Constants and Limits

Constants and Limits

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Every numeric ceiling in the protocol, with rationale. The consts module is the source of truth for these values.

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Wire format

Constant	Value	Why
PROTOCOL_VERSION	0x03	Drop frames with any other first byte.
HEADER_SIZE	16 B	12 B logical header + 4 B trailing integrity tag.
HEADER_MAC_LEN	4 B	Truncated unkeyed SHA-256 over header[0..12].
MAX_PACKET_SIZE	231 B	Meshtastic LoRa MTU. All frames MUST fit.
MAX_BODY_SIZE	215 B	MAX_PACKET_SIZE − HEADER_SIZE.
MIN_CHUNK_SIZE	16 B	Per-frame overhead floor; below this, FEC + pacing waste airtime.

Message shape

Constant	Value	Why
MAX_CHUNKS_PER_MESSAGE	255	total_data is u8; index 0..=254.
MAX_PARITY_PER_MESSAGE	128	reed-solomon-erasure GF(2⁸) coder limit.
MAX_MESSAGE_BYTES	54 825	MAX_CHUNKS_PER_MESSAGE × MAX_BODY_SIZE.

Confidentiality

V3 has no protocol-layer encryption. Confidentiality is delegated to Meshtastic's channel encryption (AES-256-CTR with the channel PSK). See Encryption.

Receiver resource bounds

Constant	Value	Why
MAX_IN_PROGRESS_GLOBAL	64	Bounds total reassembler memory.
MAX_IN_PROGRESS_PER_SENDER	4	Stops one chatty peer from starving everyone else.
BLACKLIST_TTL	600 s	Completion-memory window: how long a finalized (from, message_id) blocks late frames for itself. Sized above worst-case sender airtime (a near-MTU Long Slow clip can take 2-3 min) so the blacklist outlives the TX.
BLACKLIST_MAX	100	FIFO eviction once exceeded.
NACK_MAX_ROUNDS	400	Per-message NACK budget (consecutive rounds without progress; resets on every accepted shard) before the receiver gives up.
NACK_WINDOW_MS	3000	Quiet period after the last seen chunk before NACK'ing.
DEAD_SENDER_TIMEOUT †	120 s	Presume the sender dead after this much silence (no data/parity); suppress NACKs until message_timeout so the receiver stops NACK-flooding a peer that left the mesh.
MAX_VALIDATION_STRIKES (impl) †	3	Eviction trigger for chatty bad senders (post-template).

Sender resource bounds

Constant	Value	Why
MAX_RETRANSMITS_PER_MESSAGE †	2_400	Per-message retransmit budget; matches widened receiver NACK_MAX_ROUNDS.
DEFAULT_RETAIN_TTL †	1200 s	How long OutgoingVoiceRegistry keeps frames for late NACKs (burst + linger safety margin).
DEFAULT_LINGER (SendRequest)	600 s	How long VoiceSender stays subscribed to NACKs after burst end.
Cooldown clamp	1-30 s	Park window after each retransmit batch (pacing × frames).

† Experimental flood-control & resource-bounding limits

The constants marked † are experimental heuristic safety valves, not wire-format guarantees. They cap NACK and retransmit storms and bound reassembler/registry state so a dead, slow, or misbehaving peer can't flood the channel or exhaust local memory. The values are tuned empirically against real LoRa presets and may change between releases without a PROTOCOL_VERSION bump, so interop never depends on their exact value.

• Avoiding channel flooding:
  • DEAD_SENDER_TIMEOUT - stop NACKing a sender that has gone silent / left the mesh.
  • MAX_RETRANSMITS_PER_MESSAGE - cap a sender's retransmit storm when a peer NACKs in a loop.
• Resource bounding:
  • DEFAULT_RETAIN_TTL - garbage-collect the sender's OutgoingVoiceRegistry.
  • MAX_VALIDATION_STRIKES - evict a chatty bad sender from its scarce per-sender reassembly slot.

Sender pacing

Adaptive per modem preset (Config.LoRaConfig.modem_preset). Each value is tuned so pacing >= air_time(recommended_chunk_size) with ~30 % headroom for firmware CSMA, queue drain, and ACK windows; sending faster overruns the radio's small outbound queue and drops frames before they reach the air. Source of truth: ModemPreset::pacing.

Modem preset	Pacing
SHORT_TURBO	150 ms
SHORT_FAST	250 ms
SHORT_SLOW	400 ms
MEDIUM_FAST	500 ms
MEDIUM_SLOW	700 ms
LONG_FAST	900 ms
LONG_MODERATE	1200 ms
LONG_SLOW	1800 ms
VERY_LONG_SLOW	3000 ms
Unknown	500 ms

Recommended chunk_size per preset

Source of truth: ModemPreset::recommended_chunk_size.

Modem preset	chunk_size
SHORT_TURBO, SHORT_FAST	215 (MAX_BODY_SIZE)
SHORT_SLOW, MEDIUM_FAST	160
MEDIUM_SLOW	96
LONG_FAST	199
LONG_MODERATE, LONG_SLOW, VERY_LONG_SLOW	48

Recommended parity_count

Severe loss (>40 %) requires nearly 1:1 parity for Reed–Solomon to close without relying on NACK retransmit rounds:

Mesh profile	parity_count
Short / quiet	10 %
Medium / mixed	20 %
Long / lossy	33 %
High-loss (>40 %) / broadcast	100 % (up to 128)

Capacity reference

For quick sanity-checking message budgets:

chunk_size	Codec / bitrate	max_audio	Approx. duration
215	OPUS @ 16 kbps	54 825 B	~27 s
160	AMR-NB @ MR795 (7.95 kbps)	40 800 B	~41 s
128	AMR-NB @ MR795	32 640 B	~33 s
96	AMR-NB @ MR795	24 480 B	~25 s
48	AMR-NB @ MR795	12 240 B	~12 s

Durations are rough — they assume packed codec frames with no padding.

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