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MeshCore OTA — .mota container & LoRa protocol

This is the single source of truth for MeshCore's over-the-air firmware update system ("mOTA"). It is written for developers who want to implement an interoperable peer (server, fetcher, relay, or host tool) in another codebase or project. Everything below is implemented and hardware-verified in this repository; where a section names a source file, that file is the authoritative reference for byte-level details.

Just want to update your node? See the plain-language OTA user guide — this document is the technical/wire specification.

Design goals

  • Distribute firmware over LoRa as a self-verifying, resumable, single-source block transfer that survives reboots and never auto-applies without explicit consent.
  • Trustless mesh relay: repeaters may forward packets while the source alone serves firmware data; integrity is content-addressed against a signed merkle root, so a relay need not be trusted and never needs the signing keys.
  • Lowest priority, always: OTA traffic is enqueued behind all mesh traffic — "eventually upgradable". A busy node delays OTA indefinitely rather than competing with real traffic.
  • Portable: the engine (src/helpers/ota/OtaManager) is Arduino/radio/crypto-free and host-testable, so the same logic drives a device, a simulation, or a third-party implementation.

Source map (all under src/helpers/ota/ unless noted)

Concern File
Constants, enums, flags OtaFormat.h
Container/manifest parse MotaContainer.{h,cpp}
Merkle tree + proofs MerkleTree.{h,cpp}
EndF self-identity FirmwareInfo.{h,cpp}
Wire message codec OtaProtocol.{h,cpp}
Session engine (serve+fetch+discovery) OtaManager.{h,cpp}
Multi-mota / folder relay OtaSource.h, MotaSourceSerial.{h,cpp}, MotaSeederProto.h
Staging stores OtaStore.h, OtaStoreFlashNrf52.*, OtaStoreFlashEsp32.*
Apply OtaApply.*, bootloader Adafruit_nRF52_Bootloader_OTAFIX
Device glue (CLI/context) OtaCli.cpp, OtaContext.h
Host tooling motatool (standalone Rust CLI: build/verify/inspect/serve); tools/mota/ (Python reference lib motalib.py + build/test glue)

1. Conventions

  • Endianness: all multi-byte integers are little-endian unless stated.
  • Hashes (multihash): the hash family is declared once per manifest via hash_algo = 0x12 = SHA-256 (the multihash code for sha2-256). Truncations used:
    • sha2-256:4 — first 4 bytes of the SHA-256 digest. Merkle leaves, internal nodes, root, proofs, manifest_id, and the discovery set_digest.
    • sha2-256:8 — first 8 bytes. Base-firmware identity (base_hash, EndF.body_hash).
    • sha2-256:32 — full digest. The image security anchor (image_hash). Digests are stored bare (just the truncated bytes); the family is implied by hash_algo.
  • Signatures: Ed25519 (RFC 8032), 64-byte detached signature, 32-byte public key.

Reference constants (OtaFormat.h):

Name Value ASCII / note
Container MAGIC 6D 4F 54 41 mOTA
Container TRAILER 76 6B 34 39 36 vk496
EndF marker 45 6E 64 46 EndF
hash_algo (sha2-256) 0x12 multihash code
format_ver 0x02 this spec
approval = not approved FF FF FF FF erased NOR word
approval = approved 41 50 52 56 APRV
MFLAG_FULL 0x01 flags bit0
MFLAG_SIGNED 0x02 flags bit1
CODEC_FULL / _SEQUENTIAL / _INPLACE 0 / 1 / 2 §5
PAYLOAD_TYPE_OTA 0x0C MeshCore packet type (src/Packet.h)
MAX_PACKET_PAYLOAD 184 usable bytes per packet (src/MeshCore.h)
Default block size 1024 block_size_log2 = 0x0A
OTA TX priority 250 lowest (OTA_TX_PRIORITY, src/Mesh.h)

2. Firmware image & the EndF trailer

Every OTA-capable build appends a fixed 56-byte EndF trailer to its flashed image so a running node can discover its own size and self-describing identity on any MCU (no linker symbols needed). Every field is always present at a constant offset. Implemented by FirmwareInfo.cpp; appended at build time by tools/mota/pio_endf.py (post-build hook).

flashed image = BODY (image bytes) || EndF trailer
EndF trailer (fixed 56 bytes):
  off 0   4   "EndF"        45 6E 64 46
  off 4   4   body_len      uint32 LE — length of BODY (excludes the whole trailer)
  off 8   8   body_hash     sha2-256:8 of BODY
  off 16  4   fw_version    uint32 LE, packed MAJOR<<24|MINOR<<16|PATCH<<8|pre  (0 = unknown)
  off 20  4   target_id     uint32 LE — sha2-256:4(pio_env): hardware + role + partition (fetch routing)
  off 24  32  hw_id         NUL-padded ASCII hardware tag (brick-safety), e.g. "RAK4631" ("" = unknown)
  • Self-describing identity. pio_endf.py computes target_id from the PlatformIO env name itself (so it's correct even without build.sh's -D MOTA_TARGET_ID), hw_id from MOTA_HW_ID, and fw_version from FIRMWARE_VERSION. The device reads them back (ota_self_firmware()), so a node's advertised identity is correct regardless of how it was built — and the packaging tool reads them straight from a raw .bin (no --target-env/--fw-version flags, no reliance on filenames; §9, §13). A dev build with no dotted version simply carries fw_version = 0 / empty hw_id (= unknown) — still a full 56-byte trailer.
  • Size discovery: scan flash from the partition top downward for the EndF marker; the byte before it is the last BODY byte (the trailer is always 56 bytes). See ota_self_firmware().
  • Delta base matching: a node's body_hash is read directly from its own EndF; a delta's base_hash (§5) must equal it. body_hash is over BODY only.
  • No circularity: EndF hashes only the BODY, never itself.

The "reconstructed image" referenced by the manifest is the full BODY || EndF (what gets flashed).

Implementer note: the bootloader (and any non-Arduino consumer) MUST locate the body extent by scanning for EndF, never by trusting a stored size — see the bootloader contract in §12.

3. The .mota container

The distributed form (host-built, wire-transferred). Parsed by mota_parse() in MotaContainer.cpp.

off            size   field
0              4      MAGIC = 6D 4F 54 41
4              4      MOTA_TOTAL_SIZE  uint32 LE — total container bytes (incl. manifest, leaves[],
                                       payload, trailer). Lets a node pre-reserve staging and compute
                                       write_start = staging_region_end − MOTA_TOTAL_SIZE.
8              M      MANIFEST         (§4; M = 197 fixed + leaves[], 4*BC; no length field — BC from payload_size)
8 + M          P      PAYLOAD          (payload_size bytes; delta or full image)
8 + M + P      5      TRAILER = 76 6B 34 39 36

MOTA_TOTAL_SIZE = 4 + 4 + M + P + 5. The manifest M includes leaves[]; the manifest-minus-leaves prefix (mfl, sent over the wire as OTA_MANIFEST) is [8, leaves_off).

Staged (in-flash) form. Written bottom-aligned so TRAILER ends at staging_region_end. Identical bytes, except the device mutates two regions in place (both NOR-safe, no re-erase): the leaves[] slots (filled as blocks arrive — §7) and the 4-byte approval field (on owner consent — §4.2). Everything else is immutable.

4. The manifest

Fixed layout. Every field sits at a constant offset and is always present — base_hash, signer_pubkey and signature are zero-filled when not applicable (a full image / an unsigned container). Only leaves[] is variable (one 4-byte hash per block). So the manifest-minus-leaves (mfl) is always 197 bytes and the parser is plain offset reads — no conditionals. Parsed by mota_parse_manifest().

off  size   field            notes
0    1      format_ver       = 0x02
1    1      flags            bit0 FULL (0=delta/partial, 1=full image); bit1 SIGNED; bits2-7 reserved 0
2    1      hash_algo        0x12 = sha2-256
3    4      target_id        device/arch/role discriminator (§9)
7    4      fw_version       MAJOR<<24 | MINOR<<16 | PATCH<<8 | pre   (comparable uint32)
11   4      image_size       size of the reconstructed image (BODY||EndF)
15   4      payload_size     PAYLOAD bytes in this container
19   1      block_size_log2  e.g. 0x0A = 1024
20   4      merkle_root      sha2-256:4 over PAYLOAD blocks (§6) — also the manifest_id
24   32     image_hash       sha2-256:32 of the reconstructed image — SECURITY anchor
56   1      codec_id         0=full/raw, 1=detools-sequential, 2=detools-in-place
57   32     hw_id            NUL-padded ASCII hardware tag (e.g. "RAK4631"); same tag => bootable-compatible.
                             SIGNED. Applier refuses a mismatch (brick-safety); empty on either side = skip.
89   8      base_hash        sha2-256:8 of the BASE image's BODY (== that build's EndF.body_hash). 0 if FULL.
97   32     signer_pubkey    Ed25519 public key. 0 if not SIGNED.
129  64     signature        Ed25519 over manifest[0, 129). 0 if not SIGNED.
193  4      approval         FF FF FF FF = not approved; 41 50 52 56 ("APRV") = approved
--- end of manifest-minus-leaves: mfl = 197 (constant); leaves_off = 8 + 197 = 205 in the container ---
197  4*BC   leaves[]         BC = ceil(payload_size / 2^block_size_log2). sha2-256:4 each (the only variable field)

The signature always covers manifest[0, 129) (the head + base_hash + signer_pubkey). approval is outside the signed region so it can be flipped in place on consent without breaking the signature.

Manifest-minus-leaves size (mfl) is a constant 197 bytes for every container (full or delta, signed or unsigned). At 197 bytes the manifest exceeds one packet, so OTA_MANIFEST is always sent multi-fragment (§8.4, 2 fragments) and reassembled by the fetcher.

4.1 Signed region

signature covers manifest bytes [0, 129) — the head + base_hash + signer_pubkey. It does not cover approval or leaves[]:

  • leaves[] are verified against the signed merkle_root (§6), so they need no separate signature.
  • approval is device-local consent (§4.2), deliberately outside the signature.

4.2 The approval field

  • Distributed and forced on ingest to FF FF FF FF (a peer can never pre-approve).
  • The local owner's ota applydelta writes 41 50 52 56 ("APRV") — a single NOR-safe write (only clears bits from the erased word). Any partial/other value reads as not-approved (fail-safe).
  • Bound to this image (lives in this .mota's manifest, re-erased when a new .mota is staged).
  • A consent marker, not a security primitive. Authenticity = signature + image_hash + hw_id.

5. Payload, codecs & delta base

PAYLOAD is either the full reconstructed image (FULL) or a delta (!FULL).

codec_id Meaning Used by
0 full / raw PAYLOAD = reconstructed image (BODY‖EndF). ESP32 A/B only.
1 detools sequential random read of base + sequential write of result → ESP32 A→B inactive slot.
2 detools in-place bounded scratch; rewrites the app region in place → nRF52 single-slot.

For deltas, base_hash = the base build's EndF.body_hash (sha2-256:8 of its BODY). A node applies a delta only if base_hash matches its own EndF.body_hash. After applying, the result MUST hash (sha2-256:32) to image_hash before it is booted — the hard security gate.

A fetcher only requests firmware it can apply. Each node declares the codec(s) it can apply (set_apply_codec/set_apply_codec2): ESP32 accepts full + sequential (+ in-place), while nRF52 accepts only in-place because its single slot cannot stage a full application image. A .mota with an unsupported codec is rejected at discovery time, before any blocks are requested. A manual pull to an external folder may accept other codecs because that path captures bytes and never installs them.

Compression is internal to the detools patch and must be supported by the applier. Patches are produced by detools 0.53.0 (tools/mota → detools.create_patch) and decoded on-device by detools' embeddable C decoder, vendored verbatim at src/helpers/ota/detools/ (see its README.meshcore.txt). That build enables only the self-contained NONE + CRLE compressions (no malloc/liblzma/heatshrink), so MeshCore deltas use --compression crle. Do not reimplement the codec — use the vendored decoder.

6. Merkle tree (sha2-256:4)

Verifies each PAYLOAD block against the signed merkle_root before the whole payload exists, so corruption/forgery is localized to a block. Implemented in MerkleTree.cpp.

  • Blocks: PAYLOAD splits into BC = ceil(payload_size / B) blocks, B = 2^block_size_log2 (default 1024). The last block is its real length (no zero padding).
  • Leaf: leaves[i] = sha2-256:4( block_i_bytes ).
  • Internal node: node = sha2-256:4( left ‖ right ) (4+4 input bytes).
  • Odd level: an odd count promotes the last node unchanged to the next level (no duplication).
  • Root: reduce until one node remains. BC == 1 → root = leaves[0]. BC == 0 is invalid.

6.1 Proofs

A proof for block i is the ordered list of sibling digests from leaf to root. Promoted levels contribute no element. Verification (needs BC to know the tree shape):

h = leaf_i ; idx = i ; n = BC ; p = 0
while n > 1:
    if (n is odd) and (idx == n-1):        # this node was promoted
        pass
    else:
        sib, side = proof[p] ; p += 1
        h = sha2-256:4( sib ‖ h ) if side==left else sha2-256:4( h ‖ sib )
    idx //= 2 ; n = (n + 1) // 2
accept iff h == merkle_root and p == len(proof)

Over LoRa, leaves[] are omitted from the manifest transfer; a serving node computes a block's proof on demand from its stored leaves[] (OTA_REQ_PROOF/OTA_PROOF, §8.5), and the fetcher fills its own leaves[i] as each verified block lands.

7. Block availability, staging & resume

There is no separate availability structure. Block i is present ⟺ leaves[i] is non-erased (!= FF FF FF FF). Because leaves[] live in the staged flash region, availability survives reboot.

Commit order per block (crash-safe): (1) verify proof, (2) write block payload to its offset, (3) write leaves[i] last. A power loss before step 3 leaves the slot erased → the block is simply re-fetched (idempotent). On boot a node rebuilds an in-RAM present-bitmap by scanning leaves[].

Resume (OtaManager::resumeStaged + OtaStore::checkpoint/reopen): an interrupted fetch resumes from the staged container after a reboot — re-parse the stored manifest, recompute geometry, count present blocks, continue fetching the holes (or jump straight to COMPLETE). The checkpoint cadence (persist progress every N committed blocks) is runtime-tunable (ota config checkpoint <N>, 0 = only finalized containers resume). Stores keep leaves[] in RAM until flush and never auto-GC, preserving resumable progress.

Flash-store note (RX-safe writes): a flash page-erase halts the CPU (~85 ms on nRF52) and starves LoRa RX, so the flash stores (OtaStoreFlashNrf52/OtaStoreFlashEsp32) coalesce writes to the erase unit (4 KB page / sector) and commit each once off the per-packet path — RAM stays O(one page), not O(image). A small delta that fits page 0 does zero flash I/O until COMPLETE.

8. LoRa OTA protocol

Carried in MeshCore packets with PAYLOAD_TYPE_OTA = 0x0C. Every OTA packet payload is:

[0]    ota_msg_type      (OtaMsgType, OtaFormat.h)
[1..]  body              (fixed per type; encode/decode in OtaProtocol.cpp)

Message types:

ota_msg_type val routing purpose
OTA_ADV 0x01 flood tiny per-node beacon (discovery tier 1)
OTA_QUERY 0x02 flood ask a source for its catalog (discovery tier 2)
OTA_HAVE 0x03 flood the catalog reply (fragmented, digest-tagged)
OTA_GET_MANIFEST 0x04 direct request a manifest's fragments (want_mask) by manifest_id
OTA_MANIFEST 0x05 direct the manifest-minus-leaves, fragmented
OTA_REQ 0x06 direct request specific DATA fragments of one block (want_mask)
OTA_DATA 0x07 direct one self-describing fragment of a block's data
OTA_REQ_PROOF 0x08 direct request the merkle proof for one block
OTA_PROOF 0x09 direct the merkle proof for one block
OTA_GET_LEAVES 0x0A direct request the target's leaves[] fragments (want_mask) — warm-start only
OTA_LEAVES 0x0B direct a fragment of the leaves[] array (for host-side seed leaf-diff)
  • manifest_id = the manifest's merkle_root (4 bytes) — a compact content id present in every transfer message, so a multi-mota server dispatches each request to the right image.
  • Priority: all OTA packets enqueue at OTA_TX_PRIORITY = 250 (lowest). OTA never competes with mesh traffic; on a busy node it is delayed indefinitely.
  • Reliability is eventual: the fetcher re-requests missing fragments/blocks after a timeout, possibly from a different peer. No hard ACKs, no global ordering.
  • Relay: replies are flooded, so transparent relay needs no per-requester addressing, and the transfer is trustless (the fetcher verifies every block against the signed root). Any neighbor may serve any fragment it has.
  • Hop limit + duty cycle: OTA floods accumulate one path-hash per relay (the mesh's flood routing). A node accepts a packet only if it arrived within ota config hops hops (default 3; 0 = direct only) and relays it only while still under that limit, appending its own hash. Relays are lowest-priority and are skipped when the packet pool runs low (the source retries), so heavy OTA can never monopolise a repeater's RAM or starve real traffic.

8.1 Two-tier discovery

Because a node may serve many mOTAs (its own firmware plus an external folder — §10), discovery is split so the periodic beacon stays tiny regardless of catalog size:

Tier 1 — OTA_ADV beacon (10 bytes, constant). Flooded as a short burst at boot, then every advert_mins minutes (default 24h; runtime-tunable via ota config advert, 0 disables the periodic re-advertise). It is also emitted immediately whenever the served set changes (e.g. a motatool folder is attached/detached), so peers learn about newly-available firmware without waiting for the next interval:

seeder_id[4]    advertiser node id = pubkey[0:4]; the QUERY address + distinct-source id
n_motas         uint8 — count of complete servable mOTAs (saturates at 255)
set_digest[4]   sha2-256:4 over the SORTED set of served manifest_ids (see below)

set_digest is a content hash of the offering, not a counter: canonical across nodes, and it changes iff the set of served mids changes. A peer that has already catalogued this {seeder, set_digest} ignores the beacon (steady state is query-free). For a single served mota, set_digest = sha2-256:4(mid).

Tier 2 — OTA_QUERY → OTA_HAVE (on interest only):

OTA_QUERY  (flood):  seeder_id[4]  set_digest[4]  filter_target(uint32)   # filter_target 0 = everything
OTA_HAVE   (flood):  seeder_id[4]  set_digest[4]  frag_idx(1) frag_total(1) n_rows(1)  rows[]
  HaveRow (16 bytes, OTA_HAVE_ROW_BYTES): mid[4] target_id(4) fw_version(4) codec_id(1) flags(1) have_count(2)

have_count is the number of blocks the source holds (== block_count for a complete offered image). Receivers do not advertise partial or completed downloads as new sources.

A node interested in a source's offering schedules a QUERY; the source replies with its full catalog as OTA_HAVE rows (fragmented if they exceed one packet — up to 12 rows per fragment). The heavy manifest is fetched per-mid only on commit (§8.3).

8.2 Anti-storm (mandatory at mesh scale)

If 50 neighbours all queried a new beacon at once, the mesh would collapse. Mitigations (gossip/mDNS pattern), all in OtaManager:

  • OTA_HAVE is flooded and digest-tagged. EVERY node that overhears it caches the rows passively (keyed by {seeder, set_digest}) — no query of its own needed.
  • Jittered query: a peer needing a catalog schedules its OTA_QUERY after a random delay OTA_QUERY_MIN_MS (300) + rand(OTA_QUERY_SPREAD_MS (4000)), derived from id ⊕ digest ⊕ self.
  • Overhear suppression: during the jitter window, overhearing another QUERY or a HAVE for the same {seeder, set_digest} CANCELS the pending query.

Net effect: a digest change costs ~1 query + ~1 HAVE flood mesh-wide; a stable mesh is query-free.

8.3 Fetch handshake

fetcher                                   server (any node that has the mid)
  OTA_GET_MANIFEST(mid, want_mask)  ►     (want_mask=0xFFFF first; only missing fragments on retry)
                               ◄───────   OTA_MANIFEST(mid, frag_idx, frag_total, bytes)   × requested frags
  (reassemble manifest, verify, compute geometry: BC, block_size, payload_size)
  for each missing block:
    OTA_REQ(mid, block_idx, want_mask) ►  (want_mask=all fragments first; only the holes on retry)
                               ◄───────   OTA_DATA(mid, block_idx, frag_off, data) × requested frags
    (reassemble block from frag_off slices)
    OTA_REQ_PROOF(mid, block_idx) ────►
                               ◄───────   OTA_PROOF(mid, block_idx, n_proof, proof)
    (verify proof vs merkle_root → write block → write leaves[i])
  when all blocks present: verify full merkle_root + image_hash → COMPLETE

8.4 Message bodies (transfer)

All offsets after the 1-byte type. Encoders/decoders in OtaProtocol.cpp; constants in OtaManager.h.

OTA_GET_MANIFEST:  manifest_id[4]  want_mask(uint16)   # bit k = send manifest fragment k; 0xFFFF = all
OTA_MANIFEST:      manifest_id[4]  frag_idx(1)  frag_total(1)  bytes[]     # up to OTA_MF_FRAG=176 B/frag
OTA_REQ:           manifest_id[4]  block_idx(uint16)  want_mask(uint16)    # bit k = send fragment k of block
OTA_DATA:          manifest_id[4]  block_idx(uint16)  frag_off(uint16)  data[]   # up to OTA_FRAG_DATA=160 B
OTA_REQ_PROOF:     manifest_id[4]  block_idx(uint16)
OTA_PROOF:         manifest_id[4]  block_idx(uint16)  n_proof(1)  proof[]   # n_proof × 4 bytes
OTA_GET_LEAVES:    manifest_id[4]  want_mask(uint16)   # bit k = send leaves fragment k; 0xFFFF = all
OTA_LEAVES:        manifest_id[4]  frag_idx(1)  frag_total(1)  bytes[]      # up to OTA_LEAVES_FRAG=176 leaf bytes

  • Warm-start / leaf-diff (OTA_GET_LEAVES/OTA_LEAVES) — motatool folder-capture only. Capturing a device's firmware into a motatool serve folder is slow (a full image is hundreds of blocks). Because builds here are non-deterministic, you cannot reproduce the exact target on the host — but a similar build (e.g. a fresh recompile) is ~99% identical. So motatool serve --seed <similar.mota> stages that build's payload into the destination .part, and ota pull <#> folder validate makes the fetcher (1) bulk- fetch the target's leaves[] via OTA_GET_LEAVES/OTA_LEAVES (bitmap-fragmented with a want_mask, same anti-burst rule as OTA_MANIFEST), (2) recompute the merkle root from them and check it equals the manifest root (authenticate), then (3) keep every seeded block whose leaf matches and pull full OTA_DATA only for the blocks that differ. The want_mask is a fixed uint16, so leaves[] is capped at OTA_LEAVES_MAXFRAG=16 fragments (OTA_DIFF_MAX_BLOCKS=704 blocks); larger images just fall back to a full fetch. Normal P2P nodes never use this — they target only the blocks they want; the only always-on part is answering OTA_GET_LEAVES with leaves the node already holds, so any node's firmware can be captured.

  • Block ⇆ fragments: a 1 KB block is split into self-describing OTA_DATA fragments. frag_off is the byte offset of data within the block, so the global position is block_idx*block_size + frag_off — a fragment is self-placing when returned by the source. The fetcher tracks a per-block slice bitmap and reassembles before requesting the proof.

  • Fragment-level requests (anti-deadlock + anti-congestion): both OTA_REQ and OTA_GET_MANIFEST carry a want_mask — bit k asks for fragment k. A fetcher requests the full mask on the first ask ((1<<nf)-1, or 0xFFFF before frag_total is known) and only the still-missing bits on any retry, so recovering one lost fragment re-sends one fragment, not the whole block/manifest. This is essential on half-duplex radios: re-requesting a whole multi-fragment burst let the periodic retry (a transmit) collide with the tail of the in-flight burst and drop the same fragment forever — a hang. Requesting only the hole removes the burst, so there is nothing to collide with. The mask is 16 bits, matching the reassembly bitmap (≤16 fragments/block; 1 KB blocks = 7). OTA_HAVE (broadcast catalog gossip that self-heals via re-query) and OTA_PROOF (a single packet) have no such burst and need no mask.

  • Data and proof are separate phases. OTA_DATA carries no proof; the proof is fetched once per block via OTA_REQ_PROOF/OTA_PROOF after the block's data is complete.

8.5 Sizing against MAX_PACKET_PAYLOAD = 184

message fixed overhead payload/packet
OTA_DATA 9 B (type+mid4+idx2+off2) OTA_FRAG_DATA = 160 → 7 frags per 1 KB block
OTA_MANIFEST 7 B OTA_MF_FRAG = 176 → signed manifest ≈ 2 frags
OTA_HAVE 12 B 12 rows × 14 B per fragment
OTA_PROOF 8 B up to ~44 sibling digests (≫ any real tree)

A served mota supports up to OTA_MAX_BLOCK/4 leaves in the default 4 KB proof scratch (≤1024 blocks ≈ 1 MB payload); larger self-images pass a bigger scratch buffer.

8.6 Temporary-radio and transfer boundary

OTA packets may cross normal mesh relay hops, but each participating node processes or relays them only while its tempradio window is actually running. A receiver requests missing blocks in serial order from the offered firmware source. It never serves partial blocks and never re-advertises a completed download. This keeps each update as one transmitter and one receiver while still allowing active temporary-radio repeaters between them.

9. Identity, trust & versioning

  • target_id (4 B): sha2-256:4(pio_env_name) (little-endian uint32). The env name uniquely captures hardware and role/partition, so a node auto-fetches only matching firmware (a companion image is not fetched onto a repeater even though it shares hw_id). It is self-described in the firmware's EndF (§2, written by pio_endf.py) and read via ota_self_firmware(), so it is correct on any build; -D MOTA_TARGET_ID / MainBoard::getOtaTargetId() is the fallback when no EndF identity is present. tools/mota reads it from the firmware's EndF (or --target-env). A manual ota pull/want can override target (deliberate role switch); the hw_id brick-safety gate (§4) still applies at apply time.
  • target_id vs hw_id — complementary, not redundant: target_id is the fetch-routing key (hw + role + partition); hw_id is the human-readable brick-safety key (hardware only). Same board, two roles ⇒ same hw_id, different target_id.
  • Naming a target_id locally: only the 4-byte target_id ever travels on the wire. To show which board/role a target is, a node (and motatool) reverse-looks-it-up in src/helpers/ota/OtaTargets.h — a generated target_id → env-name table covering every ENABLE_OTA env (tools/mota/gen_targets.py, resolved from pio project config). So ota ls can render [Heltec_v3_repeater] for a neighbour's beacon without the string being transmitted. Unknown ids show as other hw / N/A.
  • fw_version: packed comparable uint32 (MAJOR<<24 | MINOR<<16 | PATCH<<8 | pre); also self-described in EndF. ota ls decodes it for display and flags each update [yours] / [other hw] / [?] by comparing the advertised target_id to the node's own.
  • hw_id: 32-byte NUL-padded ASCII hardware tag inside the signed head. The applier refuses a .mota whose hw_id differs from the device's own tag (empty on either side = permissive). Brick-safety independent of signature.
  • Signing & allowlist: a node keeps a runtime allowlist of trusted Ed25519 signer pubkeys (none embedded in firmware; ota key add/list/rm). A .mota is eligible for auto-install only if signed by an allowlisted key, the signature verifies, and image_hash matches; otherwise it is manual-apply only with explicit confirmation. Transfer needs no trust — blocks are content-addressed against the signed root.
  • Policies (persisted): autofetch ∈ {off, any, signed} (default off) gates automatic block fetching of own-target adverts; autoinstall ∈ {off, trusted} (default off) gates auto-apply of a COMPLETE signed + allowlisted fetch. Conservative defaults: a fresh node discovers + announces but never fetches/installs without operator intent.
  • Supersession: a newer version announced mid-download does not abort the in-progress transfer (finish-current).

10. Multi-mota serve & the external "folder" relay

A node serves a set of mOTAs: its own firmware plus, optionally, an external folder of .mota files it relays without holding them in flash. To peers it simply "has N mOTAs"; the relay is trustless (fetchers verify everything). The serve side (OtaManager) keeps a lightweight registry of what it advertises and two resident "views": view0 (its own firmware) and one on-demand view loaded from a source when a request targets an external mota. Every fetch message carries manifest_id, so dispatch is a registry lookup.

The same host-folder link is also a pull destination (the reverse direction): ota pull <#> folder fetches a .mota off the mesh and streams it onto the host as <mid>.mota via the seeder STORAGE ops (OP_STAT/BEGIN/WRITE/SREAD/FIN, see MotaSeederProto.h), using a FolderMotaStore as the fetch's OtaStore instead of RAM/flash. This captures an exact copy of a device's firmware — e.g. to build a delta against firmware you don't have. Resume is bookkeeping-free: BEGIN 0xFF-fills the file and, on reconnect after a link drop (the fetch PAUSES, holding progress on the host — no RAM/flash fallback), STAT+SREAD let the fetcher recompute and refill only the missing blocks.

10.1 The MotaSource abstraction (OtaSource.h)

Transport-agnostic provider of one or more complete .mota as random-access bytes. The same serve code drives USB-serial, BLE, a WiFi URL list, an NFS/samba mount, etc. — only read() differs.

struct MotaDesc {                      // catalog metadata + region offsets (no whole image in RAM)
  uint8_t mid[4]; uint32_t target_id, fw_version; uint8_t codec_id, flags;
  uint32_t total_size, leaves_off, block_count, payload_off, payload_size;
};
class MotaSource {
  virtual uint8_t count();                                  // # mOTAs offered
  virtual bool    describe(uint8_t idx, MotaDesc& out);     // metadata + offsets
  virtual bool    read(uint8_t idx, uint32_t off, uint8_t* buf, uint32_t len);   // random-access bytes
};

To serve an external mota the node reads its manifest-minus-leaves + leaves[] into RAM (≤4 KB for ≤1024 blocks) and streams payload blocks from the source on demand; proofs are generated from the read leaves.

10.2 The mota-seeder transport (MotaSeederProto.h)

A MotaSource is fed by a host that serves a folder over the device's USB serial (the same console the CLI uses — no extra hardware) or, on an ESP32 WiFi companion, over WiFi (TCP). The host is the standalone Rust tool motatool (motatool serve --serial <port> / --tcp <host[:port]>, which also builds + verifies + inspects .mota). The device only emits request frames while actively serving a fetch, and reads the reply synchronously, so over the shared USB console binary frames coexist with the text CLI/logs (resync on magic + checksum). Little-endian, XOR-checksummed:

request  (device → host):  'M' 'S'  op(1)  args...                 xsum(1 = XOR of op+args)
response (host → device):  'm' 's'  op(1)  status(1)  payload...    xsum(1 = XOR of all prior)

OP_COUNT     0x01   args: -            → payload: count(1)
OP_DESCRIBE  0x02   args: idx(1)       → payload: MotaDesc wire (38 B)
OP_READ      0x03   args: idx(1) off(4) len(2)  → payload: len bytes
MotaDesc wire (38 B): mid[4] target_id(4) fw_version(4) codec(1) flags(1)
                      total_size(4) leaves_off(4) block_count(4) payload_off(4) payload_size(4)
status: 0 = OK, non-zero = error (out of range / past EOF).

Device CLI: ota folder on (attach + announce), ota folder (list), ota folder off. Build flag OTA_FOLDER_SERIAL (default stream = console Serial; override OTA_FOLDER_SERIAL_STREAM + define OTA_FOLDER_SERIAL_BEGIN for a dedicated UART). On an ESP32 WiFi companion the node also runs a second WiFiServer on a dedicated seeder port (OTA_SEEDER_TCP_PORT, default 5001), separate from the companion app port (TCP_PORT, default 5000) — so motatool serve --tcp can feed updates while a phone app stays connected. The node auto-attaches the source when a seeder client connects and detaches when it closes (no ota folder on needed over TCP). Verified on hardware: a RAK4631 relays a host folder to a Heltec V3 over one USB cable, and a host feeds a Heltec V3 over WiFi (:5001) while the companion serves a phone on :5000 — every block merkle-checked.

Transport-agnostic by design. The request/response semantics (COUNT / DESCRIBE(idx) / READ(idx, off, len) over a folder catalog) are independent of the link. The 2-byte magic + XOR checksum + resync framing above exists for the shared USB-UART (an unframed byte stream); it is harmless over a reliable stream and the WiFi (TCP) transport reuses it as-is — both ends just treat the socket as a byte stream (on-device, SerialMotaSource runs verbatim over an Arduino Stream-compatible WiFiClient; motatool's TcpTransport mirrors its SerialTransport). A future framed link such as BLE GATT (an Android phone relaying a folder) could carry the same ops with no magic/checksum at all — a request characteristic write delivers op + args, the reply notifies status + payload. motatool reflects this split: a transport-free SeederCore (the catalog logic) under a swappable framing/transport layer.

11. CLI surface (OtaCli.cpp)

User-facing OTA data should travel via CMD_OTA_* companion binary frames; the text CLI below is debug/operator oriented and replies are snprintf-bounded into a 160-byte buffer.

Commands take intuitive aliases (matched by the first word; see is_cmd in OtaCli.cpp) so they're easy to type and read — status/neighbors/pull/drop/applydelta are the canonical names, the aliases are the recommended user-facing forms. Output is plain-language (a user-facing guide lives at ota_user_guide.md).

ota help | ?                       list the commands
ota status | st  (or bare `ota`)   plain-language: running fw, the one fetch session (state/%/id), serving, keys
ota ls | neighbors | nbrs | updates | n   discovered updates (queries sources; rows arrive async via OTA_HAVE)
ota get | pull | download <#|mid8> fetch a chosen mOTA (manual; works regardless of autofetch)
ota install | apply | applydelta   verify + approve + (ESP32) apply / (nRF52) reboot-to-bootloader
ota cancel | drop | stop           drop the current fetch session (frees the slot)
ota announce | adv                 serve self + send a beacon now
ota self | id                      print this firmware's EndF (body/image size, base_hash)
ota folder | fold [on|off]         attach/detach an external .mota folder (host daemon) ; bare = list
ota config | cfg | set [autofetch|autoinstall|checkpoint] ...   show/set persisted policy
ota key | keys [add|rm <hex>]      trusted signer allowlist ; bare = list
ota dev ...                        bring-up helpers (stage/recv/serve/verify)

12. Apply & bootloader contract

  • ESP32 (A/B): applied in-firmware via the detools decoder into the inactive OTA slot (OtaApply.cpp::ota_apply_detools_mota + OtaStoreFlashEsp32), then set-boot + reboot (power-safe, rollback-capable). No bootloader changes. Erase ranges must be sector-aligned (4096).
  • nRF52 (single-slot): the running firmware never flashes the app. ota applydelta verifies fully (image_hash, base_hash, signature/allowlist, hw_id), writes approval = "APRV", then reboots into the modified bootloader (Adafruit_nRF52_Bootloader_OTAFIX). The bootloader:
    1. scans flash for MAGIC to find the staged .mota (it must NOT trust any stored size),
    2. re-checks TRAILER, image_hash, approval == "APRV", and that the delta's base_hash equals the running firmware's EndF.body_hash (recomputed by scanning for EndF — never trust bank_0_size),
    3. applies the in-place codec over the app region and boots only if the result hashes to image_hash.

The signature proves author authenticity; approval proves local owner consent — both required to apply.

Bootloader testing note: always test apply with a real different image (base ≠ target). A same-image (X→X) "delta" trivially reproduces the target and gives a false positive.

13. Versioning of this spec

format_ver = 2. A parser accepts exactly this value and rejects anything else — there is one container format, fixed-layout, and no compatibility shims to carry. If the format ever needs to change, bump format_ver; the multihash hash_algo separately allows swapping the digest family without a format bump. Unknown codec_id / ota_msg_type values are ignored (a node simply won't fetch what it can't apply).

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