mirror of
https://github.com/jmcorgan/fips.git
synced 2026-08-09 16:24:45 +00:00
Johnathan Corgan
ae607431eb
Adds source-side TCP MSS clamping informed by per-destination path MTU learned via discovery, with a conservative IPv6-minimum-derived ceiling for cold flows where discovery has not yet completed. Closes the multi-hop default-config TCP wedges observed in production where a sender's local-floor MSS exceeds what some intermediate forwarder hop is willing to carry: silent drops, no PTB feedback through the userspace TUN to the kernel TCP stack, retransmits at the same too- large MSS, application connection times out. ## Architecture A new `Arc<RwLock<HashMap<FipsAddress, u16>>>` field `path_mtu_lookup` on Node mirrors the per-destination path MTU in a form accessible from sync TUN reader/writer threads. A new `per_flow_max_mss` helper in `src/upper/tun.rs` reads the lookup at SYN-clamp time and returns the appropriate ceiling for the flow. Three write sites populate `path_mtu_lookup`: 1. **Discovery originator branch** of `handle_lookup_response`: the path MTU bottleneck accumulated through the reverse path lands here when a LookupResponse arrives at the originator. Same value also lands in `coord_cache` per the existing `insert_with_path_mtu` API. 2. **FMP peer-promotion seed** (`seed_path_mtu_for_link_peer`): when an FMP link-layer peer is promoted to active, the local outgoing-link MTU on the peer's transport seeds the lookup. Tighter existing values (learned via discovery) are preserved; the seed only writes when no entry exists or the existing value is looser than the link MTU. Without this seed, directly-configured peers (auto_connect / static peer config) would leave `path_mtu_lookup` empty for their FipsAddress because the FSP session establishes without ever issuing a LookupRequest. 3. **Target-edge fold at `send_lookup_response`**: when a node is the discovery target, it folds its own outgoing-link MTU to the response's next-hop into `path_mtu` before sending. Without this fold, the response leaves the target with `path_mtu = u16::MAX` and only intermediate transits min-fold; the target's first reverse-path hop is never represented in the bottleneck calculation. Refactored the existing transit- side min-fold into a shared `apply_outgoing_link_mtu_to_response` helper called from both sites. ## Read-side: per_flow_max_mss Two TUN call sites consume the lookup: - Outbound `handle_tun_packet` clamps SYN MSS using packet[24..40] (IPv6 destination) as the lookup key. - Inbound `TunWriter::run` clamps SYN-ACK MSS using packet[8..24] (IPv6 source). When the lookup contains a learned value, the helper computes `min(global_max_mss, effective_ipv6_mtu(path_mtu) - 60)` where 60 is IPv6 (40) + TCP (20) headers and `effective_ipv6_mtu` accounts for the FIPS encapsulation overhead. When the lookup is empty for a destination — the cold-flow case — the helper returns `min(global_max_mss, IPv6-minimum-derived ceiling)`. RFC 8200 mandates every IPv6 path accept ≥1280-byte packets, so the IPv6-minimum-derived MSS (1280 - 77 - 60 = 1143) fits any compliant path. Without this conservative ceiling, the first SYN to a destination with no learned path MTU exits the TUN at the kernel-natural MSS (TUN MTU - 60), and the application connection wedges silently before discovery completes for a corrected second SYN to fire. The fix is provably safe: the ceiling is taken with `min` against the local global so operators with even tighter local floors are never loosened upward. Subsequent flows pick up the actual learned per-destination value once discovery (or the FMP-promotion seed for direct peers) populates the lookup. ## Diagnostic logging All write and read sites emit instrumentation suitable for operators bisecting a wedged path: - `debug!` log on every `path_mtu_lookup` write (discovery originator path and FMP-promotion seed path), showing the FipsAddress, written value, prior value, and post-write map size. `warn!` on poisoned-lock failure path. - `trace!` log per `per_flow_max_mss` call covering every fall-through branch (wrong addr_bytes length, non-fd::/8 prefix, lookup poisoned, no entry for destination, empty-lookup conservative ceiling) and the success path. trace level filters out under normal log settings; capture with `RUST_LOG=info,fips::node::handlers::discovery=debug,fips::upper::tun=trace`. ## Tests 15 new unit tests across 3 files: - `per_flow_max_mss` (8 tests in `src/upper/tun.rs::tests`): empty-lookup conservative ceiling, empty-lookup global-smaller floor, learned-value-overrides-conservative, per-destination smaller, per-destination larger capped by global, non-fips addr, short addr slice, per-destination independence. - `seed_path_mtu_for_link_peer` (4 tests in `src/node/tests/unit.rs`): seed when empty, keep tighter existing, tighten looser existing, no-op for unknown transport. - Discovery integration (3 tests in `src/node/tests/discovery.rs`): apply_outgoing_link_mtu_to_response on unknown peer no-op, two-node target-edge fold (path_mtu reflects target-edge link), three-node chain transit min-fold (existing test, updated for target-edge inclusion). Two pre-existing discovery tests had assertions updated to account for the target-edge fold: - `test_response_path_mtu_two_node`: previously asserted `u16::MAX` (no transit to min-fold); now asserts 1280 (the test transport MTU, folded in by send_lookup_response). - `test_response_path_mtu_four_node_chain`: previously asserted 1350 (transit MTUs only); now asserts 1280 (target-edge MTU is the bottleneck). - `test_transit_forwards_when_mtu_sufficient`: previously asserted 1400 (transit MTU only); now asserts 1280 (target- edge MTU is the bottleneck). ## Verification Local CI on this commit: 29/29 suites pass, 1105 lib tests pass, clippy --all-targets --all-features -D warnings clean, cargo fmt clean. Production deploy verified via trace capture across the managed fleet: cold-flow conservative ceiling branch fires on first SYN, learned-lookup branch takes over once discovery completes, both behaviors observable end-to-end at the SYN MSS on the wire. No wire-format change. No config-format change.