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ArjenandJohnathan Corgan abe3f0b40a feat(transport): bind and rebind network interfaces dynamically

An interface-bound transport is a long-lived object that is *sometimes
bound*. The interface it names may not exist when the daemon starts, may
appear minutes later, may vanish and return mid-operation, and may never
appear at all. Until now the first observation was final: an interface
missing at start was logged once and skipped for the life of the process,
and one that disappeared at runtime published a health change but was never
rebound.

On OpenWrt that is a live bug. procd starts fips before wifi has created
fips-mesh0 / fips-ap0, both transports are skipped and never retried, and
the 802.11s peer link forms anyway (that is mac80211, not the daemon) — so
the node looks healthy and reaches nothing.

Give every interface-bound transport presence state and a binder task.
start_async now returns Ok with the transport ABSENT rather than failing;
the binder binds when the interface appears, tears the socket down when it
goes away, and rebinds when it returns. Start-time absence and runtime
detach are one transition and one code path.

TransportError::InterfaceUnavailable makes absence branchable: a missing
interface and a typo'd interface name were the same flat StartFailed
(String), so nothing downstream could tell a state from a fault. A bind
failure that is *not* absence — no CAP_NET_RAW, no readable /dev/bpf* —
still fails the start, because retrying a socket that can never open behind
a Degraded nobody is watching is worse than dying loudly at boot.

**Presence is IFF_UP, not IFF_UP|IFF_RUNNING.** Carrier and bindability are
different questions and only the second belongs in a bind gate. An
AF_PACKET socket on a carrier-less bridge is valid and starts carrying
traffic the instant a member port comes up, with no rebind. Gating on
carrier would report a wifi-only router Degraded forever for an empty
br-lan, turn every carrier flap the socket would have survived into an
unbind/rebind cycle, and deadlock an 802.11s interface that reports RUNNING
only once it has peered — peering needs beacons, beacons need a bound
socket. Carrier is reported beside presence in show_transports instead.

**A name is not a device.** Both backends bind by device: AF_PACKET stores
sll_ifindex, a BPF descriptor follows the interface it was attached to. A
netdev deleted and recreated under the same name leaves the socket attached
to something gone while the name resolves perfectly well, and nothing
notices — a stale AF_PACKET socket never becomes readable, so the receive
loop neither errors nor exits, and send failures go to the caller rather
than the binder. A listen-only node (announce: false, no beacon sender to
fail) sat present and deaf indefinitely after a wifi reload. The bound index
is captured at bind and compared on every poll. A name reappearing with a
different MAC is different hardware: cached neighbors are dropped rather
than resumed onto.

**Detection is event-driven** where the kernel offers a source — netlink
RTNLGRP_LINK on Linux, PF_ROUTE on macOS and FreeBSD — with a 1 s
getifaddrs poll underneath as a backstop. Link-event payloads are not
parsed: an event is a hint to re-run the probe, which is cheap and
authoritative, and a parser's bugs would be presence bugs. Probes are
coalesced to ten a second because PF_ROUTE has no group filter and delivers
every routing message on the host; a persistently failing event source is
logged once, backed off, and abandoned for the poll after five errors
rather than spinning a core silently on ENOBUFS.

**Degraded becomes a level, not a latch.** The supervisor's reason set was
monotonic, which was correct while no child could recover; with recovery it
would have come to mean "something broke at some point since boot" rather
than "something is broken now". Absence lives in its own reversible set,
health is recomputed on every transition in both directions, and an absent
transport still counts as up so a single-interface node that boots before
its wifi degrades rather than exiting on NoTransports. There is deliberately
no restart action in the FSM: rebinding belongs next to the file descriptor,
and a supervisor-authored retry would be a second mechanism racing the first
for the same socket.

**The node's egress MTU follows the bound set.** transport_mtu filters on
is_bound(), not is_operational() — an interface-bound transport is
operational from the moment it starts, so the weaker predicate let hardware
that had never appeared clamp the whole node's IPv6 MTU. Because that
minimum now moves at runtime, the TUN reader and writer read the TCP MSS
ceiling from a shared atomic instead of a u16 captured at spawn; every other
consumer (show_status, the snapshot, the session-layer fragmentation check)
already read it live, so the clamp was the one place the daemon could report
one effective MTU and enforce another. It moves in both directions: a narrow
interface appearing tightens it, its departure releases it. MSS is
negotiated per connection, so a change binds connections opened after it.

**Logging is per edge, never per attempt**, with one deadline after it. The
edge itself is not an error — info at boot, warn on a runtime detach, since
an interface bound a moment later is the ordinary case this mechanism exists
to absorb and crying error at t=0 then "recovered" at t=0.2s is the failure
the rule exists to prevent. Ten seconds is the whole grace: past it, absence
is no longer a race against a radio or a container, so a required interface
still missing is reported once at error. Start-time absence and a runtime
detach share that one deadline, as they share everything else here. Said
once, not repeated: how long an absence has lasted is state, published as
interface.since_secs and as Degraded, and a monitor can threshold it per
deployment rather than the daemon compiling a schedule in.

Successful rebinds are damped. Backoff covers failed binds; the nastier case
is binds that keep succeeding into a socket that dies moments later, which a
receive loop giving up on a persistent error while the interface stays UP
produces once per second forever. Consecutive bindings dying inside ten
seconds back off on the 1 s → 30 s curve, and past three the binder stops
announcing each bind as a recovery until one lasts.

The receive loop backs off and exits on a dead socket instead of spinning on
Err with a warn per iteration, and the ad-hoc ENXIO socket reopen in the
beacon sender is gone: both hand recovery to the presence machine, one
mechanism for every cause rather than one hack per symptom. Beacons pause
while absent because the task simply does not exist then.

transports.ethernet.*.optional (default false) decides how loudly absence is
reported. Naming an interface is a statement that you expect it, so the
default complains: Degraded, and the error at the deadline. optional: true
is silent — info on the edge, no health impact, no error — for hardware
legitimately not always there. It describes the interface's presence, not
the transport's importance, and no value of it makes a missing interface
fatal at startup.

show_transports grows an `interface` block — presence, carrier, policy, how
long the phase has been held, bind count, failed attempts. The original boot
race was expensive precisely because nothing an operator could see said the
node was deaf.

ci: exercise the presence probe against musl and an address-less interface

Two legs, both about the assumption the presence probe rests on.

OpenWrt — the platform dynamic interface binding exists for — is musl, and
musl reimplements getifaddrs independently of glibc. `interface_present`
reads ifa_flags out of it, and the interfaces this feature exists for
(fips-mesh0, fips-ap0) are unbridged with no IP address at all, which is
exactly where getifaddrs implementations differ. Every other leg is glibc,
so without this one the probe was asserted on a libc no test had ever run it
against, on the target it was written for. Building for the musl target
rather than inside an Alpine container keeps the leg to a cross-build plus a
run, without a second toolchain image to maintain.

The address-less interface is created on both this leg and the glibc one.
Pinning the contract on both libcs is what turns "glibc and musl agree about
getifaddrs" from an assumption into a checked fact, and makes the glibc leg
fail first if glibc is the one that changes. Loopback cannot stand in: it
has 127.0.0.1, so probing it asks whether getifaddrs works rather than
whether it reports this. Deliberately not tolerant of failure — a guard that
quietly does not run is worse than no guard.

The iface-binding suite is deliberately not added to the integration
matrix here. Its files and its testing/ci-local.sh registration arrive
in the next commit, and the GitHub workflow's matrix entry now goes with
them, so both runners gain the suite together and
testing/check-ci-parity.sh holds at every point in this history.

docs: document dynamic interface binding

The transport-layer design gains an Interface Presence section: the three
deployment scenarios that motivated it, the presence machine and its two
invariants, why presence is IFF_UP and not IFF_UP|IFF_RUNNING, interface
identity and the recreated-netdev case, the detection sources and their two
rate limits, the optional policy, health as a level rather than a latch, the
egress-MTU consequence, the logging rules, and what the mechanism retires.
It records why the error deadline is a single ten-second window rather than
a repeating severity ladder, so the ladder does not come back.

The control-socket and fipsctl references described show_transports without
its `interface` block, and the transport-layer state machine still implied
that Up meant bound. Both now say what Up and presence each describe and how
they differ, since "transport up, interface absent" is a normal state an
operator will meet and would otherwise read as a contradiction.

The configuration reference documents `optional` with the absence, log and
retry behaviour of each setting, and the ground-up tutorial's dongle example
gains optional: true, which is what that example is actually for.

fix(transport): pair every presence edge the binder publishes

Six defects, all in the wiring around the presence machine rather than in
the machine itself. The state machines were well tested; what went untested
was how the binder feeds them, and every one of these lives there.

**The first detach after a clean start never reached node health.**
start_async binds inline and publishes that edge itself, before binder_loop
exists. The loop then built a fresh ChurnGuard, which had therefore recorded
no bind — and detached() takes `announced` to decide whether an edge is
owed, so it asked for no retraction. A node that booted with its interface
present and then lost it kept reporting Full for as long as the interface
stayed away, while show_transports said absent and the log said detached.
stabilized() could not repair it either: it early-returns while bound_at is
None, which it is for a bind the loop did not perform, so the guard stayed
unseeded until a second detach happened to fix it. The loop now seeds the
guard from the binding it inherits.

This is not the cable-unplug case. Presence is IFF_UP, so a carrier loss is
correctly not a detach at all; it takes an admin down, a netdev delete, or a
device removal — a wifi reload, a hostapd restart, a dongle pulled.

**An optional interface published nothing, and the MTU floor rode the same
channel.** Filtering the edge at the source conflated two questions that
happen to share a transport: whether node health should move, and whether
the bound set changed. Only the first is policy. The second determines the
node's egress MTU floor, and refresh_tun_mss_ceiling has no other trigger —
so an optional transport binding or unbinding at runtime left the TUN MSS
clamp derived from a bound set that no longer existed, reporting one
effective MTU and clamping to another. That is the defect the MssCeiling
work was written to remove, reintroduced for exactly the transports the
shipped OpenWrt config marks optional: five of seven. TransportPresence
gains health_relevant; the edge always goes out and only health is filtered.

**A permanent bind fault went unlogged if any absence race preceded it.**
record_attempt ran ahead of the InterfaceUnavailable arm, so a probe that
won a race the bind then lost burned the counter that gates the only error
a real fault ever gets — emitted on attempts == 1. A flapping interface
followed by CAP_NET_RAW being dropped or /dev/bpf* exhausting therefore
reported nothing at all, for the life of the process, and the operator got
report_sustained_absence's "still missing" instead: wrong, for an interface
that is present. An absence race is not a bind attempt and no longer counts
as one.

**The watcher's give-up did not stick.** Abandoning the event source parked
on pending() *inside the changed() future*, and that future is constructed
fresh on every pass of the binder's select! and dropped whenever the poll
ticker wins. So the next pass re-read the dead socket, re-counted the error
and re-logged "not recoverable" — once per wake-up, forever, which at a 1 s
tick across the shipped seven-transport config is seven warnings and seven
failing syscalls a second on flash-backed logging. The 100 ms backoff lived
in the dropped future too and never applied across passes. Abandonment is
now state on the watcher.

**A zero-length read livelocked the binder.** try_io clears readiness only
on WouldBlock, which the sibling error arm handles by hand and this one did
not — so breaking out left readable() instantly ready with nothing to read,
and the loop never returned Pending. That starves the select! of its ticker
entirely and takes presence detection down with it. Cleared and treated as a
fault so the give-up path applies.

**A failed presence probe read as an absent interface.** getifaddrs is a
netlink dump and fails for reasons that have nothing to do with the
interface: ENOBUFS under memory pressure, EMFILE or ENFILE under fd
exhaustion, since it opens a socket of its own. Answering "not present"
there tore down a working socket and degraded health over a transient
syscall failure, undiagnosably, and under fd exhaustion the rebind could not
have succeeded anyway. interface_has_flags now distinguishes the two; the
detach gate holds its binding when the kernel will not answer, while the
bind gate still treats it as absence and retries.

**On macOS the reader thread could not die, so a dead socket read as live.**
Any read() failure other than EBADF — ENXIO being the one that matters,
which is what BPF answers once the interface it was attached to is torn away
— reset the parse buffer and continued. The thread never returned, so the
channel never closed, so recv_from never failed, so the tokio task never
exited, so tasks_alive() reported a dead socket as a live one. Detach
detection on macOS reduced to the name and the index, and an interface reset
in place left the transport present and deaf. It now gives up after a
streak, and the return closes the channel the binder is actually watching.

Two smaller pairings while here: stop_async retracts the edge it would
otherwise leave standing for a socket that is gone, and start_async hands a
refused edge to the binder to retry rather than dropping the only edge
either consumer will see until the interface next moves. The absence
deadline is stamped at start rather than at construction, so a transport
staged for longer than the bring-up window no longer reports sustained
absence on its first tick having given the interface no window at all.

feat(config): reject impossible ethernet interface names at load

Config::validate never inspected transports.ethernet, so an empty interface
name, a name past the kernel's 15-byte limit, and two transports naming the
same netdev all loaded cleanly and failed only at runtime — the first two as
a permanent absence indistinguishable from an interface that has not been
created yet.

That indistinguishability is deliberate and worth keeping: waiting is the
right answer for an interface the operator has not made yet, and the daemon
cannot know which of the two it is looking at. Which is exactly why the
syntactic gate earns its place. A name that is *impossible* is the one case
still separable from "not there yet", and without the check a typo costs a
permanently Degraded node whose only symptom is an interface that never
arrives — the failure mode the presence machine exists to make legible,
reintroduced one level up.

Syntax only. Whether a well-formed name exists stays the binder's question,
asked once a second, forever.

The duplicate check is a different fault: two transports on one netdev means
two sockets on the same device at the same ethertype, each receiving every
frame the other does.

test(transport): close the vacuous and uncovered branches

Six tests that asserted nothing, or asserted less than they claimed.

`a_bind_fault_still_fails_the_start` returned early whenever the socket
*could* be opened, so it was vacuous as root and on any developer machine with
a group-readable /dev/bpf* — the fail-fast path it is named for went unchecked
exactly where someone was most likely to run it. It now asserts in both
halves, and the privileged half is worth more than the fix: a present,
bindable interface binding inline is the ordinary case on a booted router, and
no other unit test reaches it, because every other one here names an interface
that does not exist. The bind-success path had no unit coverage at all.

`an_interface_with_no_addresses_is_still_present` returned early when the
fixture was absent — no fixture, pass. It still has to skip on a machine with
no address-less interface, so the guard is the runner declaring that it has
fixtures: CI now sets FIPS_TEST_REQUIRE_FIXTURES beside
FIPS_TEST_ADDRLESS_IFACE, and the test fails rather than skips if the fixture
step is ever removed or renamed. Its `let _ = interface_carrier(...)` is now
asserted too: if presence and carrier ever collapsed into one read, a
carrier-less bridge would report absent and the whole IFF_UP-not-IFF_RUNNING
decision would be silently undone.

`policy_labels` checked `Required.as_str()` and not `Optional.as_str()`, so a
swapped pair would paint every expected interface as the tolerated kind and
stay green. `Presence::as_str` had no test at all — `binding` was never
observed by anything, anywhere.

Three binder branches had no coverage: a transport restarting (the second
`start_async` clearing the previous run's stop flag — only a second *stop* was
tested, so a transport that could never restart passed everything), the
episode clock being restamped at start rather than at construction, and the
refused-edge retry actually delivering. The last one matters most: the
existing test filled the channel and dropped the receiver, so a slot that
captured an edge and never re-sent it would pass while health sat on a stale
level forever.

And the hardware-change boundary `record_bind` returns, which the neighbour
flush hangs off: false on a first bind (or every clean start would drop a
cache it had just built) and true once, not stickily, on a MAC change.

Each new test was verified against the defect it guards — comment out the
`shutdown.store(false)`, the `mark_starting()`, or the seeded `unpublished`,
and the corresponding test goes red while the rest stay green.

fix(test): correct the dummy-carrier assertion, and pin Darwin's presence probe

The carrier assertion added in d6240698 was wrong and would have failed both
Linux legs. A `dummy` interface brought up reports `<BROADCAST,NOARP,UP,
LOWER_UP>` — `IFF_RUNNING` is set, so it *has* carrier. Verified against the
exact fixture CI builds, `addrgenmode none` and all, rather than against the
comment: the original code discarded the result and its comment claimed "up
but not running", which is what made asserting it look safe.

So the fixture pins address-less *presence* and cannot demonstrate the
presence-vs-carrier split at all — an interface up with no carrier is a bridge
with nothing plugged in, which no fixture here creates.
`carrier_is_reported_separately_from_presence` pins that split from the other
side. The corrected assertion is Linux-only, because the expected answer is a
property of the fixture device rather than of the code.

That is also what lets the macOS fixture land. The Linux legs pin the
address-less contract on glibc and musl, but the BSD-derived `getifaddrs` the
macOS backend actually calls had no coverage — the test skipped itself
silently on that runner, which is precisely the shape the previous commit was
removing. `feth` is macOS's fake-Ethernet pseudo-interface and is created
address-less; the step fails the leg rather than testing the wrong thing if the
runner hands it an address anyway, mirroring why the Linux step needs
`addrgenmode none`.

Both branches of the fixture guard were exercised: unset skips and passes, and
declared-but-missing fails loudly. The corrected test was run against a real
Linux dummy inside a container, not reasoned about.

fix(ci): put the macOS presence fixture on the macOS job

The `feth` fixture step added in 6b9faa0c landed on the Linux `test` job, not
on `test-macos`, and has failed CI ever since:

    create: Host name lookup failure
    ifconfig: `--help' gives usage information.

That is Linux net-tools `ifconfig`, which has no `create` subcommand. So the
Linux job ran two address-less-fixture steps — its own correct `ip link add
type dummy` one, then a macOS one that cannot work there — while `test-macos`
had none at all, leaving the Darwin `getifaddrs` path exactly as uncovered as
before.

Cause was a pattern-anchored edit: `Install cargo-nextest` followed by `Run
unit tests` appears in three jobs, and the insert hit the first match. Moving
it then hit the *last* match, which is `test-windows`. It is now placed by job
boundary rather than by pattern, and verified per job: `test` and `test-musl`
carry the `ip link`/dummy fixture, `test-macos` carries `ifconfig`/`feth`, and
`feth` appears exactly once in the file.

The verification that missed this was counting steps in `test-macos` and
reading 6 as confirmation. Six was the count *before* the insert; seven is what
a successful insert looks like. Now asserted by job and by which tool each
fixture step uses, so a step in the wrong place fails the check rather than
matching a total.

test(transport): cover the detach branch and the stop-race check

Two of the three untested branches, by two different routes.

**The detach decision is now a pure function.** `classify_detach(gone,
replaced, dead)` replaces the inline three-way `if` in the binder loop, and all
eight input combinations are asserted, plus the precedence between them and the
`reason=` labels the integration suite and operators grep for.

This does not make `Replaced` or `SocketDied` reachable from a test — both need
a bind that succeeded and then a specific external event, and the integration
suite cannot arrange the recreate deterministically either, for the reason
recorded in reference/notes.md: the netlink event from a delete is acted on
within microseconds, so `Gone` wins that race in practice. What it does is
split the untested thing in two. The three inputs each already had tests
(`interface_present`, `device_replaced`, `tasks_alive`); the branch between them
did not, and that half is now total and exhaustive.

Precedence is asserted rather than assumed: an interface that has gone away has
also trivially been "replaced" and its socket is also dead, so the most
specific true statement has to win, and callers pass `replaced` already masked
by `!gone` — the function no longer depends on them having masked correctly.

**The post-store shutdown check is now tested directly.** It needs a bind that
*succeeds*, which is why `a_stop_racing_a_bind_leaves_nothing_behind` could
never reach it: that test's interface does not exist, so `bind_and_spawn`
refuses at the presence probe several steps earlier. Binding loopback as root
reaches it, and the assertion is that a bind completing after a stop undoes its
own socket, its own loops, and its own presence.

Unprivileged runners skip it, but loudly: a runner declaring
`FIPS_TEST_PRIVILEGED` and unable to open a raw socket fails instead of
skipping, the same guard shape as `FIPS_TEST_REQUIRE_FIXTURES`. No CI leg sets
that yet — unit tests run unprivileged — so today it exercises the branch only
under a root container. Verified there, including the negative control: with
the post-store check removed the test fails, and with it present it passes.

test(transport): run the bind-success half in CI, and cover the replaced device

Adds the privileged unit-test step the rest of this depends on, then uses it.

**The privileged step.** Every unit-test leg runs unprivileged, so
`PacketSocket::open` cannot succeed on any of them and everything past a
successful bind runs nowhere in CI: the post-store shutdown check, the
`Present` arm of the binder loop, `bind_now` itself. The step builds as the
runner user and executes only the test binary under sudo — running `cargo` as
root would use root's CARGO_HOME and discard the cache the job just restored.
The binary-path extraction was verified locally before being written into the
workflow.

`FIPS_TEST_PRIVILEGED` is what makes it honest. Tests needing a raw socket skip
quietly without it; with it set, a test that cannot open one fails and says so.
A runner that stops granting the capability shows up as a red leg rather than
as silence.

**The replaced device.** `"interface replaced"` — a netdev recreated under the
same name, the `wifi reload` case in #125 — could not be reached by the
integration suite: with link events live the kernel's `RTM_DELLINK` is acted on
within microseconds, so the binder observes "gone" first and takes the branch
already covered. Attempting it there passed about one run in three.

Rather than race the binder, the test asserts the *inputs*: after a real
delete-and-recreate the name still resolves and the bound index no longer
matches. Paired with the exhaustive classifier test, which pins that
`(gone: false, replaced: true)` maps to `Replaced`, the path is covered without
depending on scheduling. A watcher-disable hook was written for the racing
approach and removed once this one made it unnecessary.

**Two smaller gaps.** `report_sustained_absence` had no direct test — the
integration suite infers it from counting ERROR lines, and ten seconds of real
time is how a deadline ends up asserted by proxy. A test-only `backdate_for_test`
ages the episode clock instead. And the probe floor was asserted only as a
relation between two constants; it now has behaviour, via a `probe_delay`
helper extracted from the loop.

That extraction was not neutral, and its own test caught it: `checked_sub`
yields `Some(0)` at exact equality where the loop used a strict `<`, so a
zero-length sleep would have replaced no sleep at all. Harmless in effect,
wrong in meaning, and fixed.

Every new test was run against the defect it guards, in a root container:
remove the post-store shutdown check, or make `device_replaced` always answer
false, and the corresponding test fails.
Mark publish_presence must_use. The defect this commit fixes was a discarded
return value, so the fix is made self-guarding: a future call site that drops
the pending edge instead of storing it now fails the lint rather than silently
reintroducing the bug. Option is not must_use in std, unlike Result, so the
attribute has to be explicit.

2026-09-10 19:18:09 +00:00

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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:

Opens a SOCKS5 connection through the local Tor proxy
Configures the socket: TCP_NODELAY, keepalive (30s)
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.

Interface Presence

Up describes the transport, not the socket. An interface-bound transport (today: Ethernet) carries a second, orthogonal state — whether it is bound right now — and the two are independent: a transport is Up from the moment it starts, whether or not the interface it names exists.

The Gap This Closes

Three deployment scenarios exercise one missing mechanism:

Boot ordering. On OpenWrt, procd starts fips before wifi has created fips-mesh0 / fips-ap0. Both transports were skipped and never retried, while the 802.11s peer link formed anyway — that is mac80211, not the daemon — so the node looked healthy and reached nothing. The failure was expensive precisely because nothing an operator could see said the node was deaf.
Intermittent hardware. A USB ethernet adapter named in fips.yaml is plugged in some days and not others. Its absence is normal and must be silent; its arrival must bind without operator action.
Mid-operation restart. wifi reload for a channel change destroys and recreates the mesh interface within a couple of seconds. The socket dies, the receive loop spun on Err with no backoff and no exit, and nothing rebound.

These are not three features. They are one presence machine plus one policy field. Before it existed, the first observation was final: an interface missing at start was logged once and skipped for the life of the process, and one that disappeared at runtime published a health change but was never rebound.

The Presence Machine

Absent ──attach──> Binding ──ok──> Present
  ^                   │              │
  └──── fail/backoff ─┘              │
  └──────────── detach ──────────────┘

start_async binds if it can and otherwise returns Ok with the transport Up and Absent; a per-transport binder task then binds when the interface appears, tears the socket down when it goes away, and rebinds when it returns. Two invariants do the work:

The transport object survives detach. Config, TransportId, statistics and the neighbor buffer persist; only the file descriptor and its loops go. A transport is never destroyed because its interface went away.
Start-time absence and runtime detach are the same transition. A node that boots before its wifi and a node whose wifi reloads at 03:00 take one code path. The old asymmetry — skip forever at start, publish health at runtime — is gone.

TransportError::InterfaceUnavailable is what makes absence branchable. A missing interface and a typo'd interface name were the same flat StartFailed(String), so nothing downstream could tell a state from a fault.

What Counts as Present

Presence means IFF_UP — the interface exists and the operator has enabled it — and deliberately not IFF_RUNNING.

Carrier and bindability are different questions, and only the second belongs in a bind gate. An AF_PACKET socket on a carrier-less bridge is valid and starts carrying traffic the instant a member port comes up, with no rebind: the socket outlives the carrier. Gating on IFF_RUNNING bought nothing and cost three things:

br-lan on a router with nothing in its LAN ports is UP with NO-CARRIER, so a healthy wifi-only router reported Degraded forever and errored for a fault it did not have;
every carrier flap the socket would have survived became an unbind/rebind cycle — churn the presence machine then has to damp, a mechanism compensating for a policy error;
an 802.11s interface that reports RUNNING only once it has peered cannot peer, because peering needs beacons, beacons need a bound socket, and the gate refuses to bind. A deadlock reachable on the hardware this mechanism was written for.

The signal IFF_RUNNING carries is not lost: show_transports reports interface.carrier beside presence, so an operator can still tell a bound transport carrying nothing from a working one. It is reported rather than obeyed.

The probe is getifaddrs plus ifa_flags rather than an SIOCGIFFLAGS ioctl: it needs no socket, so the watcher can probe before any file descriptor exists, and it is spelled the same on Linux and the BSDs.

Interface Identity

The configured name is the key, but a name is not a device. Both backends bind by device — AF_PACKET stores sll_ifindex, a BPF descriptor follows the interface it was attached to — so an interface deleted and recreated under the same name leaves the socket attached to something that no longer exists while the name resolves perfectly well.

Nothing else notices. A stale AF_PACKET socket never becomes readable, so the receive loop neither errors nor exits, and send failures go to the caller rather than to the binder. A listen-only node (announce: false, so no beacon sender to fail) therefore sat present and deaf indefinitely after a wifi reload — the original bug wearing a different hat. The bound index is captured at bind and compared on every poll; a mismatch is a detach.

Hardware can also change underneath a name. If the name reappears with a MAC other than the one last bound, that is a different device, so the cached neighbor entries for that transport are dropped rather than resumed onto, and the swap is logged at warn. Richer selectors (match: { name | mac | id_path }) are deliberately deferred; the requirement here is only that FIPS never silently resumes onto different hardware.

Detection

Platform	Source
Linux	netlink `RTNLGRP_LINK` (`RTM_NEWLINK` / `RTM_DELLINK`)
macOS, FreeBSD†	`PF_ROUTE` socket, `RTM_IFINFO`
Fallback	poll `getifaddrs` + flags, 1 s

† Aspirational: the Ethernet transport is cfg(any(target_os = "linux", target_os = "macos")), so FreeBSD has no interface-bound transport for a watcher to serve. The PF_ROUTE branch compiles for the BSD family, but only macOS reaches it.

Where an event source exists, detection is sub-second. The poll stays underneath as a backstop rather than as the mechanism, and must stay at ~1 s: the probe is cheap, and letting the interval drift to tens of seconds reintroduces exactly the latency the event source was added to remove. Construction is best-effort — a kernel or sandbox that refuses the socket yields a watcher that never fires, and the binder degrades to its poll.

Link-event payloads are not parsed. An event is a hint to re-run the presence probe, which is cheap and authoritative; decoding nlmsghdr/ifinfomsg to reach the same answer would add a parser whose bugs would be presence bugs.

Two rate limits protect the binder from its own event source. Probes are coalesced to ten a second, because PF_ROUTE has no group filter and delivers every routing message on the host — route churn, ARP, DHCP renewals, a VPN going up and down — each of which would otherwise drive a full getifaddrs walk. And a persistently failing event source is counted, logged once, backed off, and after five consecutive errors abandoned for the poll: losing events is survivable because the poll is the backstop, but burning a core on a socket that is readable-but-erroring is not.

Bind failures that are not absence back off 1 s → 30 s. Absence itself does not back off; there is nothing to poll but the probe.

Policy: `optional`

One field per transport, transports.ethernet.*.optional, default false:

	absence	log	retries
`optional: false` (default)	node reports `Degraded`	`info` at boot / `warn` on a runtime detach, then `error` once if it lasts past 10 s	forever
`optional: true`	no health impact	`info`, and nothing after	forever

Naming an interface in configuration is a statement that you expect it, so the default is to complain; silence is opted into.

optional describes the interface's presence, not the transport's importance. An optional interface that is present is used exactly as hard as any other. No value of it makes a missing interface fatal at startup: the only fatal case remains "no transports at all came up". If a deployment ever needs absence to abort startup, that arrives as an explicit on_absent: exit — never as a second meaning for optional.

A bind failure that is not absence — no CAP_NET_RAW, no readable /dev/bpf*, a buffer the kernel refused — is a fault, not a state, and still fails the daemon's start. Retrying those forever would convert a hard, actionable deployment error into a daemon that retries a socket it can never open behind a Degraded nobody is watching. Only absence is waited out at start; a non-absence failure during a later rebind does back off, since by then the node is serving and killing it would be the worse answer.

Health

Node health is recomputed on every presence transition in both directions, so a returning interface clears Degraded without a restart. That makes Degraded a level rather than a latch: the supervisor's reason set was monotonic, which was correct while no child could recover, but with recovery it would have come to mean "something broke at some point since boot" rather than "something is broken now". Absence lives in its own reversible set, separate from the one-way failed set a start failure enters.

An absent transport still counts as up. It came up — start_async returned Ok — so it does not push a single-transport node into the fatal NoTransports, which would make a node that merely booted before its wifi exit instead of waiting. Absence degrades; it never kills.

There is deliberately no restart action in the supervisor FSM. The presence watcher and the rebind loop live inside the transport, next to the file descriptor they manage, and once that exists a supervisor-authored retry has nothing left to do — it would be a second mechanism racing the first for the same socket. The supervisor learns about presence (Event::ChildAbsent / Event::ChildPresent) and republishes health; it does not drive rebinding.

Peer state needs no separate grace period. A send over an absent interface returns InterfaceUnavailable and the peer entry survives untouched, so peers are already held across a detach and resume when the interface returns; the liveness reaper is the effective linger bound. A recreated mesh interface comes back with the same MAC (it is derived from the phy) and Noise sessions are keyed on the remote peer, so a local rebind is invisible to peers. A dedicated linger timer would be a second answer to a question already answered.

Logging

Edges, never attempts. A loop that logs per attempt reproduces the hot log spin this mechanism removed, at 1–30 s intervals forever on any router with an unplugged WAN — and operators learn to filter it, which is how the next real failure gets missed.

The edge itself is not an error. An interface missing when the daemon starts and bound a moment later is the ordinary case the mechanism exists to absorb, so it is info; calling it an error at t=0 and "recovered" at t=0.2 s is the cry-wolf failure this rule exists to prevent. A runtime detach is warn — a link coming and going is ordinary weather for a mesh daemon.

There is exactly one deadline. Ten seconds is the window in which absence could still be a race — a radio, a container, a veth arriving late. Past it a required interface is a fault an operator has to fix, and it is reported once at error. Start-time absence and a runtime detach share that deadline rather than getting one each, for the same reason they share a code path everywhere else here. An optional interface never reaches error; that is what optional means.

Once, not repeated. This was a 1 m / 10 m / 1 h ladder that re-announced the same fact at rising severity and then went permanently quiet after an hour, which got both halves wrong: it used the log as a store for something already published continuously as state, and it stopped mentioning a fault that was still live. Duration belongs in interface.since_secs and in how long Degraded has been held, where a monitor can threshold it per deployment instead of the daemon compiling one in.

Node health does not wait for the deadline. Degraded publishes on the first edge, which is the signal an operator actually watches.

Successful rebinds are damped. Backoff covers failed binds; the opposite and nastier case is binds that keep succeeding into a socket that dies moments later, which a receive loop giving up on a persistent error while the interface stays UP produces once per second, forever. The binder counts consecutive bindings that die inside ten seconds, backs off on the same 1 s → 30 s curve, and past three of them stops announcing each bind as a recovery — holding health where it is until a binding lasts.

Egress MTU

transport_mtu() is the minimum across bound transports — is_bound(), not is_operational(), because an interface-bound transport is operational from the moment it starts whether or not it holds a socket. Filtering on the weaker predicate let a transport whose interface had never appeared set the whole node's IPv6 MTU from hardware that was not present.

Since a transport can now bind long after start, that minimum moves at runtime, and every consumer has to read it live. show_status, the control-socket snapshot and the session-layer fragmentation check always did. The TUN reader and writer did not: they were handed a u16 at spawn, so a narrow interface binding later never tightened the TCP MSS clamp and the node reported one effective MTU while clamping to another. The ceiling is now shared with those threads — an atomic beside the per-destination path_mtu_lookup they already read on the same packet — and recomputed on every change to the bound set.

Both directions, for the same reason Degraded is a level rather than a latch: a narrow interface arriving must tighten the clamp or traffic egressing over it is clamped too loose, and that interface leaving must release it or unplugging a low-MTU adapter leaves the node over-clamped until it restarts. MSS is negotiated per connection at SYN time, so a change binds connections opened after it and leaves established ones alone.

Observability

show_transports carries an interface block per interface-bound transport: netdev name, presence (absent / binding / present), carrier, policy (required / optional), since_secs, binds and failed_attempts. The two counters separate an interface that is flapping from one that is there and refusing to bind, and since_secs measures the absence episode rather than the phase — a bind that fails walks Absent → Binding → Absent, and restarting the clock on those edges would report a permanently unbindable interface as one second old forever.

fipstop's transports view names the netdev and the absence policy in their own columns and shows presence in the State column for these transports, because state reads up from the moment the transport starts and is therefore precisely the wrong answer in the one case someone is scanning that column for. Both render sites sort by ascending transport id — creation order, and so grouped by transport type — rather than by HashMap iteration order, which was arbitrary and differed on every daemon restart.

What This Retires

The hotplug.d/net rule that restarted the daemon when the FIPS radio interfaces appeared, and the wifi down; wifi up; sleep dance provisioning performed to sequence around the race.
The YAML comment-toggling in fips-mesh-setup / fips-ap-setup — the mesh and AP blocks ship enabled with optional: true and simply wait.
The ad-hoc ENXIO socket reopen in the beacon sender: beacons now pause while absent because the task does not exist then, and recovery is the presence machine's job. One mechanism for every cause rather than one hack per symptom.
The start-time versus runtime asymmetry in the supervisor.

It also covers the case none of those workarounds did: an interface that flaps while the daemon is running.

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

64 KiB Raw Blame History Unescape Escape

FIPS Transport Layer

Role

Services Provided to FMP

Datagram Delivery

MTU Reporting

Connection Lifecycle

Discovery (Optional)

Transport Properties

Transport Categories

Properties That Matter to FMP

Connection Model

Connectionless Transports

Connection-Oriented Transports

Implications

UDP/IP: The Primary Internet Transport

Socket Buffer Sizing

Ethernet: The Local Network Transport

Implementation

Neighbor Beacons

WiFi Compatibility

TCP/IP: Transport for UDP-Filtered Networks

Architecture

FMP Header-Based Framing

Connection Establishment

Session Independence

Connection Deduplication

Configuration

Tor: The Anonymity Transport

Architecture

Address Types

Connection Establishment

Inbound via Onion Service (Directory Mode)

Session Independence

Bridge Node Pattern

Latency Characteristics

Monitoring

Configuration

Implementation Roadmap

Statistics

Nym: The Mixnet Transport

Architecture

Outbound-Only

Address Types

Connection Establishment

Startup Readiness

Session Independence

Configuration

Statistics

BLE: The Local Radio Transport

Architecture

Backend Availability

Framing

Discovery and the PSM

Connection Establishment

Discovery

Local/Medium Discovery

Nostr Relay Discovery

Current State

Transport Interface

Trait Surface

Receive Path

Transport Metadata

Congestion Reporting

Transport Addresses

Transport State Machine

Interface Presence

The Gap This Closes

The Presence Machine

What Counts as Present

Interface Identity

Detection

Policy: optional

Health

Logging

Egress MTU

Observability

What This Retires

Implementation Status

Design Considerations

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Raw Blame History

Policy: `optional`