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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                     V. Demjanenko
Request for Comments: 8130                                  D. Satterlee
Category: Standards Track                       VOCAL Technologies, Ltd.
ISSN: 2070-1721                                               March 2017


                           RTP Payload Format
   for the Mixed Excitation Linear Prediction Enhanced (MELPe) Codec

Abstract

   This document describes the RTP payload format for the Mixed
   Excitation Linear Prediction Enhanced (MELPe) speech coder.  MELPe's
   three different speech encoding rates and sample frame sizes are
   supported.  Comfort noise procedures and packet loss concealment are
   described in detail.

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 7841.

   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/rfc8130.

Copyright Notice

   Copyright (c) 2017 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.






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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions ................................................2
   2. Background ......................................................3
   3. Payload Format ..................................................4
      3.1. MELPe Bitstream Definitions ................................5
           3.1.1. 2400 bps Bitstream Structure ........................6
           3.1.2. 1200 bps Bitstream Structure ........................9
           3.1.3. 600 bps Bitstream Structure ........................13
      3.2. MELPe Comfort Noise Bitstream Definition ..................18
      3.3. Multiple MELPe Frames in an RTP Packet ....................20
      3.4. Congestion Control Considerations .........................21
   4. Payload Format Parameters ......................................22
      4.1. Media Type Definitions ....................................22
      4.2. Mapping to SDP ............................................23
      4.3. Declarative SDP Considerations ............................25
      4.4. Offer/Answer SDP Considerations ...........................25
   5. Discontinuous Transmissions ....................................26
   6. Packet Loss Concealment ........................................26
   7. IANA Considerations ............................................26
   8. Security Considerations ........................................27
   9. References .....................................................27
      9.1. Normative References ......................................27
      9.2. Informative References ....................................29
   Authors' Addresses ................................................30

1.  Introduction

   This document describes how compressed Mixed Excitation Linear
   Prediction Enhanced (MELPe) speech as produced by the MELPe codec
   may be formatted for use as an RTP payload.  Details are provided to
   packetize the three different codec bitrate data frames (2400, 1200,
   and 600) into RTP packets.  The sender may send one or more codec
   data frames per packet, depending on the application scenario or
   based on transport network conditions, bandwidth restrictions, delay
   requirements, and packet loss tolerance.

1.1.  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 [RFC2119].

   Best current practices for writing an RTP payload format
   specification were followed [RFC2736].





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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


2.  Background

   The MELP speech coder was developed by the US military as an upgrade
   from the LPC-based CELP standard vocoder for low-bitrate
   communications [MELP].  ("LPC" stands for "Linear-Predictive Coding",
   and "CELP" stands for "Code-Excited Linear Prediction".)  MELP was
   further enhanced and subsequently adopted by NATO as MELPe for use by
   its members and Partnership for Peace countries for military and
   other governmental communications [MELPE].  The MELP speech coder
   algorithm was developed by Atlanta Signal Processing (ASPI), Texas
   Instruments (TI), SignalCom (now Microsoft), and Thales
   Communications, with noise preprocessor contributions from AT&T,
   under contract with NSA/DOD as international NATO Standard
   STANAG 4591 [MELPE].

   Commercial/civilian applications have arisen because of the
   low-bitrate property of MELPe with its (relatively) high
   intelligibility.  As such, MELPe is being used in a variety of wired
   and radio communications systems.  Voice over IP (VoIP) / SIP systems
   need to transport MELPe without decoding and re-encoding in order to
   preserve its intelligibility.  Hence, it is desirable and necessary
   to define the proper payload formatting and use conventions of MELPe
   in RTP payloads.

   The MELPe codec [MELPE] supports three different vocoder bitrates:
   2400, 1200, and 600 bps.  The basic 2400 bps bitrate vocoder uses a
   22.5 ms frame of speech consisting of 180 8000-Hz, 16-bit speech
   samples.  The 1200 and 600 bps bitrate vocoders each use three and
   four 22.5 ms frames of speech, respectively.  These reduced-bitrate
   vocoders internally use multiple 2400 bps parameter sets with further
   processing to strategically remove redundancy.  The payload sizes for
   each of the bitrates are 54, 81, and 54 bits for the 2400, 1200, and
   600 bps frames, respectively.  Dynamic bitrate switching is permitted
   but only if supported by both endpoints.

   The MELPe algorithm distinguishes between voiced and unvoiced speech
   and encodes each differently.  Unvoiced speech can be coded with
   fewer information bits for the same quality.  Forward error
   correction (FEC) is applied to the 2400 bps codec unvoiced speech for
   better protection of the subtle differences in signal reconstruction.
   The lower-bitrate coders do not allocate any bits for FEC and rely on
   strong error protection and correction in the communications channel.









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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


   Comfort noise handling for MELPe follows the procedures in Appendix B
   of SCIP-210 [SCIP210].  After Voice Activity Detection (VAD)
   no longer indicates the presence of speech/voice, a minimum of two
   comfort noise vocoder frames (serving as a grace period) are to be
   transmitted.  The contents of the comfort noise frames are described
   in the next section.

   Packet loss concealment (PLC) exploits the FEC (and, more precisely,
   any combination of two set bits in the pitch/voicing parameter) of
   the 2400 bps speech coder.  The pitch/voicing parameter has a sparse
   set of permitted values.  A value of zero indicates a non-voiced
   frame.  At least three bits are set for all valid pitch parameters.
   The PLC erasure indication utilizes any errored/erasure encodings of
   the pitch/voicing parameter with exactly two set bits, as described
   below.

3.  Payload Format

   The MELPe codec uses 22.5, 67.5, or 90 ms frames with a sampling rate
   clock of 8 kHz, so the RTP timestamp MUST be in units of 1/8000 of a
   second.

   The RTP payload for MELPe has the format shown in Figure 1.  No
   additional header specific to this payload format is needed.  This
   format is intended for situations where the sender and the receiver
   send one or more codec data frames per packet.

   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                            |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   +                  one or more frames of MELPe                  |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 1: Packet Format Diagram

   The RTP header of the packetized encoded MELPe speech has the
   expected values as described in [RFC3550].  The usage of the M bit
   SHOULD be as specified in the applicable RTP profile -- for example,
   [RFC3551], where [RFC3551] specifies that if the sender does not
   suppress silence (i.e., sends a frame on every frame interval), the
   M bit will always be zero.  When more than one codec data frame is
   present in a single RTP packet, the timestamp is, as always, that of
   the oldest data frame represented in the RTP packet.




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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


   The assignment of an RTP payload type for this new packet format is
   outside the scope of this document and will not be specified here.
   It is expected that the RTP profile for a particular class of
   applications will assign a payload type for this encoding, or if that
   is not done, then a payload type in the dynamic range shall be chosen
   by the sender.

3.1.  MELPe Bitstream Definitions

   The total number of bits used to describe one frame of 2400 bps
   speech is 54, which fits in 7 octets (with two unused bits).  For
   1200 bps speech, the total number of bits used is 81, which fits in
   11 octets (with seven unused bits).  For 600 bps speech, the total
   number of bits used is 54, which fits in 7 octets (with two unused
   bits).  Unused bits, shown below as RSVA, RSVB, etc., are coded as
   described in Section 3.3 in support of dynamic bitrate switching.

   In the MELPe bitstream definitions, the most significant bits are
   considered priority bits.  The intention was that these bits receive
   greater protection in the underlying communications channel.  For IP
   networks, such additional protection is irrelevant.  However, for the
   convenience of interoperable gateway devices, the bitstreams will be
   presented identically in IP networks.




























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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


3.1.1.  2400 bps Bitstream Structure

   According to Table 3 of [MELPE], the 2400 bps MELPe bit transmission
   order (for clarity, the bit priority is not shown) is as follows:

                +--------+-------------+-------------+
                |  Bit   |    Voiced   |   Unvoiced  |
                +--------+-------------+-------------+
                |  B_01  |       g20   |       g20   |
                |  B_02  |       BP0   |     FEC10   |
                |  B_03  |        P0   |        P0   |
                |  B_04  |     LSF20   |     LSF20   |
                |  B_05  |     LSF30   |     LSF30   |
                |  B_06  |       g23   |       g23   |
                |  B_07  |       g24   |       g24   |
                |  B_08  |     LSF35   |     LSF35   |
                +--------+-------------+-------------+
                |  B_09  |       g21   |       g21   |
                |  B_10  |       g22   |       g22   |
                |  B_11  |        P4   |        P4   |
                |  B_12  |     LSF34   |     LSF34   |
                |  B_13  |        P5   |        P5   |
                |  B_14  |        P1   |        P1   |
                |  B_15  |        P2   |        P2   |
                |  B_16  |     LSF40   |     LSF40   |
                +--------+-------------+-------------+
                |  B_17  |        P6   |        P6   |
                |  B_18  |     LSF10   |     LSF10   |
                |  B_19  |     LSF16   |     LSF16   |
                |  B_20  |     LSF45   |     LSF45   |
                |  B_21  |        P3   |        P3   |
                |  B_22  |     LSF15   |     LSF15   |
                |  B_23  |     LSF14   |     LSF14   |
                |  B_24  |     LSF25   |     LSF25   |
                +--------+-------------+-------------+
                |  B_25  |       BP3   |     FEC13   |
                |  B_26  |     LSF13   |     LSF13   |
                |  B_27  |     LSF12   |     LSF12   |
                |  B_28  |     LSF24   |     LSF24   |
                |  B_29  |     LSF44   |     LSF44   |
                |  B_30  |       FM0   |     FEC40   |
                |  B_31  |     LSF11   |     LSF11   |
                |  B_32  |     LSF23   |     LSF23   |








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                +--------+-------------+-------------+
                |  B_33  |       FM7   |     FEC22   |
                |  B_34  |       FM6   |     FEC21   |
                |  B_35  |       FM5   |     FEC20   |
                |  B_36  |       g11   |       g11   |
                |  B_37  |       g10   |       g10   |
                |  B_38  |       BP2   |     FEC12   |
                |  B_39  |       BP1   |     FEC11   |
                |  B_40  |     LSF21   |     LSF21   |
                +--------+-------------+-------------+
                |  B_41  |     LSF33   |     LSF33   |
                |  B_42  |     LSF22   |     LSF22   |
                |  B_43  |     LSF32   |     LSF32   |
                |  B_44  |     LSF31   |     LSF31   |
                |  B_45  |     LSF43   |     LSF43   |
                |  B_46  |     LSF42   |     LSF42   |
                |  B_47  |        AF   |     FEC42   |
                |  B_48  |     LSF41   |     LSF41   |
                +--------+-------------+-------------+
                |  B_49  |       FM4   |     FEC32   |
                |  B_50  |       FM3   |     FEC31   |
                |  B_51  |       FM2   |     FEC30   |
                |  B_52  |       FM1   |     FEC41   |
                |  B_53  |       g12   |       g12   |
                |  B_54  |      SYNC   |      SYNC   |
                +--------+-------------+-------------+

                Notes:
                g = Gain
                BP = Bandpass Voicing
                P = Pitch/Voicing
                LSF = Line Spectral Frequencies
                FEC = Forward Error Correction Parity Bits
                FM = Fourier Magnitudes
                AF = Aperiodic Flag
                B_01 = least significant bit of data set

             Table 1: Bitstream Definition for MELPe 2400 bps













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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


   The 2400 bps MELPe RTP payload is constructed as per Figure 2.  Note
   that bit B_01 is placed in the least significant bit (LSB) of the
   first byte with all other bits in sequence.  When filling octets, the
   least significant bits of the seventh octet are filled with bits B_49
   to B_54, respectively.

           MSB                                              LSB
            0      1      2      3      4      5      6      7
        +------+------+------+------+------+------+------+------+
        | B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
        +------+------+------+------+------+------+------+------+
        | B_16 | B_15 | B_14 | B_13 | B_12 | B_11 | B_10 | B_09 |
        +------+------+------+------+------+------+------+------+
        | B_24 | B_23 | B_22 | B_21 | B_20 | B_19 | B_18 | B_17 |
        +------+------+------+------+------+------+------+------+
        | B_32 | B_31 | B_30 | B_29 | B_28 | B_27 | B_26 | B_25 |
        +------+------+------+------+------+------+------+------+
        | B_40 | B_39 | B_38 | B_37 | B_36 | B_35 | B_34 | B_33 |
        +------+------+------+------+------+------+------+------+
        | B_48 | B_47 | B_46 | B_45 | B_44 | B_43 | B_42 | B_41 |
        +------+------+------+------+------+------+------+------+
        | RSVA | RSVB | B_54 | B_53 | B_52 | B_51 | B_50 | B_49 |
        +------+------+------+------+------+------+------+------+

              Figure 2: Packed MELPe 2400 bps Payload Octets


























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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


3.1.2.  1200 bps Bitstream Structure

   According to Tables D-9a and D-9b of [MELPE], the 1200 bps MELPe bit
   transmission order is as follows:

                 +--------+-------------+-------------+
                 |  Bit   |  Modes 1-4  |   Mode 5    |
                 |        |   (Voiced)  | (Unvoiced)  |
                 +--------+-------------+-------------+
                 |  B_01  |     Syn     |     Syn     |
                 |  B_02  |  Pitch&UV0  |  Pitch&UV0  |
                 |  B_03  |  Pitch&UV1  |  Pitch&UV1  |
                 |  B_04  |  Pitch&UV2  |  Pitch&UV2  |
                 |  B_05  |  Pitch&UV3  |  Pitch&UV3  |
                 |  B_06  |  Pitch&UV4  |  Pitch&UV4  |
                 |  B_07  |  Pitch&UV5  |  Pitch&UV5  |
                 |  B_08  |  Pitch&UV6  |  Pitch&UV6  |
                 +--------+-------------+-------------+
                 |  B_09  |  Pitch&UV7  |  Pitch&UV7  |
                 |  B_10  |  Pitch&UV8  |  Pitch&UV8  |
                 |  B_11  |  Pitch&UV9  |  Pitch&UV9  |
                 |  B_12  | Pitch&UV10  | Pitch&UV10  |
                 |  B_13  | Pitch&UV11  | Pitch&UV11  |
                 |  B_14  |    LSP0     |    LSP0     |
                 |  B_15  |    LSP1     |    LSP1     |
                 |  B_16  |    LSP2     |    LSP2     |
                 +--------+-------------+-------------+
                 |  B_17  |    LSP3     |    LSP3     |
                 |  B_18  |    LSP4     |    LSP4     |
                 |  B_19  |    LSP5     |    LSP5     |
                 |  B_20  |    LSP6     |    LSP6     |
                 |  B_21  |    LSP7     |    LSP7     |
                 |  B_22  |    LSP8     |    LSP8     |
                 |  B_23  |    LSP9     |    LSP9     |
                 |  B_24  |    LSP10    |    LSP10    |
                 +--------+-------------+-------------+
                 |  B_25  |    LSP11    |    LSP11    |
                 |  B_26  |    LSP12    |    LSP12    |
                 |  B_27  |    LSP13    |    LSP13    |
                 |  B_28  |    LSP14    |    LSP14    |
                 |  B_29  |    LSP15    |    LSP15    |
                 |  B_30  |    LSP16    |    LSP16    |
                 |  B_31  |    LSP17    |    LSP17    |
                 |  B_32  |    LSP18    |    LSP18    |







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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


                 +--------+-------------+-------------+
                 |  B_33  |    LSP19    |    LSP19    |
                 |  B_34  |    LSP20    |    LSP20    |
                 |  B_35  |    LSP21    |    LSP21    |
                 |  B_36  |    LSP22    |    LSP22    |
                 |  B_37  |    LSP23    |    LSP23    |
                 |  B_38  |    LSP24    |    LSP24    |
                 |  B_39  |    LSP25    |    LSP25    |
                 |  B_40  |    LSP26    |    LSP26    |
                 +--------+-------------+-------------+
                 |  B_41  |    LSP27    |    GAIN0    |
                 |  B_42  |    LSP28    |    GAIN1    |
                 |  B_43  |    LSP29    |    GAIN2    |
                 |  B_44  |    LSP30    |    GAIN3    |
                 |  B_45  |    LSP31    |    GAIN4    |
                 |  B_46  |    LSP32    |    GAIN5    |
                 |  B_47  |    LSP33    |    GAIN6    |
                 |  B_48  |    LSP34    |    GAIN7    |
                 +--------+-------------+-------------+
                 |  B_49  |    LSP35    |    GAIN8    |
                 |  B_50  |    LSP36    |    GAIN9    |
                 |  B_51  |    LSP37    |             |
                 |  B_52  |    LSP38    |             |
                 |  B_53  |    LSP39    |             |
                 |  B_54  |    LSP40    |             |
                 |  B_55  |    LSP41    |             |
                 |  B_56  |    LSP42    |             |
                 +--------+-------------+-------------+
                 |  B_57  |    GAIN0    |             |
                 |  B_58  |    GAIN1    |             |
                 |  B_59  |    GAIN2    |             |
                 |  B_60  |    GAIN3    |             |
                 |  B_61  |    GAIN4    |             |
                 |  B_62  |    GAIN5    |             |
                 |  B_63  |    GAIN6    |             |
                 |  B_64  |    GAIN7    |             |
                 +--------+-------------+-------------+
                 |  B_65  |    GAIN8    |             |
                 |  B_66  |    GAIN9    |             |
                 |  B_67  |     BP0     |             |
                 |  B_68  |     BP1     |             |
                 |  B_69  |     BP2     |             |
                 |  B_70  |     BP3     |             |
                 |  B_71  |     BP4     |             |
                 |  B_72  |     BP5     |             |






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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


                 +--------+-------------+-------------+
                 |  B_73  |   JITTER    |             |
                 |  B_74  |     FS0     |             |
                 |  B_75  |     FS1     |             |
                 |  B_76  |     FS2     |             |
                 |  B_77  |     FS3     |             |
                 |  B_78  |     FS4     |             |
                 |  B_79  |     FS5     |             |
                 |  B_80  |     FS6     |             |
                 +--------+-------------+-------------+
                 |  B_81  |     FS7     |             |
                 +--------+-------------+-------------+

                 Notes:
                 BP = Bandpass voicing
                 FS = Fourier magnitudes
                 LSP = Line Spectral Pair
                 Pitch&UV = Pitch/voicing
                 GAIN = Gain
                 JITTER = Jitter

             Table 2: Bitstream Definition for MELPe 1200 bps

   The 1200 bps MELPe RTP payload is constructed as per Figure 3.  Note
   that bit B_01 is placed in the LSB of the first byte with all other
   bits in sequence.  When filling octets, the least significant bit of
   the eleventh octet is filled with bit B_81.
























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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


           MSB                                              LSB
            0      1      2      3      4      5      6      7
        +------+------+------+------+------+------+------+------+
        | B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
        +------+------+------+------+------+------+------+------+
        | B_16 | B_15 | B_14 | B_13 | B_12 | B_11 | B_10 | B_09 |
        +------+------+------+------+------+------+------+------+
        | B_24 | B_23 | B_22 | B_21 | B_20 | B_19 | B_18 | B_17 |
        +------+------+------+------+------+------+------+------+
        | B_32 | B_31 | B_30 | B_29 | B_28 | B_27 | B_26 | B_25 |
        +------+------+------+------+------+------+------+------+
        | B_40 | B_39 | B_38 | B_37 | B_36 | B_35 | B_34 | B_33 |
        +------+------+------+------+------+------+------+------+
        | B_48 | B_47 | B_46 | B_45 | B_44 | B_43 | B_42 | B_41 |
        +------+------+------+------+------+------+------+------+
        | B_56 | B_55 | B_54 | B_53 | B_52 | B_51 | B_50 | B_49 |
        +------+------+------+------+------+------+------+------+
        | B_64 | B_63 | B_62 | B_61 | B_60 | B_59 | B_58 | B_57 |
        +------+------+------+------+------+------+------+------+
        | B_72 | B_71 | B_70 | B_69 | B_68 | B_67 | B_66 | B_65 |
        +------+------+------+------+------+------+------+------+
        | B_80 | B_79 | B_78 | B_77 | B_76 | B_75 | B_74 | B_73 |
        +------+------+------+------+------+------+------+------+
        | RSVA | RSVB | RSVC | RSV0 | RSV0 | RSV0 | RSV0 | B_81 |
        +------+------+------+------+------+------+------+------+

              Figure 3: Packed MELPe 1200 bps Payload Octets
























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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


3.1.3.  600 bps Bitstream Structure

   According to Tables M-11 to M-16 of [MELPE], the 600 bps MELPe bit
   transmission order (for clarity, the bit priority is not shown) is as
   follows:

          +--------+-------------+-------------+-------------+
          |  Bit   |    Mode 1   |    Mode 2   |    Mode 3   |
          |        |   (Voiced)  |   (voiced)  |   (voiced)  |
          +--------+-------------+-------------+-------------+
          |  B_01  | Voicing (4) | Voicing (4) | Voicing (4) |
          |  B_02  | Voicing (3) | Voicing (3) | Voicing (3) |
          |  B_03  | Voicing (2) | Voicing (2) | Voicing (2) |
          |  B_04  | Voicing (1) | Voicing (1) | Voicing (1) |
          |  B_05  | Voicing (0) | Voicing (0) | Voicing (0) |
          |  B_06  |  LSF1,4 (3) |  Pitch (5)  |  Pitch (7)  |
          |  B_07  |  LSF1,4 (2) |  Pitch (4)  |  Pitch (6)  |
          |  B_08  |  LSF1,4 (1) |  Pitch (3)  |  Pitch (5)  |
          +--------+-------------+-------------+-------------+
          |  B_09  |  LSF1,4 (0) |  Pitch (2)  |  Pitch (4)  |
          |  B_10  |  LSF1,3 (3) |  Pitch (1)  |  Pitch (3)  |
          |  B_11  |  LSF1,3 (2) |  Pitch (0)  |  Pitch (2)  |
          |  B_12  |  LSF1,3 (1) |  LSF1,3 (3) |  Pitch (1)  |
          |  B_13  |  LSF1,3 (0) |  LSF1,3 (2) |  Pitch (0)  |
          |  B_14  |  LSF1,2 (3) |  LSF1,3 (1) |  LSF1,3 (3) |
          |  B_15  |  LSF1,2 (2) |  LSF1,3 (0) |  LSF1,3 (2) |
          |  B_16  |  LSF1,2 (1) |  LSF1,2 (3) |  LSF1,3 (1) |
          +--------+-------------+-------------+-------------+
          |  B_17  |  LSF1,2 (0) |  LSF1,2 (2) |  LSF1,3 (0) |
          |  B_18  |  LSF1,1 (5) |  LSF1,2 (1) |  LSF1,2 (4) |
          |  B_19  |  LSF1,1 (4) |  LSF1,2 (0) |  LSF1,2 (3) |
          |  B_20  |  LSF1,1 (3) |  LSF1,1 (5) |  LSF1,2 (2) |
          |  B_21  |  LSF1,1 (2) |  LSF1,1 (4) |  LSF1,2 (1) |
          |  B_22  |  LSF1,1 (1) |  LSF1,1 (3) |  LSF1,2 (0) |
          |  B_23  |  LSF1,1 (0) |  LSF1,1 (2) |  LSF1,1 (5) |
          |  B_24  |  LSF2,4 (3) |  LSF1,1 (1) |  LSF1,1 (4) |
          +--------+-------------+-------------+-------------+
          |  B_25  |  LSF2,4 (2) |  LSF1,1 (0) |  LSF1,1 (3) |
          |  B_26  |  LSF2,4 (1) |  LSF2,3 (3) |  LSF1,1 (2) |
          |  B_27  |  LSF2,4 (0) |  LSF2,3 (2) |  LSF1,1 (1) |
          |  B_28  |  LSF2,3 (3) |  LSF2,3 (1) |  LSF1,1 (0) |
          |  B_29  |  LSF2,3 (2) |  LSF2,3 (0) |  LSF2,3 (3) |
          |  B_30  |  LSF2,3 (1) |  LSF2,2 (4) |  LSF2,3 (2) |
          |  B_31  |  LSF2,3 (0) |  LSF2,2 (3) |  LSF2,3 (1) |
          |  B_32  |  LSF2,2 (3) |  LSF2,2 (2) |  LSF2,3 (0) |






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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


          +--------+-------------+-------------+-------------+
          |  B_33  |  LSF2,2 (2) |  LSF2,2 (1) |  LSF2,2 (4) |
          |  B_34  |  LSF2,2 (1) |  LSF2,2 (0) |  LSF2,2 (3) |
          |  B_35  |  LSF2,2 (0) |  LSF2,1 (6) |  LSF2,2 (2) |
          |  B_36  |  LSF2,1 (5) |  LSF2,1 (5) |  LSF2,2 (1) |
          |  B_37  |  LSF2,1 (4) |  LSF2,1 (4) |  LSF2,2 (0) |
          |  B_38  |  LSF2,1 (3) |  LSF2,1 (3) |  LSF2,1 (5) |
          |  B_39  |  LSF2,1 (2) |  LSF2,1 (2) |  LSF2,1 (4) |
          |  B_40  |  LSF2,1 (1) |  LSF2,1 (1) |  LSF2,1 (3) |
          +--------+-------------+-------------+-------------+
          |  B_41  |  LSF2,1 (0) |  LSF2,1 (0) |  LSF2,1 (2) |
          |  B_42  |  GAIN2 (5)  |  GAIN2 (5)  |  LSF2,1 (1) |
          |  B_43  |  GAIN2 (4)  |  GAIN2 (4)  |  LSF2,1 (0) |
          |  B_44  |  GAIN2 (3)  |  GAIN2 (3)  |  GAIN2 (4)  |
          |  B_45  |  GAIN2 (2)  |  GAIN2 (2)  |  GAIN2 (3)  |
          |  B_46  |  GAIN2 (1)  |  GAIN2 (1)  |  GAIN2 (2)  |
          |  B_47  |  GAIN2 (0)  |  GAIN2 (0)  |  GAIN2 (1)  |
          |  B_48  |  GAIN1 (6)  |  GAIN1 (6)  |  GAIN2 (0)  |
          +--------+-------------+-------------+-------------+
          |  B_49  |  GAIN1 (5)  |  GAIN1 (5)  |  GAIN1 (5)  |
          |  B_50  |  GAIN1 (4)  |  GAIN1 (4)  |  GAIN1 (4)  |
          |  B_51  |  GAIN1 (3)  |  GAIN1 (3)  |  GAIN1 (3)  |
          |  B_52  |  GAIN1 (2)  |  GAIN1 (2)  |  GAIN1 (2)  |
          |  B_53  |  GAIN1 (1)  |  GAIN1 (1)  |  GAIN1 (1)  |
          |  B_54  |  GAIN1 (0)  |  GAIN1 (0)  |  GAIN1 (0)  |
          +--------+-------------+-------------+-------------+

       Table 3: Bitstream Definition for MELPe 600 bps (Part 1 of 2)























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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


          +--------+-------------+-------------+-------------+
          |  Bit   |    Mode 4   |    Mode 5   |    Mode 6   |
          |        |   (voiced)  |   (voiced)  |   (voiced)  |
          +--------+-------------+-------------+-------------+
          |  B_01  | Voicing (4) | Voicing (4) | Voicing (4) |
          |  B_02  | Voicing (3) | Voicing (3) | Voicing (3) |
          |  B_03  | Voicing (2) | Voicing (2) | Voicing (2) |
          |  B_04  | Voicing (1) | Voicing (1) | Voicing (1) |
          |  B_05  | Voicing (0) | Voicing (0) | Voicing (0) |
          |  B_06  |  Pitch (7)  |  Pitch (7)  |  Pitch (7)  |
          |  B_07  |  Pitch (6)  |  Pitch (6)  |  Pitch (6)  |
          |  B_08  |  Pitch (5)  |  Pitch (5)  |  Pitch (5)  |
          +--------+-------------+-------------+-------------+
          |  B_09  |  Pitch (4)  |  Pitch (4)  |  Pitch (4)  |
          |  B_10  |  Pitch (3)  |  Pitch (3)  |  Pitch (3)  |
          |  B_11  |  Pitch (2)  |  Pitch (2)  |  Pitch (2)  |
          |  B_12  |  Pitch (1)  |  Pitch (1)  |  Pitch (1)  |
          |  B_13  |  Pitch (0)  |  Pitch (0)  |  Pitch (0)  |
          |  B_14  |  LSF1,3 (3) |  LSF1,3 (3) |  LSF1,3 (3) |
          |  B_15  |  LSF1,3 (2) |  LSF1,3 (2) |  LSF1,3 (2) |
          |  B_16  |  LSF1,3 (1) |  LSF1,3 (1) |  LSF1,3 (1) |
          +--------+-------------+-------------+-------------+
          |  B_17  |  LSF1,3 (0) |  LSF1,3 (0) |  LSF1,3 (0) |
          |  B_18  |  LSF1,2 (3) |  LSF1,2 (4) |  LSF1,2 (4) |
          |  B_19  |  LSF1,2 (2) |  LSF1,2 (3) |  LSF1,2 (3) |
          |  B_20  |  LSF1,2 (1) |  LSF1,2 (2) |  LSF1,2 (2) |
          |  B_21  |  LSF1,2 (0) |  LSF1,2 (1) |  LSF1,2 (1) |
          |  B_22  |  LSF1,1 (5) |  LSF1,2 (0) |  LSF1,2 (0) |
          |  B_23  |  LSF1,1 (4) |  LSF1,1 (5) |  LSF1,1 (6) |
          |  B_24  |  LSF1,1 (3) |  LSF1,1 (4) |  LSF1,1 (5) |
          +--------+-------------+-------------+-------------+
          |  B_25  |  LSF1,1 (2) |  LSF1,1 (3) |  LSF1,1 (4) |
          |  B_26  |  LSF1,1 (1) |  LSF1,1 (2) |  LSF1,1 (3) |
          |  B_27  |  LSF1,1 (0) |  LSF1,1 (1) |  LSF1,1 (2) |
          |  B_28  |  LSF2,3 (3) |  LSF1,1 (0) |  LSF1,1 (1) |
          |  B_29  |  LSF2,3 (2) |  LSF2,3 (3) |  LSF1,1 (0) |
          |  B_30  |  LSF2,3 (1) |  LSF2,3 (2) |  LSF2,3 (3) |
          |  B_31  |  LSF2,3 (0) |  LSF2,3 (1) |  LSF2,3 (2) |
          |  B_32  |  LSF2,2 (4) |  LSF2,3 (0) |  LSF2,3 (1) |
          +--------+-------------+-------------+-------------+
          |  B_33  |  LSF2,2 (3) |  LSF2,2 (4) |  LSF2,3 (0) |
          |  B_34  |  LSF2,2 (2) |  LSF2,2 (3) |  LSF2,2 (4) |
          |  B_35  |  LSF2,2 (1) |  LSF2,2 (2) |  LSF2,2 (3) |
          |  B_36  |  LSF2,2 (0) |  LSF2,2 (1) |  LSF2,2 (2) |
          |  B_37  |  LSF2,1 (6) |  LSF2,2 (0) |  LSF2,2 (1) |
          |  B_38  |  LSF2,1 (5) |  LSF2,1 (5) |  LSF2,2 (0) |
          |  B_39  |  LSF2,1 (4) |  LSF2,1 (4) |  LSF2,1 (6) |
          |  B_40  |  LSF2,1 (3) |  LSF2,1 (3) |  LSF2,1 (5) |



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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


          +--------+-------------+-------------+-------------+
          |  B_41  |  LSF2,1 (2) |  LSF2,1 (2) |  LSF2,1 (4) |
          |  B_42  |  LSF2,1 (1) |  LSF2,1 (1) |  LSF2,1 (3) |
          |  B_43  |  LSF2,1 (0) |  LSF2,1 (0) |  LSF2,1 (2) |
          |  B_44  |  GAIN2 (4)  |  GAIN2 (4)  |  LSF2,1 (1) |
          |  B_45  |  GAIN2 (3)  |  GAIN2 (3)  |  LSF2,1 (0) |
          |  B_46  |  GAIN2 (2)  |  GAIN2 (2)  |  GAIN1 (8)  |
          |  B_47  |  GAIN2 (1)  |  GAIN2 (1)  |  GAIN1 (7)  |
          |  B_48  |  GAIN2 (0)  |  GAIN2 (0)  |  GAIN1 (6)  |
          +--------+-------------+-------------+-------------+
          |  B_49  |  GAIN1 (5)  |  GAIN1 (5)  |  GAIN1 (5)  |
          |  B_50  |  GAIN1 (4)  |  GAIN1 (4)  |  GAIN1 (4)  |
          |  B_51  |  GAIN1 (3)  |  GAIN1 (3)  |  GAIN1 (3)  |
          |  B_52  |  GAIN1 (2)  |  GAIN1 (2)  |  GAIN1 (2)  |
          |  B_53  |  GAIN1 (1)  |  GAIN1 (1)  |  GAIN1 (1)  |
          |  B_54  |  GAIN1 (0)  |  GAIN1 (0)  |  GAIN1 (0)  |
          +--------+-------------+-------------+-------------+

          Notes:
          xxxx (0) = LSB
          xxxx (nbits-1) = MSB
          LSF1,p = MSVQ* index of the pth stage of the two first frames
          LSF2,p = MSVQ index of the pth stage of the two last frames
          GAIN1 = VQ/MSVQ index of the 1st stage
          GAIN2 = MSVQ index of the 2nd stage
          * MSVQ: Multi-Stage Vector Quantizer

       Table 4: Bitstream Definition for MELPe 600 bps (Part 2 of 2)

   The 600 bps MELPe RTP payload is constructed as per Figure 4.  Note
   that bit B_01 is placed in the LSB of the first byte with all other
   bits in sequence.  When filling octets, the least significant bits of
   the seventh octet are filled with bits B_49 to B_54, respectively.


















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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


           MSB                                              LSB
            0      1      2      3      4      5      6      7
        +------+------+------+------+------+------+------+------+
        | B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
        +------+------+------+------+------+------+------+------+
        | B_16 | B_15 | B_14 | B_13 | B_12 | B_11 | B_10 | B_09 |
        +------+------+------+------+------+------+------+------+
        | B_24 | B_23 | B_22 | B_21 | B_20 | B_19 | B_18 | B_17 |
        +------+------+------+------+------+------+------+------+
        | B_32 | B_31 | B_30 | B_29 | B_28 | B_27 | B_26 | B_25 |
        +------+------+------+------+------+------+------+------+
        | B_40 | B_39 | B_38 | B_37 | B_36 | B_35 | B_34 | B_33 |
        +------+------+------+------+------+------+------+------+
        | B_48 | B_47 | B_46 | B_45 | B_44 | B_43 | B_42 | B_41 |
        +------+------+------+------+------+------+------+------+
        | RSVA | RSVB | B_54 | B_53 | B_52 | B_51 | B_50 | B_49 |
        +------+------+------+------+------+------+------+------+

               Figure 4: Packed MELPe 600 bps Payload Octets
































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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


3.2.  MELPe Comfort Noise Bitstream Definition

   Table B.3-1 of [SCIP210] identifies the usage of MELPe 2400 bps
   parameters for conveying comfort noise.

        +-------------------------------------+----------------+
        |           MELPe Parameter           |      Value     |
        +-------------------------------------+----------------+
        | msvq[0] (line spectral frequencies) |  * See Note    |
        +-------------------------------------+----------------+
        | msvq[1] (line spectral frequencies) |    Set to 0    |
        +-------------------------------------+----------------+
        | msvq[2] (line spectral frequencies) |    Set to 0    |
        +-------------------------------------+----------------+
        | msvq[3] (line spectral frequencies) |    Set to 0    |
        +-------------------------------------+----------------+
        |      fsvq (Fourier magnitudes)      |    Set to 0    |
        +-------------------------------------+----------------+
        |            gain[0] (gain)           |    Set to 0    |
        +-------------------------------------+----------------+
        |            gain[1] (gain)           |  * See Note    |
        +-------------------------------------+----------------+
        |   pitch (pitch - overall voicing)   |    Set to 0    |
        +-------------------------------------+----------------+
        |        bp (bandpass voicing)        |    Set to 0    |
        +-------------------------------------+----------------+
        |   af (aperiodic flag/jitter index)  |    Set to 0    |
        +-------------------------------------+----------------+
        |           sync (sync bit)           |  Alternations  |
        +-------------------------------------+----------------+

        Note:
        The default values are the respective parameters from the
        vocoder frame.  It is preferred that msvq[0] and gain[1]
        values be derived by averaging the respective parameter from
        some number of previous vocoder frames.

                  Table 5: MELPe Comfort Noise Parameters













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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


   Since only msvq[0] (also known as LSF1x or the first LSP) and gain[1]
   (also known as g2x or the second gain) are needed, the following bit
   order is used for comfort noise frames:

                       +--------+-------------+
                       |  Bit   |   Comfort   |
                       |        |    Noise    |
                       +--------+-------------+
                       |  B_01  |     LSF10   |
                       |  B_02  |     LSF11   |
                       |  B_03  |     LSF12   |
                       |  B_04  |     LSF13   |
                       |  B_05  |     LSF14   |
                       |  B_06  |     LSF15   |
                       |  B_07  |     LSF16   |
                       |  B_08  |       g20   |
                       +--------+-------------+
                       |  B_09  |       g21   |
                       |  B_10  |       g22   |
                       |  B_11  |       g23   |
                       |  B_12  |       g24   |
                       |  B_13  |      SYNC   |
                       +--------+-------------+

                       Notes:
                       g = Gain
                       LSF = Line Spectral Frequencies

           Table 6: Bitstream Definition for MELPe Comfort Noise

   The comfort noise MELPe RTP payload is constructed as per Figure 5.
   Note that bit B_01 is placed in the LSB of the first byte with all
   other bits in sequence.  When filling octets, the least significant
   bits of the second octet are filled with bits B_09 to B_13,
   respectively.

           MSB                                              LSB
            0      1      2      3      4      5      6      7
        +------+------+------+------+------+------+------+------+
        | B_08 | B_07 | B_06 | B_05 | B_04 | B_03 | B_02 | B_01 |
        +------+------+------+------+------+------+------+------+
        | RSVA | RSVB | RSVC | B_13 | B_12 | B_11 | B_10 | B_09 |
        +------+------+------+------+------+------+------+------+

            Figure 5: Packed MELPe Comfort Noise Payload Octets






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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


3.3.  Multiple MELPe Frames in an RTP Packet

   A MELPe RTP packet MAY consist of zero or more MELPe coder frames
   followed by zero or one MELPe comfort noise frame.  The presence of a
   comfort noise frame can be deduced from the length of the RTP
   payload.  The default packetization interval is one coder frame
   (22.5, 67.5, or 90 ms) according to the coder bitrate (2400, 1200, or
   600 bps).  For some applications, a longer packetization interval is
   used to reduce the packet rate.

   A MELPe RTP packet comprised of no coder frame and no comfort noise
   frame MAY be used periodically by an endpoint to indicate
   connectivity by an otherwise idle receiver.

   All MELPe frames in a single RTP packet MUST be of the same coder
   bitrate.  Dynamic switching between frame rates within an RTP stream
   may be permitted (if supported by both ends) provided that reserved
   bits RSVA, RSVB, and RSVC are filled in as per Table 7.  If bitrate
   switching is not used, all reserved bits are encoded as 0 by the
   sender and ignored by the receiver.  (RSV0 is always coded as 0.)

                 +-------------------+------+------+------+
                 |   Coder Bitrate   | RSVA | RSVB | RSVC |
                 +-------------------+------+------+------+
                 |   2400 bps        |   0  |   0  |  N/A |
                 +-------------------+------+------+------+
                 |   1200 bps        |   1  |   0  |   0  |
                 +-------------------+------+------+------+
                 |    600 bps        |   0  |   1  |  N/A |
                 +-------------------+------+------+------+
                 |   Comfort Noise   |   1  |   0  |   1  |
                 +-------------------+------+------+------+
                 |   (reserved)      |   1  |   1  |  N/A |
                 +-------------------+------+------+------+

                  Table 7: MELPe Frame Bitrate Indicators

   It is important to observe that senders have the following additional
   restrictions:

   Senders SHOULD NOT include more MELPe frames in a single RTP packet
   than will fit in the MTU of the RTP transport protocol.

   Frames MUST NOT be split between RTP packets.







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RFC 8130           RTP Payload Format for MELPe Codec         March 2017


   It is RECOMMENDED that the number of frames contained within an RTP
   packet be consistent with the application.  For example, in telephony
   and other real-time applications where delay is important, then the
   fewer frames per packet the lower the delay, whereas for bandwidth-
   constrained links or delay-insensitive streaming messaging
   applications, more than one frame per packet or many frames per
   packet would be acceptable.

   Information describing the number of frames contained in an RTP
   packet is not transmitted as part of the RTP payload.  The way to
   determine the number of MELPe frames is to count the total number of
   octets within the RTP packet and divide the octet count by the number
   of expected octets per frame (7/11/7 per frame).  Keep in mind that
   the last frame can be a 2-octet comfort noise frame.

   When dynamic bitrate switching is used and more than one frame is
   contained in an RTP packet, it is RECOMMENDED that the coder rate
   bits contained in the last octet be inspected.  If the coder bitrate
   indicates a comfort noise frame, then inspect the third last octet
   for the coder bitrate.  All MELPe speech frames in the RTP packet
   will be of this same coder bitrate.

3.4.  Congestion Control Considerations

   The target bitrate of MELPe can be adjusted at any point in time,
   thus allowing congestion management.  Furthermore, the amount of
   encoded speech or audio data encoded in a single packet can be used
   for congestion control, since the packet rate is inversely
   proportional to the packet duration.  A lower packet transmission
   rate reduces the amount of header overhead but at the same time
   increases latency and loss sensitivity, so it ought to be used
   with care.

   Since UDP does not provide congestion control, applications that use
   RTP over UDP SHOULD implement their own congestion control above the
   UDP layer [RFC8085] and MAY also implement a transport circuit
   breaker [RFC8083].  Work in the RMCAT working group [RMCAT] describes
   the interactions and conceptual interfaces necessary between the
   application components that relate to congestion control, including
   the RTP layer, the higher-level media codec control layer, and the
   lower-level transport interface, as well as components dedicated to
   congestion control functions.









Demjanenko & Satterlee       Standards Track                   [Page 21]


RFC 8130           RTP Payload Format for MELPe Codec         March 2017


4.  Payload Format Parameters

   This RTP payload format is identified using the MELP, MELP2400,
   MELP1200, and MELP600 media subtypes, which are registered in
   accordance with RFC 4855 [RFC4855] and per the media type
   registration template from RFC 6838 [RFC6838].

4.1.  Media Type Definitions

   Type name: audio

   Subtype names: MELP, MELP2400, MELP1200, and MELP600

   Required parameters: N/A

   Optional parameters:

      ptime: the recommended length of time (in milliseconds)
         represented by the media in a packet.  It SHALL use the nearest
         rounded-up ms integer packet duration.  For MELPe, this
         corresponds to the following values: 23, 45, 68, 90, 112, 135,
         156, and 180.  Larger values can be used as long as they are
         properly rounded.  See Section 6 of RFC 4566 [RFC4566].

      maxptime: the maximum length of time (in milliseconds) that can be
         encapsulated in a packet.  It SHALL use the nearest rounded-up
         ms integer packet duration.  For MELPe, this corresponds to the
         following values: 23, 45, 68, 90, 112, 135, 156, and 180.
         Larger values can be used as long as they are properly rounded.
         See Section 6 of RFC 4566 [RFC4566].

      bitrate: specifies the MELPe coder bitrates supported.  Possible
         values are a comma-separated list of rates from the following
         set: 2400, 1200, 600.  The modes are listed in order of
         preference; first is preferred.  If "bitrate" is not present,
         the fixed coder bitrate of 2400 MUST be used.  The alternate
         encoding names "MELP2400", "MELP1200", and "MELP600" directly
         specify the MELPe coder bitrates of 2400, 1200, and 600,
         respectively, and MUST NOT specify a "bitrate" parameter.

   Encoding considerations: These media subtypes are framed and binary;
      see Section 4.8 of RFC 6838 [RFC6838].

   Security considerations: Please see Section 8 of RFC 8130.

   Interoperability considerations: Early implementations used MELP2400,
      MELP1200, and MELP600 to indicate both coder type and bitrate.
      These media type names should be preserved with this registration.



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   Published specification: N/A

   Applications that use this media type: N/A

   Additional information: N/A

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      File extension(s): N/A

      Macintosh file type code(s): N/A

   Person & email address to contact for further information:

      Victor Demjanenko, Ph.D.
      VOCAL Technologies, Ltd.
      520 Lee Entrance, Suite 202
      Buffalo, NY  14228
      United States of America
      Phone: +1 716 688 4675
      Email: victor.demjanenko@vocal.com

   Intended usage: COMMON

   Restrictions on usage: These media subtypes depend on RTP framing and
      hence are only defined for transfer via RTP [RFC3550].  Transport
      within other framing protocols is not defined at this time.

   Author: Victor Demjanenko

   Change controller: IETF Payload working group delegated from the
      IESG.

   Provisional registration? (standards tree only): No

4.2.  Mapping to SDP

   The mapping of the above-defined payload format media subtypes and
   their parameters SHALL be done according to Section 3 of RFC 4855
   [RFC4855].









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   The information carried in the media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [RFC4566], which is commonly used to describe RTP sessions.  When SDP
   is used to specify sessions employing the MELPe codec, the mapping is
   as follows:

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

   o  The media subtype (payload format name) goes in SDP "a=rtpmap" as
      the encoding name.

   o  The parameter "bitrate" goes in the SDP "a=fmtp" attribute by
      copying it as a "bitrate=<value>" string.

   o  The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and
      "a=maxptime" attributes, respectively.

   When conveying information via SDP, the encoding name SHALL be "MELP"
   (the same as the media subtype).  Alternate encoding name subtypes
   "MELP2400", "MELP1200", and "MELP600" MAY be used in SDP to convey
   fixed-bitrate configurations.  These names have been observed in
   systems that do not support dynamic frame-rate switching as specified
   by the parameter "bitrate".

   An example of the media representation in SDP for describing MELPe
   might be:

      m=audio 49120 RTP/AVP 97
      a=rtpmap:97 MELP/8000

   An alternative example of SDP for fixed-bitrate configurations
   might be:

      m=audio 49120 RTP/AVP 97 100 101 102
      a=rtpmap:97 MELP/8000
      a=rtpmap:100 MELP2400/8000
      a=rtpmap:101 MELP1200/8000
      a=rtpmap:102 MELP600/8000

   If the encoding name "MELP" is received without a "bitrate"
   parameter, the fixed coder bitrate of 2400 MUST be used.  The
   alternate encoding names "MELP2400", "MELP1200", and "MELP600"
   directly specify the MELPe coder bitrates of 2400, 1200, and 600,
   respectively, and MUST NOT specify a "bitrate" parameter.

   The optional media type parameter "bitrate", when present, MUST be
   included in the "a=fmtp" attribute in the SDP, expressed as a media
   type string in the form of a semicolon-separated list of



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   parameter=value pairs.  The string "value" can be one or more of
   2400, 1200, and 600, separated by commas (where each bitrate value
   indicates the corresponding MELPe coder).  An example of the media
   representation in SDP for describing MELPe when all three coder
   bitrates are supported might be:

      m=audio 49120 RTP/AVP 97
      a=rtpmap:97 MELP/8000
      a=fmtp:97 bitrate=2400,600,1200

   Parameter "ptime" cannot be used for the purpose of specifying the
   MELPe operating mode, due to the fact that for certain values it will
   be impossible to distinguish which mode is about to be used (e.g.,
   when ptime=68, it would be impossible to distinguish if the packet is
   carrying one frame of 67.5 ms or three frames of 22.5 ms).

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

4.3.  Declarative SDP Considerations

   For declarative media, the "bitrate" parameter specifies the possible
   bitrates used by the sender.  Multiple MELPe rtpmap values (such as
   97, 98, and 99, as used below) MAY be used to convey MELPe-coded
   voice at different bitrates.  The receiver can then select an
   appropriate MELPe codec by using 97, 98, or 99.

      m=audio 49120 RTP/AVP 97 98 99
      a=rtpmap:97 MELP/8000
      a=fmtp:97 bitrate=2400
      a=rtpmap:98 MELP/8000
      a=fmtp:98 bitrate=1200
      a=rtpmap:99 MELP/8000
      a=fmtp:99 bitrate=600

4.4.  Offer/Answer SDP Considerations

   In the Offer/Answer model [RFC3264], "bitrate" is a bidirectional
   parameter.  Both sides MUST use a common "bitrate" value or values.
   The offer contains the bitrates supported by the offerer, listed in
   its preferred order.  The answerer MAY agree to any bitrate by
   listing the bitrate first in the answerer response.  Additionally,
   the answerer MAY indicate any secondary bitrate or bitrates that it
   supports.  The initial bitrate used by both parties SHALL be the
   first bitrate specified in the answerer response.



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   For example, if offerer bitrates are "2400,600" and answer bitrates
   are "600,2400", the initial bitrate is 600.  If other bitrates are
   provided by the answerer, any common bitrate between the offer and
   answer MAY be used at any time in the future.  Activation of these
   other common bitrates is beyond the scope of this document.

   The use of a lower bitrate is often important for a case such as when
   one endpoint utilizes a bandwidth-constrained link (e.g., 1200 bps
   radio link or slower), where only the lower coder bitrate will work.

5.  Discontinuous Transmissions

   A primary application of MELPe is for radio communications of voice
   conversations, and discontinuous transmissions are normal.  When
   MELPe is used in an IP network, MELPe RTP packet transmissions may
   cease and resume frequently.  RTP synchronization source (SSRC)
   sequence number gaps indicate lost packets to be filled by PLC, while
   abrupt loss of RTP packets indicates intended discontinuous
   transmissions.

   If a MELPe coder so desires, it may send a comfort noise frame as per
   Appendix B of [SCIP210] prior to ceasing transmission.  A receiver
   may optionally use comfort noise during its silence periods.  No SDP
   negotiations are required.

6.  Packet Loss Concealment

   MELPe packet loss concealment (PLC) uses the special properties and
   coding for the pitch/voicing parameter of the MELPe 2400 bps coder.
   The PLC erasure indication utilizes any of the errored encodings of a
   non-voiced frame as identified in Table 1 of [MELPE].  For the sake
   of simplicity, it is preferred that a code value of 3 for the
   pitch/voicing parameter (represented by the bits P6 to P0 in Table 1
   of this document) be used.  Hence, set bits P0 and P1 to one and bits
   P2, P3, P4, P5, and P6 to zero.

   When using PLC in 1200 bps or 600 bps mode, the MELPe 2400 bps
   decoder is called three or four times, respectively, to cover the
   loss of a MELPe frame.

7.  IANA Considerations

   IANA has registered MELP, MELP2400, MELP1200, and MELP600 as
   specified in Section 4.1.  IANA has also added these media subtypes
   to the "RTP Payload Format media types" registry
   (http://www.iana.org/assignments/rtp-parameters).





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8.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550] and in any applicable RTP profile such as
   RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or
   RTP/SAVPF [RFC5124].  However, as discussed in [RFC7202], it is not
   an RTP payload format's responsibility to discuss or mandate what
   solutions are used to meet such basic security goals as
   confidentiality, integrity, and source authenticity for RTP in
   general.  This responsibility lies with anyone using RTP in an
   application.  They can find guidance on available security mechanisms
   and important considerations in [RFC7201].  Applications SHOULD use
   one or more appropriate strong security mechanisms.  The rest of this
   section discusses the security-impacting properties of the payload
   format itself.

   This RTP payload format and the MELPe decoder do not exhibit any
   significant non-uniformity in the receiver-side computational
   complexity for packet processing and thus are unlikely to pose a
   denial-of-service threat due to the receipt of pathological data.
   Additionally, the RTP payload format does not contain any active
   content.

   Please see the security considerations discussed in [RFC6562]
   regarding VAD and its effect on bitrates.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2736]  Handley, M. and C. Perkins, "Guidelines for Writers of RTP
              Payload Format Specifications", BCP 36, RFC 2736,
              DOI 10.17487/RFC2736, December 1999,
              <http://www.rfc-editor.org/info/rfc2736>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://www.rfc-editor.org/info/rfc3264>.






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   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <http://www.rfc-editor.org/info/rfc3551>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <http://www.rfc-editor.org/info/rfc3711>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://www.rfc-editor.org/info/rfc4566>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <http://www.rfc-editor.org/info/rfc4855>.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124,
              February 2008, <http://www.rfc-editor.org/info/rfc5124>.

   [RFC6562]  Perkins, C. and JM. Valin, "Guidelines for the Use of
              Variable Bit Rate Audio with Secure RTP", RFC 6562,
              DOI 10.17487/RFC6562, March 2012,
              <http://www.rfc-editor.org/info/rfc6562>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC8083]  Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions", RFC 8083,
              DOI 10.17487/RFC8083, March 2017,
              <http://www.rfc-editor.org/info/rfc8083>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", RFC 8085, DOI 10.17487/RFC8085, March 2017,
              <http://www.rfc-editor.org/info/rfc8085>.





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   [MELP]     Department of Defense Telecommunications Standard,
              "Analog-to-Digital Conversion of Voice by 2,400 Bit/Second
              Mixed Excitation Linear Prediction (MELP)", MIL-STD-3005,
              December 1999.

   [MELPE]    North Atlantic Treaty Organization (NATO), "The 600 Bit/S,
              1200 Bit/S and 2400 Bit/S NATO Interoperable Narrow Band
              Voice Coder", STANAG No. 4591, January 2006.

   [SCIP210]  National Security Agency, "SCIP Signaling Plan", SCIP-210,
              December 2007.

9.2.  Informative References

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <http://www.rfc-editor.org/info/rfc4585>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <http://www.rfc-editor.org/info/rfc7201>.

   [RFC7202]  Perkins, C. and M. Westerlund, "Securing the RTP
              Framework: Why RTP Does Not Mandate a Single Media
              Security Solution", RFC 7202, DOI 10.17487/RFC7202,
              April 2014, <http://www.rfc-editor.org/info/rfc7202>.

   [RMCAT]    IETF, RTP Media Congestion Avoidance Techniques (rmcat)
              Working Group,
              <https://datatracker.ietf.org/wg/rmcat/about/>.



















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Authors' Addresses

   Victor Demjanenko, Ph.D.
   VOCAL Technologies, Ltd.
   520 Lee Entrance, Suite 202
   Buffalo, NY  14228
   United States of America

   Phone: +1 716 688 4675
   Email: victor.demjanenko@vocal.com


   David Satterlee
   VOCAL Technologies, Ltd.
   520 Lee Entrance, Suite 202
   Buffalo, NY  14228
   United States of America

   Phone: +1 716 688 4675
   Email: david.satterlee@vocal.com































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