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

Introduction This document describes how compressed Tactical Secure Voice Cryptographic Interoperability Specification (TSVCIS) speech as produced by the TSVCIS codec 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.

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 when, and only when, they appear in all capitals, as shown here. Best current practices for writing an RTP payload format specification were followed .

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

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 . ("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 . 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 . 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 . 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)

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 . TSVCIS specifically uses the NRL VDR in two configurations with a fixed set of 15 and 35 packed octet parameters in a standardized order .

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 . 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.

Packet Format Diagram The RTP header of the packetized encoded TSVCIS speech has the expected values as described in . The usage of the M bit SHOULD be as specified in the applicable RTP profile -- for example, 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.

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 and is repeated here for all three MELPe rates , with its recommendations now regarded as requirements. The bits previously labeled as RSVA, RSVB, and RSVC in SHOULD be filled with rate code bits CODA, CODB, and CODC, as shown in (compatible with Table 7 in ). TSVCIS/MELPe Frame Bitrate Indicators and Frame Length

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.

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 . It should be noted that CODB for MELPe 600 bps mode MAY deviate from the value in 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).

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

Packed MELPe 2400 bps Payload Octets

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

Packed MELPe 1200 bps Payload Octets

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

Packed MELPe 600 bps Payload Octets

Comfort Noise Bitstream Definition The comfort noise MELPe RTP payload is constructed as per . Note that CODA, CODB, and CODC MUST be filled with 1, 0, and 1, respectively, as per .

Packed MELPe Comfort Noise Payload Octets

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 . 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).

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 . 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.

Length Unrestricted Packed TSVCIS Payload Octets

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 . 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.

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 and MAY also implement a transport circuit breaker . Work in the RMCAT Working Group 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.

Payload Format Parameters This RTP payload format is identified using the TSVCIS media subtype, which is registered in accordance with and per the media type registration template from .

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 .
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 .
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 .

Security considerations:

Please see of RFC 8817.

Interoperability considerations:

N/A

Published specification:

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@vocal.com>

Intended usage:

COMMON

Restrictions on usage:

The media subtype depends on RTP framing and hence is only defined for transfer via RTP . Transport within other framing protocols is not defined at this time.

Author:

Change controller:

IETF; contact <avt@ietf.org>

Provisional registration? (standards tree only):

Mapping to SDP The mapping of the above-defined payload format media subtype and its parameters SHALL be done according to . The information carried in the media type specification has a specific mapping to fields in the Session Description Protocol (SDP) , 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: 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: 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: 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.

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. 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.

Offer/Answer SDP Considerations In the Offer/Answer model , "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 , "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.

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 prior to ceasing transmission. A receiver may optionally use comfort noise during its silence periods. No SDP negotiations are required.

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 . 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.

IANA Considerations IANA has registered TSVCIS as specified in . The media type has been added to the IANA registry for "RTP Payload Format Media Types" ().

Security Considerations RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification and in any applicable RTP profile such as RTP/AVP , RTP/AVPF , RTP/SAVP , or RTP/SAVPF . However, as discussed in , 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 . 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 regarding Voice Activity Detect (VAD) and its effect on bitrates.