fips/docs/design/fips-wire-formats.md

FIPS Wire Formats

This document is the comprehensive wire format reference for all three protocol layers. It covers transport framing, link-layer message formats, and session-layer message formats, with an encapsulation walkthrough showing how application data is wrapped through each layer.

Encoding Rules

• All multi-byte integers are little-endian (LE)
• NodeAddr is 16 bytes — truncated SHA-256 hash of public key
• Signatures are 64 bytes — secp256k1 Schnorr
• Variable-length arrays use a 2-byte u16 LE count prefix followed by that many items
• Public keys are 33 bytes — compressed secp256k1 (02/03 prefix + 32 bytes)

Transport Framing

UDP

FIPS packets are carried directly in UDP datagrams. No additional framing is needed — each UDP datagram contains exactly one FIPS link-layer packet.

Stream Transports (future direction)

TCP, WebSocket, and Tor transports provide a byte stream, not datagrams. The common prefix payload_len field provides integrated stream framing — the receiver reads the 4-byte common prefix, then reads exactly the number of bytes indicated by payload_len (plus any phase-specific header and AEAD tag). No separate length prefix is needed.

Link-Layer Formats

All FLP packets begin with a 4-byte common prefix that identifies the protocol version, session lifecycle phase, per-packet flags, and payload length.

Common Prefix (4 bytes)

┌──────────────────────┬───────────┬───────────────┐
│ ver(4) + phase(4)    │ flags     │ payload_len   │
│ 1 byte               │ 1 byte    │ 2 bytes LE    │
└──────────────────────┴───────────┴───────────────┘

Field	Size	Description
version	4 bits (high)	Protocol version. Currently 0x0
phase	4 bits (low)	Session lifecycle phase (see table)
flags	1 byte	Per-packet signal flags (zero during handshake)
payload_len	2 bytes LE	Length of payload after phase-specific header, excluding AEAD tag

Phase Table

Phase	Type	Description
0x0	Established frame	Post-handshake encrypted traffic
0x1	Noise IK msg1	Handshake initiation
0x2	Noise IK msg2	Handshake response

Flags (Established Phase Only)

Bit	Name	Description
0	K (key epoch)	Selects active key during rekeying
1	CE	Congestion Experienced echo
2	SP (spin bit)	RTT measurement
3-7	—	Reserved (must be zero)

Flags must be zero in handshake packets (phase 0x1 and 0x2).

Established Frame (phase 0x0)

All post-handshake traffic between authenticated peers. Contains one encrypted link-layer message.

Outer header (16 bytes, used as AEAD AAD):

┌──────────────────────┬───────────┬───────────────┬──────────────┬──────────┐
│ ver(4) + phase(4)    │ flags     │ payload_len   │ receiver_idx │ counter  │
│ 1 byte               │ 1 byte    │ 2 bytes LE    │ 4 bytes LE   │ 8 bytes LE│
└──────────────────────┴───────────┴───────────────┴──────────────┴──────────┘

Field	Size	Description
common prefix	4 bytes	ver=0, phase=0, flags, payload_len
receiver_idx	4 bytes LE	Session index for O(1) lookup
counter	8 bytes LE	Monotonic nonce, used as AEAD nonce and for replay detection

The entire 16-byte header is authenticated as Associated Data (AAD) in the ChaCha20-Poly1305 AEAD construction.

Encrypted inner header (5 bytes, first bytes of plaintext):

┌───────────────┬──────────┐
│ timestamp     │ msg_type │
│ 4 bytes LE    │ 1 byte   │
└───────────────┴──────────┘

Field	Size	Description
timestamp	4 bytes LE	Session-relative milliseconds (u32)
msg_type	1 byte	Link-layer message type

After decryption, the plaintext begins with the 4-byte timestamp followed by the 1-byte message type and message-specific fields.

Complete encrypted frame:

┌──────────────────────────────────────┬───────────────────────────┐
│ outer header (16 bytes, used as AAD) │ ciphertext + AEAD tag     │
│                                      │ (inner_hdr + body) + 16   │
└──────────────────────────────────────┴───────────────────────────┘

Total overhead: 37 bytes (16 outer + 5 inner + 16 AEAD tag)
Minimum frame: 37 bytes (empty body)

Message Type Table

Type	Message	Description
0x00	SessionDatagram	Encapsulated session-layer payload for forwarding
0x01	SenderReport	MMP sender-side report (reserved)
0x02	ReceiverReport	MMP receiver-side report (reserved)
0x10	TreeAnnounce	Spanning tree state announcement
0x20	FilterAnnounce	Bloom filter reachability update
0x30	LookupRequest	Coordinate discovery request
0x31	LookupResponse	Coordinate discovery response
0x50	Disconnect	Orderly link teardown
0x51	Keepalive	Keepalive probe (reserved)

Noise IK Message 1 (phase 0x1)

Handshake initiation from connecting party.

┌──────────────────────┬─────────────┬─────────────────────────────────────────┐
│ common prefix        │ sender_idx  │ Noise IK message 1                      │
│ 4 bytes              │ 4 bytes LE  │ 82 bytes                                │
└──────────────────────┴─────────────┴─────────────────────────────────────────┘

Total: 90 bytes

Common prefix: ver=0, phase=0x1, flags=0, payload_len=86 (4 + 82).

Field	Size	Description
common prefix	4 bytes	ver=0, phase=1, flags=0, payload_len
sender_idx	4 bytes LE	Initiator's session index (becomes receiver's receiver_idx)
noise_msg1	82 bytes	Noise IK first message

Noise msg1 breakdown (82 bytes):

Offset	Field	Size	Description
0	ephemeral_pubkey	33 bytes	Initiator's ephemeral key (compressed secp256k1)
33	encrypted_static	33 bytes	Initiator's static key (encrypted with es key)
66	tag	16 bytes	AEAD tag for encrypted_static

Noise pattern: -> e, es, s, ss

Noise IK Message 2 (phase 0x2)

Handshake response from responder.

┌──────────────────────┬─────────────┬──────────────┬──────────────────────────┐
│ common prefix        │ sender_idx  │ receiver_idx │ Noise IK message 2       │
│ 4 bytes              │ 4 bytes LE  │ 4 bytes LE   │ 33 bytes                 │
└──────────────────────┴─────────────┴──────────────┴──────────────────────────┘

Total: 45 bytes

Common prefix: ver=0, phase=0x2, flags=0, payload_len=41 (4 + 4 + 33).

Field	Size	Description
common prefix	4 bytes	ver=0, phase=2, flags=0, payload_len
sender_idx	4 bytes LE	Responder's session index
receiver_idx	4 bytes LE	Echo of initiator's sender_idx from msg1
noise_msg2	33 bytes	Noise IK second message

Noise msg2 breakdown (33 bytes):

Offset	Field	Size	Description
0	ephemeral_pubkey	33 bytes	Responder's ephemeral key (compressed secp256k1)

Noise pattern: <- e, ee, se

After msg2, both parties derive identical symmetric session keys.

Index Semantics

Each party in a link session maintains two indices:

Index	Chosen By	Used By	Purpose
our_index	Us	Them	They include this as receiver_idx in packets to us
their_index	Them	Us	We include this as receiver_idx in packets to them

Handshake Flow

Initiator                                    Responder
---------                                    ---------
generates sender_idx
generates ephemeral keypair

         [0x01|flags=0|len] | sender_idx | noise_msg1
         ------------------------------------------------>

                                              validates msg1
                                              learns initiator's static key
                                              generates sender_idx
                                              generates ephemeral keypair

         [0x02|flags=0|len] | sender_idx | receiver_idx | noise_msg2
         <------------------------------------------------

validates msg2
derives session keys

=============== HANDSHAKE COMPLETE ===============

First encrypted frame:
         [0x00|flags|len] | receiver_idx | counter=0 | ciphertext+tag
         ------------------------------------------------>

Link-Layer Message Types

These messages are carried as plaintext inside encrypted frames (phase 0x0). After decryption of the AEAD ciphertext, the plaintext begins with a 4-byte session-relative timestamp followed by the 1-byte message type and message-specific fields.

TreeAnnounce (0x10)

Spanning tree state announcement, exchanged between direct peers only.

Offset	Field	Size	Description
0	msg_type	1 byte	0x10
1	version	1 byte	0x01 (v1)
2	sequence	8 bytes LE	Monotonic counter, increments on parent change
10	timestamp	8 bytes LE	Unix seconds
18	parent	16 bytes	NodeAddr of selected parent (self = root)
34	ancestry_count	2 bytes LE	Number of AncestryEntry records
36	ancestry	32 x n bytes	AncestryEntry array (self -> root)
36 + 32n	signature	64 bytes	Schnorr signature over entire message

AncestryEntry (32 bytes):

Offset	Field	Size	Description
0	node_addr	16 bytes	Node's routing identifier
16	sequence	8 bytes LE	Node's sequence number
24	timestamp	8 bytes LE	Node's Unix timestamp

Size: 100 + (n x 32) bytes, where n = ancestry_count (depth + 1, includes self)

Tree Depth	Payload	With Link Overhead
0 (root)	132 bytes	169 bytes
3	228 bytes	265 bytes
5	292 bytes	329 bytes
10	452 bytes	489 bytes

FilterAnnounce (0x20)

Bloom filter reachability update, exchanged between direct peers only.

Offset	Field	Size	Description
0	msg_type	1 byte	0x20
1	sequence	8 bytes LE	Monotonic counter for freshness
9	hash_count	1 byte	Number of hash functions (5 in v1)
10	size_class	1 byte	Filter size: 512 << size_class bytes
11	filter_bits	variable	Bloom filter bit array

Size class table:

size_class	Bytes	Bits	Status
0	512	4,096	Reserved
1	1,024	8,192	v1 (MUST use)
2	2,048	16,384	Reserved
3	4,096	32,768	Reserved

v1 payload: 1,035 bytes (11 header + 1,024 filter). With link overhead: 1,072 bytes.

LookupRequest (0x30)

Coordinate discovery request, flooded through the mesh.

Offset	Field	Size	Description
0	msg_type	1 byte	0x30
1	request_id	8 bytes LE	Unique random identifier
9	target	16 bytes	NodeAddr being sought
25	origin	16 bytes	Requester's NodeAddr
41	ttl	1 byte	Remaining hops (default 64)
42	origin_coords_cnt	2 bytes LE	Number of coordinate entries
44	origin_coords	16 x n bytes	Requester's ancestry (NodeAddr only)
44 + 16n	visited_hash_cnt	1 byte	Hash count for visited filter
45 + 16n	visited_bits	256 bytes	Compact bloom of visited nodes

Size: 301 + (n x 16) bytes, where n = origin depth + 1

Origin Depth	Payload
3	349 bytes
5	381 bytes
10	461 bytes

LookupResponse (0x31)

Coordinate discovery response, greedy-routed back to requester.

Offset	Field	Size	Description
0	msg_type	1 byte	0x31
1	request_id	8 bytes LE	Echoes the request's ID
9	target	16 bytes	NodeAddr that was found
25	target_coords_cnt	2 bytes LE	Number of coordinate entries
27	target_coords	16 x n bytes	Target's ancestry (NodeAddr only)
27 + 16n	proof	64 bytes	Schnorr signature over (request_id || target)

Size: 91 + (n x 16) bytes

Target Depth	Payload
3	139 bytes
5	171 bytes
10	251 bytes

Proof coverage: Signs (request_id || target) only — coordinates are excluded so the proof survives tree reconvergence during the lookup round-trip.

SessionDatagram (0x00)

Encapsulated session-layer payload for multi-hop forwarding.

Offset	Field	Size	Description
0	msg_type	1 byte	0x00
1	ttl	1 byte	Remaining hops, decremented each hop
2	path_mtu	2 bytes LE	Path MTU, min'd at each forwarding hop
4	src_addr	16 bytes	Source NodeAddr
20	dest_addr	16 bytes	Destination NodeAddr
36	payload	variable	Session-layer message

Fixed header: 36 bytes (SESSION_DATAGRAM_HEADER_SIZE)

The path_mtu field is initialized to u16::MAX by the sender and each forwarding hop applies min(path_mtu, outgoing_link_mtu), giving the receiver an estimate of the minimum MTU along the path.

The payload is opaque to transit nodes — session-layer encrypted independently of link encryption.

Disconnect (0x50)

Orderly link teardown with reason code.

Offset	Field	Size	Description
0	msg_type	1 byte	0x50
1	reason	1 byte	Disconnect reason code

Reason codes:

Code	Name	Description
0x00	Shutdown	Normal operator-requested stop
0x01	Restart	Restarting, may reconnect soon
0x02	ProtocolError	Protocol error encountered
0x03	TransportFailure	Transport failure
0x04	ResourceExhaustion	Memory or connection limit
0x05	SecurityViolation	Authentication or policy violation
0x06	ConfigurationChange	Peer removed from configuration
0x07	Timeout	Keepalive or stale detection timeout
0xFF	Other	Unspecified reason

Session-Layer Message Types

These messages are carried as the payload of a SessionDatagram (0x00).

SessionSetup (0x00)

Establishes a session and warms transit coordinate caches.

Offset	Field	Size	Description
0	msg_type	1 byte	0x00
1	flags	1 byte	Bit 0: REQUEST_ACK, Bit 1: BIDIRECTIONAL
2	src_coords_count	2 bytes LE	Number of source coordinate entries
4	src_coords	16 x n bytes	Source's ancestry (NodeAddr, self -> root)
...	dest_coords_count	2 bytes LE	Number of dest coordinate entries
...	dest_coords	16 x m bytes	Destination's ancestry
...	handshake_len	2 bytes LE	Noise payload length
...	handshake_payload	variable	Noise IK msg1 (82 bytes typical)

Example (depth-3 source, depth-4 destination):

SessionDatagram header: 36 bytes
SessionSetup payload: 1 + 1 + 2 + 48 + 2 + 64 + 2 + 82 = 202 bytes
Total: 238 bytes

SessionAck (0x01)

Confirms session establishment, completes the Noise handshake.

Offset	Field	Size	Description
0	msg_type	1 byte	0x01
1	flags	1 byte	Reserved
2	src_coords_count	2 bytes LE	Number of coordinate entries
4	src_coords	16 x n bytes	Acknowledger's ancestry (for cache warming)
...	handshake_len	2 bytes LE	Noise payload length
...	handshake_payload	variable	Noise IK msg2 (33 bytes typical)

DataPacket (0x10)

Encrypted application data with explicit replay protection counter.

Minimal header (COORDS_PRESENT = 0):

Offset	Field	Size	Description
0	msg_type	1 byte	0x10
1	flags	1 byte	Bit 0: COORDS_PRESENT
2	counter	8 bytes LE	Session encryption counter / replay nonce
10	payload_length	2 bytes LE	Length of encrypted payload
12	payload	variable	Encrypted application data + 16-byte AEAD tag

Header size: 12 bytes (DATA_HEADER_SIZE)

With coordinates (COORDS_PRESENT = 1):

Offset	Field	Size	Description
0	msg_type	1 byte	0x10
1	flags	1 byte	0x01 (COORDS_PRESENT)
2	counter	8 bytes LE	Session encryption counter
10	payload_length	2 bytes LE	Length of encrypted payload
12	src_coords_count	2 bytes LE	Source coordinate entries
14	src_coords	16 x n bytes	Source's ancestry
...	dest_coords_count	2 bytes LE	Dest coordinate entries
...	dest_coords	16 x m bytes	Destination's ancestry
...	payload	variable	Encrypted application data

CoordsRequired (0x20)

Link-layer error signal — transit node lacks coordinates for destination. Plaintext (not end-to-end encrypted), generated by transit nodes.

Offset	Field	Size	Description
0	msg_type	1 byte	0x20
1	flags	1 byte	Reserved
2	dest_addr	16 bytes	NodeAddr we couldn't route to
18	reporter	16 bytes	NodeAddr of reporting router

Payload: 34 bytes. Wrapped in SessionDatagram: 70 bytes total.

PathBroken (0x21)

Link-layer error signal — greedy routing reached a dead end. Plaintext, generated by transit nodes.

Offset	Field	Size	Description
0	msg_type	1 byte	0x21
1	flags	1 byte	Reserved
2	dest_addr	16 bytes	Unreachable NodeAddr
18	reporter	16 bytes	NodeAddr of reporting router
34	last_coords_count	2 bytes LE	Number of stale coordinate entries
36	last_known_coords	16 x n bytes	Stale coordinates that failed

Encapsulation Walkthrough

A complete picture of how application data is wrapped through each layer.

Application Data -> Wire

Starting with an application sending a 1024-byte payload to a destination:

Layer 4: Application data
    1024 bytes

Layer 3: Session encryption (FSP)
    DataPacket header (12 bytes) + encrypted payload (1024) + AEAD tag (16)
    = 1052 bytes

Layer 2: SessionDatagram envelope (FLP routing)
    msg_type (1) + ttl (1) + path_mtu (2) + src_addr (16) + dest_addr (16) + payload (1052)
    = 1088 bytes

Layer 1: Link encryption (FLP per-hop)
    outer header (16) + encrypted(inner_hdr (5) + datagram (1088)) + AEAD tag (16)
    = 1125 bytes

Layer 0: Transport
    UDP datagram containing 1125 bytes

Overhead Budget

Layer	Overhead	Component
Link encryption	37 bytes	16 outer header (AAD) + 5 inner header + 16 AEAD tag
SessionDatagram	36 bytes	1 type + 1 ttl + 2 path_mtu + 16 src + 16 dest
DataPacket header	12 bytes	1 type + 1 flags + 8 counter + 2 length
Session AEAD tag	16 bytes	Poly1305 tag on session-encrypted payload
Minimal total	101 bytes
Coordinates (if present)	~43 bytes	Varies with tree depth
Worst case	144 bytes	FIPS_OVERHEAD constant

At Each Transit Node

1. Receive UDP datagram
2. Parse common prefix -> version, phase, flags, payload_len
3. Phase 0x0 -> established frame
4. Look up (transport_id, receiver_idx) -> session
5. Check replay window (counter)
6. Decrypt with link keys (16-byte header as AAD) -> plaintext
7. Strip inner header -> timestamp, msg_type
8. msg_type 0x00 -> SessionDatagram
9. Read dest_addr -> routing decision
10. Decrement ttl, min path_mtu
11. Re-encrypt with next-hop link keys
12. Send via next-hop transport

Transit nodes see the SessionDatagram envelope (src_addr, dest_addr, ttl, path_mtu) but cannot read the session-layer payload (encrypted with endpoint session keys).

Size Summary

Handshake Messages

Message	Size
Noise IK msg1	90 bytes
Noise IK msg2	45 bytes

Link-Layer Messages (inside encrypted frame)

Message	Size	Notes
TreeAnnounce	100 + 32n bytes	n = depth + 1
FilterAnnounce	1,035 bytes	v1 (1KB filter)
LookupRequest	301 + 16n bytes	n = origin depth + 1
LookupResponse	91 + 16n bytes	n = target depth + 1
SessionDatagram	36 + payload bytes	Fixed 36-byte header
Disconnect	2 bytes

Session-Layer Messages (inside SessionDatagram)

Message	Typical Size	Notes
SessionSetup	~200 bytes	Depth-dependent
SessionAck	~80 bytes	Depth-dependent
DataPacket (minimal)	12 + payload bytes	Steady state
DataPacket (with coords)	12 + ~130 + payload bytes	Warmup/recovery
CoordsRequired	34 bytes	Fixed
PathBroken	36 + 16n bytes	Includes stale coords

Complete Packet Sizes (link + session)

Scenario	Wire Size	Notes
Encrypted frame minimum	37 bytes	Empty body
SessionDatagram + DataPacket (minimal)	37 + 36 + 12 + payload + 16	101 + payload
SessionDatagram + DataPacket (with coords)	~144 + payload	Worst case
SessionDatagram + SessionSetup	~275 bytes	Depth-3, both dirs
SessionDatagram + CoordsRequired	37 + 36 + 34 = 107 bytes	Including link overhead

References

• fips-link-layer.md — FLP behavioral specification
• fips-session-layer.md — FSP behavioral specification
• fips-transport-layer.md — Transport framing
• fips-mesh-operation.md — How messages work together
• fips-ipv6-adapter.md — MTU enforcement