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rfc:rfc3558

Network Working Group A. Li Request for Comments: 3558 UCLA Category: Standards Track July 2003

    RTP Payload Format for Enhanced Variable Rate Codecs (EVRC)
                 and Selectable Mode Vocoders (SMV)

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.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This document describes the RTP payload format for Enhanced Variable
 Rate Codec (EVRC) Speech and Selectable Mode Vocoder (SMV) Speech.
 Two sub-formats are specified for different application scenarios.  A
 bundled/interleaved format is included to reduce the effect of packet
 loss on speech quality and amortize the overhead of the RTP header
 over more than one speech frame.  A non-bundled format is also
 supported for conversational applications.

Table of Contents

 1. Introduction ................................................... 2
 2. Background ..................................................... 2
 3. The Codecs Supported ........................................... 3
    3.1. EVRC ...................................................... 3
    3.2. SMV ....................................................... 3
    3.3. Other Frame-Based Vocoders ................................ 4
 4. RTP/Vocoder Packet Format ...................................... 4
    4.1. Interleaved/Bundled Packet Format ......................... 5
    4.2. Header-Free Packet Format ................................. 6
    4.3. Determining the Format of Packets ......................... 7
 5. Packet Table of Contents Entries and Codec Data Frame Format ... 7
    5.1. Packet Table of Contents entries .......................... 7
    5.2. Codec Data Frames ......................................... 8
 6. Interleaving Codec Data Frames ................................. 9
 7. Bundling Codec Data Frames .................................... 12
 8. Handling Missing Codec Data Frames ............................ 12

Li Standards Track [Page 1] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 9. Implementation Issues ......................................... 12
    9.1. Interleaving Length .......................................12
    9.2. Validation of Received Packets ............................13
    9.3. Processing the Late Packets ...............................13
 10. Mode Request ................................................. 13
 11. Storage Format ............................................... 14
 12. IANA Considerations .......................................... 15
    12.1. Registration of Media Type EVRC ..........................15
    12.2. Registration of Media Type EVRC0 .........................16
    12.3. Registration of Media Type SMV ...........................17
    12.4. Registration of Media Type SMV0 ..........................18
 13. Mapping to SDP Parameters .................................... 19
 14. Security Considerations ...................................... 20
 15. Adding Support of Other Frame-Based Vocoders ................. 20
 16. Acknowledgements ............................................. 21
 17. References ................................................... 21
    17.1 Normative ................................................ 21
    17.2 Informative .............................................. 22
 18. Author's Address ............................................. 22
 19. Full Copyright Statement ..................................... 23

1. Introduction

 This document describes how speech compressed with EVRC [1] or SMV
 [2] may be formatted for use as an RTP payload type.  The format is
 also extensible to other codecs that generate a similar set of frame
 types.  Two methods are provided to packetize the codec data frames
 into RTP packets: an interleaved/bundled format and a zero-header
 format.  The sender may choose the best format for each application
 scenario, based on network conditions, bandwidth availability, delay
 requirements, and packet-loss tolerance.
 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 [3].

2. Background

 The 3rd Generation Partnership Project 2 (3GPP2) has published two
 standards which define speech compression algorithms for CDMA
 applications: EVRC [1] and SMV [2].  EVRC is currently deployed in
 millions of first and second generation CDMA handsets.  SMV is the
 preferred speech codec standard for CDMA2000, and will be deployed in
 third generation handsets in addition to EVRC.  Improvements and new
 codecs will keep emerging as technology improves, and future handsets
 will likely support multiple codecs.

Li Standards Track [Page 2] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 The formats of the EVRC and SMV codec frames are very similar.  Many
 other vocoders also share common characteristics, and have many
 similar application scenarios.  This parallelism enables an RTP
 payload format to be designed for EVRC and SMV that may also support
 other, similar vocoders with minimal additional specification work.
 This can simplify the protocol for transporting vocoder data frames
 through RTP and reduce the complexity of implementations.

3. The Codecs Supported

3.1. EVRC

 The Enhanced Variable Rate Codec (EVRC) [1] compresses each 20
 milliseconds of 8000 Hz, 16-bit sampled speech input into output
 frames in one of the three different sizes: Rate 1 (171 bits), Rate
 1/2 (80 bits), or Rate 1/8 (16 bits).  In addition, there are two
 zero bit codec frame types: null frames and erasure frames.  Null
 frames are produced as a result of the vocoder running at rate 0.
 Null frames are zero bits long and are normally not transmitted.
 Erasure frames are the frames substituted by the receiver to the
 codec for the lost or damaged frames.  Erasure frames are also zero
 bits long and are normally not transmitted.
 The codec chooses the output frame rate based on analysis of the
 input speech and the current operating mode (either normal or one of
 several reduced rate modes).  For typical speech patterns, this
 results in an average output of 4.2 kilobits/second for normal mode
 and a lower average output for reduced rate modes.

3.2. SMV

 The Selectable Mode Vocoder (SMV) [2] compresses each 20 milliseconds
 of 8000 Hz, 16-bit sampled speech input into output frames of one of
 the four different sizes: Rate 1 (171 bits), Rate 1/2 (80 bits), Rate
 1/4 (40 bits), or Rate 1/8 (16 bits).  In addition, there are two
 zero bit codec frame types: null frames and erasure frames.  Null
 frames are produced as a result of the vocoder running at rate 0.
 Null frames are zero bits long and are normally not transmitted.
 Erasure frames are the frames substituted by the receiver to the
 codec for the lost or damaged frames.  Erasure frames are also zero
 bits long and are normally not transmitted.
 The SMV codec can operate in six modes.  Each mode may produce frames
 of any of the rates (full rate to 1/8 rate) for varying percentages
 of time, based on the characteristics of the speech samples and the
 selected mode.  The SMV mode can change on a
 frame-by-frame basis.  The SMV codec does not need additional
 information other than the codec data frames to correctly decode the

Li Standards Track [Page 3] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 data of various modes; therefore, the mode of the encoder does not
 need to be transmitted with the encoded frames.
 The SMV codec chooses the output frame rate based on analysis of the
 input speech and the current operating mode.  For typical speech
 patterns, this results in an average output of 4.2 kilobits/second
 for Mode 0 in two way conversation (approximately 50% active speech
 time and 50% in eighth rate while listening) and lower for other
 reduced rate modes.  SMV is more bandwidth efficient than EVRC.  EVRC
 is equivalent in performance to SMV mode 1.

3.3. Other Frame-Based Vocoders

 Other frame-based vocoders can be carried in the packet format
 defined in this document, as long as they possess the following
 properties:
    o The codec is frame-based;
    o blank and erasure frames are supported;
    o the total number of rates is less than 17;
    o the maximum full rate frame can be transported in a single RTP
      packet using this specific format.
 Vocoders with the characteristics listed above can be transported
 using the packet format specified in this document with some
 additional specification work; the pieces that must be defined are
 listed in Section 15.

4. RTP/Vocoder Packet Format

 The vocoder speech data may be transmitted in either of the two RTP
 packet formats specified in the following two subsections, as
 appropriate for the application scenario.  In the packet format
 diagrams shown in this document, bit 0 is the most significant bit.

Li Standards Track [Page 4] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

4.1. Interleaved/Bundled Packet Format

 This format is used to send one or more vocoder frames per packet.
 Interleaving or bundling MAY be used.  The RTP packet for this format
 is as follows:
  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 [4]                           |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |R|R| LLL | NNN | MMM |  Count  |  TOC  |  ...  |  TOC  |padding|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        one or more codec data frames, one per TOC entry       |
 |                             ....                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The RTP header has the expected values as described in the RTP
 specification [4].  The RTP timestamp is in 1/8000 of a second units
 for EVRC and SMV.  For any other vocoders that use this packet
 format, the timestamp unit needs to be defined explicitly.  The M bit
 should be set as specified in the applicable RTP profile, for
 example, RFC 3551 [5].  Note that RFC 3551 [5] specifies that if the
 sender does not suppress silence, the M bit will always be zero.
 When multiple codec data frames are present in a single RTP packet,
 the timestamp is that of the oldest data represented in the RTP
 packet.  The assignment of an RTP payload type for this packet format
 is outside the scope of this document; it is specified by the RTP
 profile under which this payload format is used.
 The first octet of a Interleaved/Bundled format packet is the
 Interleave Octet.  The second octet contains the Mode Request and
 Frame Count fields.  The Table of Contents (ToC) field then follows.
 The fields are specified as follows:
 Reserved (RR): 2 bits
    Reserved bits.  MUST be set to zero by sender, SHOULD be ignored
    by receiver.
 Interleave Length (LLL): 3 bits
    Indicates the length of interleave; a value of 0 indicates
    bundling, a special case of interleaving.  See Section 6 and
    Section 7 for more detailed discussion.

Li Standards Track [Page 5] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 Interleave Index (NNN): 3 bits
    Indicates the index within an interleave group.  MUST have a value
    less than or equal to the value of LLL.  Values of NNN greater
    than the value of LLL are invalid.  Packet with invalid NNN values
    SHOULD be ignored by the receiver.
 Mode Request (MMM): 3 bits
    The Mode Request field is used to signal Mode Request information.
    See Section 10 for details.
 Frame Count (Count): 5 bits
    The number of ToC fields (and vocoder frames) present in the
    packet is the value of the frame count field plus one.  A value of
    zero indicates that the packet contains one ToC field, while a
    value of 31 indicates that the packet contains 32 ToC fields.
 Padding (padding): 0 or 4 bits
    This padding ensures that codec data frames start on an octet
    boundary.  When the frame count is odd, the sender MUST add 4 bits
    of padding following the last TOC.  When the frame count is even,
    the sender MUST NOT add padding bits.  If padding is present, the
    padding bits MUST be set to zero by sender, and SHOULD be ignored
    by receiver.
 The Table of Contents field (ToC) provides information on the codec
 data frame(s) in the packet.  There is one ToC entry for each codec
 data frame.  The detailed formats of the ToC field and codec data
 frames are specified in Section 5.
 Multiple data frames may be included within a Interleaved/Bundled
 packet using interleaving or bundling as described in Section 6 and
 Section 7.

4.2. Header-Free Packet Format

 The Header-Free Packet Format is designed for maximum bandwidth
 efficiency and low latency.  Only one codec data frame can be sent in
 each Header-Free format packet.  None of the payload header fields
 (LLL, NNN, MMM, Count) nor ToC entries are present.  The codec rate
 for the data frame can be determined from the length of the codec
 data frame, since there is only one codec data frame in each
 Header-Free packet.
 Use of the RTP header fields for Header-Free RTP/Vocoder Packet
 Format is the same as described in Section 4.1 for
 Interleaved/Bundled RTP/Vocoder Packet Format.  The detailed format
 of the codec data frame is specified in Section 5.

Li Standards Track [Page 6] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

  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 [4]                           |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 +          ONLY one codec data frame            +-+-+-+-+-+-+-+-+
 |                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3. Determining the Format of Packets

 All receivers SHOULD be able to process both packet formats.  The
 sender MAY choose to use one or both packet formats.
 A receiver MUST have prior knowledge of the packet format to
 correctly decode the RTP packets.  When packets of both formats are
 used within the same session, different RTP payload type values MUST
 be used for each format to distinguish the packet formats.  The
 association of payload type number with the packet format is done
 out-of-band, for example by SDP during the setup of a session.

5. Packet Table of Contents Entries and Codec Data Frame Format

5.1. Packet Table of Contents entries

 Each codec data frame in a Interleaved/Bundled packet has a
 corresponding Table of Contents (ToC) entry.  The ToC entry indicates
 the rate of the codec frame.  (Header-Free packets MUST NOT have a
 ToC field.)
 Each ToC entry is occupies four bits.  The format of the bits is
 indicated below:
     0 1 2 3
    +-+-+-+-+
    |fr type|
    +-+-+-+-+
 Frame Type: 4 bits
    The frame type indicates the type of the corresponding codec data
    frame in the RTP packet.

Li Standards Track [Page 7] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 For EVRC and SMV codecs, the frame type values and size of the
 associated codec data frame are described 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)
   3     1/2       10    (80 bits)
   4     1         22    (171 bits; 5 padded at end with zeros)
   5     Erasure    0    (SHOULD NOT be transmitted by sender)
 All values not listed in the above table MUST be considered reserved.
 A ToC entry with a reserved Frame Type value SHOULD be considered
 invalid.  Note that the EVRC codec does not have 1/4 rate frames,
 thus frame type value 2 MUST be considered a reserved value when the
 EVRC codec is in use.
 Other vocoders that use this packet format need to specify their own
 table of frame types and corresponding codec data frames.

5.2. Codec Data Frames

 The output of the vocoder MUST be converted into codec data frames
 for inclusion in the RTP payload.  The conversions for EVRC and SMV
 codecs are specified below.  (Note: Because the EVRC codec does not
 have Rate 1/4 frames, the specifications of 1/4 frames does not apply
 to EVRC codec data frames).  Other vocoders that use this packet
 format need to specify how to convert vocoder output data into
 frames.
 The codec output data bits as numbered in EVRC and SMV are packed
 into octets.  The lowest numbered bit (bit 1 for Rate 1, Rate 1/2,
 Rate 1/4 and Rate 1/8) is placed in the most significant bit
 (internet bit 0) of octet 1 of the codec data frame, the second
 lowest bit is placed in the second most significant bit of the first
 octet, the third lowest in the third most significant bit of the
 first octet, and so on.  This continues until all of the bits have
 been placed in the codec data frame.
 The remaining unused bits of the last octet of the codec data frame
 MUST be set to zero.  Note that in EVRC and SMV this is only
 applicable to Rate 1 frames (171 bits) as the Rate 1/2 (80 bits),
 Rate 1/4 (40 bits, SMV only) and Rate 1/8 frames (16 bits) fit
 exactly into a whole number of octets.
 Following is a detailed listing showing a Rate 1 EVRC/SMV codec
 output frame converted into a codec data frame:

Li Standards Track [Page 8] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 The codec data frame for a EVRC/SMV codec Rate 1 frame is 22 octets
 long.  Bits 1 through 171 from the EVRC/SMV codec Rate 1 frame are
 placed as indicated, with bits marked with "Z" set to zero.  EVRC/SMV
 codec Rate 1/8, Rate 1/4 and Rate 1/2 frames are converted similarly,
 but do not require zero padding because they align on octet
 boundaries.
                      Rate 1 codec data frame
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|
 |0|0|0|0|0|0|0|0|0|1|1|1|1|1|1|1|1|1|1|2|2|2|2|2|2|2|2|2|2|3|3|3|
 |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|2|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 :                                                               :
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | | | | |
 |4|4|4|4|4|5|5|5|5|5|5|5|5|5|5|6|6|6|6|6|6|6|6|6|6|7|7|Z|Z|Z|Z|Z|
 |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| | | | | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6. Interleaving Codec Data Frames

 As indicated in Section 4.1, more than one codec data frame MAY be
 included in a single Interleaved/Bundled packet by a sender.  This is
 accomplished by interleaving or bundling.
 Bundling is used to spread the transmission overhead of the RTP and
 payload header over multiple vocoder frames.  Interleaving
 additionally reduces the listener's perception of data loss by
 spreading such loss over non-consecutive vocoder frames.  EVRC, SMV,
 and similar vocoders are able to compensate for an occasional lost
 frame, but speech quality degrades exponentially with consecutive
 frame loss.
 Bundling is signaled by setting the LLL field to zero and the Count
 field to greater than zero.  Interleaving is indicated by setting the
 LLL field to a value greater than zero.
 The discussions on general interleaving apply to the bundling (which
 can be viewed as a reduced case of interleaving) with reduced
 complexity.  The bundling case is discussed in detail in Section 7.
 Senders MAY support interleaving and/or bundling.  All receivers that
 support Interleave/Bundling packet format MUST support both
 interleaving and bundling.

Li Standards Track [Page 9] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 Given a time-ordered sequence of output frames from the codec
 numbered 0..n, a bundling value B (the value in the Count field plus
 one), and an interleave length L where n = B * (L+1) - 1, the output
 frames are placed into RTP packets as follows (the values of the
 fields LLL and NNN are indicated for each RTP packet):
 First RTP Packet in Interleave group:
    LLL=L, NNN=0
    Frame 0, Frame L+1, Frame 2(L+1), Frame 3(L+1), ... for a total of
    B frames
 Second RTP Packet in Interleave group:
    LLL=L, NNN=1
    Frame 1, Frame 1+L+1, Frame 1+2(L+1), Frame 1+3(L+1), ... for a
    total of B frames
 This continues to the last RTP packet in the interleave group:
 L+1 RTP Packet in Interleave group:
    LLL=L, NNN=L
    Frame L, Frame L+L+1, Frame L+2(L+1), Frame L+3(L+1), ... for a
    total of B frames
 Within each interleave group, the RTP packets making up the
 interleave group MUST be transmitted in value-increasing order of the
 NNN field.  While this does not guarantee reduced end-to-end delay on
 the receiving end, when packets are delivered in order by the
 underlying transport, delay will be reduced to the minimum possible.
 Receivers MAY signal the maximum number of codec data frames (i.e.,
 the maximum acceptable bundling value B) they can handle in a single
 RTP packet using the OPTIONAL maxptime RTP mode parameter identified
 in Section 12.
 Receivers MAY signal the maximum interleave length (i.e., the maximum
 acceptable LLL value in the Interleaving Octet) they will accept
 using the OPTIONAL maxinterleave RTP mode parameter identified in
 Section 12.
 The parameters maxptime and maxinterleave are exchanged at the
 initial setup of the session.  In one-to-one sessions, the sender
 MUST respect these values set be the receiver, and MUST NOT
 interleave/bundle more packets than what the receiver signals that it
 can handle.  This ensures that the receiver can allocate a known
 amount of buffer space that will be sufficient for all
 interleaving/bundling used in that session.  During the session, the
 sender may decrease the bundling value or interleaving length (so
 that less buffer space is required at the receiver), but never exceed

Li Standards Track [Page 10] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 the maximum value set by the receiver.  This prevents the situation
 where a receiver needs to allocate more buffer space in the middle of
 a session but is unable to do so.
 Additionally, senders have the following restrictions:
 o  MUST NOT bundle more codec data frames in a single RTP packet than
    indicated by maxptime (see Section 12) if it is signaled.
 o  SHOULD NOT bundle more codec data frames in a single RTP packet
    than will fit in the MTU of the underlying network.
 o  Once beginning a session with a given maximum interleaving value
    set by maxinterleave in Section 12, MUST NOT increase the
    interleaving value (LLL) to exceed the maximum interleaving value
    that is signaled.
 o  MAY change the interleaving value, but MUST do so only between
    interleave groups.
 o  Silence suppression MUST only be used between interleave groups.
    A ToC with Frame Type 0 (Blank Frame, Section 5.1) MUST be used
    within interleaving groups if the codec outputs a blank frame.
    The M bit in the RTP header is not set for these blank frames, as
    the stream is continuous in time.  Because there is only one time
    stamp for each RTP packet, silence suppression used within an
    interleave group would cause ambiguities when reconstructing the
    speech at the receiver side, and thus is prohibited.
 Given an RTP packet with sequence number S, interleave length (field
 LLL) L, interleave index value (field NNN) N, and bundling value B,
 the interleave group consists of this RTP packet and other RTP
 packets with sequence numbers from S-N mod 65536 to S-N+L mod 65536
 inclusive.  In other words, the interleave group always consists of
 L+1 RTP packets with sequential sequence numbers.  The bundling value
 for all RTP packets in an interleave group MUST be the same.
 The receiver determines the expected bundling value for all RTP
 packets in an interleave group by the number of codec data frames
 bundled in the first RTP packet of the interleave group received.
 Note that this may not be the first RTP packet of the interleave
 group if packets are delivered out of order by the underlying
 transport.

Li Standards Track [Page 11] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

7. Bundling Codec Data Frames

 As discussed in Section 6, the bundling of codec data frames is a
 special reduced case of interleaving with LLL value in the Interleave
 Octet set to 0.
 Bundling codec data frames indicates that multiple data frames are
 included consecutively in a packet, because the interleaving length
 (LLL) is 0.  The interleaving group is thus reduced to a single RTP
 packet, and the reconstruction of the codec data frames from RTP
 packets becomes a much simpler process.
 Furthermore, the additional restrictions on senders are reduced to:
 o  MUST NOT bundle more codec data frames in a single RTP packet than
    indicated by maxptime (see Section 12) if it is signaled.
 o  SHOULD NOT bundle more codec data frames in a single RTP packet
    than will fit in the MTU of the underlying network.

8. Handling Missing Codec Data Frames

 The vocoders covered by this payload format support erasure frames as
 an indication when frames are not available.  The erasure frames are
 normally used internally by a receiver to advance the state of the
 voice decoder by exactly one frame time for each missing frame.
 Using the information from packet sequence number, time stamp, and
 the M bit, the receiver can detect missing codec data frames from RTP
 packet loss and/or silence suppression, and generate corresponding
 erasure frames.  Erasure frames MUST also be used in storage format
 to record missing frames.

9. Implementation Issues

9.1. Interleaving Length

 The vocoder interpolates the missing speech content when given an
 erasure frame.  However, the best quality is perceived by the
 listener when erasure frames are not consecutive.  This makes
 interleaving desirable as it increases speech quality when packet
 loss occurs.
 On the other hand, interleaving can greatly increase the end-to-end
 delay.  Where an interactive session is desired, either
 Interleaved/Bundled packet format with interleaving length (field
 LLL) 0 or Header-Free packet format is RECOMMENDED.

Li Standards Track [Page 12] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 When end-to-end delay is not a primary concern, an interleaving
 length (field LLL) of 4 or 5 is RECOMMENDED as it offers a reasonable
 compromise between robustness and latency.

9.2. Validation of Received Packets

 When receiving an RTP packet, the receiver SHOULD check the validity
 of the ToC fields and match the length of the packet with what is
 indicated by the ToC fields.  If any invalidity or mismatch is
 detected, it is RECOMMENDED to discard the received packet to avoid
 potential severe degradation of the speech quality.  The discarded
 packet is treated following the same procedure as a lost packet, and
 the discarded data will be replaced with erasure frames.
 On receipt of an RTP packet with an invalid value of the LLL or NNN
 fields, the RTP packet SHOULD be treated as lost by the receiver for
 the purpose of generating erasure frames as described in Section 8.
 On receipt of an RTP packet in an interleave group with other than
 the expected frame count value, the receiver MAY discard codec data
 frames off the end of the RTP packet or add erasure codec data frames
 to the end of the packet in order to manufacture a substitute packet
 with the expected bundling value.  The receiver MAY instead choose to
 discard the whole interleave group.

9.3. Processing the Late Packets

 Assume that the receiver has begun playing frames from an interleave
 group.  The time has come to play frame x from packet n of the
 interleave group.  Further assume that packet n of the interleave
 group has not been received.  As described in Section 8, an erasure
 frame will be sent to the receiving vocoder.
 Now, assume that packet n of the interleave group arrives before
 frame x+1 of that packet is needed.  Receivers should use frame x+1
 of the newly received packet n rather than substituting an erasure
 frame.  In other words, just because packet n was not available the
 first time it was needed to reconstruct the interleaved speech, the
 receiver should not assume it is not available when it is
 subsequently needed for interleaved speech reconstruction.

10. Mode Request

 The Mode Request signal requests a particular encoding mode for the
 speech encoding in the reverse direction.  All implementations are
 RECOMMENDED to honor the Mode Request signal.  The Mode Request
 signal SHOULD only be used in one-to-one sessions.  In multi-party
 sessions, any received Mode Request signals SHOULD be ignored.

Li Standards Track [Page 13] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 In addition, the Mode Request signal MAY also be sent through non-RTP
 means, which is out of the scope of this specification.
 The three-bit Mode Request field is used to signal the receiver to
 set a particular encoding mode to its audio encoder.  If the Mode
 Request field is set to a valid value in RTP packets from node A to
 node B, it is a request for node B to change to the requested
 encoding mode for its audio encoder and therefore the bit rate of the
 RTP stream from node B to node A.  Once a node sets this field to a
 value, it SHOULD continue to set the field to the same value in
 subsequent packets until the requested mode is different.  This
 design helps to eliminate the scenario of getting the codec stuck in
 an unintended state if one of the packets that carries the Mode
 Request is lost.  An otherwise silent node MAY send an RTP packet
 containing a blank frame in order to send a Mode Request.
 Each codec type using this format SHOULD define its own
 interpretation of the Mode Request field.  Codecs SHOULD follow the
 convention that higher values of the three-bit field correspond to an
 equal or lower average output bit rate.
 For the EVRC codec, the Mode Request field MUST be interpreted
 according to Tables 2.2.1.2-1 and 2.2.1.2-2 of the EVRC codec
 specifications [1].
 For SMV codec, the Mode Request field MUST be interpreted according
 to Table 2.2-2 of the SMV codec specifications [2].

11. Storage Format

 The storage format is used for storing 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 corresponds to the ASCII character
 string "#!EVRC\n", i.e., "0x23 0x21 0x45 0x56 0x52 0x43 0x0A".  The
 magic number for SMV corresponds to the ASCII character string
 "#!SMV\n", i.e., "0x23 0x21 0x53 0x4d 0x56 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.
 Speech frames lost in transmission and non-received frames MUST be
 stored as erasure frames (frame type 5, see definition in Section
 5.1) to maintain synchronization with the original media.

Li Standards Track [Page 14] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

12. IANA Considerations

 Four new MIME sub-types as described in this section have been
 registered by the IANA.
 The MIME-names for the EVRC and SMV codec are allocated from the IETF
 tree since all the vocoders covered are expected to be widely used
 for Voice-over-IP applications.

12.1. Registration of Media Type EVRC

 Media Type Name:           audio
 Media Subtype Name:        EVRC
 Required Parameter:        none
 Optional parameters:
    The following parameters apply to RTP transfer only.
    ptime:    Defined as usual for RTP audio (see RFC 2327).
    maxptime: The maximum amount of media which can be encapsulated in
       each packet, expressed as time in milliseconds.  The time SHALL
       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 SHALL 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 SHALL be 5.
 Encoding considerations:
    This type is defined for transfer of EVRC-encoded data via RTP
    using the Interleaved/Bundled packet format specified in Sections
    4.1, 6, and 7 of RFC 3558.  It is also defined for other transfer
    methods using the storage format specified in Section 11 of RFC
    3558.
 Security considerations:
    See Section 14 "Security Considerations" of RFC 3558.
 Public specification:
    The EVRC vocoder is specified in 3GPP2 C.S0014.  Transfer methods
    are specified in RFC 3558.

Li Standards Track [Page 15] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 Additional information:
    The following information applies for storage format only.
    Magic number: #!EVRC\n (see Section 11 of RFC 3558)
    File extensions: evc, EVC
    Macintosh file type code: none
    Object identifier or OID: none
 Intended usage:
    COMMON.  It is expected that many VoIP applications (as well as
    mobile applications) will use this type.
 Person & email address to contact for further information:
    Adam Li
    adamli@icsl.ucla.edu
 Author/Change controller:
    Adam Li
    adamli@icsl.ucla.edu
    IETF Audio/Video Transport Working Group

12.2. Registration of Media Type EVRC0

 Media Type Name:           audio
 Media Subtype Name:        EVRC0
 Required Parameters:       none
 Optional parameters:       none
 Encoding considerations:   none
    This type is only defined for transfer of EVRC-encoded data via
    RTP using the Header-Free packet format specified in Section 4.2
    of RFC 3558.
 Security considerations:
    See Section 14 "Security Considerations" of RFC 3558.
 Public specification:
    The EVRC vocoder is specified in 3GPP2 C.S0014.  Transfer methods
    are specified in RFC 3558.
 Additional information:    none
 Intended usage:
    COMMON.  It is expected that many VoIP applications (as well as
    mobile applications) will use this type.

Li Standards Track [Page 16] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 Person & email address to contact for further information:
    Adam Li
    adamli@icsl.ucla.edu
 Author/Change controller:
    Adam Li
    adamli@icsl.ucla.edu
    IETF Audio/Video Transport Working Group

12.3. Registration of Media Type SMV

 Media Type Name:           audio
 Media Subtype Name:        SMV
 Required Parameter:        none
 Optional parameters:
 The following parameters apply to RTP transfer only.
    ptime:    Defined as usual for RTP audio (see RFC 2327).
    maxptime: The maximum amount of media which can be encapsulated
       in each packet, expressed as time in milliseconds.  The time
       SHALL 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 SHALL 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 SHALL be 5.
 Encoding considerations:
    This type is defined for transfer of SMV-encoded data via RTP
    using the Interleaved/Bundled packet format specified in Section
    4.1, 6, and 7 of RFC 3558.  It is also defined for other transfer
    methods using the storage format specified in Section 11 of RFC
    3558.
 Security considerations:
    See Section 14 "Security Considerations" of RFC 3558.
 Public specification:
    The SMV vocoder is specified in 3GPP2 C.S0030-0 v2.0.
    Transfer methods are specified in RFC 3558.

Li Standards Track [Page 17] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 Additional information:
    The following information applies to storage format only.
    Magic number: #!SMV\n (see Section 11 of RFC 3558)
    File extensions: smv, SMV
    Macintosh file type code: none
    Object identifier or OID: none
 Intended usage:
    COMMON.  It is expected that many VoIP applications (as well as
    mobile applications) will use this type.
 Person & email address to contact for further information:
    Adam Li
    adamli@icsl.ucla.edu
 Author/Change controller:
    Adam Li
    adamli@icsl.ucla.edu
    IETF Audio/Video Transport Working Group

12.4. Registration of Media Type SMV0

 Media Type Name:           audio
 Media Subtype Name:        SMV0
 Required Parameter:        none
 Optional parameters:       none
 Encoding considerations:   none
    This type is only defined for transfer of SMV-encoded data via RTP
    using the Header-Free packet format specified in Section 4.2 of
    RFC 3558.
 Security considerations:
    See Section 14 "Security Considerations" of RFC 3558.
 Public specification:
    The SMV vocoder is specified in 3GPP2 C.S0030-0 v2.0.  Transfer
    methods are specified in RFC 3558.
 Additional information:    none
 Intended usage:
    COMMON.  It is expected that many VoIP applications (as well as
    mobile applications) will use this type.

Li Standards Track [Page 18] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 Person & email address to contact for further information:
    Adam Li
    adamli@icsl.ucla.edu
 Author/Change controller:
    Adam Li
    adamli@icsl.ucla.edu
    IETF Audio/Video Transport Working Group

13. Mapping to SDP Parameters

 Please note that this section applies to the RTP transfer only.
 The information carried in the MIME media type specification has a
 specific mapping to fields in the Session Description Protocol (SDP)
 [6], which is commonly used to describe RTP sessions.  When SDP is
 used to specify sessions employing the EVRC or EMV codec, the mapping
 is as follows:
    o  The MIME type ("audio") goes in SDP "m=" as the media name.
    o  The MIME subtype (payload format name) goes in SDP "a=rtpmap"
       as the encoding name.
    o  The parameters "ptime" and "maxptime" go in the SDP "a=ptime"
       and "a=maxptime" attributes, respectively.
    o  The parameter "maxinterleave" goes in the SDP "a=fmtp"
       attribute by copying it directly from the MIME media type
       string as "maxinterleave=value".
 Some examples of SDP session descriptions for EVRC and SMV encodings
 follow below.
 Example of usage of EVRC:
    m=audio 49120 RTP/AVP 97
    a=rtpmap:97 EVRC/8000
    a=fmtp:97 maxinterleave=2
    a=maxptime:80
 Example of usage of SMV
    m=audio 49122 RTP/AVP 99
    a=rtpmap:99 SMV0/8000
    a=fmtp:99

Li Standards Track [Page 19] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 Note that the payload format (encoding) names are commonly shown in
 upper case.  MIME subtypes are commonly shown in lower case.  These
 names are case-insensitive in both places.  Similarly, parameter
 names are case-insensitive both in MIME types and in the default
 mapping to the SDP a=fmtp attribute.

14. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the security considerations discussed in the RTP
 specification [4], and any appropriate profile (for example [5]).
 This implies that confidentiality of the media streams is achieved by
 encryption.  Because the data compression used with this payload
 format is applied end-to-end, encryption may be performed after
 compression so there is no conflict between the two operations.
 A potential denial-of-service threat exists for data encoding using
 compression techniques that have non-uniform receiver-end
 computational load.  The attacker can inject pathological datagrams
 into the stream which are complex to decode and cause the receiver to
 become overloaded.  However, the encodings covered in this document
 do not exhibit any significant non-uniformity.
 As with any IP-based protocol, in some circumstances, a receiver may
 be overloaded simply by the receipt of too many packets, either
 desired or undesired.  Network-layer authentication may be used to
 discard packets from undesired sources, but the processing cost of
 the authentication itself may be too high.  In a multicast
 environment, pruning of specific sources may be implemented in future
 versions of IGMP [7] and in multicast routing protocols to allow a
 receiver to select which sources are allowed to reach it.
 Interleaving may affect encryption.  Depending on the used encryption
 scheme there may be restrictions on, for example, the time when keys
 can be changed.  Specifically, the key change may need to occur at
 the boundary between interleave groups.

15. Adding Support of Other Frame-Based Vocoders

 As described above, the RTP packet format defined in this document is
 very flexible and designed to be usable by other frame-based
 vocoders.
 Additional vocoders using this format MUST have properties as
 described in Section 3.3.

Li Standards Track [Page 20] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

 For an eligible vocoder to use the payload format mechanisms defined
 in this document, a new RTP payload format document needs to be
 published as a standards track RFC.  That document can simply refer
 to this document and then specify the following parameters:
    o Define the unit used for RTP time stamp;
    o Define the meaning of the Mode Request bits;
    o Define corresponding codec data frame type values for ToC;
    o Define the conversion procedure for vocoders output data frame;
    o Define a magic number for storage format, and complete the
      corresponding MIME registration.

16. Acknowledgements

 The following authors have made significant contributions to this
 document: Adam H. Li, John D. Villasenor, Dong-Seek Park, Jeong-Hoon
 Park, Keith Miller, S. Craig Greer, David Leon, Nikolai Leung,
 Marcello Lioy, Kyle J. McKay, Magdalena L. Espelien, Randall Gellens,
 Tom Hiller, Peter J. McCann, Stinson S. Mathai, Michael D. Turner,
 Ajay Rajkumar, Dan Gal, Magnus Westerlund, Lars-Erik Jonsson, Greg
 Sherwood, and Thomas Zeng.

17. References

17.1 Normative

 [1]  3GPP2 C.S0014, "Enhanced Variable Rate Codec, Speech Service
      Option 3 for Wideband Spread Spectrum Digital Systems", January
      1997.
 [2]  3GPP2 C.S0030-0 v2.0, "Selectable Mode Vocoder, Service Option
      for Wideband Spread Spectrum Communication Systems", May 2002.
 [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [4]  Schulzrinne, H., Casner, S., Jacobson, V. and R. Frederick,
      "RTP: A Transport Protocol for Real-Time Applications", RFC
      3550, July 2003.
 [5]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
      Conferences with Minimal Control", RFC 3551, July 2003.
 [6]  Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.

Li Standards Track [Page 21] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

17.2 Informative

 [7]  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
      1112, August 1989.

18. Author's Address

 Adam H. Li
 Image Communication Lab
 Electrical Engineering Department
 University of California
 Los Angeles, CA 90095
 USA
 Phone: +1 310 825 5178
 EMail: adamli@icsl.ucla.edu

Li Standards Track [Page 22] RFC 3558 RTP Payload Format for EVRC and SMV July 2003

19. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Li Standards Track [Page 23]

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