# Diagnose MTU Issues MTU symptoms in FIPS look like ordinary network failures: handshakes succeed but bulk transfers hang, ssh connects but stalls after the banner, an HTTP request times out on the first response. This guide walks through the diagnostic surfaces that FIPS exposes so you can distinguish a real MTU problem from its frequent imposters (bufferbloat, transport saturation, transient packet loss). For the underlying model — encapsulation overhead, proactive vs reactive PMTUD, the per-destination MTU storage layout — read [../design/fips-mtu.md](../design/fips-mtu.md) first. ## Symptom map | Application symptom | Likely cause | | ------------------- | ------------ | | `iperf3 -c ` control socket closes immediately after `Connecting to host`. | Forward-path MTU smaller than the negotiated MSS on the control connection. | | `ssh user@` shows the SSH banner then hangs forever. | First post-banner exchange exceeds the path MTU; SYN MSS clamp did not engage in time, or the path narrowed mid-session. | | `curl http:///` connects, then times out before the first response byte. | Same shape as the SSH-banner case, applied to the first server-to-client large packet. | | Throughput bursts then drops to zero, recovers, drops again, in seconds-long cycles. | Bufferbloat masquerading as MTU failure — usually the upload of the underlay link is saturated. See [Distinguishing bufferbloat](#distinguishing-bufferbloat-from-mtu-drops). | | `MtuExceeded` counters tick up under topology change but settle in seconds. | Normal: the reactive MTU mechanism doing its job. No action needed. | | `MtuExceeded` counters tick continuously under steady state. | Forward-path MTU smaller than what the source learned via `path_mtu` echo. After `mmp.path_mtu` has settled, this is a bug — see [File a bug](#file-a-bug). | The first three are MTU candidates; the fourth is usually not. The fifth is benign. The sixth is the bug shape worth filing. ## Diagnostic toolkit ### `fipsctl show sessions` The authoritative end-to-end MTU for an established session: ```sh fipsctl show sessions | jq '.sessions[] | {display_name, state, mmp: .mmp.path_mtu}' ``` `mmp.path_mtu` is the value the session-layer MMP currently believes is in force end-to-end. It updates on each PathMtuNotification echo from the destination — immediately on decrease, with hysteresis on increase. A field that starts at `1280` (the IPv6 floor) and then climbs to a higher value as echoes arrive is healthy; one that oscillates between two values may indicate a flapping path. ### `fipsctl show transports` Per-transport MTU. The `mtu` field reports the transport-wide default; for BLE, individual links may have a smaller negotiated ATT_MTU. ```sh fipsctl show transports | jq '.transports[] | {type, mtu}' ``` ### `fipsctl show cache` The coordinate cache carries reverse-path-annotated MTU per destination — the freshest "what fit on the way back from the discovery target" estimate, consulted before the session has any PathMtuNotification feedback. ```sh fipsctl show cache | jq '.entries[] | {display_name, depth, path_mtu}' ``` Entries without a `path_mtu` field are pre-discovery or were populated through a path that did not annotate the MTU. ### `fipsctl show peers` Per-peer link state, including the link-layer MMP metrics. Useful mostly for ruling out underlying loss (loss rate near zero, SRTT sane) before chasing an MTU explanation. ```sh fipsctl show peers | jq '.peers[] | {display_name, mmp: .mmp}' ``` ### Trace logging Module-scoped trace logging on the TUN reader and the MMP handler shows the per-packet decisions. The `tracing` macros default the target to the emitting module path, so the filter targets are the fully-qualified module paths under the `fips` crate. ```sh sudo systemctl edit fips # Add: # [Service] # Environment=RUST_LOG=info,fips::upper::tun=trace,fips::node::handlers::mmp=debug sudo systemctl restart fips sudo journalctl -u fips -f ``` ### tcpdump on `fips0` Capturing on the TUN reveals the IPv6 packets the daemon hands the kernel and vice-versa. Two important caveats live in the design doc and are worth restating here: - TX direction (outbound from a local app): tcpdump sees the packet **before** the daemon's TCP MSS clamp at the TUN boundary. The packet may be larger than the daemon will let leave the node. - RX direction (inbound to a local app): tcpdump sees the packet **after** the daemon's MSS clamp on inbound SYN-ACKs. The clamp fires only when `max_mss < kernel-natural-MSS`; otherwise it is a silent no-op. ```sh sudo tcpdump -ni fips0 -w /tmp/fips0.pcap port 22 or port 80 # in another terminal, reproduce the symptom, then Ctrl-C ``` Open the pcap in Wireshark and check segment sizes against what the session's `path_mtu` reports. ## Distinguishing bufferbloat from MTU drops WAN bufferbloat (sustained upload saturation on a cable or DSL link) produces a retransmit signature that looks remarkably like oversized-packet drops. Both manifest as long stalls in TCP flows, both clear when you stop pushing data, both can ramp the loss-rate counter without obvious cause. Two ways to disambiguate: 1. **Saturate the underlay first.** Run a reference upload outside FIPS (`iperf3 -c `) until it stabilises, then measure latency to the underlay's first hop with a separate `ping`. If RTT shoots up by hundreds of ms during the upload, the underlay buffer is the culprit, not FIPS MTU. Apply CAKE / fq_codel on the underlay router before continuing. 2. **Watch the FIPS counters during the symptom.** A real MTU problem ticks `MtuExceeded` (visible in `fipsctl show routing`'s `error_signals` block) and shifts the session's `mmp.path_mtu` downward. Bufferbloat ticks loss rate and RTT but leaves `path_mtu` and `MtuExceeded` alone. If both signatures fire together, you have both problems. ## Cold-flow first-SYN The MMP echo populates path-MTU state only after the first end-to-end exchange, but the TUN reader has to size the very first SYN before any echo has arrived. The cold-flow ceiling is the 1143-byte conservative fallback derived from the 1280-byte IPv6 floor. The first SYN may therefore be smaller than what the path ultimately supports; once MMP echoes arrive, subsequent flows use the larger learned value. If the first SYN of a flow is still oversized relative to the path, the receiving transit node generates an `MtuExceeded`, the source shrinks immediately, and the next packet of the flow fits. This is expected for one round trip; it becomes a problem only if it persists. ## Fixes The operator's choices, in rough order of preference: ### Pin a per-transport MTU floor in config If a known link in the path has a small MTU that discovery does not pick up promptly (e.g., a Tor hop with an unusually tight cap), set a transport-level MTU floor on the relevant `transports.*` block. See [../reference/configuration.md](../reference/configuration.md) for the per-transport MTU keys. ### Tune host UDP buffers For UDP transports specifically, undersized kernel buffers can drop oversized datagrams in a way that looks identical to MTU failure. See [tune-udp-buffers.md](tune-udp-buffers.md). ### Accept the floor on intrinsically small links Tor and BLE link MTUs are properties of the medium, not tunables. For sessions that cross those links, the path MTU will be small; the fix is to design applications around it (smaller TCP windows, fewer large RTTs) rather than fight the transport. ### File a bug The bug shape worth filing is session `mmp.path_mtu` itself oscillating, or `MtuExceeded` ticking *within* an established session after `mmp.path_mtu` has settled. The TCP-clamp mirror (`path_mtu_lookup`) is now updated on every successful proactive `PathMtuNotification` apply (tighter-only) as well as by the reactive `MtuExceeded` handler, so a steady-state divergence between the per-session `mmp.path_mtu` and the mirror used for new TCP flows is itself a defect, not an expected behavior. Capture `fipsctl show sessions`, `fipsctl show cache`, `fipsctl show routing` (for the `error_signals` block), and a tcpdump from `fips0` covering the symptom window. See [../design/fips-mtu.md](../design/fips-mtu.md#per-destination-mtu-storage) for the per-destination MTU storage layout. ## See also - [../design/fips-mtu.md](../design/fips-mtu.md) — encapsulation overhead, the proactive `path_mtu` field, the reactive `MtuExceeded` mechanism, MSS clamping, the no-fragmentation policy. - [../design/fips-mmp.md](../design/fips-mmp.md) — what the MMP metrics mean and how they are computed. - [../design/fips-ipv6-adapter.md](../design/fips-ipv6-adapter.md) — TUN-side ICMPv6 PTB generation and the MSS clamp. - [tune-udp-buffers.md](tune-udp-buffers.md) — host sysctl recipes that rule out kernel-buffer drops as a confounder.