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

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.

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

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.

   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.

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].

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.

   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.

   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.

   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
   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.

   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
   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.

   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.

   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
   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
   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.

   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).

   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.

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
     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
     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
        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.

   [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.

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

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