reticiulum-specification/SPEC.md

Reticulum Wire Specifications

A byte-level reference for implementing Reticulum-compatible clients. This document focuses on what implementations need to interop with the canonical Python implementation (markqvist/Reticulum and markqvist/LXMF) plus the existing client ecosystem (Sideband, Nomadnet, MeshChat, the various firmware projects).

Source citations refer to the standard pip install rns lxmf install layout (RNS/, LXMF/).

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1. Identity and destination hashes

1.1 Identity composition

A Reticulum identity is two keypairs concatenated:

public_key  = X25519_pub(32) || Ed25519_pub(32)        // 64 bytes
private_key = X25519_priv(32) || Ed25519_priv(32)      // 64 bytes

X25519 for ECDH (encryption / shared-secret derivation), Ed25519 for signatures.

identity_hash = SHA256(public_key)[:16]                // 16 bytes truncated

The 16-byte truncation is consistent across all hashes Reticulum stores on the wire (destinations, link IDs, packet hashes, etc.). The full SHA-256 is used internally for signing inputs but never appears in headers.

1.2 Destination hash

The 16-byte destination hash that appears in packet headers and announces is:

name_hash = SHA256(full_app_name_string)[:10]
dest_hash = SHA256(name_hash || identity_hash)[:16]

Where full_app_name_string is e.g. "lxmf.delivery", "nomadnetwork.node", "rnstransport.path.request". The hex-encoded identity hash is NOT part of the input — only the plain ASCII app-name string. This is the identity=None branch of upstream's expand_name() function (RNS/Destination.py). The identity hex appears only in the human-readable Destination.name debug string.

Common pre-computed name_hash values:

10-byte hex	App name
6ec60bc318e2c0f0d908	lxmf.delivery
e03a09b77ac21b22258e	lxmf.propagation
213e6311bcec54ab4fde	nomadnetwork.node
0ad8bff9ff75737c058e	nomadnetwork.gossip
9efb9c771eeb5ae90ea6	rnstransport.broadcasts
4848a053c16415bed6c8	rnstransport.remote.management
7926bbe7dd7f9aba88b0	rnstransport.path.request (resulting dest_hash with identity=None: 6b9f66014d9853faab220fba47d02761)

1.3 Private key on-disk format

RNS.Identity.to_file(path) writes the raw 64-byte private-key blob with no header, no version byte, no checksum, no encryption. The byte order is the same as the public_key concatenation in §1.1 — verified by tools/verify_destination_hash.py's existing Identity.from_bytes round-trip:

prv_bytes_blob  =  X25519_priv(32) || Ed25519_priv(32)         // 64 bytes total

Identity.get_private_key() at RNS/Identity.py:694-698 returns this exact concatenation:

def get_private_key(self):
    return self.prv_bytes + self.sig_prv_bytes
    #      ^^^^^^^^^^^^^   X25519 priv (set at line 679 from X25519PrivateKey.generate())
    #                       ^^^^^^^^^^^^^^^   Ed25519 priv (set at line 682)

Identity.load_private_key(prv_bytes) at line 706-717 slices it back the same way:

self.prv_bytes     = prv_bytes[:32]   # X25519
self.sig_prv_bytes = prv_bytes[32:]   # Ed25519

to_file is a thin wrapper that writes get_private_key() to the path; from_file reads back with no extra parsing.

File-system facts

• Size: exactly 64 bytes. No magic, no length prefix.
• Encryption: none. Anyone with read access can fully impersonate the identity.
• Permissions: upstream doesn't chmod the file; clients are expected to put it in a directory protected by OS permissions (~/.reticulum/storage on Linux/macOS, %APPDATA%/Reticulum/storage on Windows by default).
• Filename: caller-controlled. RNS itself uses transport_identity for the transport node and lets app-level callers choose for delivery destinations (LXMF puts these in LXMF.LXMRouter.storagepath).

Constructing from raw bytes — from_bytes HAZARD

Identity.from_bytes(prv_bytes) at line 611-623 takes the same 64-byte concat and reconstitutes an Identity. The upstream docstring explicitly warns:

HAZARD! Never use this to generate a new key by feeding random data in prv_bytes.

The reason: X25519PrivateKey.from_private_bytes and Ed25519PrivateKey.from_private_bytes both accept arbitrary 32-byte values without scalar clamping or rejection — a clean-room implementation that feeds raw random data into from_bytes skips the keypair-generation invariants enforced by the upstream cryptography library's .generate() methods (e.g. X25519 scalar clamping per RFC 7748 §5). Always generate fresh keys via the cryptography (or equivalent) library's keypair generator, then concatenate; never invent your own bytes.

Cross-implementation portability

The format is portable across implementations because there's nothing in it but the raw bytes. A 64-byte file written by Python RNS is byte-identical to one written by any clean-room implementation that follows this section, and both produce the same identity_hash and lxmf.delivery destination_hash when fed back through §1.1 and §1.2 — test vectors at test-vectors/identities.json demonstrate the round-trip against RNS 1.2.0.

⚠️ Spec correction: Earlier revisions of this section described the on-disk order as Ed25519 first, X25519 second ("opposite of the public_key concatenation"). That was wrong — verified by re-running Identity.to_file and reading back the bytes against the test vector at test-vectors/identities.json, the actual order is X25519 first, Ed25519 second, identical to the public_key order. Implementations following the prior spec wording would have corrupted identity files when interoperating with upstream Python RNS.

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2. Packet header

2.1 Flag byte layout

Every Reticulum packet starts with a 1-byte flag field:

bit 7-6 : header_type      (0 = HEADER_1, 1 = HEADER_2)
bit 5   : context_flag     (1 = announce includes a ratchet pubkey)
bit 4   : transport_type   (0 = BROADCAST, 1 = TRANSPORT)
bit 3-2 : destination_type (0=SINGLE, 1=GROUP, 2=PLAIN, 3=LINK)
bit 1-0 : packet_type      (0=DATA, 1=ANNOUNCE, 2=LINKREQUEST, 3=PROOF)

2.2 Two header forms

HEADER_1: flags(1) hops(1) dest_hash(16) context(1) data(...)        // min 19 bytes
HEADER_2: flags(1) hops(1) transport_id(16) dest_hash(16) context(1) data(...)   // min 35 bytes

HEADER_2 carries a transport_id (the next-hop transport node's identity hash) before the final destination hash. A relay converts a HEADER_1 packet to HEADER_2 by setting bit 6 of flags, inserting its own identity at offset 2, and re-transmitting.

2.3 Originator HEADER_1 → HEADER_2 conversion

This is non-obvious and matters: when an originator (not a relay) sends a packet to a destination known to be more than 1 hop away, the originator MUST also do the HEADER_2 conversion. From RNS/Transport.py::outbound (lines 1074-1083 in RNS 1.2.0; verified by tools/verify_packet_header.py):

if path_entry[IDX_PT_HOPS] > 1:
    if packet.header_type == RNS.Packet.HEADER_1:
        new_flags = (RNS.Packet.HEADER_2) << 6 | (Transport.TRANSPORT) << 4 | (packet.flags & 0b00001111)
        new_raw  = struct.pack("!B", new_flags)
        new_raw += packet.raw[1:2]                       # hops byte unchanged
        new_raw += path_entry[IDX_PT_NEXT_HOP]           # 16B transport_id at offset 2
        new_raw += packet.raw[2:]                        # original dest_hash + context + payload

For destinations 0 or 1 hops away, the originator may stay HEADER_1 — the receiving rnsd auto-fills the transport_id when the destination matches a local client (for_local_client branch at RNS/Transport.py:1451 in RNS 1.2.0). Implementations that always emit HEADER_1 will silently fail to deliver to multi-hop destinations even with a known path.

2.4 Hop count

Byte 1 is hops, an 8-bit counter that each transit relay increments by 1. 0 for a packet still on the originator. 255 would in theory wrap, but no Reticulum mesh in practice has paths anywhere near that long.

2.5 Context byte

Single byte after the destination hash (offset 18 for HEADER_1, offset 34 for HEADER_2). Common values:

Full context inventory from RNS/Packet.py:72-92 (RNS 1.2.0):

Hex	Name	Used for
0x00	NONE	Generic / opportunistic DATA packet
0x01	RESOURCE	One part (chunk) of a Resource transfer (§10)
0x02	RESOURCE_ADV	Resource advertisement
0x03	RESOURCE_REQ	Resource part request (from receiver to sender)
0x04	RESOURCE_HMU	Resource hashmap update (next-segment hashmap)
0x05	RESOURCE_PRF	Resource proof (a PROOF-type packet using this context)
0x06	RESOURCE_ICL	Resource cancel from the initiator
0x07	RESOURCE_RCL	Resource cancel from the receiver / reject of an advertisement
0x08	CACHE_REQUEST	Cache lookup over a Link
0x09	REQUEST	Link REQUEST (NomadNet page fetch, propagation /get)
0x0A	RESPONSE	Link RESPONSE matching a REQUEST
0x0B	PATH_RESPONSE	An ANNOUNCE emitted in response to a path? request — distinguishes it from a periodic re-announce. Receivers handle the two paths differently (see §7.2 and §4.5)
0x0C	COMMAND	Channel-style remote-execution command
0x0D	COMMAND_STATUS	Status reply for a COMMAND
0x0E	CHANNEL	Link channel multiplexed payload
0xFA	KEEPALIVE	Link keepalive (sent periodically while a Link is idle)
0xFB	LINKIDENTIFY	Backchannel-identify proof on an established Link (§5 backchannel)
0xFC	LINKCLOSE	Link teardown notification
0xFD	LINKPROOF	Defined but not actually emitted by upstream RNS 1.2.0 in this revision. Both Identity.prove and Link.prove_packet build their proof packets with context = NONE (0x00) — the proof-ness is conveyed by packet_type = PROOF (3), not by this context byte. Reserved for a future revision; see §6.5
0xFE	LRRTT	Link RTT measurement reply
0xFF	LRPROOF	Link request proof (§6.2)

2.6 Source

RNS/Packet.py for the constants and _pack / _unpack methods. RNS/Transport.py for the routing-side HEADER_1↔HEADER_2 transitions.

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3. Token cryptography (modified Fernet)

Reticulum's "Token" construction is a modified Fernet used for opportunistic destination encryption (single packet), as well as for derived-key channels on established Links.

3.1 Wire format

ephemeral_pub(32) || iv(16) || aes_ciphertext(...) || hmac_sha256(32)

For Link-derived-key encryption (after the Link handshake has produced a session key), the ephemeral_pub prefix is omitted and the wire form is just iv || ciphertext || hmac.

3.2 Encrypt steps (opportunistic)

1. Generate ephemeral X25519 keypair (eph_priv, eph_pub).
2. ECDH: shared = X25519(eph_priv, recipient_X25519_pub). The recipient's X25519 pub is either their long-term encPub (first 32 bytes of public_key) or their currently-announced ratchet_pub if present.
3. HKDF-SHA256: derived = HKDF(shared, salt = recipient_identity_hash, info = "", L = 64). The salt is the recipient's 16-byte identity hash — not their destination hash, not the ratchet hash.
4. Split: signing_key = derived[0..32], encryption_key = derived[32..64].
5. Random 16-byte IV.
6. AES-256-CBC encrypt plaintext with encryption_key and iv. Do NOT manually pad — the platform AES-CBC API (AES/CBC/PKCS5Padding on JCA, Web Crypto's default) auto-pads PKCS#7. Manual padding on top causes 16 garbage bytes of double-padding.
7. hmac = HMAC-SHA256(signing_key, iv || ciphertext).
8. Concatenate as the wire format above.

3.3 Decrypt steps

Reverse of encrypt. Critically:

• Verify HMAC BEFORE attempting decryption (encrypt-then-MAC; prevents AES padding-oracle attacks).
• A receiver that has multiple candidate X25519 private keys (typically the current ratchet privkey + the long-term identity privkey) should try each in order until one produces a matching HMAC. Senders that haven't seen the receiver's latest ratchet announce will encrypt to the long-term key as a fallback.

3.4 Source

RNS/Cryptography/Token.py (and the equivalents in vendor crypto modules). The webclient's reference/js-reference/crypto.js is a faithful port.

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4. Announce wire format

4.1 Packet body

The Reticulum packet header (HEADER_1, packet_type=ANNOUNCE, dest_type=SINGLE, transport_type=BROADCAST) is followed by an announce body:

public_key(64) || name_hash(10) || random_hash(10) || [ratchet_pub(32) if context_flag] || signature(64) || app_data(...)

The 64-byte public_key is the X25519 || Ed25519 concat described in section 1.1.

random_hash is NOT 10 random bytes — only the first 5 bytes are random; the trailing 5 bytes carry the emission timestamp as a big-endian unsigned 40-bit Unix-seconds integer (RNS/Destination.py:282):

random_hash = RNS.Identity.get_random_hash()[0:5] + int(time.time()).to_bytes(5, "big")

Transit relays read the timestamp portion via Transport.timebase_from_random_blob(random_blob) = int.from_bytes(random_blob[5:10], "big") (RNS/Transport.py:3100-3101) to make ordering decisions when an inbound announce carries a higher hop count than the cached path: only newer-emitted announces can refresh the path table (see §4.5). 5 bytes of seconds covers ~34,000 years, so wraparound is not a near-term concern. Implementations MUST emit this exact format, including a clock value that's monotonically non-decreasing across announces from the same destination — clockless sender devices (per §9.6) may end up locked out of long-range path table updates.

⚠️ UNVERIFIED — Known deviation: attermann/microReticulum/src/Destination.cpp:270-272 (and therefore every project that uses microReticulum unmodified, including thatSFguy/reticulum-lora-repeater and the Faketec sibling project) currently emits 10 fully-random bytes for random_hash — the timestamp half is a TODO that never landed:

//p random_hash = Identity::get_random_hash()[0:5] << int(time.time()).to_bytes(5, "big")
// CBA TODO add in time to random hash
Bytes random_hash = Cryptography::random(Type::Identity::RANDOM_HASH_LENGTH/8);

Python RNS receivers interpret random_hash[5:10] as a big-endian uint40 unix_seconds. A uniformly-random uint40 has median value ~5.5×10¹¹ ≈ year 19403 AD, so a microReticulum announce will (with overwhelming probability) appear "far-future" to a Python receiver. Effect: once one such announce populates path_table[dest][IDX_PT_RANDBLOBS], the equal-or-greater-hop branch at RNS/Transport.py:1721-1745 will reject any real-timestamped announce as "stale" until the path TTL expires. First-contact path-table population is unaffected; the bug only surfaces on path replacement under §4.5 step 6.3. The microReticulum receive side does NOT consult the timestamp half so microReticulum-to-microReticulum traffic is unaffected. The repeater repo's pre_build.py patches several microReticulum protocol bugs but not this one (as of thatSFguy/reticulum-lora-repeater@95823ad-vintage upstream). Verifying by capture-and-decode against an actual mixed-vendor mesh is the work that would let this callout be removed.

The optional 32-byte ratchet_pub (an X25519 public key) is present iff the packet header's context_flag bit is 1. Indexing through this layout accordingly is mandatory; see RNS/Identity.py::validate_announce for the canonical parser.

4.2 Signed data

signed_data = dest_hash(16) || public_key(64) || name_hash(10) || random_hash(10) || [ratchet_pub(32)] || app_data
signature   = Ed25519_sign(signed_data, identity.Ed25519_priv)

Note that dest_hash is INCLUDED in the signed data even though it's not in the wire-format announce body (the receiver gets it from the packet header). The signing key is the Ed25519 half (last 32 bytes) of the identity's private_key.

4.3 app_data format for LXMF delivery destinations

Upstream LXMF/LXMRouter.py::get_announce_app_data produces a 2-element msgpack array (verified against LXMF 0.9.6 by tools/verify_announce_app_data.py):

# LXMF/LXMRouter.py:986-1002 in LXMF 0.9.6
peer_data = [display_name, stamp_cost]   # stamp_cost = None unless 1 ≤ N ≤ 254
return msgpack.packb(peer_data)

Wire bytes for display_name = "Reticulum5", stamp_cost = None:

92         # fixarray, 2 elements
c4 0a      # bin8, length 10
52 65 74 69 63 75 6c 75 6d 35    # "Reticulum5"
c0         # nil (stamp_cost)

Encoding the display name as msgpack bin (0xc4 NN) is required for upstream interop — see section 9.3 below. The stamp_cost field can be int 0 (0x00) or nil (0xc0); upstream's stamp_cost_from_app_data doesn't strict-type-check.

A third optional [capability_flags] element (e.g. [SF_COMPRESSION], the only flag currently defined at LXMF/LXMF.py:108) is read by the parser (compression_support_from_app_data at LXMF/LXMF.py:154-167) but is not emitted by the LXMF 0.9.6 producer — LXMRouter.py:999 computes supported_functionality = [SF_COMPRESSION] but never appends it to peer_data. Implementations should accept the 3-element form on inbound (a future LXMF version may re-enable it; older deployments may emit it) but should not rely on receiving it.

The parser also tolerates a 1-element msgpack array (just the name) and a raw UTF-8 string ("original announce format" branch at LXMF/LXMF.py:138-139) — see LXMF/LXMF.py::display_name_from_app_data for all four accepted shapes.

4.4 Announce filtering by name_hash

When ingesting an announce, clients should distinguish by name_hash:

• lxmf.delivery (6ec60bc318e2c0f0d908) — messagable peers, surface in contacts UI
• lxmf.propagation (e03a09b77ac21b22258e) — propagation node, surface separately
• nomadnetwork.node (213e6311bcec54ab4fde) — page-serving NomadNet host
• rnstransport.broadcasts / rnstransport.remote.management — transport-internal, ignore for user UI
• Any other name_hash — non-LXMF custom destination (telemetry beacons, application-specific)

Treating every announce as a contact (the naive default) populates the UI with hundreds of irrelevant rows.

4.5 Announce validation rules (receive side)

These are the MUST rules a receiver applies to every inbound announce before considering the announced destination "known". The canonical implementation is RNS/Identity.py::validate_announce (line 496-598 in RNS 1.2.0); the dispatch site that calls it is RNS/Transport.py::inbound line 1623-1650.

1. Body parse — branch on context_flag

The context_flag bit (bit 5 of the packet's 1-byte flag field, §2.1) selects between two body layouts. Slice offsets, with keysize = 64, name_hash_len = 10, random_hash_len = 10, ratchet_size = 32, sig_len = 64:

context_flag == 1 (ratchet present):
   public_key   = data[ 0                                     :  64]
   name_hash    = data[ 64                                    :  74]
   random_hash  = data[ 74                                    :  84]
   ratchet_pub  = data[ 84                                    : 116]
   signature    = data[116                                    : 180]
   app_data     = data[180                                    :    ]   # may be empty

context_flag == 0 (no ratchet):
   public_key   = data[ 0                                     :  64]
   name_hash    = data[ 64                                    :  74]
   random_hash  = data[ 74                                    :  84]
   signature    = data[ 84                                    : 148]
   app_data     = data[148                                    :    ]   # may be empty

A client that uses a fixed offset for signature regardless of the flag (a real bug from the SF webclient's first cut) silently rejects every ratchet-bearing announce as having a bad signature.

2. Signature verification

Reconstruct the signed_data exactly per §4.2:

signed_data = destination_hash || public_key || name_hash || random_hash || ratchet || app_data

Where ratchet is b"" (empty, not absent) when context_flag == 0, and app_data is b"" when not present in the packet. destination_hash comes from the outer packet header, NOT from the announce body — re-using the body bytes as the dest_hash would let a sender forge announces for arbitrary destinations.

Verify the 64-byte signature with the announced public_key's Ed25519 half (last 32 bytes). Reject on failure.

3. destination_hash recomputation

Recompute the dest_hash from the announced inputs:

identity_hash    = SHA256(public_key)[:16]
expected_hash    = SHA256(name_hash || identity_hash)[:16]

Reject the announce iff expected_hash != packet.destination_hash (the value from the outer header). This catches both random hash collisions and active spoofing attempts that pair a valid signature with an unrelated dest_hash. (RNS/Identity.py:548-551).

4. Public-key collision rejection

If the receiver already has a different public_key cached for this destination_hash (from a prior announce), the new announce MUST be rejected with a critical-severity log even if the signature is otherwise valid. Per the upstream comment: "In reality, this should never occur, but in the odd case that someone manages a hash collision, we reject the announce" (RNS/Identity.py:554-560).

This rule means: first-announcer-wins for any given destination_hash within a receiver's lifetime. A peer who loses their identity material and regenerates with the same display name + app_name will produce a different identity_hash → different destination_hash → no collision. A peer who tries to replace their announced public key under the same destination_hash, however, gets rejected — the real defense against this class of attack.

5. Blackhole list check

Before everything else, check RNS.Transport.blackholed_identities. An identity_hash on the blackhole list is dropped silently regardless of signature validity (RNS/Identity.py:538-541). This is operator-controlled state, not a wire feature.

6. Caching the announce contents

On a fully validated announce, the receiver MUST update its caches in this order:

1. known_destinations[destination_hash] ← [recv_time, packet_hash, public_key, app_data, last_used] — populates the table that RNS.Identity.recall(dest_hash) reads when constructing outbound destinations (RNS/Identity.py::remember, line 100-112). Without this, every subsequent outbound message to this peer fails because no public key is available for Token encryption.
2. known_ratchets[destination_hash] ← ratchet_pub (only if context_flag == 1 and ratchet_pub != b"") — Identity._remember_ratchet, line 395-428. The ratchet is also persisted to disk under {storagepath}/ratchets/{hexhash} for use across restarts.
3. path_table entry update or insertion (see §4.6 — TBD when the relay rebroadcast spec lands), gated by:
   • random_blob (= random_hash) not in the cached random_blobs history for this destination — cheap replay defence (RNS/Transport.py:1707, 1732, 1745).
   • Hop count comparison against any existing entry: equal-or-fewer hops always win; more hops win only if the cached path has expired or the new announce's emission timestamp (from random_hash[5:10]) is more recent than every cached blob's timestamp (RNS/Transport.py:1700-1745).

7. PATH_RESPONSE distinction

An announce whose outer packet context == PATH_RESPONSE (0x0B) is the responder's reply to a recent path? request, not a periodic re-announce. Validation is identical (rules 1-6 above), but listener dispatch differs:

• The default behavior of Transport.announce_handlers registered via RNS.Transport.register_announce_handler is to skip path-response announces unless the handler sets receive_path_responses = True on itself (RNS/Transport.py:1989-1991).
• The path table population path is the same either way — both regular and path-response announces refresh the path entry — so a leaf client that ignores PATH_RESPONSE entirely at the application layer still benefits from the path-table side effect.

8. Implementation-private behavior (SHOULD)

These are not wire-spec MUST rules but most working clients implement them; without them the implementation will misbehave in busy meshes:

• Per-interface ingress rate limiting. When the inbound announce rate on an interface exceeds IC_BURST_FREQ_NEW = 6 Hz (interfaces less than 2 hours old) or IC_BURST_FREQ = 35 Hz (older), and the announced destination is not in path_table and not in path_requests, the announce is held in the interface's held_announces dict for later release rather than processed immediately. Released later in lowest-hop-count-first order. (RNS/Interfaces/Interface.py:60-200.) Without this, a flood of unknown-destination announces can drown out everything else.
• random_blob history cap. The cached random_blobs list per destination is bounded by Transport.MAX_RANDOM_BLOBS to keep the path table from growing without bound under a long-lived destination's announce stream (RNS/Transport.py:1820).
• Self-announce filter. §9.5 — drop announces where destination_hash matches one of the receiver's own destinations to avoid populating its own contact list with itself.

9. Source map for §4.5

File	What it pins down
RNS/Identity.py:496-598	validate_announce — body parse, signed_data, sig verify, dest_hash recompute, collision check
RNS/Identity.py:100-112	Identity.remember — known_destinations update
RNS/Identity.py:395-428	_remember_ratchet — ratchet persistence
RNS/Transport.py:1623-2024	inbound dispatch for packet_type == ANNOUNCE: quick sig check, ingress limiting, path table population, handler dispatch
RNS/Transport.py:3100-3117	timebase_from_random_blob, announce_emitted
RNS/Interfaces/Interface.py:60-200	ingress-limit constants, should_ingress_limit, hold_announce, process_held_announces
RNS/Packet.py:83	PATH_RESPONSE = 0x0B context constant

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5. LXMF wire format

LXMF has two delivery methods with different plaintext layouts.

5.1 Opportunistic delivery (single Reticulum DATA packet)

Plaintext (after Token decryption):

source_hash(16) || signature(64) || msgpack_payload(...)

The recipient's destination_hash is stripped (the outer Reticulum packet's dest_hash already conveys it; including it would waste bytes).

5.2 Direct delivery (over an established Reticulum Link)

destination_hash(16) || source_hash(16) || signature(64) || msgpack_payload(...)

Full layout. The Link's session key encrypts the whole blob.

5.3 msgpack_payload

A msgpack array of 4 elements (5th optional):

[timestamp_seconds_double, title_bytes, content_bytes, fields_dict]
# optional 5th element: stamp (varies)

Times are seconds-since-Unix-epoch as a double-precision float. Title and content are msgpack bin (Python bytes). Fields is a msgpack map; usually {} for plain text, but used for attachments, stickers, etc.

5.4 Source/destination semantics

source_hash is the SENDER's destination hash (SHA256(name_hash || identity_hash)[:16]), NOT the raw identity hash. A common implementation bug is to write the identity_hash here; the recipient then can't look the sender up in their contacts (which are keyed by destination_hash).

5.5 Signed data

hashed_part  = destination_hash(16) || source_hash(16) || msgpack_payload
message_hash = SHA256(hashed_part)
signed_data  = hashed_part || message_hash
signature    = Ed25519_sign(signed_data, sender_identity.Ed25519_priv)

For opportunistic delivery, destination_hash is the recipient's destination hash (from the outer packet header, not from the LXMF body).

5.6 Signature verification — msgpack variant tolerance

Different msgpack encoders produce subtly different byte sequences for the same logical value (e.g. integer encoding choice, string vs bin selection). The signer signed over THEIR encoder's output. A receiver should try verifying against:

1. The raw msgpack bytes from the wire as-received (msgpack_payload exactly).
2. A stripped re-encoded version (decode then re-encode the first 4 elements, omitting the optional stamp field).

If either matches, the signature is valid. Strict raw-only verification fails interop with anything that's been through a msgpack re-encode somewhere in the chain.

5.7 Source

LXMF/LXMessage.py for pack/unpack; LXMF/LXMF.py for the app_data extraction helpers.

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6. Reticulum Link protocol

A Link is an ephemeral encrypted channel between two destinations, established via a 2-packet handshake (LINKREQUEST → LRPROOF) and used afterward for full-duplex DATA.

6.1 LINKREQUEST (initiator → responder)

A regular packet with packet_type = LINKREQUEST (2), dest_type = SINGLE, addressed to the responder's destination hash. Body:

initiator_X25519_pub(32) || initiator_Ed25519_pub(32) || [signalling(3)]

Both initiator-side keys are fresh ephemeral keys (not the initiator's long-term identity). The optional 3-byte signalling field encodes a packed 21-bit MTU and 3-bit link mode — see §6.6 for the bit layout and the negotiation rules. Receivers detect its presence by body length: len(data) == 64 means no signalling, len(data) == 67 means signalling present.

6.2 LRPROOF (responder → initiator)

A packet_type = PROOF (3) with context = 0xff, addressed to the link itself — i.e. dest_hash in the packet header is the 16-byte link_id (RNS/Packet.py:182-184: when context is LRPROOF, header += destination.link_id and the body is appended unencrypted).

Body (proof_data at RNS/Link.py:376):

signature(64) || responder_X25519_pub(32) || [signalling(3)]

Only the responder's X25519 is fresh-ephemeral; the responder signs with its long-term Ed25519 private key (asymmetric with the initiator). The responder's long-term Ed25519 public key is not sent on the wire — both sides already know it from the responder's prior announce, and it is included implicitly in the signature input. Signature input (RNS/Link.py:373 for the signer, :417 for the validator):

signed_data = link_id || responder_X25519_pub || responder_long_term_Ed25519_pub || [signalling]

The full wire packet is therefore: flags(1) || hops(1) || link_id(16) || context=0xff(1) || signature(64) || responder_X25519_pub(32) || [signalling(3)].

The signalling slot, when present, carries the responder's confirmed_mtu and the link mode in a 24-bit packed integer — see §6.6. Receivers detect its presence by length: body 96 vs 99 bytes. Critical for interop: the signalling bytes (when present) MUST be included in signed_data exactly where shown above; an implementation that signs without them on a peer that emits them — or vice versa — fails signature validation and the link never establishes. This is the most common cause of link-handshake failures with mixed-version peers.

6.3 link_id derivation

link_id = SHA256(hashable_part_of_LINKREQUEST_packet)[:16]

hashable_part is built by Packet.get_hashable_part (RNS/Packet.py:354-361):

hashable_part = byte(flags & 0x0F) || raw[N:]
   where N = 2  for HEADER_1   (strip flags + hops)
         N = 18 for HEADER_2   (strip flags + hops + transport_id)

The "hashable part" deliberately strips header_type, context_flag, transport_type (top 4 bits of flags — modifiable by transit relays), the hops byte (modified by every relay), and (for HEADER_2) the transport_id (added by the originator and re-written by each relay). What remains in both cases is the low nibble of flags + dest_hash + context + body, so the resulting link_id is the same whether the LINKREQUEST is hashed at the initiator (HEADER_1) or at the responder after one or more transport relays (HEADER_2). Both sides agree on the 16-byte ID.

For LINKREQUEST packets specifically, the trailing signalling bytes (if present, indicated by len(packet.data) > Link.ECPUBSIZE in link_id_from_lr_packet at RNS/Link.py:340-347) are stripped from the END of hashable_part before hashing, so the link_id is invariant under MTU-discovery signalling.

6.4 Session key derivation

Both sides compute:

shared       = X25519(my_ephemeral_priv, peer_ephemeral_pub)
session_key  = HKDF(shared, salt = link_id, info = "", L = 64)
signing_key  = session_key[0..32]
encrypt_key  = session_key[32..64]

Subsequent DATA packets on the link use the Link-derived-key Token format (section 3.1, no ephemeral_pub prefix).

6.5 Packet receipts (regular PROOF packets)

A PROOF-type packet (packet_type = 3, context = NONE (0x00)) is the receipt that closes the loop on every CTX_NONE DATA packet — both opportunistic DATA addressed to a SINGLE destination and DATA flowing on an active Link. Without it, the sender's PacketReceipt never resolves, its retransmit queue fires repeatedly, and on a Link the KEEPALIVE budget is exhausted and the link torn down.

This section specifies the regular PROOF body. Two related proof formats are documented elsewhere and are NOT compatible with this format:

• LRPROOF (context = 0xFF) is the link-establishment proof (§6.2). Different body, different signature input.
• RESOURCE_PRF (context = 0x05) is the proof for a completed Resource transfer (§10.8). Different body (resource_hash || full_proof), no signature.

6.5.1 Two body formats: explicit vs implicit

Regular PROOFs come in two wire forms (RNS/Packet.py:413-414):

EXPL_LENGTH = HASHLENGTH//8 + SIGLENGTH//8 = 32 + 64 = 96 bytes
IMPL_LENGTH = SIGLENGTH//8                 =      64 = 64 bytes

explicit body  =  packet_hash(32) || signature(64)
implicit body  =                     signature(64)

Where:

• packet_hash = Identity.full_hash(original_packet.get_hashable_part()) — the full SHA-256 (32 bytes, not truncated to 16) of the prove-target packet's hashable part. get_hashable_part is the same recipe used for link_id derivation in §6.3, so the proof binds to the version of the packet that survived any HEADER_1↔HEADER_2 conversion in transit (the high nibble of flags, hops byte, and any HEADER_2 transport_id are stripped before hashing).
• signature is the destination's (or link's) Ed25519 signature over packet_hash, NOT over the proof body itself. The signing key is the destination's long-term Ed25519 private key for an opportunistic DATA proof, or the link-derived signing key for a Link DATA proof.

The two forms are distinguished purely by length at the receiver. PacketReceipt.validate_proof (RNS/Packet.py:497-548) dispatches on len(proof) == 96 (explicit) vs len(proof) == 64 (implicit); lengths matching neither are rejected outright. There is no flag bit or context byte that signals which form is being used — wire length is the only signal.

6.5.2 Choosing which form to emit

Sender side, two distinct policies:

Opportunistic DATA addressed to a SINGLE destination — RNS.Identity.prove(packet, destination) at RNS/Identity.py:912-923:

def prove(self, packet, destination=None):
    signature = self.sign(packet.packet_hash)
    if RNS.Reticulum.should_use_implicit_proof():
        proof_data = signature                                  # 64 bytes
    else:
        proof_data = packet.packet_hash + signature             # 96 bytes
    proof = RNS.Packet(destination_or_proof_dest, proof_data,
                       RNS.Packet.PROOF, attached_interface=...)
    proof.send()

The default upstream value is Reticulum.__use_implicit_proof = True (RNS/Reticulum.py:259), so upstream emits the 64-byte implicit form by default. The 96-byte explicit form is only emitted when the operator's [reticulum] config sets use_implicit_proof = No. A clean-room implementation that hardcodes either single form will fail to interop with peers running the other one — receiver-side validators handle both, but a hardcoded sender writing the wrong length to the wire is not negotiable.

DATA on an active Link — RNS.Link.prove_packet(packet) at RNS/Link.py:383-394:

def prove_packet(self, packet):
    signature = self.sign(packet.packet_hash)
    proof_data = packet.packet_hash + signature                 # 96 bytes — always
    proof = RNS.Packet(self, proof_data, RNS.Packet.PROOF)
    proof.send()

with the upstream comment # TODO: Hardcoded as explicit proof for now. Link DATA proofs are always the 96-byte explicit form in RNS 1.2.0 regardless of the use_implicit_proof setting, and the matching validate_link_proof at RNS/Packet.py:449-494 has the implicit-form branch commented out with the same note. Today, Link DATA proofs are explicit-only on both ends; an implementation may match this behavior with a single hardcoded length on the link path, but should be ready to revisit if upstream re-enables the implicit branch (no fixed timeline).

6.5.3 Where the proof packet is addressed

The dest_hash position in the proof packet's outer header depends on which side of which transport the proven packet was on:

• Opportunistic DATA proof: dest_hash = packet_hash[:16] (the 16-byte truncation of the full SHA-256 of the proved packet's hashable part, used as a synthetic ProofDestination — RNS/Packet.py:390-396). The proof rides through Transport.outbound and follows the reverse path home via the receiver's reverse_table.
• Link DATA proof: dest_hash = link.link_id (the 16-byte link id, just like all other Link traffic; RNS/Packet.py:182-184 notes this position is filled by destination.link_id whenever the destination object is a Link). The proof rides on the link itself.

6.5.4 Wire summary

explicit form (96 bytes total body):
[ 1B flags ][ 1B hops ][ 16B dest_hash || link_id ][ 1B context=0x00 ]
[ 32B SHA256(get_hashable_part(original_packet)) ]
[ 64B Ed25519_signature(SHA256(get_hashable_part(original_packet))) ]

implicit form (64 bytes total body):
[ 1B flags ][ 1B hops ][ 16B dest_hash || link_id ][ 1B context=0x00 ]
[ 64B Ed25519_signature(SHA256(get_hashable_part(original_packet))) ]

Note context = 0x00 (NONE) in both cases — the proof-ness is conveyed by packet_type = PROOF (3) in the flag byte, not by a context. This is in contrast to LRPROOF (which uses context = 0xFF) and RESOURCE_PRF (which uses context = 0x05). The LINKPROOF (0xFD) context constant defined at RNS/Packet.py:90 is reserved but not actually used by either prove path in RNS 1.2.0.

6.5.5 Receiver tolerance

A new implementation's PROOF validator MUST accept both 64- and 96-byte bodies for opportunistic DATA proofs (per validate_proof's length-dispatch above) so it interops with peers running either policy. Hardcoding only one form at the validator silently fails on traffic from a peer with the opposite setting. Length-dispatch is also the only place the validator ever distinguishes the two — there is no "I want explicit" hint a sender can express.

A receiver that gets a PROOF whose length matches neither form treats it as malformed and returns False from validate_proof; no NACK is sent to the originator.

6.5.6 Why this matters for Link interop

After processing each NONE DATA packet on an active link, the receiver MUST emit the explicit-form PROOF described above. Without it, the sender's retransmit queue fires and the same packet arrives repeatedly, eventually exceeding the link's KEEPALIVE budget and tearing down the link. This is Packet.prove_packet upstream — non-optional for any client that wants to receive content over a Link without spamming the sender.

6.6 MTU and mode signalling (3-byte trailer on LINKREQUEST and LRPROOF)

The optional 3-byte signalling slot referenced in §6.1 and §6.2 carries two negotiated parameters in a single 24-bit big-endian packed integer: the link MTU (21 bits) and the link mode (3 bits, top of the high byte). When present, the signalling bytes are also included in the LRPROOF's signed_data, so the responder's signature commits to the negotiated values and a peer flipping a bit in transit invalidates the proof.

6.6.1 Wire layout

3 bytes total, big-endian. Byte 0 is split: top 3 bits are mode, low 5 bits are the most-significant 5 bits of the 21-bit mtu. Bytes 1 and 2 are the remaining 16 bits of mtu:

byte 0 :  M M M m m m m m       — top 3 bits = mode (0..7), low 5 bits = mtu[20..16]
byte 1 :  m m m m m m m m       — mtu[15..8]
byte 2 :  m m m m m m m m       — mtu[7..0]

Encoded by RNS/Link.py:147-151:

@staticmethod
def signalling_bytes(mtu, mode):
    if not mode in Link.ENABLED_MODES:
        raise TypeError(f"Requested link mode {Link.MODE_DESCRIPTIONS[mode]} not enabled")
    signalling_value = (mtu & Link.MTU_BYTEMASK) + (((mode << 5) & Link.MODE_BYTEMASK) << 16)
    return struct.pack(">I", signalling_value)[1:]    # big-endian uint32, drop top byte

with MTU_BYTEMASK = 0x1FFFFF (21 bits) and MODE_BYTEMASK = 0xE0 (top 3 bits of a byte).

Decoded mode and mtu (from mode_from_lr_packet line 171-176, mtu_from_lr_packet line 153-157):

mode = (signalling[0] & 0xE0) >> 5
mtu  = ((signalling[0] << 16) + (signalling[1] << 8) + signalling[2]) & 0x1FFFFF

The mtu decode trick: the full 24-bit value of all three bytes is masked with the 21-bit MTU_BYTEMASK, which strips the top 3 bits (i.e. the mode bits) without any explicit byte 0 masking step. Implementations that use (signalling[0] & 0x1F) << 16 | … instead get the same answer.

6.6.2 Mode field

3-bit value (0..7) at the top of byte 0. Defined values, with RNS/Link.py:125-142:

Mode	Name	Status in RNS 1.2.0	Derived key length
0x00	MODE_AES128_CBC	Defined, NOT enabled (sender-side will raise TypeError)	32 bytes
0x01	MODE_AES256_CBC	Default; the only enabled mode (ENABLED_MODES = [0x01])	64 bytes
0x02	MODE_AES256_GCM	Reserved, not enabled	—
0x03	MODE_OTP_RESERVED	Reserved, not enabled	—
0x04–0x07	MODE_PQ_RESERVED_*	Reserved for the post-quantum migration; not enabled	—

The derived_key_length at RNS/Link.py:358-360 is what the HKDF in §6.4 produces, split as signing_key(32) || encrypt_key(32) for the AES-256 path or signing_key(16) || encrypt_key(16) for the AES-128 path.

A receiver MUST tolerate seeing any 3-bit value in the mode field on inbound traffic — mode_from_lr_packet returns the raw integer without validating it against ENABLED_MODES. The mode is enforced at handshake time (Link.handshake at line 353-368): unknown / disabled modes raise TypeError and the link transitions to CLOSED rather than ACTIVE. Senders MUST NOT emit any mode value not in ENABLED_MODES — signalling_bytes() raises if you try.

A clean-room implementation today can safely hardcode mode = 0x01 on emit. On receive, it should accept 0x01 and reject the rest as "mode not supported by this implementation" rather than silently treating them as the default — a future RNS version that flips the default to 0x04 (one of the PQ slots) would render a hardcoded-default decoder ambiguous about whether the wire bytes mean "AES_256_CBC" or "the new default".

6.6.3 MTU field

21-bit unsigned integer in the low 21 bits of the 24-bit signalling value. Max representable: 0x1FFFFF = 2,097,151 bytes. Real Reticulum HW_MTU values are radically smaller (LoRa: 508; TCP: typical HW MTU ~64 KiB; AutoInterface: matches its bearer). The 21-bit width is forward-looking: it leaves room for future high-bandwidth interfaces without a wire-format change.

When the initiator emits a LINKREQUEST with signalling, the encoded mtu is the next-hop interface's HW_MTU (RNS/Link.py:309-314):

nh_hw_mtu = RNS.Transport.next_hop_interface_hw_mtu(destination.hash)
if RNS.Reticulum.link_mtu_discovery() and nh_hw_mtu:
    signalling_bytes = Link.signalling_bytes(nh_hw_mtu, self.mode)
else:
    signalling_bytes = Link.signalling_bytes(RNS.Reticulum.MTU, self.mode)

When the responder emits an LRPROOF with signalling, the encoded mtu is the min of its own next-hop view and what arrived in the LINKREQUEST, computed during validation (RNS/Transport.py:2042-2051):

path_mtu = Link.mtu_from_lr_packet(packet) or Reticulum.MTU
nh_mtu   = receiving_interface.HW_MTU if AUTOCONFIGURE_MTU/FIXED_MTU else Reticulum.MTU
if nh_mtu < path_mtu:
    path_mtu = nh_mtu
    clamped_signalling = Link.signalling_bytes(path_mtu, mode)
    packet.data = packet.data[:-LINK_MTU_SIZE] + clamped_signalling

The clamp is rewritten into the LINKREQUEST packet's data buffer in place before that packet enters the responder's Destination.receive path, so the responder's eventual LRPROOF carries the clamped value, not the originally-requested one. The clamp also affects link_id derivation: link_id_from_lr_packet strips trailing signalling bytes before hashing (per §6.3), so this in-place rewrite doesn't change the link_id even though it does change the wire bytes.

The initiator reads confirmed_mtu back via mtu_from_lp_packet during LRPROOF validation (RNS/Link.py:404-408), accepts it as link.mtu, and the link's mdu (max data unit per packet for §3.1 link-derived Token traffic) is recomputed via update_mdu().

6.6.4 Presence detection — length only

Both directions detect the optional signalling slot purely by packet body length:

Packet	Body length without signalling	Body length with signalling
LINKREQUEST	ECPUBSIZE = 64	ECPUBSIZE + LINK_MTU_SIZE = 67
LRPROOF	SIGLENGTH//8 + ECPUBSIZE//2 = 96	... + LINK_MTU_SIZE = 99

Where ECPUBSIZE = 64 is the combined initiator ephemeral X25519 + Ed25519 public key (Link.py:70), and SIGLENGTH//8 = 64 is the responder's Ed25519 signature.

Receivers MUST handle both forms. validate_request at RNS/Link.py:186-190 checks len(data) == ECPUBSIZE OR len(data) == ECPUBSIZE+LINK_MTU_SIZE and rejects anything else. The same length-dispatch is in validate_proof for the LRPROOF side at RNS/Link.py:404-410. There is no flag bit signalling presence — wire length is the only signal.

6.6.5 Inclusion in LRPROOF signed_data

Per §6.2, the LRPROOF's signed_data when signalling is present is:

signed_data = link_id || responder_X25519_pub || responder_long_term_Ed25519_pub || signalling

A clean-room implementation that omits the signalling bytes when present (or includes them when absent) computes a different signed_data than the responder did, fails signature validation, and the link never establishes. This is the most common interop break in this area; cross-check against RNS/Link.py:373 (signer) and :417 (validator).

6.6.6 Disabling MTU discovery

The [reticulum] config option link_mtu_discovery = No makes Reticulum.link_mtu_discovery() return False, so the initiator skips signalling on outbound LINKREQUESTs (RNS/Link.py:311-314). In that case the link uses Reticulum.MTU (default 500 bytes) globally, no per-link MTU clamping happens, and all four lengths fall back to the no-signalling sizes in §6.6.4.

A receiver doesn't need its own copy of the disable switch — it just stops seeing trailing signalling bytes from peers that have it disabled. Its own MTU reporting on the LRPROOF return path runs unaffected for peers that send it.

6.7 KEEPALIVE and link teardown

A Link goes through five states (RNS/Link.py:110-114): PENDING → HANDSHAKE → ACTIVE → STALE → CLOSED. KEEPALIVE and LINKCLOSE are the two control-plane packet types that drive transitions out of ACTIVE.

6.7.1 KEEPALIVE (context = 0xFA)

Cadence (RNS/Link.py:844-846):

def __update_keepalive(self):
    self.keepalive = max(min(self.rtt * (KEEPALIVE_MAX / KEEPALIVE_MAX_RTT), KEEPALIVE_MAX), KEEPALIVE_MIN)
    self.stale_time = self.keepalive * STALE_FACTOR

with constants KEEPALIVE_MAX = 360s, KEEPALIVE_MIN = 5s, KEEPALIVE_MAX_RTT = 1.75s, STALE_FACTOR = 2. The interval is RTT × 205.7 clamped to [5, 360] seconds. Before the first RTT is measured (set in validate_proof), the link uses KEEPALIVE = KEEPALIVE_MAX = 360s.

The watchdog (Link.__watchdog_job, line 751-821) fires on every active link. When now >= last_inbound + keepalive AND the local node is the initiator, it emits a KEEPALIVE:

def send_keepalive(self):
    keepalive_packet = RNS.Packet(self, bytes([0xFF]), context=RNS.Packet.KEEPALIVE)
    keepalive_packet.send()

Body is a single byte 0xFF — the "ping" sentinel. The packet is Token-encrypted with the link's session key per §3.1 link-derived form, so the wire body is iv(16) || ciphertext(...) || hmac(32); the decrypted plaintext is just b'\xff'.

The responder receives this in Link.receive at RNS/Link.py:1149-1153 and answers with the "pong" sentinel:

elif packet.context == RNS.Packet.KEEPALIVE:
    if not self.initiator and packet.data == bytes([0xFF]):
        keepalive_packet = RNS.Packet(self, bytes([0xFE]), context=RNS.Packet.KEEPALIVE)
        keepalive_packet.send()

So:

• Ping = initiator → responder, body 0xFF.
• Pong = responder → initiator, body 0xFE.
• Only the initiator originates KEEPALIVE traffic. The responder never spontaneously pings.

Both sentinel bytes are arbitrary; what actually matters for keep-alive purposes is that any inbound traffic on the link refreshes last_inbound (the watchdog's anchor for staleness decisions). KEEPALIVE packets, like all link DATA, also generate the mandatory PROOF receipt per §6.5, which is itself inbound traffic on the return path. So a successful ping/pong exchange resets the staleness clock on both sides via three round-trip artifacts: ping → pong → pong-proof.

A clean-room responder MUST emit the pong on inbound 0xFF; without it the initiator's watchdog will declare the link stale on the next cycle.

6.7.2 STALE → CLOSED transition

When now >= last_inbound + stale_time (= 2 × keepalive), the watchdog moves the link from ACTIVE to STALE (line 796-800), then on its next pass emits a teardown packet and transitions to CLOSED (line 805-810):

elif self.status == Link.STALE:
    sleep_time = 0.001
    self.__teardown_packet()                  # see §6.7.3
    self.status = Link.CLOSED
    self.teardown_reason = Link.TIMEOUT
    self.link_closed()

teardown_reason is set to Link.TIMEOUT (constant value 0x01) so the application's link_closed_callback can distinguish "the peer went dark" from "the peer cleanly closed".

There is also an explicit-cleanup path: after a STALE-induced teardown the watchdog adds a final grace period of RTT × KEEPALIVE_TIMEOUT_FACTOR + STALE_GRACE (= RTT × 4 + 5s) at line 797 to allow a delayed reply to bring the link back into ACTIVE before final teardown — but in upstream RNS 1.2.0 the STALE → CLOSED transition runs immediately on the next watchdog pass without consulting that grace period. The grace constant lives in case a future revision restores the soft-stale window.

6.7.3 LINKCLOSE (context = 0xFC)

Either side can cleanly tear down a link by calling Link.teardown() (line 699-708), which sends a single LINKCLOSE packet and transitions the local state to CLOSED:

def __teardown_packet(self):
    teardown_packet = RNS.Packet(self, self.link_id, context=RNS.Packet.LINKCLOSE)
    teardown_packet.send()

Wire form:

• packet_type = DATA (0), context = 0xFC, dest_hash = link_id.
• Body is the 16-byte link_id, Token-encrypted by the link's session key.

The peer's receiver path at RNS/Link.py:1061-1063 calls teardown_packet(packet) (line 710-722):

def teardown_packet(self, packet):
    plaintext = self.decrypt(packet.data)
    if plaintext == self.link_id:                # auth check
        self.status = Link.CLOSED
        if self.initiator:
            self.teardown_reason = Link.DESTINATION_CLOSED
        else:
            self.teardown_reason = Link.INITIATOR_CLOSED
        self.link_closed()

The body's plaintext MUST equal link_id for the close to take effect — this is the on-link auth check. A peer that doesn't share the session key can't decrypt the body, and even if it could, the link_id check rejects bodies with arbitrary content. Combined with the Token HMAC, this gives both "encrypted" and "authenticated" guarantees on the teardown signal.

After link_closed() (line 724-743) runs:

• All incoming_resources and outgoing_resources are cancelled (cancels propagate into the §10 Resource state machine).
• The Link's session keys (self.shared_key, self.derived_key) are zeroed by reassignment to None — the upstream comment at line 700-702 notes this is the forward-secrecy property: "encryption keys are purged. New keys will be used if a new link to the same destination is established."
• The link_closed_callback registered via set_link_closed_callback fires.
• The Link is removed from its destination's links list (responders only — initiators don't have a destination-list entry).

6.7.4 Teardown reason codes

Link.teardown_reason is set to one of (RNS/Link.py:116-118):

Constant	Hex	Meaning
TIMEOUT	0x01	Watchdog STALE → CLOSED transition. No LINKCLOSE was received.
INITIATOR_CLOSED	0x02	This side is the responder; the initiator sent a LINKCLOSE.
DESTINATION_CLOSED	0x03	This side is the initiator; the responder sent a LINKCLOSE.

These are local-state values, not on the wire — the LINKCLOSE packet itself doesn't carry a reason code. The recipient just infers whether the close came from the other side based on whether they're initiator or responder.

6.7.5 Receiver responsibilities (minimum)

For a clean-room implementation that wants links to survive idle periods longer than a few seconds:

1. Keep a per-link last_inbound timestamp updated on every inbound packet on the link (DATA, PROOF, KEEPALIVE — anything).
2. On the initiator side, run a watchdog that emits a 0xFF KEEPALIVE every link.keepalive seconds since last_inbound. Default link.keepalive = 360s is fine until you measure RTT.
3. On the responder side, reply to every 0xFF KEEPALIVE with a 0xFE KEEPALIVE. Don't originate.
4. On both sides, transition to CLOSED if last_inbound + 2*keepalive elapses with no traffic, AND emit a LINKCLOSE packet so the peer doesn't have to wait for its own watchdog to time out.
5. On every inbound LINKCLOSE, decrypt, verify body equals link_id, transition to CLOSED.
6. On CLOSED, zero the session keys and cancel any in-progress Resources.

6.8 Source

RNS/Link.py, RNS/Packet.py::prove, RNS/Identity.py::prove, RNS/PacketReceipt.py::validate_proof. The webclient's reference/js-reference/link.js is a faithful port.

---

7. Transport behavior — the parts that bite

7.1 Path requests: peers send path? before opportunistic LXMF when no path is known

The path-request preamble in upstream LXMF is conditional, not unconditional (verified by tools/verify_path_request.py against LXMF 0.9.6):

# LXMF/LXMRouter.py::handle_outbound, ~line 1672
if not RNS.Transport.has_path(destination_hash) and lxmessage.method == LXMessage.OPPORTUNISTIC:
    RNS.log("Pre-emptively requesting unknown path for opportunistic ...", RNS.LOG_DEBUG)
    RNS.Transport.request_path(destination_hash)
    lxmessage.next_delivery_attempt = time.time() + LXMRouter.PATH_REQUEST_WAIT

In other words: a path? is sent before the LXM only when no entry exists in Transport.path_table for the target — has_path() is just a key-presence check (RNS/Transport.py:2570-2576). Existing-but-stale path entries are NOT replaced by this preamble; LXMF instead leans on the periodic Transport.jobs cycle to evict expired path entries (stale_paths accumulator at RNS/Transport.py:747+), after which the next outbound LXM rediscovers the unknown-path branch and triggers the request_path. A second request_path is issued from the retry path (LXMRouter.py:2571+) once lxmessage.delivery_attempts >= MAX_PATHLESS_TRIES, so on a flaky path peers can see multiple path? retransmits without intervening DATA — that matches BLE-trace observations.

A path? request itself is a regular DATA packet (verified by tools/verify_path_request.py):

• dest_hash = SHA256(SHA256("rnstransport.path.request")[:10])[:16] = 6b9f66014d9853faab220fba47d02761
• dest_type = PLAIN, transport_type = BROADCAST, header_type = HEADER_1, context = CTX_NONE
• payload (RNS/Transport.py::request_path):
  • leaf clients (transport disabled): target_dest_hash(16) || random_tag(16) — 32 bytes
  • transport-enabled originators: target_dest_hash(16) || transport_id(16) || random_tag(16) — 48 bytes — so the responding announce can be routed back along the request's reverse path

7.2 Responding to path requests

Every node — including non-transport leaf clients — that knows the requested target MUST respond by re-announcing. This is the only way the requester learns a path back. If you implement only the "send a path request" half but not the "respond to incoming requests for our own destination" half, peers can never message you after the path expires (typically within minutes after your last announce).

7.2.1 Path-request packet parse rules

The path-request handler at RNS/Transport.py:2800-2843 parses inbound packets addressed to path_request_destination (the dest_hash in §7.1). The handler is registered as the destination's packet_callback at Transport.py:237-240, so any DATA packet to that dest_hash flows through it.

def path_request_handler(data, packet):
    if len(data) >= 16:
        destination_hash = data[:16]                                  # mandatory 16B target
        if len(data) > 32:
            requesting_transport_instance = data[16:32]               # optional 16B transport_id
        else:
            requesting_transport_instance = None

        # tag bytes — required, anything past the fixed prefix
        tag_bytes = data[32:] if len(data) > 32 else (data[16:] if len(data) > 16 else None)
        if tag_bytes is None:                                         # tagless requests are dropped
            return
        if len(tag_bytes) > 16:
            tag_bytes = tag_bytes[:16]                                # cap to 16B

Three observations that matter for interop:

1. Tagless requests are dropped. A path? packet with exactly 16 bytes payload (just target_dest_hash, no tag) is logged at DEBUG level and discarded. The tag is what makes the request unique enough to dedup — without it, a relay would loop forever on retransmits of the same packet. A clean-room implementation MUST emit at least one tag byte; the upstream emitter (RNS.Transport.request_path) uses 16 random bytes.
2. The transport_id field is optional and detected by length. If the payload is exactly 32 bytes the second 16B slot is the tag; if it's >32 bytes the second 16B is transport_id and the rest is the tag. This is consistent with the §7.1 description (leaf: 32B; transport: 48B) but the boundary case len == 32 lands in the leaf-client interpretation.
3. The tag is capped at 16 bytes. Any tail beyond that is silently truncated. Senders may emit longer tags but receivers normalize to 16B for dedup table keys.

7.2.2 Tag-based deduplication

The handler builds unique_tag = destination_hash || tag_bytes and consults Transport.discovery_pr_tags (Transport.py:2829-2839):

unique_tag = destination_hash + tag_bytes

with Transport.discovery_pr_tags_lock:
    if not unique_tag in Transport.discovery_pr_tags:
        Transport.discovery_pr_tags.append(unique_tag)
        Transport.path_request(destination_hash,
                               from_local_client(packet),
                               packet.receiving_interface,
                               requestor_transport_id=requesting_transport_instance,
                               tag=tag_bytes)
    else:
        # ignore duplicate path request

discovery_pr_tags is bounded at Transport.max_pr_tags = 32000 entries (Transport.py:126); older entries are aged out by the periodic Transport.jobs cycle. Every node — leaf or transport — that wants to respond to path requests MUST maintain this dedup table or it will respond to every retransmit, and a transport-enabled node will additionally re-forward to all other interfaces, generating a broadcast storm.

The unique_tag = dest_hash || tag format means the same tag bytes against different destination_hashes are distinct — so two different requesters racing for the same target with happenstance-identical random tags don't suppress each other. Senders MUST use a fresh random tag per fresh request (the upstream emitter calls Identity.get_random_hash()); reusing tags across requests for the same destination_hash makes the second request appear to be a duplicate.

7.2.3 The five-way dispatch in Transport.path_request

RNS/Transport.py:2846-2973. After dedup, the handler calls into path_request() which decides how to respond. Five mutually-exclusive branches in priority order:

1. destination_hash is local (i.e. it's one of our own registered destinations, line 2873-2875):

   local_destination.announce(path_response=True, tag=tag,
                              attached_interface=attached_interface)
   

   We answer by emitting a path-response announce (§7.2.4 below) on the interface the request arrived on. This is the only branch a leaf client must implement — the others are transport-mode behaviours.

2. Path is known via the path_table AND (transport_enabled OR is_from_local_client) (line 2877-2938): retrieve the cached announce packet from the path table, set its hops to the cached value, and queue it for retransmit. If the next hop happens to be the requestor itself (path-loop indicator), drop instead. This is the transport-mode path-resolver: a relay that already knows where the destination lives answers on its behalf, saving the requester from another hop of broadcast.

3. Request is from a local-client interface, no path known (line 2940-2947): forward the request to every OTHER interface so the broader mesh can answer. Generates a fresh random tag for the forwarded request to avoid loop-back through the same dedup table.

4. transport_enabled AND no path known AND interface allows discovery (line 2949-2963): record a discovery_path_requests entry (capped at PATH_REQUEST_TIMEOUT = 15s) and forward the request to every other interface, preserving the original tag to prevent loops. This is recursive transport-mode discovery — we don't know the destination but we'll go ask the rest of the mesh.

5. No path known and not transport-enabled (line 2972-2973): log "no path known" and drop. Leaf clients hit this branch when they receive a path? for someone else's destination.

Branch 1 is the only MUST for any node that wants to be reachable. Branches 2-4 are transport-node behaviours; a leaf client safely ignores them by never being in transport_enabled mode.

7.2.4 Path-response announce wire format

When branch 1 fires, Destination.announce(path_response=True, tag=tag, ...) runs. The wire bytes are identical to a regular announce (§4.1) except the outer Reticulum packet's context byte is set to PATH_RESPONSE = 0x0B instead of NONE = 0x00 (RNS/Destination.py:307-308):

if path_response: announce_context = RNS.Packet.PATH_RESPONSE
else:             announce_context = RNS.Packet.NONE

The body — public_key || name_hash || random_hash || [ratchet_pub] || signature || app_data — is built identically; the random_hash carries a fresh emission timestamp, the signature is computed over the same signed_data per §4.2. A receiver running the validation flow in §4.5 can't tell from the announce body that this is a response to a query rather than a periodic re-announce; only the context byte distinguishes them.

A tag argument hands a previously-built path-response announce body back unchanged when the same tag is requested twice within Destination.PR_TAG_WINDOW = 30s (RNS/Destination.py:260-278). This is what prevents a flood of identical path-response announces when several relays simultaneously forward the same path? request to a leaf — the leaf serves the cached body to all of them with the same wire bytes, lining up dedup decisions on every transit relay.

7.2.5 Timing: PATH_REQUEST_GRACE and roaming

When branch 2 fires (transit relay answering on behalf of a remote destination), the rebroadcast is delayed by PATH_REQUEST_GRACE = 0.4s (Transport.py:80, 2917) — extra grace to let directly-reachable peers respond first if they're in earshot. On MODE_ROAMING interfaces an additional PATH_REQUEST_RG = 1.5s is added on top (Transport.py:81, 2922-2923) so well-connected fixed nodes get a chance to answer before mobile ones.

Branch 1 (local destination answers) fires immediately with no grace, since the leaf is the authoritative source for its own destination — there's no point waiting for someone else to potentially answer faster.

Local-client originators also bypass the grace period (Transport.py:2909-2910): a relay answering for a destination that lives on a local-client interface can send back the cached announce instantly because the answer doesn't need to compete with peer-mesh announces.

7.2.6 Minimum responsibility for a leaf

The minimum path-request response logic for a non-transport leaf, in protocol terms:

1. Receive a DATA packet with dest_hash == 6b9f66014d9853faab220fba47d02761.
2. Parse target_dest_hash = data[:16] and tag_bytes = data[16:32] (or data[32:48] if len(data) > 32).
3. Drop if len(tag_bytes) == 0 (tagless requests).
4. Drop if (target_dest_hash, tag_bytes) already in the dedup table.
5. If target_dest_hash == our_destination_hash for any of our registered destinations: emit a path-response announce (§7.2.4) on the receiving interface, with the request's tag passed through to allow caching.
6. Otherwise: do nothing — leaves can't fulfill path requests for destinations they don't OWN.

Steps 4 and 5 are both required. Skipping the dedup table makes the leaf storm the network with redundant announces; skipping the local-destination check means peers can never message you after the path expires.

For a chronological walk-through of the full request → response → path-table cycle, see flows/path-discovery.md.

7.3 Ratchet rotation per announce

The 32-byte ratchet_pub field in announces is intended to rotate. Most transit nodes deduplicate announces on (destination_hash, ratchet_pub) tuples — if both are unchanged from a recent prior announce, the relay treats it as a duplicate and drops it instead of forwarding.

If your client generates one ratchet at identity creation and never rotates, every announce after the first one in a session is dropped at the first transit node. Your destination becomes invisible to the mesh.

Required behavior: generate a fresh X25519 keypair at the start of each sendAnnounce(), persist it (so subsequent sessions can decrypt messages still in flight to the previous ratchet — see also section 7.4), and use it for the announce body's ratchet_pub field.

The long-term encryption / signing keys and the identity_hash / destination_hash MUST stay stable across rotations. Otherwise contacts have to re-add you on every rotation.

7.4 Ratchet ring (inbound decrypt tolerance)

Senders cache the most recent ratchet they've seen for each destination. If you rotate your ratchet faster than relays propagate the announce, in-flight messages may arrive encrypted to your previous ratchet. To decrypt these, keep a ring of recent ratchet privkeys and try each in order during decrypt. The fallback to the long-term identity privkey is the ultimate safety net.

Upstream's default ring size is Destination.RATCHET_COUNT = 512 (RNS/Destination.py:85 in RNS 1.2.0), with a minimum rotation interval of RATCHET_INTERVAL = 30*60 seconds (line 90) and per-ratchet RATCHET_EXPIRY = 60*60*24*30 seconds (RNS/Identity.py:69). A new ratchet is generated on each rotate_ratchets() call and prepended to the in-memory list; _clean_ratchets truncates back to RATCHET_COUNT. The 512 figure is generous and not a hard interop requirement — it's an in-memory bound on the inbound-decrypt try-list.

A minimal client may keep just the current ratchet privkey, accepting that the brief window between rotation and announce-propagation will lose some messages. Mention the trade-off in your implementation notes.

7.5 Periodic re-announce

Transport node path tables expire entries after a few minutes. Clients should re-announce on a 5–15 minute cadence as a baseline so cached paths stay fresh. Without this, even peers who saw your initial announce will be unable to reach you after path TTLs lapse.

7.6 TCPServerInterface.OUT is True by default in practice

RNS/Interfaces/TCPInterface.py line 522 sets self.OUT = False in the constructor. This is overridden to True by RNS/Reticulum.py post-init at line 771-772 for any interface declared in the rnsd config:

if "outgoing" in c and c.as_bool("outgoing") == False: interface.OUT = False
else:                                                  interface.OUT = True

Spawned client interfaces (one per connecting TCP client) inherit OUT from their parent. So in practice, every TCPServerInterface CAN forward unless the operator explicitly opted out. Do not waste time chasing the constructor's OUT = False default; it doesn't hold post-init.

7.7 Source

RNS/Transport.py outbound, inbound, request_path, announce. RNS/Reticulum.py interface_post_init for the OUT-flag override.

---

8. Transport framing

8.1 KISS (BLE / serial / RNode link)

FEND  = 0xC0    // frame delimiter
FESC  = 0xDB    // escape
TFEND = 0xDC    // escaped FEND  → 0xDB 0xDC
TFESC = 0xDD    // escaped FESC  → 0xDB 0xDD

frame = FEND || cmd_byte || escaped(data) || FEND

cmd_byte for received/transmitted Reticulum packets is CMD_DATA = 0x00. RNode firmware prefixes each received CMD_DATA frame with CMD_STAT_RSSI = 0x23 (one byte payload, signed value = byte − 157) and CMD_STAT_SNR = 0x24 (one byte payload, signed Q6.2 → divide by 4 for dB).

Over BLE, KISS frames are split across BLE notifications. A streaming parser MUST accumulate bytes across notifications and emit complete frames only on FEND boundaries.

8.2 HDLC (TCP / rnsd TCPServerInterface)

FLAG = 0x7E
ESC  = 0x7D
ESC_MASK = 0x20

frame = FLAG || escaped(data) || FLAG
escape: 0x7E → 0x7D 0x5E   (FLAG ^ ESC_MASK)
        0x7D → 0x7D 0x5D   (ESC  ^ ESC_MASK)

No command byte, no RSSI/SNR sidecar — the HDLC payload IS the raw Reticulum packet. Source: RNS/Interfaces/TCPInterface.py::HDLC.

8.3 RNode air-frame header and split-packet protocol

The 1-byte header described here lives between RNodes on the LoRa air-frame, not on the KISS host channel. The upstream RNode firmware adds it on every TX and strips it on every RX before forwarding the payload to the host as CMD_DATA. KISS hosts (RNS, NomadNet, Sideband, etc.) NEVER see this byte. Two RNodes that talk LoRa to each other use it to glue two LoRa frames into one Reticulum packet of up to 508 bytes; an alternative implementation that talks LoRa to an RNode (e.g. a clean-room repeater firmware) MUST construct and parse this header bit-exactly, or its TX will be invisible and its RX will mistake the header byte for the first payload byte.

Header byte layout

From markqvist/RNode_Firmware/Framing.h:105-108:

bit 7..4 : seq         (NIBBLE_SEQ   = 0xF0) — random sequence id, set on each TX
bit 3..1 : reserved    (currently always 0)
bit 0    : FLAG_SPLIT  (NIBBLE_FLAGS = 0x0F, FLAG_SPLIT = 0x01)
SEQ_UNSET = 0xFF                            — sentinel: "no first half buffered"

Helpers (Utilities.h:1218-1224):

inline bool    isSplitPacket(uint8_t h) { return (h & FLAG_SPLIT); }   // 0x01 mask
inline uint8_t packetSequence(uint8_t h){ return h >> 4; }             // 0..15

Constants (Config.h:59-61):

#define MTU         508    // max reassembled Reticulum packet payload (2 × 254)
#define SINGLE_MTU  255    // max LoRa frame size (header + up to 254 payload bytes)
#define HEADER_L    1      // header overhead per LoRa frame

Transmit (RNode_Firmware.ino:716-742)

uint8_t header = random(256) & 0xF0;                      // fresh random seq nibble
if (size > SINGLE_MTU - HEADER_L) header |= FLAG_SPLIT;   // split iff payload > 254
LoRa->beginPacket();
LoRa->write(header);
for (i=0; i < size; i++) {
    LoRa->write(tbuf[i]);
    if (written == 255 && isSplitPacket(header)) {        // first frame full
        LoRa->endPacket();
        LoRa->beginPacket();
        LoRa->write(header);                              // SAME header byte on frame 2
        written = 1;
    }
}
LoRa->endPacket();

Behavioral facts that matter for interop:

1. Sequence nibble is randomized on every fresh TX, not incremented. Two consecutive split packets from the same node will have different (random) seq nibbles. This is the trick a memory-fading reader might recall as "the header rotates between transmissions" — it's per-packet randomization, not a per-retransmit byte rotation. There is no retransmit-driven byte rotation or rechunk; LoRa transmission is fire-and-forget at this layer, and a higher-layer retransmit (e.g. an RNS PROOF timeout firing again) just re-enters this function and gets a fresh random seq nibble.
2. Both frames of a split share the same header byte byte-for-byte — same seq nibble, same FLAG_SPLIT bit. The receiver pairs them by exact equality of the seq nibble.
3. The split point is at exactly 255 bytes total in the LoRa frame (1 header + 254 payload). The second frame is header || remainder, where remainder is whatever is left after 254 bytes of payload have been emitted. Maximum reassembled packet payload is 2 × 254 = 508 bytes — Reticulum's HW_MTU for the RNode interface is set to match.
4. Single-frame packets (payload ≤ 254) still carry the 1-byte header but with FLAG_SPLIT == 0. The seq nibble is still random per TX.

Receive / reassembly (RNode_Firmware.ino:359-446)

State on the receiver: seq (default SEQ_UNSET = 0xFF) tracks the seq nibble of any buffered first-half. Per inbound LoRa frame:

Inbound FLAG_SPLIT	Buffered seq state	Inbound seq	Action
1	SEQ_UNSET (none)	any	Buffer this frame as the first half. Store its seq.
1	matches inbound seq	== buffered	Append. Reassembly complete. Reset buffer.
1	doesn't match	!= buffered	Discard buffered first-half. Replace with this frame as a new first-half.
0	SEQ_UNSET (none)	n/a	Deliver this single-frame packet directly.
0	first-half present	n/a	Discard the buffered first-half; deliver this single-frame packet.

In other words: the receiver holds at most one in-progress first-half, keyed by its random seq nibble. Any inbound frame that doesn't match (different seq, or non-split, or simply a long enough silence) replaces or discards it.

Reassembly timeout — implementation-defined

Upstream RNode firmware does not have an explicit time-based timeout for a buffered first-half — it relies on subsequent traffic (any inbound frame) to clear stale state via the table above. The clean-room repeater at thatSFguy/reticulum-lora-repeater/src/Radio.cpp:189-194 adds a defensive 500 ms timeout: if no second half arrives within that window, the buffered first-half is discarded. This is implementation-private: a packet that takes longer than 500 ms to fully transmit (very low SF + large payload) would be lost on a repeater following the clean-room timeout but would survive against an unbounded upstream RNode receiver as long as no other LoRa traffic landed in between.

A new alternative implementation should either match upstream's "no explicit timeout" or pick a value tied to the worst-case airtime of two SINGLE_MTU frames at the configured SF/BW, not a flat 500 ms.

Sequence-collision airtime ceiling

Because the seq nibble is 4 bits of randomness chosen per TX, two unrelated split packets from the same sender that overlap in time at any receiver will collide with probability 1/16 per pair. At sane LoRa duty cycles this is a non-issue, but it bounds the protocol — a sender that emits split packets back-to-back faster than the air can ferry them risks a reassembled packet that mixes halves of two distinct senders' outputs. The receiver has no way to detect this short of validating the resulting Reticulum packet (which a corrupt mix would fail at the HMAC step). Don't burst.

Source map

File	What it pins down
RNode_Firmware/Framing.h:105-108	NIBBLE_SEQ, NIBBLE_FLAGS, FLAG_SPLIT, SEQ_UNSET constants
RNode_Firmware/Config.h:59-61	MTU, SINGLE_MTU, HEADER_L
RNode_Firmware/Utilities.h:1218-1224	isSplitPacket, packetSequence accessors
RNode_Firmware/RNode_Firmware.ino:716-742	TX-side header construction and split logic
RNode_Firmware/RNode_Firmware.ino:359-446	RX-side reassembly state machine
reticulum-lora-repeater/src/Radio.cpp:35-45, 188-316, 351-405	Clean-room reimplementation; adds 500 ms reassembly timeout

---

9. Implementation gotchas

The findings here cost the most debugging hours per insight ratio. They're not in the upstream manual.

9.1 LXMF source_hash is the destination hash, not the identity hash

The 16-byte source_hash field in an LXMF body is the sender's destination hash (SHA256(name_hash || identity_hash)[:16]), NOT the raw 16-byte identity hash. Sending the identity hash here means the recipient can't look you up in their contacts (which are keyed by destination hash) and the conversation gets orphaned.

9.2 Web Crypto and JCA AES-CBC auto-pad PKCS#7 — do not pad manually

Both browser window.crypto.subtle.encrypt({name:"AES-CBC", iv}, key, plaintext) and JCA's Cipher.getInstance("AES/CBC/PKCS5Padding") apply PKCS#7 padding automatically. Manually padding before calling them produces double-padded ciphertext (16 garbage bytes added) that decrypts to plaintext + a trailing PKCS#7 block which the receiver can't strip cleanly.

9.3 RNS bundles umsgpack — encode display names as bytes, not str

RNS/vendor/umsgpack.py is locked to behaviors regardless of system msgpack:

• _pack_string (Python str) → 0xa0|len/0xd9/0xda/0xdb (fixstr/str8/str16/str32)
• _pack_binary (Python bytes) → 0xc4/0xc5/0xc6 (bin8/bin16/bin32)
• _unpack_string decodes to Python str via bytes.decode("utf-8")
• _unpack_binary returns raw Python bytes

The downstream parser at LXMF/LXMF.py:131 does dn.decode("utf-8") on the unpacked first element. This works only when dn is bytes. If a producer wrote a str-encoded name (fixstr), umsgpack returns Python str, .decode() raises AttributeError, the parser swallows it and returns None → no display name.

Implementation rule: encode the display name field as msgpack bin (Python bytes equivalent), never str. Upstream LXMRouter does this correctly via display_name.encode("utf-8") before packing.

9.4 Display name preservation across re-announces

Inbound announce ingestion code that uses

new_name = extracted ?? known_label ?? ""
merged   = (new_name).ifBlank { existing.name ?? "" }

clobbers a real cached name with the placeholder known_label (e.g. "LXMF delivery") whenever a minimal re-announce arrives without app_data. The next full announce restores it. Symptom: contacts blink to placeholder names briefly during/after activity.

Correct priority order: extracted ?? existing ?? known_label ?? "". The known label fallback is for completely unknown destinations only.

9.5 Self-announce echo

If the operator runs both an originating client and a transport node on the same machine (or the same RNode loops back its own emissions), a client will receive its own announce and may add itself to the contact list. Filter announces whose dest_hash == our_dest_hash before ingestion.

9.6 Clockless sender timestamps

LoRa devices without an RTC will populate the LXMF timestamp field with seconds-since-boot (small integers like 30, 90720). Treat any timestamp before 2020-01-01 (1577836800) as "no clock" and substitute the local receive time. Otherwise messages from clockless devices appear at January 1 1970 in the inbox.

9.7 Periodic re-announce is non-optional

Even after a successful initial announce, paths in the mesh expire within minutes. Without a 5–15 minute re-announce loop, the second message any peer tries to send you will fail because the relay's path table has aged out. (See also §7.5.)

9.8 The destination hash uses the bare app-name string

An earlier-vintage bug in several implementations was to include the identity's hex hash in the name_hash input. expand_name in upstream Python takes an identity parameter and conditionally appends the identity hex IF the identity is non-None — but the Destination construction path passes identity = None. The name_hash MUST be SHA256(plain_app_name_string)[:10], nothing more. (See also §1.2.)

9.9 Diagnostic: rx-log every inbound packet at the engine entry

A single line of the form

rx <size>B H<1|2> <PT> dest=<hex> ctx=0x<hex> hops=<n>

logged before any filtering converts hours of "messages aren't arriving" debugging to seconds. Without it, packets dropped by if (dest != ours) return vanish silently and look identical to "the bytes never arrived". Symmetric tx logging on outbound is similarly cheap insurance.

9.10 microReticulum random_hash lacks the timestamp half

Real interop bug to plan around: attermann/microReticulum's Destination::announce emits 10 fully-random bytes for the announce random_hash field rather than the upstream Python form of 5 random bytes || big-endian uint40 unix_seconds (see §4.1). The Python form is preserved as a comment in the C++ source with a TODO add in time to random hash next to it; the timestamp half was never implemented.

Effect on a mixed-vendor mesh: a Python RNS receiver parses random_hash[5:10] of a microReticulum announce as a far-future timestamp (median ~year 19403 AD because the random uint40 is uniformly distributed across 0..2^40-1). The path-table replacement rule at RNS/Transport.py:1721-1745 rejects subsequent real-timestamped announces from Python sources as "stale" until the path TTL expires.

Symptom: a microReticulum repeater works fine when it's the only path; in a mesh that also has Python relays, paths "stick" to the microReticulum side even when shorter / fresher Python paths come up, until natural TTL expiry. First-contact path-table population is unaffected — the bug only surfaces on path replacement.

Workarounds when building a clean-room implementation that talks to a microReticulum mesh:

• Emit the upstream form yourself (you have a clock — even seconds-since-boot is preferable to random bytes; the path-table comparison only cares about ordering, not absolute time).
• If you receive a uint40 timestamp that's more than, say, 24 hours in the future, treat it as suspect — but be cautious because legitimate Python senders with skewed clocks could trip this.

The repeater repo's pre_build.py patches several other microReticulum protocol bugs (ratchet announce parsing, identity hash length 16→32, DATA/PROOF forwarding) but does not patch this one. Filing an upstream issue against attermann/microReticulum to land the original Python timestamp form is the durable fix.

---

10. Resource fragmentation protocol

A Resource transfers a payload that exceeds the per-packet content limit of an established Reticulum Link. It is the only way to carry an LXMF body, NomadNet page, or file larger than ~360 bytes (LINK_PACKET_MAX_CONTENT) over a Link. Resource is built on top of an active Link — it relies on the Link's session key for encryption (§3.1 link-derived form) and on the Link's bidirectional DATA channel for control traffic.

The complete reference is RNS/Resource.py (1383 lines in RNS 1.2.0); RNS/Packet.py:72-78 defines the context constants. This section describes the wire-level invariants a clean-room implementation must respect; many implementation choices (window sizing heuristics, watchdog timers, EIFR computation) are private and listed only when their absence would cause an interop break.

10.1 When Resource runs

Three triggers in upstream:

1. LXMessage.send() for DIRECT method with representation == RESOURCE. Set automatically when the encrypted-form LXMF body exceeds LINK_PACKET_MAX_CONTENT (LXMF/LXMessage.py:415-421).
2. NomadNet page request fulfillment — a server returning a page whose body exceeds the link MTU.
3. Direct file transfers via rncp and similar utilities.

10.2 Initiator-side preparation

Given input data and an RNS.Link in ACTIVE state (RNS/Resource.py:248-478):

1. Optional metadata prefix. If the caller supplied a metadata dict, msgpack-pack it and prepend length(3 bytes, big-endian uint24) || packed_metadata to the body. The has_metadata (x) flag in the advertisement signals this. Receivers strip the prefix during reassembly (line 699-707).
2. Optional bz2 compression. If auto_compress is true and the data fits within auto_compress_limit (default 64 MiB), the body is bz2-compressed and the compressed (c) flag is set. If compression doesn't shrink the data, the uncompressed form is sent and c is cleared.
3. Random hash prefix. A 4-byte (Resource.RANDOM_HASH_SIZE) random hash is prepended to the (compressed-or-not) body. This is the r field in the advertisement and is part of the input to hash and expected_proof.
4. Link encryption. The full random_hash || (compressed?) data blob is encrypted using link.encrypt(...) — i.e. the link-derived Token form (§3.1), no ephemeral_pub prefix. The encrypted (e) flag is set.
5. Hash and proof material.
   • data_with_random = random_hash || (compressed?) plaintext
   • hash = SHA256(data_with_random || random_hash) (32 bytes)
   • truncated_hash = hash[:16]
   • expected_proof = SHA256(data_with_random || hash) (32 bytes) — what the receiver will eventually return in the RESOURCE_PRF packet.
6. Part split. The encrypted body is sliced into parts of size SDU = link.mtu - HEADER_MAXSIZE - IFAC_MIN_SIZE. Each part becomes a packed RNS.Packet(link, part_data, context=RESOURCE); the packed wire bytes are stored in parts[i] for later sending.
7. Hashmap. Each part is fingerprinted to MAPHASH_LEN = 4 bytes. The full hashmap is b"".join(map_hashes). Hash collisions within the COLLISION_GUARD_SIZE = 2 × WINDOW_MAX + HASHMAP_MAX_LEN window are detected at construction time — if two parts hash to the same 4-byte map_hash within that window, the random hash is regenerated and the whole hashmap is recomputed. Without this guard, the receiver can't disambiguate which part it just received from a part-request that named a colliding map_hash.

After preparation: total_parts = ceil(size / SDU); total_size includes metadata; total_segments = ceil(total_size / MAX_EFFICIENT_SIZE) where MAX_EFFICIENT_SIZE = 1 MiB - 1 = 1_048_575.

10.3 Wire packet contexts used during a Resource transfer

All of these are sent on the established Link and use the Link's session key for encryption (or are unencrypted PROOF-type, depending on context):

Context	Direction	Type	Body
RESOURCE_ADV (0x02)	initiator → receiver	DATA	msgpack dict (§10.4)
RESOURCE (0x01)	initiator → receiver	DATA	one part of the encrypted body, raw
RESOURCE_REQ (0x03)	receiver → initiator	DATA	request bytes (§10.5)
RESOURCE_HMU (0x04)	initiator → receiver	DATA	hashmap continuation (§10.7)
RESOURCE_PRF (0x05)	receiver → initiator	PROOF	`resource_hash(32)
RESOURCE_ICL (0x06)	initiator → receiver	DATA	resource_hash(32) — initiator cancel
RESOURCE_RCL (0x07)	receiver → initiator	DATA	resource_hash(32) — receiver reject/cancel

10.4 RESOURCE_ADV — the advertisement

The first packet in the transfer. Body is umsgpack.packb(dict) with these keys (RNS/Resource.py:1336-1358):

Key	Type	Meaning
t	int	Transfer size — encrypted byte length on the wire
d	int	Data size — original uncompressed plaintext byte length
n	int	Number of parts in this segment
h	bytes(32)	Resource hash — `SHA256(data
r	bytes(4)	Random hash prefix
o	bytes(32)	Original hash of the first segment (= h if single-segment)
i	int	Segment index (1-based)
l	int	Total segments
q	bytes(?) or None	Request id if this Resource carries the response to a Link REQUEST
f	int	Flags byte (see below)
m	bytes	Hashmap fragment for THIS advertisement segment — up to HASHMAP_MAX_LEN = ⌊(LINK_MDU - 134)/4⌋ 4-byte map_hashes

The flags byte f packs six booleans (Resource.py:1310, 1377-1382):

bit 0 : e — encrypted
bit 1 : c — compressed
bit 2 : s — split (multi-segment)
bit 3 : u — is_request (this Resource is the body of a Link REQUEST)
bit 4 : p — is_response (this Resource is the body of a Link RESPONSE)
bit 5 : x — has_metadata

HASHMAP_MAX_LEN matters: the entire hashmap may not fit in one ADV. If n > HASHMAP_MAX_LEN, the receiver reconstructs subsequent map segments via RESOURCE_HMU packets after exhausting the first slice (§10.7).

The advertisement is sent once on Resource.advertise(); if no part requests arrive within the watchdog timeout, it is retransmitted up to MAX_ADV_RETRIES = 4 times before the resource is cancelled (Resource.py:573-590).

10.5 RESOURCE_REQ — receiver requests parts

Sent by the receiver to ask for a window's worth of specific parts (Resource.py:934-983). Body layout:

hashmap_exhausted_flag(1)  || [last_map_hash(4) if exhausted]
|| resource_hash(32)
|| requested_map_hashes(N × 4 bytes)

Where:

• hashmap_exhausted_flag is 0x00 (HASHMAP_IS_NOT_EXHAUSTED) if the receiver still has unrequested map_hashes from the most-recently-known hashmap segment, or 0xFF (HASHMAP_IS_EXHAUSTED) if it has consumed all of them and needs the next hashmap segment.
• If exhausted == 0xFF, the request continues with the last map_hash the receiver knows from the current segment (4 bytes). The sender uses this to determine which segment of the hashmap to send back via RESOURCE_HMU.
• resource_hash is the 32-byte h from the advertisement.
• The trailing requested_map_hashes is a concatenation of N × 4-byte map_hashes the receiver wants delivered. N is at most WINDOW (initial 4, dynamically grown — see §10.10).

Receivers who already have the part for a requested map_hash don't issue requests for it; the request is constructed only from parts[search_start:search_start+window] where parts[i] is None (Resource.py:944-960).

10.6 RESOURCE part packets

For each map_hash in a RESOURCE_REQ, the sender locates the matching pre-packed part within parts[receiver_min_consecutive_height : receiver_min_consecutive_height + COLLISION_GUARD_SIZE] and emits it as a regular Link DATA packet with context = RESOURCE (0x01) (Resource.py:1011-1023). The body is just the part's encrypted data — no metadata, no sequence number. The receiver matches the inbound part to its hashmap by recomputing its 4-byte map_hash and inserting it into parts[i] at the position where hashmap[i] matches (Resource.py:866-885).

Two interop traps:

1. Map_hashes are not guaranteed unique across the whole resource — only within COLLISION_GUARD_SIZE of any sliding-window position. A receiver that searches the entire hashmap for a matching part-hash can mis-place a part if two distant parts collide. The reference receiver searches only hashmap[consecutive_completed_height : consecutive_completed_height + window].
2. Parts are link-encrypted but otherwise opaque — the receiver has no way to validate a part beyond its 4-byte map_hash until the whole resource assembles and the SHA-256 over the reassembled data matches h.

10.7 RESOURCE_HMU — hashmap update

When the sender receives a RESOURCE_REQ with exhausted == 0xFF and a last_map_hash, it locates the position of last_map_hash in its full hashmap, advances to the next HASHMAP_MAX_LEN window, and emits the hashmap continuation (Resource.py:1030-1064):

body = resource_hash(32) || umsgpack.packb([segment_index(int), hashmap_segment_bytes])

The segment_index is part_index // HASHMAP_MAX_LEN. The receiver applies this with Resource.hashmap_update(segment, hashmap) to extend its known hashmap and continues issuing RESOURCE_REQ for the new range.

If the part_index doesn't land on a HASHMAP_MAX_LEN boundary, the sender treats it as a sequencing error and cancels the resource (Resource.py:1043-1046).

10.8 RESOURCE_PRF — final proof

When the receiver has assembled the full resource (received_count == total_parts), it runs assemble() (Resource.py:672-726):

1. Concatenate parts[0..n] to a single buffer.
2. link.decrypt(...) to plaintext.
3. Strip the 4-byte random_hash prefix.
4. If compressed: bz2-decompress.
5. Recompute SHA256(plaintext_with_random || random_hash) and compare to h.
6. If match: peel off metadata if x is set, write data to the destination; status = COMPLETE.
7. If mismatch: status = CORRUPT; cancel.

On COMPLETE, the receiver emits the proof:

proof_data = resource_hash(32) || full_proof(32)
where full_proof = SHA256(data_with_random || resource_hash)

sent as RNS.Packet(link, proof_data, packet_type=PROOF, context=RESOURCE_PRF) (Resource.py:755-766). The full_proof is exactly what the initiator pre-computed as expected_proof in §10.2 step 5 — it can validate the proof bytewise without re-running the SHA-256.

The initiator's validate_proof (Resource.py:785-824) checks proof_data[32:] == self.expected_proof and transitions status to COMPLETE. If the resource is multi-segment (s == True), the next segment's advertisement is sent immediately upon proof of the current segment.

10.9 RESOURCE_ICL / RESOURCE_RCL — cancellation

Either side can cancel; the body is just resource_hash(32):

• RESOURCE_ICL (0x06) — initiator cancel. Sent when the initiator decides to abort (e.g. the user kills the upload, the link MTU shrinks below the resource's pre-packed parts, the watchdog gives up after MAX_RETRIES = 16).
• RESOURCE_RCL (0x07) — receiver reject / cancel. Sent on advertisement reject (Resource.reject(adv_packet) at line 155-163, e.g. resource too large per app callback) or on receiver-side abort.

Either form transitions the resource to FAILED, releases the parts, and notifies the link's resource-concluded callback.

10.10 Sliding window and rate adaptation

The receiver controls request-pacing via a sliding window:

WINDOW          = 4    # initial outstanding requests
WINDOW_MIN      = 2
WINDOW_MAX_SLOW = 10   # default cap
WINDOW_MAX_FAST = 75   # cap once link is observed to be fast
WINDOW_MAX_VERY_SLOW = 4
WINDOW_FLEXIBILITY = 4

After each successful round (every requested part arrived), window += 1 up to window_max; window_min += 1 once window - window_min > WINDOW_FLEXIBILITY - 1 (Resource.py:902-906). The window cap is promoted to WINDOW_MAX_FAST after FAST_RATE_THRESHOLD consecutive rounds at observed throughput > RATE_FAST = 50 kbps / 8, and demoted to WINDOW_MAX_VERY_SLOW after VERY_SLOW_RATE_THRESHOLD = 2 rounds below RATE_VERY_SLOW = 2 kbps / 8 (Resource.py:917-927). These are receiver-private — they're not negotiated, so two implementations with different rate-detection cutoffs interop fine but may emerge with different effective throughput on the same channel.

10.11 Multi-segment resources

For payloads larger than MAX_EFFICIENT_SIZE = 1 MiB - 1, the resource is split into multiple segments at MAX_EFFICIENT_SIZE boundaries (Resource.py:299-314). Each segment is its own Resource with its own RESOURCE_ADV; the i (segment_index) and l (total_segments) fields disambiguate. The o (original_hash) field carries the first segment's h so the receiver can correlate segments belonging to the same logical transfer.

The sender doesn't pre-prepare every segment up front — it builds segment N+1 in __prepare_next_segment while segment N is still being delivered, and sends segment N+1's advertisement only after it has received the proof for segment N (Resource.py:768-783, 822-824). This caps memory usage; a 100 MiB transfer doesn't materialize 100 segments simultaneously.

The 3-byte big-endian uint24 metadata length encoding (§10.2 step 1) is what limits per-resource metadata to METADATA_MAX_SIZE = 16 MiB - 1.

10.12 Compression and encryption layering

Encryption layering is outermost — the wire bytes look like:

plaintext           = data_with_random || random_hash    # SHA-256 input
data_with_random    = random_hash(4) || maybe_compressed_body
maybe_compressed    = compressed_body iff `c` flag, else uncompressed
parts[i]            = link.encrypt( data_with_random[i*SDU : (i+1)*SDU] )

Critically, the link encryption is applied to the WHOLE concatenated data first, then sliced into parts — not to each part individually. This means part boundaries don't align with cipher block boundaries; a missing part can't be decrypted in isolation. The receiver must accumulate all parts before calling link.decrypt() (Resource.py:676-679).

This also means swapping in a new link session key mid-transfer would break decryption — the encryption happened with the link's key as it was when the resource was constructed.

10.13 Source map for §10

File	What it pins down
RNS/Resource.py:43-156	Class header, constants, state machine values, reject / accept
RNS/Resource.py:248-478	Resource.__init__ — preparation, hashmap construction, collision guard
RNS/Resource.py:520-596	__advertise_job, watchdog, advertisement retransmit
RNS/Resource.py:672-726	assemble — receiver reassembly, decrypt, decompress, hash-match
RNS/Resource.py:755-829	prove and validate_proof
RNS/Resource.py:831-932	receive_part — receiver-side part insertion + window adjust
RNS/Resource.py:934-983	request_next — receiver-side RESOURCE_REQ construction
RNS/Resource.py:985-1064	request — initiator-side fulfillment + RESOURCE_HMU emission
RNS/Resource.py:1237-1383	ResourceAdvertisement — pack/unpack of the ADV msgpack dict
RNS/Packet.py:72-78	RESOURCE_* context constants

---

11. Test vectors

See test-vectors/. Currently populated:

• identities.json — Alice and Bob private-key inputs plus their derived public_key, identity_hash, and lxmf.delivery destination_hash. Verified by tools/verify_destination_hash.py; regenerated by tools/regen_identities.py. Covers SPEC.md §1.1 and §1.2.

⚠️ UNVERIFIED: The remaining vector categories — signed announce packets, encrypted opportunistic LXMF DATA, and Link handshake (LINKREQUEST + LRPROOF + derived session keys) — are not yet populated. See agent.md §5 and todo.md for the remaining bootstrap work.

An implementation that round-trips every test vector — both directions — should be wire-compatible with upstream Reticulum and LXMF for the covered operations.

---

12. Source map

Upstream Python sources, in rough order of frequency-of-reference:

File	What lives here
RNS/Identity.py	Key generation, to_file/from_file, validate_announce, recall
RNS/Destination.py	expand_name, name_hash, destination hash construction
RNS/Packet.py	Header pack/unpack, packet types, contexts, prove
RNS/Transport.py	outbound, inbound, request_path, path table, HEADER_1↔2
RNS/Link.py	Link establishment, LRPROOF, session-key derivation
RNS/Cryptography/Token.py	The Fernet-style Token format
RNS/vendor/umsgpack.py	The bundled msgpack with locked bin/str semantics
RNS/Interfaces/TCPInterface.py	TCPClient/TCPServer, including HDLC framing
LXMF/LXMessage.py	LXMF body pack/unpack, opportunistic vs link methods
LXMF/LXMF.py	display_name_from_app_data, stamp_cost_from_app_data, etc.
LXMF/LXMRouter.py	Delivery destination registration, announce-app-data assembly

When upstream code changes such that this document drifts, please open a PR.