Letting the kernel fragment an oversized datagram answers one question — do
fragments get through. It cannot answer the more interesting one, because the
kernel always emits them in order, first one first.
The classic middlebox fault is exactly about that ordering. Only the first
fragment carries the UDP header, and therefore the ports; a stateful firewall
or NAT that has not seen it has no flow to match the rest against, and many
drop them. That is invisible to any in-order test and shows up in the field as
"large DNS answers fail on this network" or "the tunnel breaks when the MTU
drops" — it works until the network reorders, then fails intermittently, which
is the hardest kind of fault to chase.
So the server now builds the fragments itself (raw socket, IP_HDRINCL) and
controls their order: in_order as a baseline, reversed, and first-fragment-last.
The datagram is assembled and signed whole before being cut up, so what the
client reassembles is indistinguishable from an ordinary packet — otherwise it
would be measuring our sender rather than the path.
Two details that would silently produce wrong answers:
- The UDP checksum is computed rather than left zero. A zero-checksum datagram
is dropped by some middleboxes, and that drop would be recorded as a
fragmentation failure, which is the wrong conclusion entirely.
- Fragment offsets are in 8-byte units, so non-final fragments are rounded to
a multiple of 8. A 100-byte fragment is not an error, it is a datagram no
host will ever reassemble.
frag-send is advertised only when a raw socket can actually be opened — checked
by opening one, since a permission model has more ways to say no than a
capability bit has to say yes.
Fragment header arithmetic is unit-tested (reassembly coverage, MF flags, shared
IP ID, 8-byte offsets, checksum verification), cross-compiled and run on Linux
since the code is build-tagged.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
"3 % loss" sends an engineer looking in both directions at once. The server
records every packet it received per sequence number, so the two cases are
distinguishable: sent-but-never-seen is upstream loss, seen-but-no-reply is
downstream. The findings say which, and say what is not implicated.
Downstream loss is measured against what reached the server, not against what
was sent — the other denominator counts every upstream loss twice and
overstates the return path.
Per-direction jitter comes out of the same records without needing synchronised
clocks: (server_rx - client_tx) carries a constant unknown offset, and
differencing successive samples cancels it, so RFC 3393 variation is honestly
attributable to a direction even though absolute latency is not.
Correlation is by wire sequence number, not loop index — the counter is shared
with every packet type on the session. ProbeSession exposes it even for a lost
probe, since that is precisely the packet whose direction is in question.
Live against fmr: 0.08 ms upstream jitter vs 0.85 ms downstream, an asymmetry a
round-trip test cannot see.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
POST /admin/enroll-tokens now returns the whole link, not just the token:
echolot://enroll?v=1&u=<control URL>&p=pin-sha256:<b64>&t=<token>
The server is the only party that knows all three parts at once, and the part
an operator gets wrong by hand is the base64 pin — which does not fail loudly,
it just never matches, surfacing days later as an inscrutable TLS error. The
app takes the link from a paste or from an echolot:// deep link (QR scan), and
writes URL, pin and credential together or not at all.
One trap the tests pin: an unencoded "+" in a query string decodes to a space,
so a hand-assembled link arrives with a pin wrong by one character. Base64 has
no spaces, so they are restored — unambiguous, and it cannot damage a correctly
encoded pin.
Also fixes a spec divergence: §2.1 names the field device_credential and the
first implementation shipped "credential". Both are sent now and the client
prefers the spec's; the alias goes once nothing reads it.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
The generated refusal read "point at a app within range". Also adds
LiveCompatTest, which checks the half a unit test cannot reach: that two
independently-built artifacts agree on the window, that the profile stays
readable for a version the server refuses, and that both bounds are enforced.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Three facts the client cannot produce alone, kept deliberately separate:
mtu.pmtud_down (largest datagram that arrives unfragmented — meaningful only
because the server sets DF), mtu.frag_delivery (whether larger ones arrive once
fragmentation is allowed), and train.udp_downstream (loss, reordering and
arrival spacing in the download direction, which a round trip cannot separate
from upstream loss).
ServerMeasurement now runs them on the same ProbeSession as the echo train. It
had to: a fresh session restarts client-side sequence numbers and the server's
anti-replay window discards the lot, so the re-primed source is never recorded
and every granted send goes to a socket that has already closed. That produced
four confidently-wrong FAILED tests and a RED verdict on a healthy network.
Live against fmr: path MTU 1500, fragments to 4000, 100/100 downstream, GREEN.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
fmr binds two IPv4 addresses. connFor picked whichever socket of the right
family came first in the bind list, so a downtrain for a session established on
.150 went out from .151 — and every packet was dropped by the client's NAT,
which has no mapping for that pair. tcpdump on the server showed all 50 leaving;
the client saw none. Read as "100% downstream loss", which is the worst kind of
wrong: a confident measurement of something that never happened.
Sessions now record which of our own bound addresses received their traffic, and
granted sends (and delayed echo) go back out through that socket. The fallback
to a family match is kept for the case where nothing has been received yet, and
the test pins both paths — a single-homed lab can never reproduce this.
Also: the client-side halves of the same work — anonymizer (core-privacy), local
run archive with retention (core-archive), upload client, and the app's settings
and history screens.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
big_send now forces the Don't-Fragment bit for the whole burst by default, so
the largest size that arrives IS the downstream path MTU rather than "fragments
got through" — two different measurements the schema already separates. Sizes
above our own egress MTU (from the startup self-test) are refused up front and
reported as max_df_bytes, because absence caused by our kernel must not be read
as a limit of the client's path.
Uploads: one JSON file per run under the state dir, with the policy the operator
actually cares about — who may upload (off / anonymous / account), how large,
how long to keep, and the least anonymization accepted. The profile advertises
all of it so the app can present the switch honestly instead of discovering the
rules by failing. `account` refuses today rather than falling back to anonymous:
picking the strict setting before OIDC lands must not silently mean the loose one.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Resolves the server's canary zone through the platform resolver and
compares against the spec-frozen ground truth (probe-protocol §6.1):
reference records detect answers rewritten in flight, and a per-run nonce
name (uncacheable) proves the query reached the authoritative server.
Findings: dns.answer_rewritten (high), dns.authoritative_unreachable
(medium).
Verified on the OnePlus against the deployed fmr zone: 4/4 reference
records matched exactly, nonce name answered 192.0.2.21 with
reached_authoritative=true. First full client<->server measurement loop
on real hardware; report archived.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Composes core-protocol probes into core-measurement documents. Injected
clock/UUID source keeps it pure and unit-testable. Runs a server ECHO
train and derives RTT distribution, loss, and NAT-rebinding detection
(from the server's observed source port) as train.udp_updown, then
findings + a §7.3 summary.
Verified end-to-end against fmr: 20-packet train, 0% loss, RTT
1.7/2.5/6.9ms, no rebinding → valid MeasurementDocument (2.3kB), overall
GREEN. The whole server-facing stack (protocol → engine → schema →
verdict) now produces the real product artifact against the live server,
no device required.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
