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Versions: (draft-demjanenko-payload-melpe) 00 01 02 03 04 05 06 RFC 8130

Payload Working Group                                  Victor Demjanenko
Internet-Draft                                           David Satterlee
Intended Status: Standards Track                VOCAL Technologies, Ltd.
Expires: August 11, 2017                                February 7, 2017


                   RTP Payload Format for MELPe Codec
                      draft-ietf-payload-melpe-06


Status of this Memo

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   material or to cite them other than as "work in progress."


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 frames sizes are
   supported.  Comfort noise procedures and packet loss concealment are
   detailed.





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Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Conventions, Definitions and Acronyms . . . . . . . . . . .  3
   2  Background  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3  Payload Format  . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1  MELPe Bitstream Definition  . . . . . . . . . . . . . . . .  5
       3.1.1 2400 bps Bitstream Structure . . . . . . . . . . . . . .  6
       3.1.2 1200 bps Bitstream Structure . . . . . . . . . . . . . .  8
       3.1.3 600 bps Bitstream Structure  . . . . . . . . . . . . . . 11
     3.2  MELPe Comfort Noise Bitstream Definition  . . . . . . . . . 15
     3.3  Multiple MELPe frames in a RTP packet . . . . . . . . . . . 17
     3.4  Congestion Control Considerations . . . . . . . . . . . . . 19
   4  Payload Format Parameters . . . . . . . . . . . . . . . . . . . 19
     4.1  Media Type Definition . . . . . . . . . . . . . . . . . . . 20
     4.2  Mapping to SDP  . . . . . . . . . . . . . . . . . . . . . . 23
     4.3  Declarative SDP Considerations  . . . . . . . . . . . . . . 24
     4.4  Offer/Answer SDP Considerations . . . . . . . . . . . . . . 24
   5  Discontinious Transmission  . . . . . . . . . . . . . . . . . . 25
   6  Packet Loss Concealment . . . . . . . . . . . . . . . . . . . . 25
   7  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 27
   8  Security Considerations . . . . . . . . . . . . . . . . . . . . 27
   9  RFC Editor Considerations . . . . . . . . . . . . . . . . . . . 27
   10  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 27
     10.2  Informative References . . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
























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1  Introduction

   This document describes how compressed 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 bit-rate 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 the transport network condition,
   bandwidth restriction, delay requirements and packet-loss tolerance.


1.1  Conventions, Definitions and Acronyms

   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 RTP payload format [RFC2736] were
   followed.


2  Background


   The MELP speech coder was developed by the US military as an upgrade
   from LPC-based CELP standard vocoder for low bit-rate communications
   [MELP].  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 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.

   Commercial/civilian applications have arisen because of the low bit-
   rate property of MELPe with its (relatively) high intelligibility.
   As such MELPe is being used in a variety of wired and radio
   communications systems.  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 bit rates;
   2400, 1200, and 600 bps.  The basic 2400 bps bit-rate vocoder uses a
   22.5 ms frame of speech consisting of 180 8000 Hz, 16-bit speech
   samples.  The 1200 and 600 bps bit-rate vocoders uses respectively
   three and four 22.5 ms frames of speech each.  These reduced bit-rate
   vocoders internally use multiple 2400 bps parameter sets with further



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   processing to strategically remove redundancy.  The payload sizes for
   each of the bitrates are 54, 81, and 54 bits respectively for the
   2400, 1200, and 600 bps frames.  Dynamic bit-rate switching is
   permitted but only if supported by both endpoints.

   The MELPe algorithm distinguishes between voiced and un-voiced 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 bit-rate coders do not allocate any bits for FEC and rely
   on strong error protection and correction in the communications
   channel.

   Comfort noise handling for MELPe follows the procedures in SCIP-210
   Appendix B [SCIP210].  After VAD no longer indicates the presence of
   speech/voice, a grace period of a minimum of two comfort noise
   vocoder fames are to be transmitted. The contents of the comfort
   noise frames is 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 of the two bit set
   errored/erasure encodings of a non-voiced frame as will be described
   infra.


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 the situations where the sender and the
   receiver send one or more codec data frames per packet.











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   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 M bit SHOULD
   be as specified in the applicable RTP profile, for example, RFC 3551
   [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 then 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.

   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 Definition

   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 the
   1200 bps speech the total number of bits used is 81, which fits in 11
   octets (with seven unused bits).  For the 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 3.3 in support of dynamic bit-rate switching.

   In the MELPe bitstream definition, 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
   convenience of interoperable gateway devices, the bitstreams will be
   presented identically in IP networks.





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3.1.1 2400 bps Bitstream Structure

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


          +--------+-------------+-------------+
          |  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   |
          +--------+-------------+-------------+
          |  B_33  |       FM7   |     FEC22   |
          |  B_34  |       FM6   |     FEC21   |



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          |  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 3.1 - The bitstream definition for MELPe 2400 bps.


   The 2400 bps MELPe RTP payload is constructed as per Figure 2.  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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      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.


3.1.2 1200 bps Bitstream Structure

   According to Tables D9a and D9b of [MELPE], the 1200 bit/s MELPe bit
   transmission order is the following:


          +--------+-------------+-------------+
          |  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     |



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          +--------+-------------+-------------+
          |  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    |
          +--------+-------------+-------------+
          |  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    |             |



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          |  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     |             |
          +--------+-------------+-------------+
          |  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 = Band pass voicing
          FS = Fourier magnitudes

   Table 3.2 - The 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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      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.


3.1.3 600 bps Bitstream Structure

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


          +--------+-------------+-------------+-------------+
          |  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)  |



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          +--------+-------------+-------------+-------------+
          |  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) |
          +--------+-------------+-------------+-------------+
          |  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)  |



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          |  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 3.3a - The bitstream definition for MELPe 600 bps (part 1 of
   2).

          +--------+-------------+-------------+-------------+
          |  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) |



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          +--------+-------------+-------------+-------------+
          |  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) |
          +--------+-------------+-------------+-------------+
          |  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 indice of the pth stage of the two first frames
          LSF2,p = MSVQ indice of the pth stage of the two last frames
          GAIN1 = VQ/MSVQ indice of the 1st stage
          GAIN2 = MSVQ indice of the 2nd stage

   Table 3.3b - The 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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      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.


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.


























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          +-------------------------------------+----------------+
          |           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 3.4 - MELPe Comfort Noise Parameters


   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.














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          +--------+-------------+
          |  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 3.5 - The 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 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.


3.3  Multiple MELPe frames in a RTP packet

   A MELPe RTP packet MAY consist of zero or more MELPe coder frames,



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   followed by zero or one MELPe Comfort Noise frames.  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 bit rate (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 end point to indicate
   connectivity by an otherwise idle receiver.

   All MELPe frames in a single RTP packet MUST be of the same coder bit
   rate. 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 3.6.  If bit-rate
   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 Bit Rate  | 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 3.6 - MELPe Frame Bit Rate 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.

   It is RECOMMENDED that the number of frames contained within an RTP
   packet is consistent with the application.  For example, in a
   telephony and other real time applications where delay is important,
   then the fewer frames per packet the lower the delay, whereas for



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   bandwidth constrained links or delay insensitive streaming messaging
   application, more than one 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
   the last frame can be a 2 octet comfort noise frame.

   When dynamic bit-rate switching is used and more than one frame is
   contained in a RTP packet, it is RECOMMENDED to inspect the coder
   rate bits contained in the last octet.  If the coder bit rate
   indicates a Comfort Noise frame, then inspect the third last octet
   for the coder bit rate.  All MELPe speech frames in the RTP packet
   will be of this same coder bit rate.


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 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 as well implement a transport circuit
   breaker [RFC8083] (formerly [draft-ietf-avtcore-rtp-circuit-
   breakers]).  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.


4  Payload Format Parameters

   This RTP payload format is identified using the MELP, MELP2400,
   MELP1200, and MELP600 media types which is registered in accordance
   with RFC 4855 [RFC4855] and using the template of RFC 6838 [RFC6838].





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4.1  Media Type Definition

   Type names:

      audio

   Subtype name:

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

   Encoding considerations:

      This media type is framed and binary, see section 4.8 in RFC6838
      [RFC6838].

   Security considerations:

      Please see the security considerations in section 8 of RFCxxxx
      (this RFC).



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   Interoperability considerations:

      Early implementations used MELP2400, MELP1200, and MELP600 to
      indicate both coder type and bit rate.  These media type names
      should be preserved with this registration.

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

   Intended usage:

      COMMON




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   Restrictions on usage:

      This media type depends on RTP framing, and hence is 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


































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4.2  Mapping to SDP

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

   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 by SDP, the encoding name SHALL be "MELP"
   (the same as the media subtype).  Alternative encoding name types,
   "MELP2400", "MELP1200", and "MELP600", MAY be used in SDP to convey
   fixed bit-rate 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 bit-rate 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 bit rate of 2400 MUST be used.  The
   alternate encoding names, "MELP2400", "MELP1200", and "MELP600"
   directly specify the MELPe coder bit rate of 2400, 1200, and 600
   respectively and MUST NOT specify a "bitrate" parameter.




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

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

   Parameter ptime can not be used for the purpose of specifying MELPe
   operating mode, due to fact that for the 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 packet is carrying
   1 frames of 67.5 ms or 3 frames of 22.5 ms etc.).

   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 both in media subtype name and in the
   default mapping to the SDP a=fmtp attribute


4.3  Declarative SDP Considerations

   For declarative media, the "bitrate" parameter specifes the possible
   bit rates 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 bit rates. 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 an Offer/Answer mode [RFC3264], "bitrate" is a bi-directional
   parameter. Both sides MUST use a common "bitrate" value or values.
   The offer contains the bit rates supported by the offerer listed in



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   its preferred order.  The answerer MAY agree to any bit rate by
   listing the bit rate first in the answerer response.  Additionally
   the answerer MAY indicate any secondary bit rate or bit rates that it
   supports.  The initial bit rate used by both parties SHALL be the
   first bit rate specified in the answerer response.

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

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


5  Discontinious Transmission

   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 SSRC sequence number gaps indicate lost
   packets to be filled by PLC while abrupt loss of RTP packets indicate
   intended discontinuous transmission.

   If a MELPe coder so desires, it may send a comfort noise frame as per
   SCIP-210 Appendix B [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 to use a code value of 3 for the
   pitch/voicing parameter (represented by the bits P6 to P0 in Table
   3.1).  Hence, set bits P0 and P1 to one and bits P2, P3, P4, P5, and
   P6 to zero.

   When using PLC in a 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.




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7  IANA Considerations

   This memo requests that IANA registers MELP, MELP2400, MELP1200, and
   MELP600 as specified in Section 4.1.  The media type is also
   requested to be added to the IANA registry for "RTP Payload Format
   MIME types" (http://www.iana.org/assignments/rtp-parameters).


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 [RFC4855], RTP/SAVP [RFC3711] or
   RTP/SAVPF [RFC5124].  However, as "Securing the RTP Protocol
   Framework: Why RTP Does Not Mandate a Single Media Security Solution"
   [RFC7202] discusses, it is not an RTP payload format's responsibility
   to discuss or mandate what solutions are used to meet the basic
   security goals like confidentiality, integrity and source
   authenticity for RTP in general. This responsibility lays on anyone
   using RTP in an application.  They can find guidance on available
   security mechanisms and important considerations in Options for
   Securing RTP Sessions [RFC7201].  Applications SHOULD use one or more
   appropriate strong security mechanisms.  The rest of this security
   consideration 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.
   Nor does the RTP payload format contain any active content.

   With respect to VAD and its effect on bit rate, please see security
   consideration in RFC6562 [RFC6562].


9  RFC Editor Considerations

   Note to RFC Editor: This section may be removed after carrying out
   all the instructions of this section.


10  References

10.1  Normative References

   [draft-ietf-avtcore-rtp-circuit-breakers] Perkins, C. and V. Singh,



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   "Multimedia Congestion Control: Circuit Breakers for Unicast RTP
   Sessions", draft-ietf-avtcore-rtp-circuit-breakers-18 (work in
   progress), August 18, 2016.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2736] Handley, M. and Perkins, C., "Guidelines for Writers of RTP
   Payload Format Specifications", BCP 36, RFC 2736, December 1999.

   [RFC3264] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model
   with the Session Description Protocol (SDP)" IETF RFC 3264, June
   2002.

   [RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and Jacobson,
   V., "RTP: A Transport Protocol for Real-Time Applications", IETF RFC
   3550, July 2003.

   [RFC3551] Schulzrinne, H., and Casner, S., "RTP Profile for Audio and
   Video Conferences with Minimal Control" IETF RFC 3551, July 2003.

   [RFC3711] Baugher, et al., "The Secure Real Time Transport Protocol",
   IETF RFC 3711, March 2004.

   [RFC4566] Handley, M., Jacobson, V. and Perkins, C., "SDP: Session
   Description Protocol", IETF RFC RFC4566, July 2006.

   [RFC4855] Casner, S., "Media Type Registration of RTP Payload
   Formats", RFC 4855, February 2007.

   [RFC5124] Ott, J. and Carrara, E., "Extended Secure RTP Profile for
   Real-time Transport Control Protocol (RTCP)-Based
   Feedback(RTP/SAVPF)", RFC 5124, February 2008.

   [RFC6562] Perkins, C. and Valin, J. M., "Guidelines for the Use of
   Variable Bit Rate Audio with Secure RTP", RFC 6562, March 2012.

   [RFC6838] Freed, N., Klensin, J. and Hansen, T., "Media Type
   Specifications and Registration Procedures", BCP 13, RFC 6838,
   January 2013.

   [RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
   Circuit Breakers for Unicast RTP Sessions", RFC 8083, January 2017.

   [RFC8085] Eggert, L., Fairhurst, G. and Shepherd, G., "UDP Usage
   Guidelines", RFC 8085, February 2017.

   [MELP] Department of Defense Telecommunications Standard, "Analog-to-



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   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.


10.2  Informative References

   [RFC7201] Westerlund, M. and Perkins, C., "Options for Securing RTP
   Sessions", RFC 7201, April 2014.

   [RFC7202] Perkins, C. and Westerlund, M., "Securing the RTP
   Framework: Why RTP Does Not Mandate a Single Media Security
   Solution", RFC 7202, April 2014.


Authors' Addresses


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

   David Satterlee
   VOCAL Technologies, Ltd.
   520 Lee Entrance, Suite 202
   Buffalo, NY 14228
   USA
   Phone: +1 716 688 4675
   Email: david.satterlee@vocal.com












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