# FIPS Transport Layer

<!-- markdownlint-disable MD024 -->

The transport layer is the bottom of the FIPS protocol stack. It delivers
datagrams between transport-specific endpoints over arbitrary physical or
logical media. Everything above — peer authentication, routing, encryption,
session management — is built on the services the transport layer provides.

## Role

A **transport** is a driver for a particular communication medium: a UDP
socket, an Ethernet interface, a serial line, a Tor circuit, a radio modem.
The transport layer's job is simple: accept a datagram and a transport
address, deliver the datagram to that address, and push inbound datagrams up
to the FIPS Mesh Protocol (FMP) above.

The transport layer deals exclusively in **transport addresses** — IP:port
or hostname:port addresses, MAC addresses, .onion identifiers, radio device addresses. These are
opaque to every layer above FMP. The mapping from transport address to FIPS
identity happens at the link layer after the Noise IK link handshake completes.
The word "peer" belongs to the link layer and above; the transport layer
knows only about remote endpoints identified by transport addresses.

A single transport instance can serve multiple remote endpoints
simultaneously — a UDP socket exchanges datagrams with many remote
addresses, an Ethernet interface communicates with many MAC addresses on the
same segment. Each endpoint may become a separate FMP link, but the
transport layer itself maintains no per-endpoint state.

## Services Provided to FMP

The transport layer provides four services to the FIPS Mesh Protocol above:

### Datagram Delivery

Send and receive datagrams to/from transport addresses. The transport
handles all medium-specific details: socket management, framing for stream
transports, radio configuration. FMP sees only "send bytes to address" and
"bytes arrived from address."

Inbound datagrams are pushed to FMP through a channel. The transport spawns
a receive task that pushes arriving datagrams (along with the source
transport address and transport identifier) onto a bounded channel. FMP
reads from this channel and dispatches based on the source address and
packet content.

### MTU Reporting

Report the maximum datagram size for a given link. FMP needs this to
determine how much payload can fit in a single packet after link-layer
encryption overhead.

MTU is fundamentally a per-link property. A transport with a fixed MTU
(Ethernet effective 1499, UDP default 1280) returns the same value for every
link — this is the degenerate case. Transports that negotiate MTU
per-connection (e.g., BLE ATT_MTU) report the negotiated value for each
link individually.

The transport trait exposes two MTU methods:

- `fn mtu(&self) -> u16` — Transport-wide default MTU
- `fn link_mtu(&self, addr: &TransportAddr) -> u16` — Per-link MTU for a
  specific remote address. The default implementation falls back to
  `mtu()`, so transports with uniform MTU (like UDP) need not override it.

FMP uses `link_mtu()` when computing path MTU for SessionDatagram
forwarding and LookupResponse transit annotation.

### Connection Lifecycle

For connection-oriented transports, manage the underlying connection: TCP
handshake, Tor circuit establishment, BLE pairing. FMP cannot begin
the Noise IK link handshake until the transport-layer connection is
established.

Connection-oriented transports expose a non-blocking connect interface.
`connect(addr)` initiates the connection in a background task and returns
immediately. `connection_state(addr)` reports the current status:

```text
ConnectionState {
    None        No connection attempt in progress
    Connecting  Background task running
    Connected   Ready for send()
    Failed(msg) Error message from failed attempt
}
```

Connectionless transports (UDP, raw Ethernet) return `Connected`
immediately — no async work needed.

At the node level, `PendingConnect` entries track links waiting for
transport connection. `poll_pending_connects()` runs each tick, checks
`connection_state()`, and calls `start_handshake()` on success or
`schedule_retry()` on failure. This decouples transport-layer connection
(which may take seconds for Tor circuits) from the FMP event loop.

### Discovery (Optional)

Notify FMP when FIPS-capable endpoints are discovered on the local medium.
This is an optional capability — transports that don't support it simply
don't provide discovery events.

See [Discovery](#discovery) below for details.

## Transport Properties

Transports vary widely in their characteristics. FIPS operates over all of
them because the transport interface abstracts these differences behind a
uniform datagram service.

### Transport Categories

**Overlay transports** tunnel FIPS over an existing network layer, typically
for internet connectivity:

| Transport | Addressing | MTU | Reliability | Notes |
| --------- | ---------- | --- | ----------- | ----- |
| UDP/IP | host:port | 1280–1472 | Unreliable | Primary internet transport |
| TCP/IP | host:port | Stream | Reliable | Requires length-prefix framing |
| Tor | .onion | Stream | Reliable | High latency, strong anonymity |

**Shared medium transports** operate over broadcast- or multicast-capable
media:

| Transport | Addressing | MTU | Reliability | Notes |
| --------- | ---------- | --- | ----------- | ----- |
| Ethernet | MAC | 1500 | Unreliable | Raw AF_PACKET frames |
| WiFi | MAC | 1500 | Unreliable | Infrastructure mode = Ethernet |
| BLE | BD_ADDR | 23–517 | Reliable | Negotiated ATT_MTU |
| Radio | Device addr | 51–222 | Unreliable | Low bandwidth, long range |

**Point-to-point transports** connect exactly two endpoints:

| Transport | Addressing | MTU | Reliability | Notes |
| --------- | ---------- | --- | ----------- | ----- |
| Serial | None (P2P) | 256–1500 | Reliable | SLIP/COBS framing |
| Dialup | None (P2P) | 1500 | Reliable | PPP framing |

### Properties That Matter to FMP

**MTU**: Determines how much data FMP can pack into a single datagram after
accounting for link encryption overhead. Heterogeneous MTUs across the mesh
are normal — the IPv6 minimum (1280 bytes) is the safe baseline for FIPS
packet sizing.

**Reliability**: Whether the transport guarantees delivery. FIPS prefers
unreliable transports because running TCP application traffic over a reliable
transport creates TCP-over-TCP, where retransmission and congestion control
at both layers interact adversely. FIPS tolerates packet loss, reordering,
and duplication at the routing layer.

**Connection model**: Connectionless transports (UDP, raw Ethernet) allow
immediate datagram exchange. Connection-oriented transports (TCP, Tor, BLE)
require connection setup before FMP can begin the Noise IK link handshake,
adding startup latency.

**Stream vs. datagram**: Datagram transports have natural packet boundaries.
Stream transports (TCP, Tor) require framing to delineate FIPS packets
within the byte stream. The FMP common prefix includes a payload length
field that provides this framing directly, replacing the need for a
separate length-prefix layer.

**Addressing opacity**: Transport addresses are opaque byte vectors. FMP
doesn't interpret them — it just passes them back to the transport when
sending. This means adding a new transport type with a novel address format
requires no changes to FMP or FSP.

## Connection Model

### Connectionless Transports

Datagrams can be sent to any reachable address without prior setup. Links
are lightweight — a transport address is sufficient to begin communication.

| Transport | Notes |
| --------- | ----- |
| UDP/IP | Stateless datagrams; NAT state is implicit |
| Ethernet | Send to MAC address directly |
| Radio | Raw packets to device address |

### Connection-Oriented Transports

Explicit connection setup is required before FIPS traffic can flow. The link
must complete transport-layer connection before FMP authentication can
proceed.

| Transport | Connection Setup |
| --------- | ---------------- |
| TCP/IP | TCP three-way handshake |
| Tor | Circuit establishment (typically 10–60s, default timeout 120s) |
| BLE | L2CAP CoC or GATT connection |
| Serial | Physical connection (static) |

### Implications

**Link lifecycle**: Connectionless transports use a trivial link model.
Connection-oriented transports need a real state machine: Connecting →
Connected → Disconnected. Failure can occur during connection setup, adding
error handling paths that connectionless transports don't have.

**Startup latency**: Connection-oriented transports add delay before a peer
becomes usable. This ranges from milliseconds (TCP) to tens of seconds
(Tor circuit). Peer timeout configuration must account for
transport-specific setup times.

**Framing**: Stream transports must delimit FIPS packets within the byte
stream. The FMP common prefix includes a payload length field that provides
integrated framing. Datagram transports preserve packet boundaries naturally.

## UDP/IP: The Primary Internet Transport

For internet-connected nodes, UDP/IP is the recommended transport:

- **No TCP-over-TCP**: UDP's unreliable delivery avoids the adverse
  interaction between application-layer TCP retransmission and transport-layer
  TCP retransmission
- **NAT traversal**: UDP hole punching enables peer connections through NAT
  without relay infrastructure
- **Low overhead**: 8-byte UDP header, no connection state
- **Matches FIPS model**: FIPS is datagram-oriented; UDP preserves this
  naturally without framing

Raw IP with a custom protocol number would be simpler but is blocked by most
NAT devices and firewalls, limiting deployment to networks without NAT.

### Socket Buffer Sizing

The default Linux UDP receive buffer (`net.core.rmem_default`,
typically 212 KB) is insufficient for high-throughput forwarding. At
~85 MB/s, a 212 KB buffer fills in ~2.5 ms; any stall in the async
receive loop (decryption, routing, forwarding overhead) causes the
kernel to silently drop incoming datagrams.

FIPS uses `socket2::Socket` wrapped in `tokio::io::unix::AsyncFd` for
the UDP receive path. This replaces `tokio::UdpSocket` and enables
direct `libc::recvmsg()` calls with ancillary data parsing —
specifically the `SO_RXQ_OVFL` socket option, which delivers a
cumulative kernel receive buffer drop counter on every received
packet. The drop counter feeds into the ECN congestion detection
system (see [fips-mmp.md](fips-mmp.md#ecn-congestion-signaling)).

Socket buffers (`recv_buf_size`, `send_buf_size`) are configured at
bind time via `socket2`. Linux internally doubles the requested value
(to account for kernel bookkeeping overhead) and silently clamps to
`net.core.rmem_max` / `net.core.wmem_max` if the request exceeds the
host kernel limits. The full UDP transport configuration is in
[../reference/configuration.md](../reference/configuration.md). The
host-side sysctl requirements and how to set them persistently live
in
[../how-to/tune-udp-buffers.md](../how-to/tune-udp-buffers.md).

## Ethernet: The Local Network Transport

For nodes on the same LAN segment, raw Ethernet provides a direct transport
without IP/UDP overhead — 28 bytes more FIPS payload per frame compared to
UDP (1500 vs 1472 MTU).

- **No IP dependency**: Operates below the IP layer. Nodes on the same
  Ethernet segment can communicate without IP addresses or routing
  infrastructure
- **Broadcast discovery**: Nodes discover each other via periodic beacon
  broadcasts on the shared medium, with no static peer configuration required
- **Higher MTU**: Standard Ethernet frames carry 1500 bytes of payload,
  yielding an effective FIPS MTU of 1499 after the frame type prefix
- **Matches FIPS model**: Like UDP, Ethernet is connectionless and
  unreliable — datagrams flow immediately to any MAC address on the segment

### Implementation

The Ethernet transport uses Linux AF_PACKET sockets in SOCK_DGRAM mode with
EtherType 0x2121. SOCK_DGRAM mode
lets the kernel handle Ethernet header construction and parsing — the
transport deals only with payloads and MAC addresses.

Data frames use a 3-byte header: a 1-byte frame type (`0x00`) followed by
a 2-byte little-endian payload length. The length field allows the receiver
to trim Ethernet minimum-frame padding that would otherwise corrupt AEAD
verification. Beacon frames (`0x01`) use only the 1-byte type prefix
(fixed 34-byte payload). Beacons and data share the same EtherType and
socket.

| Property | Value |
| -------- | ----- |
| EtherType | 0x2121 |
| Socket type | AF_PACKET SOCK_DGRAM |
| Data frame header | `[type:1][length:2 LE][payload]` |
| Beacon frame header | `[type:1][payload]` (fixed 34 bytes) |
| Effective MTU | Interface MTU - 3 (typically 1497) |
| Addressing | 6-byte MAC address |
| Platform | Linux only (`CAP_NET_RAW` required) |

### Beacon Discovery

Ethernet nodes discover peers via broadcast beacons sent to
ff:ff:ff:ff:ff:ff. Each beacon is a 34-byte frame containing the sender's
x-only public key. Receiving nodes extract the MAC source address from the
frame and the public key from the payload, then report the discovered peer
to FMP.

Four configuration flags control discovery behavior — `discovery`
(listen for beacons), `announce` (broadcast beacons), `auto_connect`
(initiate handshakes to discovered peers), and `accept_connections`
(accept inbound handshakes). The flag table and per-flag defaults
live in [../reference/configuration.md](../reference/configuration.md)
under `transports.ethernet.*`.

A typical discoverable node sets `announce`, `auto_connect`, and
`accept_connections` all true. A passive listener uses just
`discovery: true` to observe the network without announcing itself.

### WiFi Compatibility

WiFi interfaces in infrastructure (managed) mode work transparently for
unicast — the mac80211 subsystem handles frame translation between 802.11
and 802.3. Broadcast beacon discovery is unreliable in managed mode because
access points commonly isolate clients from each other's broadcast traffic.

Startup logging:

```text
Ethernet transport started name=eth0 interface=eth0 mac=aa:bb:cc:dd:ee:ff mtu=1499 if_mtu=1500
```

## TCP/IP: Transport for UDP-Filtered Networks

For peers whose networks filter outbound UDP, the TCP transport
provides an alternative datagram path between public endpoints. TCP
is not a NAT-traversal mechanism — there is no `tcp:nat` analogue to
the UDP hole-punch flow.

FIPS protocols (FMP, FSP, MMP) are all unreliable datagrams. Running them
over TCP introduces head-of-line blocking, which adds latency jitter. MMP
correctly measures this jitter, and cost-based parent selection naturally
penalizes TCP links (higher SRTT leads to higher link cost). ETX will be
1.0 over TCP since TCP handles retransmission.

### Architecture

Unlike UDP (one socket serves all peers), TCP requires one `TcpStream` per
peer. The transport maintains two pools: a `ConnectingPool` for background
connection attempts in progress, and an established connection pool
(`HashMap<TransportAddr, TcpConnection>`) for active connections, plus an
optional `TcpListener` for inbound connections.

| Property | Value |
| -------- | ----- |
| Addressing | host:port — IP address or DNS hostname |
| Default MTU | 1400 bytes |
| Per-link MTU | Derived from `TCP_MAXSEG` socket option |
| Framing | FMP header-based (zero overhead) |
| Connection model | Non-blocking connect, connect-on-send fallback, optional listener |
| Platform | Cross-platform (no `#[cfg]` gates) |

### FMP Header-Based Framing

TCP is a byte stream; FIPS packets need delineation. Rather than adding a
separate length-prefix layer, the TCP transport uses the existing 4-byte
FMP common prefix `[ver+phase:1][flags:1][payload_len:2 LE]` to determine
packet boundaries:

- **Phase 0x0 (established)**: remaining = 12 + payload_len + 16 (header + AEAD tag)
- **Phase 0x1 (msg1)**: remaining = payload_len (fixed at 110, total 114 bytes)
- **Phase 0x2 (msg2)**: remaining = payload_len (fixed at 65, total 69 bytes)
- **Unknown phase**: close connection (protocol error)

This provides zero framing overhead and built-in phase validation. The
stream reader is implemented in a separate module (`stream.rs`) for reuse
by the Tor transport.

### Connection Establishment

TCP connections use a non-blocking connect model. When FMP needs to reach
a configured peer address, the node calls `connect(addr)` on the transport,
which spawns a background tokio task to perform the TCP handshake and socket
configuration (TCP_NODELAY, keepalive, buffer sizes, TCP_MAXSEG query). The
call returns immediately without blocking the event loop.

The node tracks each pending connection in a `PendingConnect` entry. On
every tick, `poll_pending_connects()` calls `connection_state(addr)` to
check progress. When the transport reports `Connected`, the completed
connection is promoted to the established pool (stream split into
read/write halves, per-connection receive task spawned), and the node
initiates the Noise IK link handshake. If the transport reports `Failed`,
the node schedules a retry with exponential backoff.

As a fallback, `send(addr, data)` still performs synchronous
connect-on-send if no connection exists — this handles the case where a
send arrives before the node-level connect path runs. The non-blocking
path is the primary mechanism for configured peers.

### Session Independence

TCP connection loss does **not** tear down the FIPS peer. Noise keys, MMP
state, and FSP sessions are bound to the peer's npub, not the TCP
connection. The transport reconnects transparently via the non-blocking
connect path or connect-on-send fallback. MMP liveness timeout is the sole
authority for peer death.

### Connection Deduplication

Simultaneous outbound connections from both sides are resolved by the
existing cross-connection tie-breaker in `promote_connection`. The losing
TCP connection is closed via `Transport::close_connection(addr)`, which
removes it from the pool and aborts its receive task.

### Configuration

The TCP transport configuration block (`transports.tcp.*` — bind
address, MTU, connect timeout, TCP_NODELAY, keepalive, socket buffer
sizes, max inbound connections) is documented in
[../reference/configuration.md](../reference/configuration.md). If
`bind_addr` is configured, the transport accepts inbound connections;
without it, the transport operates in outbound-only mode (no listener
socket is created).

## Tor: The Anonymity Transport

The Tor transport routes FIPS traffic through the Tor network, hiding
a node's IP address from its peers. A node behind Tor connects outbound
through a local Tor SOCKS5 proxy; the remote peer sees the Tor exit
node's IP, not the initiator's. After the Noise IK handshake, the remote
peer knows the initiator's FIPS identity (npub) but not its network
location.

Like TCP, Tor is connection-oriented and reliable. The same TCP-over-TCP
considerations apply — MMP correctly measures the elevated latency and
cost-based parent selection naturally deprioritizes Tor links.

### Architecture

The Tor transport is a separate `TorTransport` implementation, not a TCP
variant, because it manages SOCKS5 proxy negotiation, has different
address semantics (.onion vs IP:port), and has significantly different
latency characteristics. It reuses the FMP header-based stream reader
(`tcp/stream.rs`) for packet framing on the underlying TCP connection.

The transport maintains two pools (same pattern as TCP): a
`ConnectingPool` for background SOCKS5 connection attempts, and an
established pool of `TorConnection` entries. Each `TorConnection` holds
a write half, a per-connection receive task, the negotiated MTU, and
a connection timestamp.

| Property | Value |
| -------- | ----- |
| Addressing | .onion:port or IP:port |
| Default MTU | 1400 bytes |
| Framing | FMP header-based (shared with TCP) |
| Connection model | Non-blocking connect, outbound SOCKS5 + inbound via onion service |
| Platform | Cross-platform (requires external Tor daemon) |

### Address Types

The Tor transport accepts three address formats, parsed into a `TorAddr`
enum:

- **Onion**: `.onion:port` — connects to a Tor hidden service. Both
  sides anonymous. (e.g., `abcdef...xyz.onion:8443`)
- **Clearnet IP**: `IP:port` — connects through a Tor exit node to a
  remote TCP listener. Hides the initiator's IP; the remote peer sees
  the exit node's IP.
- **Clearnet Hostname**: `hostname:port` — hostname is passed through
  SOCKS5 for Tor-side DNS resolution, avoiding local DNS leaks. Compatible
  with SafeSocks 1. (e.g., `fips.example.com:8443`)

All address types are routed through the same SOCKS5 proxy.

### Connection Establishment

Connection setup follows the same non-blocking pattern as TCP. When FMP
needs to reach a peer, the node calls `connect(addr)` on the transport.
The transport spawns a background tokio task that:

1. Opens a SOCKS5 connection through the local Tor proxy
2. Configures the socket: `TCP_NODELAY`, keepalive (30s)
3. Returns the connected stream

The call returns immediately. `connection_state(addr)` reports progress.
Tor circuit establishment typically takes 10–60 seconds (vs milliseconds
for TCP), making non-blocking connect essential — a blocking connect
would stall the entire FMP event loop.

The connect timeout defaults to 120 seconds (vs 5 seconds for TCP),
accounting for Tor circuit setup time. As a fallback, `send(addr, data)`
performs synchronous connect-on-send if no connection exists.

### Inbound via Onion Service (Directory Mode)

In `directory` mode (recommended for production), Tor manages the onion
service via `HiddenServiceDir` in `torrc`. FIPS reads the `.onion` address
from the hostname file at startup and binds a local TCP listener that the
Tor daemon forwards inbound connections to.

This mode enables Tor's `Sandbox 1` (seccomp-bpf) — the strongest single
hardening option — because no control port interaction is required for
onion service management. Tor handles key generation and persistence
directly through the `HiddenServiceDir`.

The inbound accept loop mirrors the TCP transport's pattern: accept
connection, configure socket (TCP_NODELAY, keepalive), spawn a
per-connection receive loop using the shared FMP stream reader. Inbound
connections arrive from `127.0.0.1` (Tor daemon's local forwarding); peer
identity is resolved during the Noise IK handshake, not from the transport
address.

Configuration requires coordinating `torrc` and `fips.yaml`. The
operator setup — torrc directives, `fips.yaml` `tor` section,
HiddenServiceDir permissions, and `Sandbox 1` notes — is in
[../how-to/deploy-tor-onion.md](../how-to/deploy-tor-onion.md). In
brief: the `HiddenServicePort` external port is what peers connect
to, and `tor.directory_service.bind_addr` must match the
`HiddenServicePort` target address.

### Session Independence

Same as TCP: Tor connection loss does **not** tear down the FIPS peer.
Noise keys, MMP state, and FSP sessions survive reconnection.

### Bridge Node Pattern

A node running both Tor and UDP transports acts as a bridge between
anonymous and clearnet portions of the mesh:

```text
[Anonymous node] --tor--> [Bridge node] --udp--> [Clearnet node]
```

No special code is needed — FIPS multi-transport routing handles it.
Anonymous nodes connect to the bridge via Tor; the bridge forwards
traffic to clearnet peers over UDP. Clearnet peers never see the
anonymous node's IP.

### Latency Characteristics

Tor adds 200ms–2s RTT per circuit. MMP measures this elevated latency,
and cost-based parent selection penalizes Tor links (high SRTT → high
link cost). ETX is 1.0 since TCP handles retransmission.

Tor throughput is typically 1–5 Mbps — adequate for control plane and
moderate data transfer, not for bulk transfer.

### Monitoring

In `control_port` mode and optionally in `directory` mode (when
`control_addr` is configured), the transport spawns a background
monitoring task that polls the Tor daemon every 10 seconds via the
control port. The cached monitoring data is exposed through the
`show_transports` control socket query and displayed in fipstop.

Monitoring data includes:

- **Bootstrap progress** (0–100%) with INFO logging at milestones
  (25/50/75/100%) and WARN if stalled >60s
- **Circuit status** (whether Tor has a working circuit)
- **Network liveness** (up/down) with WARN on transitions
- **Dormant mode** detection with WARN on entry
- **Tor daemon version** and **traffic counters** (bytes read/written)

The control port connection uses cookie authentication by default
(reading from `/var/run/tor/control.authcookie`). Unix socket
connections (`/run/tor/control`) are preferred over TCP for security.

### Configuration

The Tor transport block (`transports.tor.*`) is documented in
[../reference/configuration.md](../reference/configuration.md). Three
modes are available:

- **`socks5`** (default): Outbound-only through a SOCKS5 proxy. No
  control port, no inbound connections.
- **`control_port`**: Outbound via SOCKS5 plus control port connection
  for Tor daemon monitoring. No inbound connections.
- **`directory`** (recommended for inbound): Outbound via SOCKS5 plus
  inbound via Tor-managed `HiddenServiceDir` onion service.
  Optionally connects to the control port for monitoring when
  `control_addr` is set. Enables Tor's `Sandbox 1` for maximum
  security.

The Tor transport requires an external Tor daemon. Named instances
are supported for multiple proxy endpoints.

### Implementation Roadmap

- Outbound SOCKS5 connections to .onion, clearnet IP, and clearnet
  hostname addresses *(implemented)*
- Inbound connections via Tor onion service using `HiddenServiceDir`
  directory mode *(implemented)*
- Operator visibility: cached monitoring snapshot, control socket
  exposure, fipstop display, bootstrap/liveness logging *(implemented)*
- Embedded `arti` (Rust Tor implementation) for self-contained operation
  without an external Tor daemon *(future)*

### Statistics

The Tor transport exposes per-instance counters covering successful
send/receive, send/receive errors, connection establishment,
SOCKS5-level errors, MTU rejections, accepted/rejected inbound
connections, and Tor control-port errors. The full counter table
lives in [../reference/transports.md](../reference/transports.md).

## Discovery

Discovery determines that a FIPS-capable endpoint is reachable at a given
transport address. It is distinct from raw transport-level endpoint
detection — a new TCP connection or UDP packet from an unknown source is not
discovery; a FIPS-specific announcement or response is.

Discovery is an optional transport capability. Transports that don't support
it (configured UDP endpoints, TCP, Tor) simply don't provide discovery events.
FMP handles both cases uniformly: with discovery, it waits for events then
initiates link setup; without discovery, it initiates link setup directly to
configured addresses.

### Local/Medium Discovery

For transports where endpoints share a physical or link-layer medium — LAN
broadcast, radio, BLE — discovery uses beacon and query mechanisms:

- **Beacon**: A node periodically broadcasts its FIPS presence on the shared
  medium. Content is a FIPS-defined discovery frame carrying enough
  information to initiate a link. Non-FIPS endpoints ignore the frame.
- **Query**: A node broadcasts a one-shot solicitation. FIPS-capable nodes
  respond. Responses arrive on the same channel as beacon events.

Both produce the same result: "FIPS endpoint available at transport address
X." FMP does not need to distinguish beacons from query responses.

| Transport | Discovery | Notes |
| --------- | --------- | ----- |
| UDP (LAN) | Broadcast/multicast | On local network segment |
| Ethernet | Broadcast | Custom EtherType, ff:ff:ff:ff:ff:ff |
| Radio | Beacon | Shared RF channel, natural fit |
| BLE | Advertising | GATT service UUID |

### Nostr Relay Discovery

For internet-reachable transports, a node publishes a signed Nostr event
containing its FIPS discovery information — public key and reachable
transport endpoints (UDP host:port, TCP host:port, .onion address). Other FIPS
nodes subscribing on the same relays learn about available peers.

Nostr relay discovery is not a transport — it is a discovery service that
feeds addresses to other transports. A node discovers via Nostr that a peer
is reachable at UDP 1.2.3.4:9735, then establishes the link over the UDP
transport.

For NAT'd UDP endpoints, a node may advertise `addr: "nat"` instead of a
concrete address, signaling that peers should initiate STUN-assisted UDP
hole punching. Offer/answer exchange uses Nostr gift-wrap (NIP-59) events
on the configured DM relays; the resulting punched socket is adopted into
the standard UDP transport via the bootstrap handoff path.

Key properties:

- Identity is built in — Nostr events are signed, so discovery information
  is authenticated
- Relay selection acts as scoping — which relays a node publishes to and
  subscribes on determines its discovery neighborhood
- Can only advertise IP-reachable endpoints (not radio, BLE, serial)
- Higher latency than local discovery (relay propagation delays)

### Current State

> **Implemented**: UDP, TCP, Tor, and Ethernet peers can be configured
> statically via YAML. Ethernet peers can also be discovered via beacon
> broadcast — the `discover()` trait method returns newly seen endpoints,
> and per-transport `auto_connect()` / `accept_connections()` policies
> control whether discovered peers are connected automatically or require
> explicit configuration. TCP and Tor have no built-in discovery mechanism.
> Nostr relay discovery and STUN-assisted UDP hole punching are
> implemented and toggled via configuration; see
> [../reference/configuration.md](../reference/configuration.md) for the
> `node.discovery.nostr.*` configuration tree.

## Transport Interface

The transport interface defines what every transport driver must provide.

### Trait Surface

```text
transport_id()        → TransportId         Unique identifier for this transport instance
transport_type()      → &TransportType      Static metadata (name, connection-oriented, reliable)
name()                → Option<&str>        Instance name (for multi-instance transports)
state()               → TransportState      Current lifecycle state
mtu()                 → u16                 Transport-wide default MTU
link_mtu(addr)        → u16                 Per-link MTU (defaults to mtu())
start()               → lifecycle           Bring transport up (bind socket, open device)
stop()                → lifecycle           Bring transport down
send(addr, data)      → delivery            Send datagram to transport address
connect(addr)         → ()                  Initiate non-blocking connection (connection-oriented only)
connection_state(addr)→ ConnectionState     Poll connection status (None/Connecting/Connected/Failed)
close_connection(addr)→ ()                  Close a specific connection (no-op for connectionless)
congestion()          → TransportCongestion  Local congestion indicators (optional)
discover()            → Vec<DiscoveredPeer> Report discovered FIPS endpoints (optional)
auto_connect()        → bool                Auto-connect discovered peers (default: false)
accept_connections()  → bool                Accept inbound handshakes (default: true)
```

### Receive Path

Rather than a synchronous receive method, transports use a channel-push
model. Each transport takes a sender handle at construction and spawns an
internal receive loop that pushes inbound datagrams onto the channel. The
node's main event loop reads from the corresponding receiver, which
aggregates datagrams from all active transports into a single stream.

Each inbound datagram carries:

- **transport_id** — which transport it arrived on
- **remote_addr** — the transport address of the sender
- **data** — the raw datagram bytes
- **timestamp** — arrival time

### Transport Metadata

Transport types carry static metadata that FMP can query:

```text
TransportType {
    name              "udp", "ethernet", "tor", etc.
    connection_oriented   bool
    reliable              bool
}
```

Predefined types exist for UDP, TCP, Ethernet, WiFi, Tor, and Serial.

### Congestion Reporting

Transports optionally report local congestion indicators via a
`TransportCongestion` struct, providing a transport-agnostic interface for
the node layer's ECN congestion detection:

```text
TransportCongestion {
    recv_drops: Option<u64>    Cumulative kernel-dropped packets (monotonic)
}
```

The node samples each transport's congestion state on a 1-second tick via
`sample_transport_congestion()`. `TransportDropState` tracks per-transport
drop deltas: when new drops appear (rising edge), the `dropping` flag is
set, and `detect_congestion()` in the forwarding path triggers CE marking
on all forwarded datagrams.

| Transport | Congestion Source | Mechanism |
| --------- | ----------------- | --------- |
| UDP | `SO_RXQ_OVFL` kernel drop counter | `recvmsg()` ancillary data on every packet |
| TCP | Not implemented | Returns `None` (TCP handles congestion internally) |
| Tor | Not implemented | Returns `None` (TCP handles congestion internally) |
| Ethernet | Not implemented | Returns `None` |

### Transport Addresses

Transport addresses (`TransportAddr`) are opaque byte vectors. The transport
layer interprets them — e.g. UDP and TCP resolve `host:port` strings (IP
fast path, DNS fallback with a 60s cache on UDP). All layers above treat
them as opaque handles passed back to the transport for sending.

### Transport State Machine

```text
Configured → Starting → Up → Down
                         ↓
                       Failed
```

Transports begin in `Configured` state with all parameters set. `start()`
transitions through `Starting` to `Up` (operational). `stop()` moves to
`Down`. Transport failures move to `Failed`.

## Implementation Status

| Transport | Status | Notes |
| --------- | ------ | ----- |
| UDP/IP | **Implemented** | Primary transport, AsyncFd/recvmsg, SO_RXQ_OVFL kernel drop detection |
| TCP/IP | **Implemented** | FMP header-based framing, non-blocking connect, per-connection MSS MTU |
| Ethernet | **Implemented** | AF_PACKET SOCK_DGRAM, EtherType 0x2121, beacon discovery, Linux only |
| WiFi | **Implemented** (via Ethernet transport, infrastructure mode) | mac80211 translates 802.11↔802.3; broadcast beacons unreliable through APs |
| Tor | **Implemented** | Outbound SOCKS5, inbound via onion service, .onion and clearnet addressing |
| BLE | **Implemented** (Linux/glibc only; experimental) | L2CAP CoC, ATT_MTU negotiation, per-link MTU; musl/macOS/Windows skip |
| Radio | Future direction | Constrained MTU (51–222 bytes) |
| Serial | Future direction | SLIP/COBS framing, point-to-point |

## Design Considerations

### TCP-over-TCP Avoidance

Running TCP application traffic over a reliable transport (TCP, Tor)
creates a layering violation where retransmission and congestion control
operate at both levels. When the inner TCP detects loss (which may just be
transport-layer retransmission delay), it retransmits, creating more traffic
for the outer TCP, which may itself be retransmitting. This amplification
loop degrades performance severely under any packet loss.

FIPS prefers unreliable transports for this reason. When a reliable transport
must be used (e.g., Tor), applications should be aware of the performance
implications.

### Multi-Transport Operation

A node can run multiple transports simultaneously. Peers from all transports
feed into a single spanning tree and routing table. If one transport fails,
traffic automatically routes through alternatives. A node with both UDP and
Ethernet transports bridges between internet-connected and local-only
networks transparently.

Multiple links to the same peer over different transports are possible. FMP
manages these independently — each link has its own Noise session, its own
MTU, and its own liveness tracking.

### Transport Quality and Path Selection

Transport characteristics (latency, bandwidth, reliability) affect path
quality. The spanning tree parent selection factors in link quality through
cost-based effective depth (`effective_depth = depth + link_cost`), where
`link_cost` is derived from locally measured MMP metrics (ETX and SRTT).
This allows the tree to prefer lower-latency, lower-loss links when the
quality difference is significant. Link cost is not yet used in
`find_next_hop()` candidate ranking for data forwarding.

## References

- [fips-concepts.md](fips-concepts.md) — Protocol overview
- [fips-architecture.md](fips-architecture.md) — Layer architecture
- [fips-mesh-layer.md](fips-mesh-layer.md) — FMP specification (the
  layer above)
- [fips-mtu.md](fips-mtu.md) — How transport-reported `link_mtu`
  feeds the unified path-MTU model
- [../reference/wire-formats.md](../reference/wire-formats.md) —
  Transport framing details
- [../reference/configuration.md](../reference/configuration.md) —
  Per-transport configuration blocks
- [../reference/transports.md](../reference/transports.md) —
  Per-transport statistics counter inventory
