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Arjen
dca938c104
A peer reachable over more than one transport keeps one Noise session and moves its traffic between transports on failure or degradation. Implements docs/design/fips-multi-path-switchover.md §4-§10 and closes the two-interface case in #143. Three inner link messages next to Heartbeat: `0x52 PathProbe` and `0x53 PathAck`, carrying a probe id, the sender's path id and a `remote_active` bit, and `0x54 PathClose`, naming the receiver's path id and a reason. A probe is an ordinary encrypted frame sent on a candidate transport; the receiver, having decrypted it against the session found by index, adds the path as `Probing`, marks it `rx_live` and answers on that same path. The prober's receipt of the ack marks the path `Live`, `tx_live`, and takes an RTT sample. No handshake, no key material, no index allocation. Old nodes drop the unknown types at debug, so a path to one stays `Probing` and never becomes eligible. The discovery gate changes shape: a live peer beaconing on a transport we hold no path to it over becomes a path candidate rather than being skipped, and the heartbeat tick probes it. The active path's first probe is small — the handshake proved it and seeded its MTU — while a standby's discovery probes are padded to the link MTU, as is one a minute on every path, so a medium that passes small frames and drops large ones never proves itself. A standby the peer never answers on is given up after eight probes; the active path never is. Detection is per path and takes each medium's own failure signal: a carrier edge, an unreachable-on-send (`ENETUNREACH`/`EHOSTUNREACH`), an interface going away, or two unanswered heartbeats on a path the peer is also silent on. Any of them marks the path `Suspect` and selection leaves it at once, because the standby is warm: heartbeats run at `node.path.active_heartbeat_ms` where either side sends and `standby_heartbeat_ms` elsewhere, both stretched by the path's own round trip so a Tor or Nym path is neither flooded nor declared dead every round trip. A peer holding one live path is not heartbeated here at all — selection has nothing to move to, and the link heartbeat keeps its liveness. Soft signals (the peer's `remote_active` flipping away, silence here while a standby hears the peer) trigger a probe, never `Suspect`: reading them as a verdict forces both sides onto one path and loops under a one-way failure. A node that loses a path tells the peer with a `PathClose` on a surviving one, so the peer moves at once rather than after its own timeout. A transport that returns inside the five-minute grace revives its dead paths as `Probing` with their RTT window and ETX intact. Selection is measured, not configured: each path scores `quality_index(etx, min_rtt)`, and traffic moves when the active path is no longer eligible (mandatory) or when a standby beats it by `switch_margin` for `switch_dwell_secs` (discretionary). Min RTT over a window rather than SRTT, because SRTT inflates under load on the path carrying traffic while an idle standby looks pristine — a ping-pong generator. A `role: backup` transport carries a peer's traffic only while no normal path is eligible, and yields outright when one becomes eligible. `fipsctl path pin` overrides both while its path is eligible. A switch re-seeds the path MTU from the new path, tightens the session MTUs and refreshes the MSS ceiling, so the first frames after a switch are not black-holed. The link record and `addr_to_link` follow the active path. The link cost the tree sees is held at its pre-switch value for the dwell, and until the two receiver reports that span the switch have arrived — the first counts every frame in flight on the old path as lost and spikes the per-report ETX for one interval, the second replaces it — so neither a short flap nor that spike ripples mesh-wide through parent selection or the next-hop order. Operator surface: `role: backup` on any transport, `node.path.*` (`switch_margin`, validated finite and at least 1.0, `switch_dwell_secs`, `min_samples`, `active_heartbeat_ms`, `standby_heartbeat_ms`), and `fipsctl path show|pin|unpin` over the `path_show`, `path_pin` and `path_unpin` control commands. Two chaos scenarios calibrate the defaults and are wired into both runners: `dual-path-flap` (a raw-Ethernet veth as the cable, the Docker bridge over UDP as the wifi) and `dual-udp-flap` (two interface-bound UDP instances). Each flaps one path under iperf and carries detectors that can fail on a switchover that did not carry traffic — a per-node ceiling on "Peer promoted to active" (a second is a re-peering), a one-second ceiling from link-down to the first switch, and a two-second ceiling on any zero-byte iperf interval run — alongside the `path_switches` band. Neither has been run to calibrate; the defaults are chosen, not derived, and the design doc says so. Refs #143