport working group delegated from the IESG.














































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10.  EVRC-B RFC XXXX Interoperability with legacy EVRC-B (RFC 4788)
     implementations

   This document adds new optional parameters "recvmode" and "sendmode"
   to the original EVRC-B payload subtypes "EVRCB", "EVRCB0" and
   "EVRCB1" defined in RFC 4788.  Since the new parameters are receiver
   options, the existing RFC 4788 implementations will not send these
   parameters in SDP and will ignore if they are received.  This will
   allow interoperability between RFC 4788 and RFC XXXX implementations
   of EVRC-B.  For an example offer and answer exchange, see Section 15.









































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11.  Mapping MIME Parameters into SDP

   Information carried in the MIME media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [10], 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:

   The MIME type ("audio") goes in SDP "m=" as the media name.

   o  The MIME subtype ("EVRCWB", "EVRCWB0" or "EVRCWB1") goes in SDP
      "a=rtpmap" as the encoding name.

   o  The optional parameters "mode-set-recv" and "sendmode" (for
      subtypes EVRCWB, EVRCWB0 and EVRCWB1) go in the SDP "a=fmtp"
      attribute by copying them directly from the MIME media type
      string.

   o  The optional parameters "ptime" and "maxptime" (for subtypes
      EVRCWB, EVRCWB1) go in the SDP "a=ptime" and "a=maxptime"
      attributes, respectively.

   o  The optional parameter "maxinterleave" for subtype EVRCWB goes in
      the SDP "a=fmtp" attribute by copying it directly from the MIME
      media type string as "maxinterleave=value".

   o  The optional parameter "fixedrate" for subtypes EVRCWB1 goes in
      "a=fmtp" attribute by copying it directly from the MIME media type
      string as "fixedrate=value".

   o  The optional parameters "silencesupp", "dtxmax", "dtxmin", and
      "hangover" go in "a=fmtp" attribute by copying them directly from
      the MIME media type string as "silencesupp=value", "dtxmax=value",
      "dtxmin=value", and "hangover=value", respectively.

   o  The optional parameters "recvmode" and "sendmode" (for subtypes
      EVRCB, EVRCB0 and EVRCB1) go in the SDP "a=fmtp" attribute by
      copying them directly from the MIME media type string.













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12.  Offer-Answer Model Considerations for EVRC-WB

   The following considerations apply when using SDP offer-answer
   procedures [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 which supports only EVRC-B.

   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:

          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 uni-directional receive only parameter.

   o  "sendmode" is a uni-directional send only parameter.

   o  Using "sendmode", a sender can signal its current mode of
      operation.

   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 RFC3264 [7].
      The "maxptime" parameter MUST be handled in the same way.




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   o  For a sendonly stream, "mode-set-recv" parameter is not useful and
      SHOULD NOT be used.

   o  For a recvonly stream, "sendmode" parameter is not useful and
      SHOULD NOT be used.

   o  For additional rules which MUST be followed while negotiating DTX
      parameters, see Section 6.8 in [2].

   o  Any unknown parameter in an SDP offer MUST be ignored by the
      receiver and MUST NOT be included in the SDP answer.








































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13.  Offer-Answer Model Considerations for EVRC-B

   See Section 6.8 of [2] for offer-answer usage of EVRC-B.  The
   following are several additional considerations for EVRC-B.

   o  "recvmode" is a uni-directional receive only parameter.

   o  "sendmode" is a uni-directional 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.

   o  For a sendonly stream, "recvmode" parameter is not useful and
      SHOULD NOT be used.

   o  For a recvonly stream, "sendmode" parameter is not useful and
      SHOULD NOT be used.






























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14.  Declarative SDP Considerations

   For declarative use of SDP in SAP [15] and RTSP [16] , 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.





































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15.  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 MIME subtype names are case-
   insensitive.  Parameter names are case-insensitive both in MIME 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=rtpmap:98 EVRCB0/8000
       a=fmtp:97 fixedrate=0.5 mode-set-recv=4 sendmode=7
       a=fmtp:98 recvmode=7 sendmode=7
       a=maxptime:100



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     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


     Examples 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 usage of EVRCB1:

        m=audio 49120 RTP/AVP 97
        a=rtpmap:97 EVRCB1/8000
        a=fmtp:97 fixedrate=0.5 recvmode=7 sendmode=7
        a=maxptime:100








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     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




     Example offer answer exchange between EVRC-WB and
     updated EVRC-B (RFC XXXX):

      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
















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     Example offer answer exchanges for interoperability between
     legacy (RFC XXXX) and updated EVRC-B(RFC 4788) implementations:

        Offer from an offer which supports updated EVRC-B (RFC XXXX)
        implementation:

          m=audio 55954 RTP/AVP 99
          a=rtpmap:99 EVRCB0/8000
          a=fmtp:99 recvmode=0 sendmode=4

        Answer from an answerer which supports only
        legacy EVRC-B (RFC 4788) implementation:

          m=audio 55954 RTP/AVP 99
          a=rtpmap:99 EVRCB0/8000

        Offer from an offer which supports only
        legacy EVRC-B (RFC 4788) implementation:

          m=audio 55954 RTP/AVP  99
          a=rtpmap:99 EVRCB0/8000

        Answer from an answerer which supports updated
        EVRC-B (RFC XXXX) implementation:

          m=audio 55954 RTP/AVP 99
          a=rtpmap:99 EVRCB0/8000
          a=fmtp:99 recvmode=0 sendmode=4























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16.  IANA Considerations

   Three new MIME subtype registrations ("EVRCWB", "EVRCWB0" and
   "EVRCWB1") are defined in this document.

   The MIME subtype registrations "EVRCB","EVRCB0" and "EVRCB1"
   originally defined in [2] are updated with optional "recvmode" and
   "sendmode" parameters (See Section 9.4, Section 9.5 and Section 9.6).

   [-- RFC Editor: Please replace all instances of "RFC XXXX" in this
   document with the RFC number of this document prior to IANA
   registration and RFC publication, and remove this note.]

   [-- RFC Editor:Please add MIME subtypes "EVRCWB","EVRCWB0", and
   "EVRCWB1" to the IANA registry for "RTP Payload Format MIME types" at
   http://www.iana.org/assignments/rtp-parameters, and remove this
   note.]


































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17.  Security Considerations

   Implementations using the payload defined in this specification are
   subject to the security considerations discussed in RFC3558 [14],
   RFC3550 [6], and any appropriate profile (for example RFC3551 [13]).
   This payload does not specify any different security services.













































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18.  References

18.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Xie, Q., "Enhancements to RTP Payload Formats for EVRC Family
         Codecs", RFC 4788, January 2007.

   [3]   "Enhanced Variable Rate Codec, Speech Service Option 3 for
         Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014 ,
         January 1997.

   [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.

   [5]   "Enhanced Variable Rate Codec, Speech Service Option 3,68 and
         70 for Wideband Spread Spectrum Digital Systems", 3GPP2  Work
         in Progress, October 2006.

   [6]   Schulzrinne, H., "RTP: A Transport Protocol for Real-Time
         Applications", STD 64, RFC 3550, March 1997.

   [7]   Rosenberg, J., "An Offer/Answer Model with Session Description
         Protocol (SDP)", RFC 3264, June 2002.

   [8]   Narten, T., "Guidelines for Writing an IANA Considerations
         Section in RFCs", RFC 2434, October 1998.

   [9]   Johnston, A., "SDP Offer/Answer Examples", RFC 4317,
         December 2005.

   [10]  Handley, M., "SDP: Session Description Protocol", RFC 4566,
         July 2006.

   [11]  Garudadri, H., "3GPP2 Multimedia Registrations", RFC 4393,
         March 2006.

   [12]  "3GPP2 File Formats for Multimedia Services", 3GPP2 C.S0050-A ,
         March 2006.

18.2.  Informative References

   [13]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences
         with Minimal Control", STD 65, RFC 3551, July 2003.




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   [14]  Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs
         (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558,
         July 2003.

   [15]  Handley, M., "Session Announcement Protocol", RFC 2974,
         October 2000.

   [16]  Schulzrinne, H., "Real Time Streaming Protocol (RTSP)",
         RFC 2326, April 1998.

   [17]  Westerlund, M., "How to Write an RTP Payload Format",
         draft-ietf-avt-rtp-howto-01.txt(Work in Progress) , Dec 2006.







































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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: +1-847-632-3028
   Email: Qiaobing.Xie@Motorola.com
   URI:   http://www.motorola.com





























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