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

FIPS Session Protocol (FSP)

The FIPS Session Protocol is the top protocol layer in the FIPS stack. It sits above the FIPS Link Protocol (FLP) and below applications (native FIPS API or IPv6 adapter). FSP provides end-to-end authenticated, encrypted datagram delivery between any two FIPS nodes, regardless of how many intermediate hops separate them.

Role

FSP manages end-to-end communication sessions between FIPS nodes identified by their public keys (npubs). Each session provides:

• End-to-end encryption: Payload confidentiality independent of how many intermediate nodes handle the traffic
• Mutual authentication: Both parties prove they control the private key for their claimed identity
• Replay protection: Counter-based nonces with sliding window, tolerant of UDP packet loss and reordering
• Transport independence: Sessions survive transport changes, route changes, and address changes — they are bound to npub identities, not to transport paths

FSP is a datagram session protocol. It provides encrypted datagrams, not reliable streams. There is no FIPS equivalent of TCP; if applications need reliability, ordering, or flow control, they provide it themselves (typically by running TCP over the FIPS IPv6 adapter).

Services Provided to Applications

Applications access the FIPS mesh through two interfaces, both served by FSP:

Native FIPS API

Applications address destinations directly by npub or public key. The FIPS stack resolves the destination's node_addr, establishes or reuses a session, encrypts the payload, and routes through FLP. No DNS involvement.

IPv6 Adapter

Unmodified IPv6 applications use a TUN device with fd::/8 routing. A local DNS service maps npub → IPv6 address and primes the identity cache. Packets arriving at the TUN are translated to FIPS datagrams and routed through FSP.

See fips-ipv6-adapter.md for the IPv6 adaptation layer.

What Applications Get

• Authenticated datagram delivery: Each datagram is encrypted and authenticated with session keys bound to both parties' npubs
• Session transparency: Sessions are established on demand and maintained automatically. Applications send packets; FSP handles session setup, encryption, and teardown.
• Endpoint identity: Applications address destinations by npub. The FIPS address is the public key.

What Applications Do Not Get

• Reliability: Datagrams may be lost, duplicated, or delivered out of order. FSP provides no retransmission or ordering.
• Path MTU discovery: FSP does not signal MTU to applications. The IPv6 adapter handles MTU enforcement via ICMP Packet Too Big and TCP MSS clamping.
• Congestion control: FSP does not throttle traffic. Applications running TCP over IPv6 get TCP's congestion control; native API applications must manage their own sending rate.

Services Required from FLP

FSP treats FLP as a black box providing three services. FSP knows nothing about transports, transport addresses, links, peers, spanning trees, coordinates, bloom filters, hop counts, or network topology.

SessionDatagram Forwarding

FLP accepts a SessionDatagram (source node_addr, destination node_addr, TTL, path MTU, payload) and delivers it best-effort toward the destination. Delivery may traverse multiple hops, each with independent link encryption.

Error Signaling

FLP signals routing failures asynchronously:

• CoordsRequired: A transit node lacks the destination's tree coordinates. FSP responds by re-initiating discovery and resetting the coordinate warmup strategy.
• PathBroken: Greedy routing reached a dead end. FSP responds by re-discovering the destination's current coordinates and resetting warmup.

Both signals are generated by transit nodes (not the destination) and travel back to the source inside a new SessionDatagram. They are plaintext (not end-to-end encrypted) because transit nodes have no session with the source.

Local Delivery

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

Session Lifecycle

Session Establishment

Sessions are established on demand when the first datagram needs to be sent to a destination with no existing session.

FSP uses Noise IK for session key agreement. The initiator knows the destination's npub (from DNS lookup or native API); the responder learns the initiator's identity from the handshake. This is the same asymmetry as link-layer peer connections.

The handshake is carried in SessionSetup and SessionAck messages:

1. Initiator sends SessionSetup containing Noise IK msg1 and both parties' tree coordinates
2. Responder processes msg1, learns initiator identity, sends SessionAck containing Noise IK msg2 and its own coordinates
3. Both parties derive identical symmetric session keys

Packets that trigger session establishment are queued (with bounded buffer) and transmitted after the session is established.

Self-Bootstrapping

SessionSetup is self-bootstrapping for routing. It carries the source's and destination's tree coordinates in the clear (not inside the Noise payload). As the message transits intermediate nodes, each node caches these coordinates, warming the path for subsequent data packets that carry only addresses (no coordinates).

SessionAck carries the responder's coordinates back along the reverse path, warming caches in the other direction.

Simultaneous Initiation

When both nodes attempt to establish a session simultaneously ("crossing hellos"), a deterministic tie-breaker resolves the conflict:

• If local_node_addr < remote_node_addr: Continue as initiator, ignore incoming setup
• If local_node_addr > remote_node_addr: Abort own initiation, switch to responder role

This ensures exactly one handshake completes.

Data Transfer

Once established, sessions carry encrypted data using the FSP pipeline. Each encrypted message includes:

• A 12-byte cleartext header (used as AEAD AAD) containing the counter and flags (including the CP flag for coordinate cache warming)
• Optional cleartext coordinates when the CP flag is set
• An AEAD-encrypted payload containing a 6-byte inner header (session-relative timestamp, message type, inner flags) followed by the application data

Session Idle Timeout

Sessions that see no traffic for a configurable duration (default 90s) are torn down. When traffic resumes, a new session is established automatically.

The idle timeout is deliberately shorter than the coordinate cache TTL (300s). This ordering ensures that when traffic stops and the session tears down, the transit node coordinate caches are still warm when a new session is established. The fresh SessionSetup re-warms the caches, maintaining routing continuity.

Session Independence from Transport

Sessions exist above the routing layer and are bound to npub identities, not transport addresses or routing paths. A session survives:

• Transport failover (UDP → Ethernet → back to UDP)
• Route changes (different intermediate hops)
• Transport address changes (IP address or port changes)
• Topology changes (direct peer becomes multi-hop or vice versa)

End-to-End Encryption

Noise IK Pattern

FSP uses the same Noise IK pattern as FLP link encryption, but with independent keys and sessions. The full Noise descriptor is Noise_IK_secp256k1_ChaChaPoly_SHA256.

The IK pattern:

• msg1 (→ e, es, s, ss): Initiator sends ephemeral key, encrypts static key to responder. Four DH operations establish session keys.
• msg2 (← e, ee, se): Responder sends ephemeral key. Both parties now share identical session keys.

After the handshake, Noise produces two directional symmetric keys (send_key, recv_key) used with ChaCha20-Poly1305 for all subsequent data.

Cryptographic Primitives

Component	Choice	Notes
Curve	secp256k1	Nostr-native
DH	ECDH on secp256k1	Standard EC Diffie-Hellman
Cipher	ChaCha20-Poly1305	AEAD, same as NIP-44
Hash	SHA-256	Nostr-native
Key derivation	HKDF-SHA256	Standard Noise KDF

These choices prioritize compatibility with the Nostr cryptographic stack.

secp256k1 Parity Normalization

Nostr npubs encode x-only public keys (32 bytes, no y-coordinate parity). The Noise IK pre-message mixes the responder's static key as a 33-byte compressed key, and the default secp256k1 ECDH hash includes a parity-dependent version byte.

Both operations are normalized to be parity-independent: the pre-message hash uses even parity (0x02 prefix), and ECDH hashes only the x-coordinate of the result point. This ensures handshakes succeed regardless of the responder's actual key parity.

Privacy Note

Noise IK does not provide initiator anonymity if the responder's static key is compromised. An attacker who obtains the responder's nsec can decrypt the initiator's identity from captured handshake messages. Noise XK would protect initiator identity in this scenario but requires an additional round-trip (3 handshake messages vs. 2). The privacy/latency tradeoff may be revisited with deployment experience.

Data Packet Authentication

FSP uses AEAD authentication only — no per-packet signatures. The Noise handshake binds session keys to both parties' static keys, so only holders of the corresponding nsecs can derive the session keys. This provides implicit authentication for every packet, matching WireGuard and Lightning's approach.

Forward Secrecy

Ephemeral keys in the Noise handshake provide forward secrecy. Compromise of static keys (nsec) does not reveal past session keys, because session keys are derived in part from ephemeral-ephemeral DH (ee), and ephemeral keys are discarded after the handshake.

Replay Protection

FSP uses explicit 8-byte counters on the wire for replay protection. Each side maintains a monotonically increasing send counter, included in the 12-byte cleartext header of every encrypted message. The receiver maintains a sliding window (2048-entry bitmap) tracking which counters have been seen.

This design is critical for operation over unreliable transports. Under UDP packet loss or reordering, implicit nonce counters (where the receiver increments on each decrypt attempt) would desynchronize permanently — a failed decrypt() increments the nonce, and the desync grows with each lost packet. Explicit counters allow the receiver to decrypt any packet independently, regardless of what packets were lost or reordered.

The same ReplayWindow and decrypt_with_replay_check() implementation is used at both the link and session layers.

CP (Coords Present) Warmup Strategy

Session establishment (SessionSetup/SessionAck) warms transit node coordinate caches along the path. But coordinate caches have a finite TTL (default 300s), and entries may be evicted under memory pressure. When a transit node's cache entry expires, it cannot forward data packets (which carry only addresses, not coordinates) and sends a CoordsRequired error.

FSP uses a warmup-then-reactive strategy to keep transit caches populated:

Warmup Phase

After session establishment, the first N data packets (configurable, default 5) include both source and destination coordinates via the CP flag in the FSP common prefix. The coordinates appear in cleartext between the 12-byte header and the ciphertext, allowing transit nodes to cache them without decryption.

Steady State

After the warmup count is reached, FSP clears the CP flag and sends minimal data packets (12-byte header + ciphertext). Transit nodes serve from their coordinate caches.

Reactive Recovery

When FSP receives a CoordsRequired signal:

1. The warmup counter resets — subsequent data packets include coordinates again
2. A new LookupRequest may be initiated to rediscover the destination's current coordinates
3. When the LookupResponse arrives for an established session, the warmup counter resets again (handling the timing gap where warmup packets might fire before transit caches are repopulated by discovery)

When FSP receives a PathBroken signal:

1. A LookupRequest is initiated to discover the destination's current coordinates (which may have changed due to topology change)
2. The warmup counter resets

Both signals are rate-limited at transit nodes (100ms per destination) to prevent storms during topology changes.

Warmup State Machine

        ┌──────────────┐
        │    WARMUP    │ ◄── Send first N packets with coords
        └──────┬───────┘
               │ N packets sent without CoordsRequired
               ▼
        ┌──────────────┐
        │   MINIMAL    │ ◄── Send packets without coords
        └──────┬───────┘
               │ CoordsRequired or PathBroken received
               ▼
        ┌──────────────┐
        │    WARMUP    │ ◄── Counter reset, send coords again
        └──────────────┘

Identity Cache

The identity cache maps FIPS address prefix (15 bytes, the fd::/8 IPv6 address minus the fd prefix) to (NodeAddr, PublicKey). This cache is needed only when using the IPv6 adapter — the native FIPS API provides the public key directly.

The mapping is deterministic (derived from the public key via SHA-256) and never becomes stale. The cache uses LRU-only eviction bounded by a configurable size (default 10K entries). There is no TTL — entries are evicted only when the cache is full and space is needed for a new entry.

Cache population mechanisms:

• DNS lookup: The primary path. Resolving npub1xxx...xxx.fips derives the IPv6 address and populates the identity cache.
• Inbound traffic: Authenticated sessions from other nodes populate the cache with their identity information.

Coordinate Cache

The coordinate cache maps NodeAddr → TreeCoordinate and is the critical data structure that enables efficient multi-hop routing. Without cached coordinates for a destination, FLP cannot make forwarding decisions and must either fall back to bloom-filter-only routing or signal CoordsRequired.

Unified Cache

The coordinate cache is a single unified cache (merged from previously separate coord_cache and route_cache). All coordinate sources — SessionSetup transit, CP-flagged data packets, LookupResponse — write to the same cache.

Eviction Policy

• TTL-based expiration: Entries expire after a configurable duration (default 300s)
• Refresh on use: Active routing through a cache entry resets its TTL, keeping hot entries alive
• LRU eviction: When the cache is full, least recently used entries are evicted first
• Flush on parent change: When the local node's tree parent changes, the entire coordinate cache is flushed. Tree parent changes mean the node's own coordinates have changed, making cached coordinates for other nodes potentially stale for routing purposes.

Timer Ordering

Cache and session timers are ordered so that idle sessions tear down before transit caches expire:

Timer	Default	Purpose
Session idle timeout	90s	Tear down unused sessions
Coordinate cache TTL	300s	Expire stale coordinates
DNS TTL	300s	Expire DNS resolutions

When traffic stops: the session tears down at 90s. When traffic resumes: DNS re-resolves the identity, a fresh SessionSetup carries coordinates, and transit node caches (still within their 300s TTL) are re-warmed.

Implementation Status

Feature	Status
Session establishment (Noise IK)	Implemented
End-to-end encryption (ChaCha20-Poly1305)	Implemented
Explicit counter replay protection	Implemented
CP warmup-then-reactive	Implemented
FSP wire format (prefix, AAD, inner header)	Implemented
Session-layer MMP report types	Implemented (wire format)
Identity cache (LRU-only)	Implemented
Coordinate cache (unified, TTL + refresh)	Implemented
Session idle timeout	Implemented
CoordsRequired handling	Implemented
PathBroken handling	Implemented
Simultaneous initiation tie-breaker	Implemented
Flush coord cache on parent change	Implemented
Rekey	Planned
Path MTU tracking (FLP SessionDatagram field)	Implemented
Path MTU notification (end-to-end echo)	Planned

References

• fips-intro.md — Protocol overview and architecture
• fips-link-layer.md — FLP specification (below FSP)
• fips-ipv6-adapter.md — IPv6 adaptation layer (above FSP)
• fips-mesh-operation.md — Routing, discovery, and error recovery
• fips-wire-formats.md — Wire format reference for all session message types