fips/docs/how-to/diagnose-mtu-issues.md

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 first.

Symptom map

Application symptom	Likely cause
iperf3 -c <host.fips> control socket closes immediately after Connecting to host.	Forward-path MTU smaller than the negotiated MSS on the control connection.
ssh user@<host.fips> 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://<host.fips>/ 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.
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.

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:

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.

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.

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.

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.

sudo systemctl edit fips
# Add:
# [Service]
# Environment=RUST_LOG=info,fips::ipv6tun::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.

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 <internet-target>) 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 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.

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 for the per-destination MTU storage layout.

See also

• ../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 — what the MMP metrics mean and how they are computed.
• ../design/fips-ipv6-adapter.md — TUN-side ICMPv6 PTB generation and the MSS clamp.
• tune-udp-buffers.md — host sysctl recipes that rule out kernel-buffer drops as a confounder.