MLow RED and Reed-Solomon FEC

Encodings - mlow-red-fec

ENC-02 - status: draft - audio

The optional "SplitRed" redundancy envelope wrapping an MLow RTP audio payload: one main frame plus zero or more time-shifted redundant copies of earlier frames, recoverable without retransmission.

Applicability

SplitRed is the OUTERMOST layer of an MLow RTP audio payload. It MUST be applied only when mlow_red_redundancy_level > 0. Otherwise the RTP payload is a single bare MLow frame (see mlow-frame) with no wrapper and the SplitRed parse MUST NOT be applied.

A decoder MUST choose between SplitRed and bare-frame based on the negotiated redundancy level, NOT by sniffing the first byte (a bare frame's first byte has its high bit set and is misread as a redundant block header). Feeding a bare frame to the SplitRed parser is malformed and MUST be rejected.

Wire layout

With N = number of redundant blocks in the packet:

[ red_hdr[0] (2B) ] ... [ red_hdr[N-1] (2B) ]   ; N redundant-block headers
[ main_marker     (1B) ]                         ; terminates the header run
[ red_payload[0] ] ... [ red_payload[N-1] ]      ; redundant bodies, in header order
[ main_payload ]                                 ; main frame body, last

Redundant block header (red_hdr[i], 2 bytes)

byte 0:  1 t t t t t t t      ; high bit MUST be 1; low 7 bits = time_code
byte 1:  s s s s s s s s      ; size = byte length of red_payload[i]

• byte0 MUST have 0x80 set; byte0 & 0x7f is time_code.
• byte1 is the unsigned byte length of the redundant body (max 255 bytes).

Main marker (main_marker, 1 byte)

byte 0:  0 t t t t t t t      ; high bit MUST be 0; low 7 bits = main time_code

0x80 MUST be clear; the parser detects end-of-header-run by a byte with the high bit clear. byte & 0x7f is the main frame's time_code. The main payload length is NOT in the header; it is whatever bytes remain after the header run and all redundant payloads.

Parse procedure

Let p be the payload, n = len(p). A decoder MUST parse as follows and MUST reject on each error condition:

1. If n == 0, reject (PktSizeZero).
2. Walk the header run from cur = 0, tracking rem = bytes from cur to end (rem starts at n):
3. If rem == 0 before a main marker is seen, reject (HeaderTooShort).
4. Read b0 = p[cur].
5. If b0 < 0x80, this is the main marker: if rem <= 1, reject (MainTooShort); otherwise stop the walk.
6. Else redundant header: if rem <= 2, reject (RedundantTooShort). Read size = p[cur+1]. If size + 2 >= rem, reject (RedundantTooShort). Record { code: b0 & 0x7f, size }. Advance cur += 2; rem -= size + 2.
7. Read main marker: main_code = p[cur] & 0x7f; advance cur += 1.
8. For each recorded redundant block r, in header order, the next r.size bytes at cur are its body (is_main = false, time_code = r.code); advance cur += r.size.
9. The remaining main_size = rem - 1 bytes are the main body (is_main = true, time_code = main_code).

Redundant frames are yielded first in header order, main frame last. Each body is a complete MLow frame (TOC plus body), decoded per mlow-frame. A decoder SHOULD use a redundant copy only when the main frame for that time_code was lost.

Reed-Solomon FEC

A Reed-Solomon FEC tier exists alongside the RED envelope. Its on-wire encoding, generator polynomial, symbol size, and negotiation are not specified here.

Notes. The header run has no count field for N: the terminator is structural, found by the high bit of the leading byte (set = redundant, clear = main). The size + 2 >= rem check guarantees the one-byte main marker plus a non-empty main payload remain after every redundant block.

Worked example (one redundant block + main):

p = 85 03 00  AA BB CC  50 11 22 33
    |  |  |   \______/  \_________/
    |  |  |   red body  main body (4B)
    |  |  main_marker (time_code 0)
    |  size = 3
    red hdr byte0 = 0x85 -> high bit set, time_code = 5

yields redundant frame AA BB CC (time_code 5) then main 50 11 22 33 (time_code 0). A lone main marker (e.g. 00 50 11 22) yields a single main frame with no redundancy.

Parent: mlow
Requires: mlow, mlow-frame
Breakdown: mlow-decoder

Implemented by

Flavor	Status	Source	Notes
whatsapp-rust	working	history - blame - commits 674e851	depack_split_red parses the RED envelope; bare-frame streams bypass it
meowcaller	partial	—	codec modules partial

Annotation wacrg:ENC-02 — a flavor marks its implementation site in source with this comment; a script clones the source, finds it, and attaches the commit blame/permalink.

Contributors

Contributor	Role
Rajeh Taher	wrote initial spec

protocol history / diff - blame

Open questions - On-wire encoding of the Reed-Solomon FEC tier: generator polynomial, symbol/block size, and parity layout. - Negotiation signalling that sets mlow_red_redundancy_level and enables Reed-Solomon FEC. - Semantics of time_code relative to RTP timestamp / frame sequence, and the receiver's repair (jitter-buffer) policy for selecting a redundant copy. - Encoder side: how many redundant copies are emitted per level and which past frames are chosen.

References - RFC 2198 — RTP Payload for Redundant Audio Data

Changelog

• 2026-06-21 — Initial spec entry.

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