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acarteron edited this page May 20, 2026 · 2 revisions

Overview

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The Voicetastic Voice Protocol layers short voice messages on top of the Meshtastic mesh. It runs as a tenant of PortNum::PRIVATE_APP (256), fragments codec frames into LoRa-sized chunks, recovers loss with Reed-Solomon FEC and selective NACKs, and delegates confidentiality to Meshtastic's channel encryption (AES-256-CTR with the channel PSK).

What it does

  • Carries pre-encoded codec frames (AMR-NB, Opus, …) — never raw audio.
  • Fragments an audio message into ≤ 219-byte chunks (LoRa MTU minus header).
  • Forward Error Correction (Reed-Solomon over GF(2⁸)) tolerates loss up to parity_count chunks without retransmission.
  • Selective NACKs with bitmap recover heavier loss in one round-trip.
  • Confidentiality on the air comes from Meshtastic's channel encryption (AES-256-CTR with the channel PSK). This protocol does not add its own envelope — V2 did, V3 dropped it (see Encryption).
  • Resource-bounded receiver: per-sender + global in-flight caps, blacklist for recently-finalized messages, validation-strike eviction for chatty bad senders.

What it does not do

  • No built-in audio codec — bring your own encoder/decoder.
  • No protocol-layer encryption or authentication. Confidentiality on the air is delegated to Meshtastic; any peer with the channel PSK can read or forge voice frames at this layer.
  • No congestion control beyond adaptive pacing per modem preset.
  • No cross-message ordering — only within a stream via stream_seq.
  • No authenticated NACKs — see Reliability.

Design priorities

  1. Survive packet loss without round-trip-bound recovery whenever possible (FEC).
  2. Bounded airtime — every message has a hard NACK-round cap and an absolute timeout.
  3. Codec-agnostic wire format so codec evolution doesn't bump the protocol version.
  4. Resist abuse — bound per-sender resource use, reject malformed frames at the earliest possible point.
  5. Forward-compatible — a single version byte at offset 0 lets future revisions coexist on the same port.

When not to use it

  • Real-time interactive voice. The protocol is store-and-forward: recordings of a few seconds, transmitted asynchronously. Latency is dominated by airtime + pacing, which on LONG_SLOW can exceed the duration of the recording itself.
  • Anything requiring guaranteed delivery. The protocol gives up after NACK_MAX_ROUNDS = 400 consecutive rounds without progress.
  • High-fidelity audio. The MTU + airtime budget caps usable bitrates to the 5–16 kbps range.

Where it fits

┌────────────────────────────────────────────────────┐
│ Application (recording UI, playback, message UX)   │
├────────────────────────────────────────────────────┤
│ Voice protocol (this document)                     │
│   • build_message  • VoiceAssembler  • NACK loop   │
├────────────────────────────────────────────────────┤
│ Meshtastic application layer                       │
│   • MeshPacket  • PortNum::PRIVATE_APP             │
├────────────────────────────────────────────────────┤
│ Meshtastic channel crypto (AES-256-CTR, per-PSK)   │
├────────────────────────────────────────────────────┤
│ LoRa PHY (modem preset chosen by the user)         │
└────────────────────────────────────────────────────┘

The voice protocol is one of many PRIVATE_APP tenants. The leading version byte (0x03) lets receivers triage frames before parsing.

→ Continue to Frame Format.

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