# 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](ota_user_guide.md) — 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`](https://github.com/vk496/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](https://github.com/multiformats/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 (and any board for a full image). | | 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`), nRF52 accepts `full` + `in-place`. `CODEC_FULL` is always acceptable. A `.mota` with an unsupported codec is rejected at discovery time, before any blocks are requested. 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 `, 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 ` 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< folder` fetches a `.mota` off the mesh and streams it onto the host as `.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. ```cpp 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`](https://github.com/vk496/motatool) (`motatool serve --serial ` / `--tcp `, 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_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 ] 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).