fips/docs/design/fips-mesh-layer.md

FIPS Mesh Protocol (FMP)

The FIPS Mesh Protocol is the middle layer of the FIPS protocol stack. It sits between the transport layer below and the FIPS Session Protocol (FSP) above. FMP is where anonymous transport addresses become authenticated peers, where the mesh self-organizes, and where forwarding decisions are made.

Role

FMP manages direct peer connections over transports. When a transport delivers a datagram from an unknown address, FMP authenticates the sender through a Noise XX handshake, establishing a cryptographic link. Once authenticated, the link carries all inter-peer communication: spanning tree gossip, bloom filter updates, coordinate discovery, and forwarded session datagrams — all encrypted per-hop.

FMP is the boundary between opaque transport addresses and identified peers. Below FMP, everything is transport-specific addresses (host:port, MAC, .onion). Above FMP, everything is peers identified by public keys and routable by node_addr. The transport layer never sees FIPS-level structure; FSP never sees transport addresses or routing details.

Services Provided to FSP

From the session layer's perspective, FMP is a black box providing three services. FSP knows nothing about transports, links, peers, spanning trees, coordinates, bloom filters, hop counts, or network topology.

Datagram Forwarding

FMP accepts a datagram addressed by source and destination node_addr and delivers it best-effort toward the destination. The datagram travels hop by hop — at each node, FMP decrypts the link layer, reads the destination node_addr, makes a local forwarding decision, and re-encrypts onto the next-hop link.

FSP provides: source node_addr, destination node_addr, hop limit, and an opaque payload (the session-layer encrypted message).

FMP provides: best-effort delivery. No acknowledgment, no retransmission, no ordering guarantee. Datagrams may be dropped, duplicated, or delivered out of order.

Error Signaling

When forwarding fails, FMP signals the source endpoint asynchronously:

• CoordsRequired: A transit node lacks the destination's tree coordinates and cannot make a forwarding decision. The source should re-initiate discovery and reset its coordinate warmup strategy.
• PathBroken: Greedy routing reached a dead end — no peer is closer to the destination than the current node. The source should re-discover the destination's current coordinates.

Both signals travel inside the SessionDatagram envelope (using the existing src/dest/hop_limit addressing) but are generated by transit nodes and are not end-to-end encrypted. They are rate-limited at 100ms per destination to prevent storms during topology changes.

Local Delivery

When a datagram arrives with a destination node_addr matching the local node, FMP delivers it up to FSP for session-layer processing.

Services Required from Transport Layer

FMP requires the following from each transport:

Datagram Delivery

Send and receive raw datagrams to/from transport addresses. The transport handles all medium-specific details. FMP sees only "send bytes to address" and "bytes arrived from address."

MTU Reporting

The maximum datagram size for a given link. FMP needs this to determine how much payload fits in a single packet after link encryption overhead (37 bytes for the encrypted frame wrapper: 16-byte outer header + 5-byte inner header + 16-byte AEAD tag).

Connection Lifecycle

For connection-oriented transports, the transport must establish the underlying connection before FMP can begin the Noise XX handshake. For connectionless transports, datagrams can flow immediately.

Endpoint Discovery (Optional)

When a transport discovers a FIPS-capable endpoint (via beacon, query, or other transport-specific mechanism), it notifies FMP so that link setup can be initiated. Transports without discovery support provide peer addresses through configuration.

Peer Authentication

Noise XX Handshake

Every peer connection begins with a Noise XX handshake that mutually authenticates both parties and establishes symmetric keys for link encryption.

The XX pattern is chosen because:

• Neither side requires prior knowledge of the other's static public key
• The responder reveals its identity in msg2; the initiator reveals its identity in msg3
• A protocol negotiation payload (version range, feature bitfield, TLV extensions) is appended to msg2 and msg3, enabling rolling protocol upgrades without additional round-trips

After the three-message handshake completes, both parties share symmetric session keys derived from three DH operations (ee, es, se). The handshake provides mutual authentication, forward secrecy, and identity hiding for both parties until they choose to reveal.

Epoch Exchange and Peer Restart Detection

XX handshake messages msg2 and msg3 carry an encrypted epoch payload — an 8-byte random value generated once at node startup:

• msg1: Ephemeral key only (33 bytes) — no identity or epoch
• msg2: Ephemeral key (33 bytes) + encrypted static key (49 bytes) + encrypted epoch (24 bytes) = 106 bytes base, plus negotiation payload
• msg3: Encrypted static key (49 bytes) + encrypted epoch (24 bytes) = 73 bytes base, plus negotiation payload

The encrypted epoch (EPOCH_ENCRYPTED_SIZE = 24 bytes) consists of the 8-byte epoch value plus a 16-byte AEAD tag.

Because msg1 carries no identity, restart detection is deferred: the initiator checks the responder's epoch after msg2, and the responder checks the initiator's epoch after msg3. An epoch mismatch indicates the peer has restarted, triggering full link re-establishment rather than treating the handshake as a simple reconnection. This prevents stale session state from persisting across restarts.

Identity Binding

The Noise handshake binds the link to the peer's cryptographic identity. With XX, identity confirmation happens at different points: the initiator learns the responder's identity from msg2, and the responder learns the initiator's identity from msg3. After handshake completion:

• The peer's public key (FIPS identity) is confirmed
• The node_addr is computed from the public key (SHA-256, truncated to 128 bits)
• All subsequent traffic on the link is authenticated by the Noise session — successful decryption proves the sender is the authenticated peer
• The link is registered in the dispatch table for O(1) packet routing

Reconnection

When a Noise XX msg1 arrives from an address that already has an authenticated link, FMP accepts the new handshake alongside the existing session. With XX, the peer's identity is not known at msg1 time — FMP can only detect the duplicate by transport address. Identity-based checks (restart detection, rekey recognition, cross-connection resolution) are deferred to msg3 when the initiator's identity is revealed. If the new handshake completes successfully, it replaces the old session. This handles legitimate reconnection (network change, process restart, NAT rebinding) without disrupting ongoing traffic until the new session is confirmed.

Auto-Reconnect

When MMP's liveness detection removes a peer (dead timeout exceeded), FMP automatically re-initiates the connection if the peer is configured for it. The auto-reconnect path:

1. check_link_heartbeats() detects the dead peer and calls remove_active_peer(), tearing down the link and triggering tree/bloom reconvergence
2. schedule_reconnect() checks whether the peer is in the auto-connect list with auto_reconnect: true (the default)
3. If eligible, the peer is fed into the retry system with unlimited retries and exponential backoff (same base interval and max backoff as startup retries, configured via node.retry.*)
4. On each retry tick, a fresh Noise XX handshake is initiated toward the peer's configured transport addresses

Auto-reconnect only applies to peers in the static peer list with connect_policy: auto_connect. Inbound-only peers (those not in the local config) are not reconnected — the owning node (the one with the outbound config) is responsible for re-establishing the link.

Handshake Message Retry

Both link-layer (Noise XX msg1/msg2/msg3) and session-layer (SessionSetup/ SessionAck/SessionMsg3) handshakes use message-level retry with exponential backoff within the handshake timeout window. This handles packet loss on the underlying transport without waiting for the full handshake timeout to expire.

Configuration (under node.rate_limit.*):

• handshake_resend_interval_ms (default 1000): initial resend interval
• handshake_resend_backoff (default 2.0): backoff multiplier per resend
• handshake_max_resends (default 5): max resends per handshake attempt

With defaults, resends occur at 1s, 2s, 4s, 8s, 16s — all within the 30s handshake timeout. Under 19% per-attempt loss (10% bidirectional), the probability of all 6 attempts (initial + 5 resends) failing is ~0.005%.

Session-layer resends wrap the stored payload in a fresh SessionDatagram so routing adapts to topology changes between resends. Responder idempotency: duplicate msg1/SessionSetup triggers resend of the stored msg2/SessionAck.

Link Encryption

All traffic between authenticated peers is encrypted. Every packet on a link — gossip messages, routing queries, forwarded session datagrams, disconnect notifications — passes through Noise's ChaCha20-Poly1305 AEAD.

Encrypted Frame Structure

Post-handshake packets are wrapped in an encrypted frame consisting of:

• A 4-byte common prefix (version, phase, flags, payload length)
• A receiver index for O(1) session lookup
• An explicit counter used as the AEAD nonce
• The ciphertext with a Poly1305 authentication tag

The 16-byte outer header (common prefix + receiver index + counter) is used as AAD for the AEAD, binding the header to the ciphertext without encrypting it.

The plaintext inside the encrypted frame begins with a 5-byte inner header (4-byte session-relative timestamp followed by a message type byte), then the message-specific payload.

See ../reference/wire-formats.md for the complete wire format specification.

What Encryption Provides

• Confidentiality: An observer on the underlying transport sees only encrypted packets, packet timing, and packet sizes
• Integrity: Any modification to a packet is detected by the AEAD tag
• Authentication: Only the authenticated peer can produce valid ciphertext for this link's session keys

What Encryption Does Not Provide

• End-to-end confidentiality: Link encryption protects traffic on a single hop. Each transit node decrypts, reads the routing envelope, and re-encrypts for the next hop. Session-layer encryption (FSP) provides end-to-end confidentiality.
• Traffic analysis protection: Packet timing, sizes, and volume are visible to transport-layer observers.

Index-Based Session Dispatch

Incoming packets are dispatched to the correct Noise session using a receiver index — a random 32-bit value chosen by the receiver during the handshake. This enables O(1) lookup without relying on source addresses.

Each party in a link has two indices:

• our_index: Chosen by us, included by the peer in packets sent to us
• their_index: Chosen by them, included by us in packets sent to them

The tuple (transport_id, receiver_idx) uniquely identifies a session.

Index Properties

• Random: Cryptographically random to prevent guessing
• Unique per transport: No two active sessions on the same transport share an index
• Scoped to transport: The same index value may appear on different transports
• Rotated on rekey: New indices allocated on rekey to prevent cross-session correlation by passive observers

Dispatch Flow

1. Read the 4-byte common prefix to determine the phase (established, handshake msg1, msg2)
2. For established frames (phase 0x0): look up (transport_id, receiver_idx) in the session table — O(1) hash lookup. Unknown indices are rejected before any cryptographic operation.
3. For handshake msg2 (phase 0x2): look up by our sender index to match to a pending outbound handshake
4. For handshake msg1 (phase 0x1): rate-limited processing, creates new state

This approach follows WireGuard's design: source address is informational, not authoritative. Only successful cryptographic verification establishes authenticity.

Roaming

When an encrypted packet successfully decrypts, the sender is the authenticated peer regardless of what transport address the packet arrived from. FMP updates the peer's current address to the packet's source address, and subsequent outbound packets use the updated address.

This allows peers to change transport addresses (e.g., host:port for UDP) without session interruption. The mechanism is:

1. Packet arrives from a different address than expected
2. Receiver index lookup finds the session
3. AEAD decryption succeeds — the sender is cryptographically authenticated
4. Peer's address is updated to the new source address

Roaming is most useful for UDP, where source addresses can change due to NAT rebinding or network changes. For connection-oriented transports, "roaming" manifests as reconnection rather than mid-session address change.

Roaming addresses mid-session NAT rebinding. Establishing the initial UDP path through NAT is a separate concern, addressed by the optional Nostr-mediated overlay discovery and STUN-assisted hole punching feature (see fips-transport-layer.md and ../reference/configuration.md).

Replay Protection

Each link session maintains per-direction counters:

• Send counter: Monotonically increasing, used as the AEAD nonce for each outbound packet
• Receive window: A sliding bitmap (2048 entries) tracking which counters have been seen

The receive window handles the realities of unreliable transports: packets may arrive out of order, be duplicated, or be lost. The window accepts any counter not yet seen and within 2048 of the highest counter received. Counters older than the window are rejected.

The replay check is performed before decryption to prevent CPU exhaustion from replayed packets that would pass the index lookup but fail decryption.

During link transitions, stale packets from a previous Noise session arrive encrypted with old counters and are correctly rejected. To avoid excessive log volume from these benign bursts, replay detection logging is suppressed after the first 3 occurrences per peer. A summary is emitted when the peer's session is re-established or the peer is removed.

Rate Limiting

Handshake initiation (msg1) is the primary attack surface for unauthenticated traffic. Each msg1 requires Noise DH operations (~200µs on modern CPUs), state allocation, and response generation.

FMP uses a global token bucket rate limiter:

• Burst capacity: Handles legitimate connection storms (e.g., node restart with many configured peers)
• Sustained rate: Limits steady-state new connections per second
• Global scope: Rate limiting is global, not per-source, because UDP source addresses are trivially spoofable

Additional protections:

• Connection limit: Maximum number of pending inbound handshakes, capping memory usage
• Handshake timeout: Stale pending handshakes are cleaned up after a configurable timeout
• Peer ACL: Optional allowlist / denylist filtering of peer npubs before handshake processing (loaded from /etc/fips/peers.allow and /etc/fips/peers.deny, mtime-watched and reloaded automatically)

Disconnect

FMP supports orderly link teardown via a Disconnect message carrying a reason code (shutdown, restart, protocol error, transport failure, resource exhaustion, security violation, configuration change, timeout).

On receiving Disconnect, FMP immediately cleans up state: removes the peer from the peer table, frees the session index, removes the link, and cleans up address mappings. If the departed peer was the tree parent, FMP triggers parent reselection.

Disconnect is best-effort — if the transport is broken, the message won't arrive. Timeout-based detection remains the fallback for detecting failed links.

On node shutdown, Disconnect is sent to all active peers before transports are stopped.

Rekey

FMP periodically negotiates a fresh Noise session over each established link to bound forward-secrecy exposure: limiting AEAD nonce reuse risk, bounding the volume of ciphertext recoverable from a stolen long-term static key, and rotating the session indices that addressed packets carry on the wire.

A rekey is initiated when either threshold is reached on the link's current session: node.rekey.after_secs (default 120) elapsed since the link came up or last rekeyed, or node.rekey.after_messages (default 65536) frames sent. Either side can be the initiator independently. Rekey is on by default and can be disabled via node.rekey.enabled: false (the configuration tree is documented in ../reference/configuration.md).

Mechanism

A rekey reuses the Noise XX pattern of the initial handshake, but the three messages travel over the existing link as ordinary encrypted FMP frames rather than as plaintext bootstrap packets. The initiator builds a fresh HandshakeState, generates msg1, and sends it through the current session; the responder consumes msg1, builds msg2, and replies; the initiator then sends msg3. After both sides have exchanged messages and finalised the new keys, traffic transitions from the old session to the new one.

Cutover is signalled in-band by the K-bit in the FMP flags byte. Each side starts emitting frames under the new session with K set; on receipt of the first K-marked frame the peer accepts the cutover and follows suit. A new pair of session indices is allocated as part of the new session, replacing the old indices on subsequent frames (see Index Properties).

Drain Window

To absorb in-flight reordering across the cutover, the old session is not discarded immediately. Each peer retains it in a previous_session slot on the active-peer state for DRAIN_WINDOW_SECS = 10 seconds (a compile-time constant in src/node/handlers/rekey.rs). During the window, decrypt attempts fall back to previous_session when the new session rejects a frame, so a packet sent under the old keys that arrives a few hundred milliseconds late still decrypts. After the window expires, the old session is dropped.

Dual-Initiation Race

On high-latency links, both sides' rekey timers can fire close enough together that each peer's msg1 crosses the other in flight. Without arbitration, each side would act as both initiator and responder, end up with two different Noise sessions, and lose connectivity at cutover. FMP arbitrates with a deterministic tie-breaker: the peer with the numerically smaller NodeAddr wins the role of initiator and discards any inbound msg1 it sees during the race; the larger-NodeAddr peer abandons its own initiation and processes the inbound msg1 as responder. The same tie-breaker is applied to cross-connection races during initial handshake.

Operator Visibility

Successful cutover is reported at INFO level on the K-bit observation; intermediate steps (handshake start, msg1/msg2 exchange, drain-window fallback decrypts) log at DEBUG/TRACE. Failures (handshake error, drain-window expiry without cutover) log at WARN.

The end-to-end rekey at the session layer follows a parallel design; see fips-session-layer.md.

Liveness Detection

FMP detects link liveness through a combination of explicit heartbeats and traffic observation.

Heartbeat

A Heartbeat message (0x51) is sent to each active peer at a configurable interval (node.heartbeat_interval_secs). The heartbeat is a minimal encrypted frame with no payload beyond the standard inner header (timestamp + message type). Any successfully decrypted frame — data, gossip, MMP report, or heartbeat — resets the peer's last-receive timestamp tracked by the MMP receiver.

Dead Timeout

When no traffic (of any kind) is received from a peer for the configured node.link_dead_timeout_secs window, the peer is declared dead and removed via remove_active_peer(). This triggers the full teardown cascade: spanning tree parent reselection (if the dead peer was the parent), TreeAnnounce propagation, coordinate cache flush, and bloom filter recompute.

If the dead peer is eligible for auto-reconnect (see [Auto-Reconnect] (#auto-reconnect)), reconnection is scheduled immediately after removal.

The heartbeat is independent of MMP. Gossip is event-driven, Lightweight produces receiver reports only when traffic arrives, and Minimal emits no reports at all — so on a fully idle link no MMP-mode combination guarantees periodic activity. The heartbeat is the always-on liveness signal.

Link Message Types

FMP defines several encrypted message types carried inside the established-frame envelope. They group naturally by purpose:

• Routing gossip: TreeAnnounce carries spanning-tree announcements between direct peers; FilterAnnounce carries bloom-filter reachability updates between direct peers. Both are peer-to-peer (not forwarded).
• Discovery: LookupRequest is forwarded through tree peers under bloom-filter guidance to find a destination's coordinates; LookupResponse routes back to the requester via reverse-path lookup in recent_requests.
• Forwarded payload: SessionDatagram carries a session-layer payload hop-by-hop toward the destination.
• Metrics: SenderReport and ReceiverReport carry the link-layer MMP report stream peer-to-peer.
• Liveness and lifecycle: Heartbeat is a minimal frame sent peer-to-peer to keep the link alive; Disconnect carries an orderly teardown reason code peer-to-peer.

Handshake messages (phase 0x1 msg1, phase 0x2 msg2) travel before encryption is established and are identified by the FMP common-prefix phase field rather than a msg_type byte.

See ../reference/wire-formats.md for byte-level message layouts and the canonical FMP message type catalog, and fips-mesh-operation.md for how these messages work together to build and maintain the mesh.

Metrics Measurement Protocol (MMP)

MMP runs on every active link to provide per-link quality metrics (SRTT, loss, jitter, goodput, OWD trend, ETX) to the operator and to the spanning tree layer for cost-based parent selection. Reports are exchanged peer-to-peer between direct neighbors at RTT-adaptive intervals clamped to [1s, 5s], with a 200 ms cold-start floor for the first five SRTT samples.

The CE (Congestion Experienced) bit in the FMP flags byte carries hop-by-hop ECN signaling: transit nodes detect congestion on outgoing links (via MMP loss/ETX or SO_RXQ_OVFL kernel drops) and set CE on forwarded packets, which the destination then mirrors to the IPv6 Traffic Class for ECN-capable flows.

For the full MMP design — operating modes, report scheduling, spin bit interaction, ECN, and the algorithmic details shared with session-layer MMP — see fips-mmp.md. For the SenderReport and ReceiverReport byte layouts, see ../reference/wire-formats.md. Configuration knobs live under node.mmp.* and node.ecn.* in ../reference/configuration.md.

Security Properties

Threat Resistance

The link-layer threat-resistance matrix (connection exhaustion, CPU exhaustion, replay, state confusion, spoofing variants, address spoofing, session correlation) is consolidated in ../reference/security.md along with the session-layer matrix and operator-facing controls.

Unauthenticated Attack Surface

Only handshake msg1 can be sent by unauthenticated parties. Encrypted frames require a known session index, and msg2 requires a response to a specific ephemeral key. The msg1 attack surface is protected by rate limiting, connection limits, and handshake timeouts.

Authenticated Peer Misbehavior

Authentication establishes identity but does not grant trust. An authenticated peer can send malformed packets (which fail AEAD and are dropped) or high-frequency traffic (rate-limited by higher layers). False tree coordinate claims are constrained by signature verification on TreeAnnounce messages.

Silent Drop Policy

Invalid packets are silently dropped without error responses. This prevents information leakage about internal state and avoids amplification attacks where an attacker sends invalid packets to elicit responses.

Implementation Status

Feature	Status
Noise XX handshake (with epoch and negotiation)	Implemented
Protocol negotiation (version + features + TLV)	Implemented
Node profiles (Full, NonRouting, Leaf)	Implemented
Peer restart detection (epoch mismatch)	Implemented
Link encryption (ChaCha20-Poly1305)	Implemented
Index-based session dispatch	Implemented
Replay protection (sliding window)	Implemented
Roaming (address-follows-crypto)	Implemented
Rate limiting (token bucket)	Implemented
Disconnect with reason codes	Implemented
Heartbeat liveness detection	Implemented
Reconnection handling	Implemented
Auto-reconnect after link-dead removal	Implemented
Handshake message retry (link + session layer)	Implemented
Common prefix framing	Implemented
AAD binding on encrypted frames	Implemented
Inner header timestamps	Implemented
Path MTU tracking (SessionDatagram)	Implemented
Metrics Measurement Protocol (MMP)	Implemented
ECN congestion signaling (CE relay, IPv6 marking)	Implemented
Rekey with index rotation	Implemented
Peer ACL (allowlist / denylist)	Implemented

References

• fips-concepts.md — Protocol overview
• fips-architecture.md — Layer architecture and identity model
• fips-transport-layer.md — Transport layer (below FMP)
• fips-session-layer.md — FSP (above FMP)
• fips-mmp.md — Metrics Measurement Protocol (link + session)
• fips-mesh-operation.md — How FMP's routing and self-organization work in practice
• ../reference/wire-formats.md — Byte-level wire format reference