



Internet Engineering Task Force (IETF)                     V. Demjanenko
Request for Comments: 8817                                     J. Punaro
Category: Standards Track                                   D. Satterlee
ISSN: 2070-1721                                 VOCAL Technologies, Ltd.
                                                             August 2020


       RTP Payload Format for Tactical Secure Voice Cryptographic
             Interoperability Specification (TSVCIS) Codec

Abstract

   This document describes the RTP payload format for the Tactical
   Secure Voice Cryptographic Interoperability Specification (TSVCIS)
   speech coder.  TSVCIS is a scalable narrowband voice coder supporting
   varying encoder data rates and fallbacks.  It is implemented as an
   augmentation to the Mixed Excitation Linear Prediction Enhanced
   (MELPe) speech coder by conveying additional speech coder parameters
   to enhance voice quality.  TSVCIS augmented speech data is processed
   in conjunction with its temporally matched Mixed Excitation Linear
   Prediction (MELP) 2400 speech data.  The RTP packetization of TSVCIS
   and MELPe speech coder data is 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
   https://www.rfc-editor.org/info/rfc8817.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

1.  Introduction

   This document describes how compressed Tactical Secure Voice
   Cryptographic Interoperability Specification (TSVCIS) speech as
   produced by the TSVCIS codec [TSVCIS] [NRLVDR] may be formatted for
   use as an RTP payload.  The TSVCIS speech coder (or TSVCIS speech-
   aware communications equipment on any intervening transport link) may
   adjust to restricted bandwidth conditions by reducing the amount of
   augmented speech data and relying on the underlying MELPe speech
   coder for the most constrained bandwidth links.

   Details are provided for packetizing the TSVCIS augmented speech data
   along with MELPe 2400 bps speech parameters in an RTP packet.  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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Best current practices for writing an RTP payload format
   specification were followed [RFC2736] [RFC8088].

1.2.  Abbreviations

   The following abbreviations are used in this document.

   AVP:      Audio/Video Profile

   AVPF:     Audio/Video Profile Feedback

   CELP:     Code-Excited Linear Prediction

   FEC:      Forward Error Correction

   LPC:      Linear-Predictive Coding

   LSB:      Least Significant Bit

   MELP:     Mixed Excitation Linear Prediction

   MELPe:    Mixed Excitation Linear Prediction Enhanced

   MSB:      Most Significant Bit

   MTC:      Modified Count

   NATO:     North American Treaty Organization

   NRL:      Naval Research Lab

   PLC:      Packet Loss Concealment

   SAVP:     Secure Audio/Video Profile

   SAVPF:    Secure Audio/Video Profile Feedback

   SDP:      Session Description Protocol

   SSRC:     Synchronization Source

   SRTP:     Secure Real-Time Transport Protocol

   TSVCIS:   Tactical Secure Voice Cryptographic Interoperability
             Specification

   VAD:      Voice Activity Detect

   VDR:      Variable Date Rate

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 as international NATO Standard
   STANAG 4591 [MELPE].

   The Tactical Secure Voice Cryptographic Interoperability
   Specification (TSVCIS) is a specification written by the Tactical
   Secure Voice Working Group (TSVWG) to enable all modern tactical
   secure voice devices to be interoperable across the US Department of
   Defense [TSVCIS].  One of the most important aspects is that the
   voice modes defined in TSVCIS are based on specific fixed rates of
   the Naval Research Lab's (NRL's) Variable Date Rate (VDR) Vocoder,
   which uses the MELPe standard as its base [NRLVDR].  A complete
   TSVCIS speech frame consists of MELPe speech parameters and
   corresponding TSVCIS augmented speech data.

   In addition to the augmented speech data, the TSVCIS specification
   identifies which speech coder and framing bits are to be encrypted
   and how they are protected by forward error correction (FEC)
   techniques (using block codes).  At the RTP transport layer, only the
   speech coder-related bits need to be considered and are conveyed in
   unencrypted form.  In most IP-based network deployments, standard
   link encryption methods (Secure Real-Time Transport Protocol (SRTP),
   VPNs, FIPS 140 link encryptors, or Type 1 Ethernet encryptors) would
   be used to secure the RTP speech contents.

   TSVCIS augmented speech data is derived from the signal processing
   and data generated by the MELPe speech coder.  For the purposes of
   this specification, only the general parameter nature of TSVCIS will
   be characterized.  Depending on the bandwidth available (and FEC
   requirements), a varying number of TSVCIS-specific speech coder
   parameters need to be transported.  These are first byte-packed and
   then conveyed from encoder to decoder.

   Byte packing of TSVCIS speech data into packed parameters is
   processed as per the following example, where

   Three-bit field:  Bits A, B, and C (A is MSB; C is LSB)

   Five-bit field:  Bits D, E, F, G, and H (D is MSB; H is LSB)

           MSB                                              LSB
            0      1      2      3      4      5      6      7
        +------+------+------+------+------+------+------+------+
        |   H  |   G  |   F  |   E  |   D  |   C  |   B  |   A  |
        +------+------+------+------+------+------+------+------+

   This packing method places the three-bit field "first" in the lowest
   bits followed by the next five-bit field.  Parameters may be split
   between octets with the most significant bits in the earlier octet.
   Any unfilled bits in the last octet MUST be filled with zero.

   In order to accommodate a varying amount of TSVCIS augmented speech
   data, an octet count specifies the number of octets representing the
   TSVCIS packed parameters.  The encoding to do so is presented in
   Section 3.2.  TSVCIS specifically uses the NRL VDR in two
   configurations with a fixed set of 15 and 35 packed octet parameters
   in a standardized order [TSVCIS].

3.  Payload Format

   The TSVCIS codec augments the standard MELP 2400, 1200, and 600
   bitrates and hence 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 TSVCIS 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 TSVCIS                  |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 1: Packet Format Diagram

   The RTP header of the packetized encoded TSVCIS 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] 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 specified is 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; 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 TSVCIS speech coder includes all three MELPe coder rates used as
   base speech parameters or as speech coders for bandwidth-restricted
   links.  RTP packetization of MELPe follows [RFC8130] and is repeated
   here for all three MELPe rates [RFC8130], with its recommendations
   now regarded as requirements.  The bits previously labeled as RSVA,
   RSVB, and RSVC in [RFC8130] SHOULD be filled with rate code bits
   CODA, CODB, and CODC, as shown in Table 1 (compatible with Table 7 in
   Section 3.3 of [RFC8130]).

              +===============+======+======+======+========+
              | Coder Bitrate | CODA | CODB | CODC | Length |
              +===============+======+======+======+========+
              | 2400 bps      | 0    | 0    | N/A  | 7      |
              +---------------+------+------+------+--------+
              | 1200 bps      | 1    | 0    | 0    | 11     |
              +---------------+------+------+------+--------+
              | 600 bps       | 0    | 1    | N/A  | 7      |
              +---------------+------+------+------+--------+
              | Comfort Noise | 1    | 0    | 1    | 2      |
              +---------------+------+------+------+--------+
              | TSVCIS Data   | 1    | 1    | N/A  | var.   |
              +---------------+------+------+------+--------+

                    Table 1: TSVCIS/MELPe Frame Bitrate
                        Indicators and Frame Length

   The total number of bits used to describe one MELPe frame of 2400 bps
   speech is 54, which fits in 7 octets (with two rate code bits).  For
   MELPe 1200 bps speech, the total number of bits used is 81, which
   fits in 11 octets (with three rate code bits and four unused bits).
   For MELPe 600 bps speech, the total number of bits used is 54, which
   fits in 7 octets (with two rate code bits).  The comfort noise frame
   consists of 13 bits, which fits in 2 octets (with three rate code
   bits).  TSVCIS packed parameters will use the last code combination
   in a trailing byte as discussed in Section 3.2.

   It should be noted that CODB for MELPe 600 bps mode MAY deviate from
   the value in Table 1 when bit 55 is used as an alternating 1/0 end-
   to-end framing bit.  Frame decoding would remain distinct as CODA
   being zero on its own would indicate a 7-byte frame for either a 2400
   or 600 bps rate, and the use of 600 bps speech coding could be
   deduced from the RTP timestamp (and anticipated by the Session
   Description Protocol (SDP) negotiations).

3.1.1.  2400 bps Bitstream Structure

   The 2400 bps MELPe RTP payload is constructed as per Figure 2.  Note
   that CODA MUST be filled with 0 and CODB SHOULD be filled with 0 as
   per Section 3.1.  CODB MAY contain an end-to-end framing bit if
   required by the endpoints.

           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 |
        +------+------+------+------+------+------+------+------+
        | CODA | CODB | 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

   The 1200 bps MELPe RTP payload is constructed as per Figure 3.  Note
   that CODA, CODB, and CODC MUST be filled with 1, 0, and 0,
   respectively, as per Section 3.1.  RSV0 MUST be coded as 0.

           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 |
        +------+------+------+------+------+------+------+------+
        | CODA | CODB | CODC | RSV0 | RSV0 | RSV0 | RSV0 | B_81 |
        +------+------+------+------+------+------+------+------+

               Figure 3: Packed MELPe 1200 bps Payload Octets

3.1.3.  600 bps Bitstream Structure

   The 600 bps MELPe RTP payload is constructed as per Figure 4.  Note
   CODA MUST be filled with 0 and CODB SHOULD be filled with 1 as per
   Section 3.1.  CODB MAY contain an end-to-end framing bit if required
   by the endpoints.

           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 |
        +------+------+------+------+------+------+------+------+
        | CODA | CODB | B_54 | B_53 | B_52 | B_51 | B_50 | B_49 |
        +------+------+------+------+------+------+------+------+

               Figure 4: Packed MELPe 600 bps Payload Octets

3.1.4.  Comfort Noise Bitstream Definition

   The comfort noise MELPe RTP payload is constructed as per Figure 5.
   Note that CODA, CODB, and CODC MUST be filled with 1, 0, and 1,
   respectively, as per Section 3.1.

           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 |
        +------+------+------+------+------+------+------+------+
        | CODA | CODB | CODC | B_13 | B_12 | B_11 | B_10 | B_09 |
        +------+------+------+------+------+------+------+------+

            Figure 5: Packed MELPe Comfort Noise Payload Octets

3.2.  TSVCIS Bitstream Definition

   The TSVCIS augmented speech data as packed parameters MUST be placed
   immediately after a corresponding MELPe 2400 bps payload in the same
   RTP packet.  The packed parameters are counted in octets (TC).  The
   preferred placement SHOULD be used for TSVCIS payloads with TC less
   than or equal to 77 octets; this is shown in Figure 6.  In the
   preferred placement, a single trailing octet SHALL be appended to
   include a two-bit rate code, CODA and CODB (both bits set to one),
   and a six-bit modified count (MTC).  The special modified count value
   of all ones (representing an MTC value of 63) SHALL NOT be used for
   this format as it is used as the indicator for the alternate packing
   format shown next.  In a standard implementation, the TSVCIS speech
   coder uses a minimum of 15 octets for parameters in octet packed
   form.  The modified count (MTC) MUST be reduced by 15 from the full
   octet count (TC).  Computed MTC = TC-15.  This accommodates a maximum
   of 77 parameter octets (the maximum value of MTC is 62; 77 is the sum
   of 62+15).

           MSB                                              LSB
            0      1      2      3      4      5      6      7
        +------+------+------+------+------+------+------+------+
     1  | T008 | T007 | T006 | T005 | T004 | T003 | T002 | T001 |
        +------+------+------+------+------+------+------+------+
     2  | T016 | T015 | T014 | T013 | T012 | T011 | T010 | T009 |
        +------+------+------+------+------+------+------+------+
     3  | T024 | T023 | T022 | T021 | T020 | T019 | T018 | T017 |
        +------+------+------+------+------+------+------+------+
     4  | T032 | T031 | T030 | T029 | T028 | T027 | T026 | T025 |
        +------+------+------+------+------+------+------+------+
     5  | T040 | T039 | T038 | T037 | T036 | T035 | T034 | T033 |
        +------+------+------+------+------+------+------+------+
     6  | T048 | T047 | T046 | T045 | T044 | T043 | T042 | T041 |
        +------+------+------+------+------+------+------+------+
     7  | TO56 | TO55 | T054 | T053 | T052 | T051 | T050 | T049 |
        +------+------+------+------+------+------+------+------+
     8  | T064 | T063 | T062 | T061 | T060 | T059 | T058 | T057 |
        +------+------+------+------+------+------+------+------+
     9  | T072 | T071 | T070 | T069 | T068 | T067 | T066 | T065 |
        +------+------+------+------+------+------+------+------+
    10  | T080 | T079 | T078 | T077 | T076 | T075 | T074 | T073 |
        +------+------+------+------+------+------+------+------+
    11  | T088 | T087 | T086 | T085 | T084 | T083 | T082 | T081 |
        +------+------+------+------+------+------+------+------+
    12  | TO96 | TO95 | T094 | T093 | T092 | T091 | T090 | T089 |
        +------+------+------+------+------+------+------+------+
    13  | T104 | T103 | T102 | T101 | T100 | T099 | T098 | T097 |
        +------+------+------+------+------+------+------+------+
    14  | T112 | T111 | T110 | T109 | T108 | T107 | T106 | T105 |
        +------+------+------+------+------+------+------+------+
    15  | T120 | T119 | T118 | T117 | T116 | T115 | T114 | T113 |
        +------+------+------+------+------+------+------+------+
        |                      .  .  .  .                       |
        +------+------+------+------+------+------+------+------+
   TC+1 | CODA | CODB |          modified octet count           |
        +------+------+------+------+------+------+------+------+

              Figure 6: Preferred Packed TSVCIS Payload Octets

   In order to accommodate all other NRL VDR configurations, an
   alternate parameter placement MUST use two trailing bytes as shown in
   Figure 7.  The last trailing byte MUST be filled with a two-bit rate
   code, CODA and CODB (both bits set to one), and its six-bit count
   field MUST be filled with ones.  The second to last trailing byte
   MUST contain the parameter count (TC) in octets (a value from 1 and
   255, inclusive).  The value of zero SHALL be considered as reserved.

           MSB                                              LSB
            0      1      2      3      4      5      6      7
        +------+------+------+------+------+------+------+------+
     1  | T008 | T007 | T006 | T005 | T004 | T003 | T002 | T001 |
        +------+------+------+------+------+------+------+------+
     2  | T016 | T015 | T014 | T013 | T012 | T011 | T010 | T009 |
        +------+------+------+------+------+------+------+------+
        |                      .  .  .  .                       |
        +------+------+------+------+------+------+------+------+
   TC+1 |                      octet count                      |
        +------+------+------+------+------+------+------+------+
   TC+2 | CODA | CODB |   1  |   1  |   1  |   1  |   1  |   1  |
        +------+------+------+------+------+------+------+------+

         Figure 7: Length Unrestricted Packed TSVCIS Payload Octets

3.3.  Multiple TSVCIS Frames in an RTP Packet

   A TSVCIS RTP packet payload consists of zero or more consecutive
   TSVCIS coder frames (each consisting of MELPe 2400 and TSVCIS coder
   data), with the oldest frame first, followed by zero or one MELPe
   comfort noise frame.  The presence of a comfort noise frame can be
   determined by its rate code bits in its last octet.

   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 TSVCIS RTP packet without coder and comfort noise frames MAY be
   used periodically by an endpoint to indicate connectivity by an
   otherwise idle receiver.

   TSVCIS coder frames in a single RTP packet MAY have varying TSVCIS
   parameter octet counts.  Its packed parameter octet count (length) is
   indicated in the trailing byte(s).  All MELPe frames in a single RTP
   packet MUST be of the same coder bitrate.  For all MELPe coder
   frames, the coder rate bits in the trailing byte identify the
   contents and length as per Table 1.

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

   *  Senders SHOULD NOT include more TSVCIS or 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 be consistent with the application.  For example, in telephony
   and other real-time applications where delay is important, the fewer
   frames per packet, the lower the delay.  However, 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 TSVCIS/MELPe frames is to identify each frame
   type and length, thereby counting the total number of octets within
   the RTP packet.

3.4.  Congestion Control Considerations

   The target bitrate of TSVCIS 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.

4.  Payload Format Parameters

   This RTP payload format is identified using the TSVCIS media subtype,
   which is registered in accordance with [RFC4855] and per the media
   type registration template from [RFC6838].

4.1.  Media Type Definitions

   Type name:  audio

   Subtype name:  TSVCIS

   Required parameters:  Clock Rate (Hz): 8000

   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 TSVCIS, 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 [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 TSVCIS, 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 [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;
         the first is preferred.  If "bitrate" is not present, the fixed
         coder bitrate of 2400 MUST be used.

      tcmax:
         specifies the TSVCIS maximum value for the TC supported or
         desired, ranging from 1 to 255.  If "tcmax" is not present, a
         default value of 35 is used.

      Channels:
         1

   Encoding considerations:  This media subtype is framed and binary;
      see Section 4.8 of [RFC6838].

   Security considerations:  Please see Section 8 of RFC 8817.

   Interoperability considerations:  N/A

   Published specification:  [TSVCIS]

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:

      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.  <victor.demjanenko@vocal.com>

   Intended usage:  COMMON

   Restrictions on usage:  The media subtype 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, Ph.D.

   Change controller:  IETF; contact <avt@ietf.org>

   Provisional registration? (standards tree only):  No

4.2.  Mapping to SDP

   The mapping of the above-defined payload format media subtype and its
   parameters SHALL be done according to Section 3 of [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 TSVCIS codec, the mapping
   is as follows:

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

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

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

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

   *  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
   "TSVCIS" (the same as the media subtype).

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

      m=audio 49120 RTP/AVP 96
      a=rtpmap:96 TSVCIS/8000

   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 bitrate value
   indicates the corresponding MELPe coder).  An example of the media
   representation in SDP for describing TSVCIS when all three coder
   bitrates are supported might be:

      m=audio 49120 RTP/AVP 96
      a=rtpmap:96 TSVCIS/8000
      a=fmtp:96 bitrate=2400,600,1200

   The optional media type parameter "tcmax", 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" is an integer number in
   the range of 1 to 255 representing the maximum number of TSVCIS
   parameter octets supported.  An example of the media representation
   in SDP for describing TSVCIS with a maximum of 101 octets supported
   is as follows:

      m=audio 49120 RTP/AVP 96
      a=rtpmap:96 TSVCIS/8000
      a=fmtp:96 tcmax=101

   The parameter "ptime" cannot be used for the purpose of specifying
   the TSVCIS 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 whether
   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 TSVCIS rtpmap values (such as
   97, 98, and 99, as used below) MAY be used to convey TSVCIS-coded
   voice at different bitrates.  The receiver can then select an
   appropriate TSVCIS codec by using 97, 98, or 99.

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

   For declarative media, the "tcmax" parameter specifies the maximum
   number of octets of TSVCIS packed parameters used by the sender or
   the sender's communications channel.

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.

   For example, if offerer bitrates are "2400,600" and answerer 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.

   In the Offer/Answer model [RFC3264], "tcmax" is a bidirectional
   parameter.  Both sides SHOULD use a common "tcmax" value.  The offer
   contains the tcmax supported by the offerer.  The answerer MAY agree
   to any tcmax equal to or less than this value by stating the desired
   tcmax in the answerer response.  The answerer alternatively MAY
   identify its own tcmax and rely on TSVCIS ignoring any augmented data
   it cannot use.

5.  Discontinuous Transmissions

   A primary application of TSVCIS is for radio communications of voice
   conversations, and discontinuous transmissions are normal.  When
   TSVCIS is used in an IP network, TSVCIS RTP packet transmissions may
   cease and resume frequently.  RTP synchronization source (SSRC)
   sequence number gaps indicate lost packets to be filled by Packet
   Loss Concealment (PLC), while abrupt loss of RTP packets indicates
   intended discontinuous transmissions.  Resumption of voice
   transmission SHOULD be indicated by the RTP marker bit (M) set to 1.

   If a TSVCIS coder so desires, it may send a MELPe 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

   TSVCIS 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 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 low bitrate MELPe frame.

7.  IANA Considerations

   IANA has registered TSVCIS as specified in Section 4.1.  The media
   type has been added to the IANA registry for "RTP Payload Format
   Media Types" (https://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 [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 TSVCIS decoder, to the best of our
   knowledge, 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 Voice Activity Detect (VAD) and its effect on bitrates.

9.  References

9.1.  Normative References

   [MELP]     Department of Defense, "Analog-to-Digital Conversion of
              Voice by 2,400 Bit/Second Mixed Excitation Linear
              Prediction (MELP)", Department of Defense
              Telecommunications Standard 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, October 2008.

   [NRLVDR]   Heide, D., Cohen, A., Lee, Y., and T. Moran, "Universal
              Vocoder Using Variable Data Rate Vocoding",
              DOI 10.21236/ada588068, Naval Research Lab NRL/FR/5555--
              13-10, 239, June 2013,
              <https://doi.org/10.21236/ada588068>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc3264>.

   [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, <https://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,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <https://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, <https://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,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc8083>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8088]  Westerlund, M., "How to Write an RTP Payload Format",
              RFC 8088, DOI 10.17487/RFC8088, May 2017,
              <https://www.rfc-editor.org/info/rfc8088>.

   [RFC8130]  Demjanenko, V. and D. Satterlee, "RTP Payload Format for
              the Mixed Excitation Linear Prediction Enhanced (MELPe)
              Codec", RFC 8130, DOI 10.17487/RFC8130, March 2017,
              <https://www.rfc-editor.org/info/rfc8130>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SCIP210]  National Security Agency, "SCIP Signaling Plan", SCIP-210,
              January 2013.

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,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc7202>.

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

   [TSVCIS]   National Security Agency, "Tactical Secure Voice
              Cryptographic Interoperability Specification (TSVCIS)
              Version 3.1", NSA 09-01A, March 2019.

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


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

   Phone: +1 716 688 4675
   Email: john.punaro@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
