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

# FIPS Session Protocol Flow

## Overview

This document captures design considerations for FIPS protocol message flow,
including peer discovery, authentication, tree announcements, and data routing.

### Access to the FIPS Datagram Service

Applications can access FIPS datagram delivery through two interfaces:

- **Native FIPS API**: Applications address destinations directly by npub or
  public key. The FIPS stack resolves the destination's node_addr and routes
  without any DNS involvement. This is the preferred interface for
  FIPS-aware applications.

- **IPv6 adapter (TUN interface)**: Unmodified IPv6 applications use a TUN
  device with `fd::/8` routing. Because IPv6 addresses are one-way hashes of
  public keys, a local DNS service maps npub → IPv6 address and primes the
  identity cache so the TUN can route arriving packets.

The remainder of this document details the sequence of actions initiated through
the IPv6 adapter path. The native FIPS API bypasses sections 1.1–1.4 (DNS
entry point, identity cache) entirely, entering the flow at route discovery.

---

## 1. Application-Initiated Traffic Flow (IPv6 Adapter)

> **Note**: This section applies to traditional IP-based applications using the
> TUN interface. Applications using the native FIPS API address destinations
> directly by npub or public key; routing proceeds from there without DNS.

Traffic flow begins at the application layer with a DNS query, which triggers
a cascade of events through the FIPS stack.

### 1.1 DNS as Entry Point

An application wants to send IPv6 traffic to another FIPS node, identified by
an npub. The flow:

1. **DNS Query**: Application queries the local FIPS DNS service for the npub
   mapping to an IPv6 address

2. **FIPS DNS service** performs two functions:
   - **Address derivation**: Converts the npub to an identity and derives the
     corresponding `fd::/8` IPv6 address
   - **Cache priming**: Stores the identity mapping (IPv6 address ↔ npub ↔ node_addr)
     in the local FIPS routing cache

3. **DNS Response**: Returns the derived IPv6 address to the application

4. **Packet Transmission**: Application sends IPv6 packet to the returned address,
   which routes to the TUN interface via the `fd::/8` route

5. **TUN Processing**: When the packet arrives at the TUN, FIPS already has the
   cached mapping from the DNS lookup, enabling immediate routing decisions

6. **Note**: This identity cache is only necessary when using the FIPS IPv6 shim

### 1.2 Design Rationale

Using DNS as the trigger point ensures the routing cache is populated *before*
packets arrive. This avoids:

- Blocking packets while performing identity lookups
- Packet drops during cold-cache scenarios
- Complex async lookup machinery in the hot path

The DNS server acts as a "routing intent" signal - if an application queries
for a destination, it likely intends to send traffic there.

### 1.3 DNS Name Format

NPUBs are represented as DNS names in the format:

```text
npub1xxxxxx...xxxxx.fips
```

The FIPS DNS server recognizes names ending in `.fips` and extracts the npub
for address derivation.

### 1.4 Identity Cache Lifetime

The identity cache (IPv6 address ↔ npub ↔ node_addr) has the following lifetime
semantics:

- **Configurable timeout**: Cache entries expire after a configured duration
- **Traffic refresh**: Timer resets to zero whenever traffic is sent to that
  destination (LRU-style keep-alive)
- **TTL relationship**: Cache timeout MUST be longer than DNS TTL

The TTL constraint ensures that while an application believes its DNS resolution
is valid (within TTL), the corresponding FIPS routing entry remains present.
Example: DNS TTL = 300s, Cache timeout = 600s.

```text
DNS query → cache entry created (timeout = 600s)
   ...traffic... → timeout reset to 600s
   ...traffic... → timeout reset to 600s
DNS TTL expires (300s) → app may re-query, but cache still valid
   ...no traffic for 600s...
Cache entry expires
```

### 1.5 Traffic Without Prior DNS Lookup

A packet may arrive at the TUN for an `fd::/8` destination without a prior
DNS lookup (cached address, manual configuration, etc.). Since address
derivation is one-way (SHA-256), the npub cannot be recovered from the address,
and without the npub we cannot determine the node_addr needed for routing.

FIPS returns ICMPv6 Destination Unreachable (Code 0: No route to destination)
for packets to unknown addresses. The identity cache must be populated before
traffic can be routed.

Known cache population mechanisms:

- DNS lookup (primary path, described above)
- Inbound traffic from authenticated peers

---

## 2. TUN Reader Processing

After DNS resolution, the application sends an IPv6 datagram to the destination
address. The kernel routes it to the TUN interface (via the `fd::/8` route),
where the FIPS TUN reader receives it.

### 2.1 Packet Arrival

```text
Application
    │
    ▼
IPv6 datagram (src=local_addr, dst=target_addr)
    │
    ▼
Kernel routing table: fd::/8 → fips0
    │
    ▼
TUN reader receives raw IPv6 packet
```

### 2.2 TUN Reader Actions

On receiving a packet, the TUN reader:

1. **Validate IPv6 header**: Version = 6, payload length sane, etc.

2. **Extract destination address**: The `fd::/8` address from the IPv6 header

3. **Identity cache lookup**: Query cache for destination address
   - **Miss**: Return ICMPv6 Destination Unreachable (see §1.5)
   - **Hit**: Proceed with routing

4. **Retrieve routing identity**: Cache hit provides:
   - `npub`: The Nostr public key of the destination
   - `node_addr`: SHA-256(pubkey), used for spanning tree routing

5. **Session lookup**: Check for existing FIPS session with destination npub
   - **Hit**: Use existing session for encryption/signing
   - **Miss**: Initiate session establishment (see §3)

6. **Route determination**: Using node_addr, determine the next hop peer:
   - Check route cache for destination's spanning tree coordinates
   - If cache miss, initiate route discovery (see §4.4)
   - Select next hop via greedy routing toward destination coordinates

7. **Packet forwarding**: Encapsulate and send via appropriate transport:
   - Encrypt payload with session keys (end-to-end)
   - Wrap in link-layer frame for next hop peer
   - Encrypt with link keys and transmit via peer's transport

---

## 3. FIPS Sessions

A FIPS session represents a bidirectionally authenticated, encrypted channel
between two FIPS nodes.

### 3.1 Session Properties

Each session contains:

- **Peer identity**: The remote node's npub and node_addr
- **Symmetric session keys**: Directional keys for encryption (send_key, recv_key)
- **Nonce counters**: Per-direction counters for replay protection

Payloads within a session are:

1. **Encrypted** with the session key (provides confidentiality)
2. **Authenticated** via AEAD tag (session keys bound to npub identities)

Authentication derives from the Noise IK handshake binding session keys to
both parties' static keys. See §6 for cryptographic details.

### 3.2 Session Establishment Trigger

When the TUN reader has a packet for a destination with no existing session:

```text
TUN reader
    │
    ├─► Identity cache lookup → node_addr
    │
    ├─► Session lookup (by npub) → MISS
    │
    └─► Initiate session establishment
```

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

### 3.3 Session Independence from Transport

FIPS sessions exist above the routing layer. A session between two npubs
survives:

- Transport failover (UDP → WiFi → back to UDP)
- Route changes (different intermediate hops)
- Transport address changes on either end (WiFi → LTE → WiFi)

The session is bound to **npub identities**, not transport addresses or routing
paths. This allows FIPS endpoints to roam over their transports as needed while
maintaining an established session.

### 3.4 Session Establishment Flow

FIPS uses Noise IK for session establishment. The initiator knows the
destination's npub; the responder learns the initiator's identity from the
handshake. This is the same asymmetry as link-layer connections.

The handshake is carried inside SessionSetup/SessionAck messages (see §5.1),
which also establish routing session state at intermediate nodes.

### 3.5 Simultaneous Session Initiation (Crossing Hellos)

When both nodes attempt to establish a session simultaneously, a deterministic
tie-breaker resolves the conflict using node_addr ordering (consistent
with the link-layer cross-connection tie-breaker):

- 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 with minimal wasted effort.

---

## 4. FIPS Mesh Routing

Below the session layer, all FIPS packets (session handshake messages, encrypted
payloads, control traffic) must be routed through the mesh to their destination.
See [fips-routing.md](fips-routing.md) for the full routing design.

### 4.1 Routing Overview

FIPS routing combines three mechanisms:

1. **Bloom filters**: Fast reachability lookup for nearby destinations
2. **Discovery protocol**: Query-based lookup for distant destinations
3. **Greedy tree routing**: Coordinate-based forwarding using spanning tree position

The routing layer maintains coordinate caches mapping `node_addr →
TreeCoordinate` (see [fips-routing.md](fips-routing.md) §4 for details).
Cache hits enable immediate routing via `find_next_hop()`; cache misses
trigger route discovery via bloom filter queries or LookupRequest flooding.

### 4.2 Packet Handling During Discovery

Packets are queued (with bounded buffer) while route discovery is in progress
and transmitted once coordinates are obtained.

### 4.3 Coordinate Cache Lifetime

Two caches store `node_addr → TreeCoordinate` mappings (see
[fips-routing.md](fips-routing.md) §4):

- **CoordCache** (session-populated): TTL-based eviction (300s default), 50K entries
- **RouteCache** (discovery-populated): LRU eviction only, 10K entries

Neither cache stores next-hop information — routing decisions are computed at
lookup time by `find_next_hop()` using cached coordinates.

---

## 5. Route Cache Warming

### 5.1 Initial Warming via Handshake

The crypto session handshake (SessionSetup/SessionAck) warms route caches at
intermediate routers as it transits. Each message carries the sender's
coordinates; routers extract and cache `node_addr → TreeCoordinate` in the
CoordCache. After the handshake completes, data packets use minimal 38-byte
headers (34 envelope + 4 DataPacket) and routers forward via `find_next_hop()`.

### 5.2 Cache Miss Recovery

When an intermediate router's cache entry expires or is evicted, it cannot
forward data packets (which carry only addresses, not coordinates). The router
returns a CoordsRequired error to the sender.

The crypto session remains valid—only the routing state is lost. Recovery uses
coords-on-demand: data packets include an optional coordinates field. When the
sender receives CoordsRequired:

1. Sender marks the route as "cold"
2. Subsequent data packets include coordinates (flag bit set)
3. Routers along the path cache coordinates as packets transit
4. Once route is warm again, sender clears the flag and resumes minimal headers

This avoids a full SessionSetup round-trip for what is purely a routing cache
refresh.

### 5.3 DataPacket Coordinate Flag

The DataPacket `flags` field includes a `COORDS_PRESENT` bit:

| Bit | Meaning                                              |
|-----|------------------------------------------------------|
| 0   | COORDS_PRESENT - coordinates follow the fixed header |

When set, the packet includes `src_coords` and `dest_coords` after the standard
header fields. Routers process these coordinates the same way as SessionSetup:
cache both directions and forward using greedy routing.

### 5.4 Sender State Machine

```text
        ┌──────────────┐
        │    WARM      │ ◄── Normal: send minimal headers
        └──────┬───────┘
               │ CoordsRequired received
               ▼
        ┌──────────────┐
        │    COLD      │ ◄── Send packets with coords
        └──────┬───────┘
               │ N packets sent successfully
               ▼
        ┌──────────────┐
        │    WARM      │
        └──────────────┘
```

The sender transitions back to WARM after sending a configurable number of
packets with coordinates (e.g., 3) without receiving CoordsRequired. This
provides confidence that caches along the path are populated.

---

## 6. Session-Layer Encryption

FIPS uses two independent Noise Protocol handshakes at different layers:

| Layer   | Scope       | Pattern  | Purpose                                   |
|---------|-------------|----------|-------------------------------------------|
| Link    | Hop-by-hop  | Noise IK | Authenticate peers, encrypt link          |
| Session | End-to-end  | Noise IK | Authenticate endpoints, encrypt payload   |

Both use `Noise_IK_secp256k1_ChaChaPoly_SHA256` with the same cryptographic
primitives, but with separate keys and sessions.

> **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), increasing session setup
> from 3 packets to 4. Further deployment experience is needed to evaluate
> whether the privacy benefit justifies the latency cost.

### 6.1 Why Two Layers?

**Link encryption** protects against passive observers on each hop but allows
intermediate nodes to see routing information (destination address).

**Session encryption** protects the actual payload end-to-end. Intermediate
nodes forward opaque ciphertext without being able to read the contents.

### 6.2 Session Noise Handshake

The session-layer Noise IK handshake is carried inside `SessionSetup`/`SessionAck`
messages, which themselves travel through the link-encrypted channel:

```text
Initiator knows destination npub (from DNS lookup)
    │
    ▼
SessionSetup { coords, handshake_payload: Noise IK msg1 }
    │
    ▼  (travels through link-encrypted hops)
    │
Responder processes msg1, learns initiator identity
    │
    ▼
SessionAck { coords, handshake_payload: Noise IK msg2 }
    │
    ▼
Session keys established (independent of link keys)
```

### 6.3 Cryptographic Primitives

Both link and session layers use the same cryptographic stack:

| 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 existing Nostr infrastructure.
Secp256k1 and SHA-256 are already used for Nostr identities, and
ChaCha20-Poly1305 matches NIP-44 encryption. Lightning's BOLT 8 provides a
proven reference for adapting Noise Protocol to secp256k1.

**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. Since the
initiator may only have the x-only key, 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. See
`secp256k1-parity-fix.md` for detailed analysis.

### 6.4 Handshake Integration with SessionSetup

The Noise handshake messages embed in SessionSetup/SessionAck, which are
carried inside a SessionDatagram envelope that provides addressing:

```text
SessionDatagram {
    // Addressing (used by routers for forwarding and error routing)
    src_addr: NodeAddr,
    dest_addr: NodeAddr,
    hop_limit: u8,

    // Payload: SessionSetup
    payload: SessionSetup {
        // Routing portion (processed by routers for cache warming)
        src_coords: Vec<NodeAddr>,
        dest_coords: Vec<NodeAddr>,

        // Crypto portion (opaque to routers, processed by destination)
        handshake_payload: Vec<u8>,  // Noise IK message 1
    }
}

SessionDatagram {
    src_addr: NodeAddr,   // Responder
    dest_addr: NodeAddr,  // Original sender
    hop_limit: u8,

    payload: SessionAck {
        // Routing portion
        src_coords: Vec<NodeAddr>,  // Responder's coordinates

        // Crypto portion
        handshake_payload: Vec<u8>,  // Noise IK message 2
    }
}
```

### 6.5 Session Keys

After handshake completion, Noise produces two symmetric keys:

- `send_key`: For encrypting outbound packets
- `recv_key`: For decrypting inbound packets

These are used with ChaCha20-Poly1305 for all subsequent data packets.

### 6.6 Nonce Management

FIPS uses counter-based nonces for ChaCha20-Poly1305. Each side maintains a
64-bit send counter, incremented per packet. No coordination is needed since
keys are directional. The counter also enables replay detection by rejecting
packets with nonce ≤ last seen.

### 6.7 Forward Secrecy

The ephemeral keys (`e` in Noise notation) provide forward secrecy:

- Compromise of static keys (npub/nsec) doesn't reveal past session keys
- Each session has unique ephemeral keys
- Session keys derived from ephemeral-ephemeral DH (`ee`)

### 6.8 Reference: Lightning BOLT 8

Lightning's adaptation of Noise for secp256k1 (BOLT 8) provides a proven
reference implementation:

- Uses Noise XK pattern (different from our IK)
- Same secp256k1 + ChaCha20-Poly1305 + SHA-256 stack
- Handles the secp256k1 ECDH correctly
- Open source implementations available in multiple languages

FIPS can reference BOLT 8's cryptographic details while using the IK pattern.

### 6.9 Data Packet Authentication

**Decision**: Use AEAD authentication only (no per-packet signatures).

The Noise handshake binds session keys to both parties' static keys. After
handshake completion:

- Session keys are cryptographically tied to both npubs
- AEAD (ChaCha20-Poly1305) provides integrity and authenticity
- Only the holder of the session key can produce valid ciphertext
- Session keys can only be derived by holders of the corresponding nsecs

Per-packet signatures would add:

- 64 bytes overhead per packet
- Signing CPU cost (secp256k1 Schnorr)
- Verification CPU cost at receiver

Since Noise already provides authentication through key binding, signatures
are redundant. This matches WireGuard and Lightning's approach.

---

## 7. Peer Connection Establishment

Before any session-layer traffic can flow, nodes must establish authenticated
link-layer connections with their peers using Noise IK. See
[fips-wire-protocol.md](fips-wire-protocol.md) for the complete wire protocol
specification including handshake flow, session lifecycle, index management,
roaming support, and transport-specific considerations.

After successful Noise IK handshake:

1. **Link encrypted**: All subsequent messages use AEAD encryption
2. **TreeAnnounce exchange**: Both peers send their current spanning tree state
3. **FilterAnnounce exchange**: Both peers send their bloom filters
4. **Peer is Active**: Can now participate in routing and forwarding

The first TreeAnnounce from a new peer may trigger parent reselection if that
peer offers a better path to root. See [fips-gossip-protocol.md](fips-gossip-protocol.md)
for TreeAnnounce and FilterAnnounce wire formats.

---

## 8. Session Layer Wire Format

Session layer messages are carried inside `SessionDatagram` (type 0x40) at the
link layer. The session datagram provides source and destination addressing for
multi-hop forwarding: it is encrypted hop-by-hop with link keys, and the inner
payload is either encrypted end-to-end with session keys (for data traffic) or
plaintext (for link-layer error signals from transit routers).

### 8.0 SessionDatagram Envelope

The SessionDatagram is a link-layer message (type 0x40) that carries all
routable traffic through the mesh. It provides the source and destination
addresses that transit routers use for forwarding decisions.

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                        SESSION DATAGRAM (0x40)                              │
├────────┬──────────────────┬───────────┬─────────────────────────────────────┤
│ Offset │ Field            │ Size      │ Description                         │
├────────┼──────────────────┼───────────┼─────────────────────────────────────┤
│   0    │ msg_type         │ 1 byte    │ 0x40 (link-layer SessionDatagram)   │
│   1    │ src_addr         │ 16 bytes  │ Source node_addr                    │
│  17    │ dest_addr        │ 16 bytes  │ Destination node_addr               │
│  33    │ hop_limit        │ 1 byte    │ Decremented each hop                │
│  34    │ payload          │ variable  │ Session-layer message (see §8.1)    │
└────────┴──────────────────┴───────────┴─────────────────────────────────────┘

Fixed header: 34 bytes
```

**src_addr**: The originator of the datagram. For data traffic, this is the
source endpoint. For error signals (CoordsRequired, PathBroken), this is the
transit router that generated the error. Transit routers use `src_addr` to
route error signals back to the packet's originator.

**dest_addr**: The intended recipient. Transit routers use this for next-hop
selection via `find_next_hop()`.

**hop_limit**: Decremented at each forwarding hop. When it reaches zero, the
datagram is dropped silently. This prevents infinite forwarding loops. Initial
value is configurable (default 64).

### 8.1 Payload Message Types

The SessionDatagram payload begins with a message type byte, followed by the
message-specific content.

**Session-layer messages** (end-to-end encrypted with session keys):

| Type Code | Message        | Direction | Purpose                           |
|-----------|----------------|-----------|-----------------------------------|
| 0x00      | SessionSetup   | S → D     | Establish session + warm caches   |
| 0x01      | SessionAck     | D → S     | Confirm session establishment     |
| 0x10      | DataPacket     | Both      | Application data                  |

**Link-layer error signals** (plaintext, generated by transit routers):

| Type Code | Message        | Direction | Purpose                           |
|-----------|----------------|-----------|-----------------------------------|
| 0x20      | CoordsRequired | R → S     | Router cache miss                 |
| 0x21      | PathBroken     | R → S     | Greedy routing failed             |

Error signals are generated by transit router R and sent back to the source S
inside a *new* SessionDatagram with `src_addr=R, dest_addr=S`. They are
plaintext (not end-to-end encrypted) because the transit router has no session
with the source. Link-layer encryption protects them hop-by-hop.

> **Address terminology**: The `src_addr` and `dest_addr` fields in the
> SessionDatagram header are node_addrs (16-byte truncated SHA-256 hashes of
> pubkeys). These are visible to intermediate routers for routing decisions. The
> actual FIPS addresses (pubkeys/npubs) are exchanged only during the Noise IK
> handshake and never appear in packet headers—routers cannot determine endpoint
> identities from the node_addrs they see.

### 8.2 SessionSetup (0x00)

Establishes a crypto session and warms router coordinate caches along the path.
The `src_addr` and `dest_addr` are carried in the enclosing SessionDatagram
envelope (§8.0); the SessionSetup payload carries only coordinates and the
Noise handshake.

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                    SESSION SETUP PAYLOAD (inside SessionDatagram)            │
├────────┬──────────────────┬───────────┬─────────────────────────────────────┤
│ 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   │ u16 LE, number of src coord entries │
│   4    │ src_coords       │ 16 × n    │ NodeAddr array (self → root)        │
│  ...   │ dest_coords_count│ 2 bytes   │ u16 LE, number of dest coord entries│
│  ...   │ dest_coords      │ 16 × m    │ NodeAddr array (dest → root)        │
│  ...   │ handshake_len    │ 2 bytes   │ u16 LE, Noise payload length        │
│  ...   │ handshake_payload│ variable  │ Noise IK msg1 (82 bytes typical)    │
└────────┴──────────────────┴───────────┴─────────────────────────────────────┘
```

**Example** (depth 3 source, depth 4 destination, with Noise handshake):

```text
SessionDatagram { src: S, dest: D, hop_limit: 64 } +
┌──────┬───────┬───────┬─────────────┬───────┬───────────────┬───────┬───────┐
│ 0x00 │ 0x01  │ 0x03  │ src_coords  │ 0x04  │ dest_coords   │ 0x52  │ noise │
│ type │ flags │ count │ 3 × 16 bytes│ count │ 4 × 16 bytes  │ len=82│ msg1  │
└──────┴───────┴───────┴─────────────┴───────┴───────────────┴───────┴───────┘

SessionDatagram header: 34 bytes
SessionSetup payload: 1 + 1 + 2 + 48 + 2 + 64 + 2 + 82 = 202 bytes
Total: 236 bytes
```

### 8.3 SessionAck (0x01)

Confirms session establishment and completes the Noise handshake. Addressing
is in the enclosing SessionDatagram envelope.

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                    SESSION ACK PAYLOAD (inside SessionDatagram)              │
├────────┬──────────────────┬───────────┬─────────────────────────────────────┤
│ Offset │ Field            │ Size      │ Description                         │
├────────┼──────────────────┼───────────┼─────────────────────────────────────┤
│   0    │ msg_type         │ 1 byte    │ 0x01                                │
│   1    │ flags            │ 1 byte    │ Reserved                            │
│   2    │ src_coords_count │ 2 bytes   │ u16 LE                              │
│   4    │ src_coords       │ 16 × n    │ Acknowledger's coords (for caching) │
│  ...   │ handshake_len    │ 2 bytes   │ u16 LE, Noise payload length        │
│  ...   │ handshake_payload│ variable  │ Noise IK msg2 (33 bytes typical)    │
└────────┴──────────────────┴───────────┴─────────────────────────────────────┘
```

### 8.4 DataPacket (0x10)

Carries encrypted application data (typically IPv6 payloads). Addressing and
hop limit are in the enclosing SessionDatagram envelope.

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                    DATA PACKET (Minimal Header, inside SessionDatagram)      │
├────────┬──────────────────┬───────────┬─────────────────────────────────────┤
│ Offset │ Field            │ Size      │ Description                         │
├────────┼──────────────────┼───────────┼─────────────────────────────────────┤
│   0    │ msg_type         │ 1 byte    │ 0x10                                │
│   1    │ flags            │ 1 byte    │ Bit 0: COORDS_PRESENT               │
│   2    │ payload_length   │ 2 bytes   │ u16 LE                              │
│   4    │ payload          │ variable  │ Encrypted application data          │
└────────┴──────────────────┴───────────┴─────────────────────────────────────┘

Minimal header: 4 bytes (+ 34 SessionDatagram header = 38 bytes total)
```

When `COORDS_PRESENT` flag is set (route warming after CoordsRequired):

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                    DATA PACKET (With Coordinates, inside SessionDatagram)    │
├────────┬──────────────────┬───────────┬─────────────────────────────────────┤
│ Offset │ Field            │ Size      │ Description                         │
├────────┼──────────────────┼───────────┼─────────────────────────────────────┤
│   0    │ msg_type         │ 1 byte    │ 0x10                                │
│   1    │ flags            │ 1 byte    │ 0x01 (COORDS_PRESENT)               │
│   2    │ payload_length   │ 2 bytes   │ u16 LE                              │
│   4    │ src_coords_count │ 2 bytes   │ u16 LE                              │
│   6    │ src_coords       │ 16 × n    │ Source coordinates                  │
│  ...   │ dest_coords_count│ 2 bytes   │ u16 LE                              │
│  ...   │ dest_coords      │ 16 × m    │ Destination coordinates             │
│  ...   │ payload          │ variable  │ Encrypted application data          │
└────────┴──────────────────┴───────────┴─────────────────────────────────────┘

With depth-4 coords both directions:
  DataPacket: 4 + 2 + 64 + 2 + 64 = 136 bytes header
  + SessionDatagram: 34 bytes
  Total: 170 bytes header
```

### 8.5 CoordsRequired (0x20)

Sent by an intermediate router when it cannot forward a SessionDatagram due to
coordinate cache miss for the destination. This is a **link-layer error signal**:
the transit router generates a new SessionDatagram addressed back to the
original source, with the CoordsRequired payload in plaintext (not end-to-end
encrypted, since the transit router has no session with the source).

**Error routing**: Router R receives a SessionDatagram `{src: S, dest: D}` but
cannot route to D. R creates a new SessionDatagram `{src: R, dest: S}` carrying
a CoordsRequired payload, and calls `find_next_hop(S)` to route it back to S.
If R also cannot route to S, the error is dropped silently (no cascading errors).

**CoordsRequired payload** (inside SessionDatagram):

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                        COORDS REQUIRED PAYLOAD                              │
├────────┬──────────────────┬───────────┬─────────────────────────────────────┤
│ Offset │ Field            │ Size      │ Description                         │
├────────┼──────────────────┼───────────┼─────────────────────────────────────┤
│   0    │ msg_type         │ 1 byte    │ 0x20                                │
│   1    │ flags            │ 1 byte    │ Reserved                            │
│   2    │ dest_addr        │ 16 bytes  │ The node_addr we couldn't route to  │
│  18    │ reporter         │ 16 bytes  │ NodeAddr of reporting router        │
└────────┴──────────────────┴───────────┴─────────────────────────────────────┘

Payload: 34 bytes
Wrapped in SessionDatagram: 34 + 34 = 68 bytes total
```

### 8.6 PathBroken (0x21)

Sent when greedy routing fails (no peer is closer to destination). Like
CoordsRequired, this is a **link-layer error signal** carried inside a new
SessionDatagram addressed back to the original source.

**PathBroken payload** (inside SessionDatagram):

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                         PATH BROKEN PAYLOAD                                 │
├────────┬──────────────────┬───────────┬─────────────────────────────────────┤
│ Offset │ Field            │ Size      │ Description                         │
├────────┼──────────────────┼───────────┼─────────────────────────────────────┤
│   0    │ msg_type         │ 1 byte    │ 0x21                                │
│   1    │ flags            │ 1 byte    │ Reserved                            │
│   2    │ dest_addr        │ 16 bytes  │ The unreachable node_addr           │
│  18    │ reporter         │ 16 bytes  │ NodeAddr of reporting router        │
│  34    │ last_coords_count│ 2 bytes   │ u16 LE                              │
│  36    │ last_known_coords│ 16 × n    │ Stale coords that failed            │
└────────┴──────────────────┴───────────┴─────────────────────────────────────┘
```

### 8.7 Full Packet Layout Example

A DataPacket from source S to destination D, transiting router R:

```text
┌─────────────────────────────────────────────────────────────────────────────┐
│                           UDP DATAGRAM                                      │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                             │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │              FIPS LINK LAYER (S→R encrypted)                          │  │
│  ├───────────┬──────────────┬────────────┬───────────────────────────────┤  │
│  │ 0x00      │ R's recv_idx │ counter    │ ciphertext + tag              │  │
│  │ 1 byte    │ 4 bytes LE   │ 8 bytes LE │ N + 16 bytes                  │  │
│  └───────────┴──────────────┴────────────┴───────────────────────────────┘  │
│                                            │                                │
│                    ┌───────────────────────┘                                │
│                    │ Decrypt with S↔R link keys                             │
│                    ▼                                                        │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │              SESSION DATAGRAM (0x40) — plaintext for R                 │  │
│  ├───────────┬──────────────┬──────────────┬──────────┬──────────────────┤  │
│  │ 0x40      │ src_addr     │ dest_addr    │ hop_limit│ payload          │  │
│  │ msg_type  │ S (16 bytes) │ D (16 bytes) │ 64       │ (session msg)    │  │
│  └───────────┴──────────────┴──────────────┴──────────┴──────────────────┘  │
│                    │                                                        │
│                    │ R reads src_addr + dest_addr for routing               │
│                    ▼                                                        │
│  ┌───────────────────────────────────────────────────────────────────────┐  │
│  │              SESSION LAYER PAYLOAD (S↔D encrypted)                    │  │
│  ├───────────┬───────┬──────────┬──────────┬─────────────────────────────┤  │
│  │ 0x10      │ flags │ reserved │ pay_len  │ encrypted application data  │  │
│  │ DataPacket│ 0x00  │ 0x00     │ 1400     │ (S↔D session keys + tag)    │  │
│  └───────────┴───────┴──────────┴──────────┴─────────────────────────────┘  │
│                                                                             │
└─────────────────────────────────────────────────────────────────────────────┘

Router R can see: src_addr, dest_addr, hop_limit (for routing/error signaling)
Router R cannot see: payload contents (encrypted with S↔D session keys)
```

### 8.7.1 Error Routing Example

When router R cannot forward a SessionDatagram from S to D:

```text
1. R receives SessionDatagram { src: S, dest: D, payload: DataPacket{...} }
2. R has no cached coordinates for D and no bloom filter hit
3. R creates NEW SessionDatagram:
   { src: R, dest: S, hop_limit: 64,
     payload: CoordsRequired { dest: D, reporter: R } }
4. R calls find_next_hop(S) to route the error back
5. If R can also not route to S: drop silently (no cascading errors)
```

This avoids the need for transit routers to have end-to-end sessions with
the source. The `src_addr` field in the original SessionDatagram tells the
transit router where to send the error.

### 8.8 Encoding Rules

- All multi-byte integers are **little-endian**
- NodeAddr is 16 bytes (truncated SHA-256 hash of npub)
- IPv6 addresses are 16 bytes (network byte order)
- Variable-length coordinate arrays use 2-byte u16 count prefix

```text
Vec<NodeAddr> encoding:
  count: u16 (little-endian)
  items: [u8; 16] × count
```