# Protocol

Module boundaries are in [architecture.md](architecture.md); the threat model
is in [threat-model.md](threat-model.md).

## Versioning

Two versions exist and are independent:

- **Identity scheme**, `tsunagi-network-id-v1`. Frozen. Changing it creates a
  different network space for the same name and secret.
- **Control protocol**, ALPN `tsunagi/ctrl/1`, `PROTOCOL_VERSION = 1`.

Upgrading the crate or bumping the control protocol must never change an
existing `NetworkId`.

## Canonical encoding

Everything that is hashed or MAC'd uses length-prefixed fields, written here as
`LP(x) = u32_be(len(x)) || x`. String concatenation is never used, so no two
different field splits can produce the same bytes.

## Network space identity

```text
salt = SHA-256( LP("tsunagi-network-id-v1") || LP(name_utf8) )
prk  = HKDF-SHA256-Extract(salt, ikm = secret_bytes)

info(label)   = LP("tsunagi-network-id-v1") || LP(label)
network_id    = HKDF-Expand(prk, info("network-id"),     32)
discovery_key = HKDF-Expand(prk, info("discovery-key"),  32)
auth_key      = HKDF-Expand(prk, info("handshake-auth"), 32)
```

HKDF's `info` parameter is what separates the three outputs (RFC 5869 §3.2).
The labels and this encoding are frozen.

**Name rules.** 1..=64 bytes of UTF-8, no control characters, no leading or
trailing ASCII whitespace — such a name is *rejected*, not trimmed. The name is
used verbatim: no case folding, no Unicode normalisation. `Home` and `home` are
different network spaces.

**Secret rules.** Used verbatim: never trimmed, case-folded, normalised or
truncated. At least 16 bytes; `NetworkSecret::generate()` produces 32 random
bytes. The canonical text form is `tsn1` followed by lowercase unpadded base32.

**Key separation.** `network_id` is public. `discovery_key` is what a discovery
backend is told; it is secret-derived but is **not** a credential — learning it
does not help pass the handshake, and it must never be used as a password or
bearer token. `auth_key` never leaves the process.

## No competing genesis

A network's description is `NetworkDescriptor { scheme, name, network_id }`. It
contains no creator identity, no creation time and no owner signature, so two
agents started independently with the same parameters produce byte-identical
descriptors. No minimum-hash rule, no vote, no leader. The secret is not part
of it.

A network's name does not change inside an existing space. A different name or
secret is a different space.

## Framing

One QUIC bidirectional stream per session. A frame is `u32_be(len) || payload`.
The announced length is checked against `Limits::max_frame_len` (64 KiB by
default) **before** any buffer of that size is allocated. Payloads are
[postcard], a compact deterministic serde format — not a general RPC framework.

No encryption is layered on top of iroh: QUIC/TLS already provides
confidentiality, integrity and endpoint authentication.

[postcard]: https://docs.rs/postcard

## Handshake

A successful iroh connection proves only *which endpoint* is on the other side,
because the endpoint id is the public key in the TLS certificate. Anyone can
dial us. Membership of a specific network is proved separately.

iroh exposes the TLS exporter (RFC 5705) through
`Connection::export_keying_material`. That yields the same secret bytes on both
ends of *this* connection, which is what stops a proof being replayed
elsewhere. **It proves nothing about the shared secret on its own**, because
both ends of any connection can compute it. The membership proof is the HMAC
keyed by `auth_key`; the exporter output is just one of its inputs.

```text
cb = TLS-Exporter(label = "tsunagi/handshake/v1", context = network_id, 32)

transcript(role) = LP("tsunagi-handshake-v1")
                || LP(role)                    // "initiator-proof" | "responder-proof"
                || LP(u16_be(protocol_version))
                || LP(network_id)              // 32 bytes
                || LP(initiator_endpoint_id)   // 32 bytes
                || LP(responder_endpoint_id)   // 32 bytes
                || LP(cb)                      // 32 bytes
                || LP(nonce_initiator)         // 16 bytes
                || LP(nonce_responder)         // 16 bytes

proof(role) = HMAC-SHA256(auth_key, transcript(role))
```

What each input buys:

| input | property |
|---|---|
| `auth_key` | membership in this network space |
| `cb` | binding to this connection; a captured proof is useless on another |
| `network_id` | binding to this network space |
| both endpoint ids | binding to these two identities |
| distinct `role` labels | no reflection: a proof cannot be bounced back |
| both nonces | freshness contributed by each side |

Message order:

```text
initiator -> responder : Hello     { version, network_id, nonce_i }
initiator <- responder : HelloAck  { version, nonce_r }
initiator -> responder : AuthProof { proof(initiator) }
initiator <- responder : AuthProof { proof(responder) }   // only after the first verified
```

The responder emits nothing derived from `auth_key` until the initiator's proof
verifies, so a caller who does not know the secret learns nothing. Comparison
is constant time. Endpoint ids always come from the TLS certificate, never from
a peer's claim. Until both steps complete, no regular control message is
accepted in either direction. The whole exchange is bounded by
`Limits::handshake_timeout`.

A `Hello` naming a network this agent does not have active is rejected with
"unknown network". Because the claim is unverified at that point, the rejection
event does not report a network id.

## The data plane protocol

IP plugin packets never travel on a control connection. They use their own
ALPN, `tsunagi/data/3`, on their own iroh connection:

```text
initiator -> responder : (the same membership handshake as above)
initiator -> responder : DataOpen    { protocol, max_datagram }
initiator <- responder : DataOpenAck { accepted, max_datagram }
thereafter             : QUIC datagrams carrying fragments of opaque payloads
```

The membership handshake is identical and bound to the same network, so a data
channel cannot be opened by somebody who does not know the secret. `protocol`
is bounded and must name a plugin the responder actually runs; otherwise the
channel is declined, which is an ordinary outcome rather than an error.

Only one side dials — the one with the smaller endpoint id — so two agents
never open two channels for the same thing.

Packets ride as QUIC **datagrams**: unreliable and unordered, which is what a
tunnelled protocol wants, and free of the head-of-line blocking a stream would
add. `max_datagram` negotiates the reassembled payload limit (at most 65536
bytes), independently of the current QUIC path MTU. The transport reads QUIC's
current datagram capacity for every fragment, including after migration.

Every datagram starts with a 16-byte header: packet ID (`u64`), total payload
length (`u32`), and byte offset (`u32`), all big endian. The payload is the
complete peer-relay envelope containing opaque plugin bytes. Relays reassemble
before forwarding and fragment again for their outgoing path; they never
decrypt the plugin's payload. Small packets use the same framing in one part.

Reassembly tolerates reordering and exact duplicates. Conflicting lengths,
overlapping data and excessive fragment counts discard that packet. Each link
allows at most 64 incomplete packets, 256 KiB of their payload buffers, and 128
fragments per packet. Incomplete packets expire after five seconds, without
holding up subsequent packets. Packet IDs are scoped to a QUIC connection;
reassembly is created only after the membership handshake. All limits live in
`config.rs`. There is no transport-layer retransmission added by this framing.

Version 3 is incompatible with previous data ALPNs. Upgrade both endpoints
and intermediate peers together. The control protocol, network secret,
device identity and stored network configuration are unchanged.

Separate connections mean separate congestion control, so a saturated data
plane cannot delay control messages, and a data plane failure cannot take the
control plane down with it.

## Control messages

After authentication, every frame is an `Envelope { network_id, message }` and
the `network_id` is re-checked against the session's network on every message.
A mismatch ends that session and is counted as a protocol violation; it does
not affect other networks.

| message | meaning |
|---|---|
| `Announce { hostname, capabilities }` | this agent's hostname and IP-plugin capabilities |
| `Ping { seq, payload }` | small request used to verify the exchange |
| `Pong { seq, payload }` | the echoed reply |
| `State { records }` | a snapshot of signed records, merged into what the receiver holds |
| `Bye { reason }` | graceful goodbye; not a revocation of anything |

A `State` snapshot is merged, never substituted: an author missing from it is
left untouched. Each record carries its own signature, so a peer forwarding
somebody else's record cannot alter it, and a record that fails verification
is dropped without affecting the rest of the batch. See
[sync-model.md](sync-model.md).

`PluginCapability { protocol, version, enabled, data }` is opaque to the core:
`data` is bounded and handed to the matching plugin unparsed. Nothing in it is
ever treated as a shell command, filesystem path or OS setting.

Every decoded message is validated against `Limits` before it reaches anything
else. A rejected message never stops a network or the agent.

## Limits

Defaults from `Limits`, all configurable:

| limit | default |
|---|---|
| frame payload | 64 KiB |
| hostname | 255 bytes |
| capabilities per announcement | 16 |
| capability payload | 4 KiB |
| echo payload | 4 KiB |
| reason string | 256 bytes |
| handshake timeout | 10 s |
| dial timeout | 10 s |
| write timeout | 30 s |
| concurrent dials per network | 8 |
| sessions per network | 64 |
| inbound handshakes in flight | 32 |
| outbound queue per session | 64 |

Liveness of an established session is delegated to QUIC: iroh configures
keep-alives and an idle timeout, so a dead peer surfaces as a read error rather
than needing a heartbeat in this protocol.