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rfc:rfc6884

Internet Engineering Task Force (IETF) Z. Fang Request for Comments: 6884 Qualcomm Incorporated Category: Standards Track March 2013 ISSN: 2070-1721

                         RTP Payload Format
 for the Enhanced Variable Rate Narrowband-Wideband Codec (EVRC-NW)

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6884.

Copyright Notice

 Copyright (c) 2013 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Fang Standards Track [Page 1] RFC 6884 EVRC-NW RTP Payload Format March 2013

Table of Contents

 1. Introduction ....................................................2
 2. Conventions .....................................................2
 3. Background ......................................................3
 4. EVRC-NW Codec ...................................................3
 5. RTP Header Usage ................................................4
 6. Payload Format ..................................................4
    6.1. Encoding Capability Identification in EVRC-NW
         Interleaved/Bundled Format .................................5
 7. Congestion Control Considerations ...............................6
 8. Storage Format for the EVRC-NW Codec ............................6
 9. IANA Considerations .............................................7
    9.1. Media Type Registrations ...................................7
         9.1.1. Registration of Media Type audio/EVRCNW .............7
         9.1.2. Registration of Media Type audio/EVRCNW0 ............9
         9.1.3. Registration of Media Type audio/EVRCNW1 ...........10
 10. SDP Mode Attributes for EVRC-NW ...............................12
 11. Mode Change Request/Response Considerations ...................13
 12. Mapping EVRC-NW Media Type Parameters into SDP ................14
 13. Offer-Answer Model Considerations for EVRC-NW .................14
 14. Declarative SDP Considerations ................................16
 15. Examples ......................................................16
 16. Security Considerations .......................................19
 17. References ....................................................19
    17.1. Normative References .....................................19
    17.2. Informative References ...................................20

1. Introduction

 This document specifies the payload formats for packetization of
 EVRC-NW 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-NW codec as well as
 discontinuous transmission (DTX) support for EVRC-NW encoded speech
 transported via RTP.  The EVRC-NW codec offers better speech quality
 than the EVRC and EVRC-B codecs and better capacity than the Enhanced
 Variable Rate Wideband Codec (EVRC-WB).  EVRC-NW belongs to the EVRC
 family of codecs.

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].

Fang Standards Track [Page 2] RFC 6884 EVRC-NW RTP Payload Format March 2013

3. Background

 EVRC-NW is an extension of both the EVRC-B [2] and EVRC-WB [3] speech
 codecs developed in the Third Generation Partnership Project 2
 (3GPP2) with support for DTX.  It provides enhanced voice quality and
 high spectral efficiency.
 The EVRC-NW codec operates on 20 ms frames, and the default sampling
 rate is 16 kHz (wideband).  Input and output at the 8 kHz sampling
 rate (narrowband) is also supported.  The EVRC-NW codec can operate
 in eight modes (0 to 7) as defined in 3GPP2 C.S0014-D [4].  EVRC-NW
 modes 0, 1, and 7 are interoperable with EVRC-WB.  EVRC-NW modes 1 to
 7 are interoperable with EVRC-B.  EVRC-NW modes 0 to 6 use the full
 set or a subset of full rate, 1/2 rate, 1/4 rate, and 1/8 rate
 frames.  EVRC-NW mode 7 uses only 1/2 rate and 1/8 rate frames.  By
 default, EVRC-NW supports all narrowband modes (modes 1 to 7).  The
 support of wideband mode (mode 0) is optional.  Mode change among
 modes 1 to 7 (or among modes 0 to 7 if the receiver supports wideband
 mode) results in codec output bit-rate change but does not cause any
 decoding problems at the receiver.  EVRC-NW provides a standardized
 solution for packetized voice applications that allow transitions
 between enhanced quality and increased capacity.  The most important
 service addressed is IP telephony.  Target devices can be IP phones
 or VoIP handsets, media gateways, voice messaging servers, etc.

4. EVRC-NW Codec

 The EVRC-NW codec operates on 20 ms frames.  It produces output
 frames of one of the four different sizes: 171 bits (Rate 1), 80 bits
 (Rate 1/2), 40 bits (Rate 1/4), or 16 bits (Rate 1/8).  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 the 8 kHz sampling rate is 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 (Null)   0    (0 bits)
   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)

Fang Standards Track [Page 3] RFC 6884 EVRC-NW RTP Payload Format March 2013

5. RTP Header Usage

 The format of the RTP header is specified in RFC 3550 [5].  The
 EVRC-NW payload formats (Section 6) use the fields of the RTP header
 as specified in RFC 3550 [5].
 EVRC-NW 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 electroacoustic 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-NW 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 of EVRC-NW, such as TOC,
 interleaving, DTX, and bundling, are exactly the same as those
 defined in EVRC [6], EVRC-B [2], and EVRC-WB [3], except that
 1.  the mode change request field in the interleaved/bundled packet
     format MUST be interpreted according to the definition of the
     RATE_REDUC parameter as described for EVRC-NW in
     3GPP2 C.S0014-D [4].
 2.  the mode change request field in the interleaved/bundled packet
     format SHOULD be honored by an EVRC-NW encoding endpoint in a
     one-to-one session with a dedicated EVRC-NW decoding endpoint,
     such as in a two-party call or in a conference leg.
 3.  the reserved bit field in the first octet of the interleaved/
     bundled format has only one bit.  Bit 1 of the first octet is an
     EVRC-NW wideband/narrowband encoding capability identification
     flag.

Fang Standards Track [Page 4] RFC 6884 EVRC-NW RTP Payload Format March 2013

 The media type audio/EVRCNW maps to the interleaved/bundled packet
 format, audio/EVRCNW0 maps to the header-free packet format, and
 audio/EVRCNW1 maps to the compact bundled packet format.

6.1. Encoding Capability Identification in EVRC-NW Interleaved/Bundled

    Format
 The EVRC-NW interleaved/bundled format defines an encoding capability
 identification flag, which is used to signal the local EVRC-NW
 wideband/narrowband encoding capability at the time of construction
 of an RTP packet to the far end of a communication session.  This
 capability identification flag allows the far end to use the MMM
 field in its outgoing (returning) EVRC-NW interleaved/bundled format
 packets to request the desired EVRC-NW wideband or narrowband
 encoding mode in accordance with the dynamic/instantaneous encoding
 capability information.  See RFC 3558 [6] for the definition of the
 MMM field.  The following examples illustrate a few scenarios where
 the encoding capability information is used:
 o  An end-to-end wideband communication is established first between
    two communication endpoints using the EVRC-NW interleaved/bundled
    format.  The called endpoint becomes wideband encoding incapable
    during the call and makes the other end aware of this change by
    using the encoding capability identification flag.  Based on the
    new information, the calling endpoint could change the MMM value
    in its outgoing EVRC-NW packets from mode 0 to mode 4 to request
    narrowband encoded traffic for bandwidth efficiency or from mode 0
    to mode 1 for best perceptual quality.
 o  An end-to-end narrowband communication is established between a
    calling endpoint that is EVRC-NW wideband encoding capable and a
    called endpoint that is EVRC-NW wideband encoding incapable.  The
    called endpoint becomes EVRC-NW wideband encoding capable during
    the call and makes the other end aware of this change using the
    encoding capability identification flag.  Based on the new
    information, the calling endpoint could change the MMM value in
    its outgoing EVRC-NW packets from non-mode-0 to mode 0 to request
    wideband traffic.
 The EVRC-NW interleaved/bundled format defines the encoding
 capability identification flag in bit 1 of the first octet, as
 illustrated in the figure below.  The flag shall be set to zero (C=0)
 when the local EVRC-NW encoder is capable of mode 0 wideband
 encoding.  The flag shall be set to one (C=1) when the local EVRC-NW
 encoder is capable of non-mode-0 narrowband encoding only.  See
 RFC 3558 [6] for original definitions of other fields in the
 interleaved/bundled format.

Fang Standards Track [Page 5] RFC 6884 EVRC-NW RTP Payload Format March 2013

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        RTP Header                             |
  +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
  |R|C| LLL | NNN | MMM |  Count  |  TOC  |  ...  |  TOC  |padding|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        one or more codec data frames, one per TOC entry       |
  |                             ....                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Reserved (R): 1 bit
    Reserved bit.  MUST be set to zero by sender; SHOULD be ignored by
    receiver.
 Encoding capability identification (C): 1 bit
    Must be set to zero by sender to indicate wideband encoding
    capable or set to one to indicate narrowband encoding capable
    only.
    C = 0 :  mode 0 wideband encoding capable
      = 1 :  mode 0 wideband encoding incapable, i.e., narrowband
             encoding only.

7. Congestion Control Considerations

 Congestion control for RTP is discussed in RFC 3550 [5] and in
 applicable RTP profiles, e.g., RFC3551 [7].  This document does not
 change those considerations.
 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-NW Codec

 The storage format is used for storing EVRC-NW 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-NW corresponds to the ASCII
 character string "#!EVRCNW\n", i.e., "0x23 0x21 0x45 0x56 0x52 0x43
 0x4E 0x57 0x0A".

Fang Standards Track [Page 6] RFC 6884 EVRC-NW RTP Payload Format March 2013

 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.  The Value column in the
 table in Section 4 provides the TOC values for corresponding frame
 types.
 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 introduces a new EVRC-NW 'audio' media subtype.

9.1. Media Type Registrations

 Following the guidelines in RFC 4855 [8] and RFC 6838 [9], this
 section registers new 'audio' media subtypes for EVRC-NW.

9.1.1. Registration of Media Type audio/EVRCNW

 Type name: audio
 Subtype name: EVRCNW
 Required parameters: None
 Optional parameters:  These parameters apply to RTP transfer only.
    mode-set-recv:  A subset of EVRC-NW modes.  Possible values are a
       comma-separated list of modes from the set {0,1,2,3,4,5,6,7}
       (see Table 2.6.1.2-1 in 3GPP2 C.S0014-D [4]).  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 {1,2,3,4,5,6,7}.
    ptime:  See RFC 4566 [10].
    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.

Fang Standards Track [Page 7] RFC 6884 EVRC-NW RTP Payload Format March 2013

    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 6838, Section 4.8)
    and is defined for transfer of EVRC-NW encoded data via RTP using
    the interleaved/bundled packet format specified in RFC 3558 [6].
 Security considerations:  See Section 16.
 Interoperability considerations:  None
 Published specification:
    The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D [4].  The
    transfer method with the interleaved/bundled packet format via RTP
    is specified in RFC 3558 [6].  See Section 6 of RFC 6884 for
    details for EVRC-NW.
 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: #!EVRCNW\n (see Section 8)
       File extensions: enw, ENW
       Macintosh file type code: None
       Object identifier or OID: None
 EVRC-NW speech frames may also be stored in the file format "3g2" as
 defined in 3GPP2 C.S0050-B [14], which is identified using the media
 types "audio/3gpp2" or "video/3gpp2" registered by RFC 4393 [11].
 Person & email address to contact for further information:
    Zheng Fang <zfang@qualcomm.com>
 Intended usage:  COMMON

Fang Standards Track [Page 8] RFC 6884 EVRC-NW RTP Payload Format March 2013

 Restrictions on usage:
    This media type can be used with the file format defined in
    Section 8 of RFC 6884 in contexts other than RTP.  In the context
    of transfers over RTP, the RTP payload format specified in
    Section 4.1 of RFC 3558 [6] is used for this media type.
 Author:  Zheng Fang <zfang@qualcomm.com>
 Change controller:
    IETF Payload working group delegated from the IESG.

9.1.2. Registration of Media Type audio/EVRCNW0

 Type name: audio
 Subtype name: EVRCNW0
 Required parameters: None
 Optional parameters:  These parameters apply to RTP transfer only.
    mode-set-recv:  A subset of EVRC-NW modes.  Possible values are a
       comma-separated list of modes from the set {0,1,2,3,4,5,6,7}
       (see Table 2.6.1.2-1 in 3GPP2 C.S0014-D [4]).  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 {1,2,3,4,5,6,7}.
    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 6838, Section 4.8)
    and is defined for transfer of EVRC-NW encoded data via RTP using
    the header-free packet format specified in RFC 3558 [6].
 Security considerations:  See Section 16.
 Interoperability considerations:  None

Fang Standards Track [Page 9] RFC 6884 EVRC-NW RTP Payload Format March 2013

 Published specification:
    The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D [4].  The
    transfer method with the header-free packet format via RTP is
    specified in RFC 3558 [6].
 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:
    Zheng Fang <zfang@qualcomm.com>
 Intended usage:  COMMON
 Restrictions on usage:
    This media type depends on RTP framing and hence is only defined
    for transfer via RTP [5].  The RTP payload format specified in
    Section 4.2 of RFC 3558 [6] SHALL be used.  This media type SHALL
    NOT be used for storage or file transfer; instead, audio/EVRCNW
    SHALL be used.
 Author:  Zheng Fang <zfang@qualcomm.com>
 Change controller:
    IETF Payload working group delegated from the IESG.

9.1.3. Registration of Media Type audio/EVRCNW1

 Type name: audio
 Subtype name: EVRCNW1
 Required parameters: None
 Optional parameters:  These parameters apply to RTP transfer only.
    mode-set-recv:  A subset of EVRC-NW modes.  Possible values are a
       comma-separated list of modes from the set {0,1} (see Table
       2.6.1.2-1 in 3GPP2 C.S0014-D [4]).  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 support for
       wideband fixed rate (full or half rate, depending on the value
       of the 'fixedrate' parameter).  A value of 1 signals narrowband
       fixed rate (full or half rate, depending on the value of the
       'fixedrate' parameter).  Absence of this parameter signals
       mode 1.

Fang Standards Track [Page 10] RFC 6884 EVRC-NW RTP Payload Format March 2013

    ptime:  See RFC 4566.
    maxptime:  See RFC 4566.
    fixedrate:  Indicates the EVRC-NW 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 6838, Section 4.8)
    and is defined for transfer of EVRC-NW encoded data via RTP using
    the compact bundled packet format specified in RFC 4788.
 Security considerations:  See Section 16.
 Interoperability considerations:  None
 Published specification:
    The EVRC-NW vocoder is specified in 3GPP2 C.S0014-D [4].  The
    transfer method with the compact bundled packet format via RTP is
    specified in 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: None
 Person & email address to contact for further information:
    Zheng Fang <zfang@qualcomm.com>
 Intended usage:  COMMON

Fang Standards Track [Page 11] RFC 6884 EVRC-NW RTP Payload Format March 2013

 Restrictions on usage:
    This media type depends on RTP framing and hence is only defined
    for transfer via RTP [5].  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; instead, audio/EVRCNW SHALL be
    used.
 Author:  Zheng Fang <zfang@qualcomm.com>
 Change controller:
    IETF Payload working group delegated from the IESG.

10. SDP Mode Attributes for EVRC-NW

 'mode-set-recv' can be used by a decoder to inform an encoder of its
 preference to operate in a specified subset of modes.  Note that
 indicating a preference implicitly indicates support for that
 capability.  If mode 0 is not preferred for media type EVRCNW0 or
 EVRCNW1, then there is no indication that mode 0 is supported.
 However, absence of this parameter or absence of mode 0 in this
 parameter for media type EVRCNW shall not preclude mode 0 support
 during a call where mode 0 may be requested via the MMM field.
 1.  To inform other nodes of its capability for wideband mode
     support: a decoder can always decode all the narrowband modes
     (modes 1 to 7).  Unless the decoder indicates support of mode 0
     (i.e., preference) in this parameter or in the MMM mode request
     field in the interleaved/bundled payload format, an encoder at
     the other side shall not operate in mode 0.
 2.  To indicate a preference to operate in a subset of modes: a set
     has been defined so that several modes can be expressed as a
     preference in one attempt.  For instance, the set {4,5,6,7}
     signals that the receiver prefers that the sender operate in
     bandwidth-efficient narrowband modes of EVRC-NW.
 Note that during an active call session using the interleaved/bundled
 packet format, the MMM mode request received from a communication
 partner can contain a mode request different than the values in the
 last mode-set-recv attribute.  The partner's EVRC-NW wideband
 decoding capability is determined by the latest mode-set-recv
 attribute or MMM mode request field.  For example, a mode request
 with MMM=0 from a communication partner is an implicit indication of
 the partner's EVRC-NW wideband decoding capability and preference.
 An EVRC-NW wideband-capable node receiving the request can operate in
 wideband mode.  A mode request with MMM=1, 2, ..., or 7 from a
 communication partner is an implicit indication of the partner's

Fang Standards Track [Page 12] RFC 6884 EVRC-NW RTP Payload Format March 2013

 EVRC-NW narrowband decoding preference.  The encoder of an EVRC-NW
 node receiving the request shall honor the request and operate in
 narrowband mode.
 'sendmode' is used as a Session Description Protocol (SDP) mode
 attribute in EVRC [6], EVRC-B [2], and EVRC-WB [3].  However, it is
 deprecated in EVRC-NW.

11. Mode Change Request/Response Considerations

 The interleaved/bundled packet format for the EVRC family of vocoders
 supports a 3-bit field (MMM) that a communication node can use to
 indicate its preferred compression mode to an opposite node.  The
 concept of the compression mode (also known as Capacity Operating
 Point) was introduced to allow a controlled trade-off between voice
 quality and channel capacity.  The notion makes it possible to
 exercise vocoders at the highest possible (average) bit-rate (hence,
 highest voice quality) when the network is lightly loaded.
 Conversely, once the network load increases, the vocoders can be
 requested to operate at lower average bit-rates so as to absorb the
 additional network load without causing an undue increase in the
 frame-erasure rates; the underlying premise is that while a higher
 bit-rate improves vocoder performance, it also increases the network
 load, risking a sharp decline in voice quality should the frame-
 erasure rate be too high.  By contrast, a lower bit-rate mode of
 operation can result in accommodation of the additional network load
 without causing unduly high frame-erasure rates, resulting in better
 overall quality despite the inherently lower voice quality of the
 lower bit-rate mode of the vocoder.
 Accordingly, the MMM field should be used to request the far end to
 transmit compressed speech using a mode that provides the best
 balance between voice quality and capacity.  However, in the case of
 mobile-mobile calls, for example, there are two wireless sides
 involved, each with a potentially different network load level and
 hence a different preferred mode.  In such cases, achieving optimal
 end-to-end performance depends on coherent management of the
 operative mode by the two sides.  This requires that even if the
 local node prefers a higher bit-rate vocoder mode, it should adjust
 to a lower bit-rate mode if requested by the far end, in order to
 avoid potentially high frame-erasure rates due to heavy load at the
 far-end network.  For similar reasons, in cases where a mode
 requested by the far end should not be supported, it might still be
 beneficial to consider switching to a supported vocoder mode
 corresponding to a lower average bit-rate than requested.  It is
 recommended that the next lower average bit-rate supported vocoder
 mode be used for encoding when a mode requested by the far end is not
 supported.

Fang Standards Track [Page 13] RFC 6884 EVRC-NW RTP Payload Format March 2013

 A wideband-capable endpoint can use the information conveyed by the
 C-bit of the RTP payload header to determine the optimal mode to
 request of the far end.  If the far end cannot provide mode 0 packets
 (C-bit=1), then the choice of MMM can be based strictly on the local
 network load.  If the C-bit indicates the remote end's mode 0
 encoding capability (C-bit=0), then even if the local network load is
 not light, mode 0 can be requested knowing definitively that it will
 be supported.  This will permit operators to treat wideband-capable
 mobiles preferentially, should they wish to adopt such policy.

12. Mapping EVRC-NW Media Type Parameters into SDP

 Information carried in the 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-NW encoded speech, the mapping is as
 follows.
 o  The media type ("audio") goes in SDP "m=" as the media name.
 o  The media subtype ("EVRCNW", "EVRCNW0", or "EVRCNW1") goes in SDP
    "a=rtpmap" as the encoding name.
 o  The optional parameters 'ptime and 'maxptime' (for subtypes EVRCNW
    and EVRCNW1) go in the SDP "a=ptime" and "a=maxptime" attributes,
    respectively.
 o  Any remaining parameters (for subtypes EVRCNW, EVRCNW0, and
    EVRCNW1) 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. Offer-Answer Model Considerations for EVRC-NW

 The following considerations apply when using the SDP offer-answer
 procedures of RFC 3264 [12] to negotiate the use of EVRC-NW payload
 in RTP:
 o  Since EVRC-NW is an extension of both EVRC-B and EVRC-WB, the
    offerer SHOULD also announce EVRC-B and EVRC-WB support in its
    "m=audio" lines, with EVRC-NW as the preferred codec.  This will
    allow interoperability with an answerer that supports only EVRC-B
    and/or EVRC-WB.

Fang Standards Track [Page 14] RFC 6884 EVRC-NW RTP Payload Format March 2013

 Below is an example of such an offer:
        m=audio 55954 RTP/AVP 98 99 100
        a=rtpmap:98 EVRCNW0/16000
        a=rtpmap:99 EVRCWB0/16000
        a=rtpmap:100 EVRCB0/8000
        a=fmtp:98 mode-set-recv=0,1,2,3,4,5,6
        a=fmtp:99 mode-set-recv=0,4
        a=fmtp:100 recvmode=0
 If the answerer supports EVRC-NW, then the answerer can keep the
 payload type 98 in its answer and the conversation can be done using
 EVRC-NW.  Otherwise, if the answerer supports only EVRC-WB and/or
 EVRC-B, then the answerer will leave only the payload type 99 and/or
 100, respectively, in its answer and the conversation will be done
 using EVRC-WB and/or EVRC-B, respectively.
 An example answer for the above offer:
        m=audio 55954 RTP/AVP 98
        a=rtpmap:98 EVRCNW0/16000
        a=fmtp:98 mode-set-recv=4
 o  'mode-set-recv' is a unidirectional receive-only 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.  However, a remote sender shall not assume the other
    side can support mode 0, unless the offer includes mode 0
    explicitly in 'mode-set-recv' or the remote sender receives mode
    requests with MMM=0 from the communication partner during an
    active call using the EVRC-NW interleaved/bundled format.
 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
    [12].  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  When using EVRCNW1, the entire session MUST use the same fixed
    rate and mode (0-Wideband or 1-Narrowband).

Fang Standards Track [Page 15] RFC 6884 EVRC-NW RTP Payload Format March 2013

 o  For additional rules that MUST be followed while negotiating DTX
    parameters, see Section 6.8 in RFC 4788 [2].
 o  Any unknown parameter in an SDP offer MUST be ignored by the
    receiver and MUST NOT be included in the SDP answer.

14. Declarative SDP Considerations

 For declarative use of SDP in the Session Announcement Protocol (SAP)
 [15] and the Real Time Streaming Protocol (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.  For declarative examples,
    see Section 15.
 o  The usage of unidirectional receive-only parameters, such as
    'mode-set-recv', should be excluded in any declarations, since
    these parameters are meaningless in one-way streaming
    applications.

15. Examples

 Some example SDP session descriptions utilizing EVRC-NW 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 EVRCNW if wideband mode is supported:
        m=audio 49120 RTP/AVP 97 98 99
        a=rtpmap:97 EVRCNW/16000
        a=rtpmap:98 EVRCWB/16000
        a=rtpmap:99 EVRCB/8000
        a=fmtp:97 mode-set-recv=0,1,2,3,4,5,6
        a=fmtp:98 mode-set-recv=0,4
        a=fmtp:99 recvmode=0
        a=maxptime:120

Fang Standards Track [Page 16] RFC 6884 EVRC-NW RTP Payload Format March 2013

 Example usage of EVRCNW if wideband mode is not supported:
        m=audio 49120 RTP/AVP 97 98 99
        a=rtpmap:97 EVRCNW/16000
        a=rtpmap:98 EVRCWB/16000
        a=rtpmap:99 EVRCB/8000
        a=fmtp:97 mode-set-recv=1,2,3,4,5,6
        a=fmtp:98 mode-set-recv=4
        a=fmtp:99 recvmode=0
        a=maxptime:120
 Example usage of EVRCNW0:
        m=audio 49120 RTP/AVP 97 98 99
        a=rtpmap:97 EVRCNW0/16000
        a=rtpmap:98 EVRCWB0/16000
        a=rtpmap:99 EVRCB0/8000
        a=fmtp:97 mode-set-recv=0,1,2,3,4,5,6
        a=fmtp:98 mode-set-recv=0,4
        a=fmtp:99 recvmode=0
 Example SDP answer from a media gateway requesting a terminal to
 limit its encoder operation to EVRC-NW mode 4.
        m=audio 49120 RTP/AVP 97
        a=rtpmap:97 EVRCNW0/16000
        a=fmtp:97 mode-set-recv=4
 Example usage of EVRCNW1:
        m=audio 49120 RTP/AVP 97 98 99
        a=rtpmap:97 EVRCNW1/16000
        a=rtpmap:98 EVRCWB1/16000
        a=rtpmap:99 EVRCB1/8000
        a=fmtp:97 fixedrate=0.5
        a=fmtp:98 fixedrate=0.5
        a=fmtp:99 fixedrate=0.5
        a=maxptime:100

Fang Standards Track [Page 17] RFC 6884 EVRC-NW RTP Payload Format March 2013

 Example usage of EVRCNW with DTX with silencesupp=1:
        m=audio 49120 RTP/AVP 97 98 99
        a=rtpmap:97 EVRCNW/16000
        a=rtpmap:98 EVRCWB/16000
        a=rtpmap:99 EVRCB/8000
        a=fmtp:97 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1; \
        mode-set-recv=0,1,2,3,4,5,6
        a=fmtp:98 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1; \
        mode-set-recv=0,4
        a=fmtp:99 recvmode=0
        a=maxptime:120
 Example usage of EVRCNW with DTX with silencesupp=0:
        m=audio 49120 RTP/AVP 97 98 99
        a=rtpmap:97 EVRCNW/16000
        a=rtpmap:98 EVRCWB/16000
        a=rtpmap:99 EVRCB/8000
        a=fmtp:97 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1; \
        mode-set-recv=0,1,2,3,4,5,6
        a=fmtp:98 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1; \
        mode-set-recv=0,4
        a=fmtp:99 recvmode=0
        a=maxptime:120
 Example offer-answer exchange between EVRC-NW and legacy EVRC-B
 (RFC 4788):
       Offer:
         m=audio 55954 RTP/AVP 97 98 99
         a=rtpmap:97 EVRCNW0/16000
         a=rtpmap:98 EVRCWB0/16000
         a=rtpmap:99 EVRCB0/8000
         a=rtpmap:97 mode-set-recv=0,1,2,3,4,5,6
         a=fmtp:98 mode-set-recv=0,4
         a=fmtp:99 recvmode=0
       Answer:
         m=audio 55954 RTP/AVP 99
         a=rtpmap:99 EVRCB0/8000

Fang Standards Track [Page 18] RFC 6884 EVRC-NW RTP Payload Format March 2013

 Example offer-answer exchange between EVRC-NW and legacy EVRC-WB
 (RFC 5188):
       Offer:
         m=audio 55954 RTP/AVP 97 98 99
         a=rtpmap:97 EVRCNW0/16000
         a=rtpmap:98 EVRCWB0/16000
         a=rtpmap:99 EVRCB0/8000
         a=rtpmap:97 mode-set-recv=0,1,2,3,4,5,6
         a=fmtp:98 mode-set-recv=0,4
         a=fmtp:99 recvmode=0
       Answer:
         m=audio 55954 RTP/AVP 98 99
         a=rtpmap:98 EVRCWB0/16000

16. 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 [6].  Implementations using the
 payload defined in this specification are subject to the security
 considerations discussed in RFC 3558 [6], RFC 3550 [5], and any
 appropriate profile (for example, RFC 3551 [7]).  Additional security
 considerations are described in RFC 6562 [13].

17. References

17.1. Normative References

 [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [2]   Xie, Q. and R. Kapoor, "Enhancements to RTP Payload Formats for
       EVRC Family Codecs", RFC 4788, January 2007.
 [3]   Desineni, H. and Q. Xie, "RTP Payload Format for the Enhanced
       Variable Rate Wideband Codec (EVRC-WB) and the Media Subtype
       Updates for EVRC-B Codec", RFC 5188, February 2008.
 [4]   "Enhanced Variable Rate Codec, Speech Service Options 3, 68,
       70, and 73 for Wideband Spread Spectrum Digital Systems",
       3GPP2 C.S0014-D v3.0, October 2010, <http://www.3gpp2.org/
       public_html/specs/C.S0014-D_v3.0_EVRC.pdf>.

Fang Standards Track [Page 19] RFC 6884 EVRC-NW RTP Payload Format March 2013

 [5]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", STD 64,
       RFC 3550, July 2003.
 [6]   Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs
       (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558,
       July 2003.
 [7]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
       Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
 [8]   Casner, S., "Media Type Registration of RTP Payload Formats",
       RFC 4855, February 2007.
 [9]   Freed, N., Klensin, J., and T. Hansen, "Media Type
       Specifications and Registration Procedures", BCP 13, RFC 6838,
       January 2013.
 [10]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
       Description Protocol", RFC 4566, July 2006.
 [11]  Garudadri, H., "MIME Type Registrations for 3GPP2 Multimedia
       Files", RFC 4393, March 2006.
 [12]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
       Session Description Protocol (SDP)", RFC 3264, June 2002.
 [13]  Perkins, C. and JM. Valin, "Guidelines for the Use of Variable
       Bit Rate Audio with Secure RTP", RFC 6562, March 2012.

17.2. Informative References

 [14]  "3GPP2 File Formats for Multimedia Services", 3GPP2 C.S0050-B
       v1.0, May 2007, <http://www.3gpp2.org/public_html/specs/
       C.S0050-B_v1.0_070521.pdf>.
 [15]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
       Protocol", RFC 2974, October 2000.
 [16]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
       Protocol (RTSP)", RFC 2326, April 1998.

Fang Standards Track [Page 20] RFC 6884 EVRC-NW RTP Payload Format March 2013

Author's Address

 Zheng Fang
 Qualcomm Incorporated
 5775 Morehouse Drive
 San Diego, CA  92126
 USA
 Phone: +1 858 651 9484
 EMail: zfang@qualcomm.com
 URI:   http://www.qualcomm.com

Fang Standards Track [Page 21]

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