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 FMP 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 metrics report (48 bytes)
0x02	ReceiverReport	MMP receiver-side metrics report (68 bytes)
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	Heartbeat	Link liveness probe

Noise IK Message 1 (phase 0x1)

Handshake initiation from connecting party.

┌──────────────────────┬─────────────┬─────────────────────────────────────────┐
│ common prefix        │ sender_idx  │ Noise IK message 1                      │
│ 4 bytes              │ 4 bytes LE  │ 106 bytes                               │
└──────────────────────┴─────────────┴─────────────────────────────────────────┘

Total: 114 bytes

Common prefix: ver=0, phase=0x1, flags=0, payload_len=110 (4 + 106).

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	106 bytes	Noise IK first message

Noise msg1 breakdown (106 bytes):

Offset	Field	Size	Description
0	ephemeral_pubkey	33 bytes	Initiator's ephemeral key (compressed secp256k1)
33	encrypted_static	49 bytes	Initiator's static key (33) + AEAD tag (16)
82	encrypted_epoch	24 bytes	Startup epoch (8) + AEAD tag (16)

Noise pattern: -> e, es, s, ss with epoch payload

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   │ 57 bytes                 │
└──────────────────────┴─────────────┴──────────────┴──────────────────────────┘

Total: 69 bytes

Common prefix: ver=0, phase=0x2, flags=0, payload_len=65 (4 + 4 + 57).

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	57 bytes	Noise IK second message

Noise msg2 breakdown (57 bytes):

Offset	Field	Size	Description
0	ephemeral_pubkey	33 bytes	Responder's ephemeral key (compressed secp256k1)
33	encrypted_epoch	24 bytes	Startup epoch (8) + AEAD tag (16)

Noise pattern: <- e, ee, se with epoch payload

After msg2, both parties derive identical symmetric session keys. The encrypted epoch in msg1 and msg2 enables peer restart detection — if a peer's epoch changes, the other side knows it restarted and must re-establish the link.

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	min_mtu	2 bytes LE	Minimum transport MTU the origin requires (0 = no requirement)
44	origin_coords_cnt	2 bytes LE	Number of coordinate entries
46	origin_coords	16 x n bytes	Requester's ancestry (NodeAddr only)
46 + 16n	visited_hash_cnt	1 byte	Hash count for visited filter
47 + 16n	visited_bits	256 bytes	Compact bloom of visited nodes

Size: 303 + (n x 16) bytes, where n = origin depth + 1

Origin Depth	Payload
3	351 bytes
5	383 bytes
10	463 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	path_mtu	2 bytes LE	Minimum MTU along response path (transit-annotated)
27	target_coords_cnt	2 bytes LE	Number of coordinate entries
29	target_coords	16 x n bytes	Target's ancestry (NodeAddr only)
29 + 16n	proof	64 bytes	Schnorr signature over (request_id || target || target_coords)

Size: 93 + (n x 16) bytes

Target Depth	Payload
3	141 bytes
5	173 bytes
10	253 bytes

The path_mtu field is initialized to u16::MAX by the target and each transit hop applies min(path_mtu, outgoing_link_mtu), giving the originator an MTU estimate for the discovered path.

Proof coverage: Signs (request_id || target || target_coords) — path_mtu is excluded from the proof because it is a transit annotation modified at each hop. Coordinates are included because proof verification at the source confirms the target actually holds the claimed tree position. The source verifies the proof upon receipt.

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	Heartbeat liveness timeout
0xFF	Other	Unspecified reason

SenderReport (0x01)

Sent by the frame sender to provide interval-based transmission statistics.

Offset	Field	Size	Encoding
0	msg_type	1	0x01
1	reserved	3	Zero
4	interval_start_counter	8	u64 LE — first counter in this interval
12	interval_end_counter	8	u64 LE — last counter in this interval
20	interval_start_timestamp	4	u32 LE — timestamp at interval start
24	interval_end_timestamp	4	u32 LE — timestamp at interval end
28	interval_bytes_sent	4	u32 LE — payload bytes sent in interval
32	cumulative_packets_sent	8	u64 LE — total packets sent on this link
40	cumulative_bytes_sent	8	u64 LE — total bytes sent on this link

Total: 48 bytes.

ReceiverReport (0x02)

Sent by the frame receiver to provide loss, jitter, and timing feedback.

Offset	Field	Size	Encoding
0	msg_type	1	0x02
1	reserved	3	Zero
4	highest_counter	8	u64 LE — highest counter value received
12	cumulative_packets_recv	8	u64 LE — total packets received
20	cumulative_bytes_recv	8	u64 LE — total bytes received
28	timestamp_echo	4	u32 LE — echoed sender timestamp for RTT
32	dwell_time	2	u16 LE — time between receive and echo (ms)
34	max_burst_loss	2	u16 LE — largest loss burst in interval
36	mean_burst_loss	2	u16 LE — mean burst length (u8.8 fixed-point)
38	reserved	2	Zero
40	jitter	4	u32 LE — interarrival jitter (microseconds)
44	ecn_ce_count	4	u32 LE — cumulative ECN-CE marked packets
48	owd_trend	4	i32 LE — one-way delay trend (µs/s, signed)
52	burst_loss_count	4	u32 LE — number of loss bursts in interval
56	cumulative_reorder_count	4	u32 LE — total reordered packets
60	interval_packets_recv	4	u32 LE — packets received in interval
64	interval_bytes_recv	4	u32 LE — bytes received in interval

Total: 68 bytes.

Session-Layer Message Formats

Session-layer messages are carried as the payload of a SessionDatagram (0x00). All FSP messages begin with a 4-byte common prefix that identifies the protocol version, session lifecycle phase, per-packet flags, and payload length.

FSP Common Prefix (4 bytes)

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

FSP Phase Table

Phase	Type	Description
0x0	Established	Post-handshake encrypted traffic or plaintext error signals
0x1	Handshake msg1	SessionSetup (Noise XK msg1)
0x2	Handshake msg2	SessionAck (Noise XK msg2)
0x3	Handshake msg3	SessionMsg3 (Noise XK msg3)

FSP Flags (Established Phase Only)

Bit	Name	Description
0	CP (coords present)	Source and destination coordinates follow the header in cleartext
1	K (key epoch)	Selects active key during rekeying
2	U (unencrypted)	Payload is plaintext (error signals)
3-7	—	Reserved (must be zero)

Flags must be zero in handshake packets (phase 0x1, 0x2, and 0x3).

FSP Encrypted Message (phase 0x0, U flag clear)

Post-handshake encrypted data. The 12-byte cleartext header is used as AEAD AAD. Coordinates may appear in cleartext between the header and ciphertext when the CP flag is set.

Cleartext header (12 bytes, used as AEAD AAD):

Field	Size	Description
common prefix	4 bytes	ver=0, phase=0, flags, payload_len
counter	8 bytes LE	Monotonic nonce, used as AEAD nonce and for replay detection

Optional cleartext coordinates (when CP flag is set):

Field	Size	Description
src_coords_count	2 bytes LE	Number of source coordinate entries
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

Transit nodes parse the CP flag and extract coordinates without decryption.

Encrypted inner header (6 bytes, first bytes of AEAD plaintext):

Field	Size	Description
timestamp	4 bytes LE	Session-relative milliseconds (u32)
msg_type	1 byte	Session-layer message type
inner_flags	1 byte	Bit 0: SP (spin bit for RTT measurement)

After the inner header, the remaining plaintext is the message-type-specific body.

Complete encrypted message:

┌─────────────────────────────────┬─────────────────┬───────────────────────────┐
│ header (12 bytes, used as AAD)  │ [coords if CP]  │ ciphertext + AEAD tag     │
│                                 │                 │ (inner_hdr + body) + 16   │
└─────────────────────────────────┴─────────────────┴───────────────────────────┘

FSP Session Message Types

Type	Message	Description
0x10	Data	Application data (IPv6 payload via TUN)
0x11	SenderReport	MMP sender-side metrics report
0x12	ReceiverReport	MMP receiver-side metrics report
0x13	PathMtuNotification	End-to-end path MTU echo
0x14	CoordsWarmup	Standalone coordinate cache warming
0x20	CoordsRequired	Error: transit node lacks destination coordinates
0x21	PathBroken	Error: greedy routing reached dead end
0x22	MtuExceeded	Error: forwarded packet exceeds next-hop MTU

Message types 0x10-0x14 are carried inside the AEAD ciphertext (dispatched by the msg_type field in the encrypted inner header). Types 0x20-0x22 are plaintext error signals (U flag set, no encryption).

Session-layer SenderReport (0x11) and ReceiverReport (0x12) use the same body format as their link-layer counterparts (0x01 and 0x02). The msg_type byte in the body matches the link-layer value; dispatch to the correct layer happens at the session level based on the FSP message type.

SessionSetup (phase 0x1)

Establishes a session and warms transit coordinate caches. Contains the first message of the Noise XK handshake (ephemeral key only — the initiator's static identity is not revealed until msg3). Encoded with FSP prefix: ver=0, phase=0x1, flags=0, payload_len.

Body (after 4-byte FSP prefix):

Offset	Field	Size	Description
0	flags	1 byte	Bit 0: REQUEST_ACK, Bit 1: BIDIRECTIONAL
1	src_coords_count	2 bytes LE	Number of source coordinate entries
3	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 XK msg1 (33 bytes — ephemeral key only)

SessionAck (phase 0x2)

Second message of the Noise XK handshake. The responder sends its ephemeral key and encrypted epoch. Encoded with FSP prefix: ver=0, phase=0x2, flags=0, payload_len.

Body (after 4-byte FSP prefix):

Offset	Field	Size	Description
0	flags	1 byte	Reserved
1	src_coords_count	2 bytes LE	Number of acknowledger coordinate entries
3	src_coords	16 x n bytes	Acknowledger's ancestry (for cache warming)
...	dest_coords_count	2 bytes LE	Number of initiator coordinate entries
...	dest_coords	16 x m bytes	Initiator's ancestry (for return-path cache warming)
...	handshake_len	2 bytes LE	Noise payload length
...	handshake_payload	variable	Noise XK msg2 (57 bytes — ephemeral key + encrypted epoch)

SessionMsg3 (phase 0x3)

Third and final message of the Noise XK handshake. The initiator reveals its encrypted static identity and epoch. After msg3, both parties derive identical symmetric session keys and the session is established. Encoded with FSP prefix: ver=0, phase=0x3, flags=0, payload_len.

Body (after 4-byte FSP prefix):

Offset	Field	Size	Description
0	flags	1 byte	Reserved
1	handshake_len	2 bytes LE	Noise payload length
3	handshake_payload	variable	Noise XK msg3 (73 bytes — encrypted static + encrypted epoch)

Noise XK msg3 breakdown (73 bytes):

Offset	Field	Size	Description
0	encrypted_static	49 bytes	Initiator's static key (33) + AEAD tag (16)
49	encrypted_epoch	24 bytes	Startup epoch (8) + AEAD tag (16)

SessionMsg3 does not carry coordinates — both endpoints already have each other's coordinates from SessionSetup (msg1) and SessionAck (msg2).

Data (0x10)

Application data (typically IPv6 payload). This is the msg_type byte inside the encrypted inner header — there is no separate DataPacket struct. The body after the inner header is delivered directly to the TUN interface.

PathMtuNotification (0x13)

Sent by the destination to report the observed forward-path MTU.

Offset	Field	Size	Encoding
0	path_mtu	2	u16 LE — minimum MTU observed along the forward path

Total body: 2 bytes (plus FSP common prefix + encrypted header + AEAD tag).

CoordsWarmup (0x14)

Standalone coordinate cache warming message. Sent when piggybacking coordinates via the CP flag on a data packet would exceed the transport MTU, or as an immediate response to CoordsRequired/PathBroken signals (rate-limited).

CoordsWarmup is an encrypted FSP message with the CP flag set and an empty body. Transit nodes extract coordinates via the existing CP-flag parsing path — no transit-side changes required.

Wire format:

FSP header (12 bytes, AAD): ver=0, phase=0, flags=CP, counter, payload_len
Cleartext coords: src_coords + dst_coords (same encoding as CP flag)
AEAD ciphertext: inner_header(6) + Poly1305 tag(16) = 22 bytes

Total FSP payload: 12 + coords + 22

The cleartext coords section uses the same variable-length encoding as any CP-flagged message: src_coords_count(2) + src_coords(16×n) + dest_coords_count(2) + dest_coords(16×m).

Typical size (depth-3 tree): 12 + (2+64+2+64) + 22 = 166 bytes FSP payload. With SessionDatagram + link overhead: 166 + 36 + 37 = 239 bytes on the wire.

CoordsRequired (0x20)

Plaintext error signal — transit node lacks coordinates for destination. Encoded with FSP prefix: ver=0, phase=0x0, U flag set, payload_len.

Body (after 4-byte FSP prefix + 1-byte msg_type):

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

Body size: 33 bytes. Total with prefix + msg_type: 38 bytes.

PathBroken (0x21)

Plaintext error signal — greedy routing reached a dead end. Encoded with FSP prefix: ver=0, phase=0x0, U flag set, payload_len.

Body (after 4-byte FSP prefix + 1-byte msg_type):

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

MtuExceeded (0x22)

Plaintext error signal — forwarded packet exceeds the next-hop link MTU. Sent by a transit router back to the source when a SessionDatagram cannot be forwarded because its size exceeds the outgoing link's MTU. Encoded with FSP prefix: ver=0, phase=0x0, U flag set, payload_len.

Body (after 4-byte FSP prefix + 1-byte msg_type):

Offset	Field	Size	Description
0	flags	1 byte	Reserved
1	dest_addr	16 bytes	NodeAddr of the destination being forwarded to
17	reporter	16 bytes	NodeAddr of the router that detected the MTU violation
33	mtu	2 bytes LE	Bottleneck MTU at the reporting router

Body size: 35 bytes. Total with prefix + msg_type: 40 bytes.

The source uses the reported MTU to adjust its session-layer path MTU estimate. MtuExceeded is the reactive complement to the proactive path_mtu field in SessionDatagram and LookupResponse.

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)
    FSP header (12 bytes) + AEAD(inner_hdr (6) + payload (1024)) + AEAD tag (16)
    = 1058 bytes

Layer 2: SessionDatagram envelope (FMP routing)
    msg_type (1) + ttl (1) + path_mtu (2) + src_addr (16) + dest_addr (16) + payload (1058)
    = 1094 bytes

Layer 1: Link encryption (FMP per-hop)
    outer header (16) + encrypted(inner_hdr (5) + datagram (1094)) + AEAD tag (16)
    = 1131 bytes

Layer 0: Transport
    UDP datagram containing 1131 bytes

Overhead Budget

Layer	Overhead	Component
Link encryption	37 bytes	16 outer header (AAD) + 5 inner header (timestamp + msg_type) + 16 AEAD tag
SessionDatagram body	35 bytes	1 ttl + 2 path_mtu + 16 src + 16 dest (msg_type counted in inner header)
FSP header	12 bytes	4 prefix + 8 counter
FSP inner header	6 bytes	4 timestamp + 1 msg_type + 1 inner_flags (inside AEAD)
Session AEAD tag	16 bytes	Poly1305 tag on session-encrypted payload
Data path total	106 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

FMP Handshake Messages (Noise IK)

Message	Raw Noise	Wire Frame
IK msg1 (ephemeral + encrypted static + encrypted epoch)	106 bytes	114 bytes
IK msg2 (ephemeral + encrypted epoch)	57 bytes	69 bytes

FSP Handshake Messages (Noise XK)

Message	Raw Noise	Notes
XK msg1 (ephemeral only)	33 bytes	Carried in SessionSetup
XK msg2 (ephemeral + encrypted epoch)	57 bytes	Carried in SessionAck
XK msg3 (encrypted static + encrypted epoch)	73 bytes	Carried in SessionMsg3

Link-Layer Messages (inside encrypted frame)

Message	Size	Notes
TreeAnnounce	100 + 32n bytes	n = depth + 1
FilterAnnounce	1,035 bytes	v1 (1KB filter)
LookupRequest	303 + 16n bytes	n = origin depth + 1
LookupResponse	93 + 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	~170 bytes	Depth-dependent (XK msg1 = 33 bytes)
SessionAck	~190 bytes	Depth-dependent, carries both endpoints' coords (XK msg2 = 57 bytes)
SessionMsg3	~80 bytes	Fixed (XK msg3 = 73 bytes, no coords)
Data (minimal)	12 + 6 + payload + 16 bytes	Steady state
Data (with coords)	12 + ~130 + 6 + payload + 16 bytes	Warmup/recovery
SenderReport	12 + 6 + 46 + 16 bytes	MMP metrics
ReceiverReport	12 + 6 + 66 + 16 bytes	MMP metrics
PathMtuNotification	12 + 6 + 2 + 16 bytes	MTU signal
CoordsWarmup	12 + coords + 6 + 16 bytes	Standalone warmup (empty body)
CoordsRequired	38 bytes	Fixed (prefix + msg_type + body)
PathBroken	35 + 16n bytes	Includes stale coords
MtuExceeded	40 bytes	Fixed (prefix + msg_type + body)

Complete Packet Sizes (link + session)

Scenario	Wire Size	Notes
Encrypted frame minimum	37 bytes	Empty body
SessionDatagram + Data (minimal)	37 + 35 + 12 + 6 + payload + 16	106 + payload
SessionDatagram + Data (with coords)	106 + coords + payload	Coords vary with tree depth
SessionDatagram + SessionSetup	~275 bytes	Depth-3, both dirs
SessionDatagram + CoordsRequired	37 + 36 + 38 = 111 bytes	Including link overhead

References

• fips-mesh-layer.md — FMP 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