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

FIPS Transport Layer

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 1497, UDP default 1280) returns the same value for every link — this is the degenerate case. Transports that negotiate MTU per-connection (e.g., the BLE L2CAP CoC 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:

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 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
Nym	host:port	Stream	Reliable	Mixnet, outbound-only, 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	2048 default	Reliable	Per-connection L2CAP CoC 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)
Nym	SOCKS5 connect through mixnet (minutes possible, default timeout 300s)
BLE	L2CAP CoC 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).

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. The host-side sysctl requirements and how to set them persistently live in ../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 — 25 bytes more FIPS payload per frame compared to UDP (1497 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 neighbor detection: 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 1497 after the 3-byte frame header
• 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, and BPF devices (/dev/bpf*) on macOS. SOCK_DGRAM mode lets the kernel handle Ethernet header construction and parsing — the transport deals only with payloads and MAC addresses; the macOS BPF backend presents the same API and handles the 14-byte Ethernet header itself.

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 (AF_PACKET, CAP_NET_RAW required) and macOS (BPF /dev/bpf*)

Neighbor Beacons

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 neighbor behavior — listen (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 under transports.ethernet.*.

A typical discoverable node sets announce, auto_connect, and accept_connections all true. A passive listener uses just listen: 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 neighbor detection is unreliable in managed mode because access points commonly isolate clients from each other's broadcast traffic.

Startup logging:

Ethernet transport started name=eth0 interface=eth0 mac=aa:bb:cc:dd:ee:ff mtu=1497 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. 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. 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:

[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. 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.

Nym: The Mixnet Transport

The Nym transport routes FIPS traffic through the Nym mixnet, providing network-level anonymity via Sphinx packet routing and timing obfuscation. It uses the "mixnet-as-proxy" pattern: a node connects outbound through a local nym-socks5-client SOCKS5 proxy, which carries the traffic into the mixnet. The nym-socks5-client runs as a separate process alongside the fips daemon and must be started independently.

Like Tor, Nym is a privacy-oriented deployment mode chosen for the anonymity properties of the mixnet, not a failover for other transports. Like TCP and Tor, it is connection-oriented and reliable; the same TCP-over-TCP considerations apply, and cost-based parent selection naturally deprioritizes the high-latency Nym links.

Architecture

The Nym transport is a separate NymTransport implementation. It reuses the FMP header-based stream reader (tcp/stream.rs) for packet framing on the underlying byte stream, and follows the same connection-pool pattern as the TCP and Tor transports.

It maintains two pools: a ConnectingPool for background SOCKS5 connection attempts, and an established pool of NymConnection entries. Each NymConnection holds a write half, a per-connection receive task, the configured MTU, and a connection timestamp.

Property	Value
Addressing	IP:port or hostname:port
Default MTU	1400 bytes
Framing	FMP header-based (shared with TCP)
Connection model	Outbound-only, non-blocking connect through SOCKS5
Platform	Cross-platform (requires external nym-socks5-client)

Outbound-Only

The Nym transport is strictly outbound. It supports no inbound service: accept_connections() returns false and discover() returns no peers. A node using the Nym transport can initiate links to remote peers through the mixnet, but cannot accept inbound connections over Nym. (A node can still accept inbound links over other transports it runs.)

Address Types

The Nym transport accepts two address formats, parsed into an internal target address:

• IP:port — a numeric IP and port, sent to the SOCKS5 proxy as a numeric target.
• Hostname:port — the hostname is passed through SOCKS5 so it is resolved on the exit side rather than locally.

Both forms are routed through the same SOCKS5 proxy.

Connection Establishment

Connection setup follows the same non-blocking pattern as the TCP and Tor transports. When FMP needs to reach a peer, the node initiates a background connect (connect_async). The transport spawns a background tokio task that opens a SOCKS5 connection through the local nym-socks5-client, configures the socket (including TCP keepalive), splits the stream, and spawns a per-connection receive loop using the shared FMP stream reader. The call returns immediately while the connect proceeds in the background.

SOCKS5 connection setup through the mixnet can take much longer than a direct TCP connection because each connection traverses multiple mix nodes with timing obfuscation. Accordingly the connect timeout defaults to 300 seconds (connect_timeout_ms). Non-blocking connect is essential here — a blocking connect would stall the FMP event loop for the duration of mixnet setup. As a fallback, send_async(addr, data) performs a connect-on-send if no connection to the address yet exists.

Each outbound packet is checked against the configured MTU before being written; an oversized packet is rejected with an MTU-exceeded error rather than being sent.

Startup Readiness

At startup the transport validates the configured socks5_addr and then probes the SOCKS5 port to wait for nym-socks5-client to become ready, using exponential backoff (starting at 1 second, capped at 10 seconds between attempts) up to startup_timeout_secs (default 120 seconds). If the proxy does not become reachable within that window, the transport logs a warning and starts anyway; outbound connections then fail until the nym-socks5-client becomes available.

Session Independence

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

Configuration

The Nym transport block (transports.nym.*) has the following fields:

Field	Default	Description
socks5_addr	127.0.0.1:1080	Address (host:port) of the local nym-socks5-client SOCKS5 proxy
connect_timeout_ms	300000	Outbound SOCKS5 connect timeout in milliseconds (300s)
mtu	1400	Maximum FIPS packet size for Nym connections, in bytes
startup_timeout_secs	120	Seconds to wait for nym-socks5-client to become ready at startup

The Nym transport requires an external nym-socks5-client. Named instances are supported for multiple proxy endpoints. Unknown configuration keys are rejected.

Statistics

The Nym transport exposes per-instance counters covering successful send/receive, send/receive errors, connection establishment, SOCKS5-level errors, connect timeouts, and MTU rejections.

BLE: The Local Radio Transport

The BLE transport peers two nodes over Bluetooth Low Energy with no IP network between them, using an L2CAP connection-oriented channel as the byte pipe. It is the only transport whose reach is a radio horizon rather than a route, which makes it the fallback when there is no infrastructure at all: two phones in a room, a node and a handset, a mesh with its uplink cut.

Like TCP, Tor and Nym it is connection-oriented and reliable, so the same TCP-over-TCP considerations apply. Unlike them, its peer set is discovered rather than configured, and the addresses it discovers are not stable.

Architecture

Nothing above the radio has a platform dependency. BleTransport<I> is generic over a BleIo seam (ble/io.rs) that covers listening, connecting, advertising, scanning and the stream I/O itself; the connection pool, the PSM wire format, the stream framer and the scan/probe loop are shared by every backend.

The backends live one per file and are selected by a three-way cascade in ble/mod.rs: BluerIo (io_linux.rs) talks to BlueZ over D-Bus, AndroidIo (io_android.rs) drives a radio the embedding application installs, and MockBleIo (io.rs) is an in-memory double compiled only under cfg(test). A build that matches none of the three fails with a compile_error! rather than silently selecting the mock.

That failure is deliberate. An earlier arrangement wrote the mock arm as "anything that is not BlueZ", which meant a new platform got a transport that compiled, started, reported itself Up and never peered, with no error anywhere to find it.

Backend Availability

build.rs sets ble_available for glibc Linux or Android, which is the set of platforms with a concrete backend rather than the set that could plausibly have Bluetooth. bluer_available, the BlueZ sub-condition, is glibc Linux alone: musl cannot satisfy libdbus-sys's pkg-config cross-compile requirement, and musl router targets do not run BlueZ by default. macOS, FreeBSD and Windows have no backend and so have no BLE transport at all.

On glibc Linux the build needs libdbus-1-dev and pkg-config; the BlueZ daemon itself is a runtime dependency. On Android the radio is supplied by the application: scanning, advertising, L2CAP listen and connect all sit behind Java APIs held under a permission and foreground-service model that only the app can satisfy, so the embedder implements AndroidRadio and installs it into a per-node slot which the backend resolves per operation.

Framing

The channel is L2CAP CoC, not GATT, so there is no ATT_MTU to negotiate. The per-connection CoC MTU applies, defaulting to 2048, and it overrides the transport-wide default per link.

Packet boundaries are recovered from the byte stream rather than assumed from the socket. BlueZ's SOCK_SEQPACKET preserves SDU boundaries, but that is a property of one backend's socket type and not of L2CAP: Android's BluetoothSocket input stream and macOS's CBL2CAPChannel may return a fragment of a packet or several packets coalesced in one read. FIPS packets are self-delimiting through the 4-byte FMP common prefix, so stream_read.rs adapts the datagram-shaped stream into the AsyncRead that transport::framing::read_fmp_packet already expects, shared with every other stream-oriented transport.

Discovery and the PSM

Discovery is an LE advertisement, received passively, carrying the 128-bit FIPS service UUID plus the listener's L2CAP PSM as service data.

The PSM has to ride the advertisement because it is not knowable any other way. BlueZ lets an application choose the PSM it binds, and BlueZ is the exception: Android's listenUsingInsecureL2capChannel and macOS's CBPeripheralManager.publishL2CAPChannel both return an OS-assigned PSM the application cannot request. A dialer cannot guess it, and before a connection exists there is no channel on which to be told. So BleIo::listen reports the PSM it actually bound, start_advertising takes that PSM, and the scanner yields it alongside the address.

The wire layout is fixed by a byte budget and specified in ble/psm.rs. A legacy advertising PDU carries 31 bytes of AD payload. Flags take 3 and the 128-bit service UUID list takes 18, so keying the service data on the full 128-bit UUID would need 20 more and overrun by 10. Keying it on the 16-bit UUID 0x9C90, which is the leading 16 bits of the FIPS service UUID expanded through the Bluetooth base UUID, takes 6 and fits at 27. The budget is asserted at compile time. It leaves no room for a local name, and it must ride the primary advertisement rather than the scan response, because a scan response arrives only after an active-scan round trip that drops asymmetrically across chipsets.

Connection Establishment

A scan/probe loop dials discovered addresses, keeping the learned PSM per address beside a probe-cooldown book and falling back to the configured DEFAULT_PSM for a peer that advertises none.

Peers are identified by node address, not by link address. A device using resolvable private addresses rotates continually, and modern phones do so by default, so an address-keyed pool sees every rotation as a new device and every already-connected guard fails to fire.

Failing addresses back off by powers of two up to MAX_PROBE_BACKOFF_SHIFT, and the retry book is capped at MAX_PENDING_PROBES so that rotating addresses cannot grow it without bound. Both bounds matter more here than on other transports because BLE hardware caps concurrent connections at roughly four to ten, so a handful of unreachable addresses can starve discovery of everything behind them.

Inbound connections are admitted off the accept loop, with INBOUND_HANDSHAKE_INFLIGHT handshakes allowed at once and the oldest aborted at the bound rather than the loop waiting for a slot.

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	LE advertisement: 128-bit FIPS service UUID plus service-data PSM

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, Ethernet, and BLE peers can be configured statically via YAML. Ethernet peers can also be discovered via beacon broadcast and BLE peers via LE scanning — 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 for the node.rendezvous.nostr.* configuration tree. LAN/mDNS peer rendezvous is implemented as a separate subsystem and documented in fips-nostr-discovery.md.

Transport Interface

The transport interface defines what every transport driver must provide.

Trait Surface

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:

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

Predefined types exist for UDP, TCP, Ethernet, WiFi, Tor, Nym, BLE, 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:

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)
Nym	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

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, neighbor beacons; Linux (AF_PACKET) and macOS (BPF)
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
Nym	Implemented	Outbound-only SOCKS5 through nym-socks5-client, mixnet anonymity, IP/hostname addressing
BLE	Implemented (glibc Linux and Android; experimental)	L2CAP CoC, per-connection MTU (2048 default), per-link MTU; musl, macOS, FreeBSD and Windows have no backend
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 also the primary key in find_next_hop() candidate ranking for data forwarding, which orders candidates by (link_cost, distance_to_dest, node_addr).

References

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