throughput: the upstream direction, counted by the only party that can

The client generates the traffic and the server counts it. No grant is involved
- the client is sending its own packets, so there is nothing to amplify - but it
does need the server's tally, because only the far end knows how much arrived.
Without that number a sender measures how fast it can transmit, which is usually
just the speed of the local NIC and is not the question being asked.

A new wire type the server counts and deliberately never answers: a reply would
double the traffic and drag the return path into a measurement that is
specifically about the outbound one.

The tally is a counter, not a list, and short-circuits before the observation
log. A five-second run at 20 Mbps is around ten thousand packets; one struct
each would turn a measurement into an allocation storm on a shared server, and
nothing needs the per-packet detail since the client holds the send-side record.
The gap between the two counts is the loss.

direction=up on the throughput action sends nothing - it zeroes the counter, so
a second run in one session measures itself instead of inheriting the first.

Same honesty rule as downstream: measures_network is false when what arrived
matches what was offered, because then the path was never the constraint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -162,3 +162,11 @@ First build downloads AGP/Compose/Shizuku from Google Maven + Maven Central.
    are SUPPORTED on both known devices via `Os.recvmsg` + `StructMsghdr` reflection.
 3. Fold the confirmed capabilities + Shizuku dump-format samples back into the production
    `core-probe` / `core-shizuku` modules.
+
+## Enrolling a device with a server
+
+`echolot-app/scripts/enroll-link.sh [note]` mints a §2.1 bootstrap link on fmr over SSH and prints
+it (plus a QR if `qrencode` is installed, plus the `adb shell am start -a …VIEW -d '<uri>'` command
+when a device is attached). The link carries a single-use token — treat it as a secret until spent.
+Never hand-assemble one: the base64 pin needs percent-encoding, and a pin wrong by one character
+fails as an inscrutable TLS error rather than as a bad pin.

docs/build-status.md

@@ -660,3 +660,284 @@ One user-visible bug caught in the process: Go's JSON encoder HTML-escapes `<`,
 default, so the refusal reached the client as `needs \u003e= 0.2.0`. Disabled at the encoder (this
 is an API, not a page), and the client now *parses* the error field instead of pattern-matching it,
 so it survives whatever a future encoder decides to escape.
+
+### Enrollment: the server mints the bootstrap link (server-v0.5.3 … v0.5.4, 2026-08-01)
+Until now a device was configured by hand-typing a control URL, a base64 SPKI pin and a
+credential. That is the step that goes wrong, and it goes wrong quietly: a pin off by one
+character does not fail loudly, it just never matches, and surfaces days later as an inscrutable
+TLS error.
+
+`POST /admin/enroll-tokens` now returns the whole §2.1 bootstrap link alongside the token, because
+the server is the only party holding all three parts at once. The app takes it from a paste or an
+`echolot://enroll` deep link (so a QR scan configures a server in one action) and writes URL, pin
+and credential **together or not at all** — a half-applied server fails later, somewhere else,
+with an error pointing at the wrong thing.
+
+The control URL comes from `ECHOLOT_PUBLIC_URL` (set on fmr to `https://fmr-1.echo-lot.app:8443`),
+falling back to the first control listen address; a wildcard bind warns rather than emitting a
+link to `0.0.0.0`.
+
+**The encoding trap, which is the whole reason this is tested across both languages.** The pin is
+base64, so it contains `+`, `/` and `=` — each of which means something else in a query string. An
+unencoded `+` decodes to a space, leaving the pin wrong by exactly one character. Base64 has no
+spaces, so the parser restores them; that cannot damage a correctly-encoded pin and it rescues
+every hand-assembled link. `LiveEnrollmentTest` redeems a link the *server* produced, which is the
+only way to catch a disagreement between the Go assembler and the Kotlin parser — a unit test on
+either side alone cannot see it. It also asserts the token is refused the second time.
+
+Also fixed a spec divergence found while reading §2.1: the spec names the field
+`device_credential`, the first implementation shipped `credential`. The server now sends both and
+the client prefers the spec's; the alias goes once nothing reads it.
+
+Two process notes from this round:
+- An edit to the admin handler silently failed to apply and the endpoint kept returning just the
+  token. Caught by deploying and *looking at the response*, not by trusting a green build.
+- The live suite is now six tests (`LiveServerTest`, `LiveMeasurement`, `LiveGranted`,
+  `LiveUpload`, `LiveCompat`, `LiveEnrollment`), all green against fmr from the PC with no device.
+
+### Directional loss: which way is the packet loss? (2026-08-01)
+A round trip can only report that *something* was lost somewhere, which is the least useful form
+of the answer — "3 % loss" sends an engineer looking in both directions at once. The server
+already records every packet it received per sequence number (§6), so the two cases are actually
+distinguishable, and `train.udp_updown` now reports them separately:
+
+- sent, never seen by the server → **upstream** loss
+- seen by the server, reply never arrived → **downstream** loss
+
+Findings name the direction and say what is *not* implicated, which is half the value:
+`connectivity.loss_upstream` ("the return path is not implicated: replies came back for everything
+that arrived"), `connectivity.loss_downstream`, `nat.udp_unreachable_upstream`.
+
+Two things the implementation gets deliberately right:
+- **Downstream loss is measured against what reached the server**, not against what was sent.
+  Using "sent" as the denominator counts every upstream loss a second time and overstates the
+  return path. Pinned by a test with loss in both directions at once.
+- **Per-direction jitter without synchronised clocks.** Absolute one-way delay would need clock
+  sync and we deliberately have none (the two-clock rule). But `server_rx − client_tx` carries a
+  constant unknown offset, and differencing successive samples cancels it — so RFC 3393 one-way
+  delay variation *is* honestly attributable to a direction even though latency is not. A test
+  pins that a 10-second clock offset changes nothing.
+
+Correlation is by **wire sequence number**, which is not the loop index: the counter is shared
+with every other packet type on the session, so "the nth echo" is not "sequence n". `ProbeSession`
+now exposes `lastSeq`, including for a probe that was lost — a lost packet still has a sequence
+number, and that number is exactly what tells you which way it was lost.
+
+Live against fmr: 20/20 both ways, and jitter of **0.08 ms upstream vs 0.85 ms downstream** — a
+tenfold asymmetry that a round-trip measurement cannot see at all.
+
+10 unit tests on the arithmetic (a wrong denominator here does not crash, it produces a plausible
+number pointing at the wrong half of the network) plus the live correlation check.
+
+### frag_send: crafted IP fragments, so *ordering* is testable (server-v0.6.0, 2026-08-01)
+`big_send` with `df=false` answers one question — do fragments get through. It cannot answer the
+more interesting one, because the kernel always emits fragments 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 simply drop them. That is invisible to every in-order test, and in
+the field it looks like "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 builds the fragments itself (raw socket, `IP_HDRINCL`) and controls their order:
+`in_order` (baseline), `reversed` (last fragment first), `first_last` (first fragment held back
+250 ms). The datagram is assembled and **signed whole** before being cut up, so what the client
+reassembles is indistinguishable from an ordinary packet — otherwise the test would be measuring
+our sender rather than the path. New test type `mtu.frag_ordering`; findings
+`mtu.fragments_blocked` and `mtu.fragment_reorder_sensitive`.
+
+Two details that would otherwise produce confidently wrong answers:
+- **The UDP checksum is computed, not left zero.** Zero is legal in IPv4 and would be less code,
+  but zero-checksum datagrams are 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 down 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,
+because a permission model has more ways to say no (userns, seccomp, LSM) than a capability bit has
+to say yes. fmr runs as root with `cap_net_raw` in its bounding set, so it is available there.
+
+Fragment ordering runs only after `mtu.frag_delivery` shows fragments arrive at all; otherwise the
+three orderings would each report "not delivered" and read as three faults instead of one.
+
+The header arithmetic is unit-tested (reassembly coverage with no gaps or double-delivery, MF
+flags, shared IP ID, 8-byte offsets, checksum verification over odd and even lengths). Because the
+code is `//go:build linux`, the tests are **cross-compiled and run on fmr** — there is no Go
+toolchain there, so `go test -c` plus scp is the loop.
+
+Live against fmr: 4 fragments per burst, and all three orderings reassembled — a healthy path, and
+the baseline against which a mobile network will be interesting.
+
+### Testing state (2026-08-01)
+Six live tests against fmr, all green, no device involved: `LiveServerTest`, `LiveMeasurement`,
+`LiveGranted`, `LiveDownstream`, `LiveUpload`, `LiveCompat`, `LiveEnrollment`. Plus 74 client unit
+tests and the full Go suite. Everything in the last several entries is verified from the PC; the
+app's UI (settings, history, deep-link enrollment) and `mtu.pmtud_up` remain device-only.
+
+### throughput: a rate, plus the qualifier that makes it a measurement (server-v0.6.1 … v0.6.2)
+A throughput test reports the *smallest* limit on the path — and the sender's own ceiling is one of
+the candidates. If the server is asked for 50 Mbps and 50 Mbps arrives, the network was never the
+constraint and "50 Mbps" says nothing about it. So `perf.throughput_udp` always carries
+`limited_by` (duration | budget | rate | send_error) and `measures_network`, and a finding is
+raised only when the path is actually implicated. The live run against fmr reports 20 Mbit/s with
+`measures_network: false`, which is the correct and useful answer.
+
+Loss is computed against the **sender's own count**, fetched from the observations API, not against
+the requested rate. A receiver alone cannot tell "the network dropped it" from "the sender never
+sent it", and guessing turns a healthy server-side limit into a phantom network fault. The server
+keeps one summary per action rather than per-packet records — a ten-second run at 50 Mbps is half a
+million packets, and a struct each would turn a measurement into memory exhaustion.
+
+Sending is **paced**, on an absolute schedule. Unpaced would measure the server's NIC and the first
+queue it meets, then collapse into loss that reads as a network fault; sleep-per-packet would
+accumulate scheduler error and drift the rate down over a ten-second run.
+
+Throughput gets its own grant budget sized from the request, so every *other* action stays bounded
+at 8 MiB. When the byte cap binds before the clock does, the **duration is shortened and reported**
+rather than the run being truncated: promising thirty seconds and delivering twenty-one is the same
+information with a surprise attached, and it keeps "the clock ended the run" as the normal case —
+the only case where the rate is a clean property of the path. That behaviour came out of a test
+that failed honestly (30 s at 100 Mbps needs 375 MB against a 256 MB cap).
+
+It is **opt-in** in the run config, default off. A 5-second run at 50 Mbps moves ~30 MB; on a
+metered mobile connection that is the user's money, and a tool that spends it without being asked
+is not one people keep installed.
+
+#### The bug the live test found
+The first live run delivered 104 packets and stopped after 50 ms. The grant's rate check exempted
+the first 50 ms entirely, meaning to be lenient at startup — the effect was the opposite. A sender
+could dump an unbounded burst into that free window, and the instant the check switched on it
+compared those bytes against 50 ms worth of allowance and refused everything until real time caught
+up. **Every short test passed** (downtrain sends 50 packets, big_send seven); every sustained send
+died fifty milliseconds in.
+
+Replaced with a token bucket (`allowance = burst + rate × elapsed`), which is smooth from t=0.
+The burst is 100 ms of the allowed rate, floored at one ordinary datagram — deliberately one, since
+at 8 kbps a 64 KB floor is sixty-four seconds' worth, exactly the instant dump the ceiling exists to
+prevent. The pre-existing rate test caught that when I first tried the generous floor, and it was
+right to. Second half of the same bug: callers treated *any* refusal as terminal, so `TryAllow` now
+says why — a sender paces through a transient "too fast just now" and still stops dead on a spent
+budget or an expired grant. Both halves are pinned by regression tests.
+
+### Findings registry (2026-08-01)
+Closes open item 1 of measurement-schema.md §9. A finding code is the stable, machine-readable half
+of a result — what a dashboard groups by and what someone greps a year of archived runs for — and
+that only holds if a code means exactly one thing forever. Ad-hoc string literals at fifteen call
+sites cannot promise that, and by the time the registry was written the failure had already
+happened.
+
+**Two emitters had independently produced `connectivity.downstream_loss` and
+`connectivity.loss_downstream` for the same claim**, and nothing anywhere objected. Anyone
+aggregating either one would have silently seen half their data. Merged into
+`connectivity.loss_downstream`, paired with `loss_upstream` so the two directions read as a set.
+
+**Two codes were also renamed out of `nat.*`.** `nat.udp_unreachable` is not about NAT — it means
+no replies came back — but the prefix determines the category, and the category determines which
+verdict light the finding rolls up into (§7.3). A `nat.*` code landing under *connectivity* is not
+a naming quibble; it changes which light turns red. Cheap to fix now, a breaking change later.
+
+Codes are now declared as typed `FindingSpec`s carrying their category and default severity, and
+emitters reference the spec instead of retyping the string — so a typo is a compile error and two
+call sites cannot disagree about a finding's category.
+
+`docs/findings-registry.md` is the contract, and a test reads it: it fails when the document and
+the registry have codes the other lacks, or when a severity differs. Documentation that drifts from
+its implementation is worse than none, because it still looks authoritative. The check scopes
+itself to table rows, so the prose can keep explaining which codes were retired and why.
+
+Six tests: uniqueness, declared-vs-listed, prefix↔category agreement, naming convention, a
+word-order-anagram check (the shape the duplication actually took), and the document agreement.
+
+### A real privacy leak, found by starting on the machine-readable schema (2026-08-01)
+The intent was `measurement.schema.json` (§8's promised companion). The first step — checking
+whether the anonymizer actually covers the fields the schema declares as sensitive — found that it
+did not, so that became the work.
+
+**At the `balanced` level, five identifying values were being uploaded verbatim:**
+
+| value | field | why it matters |
+|---|---|---|
+| `2001:…::150` | `networks[].link.addresses[].addr` | the device's own global IPv6 address — a strong, geolocatable device identifier |
+| `2a02:…::1` | `networks[].link.routes[].gateway` | identifies the ISP allocation |
+| `203.0.113.77` | `networks[].link.dns.servers[]` | the configured resolver |
+| `nas.example.lan` | `private_dns_hostname` | an internal hostname |
+| `example.lan` | `search_domains[]` | the internal domain |
+
+The settings screen describes that level as pseudonymizing addresses. It was not.
+
+**Root cause:** classification keyed on field *names*, and the schema's actual names (`addr`,
+`gateway`, `dst`, `servers`, `search_domains`, `private_dns_hostname`) had never been added to the
+table. Not a subtle bug — just an unfalsifiable design. The existing tests all passed, because each
+one checked a field somebody had remembered to write a case for.
+
+**Two fixes, one of them structural:**
+1. The missing names were added.
+2. More importantly, a **shape-based backstop**: when a field name is unrecognised, the *value* is
+   inspected, and anything shaped like an IPv4/IPv6 address or a MAC is treated as one. A name
+   table can only protect fields someone thought of, which is precisely the wrong property for a
+   privacy control. Hostnames are deliberately *not* inferred by shape — `train.udp_updown` is
+   indistinguishable from a domain, and mangling a test type would corrupt the document to protect
+   nothing.
+
+`LeakTest` is the new guard and is written to fail for fields nobody has considered: it plants
+identifying values wherever one can actually occur and asserts none survive, rather than checking
+a list of known cases. It also pins that RFC1918 addresses still come through readable, so the
+test cannot pass by over-redacting everything.
+
+Route prefixes and the unspecified address needed care in the transform: `0.0.0.0/0` and `::/0`
+must stay themselves, or a routing table becomes unreadable for no privacy gain.
+
+**Still outstanding:** `measurement.schema.json` itself. Worth noting what this episode implies for
+it — much of a document's payload lives in `evidence`/`metrics`/`params`, which are per-test-type
+`JsonObject` by design and therefore *outside* any schema. A schema-driven anonymizer would have
+less coverage there than the name-plus-shape one now does, so the schema should be built for
+validation and external tooling, not as a replacement for the classifier.
+
+### ULA prefixes are pseudonymized whole (2026-08-01)
+Spotted in a real uploaded run from the phone: the server had
+`fda1:3fb1:ff92:6696::2662` for a DNS server. The general IPv6 path preserves the leading two
+groups (deliberately — for a global address that keeps the ISP allocation, which is the
+diagnostically useful part), and for a ULA that passed through **32 of the 40 random bits** of the
+global ID.
+
+ULA looks like the v6 equivalent of RFC1918 and the instinct is to treat it the same. That
+reasoning does not carry over, and the difference is the whole point: an RFC1918 prefix is shared
+by millions of networks and identifies none of them, while a ULA global ID is random and unique to
+one network by construction (RFC 4193). The prefix *is* the identifier — it is a network
+fingerprint that was surviving redaction.
+
+Now pseudonymized as a unit, so two addresses on the same ULA subnet still land on the same
+pseudonymous prefix: "these hosts are on one network" survives, "this is *that* network" does not.
+Three tests, one of which uses the exact value observed on the wire.
+
+Worth recording as a reasoning trap: I had originally raised this as "ULA should probably be kept
+verbatim, like RFC1918, for consistency". The surface analogy pointed the wrong way, and the
+correct answer was the opposite.
+
+### Registry adopted everywhere; v6 findings renamed; Back works (2026-08-01)
+The findings registry was only adopted in `core-engine`. The app module still emitted seven codes
+as raw strings, so the registry test passed while codes existed outside it — including
+`ipv6.broken`, which fired on a real network and was in no registry at all.
+
+All seven now reference registry entries for code, category and severity, so those three cannot
+disagree at a call site. A grep for `code = "…"` across the app, engine and probe modules returns
+nothing.
+
+**`ipv6.*` → `v6.*`.** The third instance of rule 1: they declared `Category.IPV6` while the prefix
+map only knows `v6`, so `TestType.category("ipv6.broken")` fell through to *connectivity* and the
+finding rolled up under the wrong verdict light. The test-type registry already used `v6.`.
+
+Two severities reconciled while merging:
+- `connectivity.captive_portal` is **medium**, not high. The registry had guessed high; the probe
+  that emits it had always said medium, and the probe was the considered value — a captive portal
+  on hotel wifi is what should be there, and logging in clears it. `connectivity.no_internet` is
+  the high one, because nothing the user does locally fixes that.
+- `v6.not_offered` is **info, and the registry says it must stay info**. Most networks still do not
+  offer IPv6 and that is not a fault; a warning here lights a yellow verdict on a healthy network,
+  which teaches people to ignore the light.
+
+Also: a `BackHandler` now returns from Settings/History to the run screen. The screen was a plain
+state variable with nothing connecting it to the back stack, so the system Back gesture left the
+app entirely. Enabled only when there is somewhere to go back to, so Back still exits from the run
+screen.

docs/findings-registry.md

@@ -0,0 +1,106 @@
+<!--
+SPDX-FileCopyrightText: 2026 Echolot contributors
+SPDX-License-Identifier: CC-BY-4.0
+-->
+
+# Echolot findings registry
+
+Closes open item 1 of `measurement-schema.md` §9.
+
+A **finding code** is the stable, machine-readable half of a result. The prose around it changes
+freely; the code is what a dashboard groups by, what a diff between two runs keys on, and what
+someone greps a year of archived runs for. That only works if a code means exactly one thing,
+forever.
+
+This document is the contract. It is kept in step with
+`echolot-app/core-measurement/.../FindingRegistry.kt` by a test that fails when either side has a
+code the other does not — a registry that drifts from its documentation is worse than none,
+because it looks authoritative.
+
+## Rules
+
+1. **The prefix determines the category**, and the category determines which verdict light the
+   finding rolls up into (§7.3). A `nat.*` code appearing under *connectivity* is not a naming
+   quibble; it changes which light turns red. Two codes were renamed from `nat.*` to
+   `connectivity.*` for exactly this reason.
+2. **One code per concept.** Two emitters independently produced `connectivity.downstream_loss`
+   and `connectivity.loss_downstream` for the same claim before this registry existed. Anyone
+   aggregating either would have silently seen half their data.
+3. **Codes are declared, not typed.** Emitters reference a `FindingSpec`, so a typo is a compile
+   error and no two call sites can disagree about a finding's category or default severity.
+4. **Severity in the registry is the default.** An emitter may escalate for a specific run; it may
+   not quietly reclassify the finding in general.
+5. **Say what is ruled out**, where that is the useful half. "Loss upstream" is worth far more
+   when it also states that the return path is clean, because that halves where to look next.
+6. **Renaming a code is a breaking change** once runs are archived at scale. Before 1.0 it is
+   cheap; after, it needs an alias and a deprecation window.
+
+## Registry
+
+### connectivity
+
+| code | severity | means | rules out |
+|---|---|---|---|
+| `connectivity.udp_unreachable` | high | No UDP echo replies came back from the server at all. | — |
+| `connectivity.udp_unreachable_upstream` | high | The server received none of the probes, so traffic is dropped on the way out. | The return path: nothing arrived to be replied to. |
+| `connectivity.udp_loss` | medium | A large fraction of round-trip probes were lost, direction unknown. | — |
+| `connectivity.loss_upstream` | medium | Probes were lost on the way to the server. | The return path: replies came back for everything that arrived. |
+| `connectivity.loss_downstream` | medium | Packets were lost on the way back from the server. | The outbound path: the server received what it was answering. |
+| `connectivity.downstream_blocked` | high | Server-initiated packets never arrive, although round trips work. | Basic reachability: the path forwards replies, just not unsolicited traffic. |
+| `connectivity.downstream_reorder` | low | Downstream packets arrive in a different order than they were sent. | — |
+| `connectivity.captive_portal` | medium | A captive portal is intercepting connectivity checks. | — |
+| `connectivity.no_internet` | high | Android's own connectivity checks fail on this network. | — |
+
+### mtu
+
+| code | severity | means | rules out |
+|---|---|---|---|
+| `mtu.reduced_downstream` | low | The downstream path MTU is below the usual 1500 bytes. | — |
+| `mtu.downstream_blackhole` | medium | Datagrams above the path MTU are dropped downstream, fragmented or not. | — |
+| `mtu.fragments_blocked` | medium | IP fragments do not reach this device even when sent in order. | — |
+| `mtu.fragment_reorder_sensitive` | low | Fragments are delivered in order but dropped when reordered or delayed. | Fragmentation itself: in-order fragments arrive fine. |
+
+### nat
+
+| code | severity | means | rules out |
+|---|---|---|---|
+| `nat.udp_rebinding` | medium | A NAT remapped the UDP source port mid-flow. | — |
+| `nat.symmetric` | medium | The NAT assigns a different external port per destination. | — |
+
+### perf
+
+| code | severity | means | rules out |
+|---|---|---|---|
+| `perf.throughput_no_delivery` | high | No throughput traffic arrived, although the server sent it. | — |
+| `perf.throughput_below_offered` | low | Less throughput arrived than the server sent for the whole run. | — |
+
+### dns
+
+| code | severity | means | rules out |
+|---|---|---|---|
+| `dns.answer_rewritten` | high | A resolver returned an answer that differs from the authoritative record. | — |
+| `dns.authoritative_unreachable` | medium | The canary zone's authoritative server could not be reached. | — |
+
+### v6
+
+The prefix is `v6.`, matching the test-type registry (`v6.brokenness`, `v6.happy_eyeballs`, …).
+These were `ipv6.*` while declaring `Category.IPV6`; since the prefix map only knows `v6`, they
+rolled up under *connectivity* instead — the third occurrence of rule 1 being broken.
+
+| code | severity | means | rules out |
+|---|---|---|---|
+| `v6.broken` | medium | IPv6 is configured on this network but does not work. | Absence of IPv6: it is provisioned, it simply fails. |
+| `v6.not_offered` | info | This network does not offer IPv6. | — |
+
+`v6.not_offered` is **info and must stay info**. Most networks still do not offer IPv6 and that is
+not a fault; reporting it as a warning lights a yellow verdict on a healthy network, which teaches
+people to ignore the light — the one thing a diagnostic must never do.
+
+## Adding a finding
+
+1. Add a `FindingSpec` to `FindingRegistry`, and to its `all` list.
+2. Add the row here, under the section its prefix names.
+3. Emit it with `finding(FindingRegistry.YOUR_CODE, …)`.
+
+The registry test checks 1 and 2 agree, that every prefix maps to the category it claims, and that
+no two entries share a code.

docs/measurement-schema.md

@@ -269,7 +269,7 @@ The JSON Schema (machine-readable companion, `measurement.schema.json`, generate
 
 | type | example fields | v2 anonymizer transform |
 |---|---|---|
-| `ip4`, `ip6` | addresses, routes, hops, DNS answers | prefix-preserving pseudonymization, consistent per document; well-known/reserved ranges kept verbatim |
+| `ip4`, `ip6` | addresses, routes, hops, DNS answers | prefix-preserving pseudonymization, consistent per document; well-known/reserved ranges kept verbatim. **Exception: ULA (`fc00::/7`) has its whole prefix pseudonymized as a unit.** It resembles RFC1918 but is not analogous: a ULA global ID is 40 random bits, unique to one network by construction (RFC 4193), so the prefix *is* the identifier, whereas `192.168.0.0/16` is shared by millions of networks and identifies none. Pseudonymizing it as a unit keeps "these hosts are on one subnet" while dropping "this is that subnet". |
 | `mac`, `bssid` | wifi, arp_watch | OUI kept, NIC part pseudonymized |
 | `fqdn` | DNS names, reverse lookups | per-label pseudonyms, public-suffix kept |
 | `ssid` | wifi | pseudonym |
@@ -279,7 +279,8 @@ Free-text fields (`notes`, `error.detail`, dump excerpts from Shizuku parsers) c
 
 ## 9. Open items
 
-1. Findings registry document — start alongside the first implemented tests.
+1. ~~Findings registry document~~ — done: `findings-registry.md`, kept in step with
+   `FindingRegistry.kt` by a test that fails when the two disagree.
 2. Whether Shizuku raw-dump excerpts (dumpsys/ip output) are embedded in `evidence` verbatim (auditable, but large and hard to anonymize) or parsed-only with an optional "attach raw dumps" toggle. Proposal: toggle, default on for local archive, default off for export.
 3. Peer-mode documents: each device produces its own run; the coordinator embeds the peer's findings summary and cross-references by `run.id`. Full merge format deferred.
 4. Size guardrails: soft cap 20 MB uncompressed per run; trains beyond that downsample evidence (keep aggregates + first/last N + all anomalies) and record `"evidence_truncated": true`.

docs/probe-protocol.md

@@ -24,11 +24,34 @@ echolot://enroll?v=1&u=<control-URL, urlencoded>&p=pin-sha256:<b64 SPKI hash>&t=
 
 ```
 POST /v1/enroll        Authorization: Bearer <enrollment-token>
-→ 200 { "device_credential": "<random 256-bit, b64url>",
-        "device_id": "uuid",
-        "profile": { ... §2.2 ... } }
+→ 201 { "device_credential": "<random 256-bit, b64url>",
+        "device_id": "uuid" }
 ```
 
+The **server assembles the bootstrap link**, because it is the only party holding 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, and surfaces later as an inscrutable TLS error:
+
+```
+POST /admin/enroll-tokens
+→ { "token": "…", "expires_in_s": 86400,
+    "enroll_uri": "echolot://enroll?v=1&u=…&p=…&t=…" }
+```
+
+The control URL in the link comes from `ECHOLOT_PUBLIC_URL`, falling back to the first control
+listen address. A wildcard bind has no single right answer, so it warns rather than guessing.
+
+Encoding notes that matter in practice:
+- `u`, `p` and `t` are **percent-encoded**. The pin is base64, so it contains `+`, `/` and `=`,
+  every one of which means something else in a query string.
+- A `+` that was *not* encoded decodes to a space. Base64 contains no spaces, so a parser SHOULD
+  restore them — the alternative is a pin wrong by one character and a failure that points nowhere
+  near the cause.
+- The control URL MUST be `https://`. The pin only protects a TLS connection; a cleartext URL
+  would hand the token to anyone on the path.
+- **The link is a secret** while it is live: it carries a bearer token, so anyone who sees it
+  before the device does can enroll instead.
+
 Enrollment tokens are single-use with expiry, created in the admin UI, scoped `enroll`. The device credential is a long-lived bearer secret, scoped `run-tests`; it is also the HKDF input for session keys. Revocation = deleting the device in the admin UI.
 
 ### 2.2 Profile

echolot-app/app/src/main/kotlin/app/echo_lot/app/HistoryScreen.kt

@@ -4,6 +4,7 @@
 package app.echo_lot.app
 
 import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.safeDrawingPadding
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxWidth
@@ -39,7 +40,7 @@ fun HistoryScreen(
     onDelete: (String) -> Unit,
     onBack: () -> Unit,
 ) {
-    Column(Modifier.fillMaxWidth().padding(16.dp), verticalArrangement = Arrangement.spacedBy(10.dp)) {
+    Column(Modifier.fillMaxWidth().safeDrawingPadding().padding(16.dp), verticalArrangement = Arrangement.spacedBy(10.dp)) {
         Row(verticalAlignment = Alignment.CenterVertically) {
             TextButton(onClick = onBack) { Text("‹ Back") }
             Text("History", style = MaterialTheme.typography.titleLarge)
@@ -71,12 +72,20 @@ fun HistoryScreen(
                             )
                         }
                         Text(
-                            "${r.findingCount} finding(s) · ${r.sizeBytes / 1024} kB · ${r.anonymization}",
+                            "${r.findingCount} finding(s) · ${r.sizeBytes / 1024} kB · " +
+                                "kept complete on this device",
                             style = MaterialTheme.typography.bodySmall,
                         )
+                        // The upload line names the level the upload was made at, not the
+                        // archive's. They describe different documents, and showing the archive's
+                        // level here claimed more had left the device than actually did.
                         Text(
-                            if (r.uploaded) "uploaded to ${r.uploadedTo ?: "a server"}"
-                            else "on this device only",
+                            if (r.uploaded) {
+                                "uploaded to ${r.uploadedTo ?: "a server"}" +
+                                    (r.uploadedAs?.let { " as $it" } ?: "")
+                            } else {
+                                "on this device only"
+                            },
                             style = MaterialTheme.typography.bodySmall,
                             color = if (r.uploaded) Color(0xFF7FD17F) else Color(0xFFBBBBBB),
                         )

echolot-app/app/src/main/kotlin/app/echo_lot/app/MainActivity.kt

@@ -85,10 +85,18 @@ class MainActivity : ComponentActivity() {
                             finish()
                         }
                     }
+                    // Without this, the system Back gesture leaves the activity from Settings or
+                    // History instead of returning to the run screen — the screen is a plain state
+                    // variable, so nothing connects it to the back stack. Registered only when
+                    // there is somewhere to go back to, so Back still exits from the run screen.
+                    androidx.activity.compose.BackHandler(enabled = screen != Screen.RUN) {
+                        screen = Screen.RUN
+                    }
+
                     when (screen) {
                       Screen.SETTINGS -> SettingsScreen(
                         settings = vm.settings,
-                        archivedRuns = vm.state.history.size,
+                        archivedRuns = vm.archivedRunCount(),
                         archivedBytes = vm.archivedBytes(),
                         onApplyRetention = vm::applyRetention,
                         onDeleteAll = vm::deleteAllRuns,

echolot-app/app/src/main/kotlin/app/echo_lot/app/RunStore.kt

@@ -164,8 +164,10 @@ class RunStore(context: Context, private val settings: Settings) {
             )
             val body = redactedForUpload(docJson, level)
             val reply = client.uploadRun(settings.serverCredential, body)
-            archive.markUploaded(runId, profile.name)
-            UploadOutcome.Sent(profile.name, "as $level, ${body.toByteArray().size} bytes: ${reply.take(120)}")
+            archive.markUploaded(runId, profile.name, level.wire)
+            // Deliberately not echoing `reply`: it is the server's index entry as raw JSON, and
+            // it ended up rendered verbatim in the UI. Size and level are what a person wants.
+            UploadOutcome.Sent(profile.name, "as $level, ${body.toByteArray().size} bytes")
         } catch (e: VersionRefused) {
             UploadOutcome.Incompatible(e.message ?: "the server refused this app's version")
         } catch (e: UploadRefused) {

echolot-app/app/src/main/kotlin/app/echo_lot/app/RunViewModel.kt

@@ -196,6 +196,10 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
 
     fun archivedBytes(): Long = store.totalBytes()
 
+    /** Counted from the archive itself, not from [UiState.history], which is empty until the
+     *  history screen has been opened - the two disagreeing read as data loss. */
+    fun archivedRunCount(): Int = store.list().size
+
     /** Redeems an enrollment link, from a paste or from an echolot:// deep link. */
     fun enroll(link: String, deviceName: String? = android.os.Build.MODEL) {
         viewModelScope.launch {
@@ -352,8 +356,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 when {
                     ev.contains("\"captive_portal\"") -> out.add(
                         Finding(
-                            id = ids.uuid(), code = "connectivity.captive_portal", category = Category.CONNECTIVITY,
-                            severity = Severity.MEDIUM, confidence = Confidence.HIGH,
+                            id = ids.uuid(), code = FindingRegistry.CAPTIVE_PORTAL.code,
+                            category = FindingRegistry.CAPTIVE_PORTAL.category,
+                            severity = FindingRegistry.CAPTIVE_PORTAL.severity, confidence = Confidence.HIGH,
                             title = "Captive portal intercepting connections",
                             description = "The generate_204 check returned a redirect or a page instead of HTTP 204 — a captive portal (login/splash page) is intercepting traffic on this network.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -361,8 +366,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                     )
                     t.status == TestStatus.FAILED -> out.add(
                         Finding(
-                            id = ids.uuid(), code = "connectivity.no_internet", category = Category.CONNECTIVITY,
-                            severity = Severity.HIGH, confidence = Confidence.HIGH,
+                            id = ids.uuid(), code = FindingRegistry.NO_INTERNET.code,
+                            category = FindingRegistry.NO_INTERNET.category,
+                            severity = FindingRegistry.NO_INTERNET.severity, confidence = Confidence.HIGH,
                             title = "No working internet on any network",
                             description = "Android's own generate_204 connectivity checks failed on every active network (no HTTP 204) — this device has no validated internet path.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -375,8 +381,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 if (ev.contains("MISMATCH")) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "dns.answer_rewritten", category = Category.DNS,
-                            severity = Severity.HIGH, confidence = Confidence.HIGH,
+                            id = ids.uuid(), code = FindingRegistry.DNS_ANSWER_REWRITTEN.code,
+                            category = FindingRegistry.DNS_ANSWER_REWRITTEN.category,
+                            severity = FindingRegistry.DNS_ANSWER_REWRITTEN.severity, confidence = Confidence.HIGH,
                             title = "DNS answers are being rewritten",
                             description = "A canary reference record returned different RDATA than the spec-defined ground truth — something on the path is rewriting DNS answers (interception, filtering, or a middlebox).",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -385,8 +392,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 } else if (ev.contains("\"reached_authoritative\":false")) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "dns.authoritative_unreachable", category = Category.DNS,
-                            severity = Severity.MEDIUM, confidence = Confidence.MEDIUM,
+                            id = ids.uuid(), code = FindingRegistry.DNS_AUTHORITATIVE_UNREACHABLE.code,
+                            category = FindingRegistry.DNS_AUTHORITATIVE_UNREACHABLE.category,
+                            severity = FindingRegistry.DNS_AUTHORITATIVE_UNREACHABLE.severity, confidence = Confidence.MEDIUM,
                             title = "Canary queries don't reach the authoritative server",
                             description = "A per-run nonce name (which cannot be cached) was not answered by the canary server — the resolver is intercepting or failing to reach it.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -399,8 +407,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 if (ev.contains("address/port-dependent (symmetric NAT")) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "nat.symmetric", category = Category.NAT,
-                            severity = Severity.MEDIUM, confidence = Confidence.HIGH,
+                            id = ids.uuid(), code = FindingRegistry.NAT_SYMMETRIC.code,
+                            category = FindingRegistry.NAT_SYMMETRIC.category,
+                            severity = FindingRegistry.NAT_SYMMETRIC.severity, confidence = Confidence.HIGH,
                             title = "Symmetric NAT — peer-to-peer connections need a relay",
                             description = "The NAT assigns a different external port per destination (address/port-dependent mapping). Direct peer-to-peer connections (calls, games, file transfer) will usually fail and fall back to relays.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -417,8 +426,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 if (ipv6Provisioned(networks)) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "ipv6.broken", category = Category.IPV6,
-                            severity = Severity.MEDIUM, confidence = Confidence.HIGH,
+                            id = ids.uuid(), code = FindingRegistry.V6_BROKEN.code,
+                            category = FindingRegistry.V6_BROKEN.category,
+                            severity = FindingRegistry.V6_BROKEN.severity, confidence = Confidence.HIGH,
                             title = "IPv6 is configured but not working",
                             description = "This network advertises IPv6 (a global address and/or a default route), but ICMPv6 got no reply on any network. Half-configured IPv6 is worse than none: connections try IPv6 first and stall before falling back.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -427,8 +437,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 } else {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "ipv6.not_offered", category = Category.IPV6,
-                            severity = Severity.INFO, confidence = Confidence.HIGH,
+                            id = ids.uuid(), code = FindingRegistry.V6_NOT_OFFERED.code,
+                            category = FindingRegistry.V6_NOT_OFFERED.category,
+                            severity = FindingRegistry.V6_NOT_OFFERED.severity, confidence = Confidence.HIGH,
                             title = "IPv4-only network (no IPv6 offered)",
                             description = "No IPv6 address or default route was provisioned, so IPv6 tests could not run. This is normal — many networks are still IPv4-only and it is not a fault.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),

echolot-app/app/src/main/kotlin/app/echo_lot/app/SettingsScreen.kt

@@ -4,6 +4,7 @@
 package app.echo_lot.app
 
 import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.safeDrawingPadding
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.Spacer
@@ -66,7 +67,7 @@ fun SettingsScreen(
     var serverCred by remember { mutableStateOf(settings.serverCredential) }
 
     Column(
-        Modifier.fillMaxWidth().verticalScroll(rememberScrollState()).padding(16.dp),
+        Modifier.fillMaxWidth().safeDrawingPadding().verticalScroll(rememberScrollState()).padding(16.dp),
         verticalArrangement = Arrangement.spacedBy(12.dp),
     ) {
         Row(verticalAlignment = Alignment.CenterVertically) {

echolot-app/core-archive/src/main/kotlin/app/echo_lot/archive/RunArchive.kt

@@ -31,10 +31,23 @@ data class ArchivedRun(
     val verdict: String? = null,
     @SerialName("finding_count") val findingCount: Int = 0,
     @SerialName("size_bytes") val sizeBytes: Long = 0,
+    /**
+     * How the *archived* document is redacted. Always "full" in practice, because the archive
+     * deliberately keeps the unredacted run - see the package doc. This is not what was uploaded.
+     */
     val anonymization: String = "full",
     /** Whether this run has been accepted by a server, so history can show what is backed up. */
     val uploaded: Boolean = false,
     @SerialName("uploaded_to") val uploadedTo: String? = null,
+    /**
+     * The level the run was *uploaded* at, which is a different document from the archived one.
+     *
+     * Kept separately because conflating the two is actively misleading: the history row showed
+     * the archive's own level ("full") directly beneath "uploaded to fmr", which reads as "the
+     * complete data was uploaded" when a redacted copy had been sent. A privacy display that
+     * overstates what left the device is worse than none.
+     */
+    @SerialName("uploaded_as") val uploadedAs: String? = null,
 )
 
 /**
@@ -119,7 +132,7 @@ class RunArchive(private val dir: File, private val now: () -> Long = System::cu
     fun deleteAll(): Int = list().count { delete(it.id) }
 
     /** Records that a server accepted this run, so history can distinguish backed-up from local. */
-    fun markUploaded(id: String, serverName: String) {
+    fun markUploaded(id: String, serverName: String, uploadedAs: String? = null) {
         val f = File(dir, safe(id) + META_EXT)
         val meta = runCatching { json.decodeFromString(ArchivedRun.serializer(), f.readText()) }.getOrNull()
             ?: return
@@ -127,7 +140,7 @@ class RunArchive(private val dir: File, private val now: () -> Long = System::cu
             f,
             json.encodeToString(
                 ArchivedRun.serializer(),
-                meta.copy(uploaded = true, uploadedTo = serverName),
+                meta.copy(uploaded = true, uploadedTo = serverName, uploadedAs = uploadedAs),
             ),
         )
     }
@@ -181,7 +194,10 @@ class RunArchive(private val dir: File, private val now: () -> Long = System::cu
             id = id,
             savedAtEpochMs = now(),
             startedAt = run["started_at"]?.jsonPrimitive?.content,
-            verdict = doc["summary"]?.jsonObject?.get("verdict")?.jsonPrimitive?.content,
+            // The schema calls it `overall` (Summary.overall); reading `verdict` here silently
+            // yielded null for every run, so the history list's most prominent element - the
+            // coloured verdict - was blank on every row.
+            verdict = doc["summary"]?.jsonObject?.get("overall")?.jsonPrimitive?.content,
             findingCount = (doc["findings"] as? kotlinx.serialization.json.JsonArray)?.size ?: 0,
             sizeBytes = size,
             anonymization = run["privacy"]?.jsonObject?.get("anonymization")?.jsonPrimitive?.content ?: "full",

echolot-app/core-archive/src/test/kotlin/app/echo_lot/archive/RunArchiveTest.kt

@@ -25,7 +25,7 @@ class RunArchiveTest {
     private fun doc(id: String, findings: Int = 1, pad: Int = 0): String {
         val f = (1..findings).joinToString(",") { """{"id":"f$it"}""" }
         return """{"run":{"id":"$id","started_at":"2026-08-01T10:00:00Z","privacy":{"anonymization":"balanced"}},""" +
-            """"findings":[$f],"summary":{"verdict":"warn"},"pad":"${"x".repeat(pad)}"}"""
+            """"findings":[$f],"summary":{"overall":"warn"},"pad":"${"x".repeat(pad)}"}"""
     }
 
     @Test
@@ -120,13 +120,40 @@ class RunArchiveTest {
     fun uploadStateIsRecorded() {
         val a = archive()
         a.save(doc("run-1"))
-        a.markUploaded("run-1", "fmr")
+        a.markUploaded("run-1", "fmr", "balanced")
         val meta = a.list().single()
         assertTrue(meta.uploaded)
         assertEquals("fmr", meta.uploadedTo)
         assertEquals("run-1", meta.id, "marking upload must not disturb the rest of the entry")
     }
 
+    // The archive's own level and the level a run was uploaded at describe *different documents*.
+    // Showing the archive's ("full", because the archive is deliberately unredacted) next to
+    // "uploaded to fmr" reads as "the complete data was uploaded" when a redacted copy was sent —
+    // a privacy display that overstates what left the device is worse than none.
+    @Test
+    fun theUploadedLevelIsRecordedSeparatelyFromTheArchivedOne() {
+        val a = archive()
+        // A real archived document carries no privacy stamp: the anonymizer never runs on the
+        // archive. The shared doc() fixture has one, which is exactly the unrealism that let this
+        // confusion through in the first place.
+        a.save("""{"run":{"id":"run-1"},"findings":[],"summary":{"overall":"green"}}""")
+        a.markUploaded("run-1", "fmr", "balanced")
+        val meta = a.list().single()
+        assertEquals("full", meta.anonymization, "the archived copy is unredacted, by design")
+        assertEquals("balanced", meta.uploadedAs, "the uploaded copy was redacted, and must say so")
+    }
+
+    // The verdict is read from `summary.overall` — the schema's actual field name. Reading
+    // `summary.verdict` silently yielded null for every run, so the history list's most prominent
+    // element was blank on every row while everything else looked fine.
+    @Test
+    fun theVerdictComesFromTheSchemasOverallField() {
+        val a = archive()
+        a.save("""{"run":{"id":"r1"},"findings":[],"summary":{"overall":"yellow"}}""")
+        assertEquals("yellow", a.list().single().verdict)
+    }
+
     @Test
     fun deleteRemovesBothFiles() {
         val a = archive()

echolot-app/core-engine/src/main/kotlin/app/echo_lot/engine/Directional.kt

@@ -0,0 +1,107 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.engine
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * Splits a round-trip train into its two directions using what the server witnessed.
+ *
+ * A round trip can only report that *something* was lost somewhere. That is the least useful form
+ * of the answer: "3 % loss" sends an engineer looking in both directions at once. The server
+ * records every packet it received, per sequence number (probe-protocol.md §6), so the two cases
+ * are actually distinguishable:
+ *
+ *   - sent, never seen by the server  → **upstream** loss
+ *   - seen by the server, reply never arrived → **downstream** loss
+ *
+ * The same records give one-way delay *variation* per direction. Absolute one-way delay would
+ * need synchronised clocks and we deliberately have none (measurement-schema.md's two-clock rule),
+ * but the variation does not: (server_rx − client_tx) contains an unknown constant clock offset,
+ * and differencing successive samples cancels it. So jitter is honestly attributable to a
+ * direction even though latency is not.
+ */
+object Directional {
+
+    /** One probe as the client saw it. [tRxNs] null means no reply came back. */
+    data class Sample(val seq: Int, val tTxNs: Long, val tRxNs: Long?)
+
+    /** One probe as the server saw it: its own receive and transmit stamps, on its own clock. */
+    data class ServerSighting(val seq: Int, val tRxNs: Long, val tTxNs: Long)
+
+    fun analyse(sent: List<Sample>, seen: List<ServerSighting>): DirectionalMetrics {
+        val byServerSeq = seen.associateBy { it.seq }
+        // Only sequences we actually sent count. A server record for a sequence we have no note
+        // of is not evidence about this train — it is a bug or a stray, and silently folding it
+        // in would produce loss percentages above 100 or below zero.
+        val relevant = sent.filter { byServerSeq.containsKey(it.seq) }
+
+        val nSent = sent.size
+        val nSeen = relevant.size
+        val nReplied = sent.count { it.tRxNs != null }
+
+        // A reply can only exist if the request arrived, so downstream loss is measured against
+        // what the server saw, not against what we sent — otherwise upstream loss is counted twice.
+        val lostUp = nSent - nSeen
+        val lostDown = (nSeen - nReplied).coerceAtLeast(0)
+
+        val upDeltas = relevant.sortedBy { it.seq }
+            .map { byServerSeq.getValue(it.seq).tRxNs - it.tTxNs }
+        val downDeltas = sent.filter { it.tRxNs != null && byServerSeq.containsKey(it.seq) }
+            .sortedBy { it.seq }
+            .map { it.tRxNs!! - byServerSeq.getValue(it.seq).tTxNs }
+
+        return DirectionalMetrics(
+            sent = nSent,
+            seenByServer = nSeen,
+            repliesReceived = nReplied,
+            lostUpstream = lostUp,
+            lostDownstream = lostDown,
+            lossUpstreamPct = pct(lostUp, nSent),
+            // Denominator is what reached the server: of the packets that got there, how many
+            // replies came back.
+            lossDownstreamPct = pct(lostDown, nSeen),
+            jitterUpstreamMs = jitterMs(upDeltas),
+            jitterDownstreamMs = jitterMs(downDeltas),
+            /** True when the server saw nothing at all, which is a different fault from loss. */
+            noneReachedServer = nSent > 0 && nSeen == 0,
+        )
+    }
+
+    /**
+     * Mean absolute difference between consecutive one-way samples (RFC 3393 IPDV, averaged).
+     *
+     * Differencing is what makes this legitimate without synchronised clocks: each sample carries
+     * the same unknown offset between the two clocks, and the difference cancels it. Fewer than
+     * two samples yields null rather than zero — "no jitter" and "not enough data to say" are
+     * different claims and only one of them is true here.
+     */
+    private fun jitterMs(oneWayNs: List<Long>): Double? {
+        if (oneWayNs.size < 2) return null
+        val deltas = oneWayNs.zipWithNext { a, b -> kotlin.math.abs(b - a) }
+        return round2(deltas.average() / 1_000_000.0)
+    }
+
+    private fun pct(part: Int, whole: Int): Double =
+        if (whole <= 0) 0.0 else round2(part * 100.0 / whole)
+
+    private fun round2(v: Double) = Math.round(v * 100.0) / 100.0
+}
+
+/** Directional metrics for train.udp_updown; recomputable from the columnar evidence. */
+@Serializable
+data class DirectionalMetrics(
+    val sent: Int,
+    @SerialName("seen_by_server") val seenByServer: Int,
+    @SerialName("replies_received") val repliesReceived: Int,
+    @SerialName("lost_upstream") val lostUpstream: Int,
+    @SerialName("lost_downstream") val lostDownstream: Int,
+    @SerialName("loss_upstream_pct") val lossUpstreamPct: Double,
+    @SerialName("loss_downstream_pct") val lossDownstreamPct: Double,
+    /** One-way delay variation (RFC 3393), per direction. Null when there were too few samples. */
+    @SerialName("jitter_upstream_ms") val jitterUpstreamMs: Double? = null,
+    @SerialName("jitter_downstream_ms") val jitterDownstreamMs: Double? = null,
+    @SerialName("none_reached_server") val noneReachedServer: Boolean = false,
+)

echolot-app/core-engine/src/main/kotlin/app/echo_lot/engine/DownstreamMeasurement.kt

@@ -37,6 +37,120 @@ class DownstreamMeasurement(private val ids: IdSource) {
     /** How long to wait for a granted burst after the server accepts the action. */
     private val collectWindowMs = 4_000L
 
+    /**
+     * Shorter, but long enough to cover the first_last mode's deliberate 250 ms hold plus a
+     * reassembly. A fragment burst is one datagram: it is here quickly or not at all.
+     */
+    private val fragWindowMs = 1_500L
+
+    /**
+     * Asks the server to send one deliberately-fragmented datagram per ordering, and reports
+     * which orderings survive the path.
+     *
+     * Kernel fragmentation always emits fragments in order, first one first, so an oversized
+     * datagram can only answer "do fragments get through at all". The interesting fault is about
+     * ordering: only the *first* fragment carries the UDP ports, so a stateful firewall that has
+     * not seen it has nothing to match the rest against, and many drop them. That failure is
+     * invisible to every in-order test and shows up in the field as "large DNS answers fail here"
+     * or "the tunnel breaks when the MTU drops".
+     */
+    fun fragmentOrdering(
+        credential: String,
+        sessionId: String,
+        control: ControlClient,
+        probe: ProbeSession,
+        sessionRef: String,
+        sizeBytes: Int = 2000,
+        fragBytes: Int = 576,
+    ): Pair<Test, List<Finding>> {
+        val testId = ids.uuid()
+        val started = ids.monoNs()
+        val delivered = LinkedHashMap<String, Boolean>()
+        val fragmentCounts = LinkedHashMap<String, Int>()
+        var unsupported = false
+
+        for (mode in FRAG_MODES) {
+            val reply = runCatching {
+                control.action(
+                    credential, sessionId,
+                    """{"action":"frag_send","size_bytes":$sizeBytes,"mode":"$mode","frag_bytes":$fragBytes}""",
+                )
+            }
+            if (reply.isFailure) {
+                // A server without a raw socket says so; that is a missing capability, not a
+                // property of the network, and must not be recorded as a failed delivery.
+                unsupported = true
+                break
+            }
+            parseInt(reply.getOrNull(), "fragments")?.let { fragmentCounts[mode] = it }
+            // The burst is already on the wire when the action returns (it is sent
+            // synchronously), so anything that survived is either here or lost.
+            val got = probe.collectGranted(fragWindowMs).any { it.type == Wire.TYPE_FRAG_DATA }
+            delivered[mode] = got
+        }
+
+        if (unsupported) {
+            return Test(
+                id = testId, type = TestType.MTU_FRAG_ORDERING, sessionRef = sessionRef, tier = Tier.APP,
+                startedMonoNs = started, endedMonoNs = ids.monoNs(),
+                status = TestStatus.UNSUPPORTED,
+                error = TestError("no_raw_socket", "this server cannot craft fragments"),
+            ) to emptyList()
+        }
+
+        val metrics = json.encodeToJsonElement(
+            FragOrderingMetrics(
+                sizeBytes = sizeBytes,
+                fragBytes = fragBytes,
+                fragmentsPerBurst = fragmentCounts,
+                deliveredByMode = delivered,
+                inOrderDelivered = delivered[FRAG_IN_ORDER] == true,
+                reorderedDelivered = delivered[FRAG_REVERSED] == true,
+                delayedFirstDelivered = delivered[FRAG_FIRST_LAST] == true,
+            ),
+        ) as JsonObject
+
+        val findings = ArrayList<Finding>()
+        val inOrder = delivered[FRAG_IN_ORDER] == true
+        val reversed = delivered[FRAG_REVERSED] == true
+        val firstLast = delivered[FRAG_FIRST_LAST] == true
+
+        if (!inOrder) {
+            findings.add(
+                finding(
+                    FindingRegistry.FRAGMENTS_BLOCKED, testId,
+                    "IP fragments do not reach this device",
+                    "A fragmented datagram sent in the normal order never arrived. Anything that " +
+                        "relies on fragmentation — large DNS answers over UDP, some VPN traffic — " +
+                        "will fail here rather than slow down.",
+                ),
+            )
+        } else if (!reversed || !firstLast) {
+            // The precise and useful finding: fragments work, but only if they arrive tidily.
+            val which = buildList {
+                if (!reversed) add("out of order")
+                if (!firstLast) add("with the first fragment delayed")
+            }.joinToString(" or ")
+            findings.add(
+                finding(
+                    FindingRegistry.FRAGMENT_REORDER_SENSITIVE, testId,
+                    "Fragments are dropped when they arrive $which",
+                    "In-order fragments are delivered, but the same datagram sent $which is not. " +
+                        "Something on the path only reassembles when the first fragment (the one " +
+                        "carrying the UDP ports) arrives first — typical of a stateful firewall " +
+                        "or NAT. It works until the network reorders, then fails intermittently, " +
+                        "which is the hardest kind of fault to chase.",
+                ),
+            )
+        }
+        return Test(
+            id = testId, type = TestType.MTU_FRAG_ORDERING, sessionRef = sessionRef, tier = Tier.APP,
+            startedMonoNs = started, endedMonoNs = ids.monoNs(),
+            status = if (inOrder) TestStatus.OK else TestStatus.PARTIAL,
+            metrics = metrics,
+        ) to findings
+    }
+
     /**
      * Runs all three against an already-primed session.
      *
@@ -66,6 +180,16 @@ class DownstreamMeasurement(private val ids: IdSource) {
 
         tests.add(df.test); tests.add(frag.test); tests.add(train.test)
 
+        // Fragment ordering only makes sense once we know fragments arrive at all; when they do
+        // not, the ordering variants would all report "not delivered" and read as three faults
+        // instead of one.
+        if (frag.largestDelivered != null) {
+            val (fragTest, fragFindings) =
+                fragmentOrdering(credential, sessionId, control, probe, sessionRef)
+            tests.add(fragTest)
+            findings.addAll(fragFindings)
+        }
+
         // A downstream MTU below the classic 1500-byte Ethernet payload is worth saying out loud:
         // it is the usual cause of "small requests work, large responses hang".
         val pathMtu = df.largestDelivered
@@ -74,7 +198,7 @@ class DownstreamMeasurement(private val ids: IdSource) {
             if (ipMtu < 1500) {
                 findings.add(
                     finding(
-                        "mtu.reduced_downstream", Category.MTU, Severity.LOW, df.test.id,
+                        FindingRegistry.MTU_REDUCED_DOWNSTREAM, df.test.id,
                         "Downstream path MTU is $ipMtu bytes, below 1500",
                         "The largest datagram that reached this device without fragmenting was " +
                             "$pathMtu bytes of payload ($ipMtu on the wire). Tunnels (PPPoE, VPN, " +
@@ -89,7 +213,7 @@ class DownstreamMeasurement(private val ids: IdSource) {
             if (fragLargest <= pathMtu && sizes.any { it > pathMtu }) {
                 findings.add(
                     finding(
-                        "mtu.downstream_blackhole", Category.MTU, Severity.MEDIUM, frag.test.id,
+                        FindingRegistry.MTU_DOWNSTREAM_BLACKHOLE, frag.test.id,
                         "Datagrams above $pathMtu bytes are dropped downstream, fragmented or not",
                         "Nothing larger than $pathMtu bytes arrived, even when the network was " +
                             "free to fragment it. Traffic that relies on large responses will " +
@@ -102,7 +226,7 @@ class DownstreamMeasurement(private val ids: IdSource) {
         if (train.received == 0) {
             findings.add(
                 finding(
-                    "connectivity.downstream_blocked", Category.CONNECTIVITY, Severity.HIGH, train.test.id,
+                    FindingRegistry.DOWNSTREAM_BLOCKED, train.test.id,
                     "No server-initiated packets arrived",
                     "The server sent ${train.sent} packets toward this device and none arrived, " +
                         "while the round-trip echo worked. Something on the path forwards replies " +
@@ -112,7 +236,7 @@ class DownstreamMeasurement(private val ids: IdSource) {
         } else if (train.lossPct >= 5.0) {
             findings.add(
                 finding(
-                    "connectivity.downstream_loss", Category.CONNECTIVITY, Severity.MEDIUM, train.test.id,
+                    FindingRegistry.LOSS_DOWNSTREAM, train.test.id,
                     "Downstream loss of ${round1(train.lossPct)}%",
                     "${train.sent - train.received} of ${train.sent} packets sent toward this " +
                         "device were lost. Downstream loss is invisible to a round-trip test, " +
@@ -123,7 +247,7 @@ class DownstreamMeasurement(private val ids: IdSource) {
         if (train.reordered > 0) {
             findings.add(
                 finding(
-                    "connectivity.downstream_reorder", Category.CONNECTIVITY, Severity.LOW, train.test.id,
+                    FindingRegistry.DOWNSTREAM_REORDER, train.test.id,
                     "${train.reordered} downstream packet(s) arrived out of order",
                     "Packets arrived in a different order than they were sent. Usually per-packet " +
                         "load balancing across links; harmless for most traffic, not for all of it.",
@@ -290,9 +414,17 @@ class DownstreamMeasurement(private val ids: IdSource) {
 
     // ---- helpers ----------------------------------------------------------------------
 
-    private fun finding(code: String, cat: Category, sev: Severity, testId: String, title: String, desc: String) =
+    /**
+     * Builds a finding from a registry entry, which supplies the code, category and severity.
+     *
+     * Taking a [FindingSpec] rather than three loose values is the point: a typo becomes a
+     * compile error, and two call sites cannot disagree about which category a finding belongs
+     * to - a disagreement that would split one fault across two verdict lights.
+     */
+    private fun finding(spec: FindingSpec, testId: String, title: String, desc: String) =
         Finding(
-            id = ids.uuid(), code = code, category = cat, severity = sev, confidence = Confidence.HIGH,
+            id = ids.uuid(), code = spec.code, category = spec.category, severity = spec.severity,
+            confidence = Confidence.HIGH,
             title = title, description = desc, evidenceRefs = listOf(EvidenceRef(testId)),
         )
 
@@ -310,6 +442,11 @@ class DownstreamMeasurement(private val ids: IdSource) {
         /** IPv4 (20) + UDP (8). The v6 case is 48; reported per-family once v6 sessions land. */
         const val IP_UDP_OVERHEAD4 = 28
 
+        const val FRAG_IN_ORDER = "in_order"
+        const val FRAG_REVERSED = "reversed"
+        const val FRAG_FIRST_LAST = "first_last"
+        val FRAG_MODES = listOf(FRAG_IN_ORDER, FRAG_REVERSED, FRAG_FIRST_LAST)
+
         /** Straddles the usual suspects: 1500 Ethernet, 1492 PPPoE, 1400-ish tunnels. */
         val DEFAULT_SIZES = listOf(600, 1200, 1372, 1400, 1450, 1472, 1500, 2000, 4000)
 
@@ -330,6 +467,18 @@ data class BigSendMetrics(
     @SerialName("path_mtu_bytes") val pathMtuBytes: Int? = null,
 )
 
+/** Metrics for mtu.frag_ordering. */
+@Serializable
+data class FragOrderingMetrics(
+    @SerialName("size_bytes") val sizeBytes: Int,
+    @SerialName("frag_bytes") val fragBytes: Int,
+    @SerialName("fragments_per_burst") val fragmentsPerBurst: Map<String, Int>,
+    @SerialName("delivered_by_mode") val deliveredByMode: Map<String, Boolean>,
+    @SerialName("in_order_delivered") val inOrderDelivered: Boolean,
+    @SerialName("reordered_delivered") val reorderedDelivered: Boolean,
+    @SerialName("delayed_first_delivered") val delayedFirstDelivered: Boolean,
+)
+
 /** Metrics for train.udp_downstream. */
 @Serializable
 data class DownTrainMetrics(

echolot-app/core-engine/src/main/kotlin/app/echo_lot/engine/ServerMeasurement.kt

@@ -10,8 +10,13 @@ import app.echo_lot.measurement.*
 import app.echo_lot.protocol.ControlClient
 import app.echo_lot.protocol.ProbeSession
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonArray
 import kotlinx.serialization.json.JsonObject
 import kotlinx.serialization.json.encodeToJsonElement
+import kotlinx.serialization.json.intOrNull
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import kotlinx.serialization.json.longOrNull
 
 /**
  * Runs the server-facing measurements against one target and assembles a [MeasurementDocument]:
@@ -42,6 +47,14 @@ class ServerMeasurement(
          * it is a flag rather than an assumption.
          */
         val downstream: Boolean = true,
+        /**
+         * Throughput moves real data — a 5-second run at 50 Mbps is about 30 MB — so it is off
+         * unless asked for. On a metered mobile connection that is the user's money, and a
+         * measurement tool that spends it without being told to is not one people keep installed.
+         */
+        val throughput: Boolean = false,
+        @Suppress("unused") val throughputSeconds: Int = 5,
+        @Suppress("unused") val throughputKbps: Int = 50_000,
     )
 
     fun run(cfg: Config): MeasurementDocument {
@@ -71,7 +84,7 @@ class ServerMeasurement(
         // re-primed source is never recorded and every granted send goes to the old, closed port.
         // Session identity lives on the server; the socket must live as long as it does.
         ProbeSession(cfg.credential, session, cfg.udpHost, cfg.udpPort).use { ps ->
-            val (test, findings) = echoTrain(cfg, ps, startMono)
+            val (test, findings) = echoTrain(cfg, ps, startMono, control, session.sessionId)
             tests.add(test)
             allFindings.addAll(findings)
 
@@ -84,6 +97,15 @@ class ServerMeasurement(
                 tests.addAll(dsTests)
                 allFindings.addAll(dsFindings)
             }
+
+            if (cfg.throughput && profile.supports("throughput")) {
+                val (tpTest, tpFindings) = ThroughputMeasurement(ids).run(
+                    cfg.credential, session.sessionId, control, ps, sessionRef = "sess-1",
+                    durationS = cfg.throughputSeconds, kbps = cfg.throughputKbps,
+                )
+                tests.add(tpTest)
+                allFindings.addAll(tpFindings)
+            }
         }
 
         control.deleteSession(cfg.credential, session.sessionId)
@@ -105,6 +127,7 @@ class ServerMeasurement(
 
     private fun echoTrain(
         cfg: Config, ps: ProbeSession, startMono: Long,
+        control: ControlClient? = null, sessionId: String? = null,
     ): Pair<Test, List<Finding>> {
         val testId = ids.uuid()
         val seqs = ArrayList<Int>()
@@ -114,9 +137,15 @@ class ServerMeasurement(
         val rtts = ArrayList<Double>()
         val observedPorts = LinkedHashSet<Int>()
 
+        // Wire sequence numbers, kept so the server's observations can be correlated packet by
+        // packet. They are not 0..n-1: the counter is shared with every other packet type on the
+        // session, so "the nth echo" is not "sequence n".
+        val wireSeqs = ArrayList<Int>()
+
         for (i in 0 until cfg.echoCount) {
             val txMono = ids.monoNs() - startMono
             val r = ps.echo(cfg.echoPaddingBytes)
+            wireSeqs.add(ps.lastSeq)
             seqs.add(i)
             tTx.add(txMono)
             sizes.add(Wire_HEADER + cfg.echoPaddingBytes)
@@ -129,6 +158,20 @@ class ServerMeasurement(
             }
         }
 
+        // Ask the server what it actually received. This is what turns "3 % loss somewhere" into
+        // "3 % loss upstream" - the least useful form of the answer into a usable one.
+        val directional: DirectionalMetrics? =
+            if (control != null && sessionId != null) {
+                runCatching {
+                    val samples = wireSeqs.indices.map {
+                        Directional.Sample(wireSeqs[it], tTx[it] ?: 0L, tRx[it])
+                    }
+                    Directional.analyse(samples, serverSightings(control, cfg, sessionId))
+                }.getOrNull() // an older server without the endpoint simply yields no split
+            } else {
+                null
+            }
+
         val sent = cfg.echoCount
         val received = rtts.size
         val lossPct = if (sent == 0) 0.0 else (sent - received) * 100.0 / sent
@@ -138,6 +181,9 @@ class ServerMeasurement(
             epochMonoNs = startMono, seq = seqs, tTxNs = tTx, tRxNs = tRx, sizeBytes = sizes,
         ).toEvidence()
 
+        val directionalJson = directional?.let {
+            json.encodeToJsonElement(DirectionalMetrics.serializer(), it) as JsonObject
+        }
         val metrics: JsonObject = json.encodeToJsonElement(
             EchoMetrics(
                 sent = sent, received = received, lossPct = round1(lossPct),
@@ -147,7 +193,7 @@ class ServerMeasurement(
                 observedPorts = observedPorts.toList(),
                 natRebindingDetected = natRebinding,
             )
-        ) as JsonObject
+        ).let { base -> JsonObject((base as JsonObject) + (directionalJson ?: JsonObject(emptyMap()))) }
 
         val status = when {
             received == 0 -> TestStatus.FAILED
@@ -162,30 +208,91 @@ class ServerMeasurement(
 
         val findings = ArrayList<Finding>()
         if (received == 0) {
-            findings.add(finding("nat.udp_unreachable", Category.CONNECTIVITY, Severity.HIGH, testId,
+            findings.add(finding(FindingRegistry.UDP_UNREACHABLE, testId,
                 "No UDP echo replies from the server",
                 "Every ECHO probe to the server's UDP data plane was lost — the path blocks or drops the session's UDP traffic."))
         } else if (lossPct >= 20.0) {
-            findings.add(finding("connectivity.udp_loss", Category.CONNECTIVITY, Severity.MEDIUM, testId,
+            findings.add(finding(FindingRegistry.UDP_LOSS, testId,
                 "High UDP loss to the server (${round1(lossPct)}%)",
                 "A large fraction of ECHO probes were lost, indicating an unreliable UDP path."))
         }
+        // Naming the direction is the entire value of the split, so the findings do.
+        directional?.let { d ->
+            when {
+                d.noneReachedServer && received == 0 -> findings.add(
+                    finding(FindingRegistry.UDP_UNREACHABLE_UPSTREAM, testId,
+                        "Nothing reached the server",
+                        "The server received none of the ${d.sent} probes, so the traffic is being " +
+                            "dropped on the way out, not on the way back. A firewall or NAT on " +
+                            "this side of the path is the place to look."),
+                )
+                d.lossUpstreamPct >= 2.0 -> findings.add(
+                    finding(FindingRegistry.LOSS_UPSTREAM, testId,
+                        "${d.lossUpstreamPct} % of probes were lost on the way to the server",
+                        "${d.lostUpstream} of ${d.sent} probes never reached the server. The " +
+                            "return path is not implicated: replies came back for everything that " +
+                            "arrived."),
+                )
+            }
+            if (d.lossDownstreamPct >= 2.0) {
+                findings.add(
+                    finding(FindingRegistry.LOSS_DOWNSTREAM, testId,
+                        "${d.lossDownstreamPct} % of replies were lost on the way back",
+                        "The server received ${d.seenByServer} probes and answered them, but " +
+                            "${d.lostDownstream} of those replies never arrived. The outbound path " +
+                            "is fine; the fault is on the return leg."),
+                )
+            }
+        }
+
         if (natRebinding) {
-            findings.add(finding("nat.udp_rebinding", Category.NAT, Severity.MEDIUM, testId,
+            findings.add(finding(FindingRegistry.NAT_UDP_REBINDING, testId,
                 "NAT remapped the UDP source port mid-flow",
                 "The server observed more than one source port for this session (${observedPorts.joinToString()}), i.e. a NAT with a short UDP mapping or per-packet remapping."))
         }
         return test to findings
     }
 
-    private fun finding(code: String, cat: Category, sev: Severity, testId: String, title: String, desc: String) =
+    /**
+     * The server's per-packet record of this session's echoes (spec section 6). Filtered to
+     * ECHO_REQ, because the observation list also holds MTU probes and anything else we sent -
+     * counting those as train packets would invent loss that is not there.
+     */
+    private fun serverSightings(
+        control: ControlClient, cfg: Config, sessionId: String,
+    ): List<Directional.ServerSighting> {
+        val body = control.observations(cfg.credential, sessionId)
+        val packets = Json.parseToJsonElement(body).jsonObject["udp"]
+            ?.jsonObject?.get("packets") as? JsonArray ?: return emptyList()
+        return packets.mapNotNull { el ->
+            val o = el as? JsonObject ?: return@mapNotNull null
+            val type = o["type"]?.jsonPrimitive?.intOrNull ?: return@mapNotNull null
+            if (type != ECHO_REQ_TYPE) return@mapNotNull null
+            Directional.ServerSighting(
+                seq = o["seq"]?.jsonPrimitive?.intOrNull ?: return@mapNotNull null,
+                tRxNs = o["t_rx_ns"]?.jsonPrimitive?.longOrNull ?: return@mapNotNull null,
+                tTxNs = o["t_tx_ns"]?.jsonPrimitive?.longOrNull ?: return@mapNotNull null,
+            )
+        }
+    }
+
+    /**
+     * Builds a finding from a registry entry, which supplies the code, category and severity.
+     *
+     * Taking a [FindingSpec] rather than three loose values is the point: a typo becomes a
+     * compile error, and two call sites cannot disagree about which category a finding belongs
+     * to - a disagreement that would split one fault across two verdict lights.
+     */
+    private fun finding(spec: FindingSpec, testId: String, title: String, desc: String) =
         Finding(
-            id = ids.uuid(), code = code, category = cat, severity = sev, confidence = Confidence.HIGH,
+            id = ids.uuid(), code = spec.code, category = spec.category, severity = spec.severity,
+            confidence = Confidence.HIGH,
             title = title, description = desc, evidenceRefs = listOf(EvidenceRef(testId)),
         )
 
     private companion object {
         const val Wire_HEADER = 32
+        const val ECHO_REQ_TYPE = 0x01
         fun round1(v: Double) = Math.round(v * 10.0) / 10.0
     }
 }

echolot-app/core-engine/src/main/kotlin/app/echo_lot/engine/ThroughputMeasurement.kt

@@ -0,0 +1,338 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.engine
+
+import app.echo_lot.measurement.*
+import app.echo_lot.protocol.ControlClient
+import app.echo_lot.protocol.ProbeSession
+import app.echo_lot.protocol.Wire
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.encodeToJsonElement
+import kotlinx.serialization.json.jsonArray
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+
+/**
+ * Downstream throughput: the server sends at a paced rate for a bounded time and the client
+ * measures what arrives (`perf.throughput_udp`).
+ *
+ * The number this produces is only meaningful with a qualifier attached, and getting that
+ * qualifier right is most of the work here. A throughput test reports the *smallest* limit on the
+ * path, and the sender's own ceiling is one of the candidates: if the server was asked for 50 Mbps
+ * and 50 Mbps arrived, the network was never the constraint and "50 Mbps" says nothing about it.
+ * Reporting that as a capacity measurement would be a confident lie, so the result always carries
+ * [ThroughputMetrics.limitedBy] and a finding is only raised when the network is actually
+ * implicated.
+ *
+ * Comparing against the *sender's* count rather than the requested rate is the other half: the
+ * server reports how much it actually put on the wire, and the gap between that and what arrived
+ * is the loss. A receiver alone cannot tell "the network dropped it" from "the sender never sent
+ * it", and guessing turns a healthy server-side limit into a phantom network fault.
+ */
+class ThroughputMeasurement(private val ids: IdSource) {
+
+    private val json = Json { encodeDefaults = true; explicitNulls = true }
+
+    fun run(
+        credential: String,
+        sessionId: String,
+        control: ControlClient,
+        probe: ProbeSession,
+        sessionRef: String,
+        durationS: Int = 5,
+        kbps: Int = 50_000,
+        sizeBytes: Int = 1200,
+    ): Pair<Test, List<Finding>> {
+        val testId = ids.uuid()
+        val started = ids.monoNs()
+
+        val reply = runCatching {
+            control.action(
+                credential, sessionId,
+                """{"action":"throughput","direction":"down","duration_s":$durationS,""" +
+                    """"kbps":$kbps,"size_bytes":$sizeBytes}""",
+            )
+        }
+        if (reply.isFailure) {
+            return Test(
+                id = testId, type = TestType.PERF_THROUGHPUT_UDP, sessionRef = sessionRef, tier = Tier.APP,
+                startedMonoNs = started, endedMonoNs = ids.monoNs(),
+                status = TestStatus.UNSUPPORTED,
+                error = TestError("action_refused", reply.exceptionOrNull()?.message ?: "throughput refused"),
+            ) to emptyList()
+        }
+
+        // The server may have shortened the run to fit its own byte budget; listen for what it
+        // actually promised, not for what we asked.
+        val plannedMs = parseInt(reply.getOrNull(), "duration_ms") ?: (durationS * 1000)
+
+        // A margin past the planned end so the tail of the run is not counted as loss: packets
+        // still in flight when we stop listening were not dropped, they were merely late.
+        val received = probe.collectGranted(plannedMs + 1_500L)
+            .filter { it.type == Wire.TYPE_THROUGHPUT_DATA }
+
+        val bytes = received.sumOf { it.sizeBytes.toLong() }
+        val spanNs = if (received.size >= 2) {
+            received.maxOf { it.tRxNs } - received.minOf { it.tRxNs }
+        } else {
+            0L
+        }
+        // Measured over the arrival span rather than our listening window, which includes the
+        // request round trip and the trailing margin and would understate the rate.
+        val receivedKbps = if (spanNs > 0) (bytes * 8 * 1_000_000 / spanNs).toInt() else 0
+
+        val sender = senderReport(control, credential, sessionId)
+        val sentPackets = sender?.packets ?: 0
+        val lossPct = if (sentPackets > 0) {
+            round2((sentPackets - received.size).coerceAtLeast(0) * 100.0 / sentPackets)
+        } else {
+            null
+        }
+
+        // Only a run the *clock* ended measured the network. One stopped by our own byte budget
+        // or rate ceiling measured this server.
+        val limitedBy = sender?.limitedBy ?: "unknown"
+        val networkLimited = limitedBy == "duration" &&
+            sender != null && receivedKbps > 0 && receivedKbps < sender.kbps * 9 / 10
+
+        val metrics = json.encodeToJsonElement(
+            ThroughputMetrics(
+                requestedKbps = kbps,
+                plannedDurationMs = plannedMs,
+                packetsReceived = received.size,
+                bytesReceived = bytes,
+                receivedKbps = receivedKbps,
+                senderPackets = sender?.packets,
+                senderBytes = sender?.bytes,
+                senderKbps = sender?.kbps,
+                lossPct = lossPct,
+                limitedBy = limitedBy,
+                measuresNetwork = networkLimited,
+            ),
+        ) as JsonObject
+
+        val findings = ArrayList<Finding>()
+        when {
+            sender == null -> Unit // no sender report: nothing can be concluded, so nothing is
+            received.isEmpty() -> findings.add(
+                finding(
+                    FindingRegistry.THROUGHPUT_NO_DELIVERY, testId,
+                    "No throughput traffic arrived",
+                    "The server sent ${sender.packets} packets and none arrived. This is a " +
+                        "connectivity fault rather than a slow link.",
+                ),
+            )
+            networkLimited -> findings.add(
+                finding(
+                    FindingRegistry.THROUGHPUT_BELOW_OFFERED, testId,
+                    "Downstream throughput ${receivedKbps / 1000} Mbit/s, below the " +
+                        "${sender.kbps / 1000} Mbit/s offered",
+                    "The server sent at ${sender.kbps / 1000} Mbit/s for the full run and " +
+                        "${receivedKbps / 1000} Mbit/s arrived" +
+                        (lossPct?.let { ", losing $it % of packets" } ?: "") +
+                        ". The path could not carry what was offered.",
+                ),
+            )
+        }
+
+        return Test(
+            id = testId, type = TestType.PERF_THROUGHPUT_UDP, sessionRef = sessionRef, tier = Tier.APP,
+            startedMonoNs = started, endedMonoNs = ids.monoNs(),
+            status = if (received.isEmpty()) TestStatus.FAILED else TestStatus.OK,
+            metrics = metrics,
+        ) to findings
+    }
+
+    /**
+     * Upstream throughput: the client sends, the server counts.
+     *
+     * The mirror image of the downstream case, and it needs no grant — the client is generating
+     * its own traffic, so there is no amplification to gate. What it does need is the server's
+     * count: only the far end knows how much arrived, and without that number a sender can
+     * measure how fast it can *transmit*, which is not the same question and is usually just the
+     * speed of the local NIC.
+     */
+    fun runUpstream(
+        credential: String,
+        sessionId: String,
+        control: ControlClient,
+        probe: ProbeSession,
+        sessionRef: String,
+        durationS: Int = 5,
+        kbps: Int = 20_000,
+        sizeBytes: Int = 1200,
+    ): Pair<Test, List<Finding>> {
+        val testId = ids.uuid()
+        val started = ids.monoNs()
+
+        // Zeroes the server's counter so this run measures itself rather than inheriting the
+        // packets of an earlier one on the same session.
+        val reply = runCatching {
+            control.action(credential, sessionId, """{"action":"throughput","direction":"up"}""")
+        }
+        if (reply.isFailure) {
+            return Test(
+                id = testId, type = TestType.PERF_THROUGHPUT_UDP, sessionRef = sessionRef, tier = Tier.APP,
+                startedMonoNs = started, endedMonoNs = ids.monoNs(),
+                status = TestStatus.UNSUPPORTED,
+                error = TestError("action_refused", reply.exceptionOrNull()?.message ?: "refused"),
+            ) to emptyList()
+        }
+
+        val sent = probe.sendThroughput(durationS * 1000L, kbps, sizeBytes)
+        // A moment for the tail of the run to arrive; counting still-in-flight packets as lost
+        // would inflate the loss figure by whatever the path's delay happens to be.
+        Thread.sleep(500)
+        val seen = upstreamCount(control, credential, sessionId)
+
+        val lossPct = if (sent.packets > 0 && seen != null) {
+            round2((sent.packets - seen.packets).coerceAtLeast(0) * 100.0 / sent.packets)
+        } else {
+            null
+        }
+        // The receiver's rate is the measurement. The sender's is what we managed to emit, which
+        // is a property of this phone and its radio, not of the network.
+        val achievedKbps = seen?.kbps ?: 0
+
+        val metrics = json.encodeToJsonElement(
+            UpstreamThroughputMetrics(
+                requestedKbps = kbps,
+                sentPackets = sent.packets,
+                sentBytes = sent.bytes,
+                sentKbps = sent.kbps,
+                receivedPackets = seen?.packets,
+                receivedBytes = seen?.bytes,
+                receivedKbps = achievedKbps,
+                lossPct = lossPct,
+                // Same honesty rule as downstream: if what arrived matches what we offered, the
+                // path was never the constraint and this number says nothing about it.
+                measuresNetwork = seen != null && achievedKbps > 0 && achievedKbps < sent.kbps * 9 / 10,
+            ),
+        ) as JsonObject
+
+        val findings = ArrayList<Finding>()
+        if (seen != null && seen.packets == 0 && sent.packets > 0) {
+            findings.add(
+                finding(
+                    FindingRegistry.THROUGHPUT_NO_DELIVERY, testId,
+                    "No upstream traffic reached the server",
+                    "This device sent ${sent.packets} packets and the server received none. " +
+                        "That is a connectivity fault on the outbound path rather than a slow link.",
+                ),
+            )
+        } else if (lossPct != null && lossPct >= 2.0) {
+            findings.add(
+                finding(
+                    FindingRegistry.THROUGHPUT_BELOW_OFFERED, testId,
+                    "Upstream loss of $lossPct % at ${sent.kbps / 1000} Mbit/s",
+                    "The server received ${seen?.packets} of the ${sent.packets} packets this " +
+                        "device sent. The outbound path could not carry what was offered.",
+                ),
+            )
+        }
+
+        return Test(
+            id = testId, type = TestType.PERF_THROUGHPUT_UDP, sessionRef = sessionRef, tier = Tier.APP,
+            startedMonoNs = started, endedMonoNs = ids.monoNs(),
+            status = if (seen == null || seen.packets == 0) TestStatus.FAILED else TestStatus.OK,
+            metrics = metrics,
+        ) to findings
+    }
+
+    private data class UpstreamCount(val packets: Int, val bytes: Long, val kbps: Int)
+
+    /** The server's tally for this session's upstream run. */
+    private fun upstreamCount(
+        control: ControlClient, credential: String, sessionId: String,
+    ): UpstreamCount? = runCatching {
+        val o = Json.parseToJsonElement(control.observations(credential, sessionId))
+            .jsonObject["throughput_up"]?.jsonObject ?: return null
+        UpstreamCount(
+            packets = o["packets"]?.jsonPrimitive?.content?.toIntOrNull() ?: 0,
+            bytes = o["bytes"]?.jsonPrimitive?.content?.toLongOrNull() ?: 0,
+            kbps = o["kbps"]?.jsonPrimitive?.content?.toIntOrNull() ?: 0,
+        )
+    }.getOrNull()
+
+    private data class SenderReport(
+        val packets: Int, val bytes: Long, val kbps: Int, val limitedBy: String,
+    )
+
+    /** The server's own account of the run, from the observations API. */
+    private fun senderReport(
+        control: ControlClient, credential: String, sessionId: String,
+    ): SenderReport? = runCatching {
+        val arr = Json.parseToJsonElement(control.observations(credential, sessionId))
+            .jsonObject["throughput"]?.jsonArray ?: return null
+        val last = arr.lastOrNull()?.jsonObject ?: return null
+        SenderReport(
+            packets = last["packets"]?.jsonPrimitive?.content?.toIntOrNull() ?: 0,
+            bytes = last["bytes"]?.jsonPrimitive?.content?.toLongOrNull() ?: 0,
+            kbps = last["kbps"]?.jsonPrimitive?.content?.toIntOrNull() ?: 0,
+            limitedBy = last["limited_by"]?.jsonPrimitive?.content ?: "unknown",
+        )
+    }.getOrNull()
+
+    private fun parseInt(body: String?, key: String): Int? =
+        body?.let { Regex("\"$key\"\\s*:\\s*(-?\\d+)").find(it)?.groupValues?.get(1)?.toIntOrNull() }
+
+    /**
+     * Builds a finding from a registry entry, which supplies the code, category and severity.
+     *
+     * Taking a [FindingSpec] rather than three loose values is the point: a typo becomes a
+     * compile error, and two call sites cannot disagree about which category a finding belongs
+     * to - a disagreement that would split one fault across two verdict lights.
+     */
+    private fun finding(spec: FindingSpec, testId: String, title: String, desc: String) =
+        Finding(
+            id = ids.uuid(), code = spec.code, category = spec.category, severity = spec.severity,
+            confidence = Confidence.HIGH,
+            title = title, description = desc, evidenceRefs = listOf(EvidenceRef(testId)),
+        )
+
+    private fun round2(v: Double) = Math.round(v * 100.0) / 100.0
+}
+
+/** Metrics for perf.throughput_udp in the upstream direction. */
+@Serializable
+data class UpstreamThroughputMetrics(
+    val direction: String = "up",
+    @SerialName("requested_kbps") val requestedKbps: Int,
+    @SerialName("sent_packets") val sentPackets: Int,
+    @SerialName("sent_bytes") val sentBytes: Long,
+    /** What this device managed to emit — a property of the phone and its radio, not the path. */
+    @SerialName("sent_kbps") val sentKbps: Int,
+    @SerialName("received_packets") val receivedPackets: Int? = null,
+    @SerialName("received_bytes") val receivedBytes: Long? = null,
+    /** What arrived, measured by the only party that can measure it. This is the result. */
+    @SerialName("received_kbps") val receivedKbps: Int,
+    @SerialName("loss_pct") val lossPct: Double? = null,
+    @SerialName("measures_network") val measuresNetwork: Boolean,
+)
+
+/** Metrics for perf.throughput_udp. */
+@Serializable
+data class ThroughputMetrics(
+    val direction: String = "down",
+    @SerialName("requested_kbps") val requestedKbps: Int,
+    @SerialName("planned_duration_ms") val plannedDurationMs: Int,
+    @SerialName("packets_received") val packetsReceived: Int,
+    @SerialName("bytes_received") val bytesReceived: Long,
+    @SerialName("received_kbps") val receivedKbps: Int,
+    @SerialName("sender_packets") val senderPackets: Int? = null,
+    @SerialName("sender_bytes") val senderBytes: Long? = null,
+    @SerialName("sender_kbps") val senderKbps: Int? = null,
+    /** Against the sender's count, so a server-side limit is never counted as network loss. */
+    @SerialName("loss_pct") val lossPct: Double? = null,
+    /** What ended the run: duration | budget | rate | send_error | unknown. */
+    @SerialName("limited_by") val limitedBy: String,
+    /**
+     * Whether this number says anything about the network. False when the sender's own ceiling
+     * was the binding constraint — in which case the rate is a property of the test, not the path.
+     */
+    @SerialName("measures_network") val measuresNetwork: Boolean,
+)

echolot-app/core-engine/src/test/kotlin/app/echo_lot/engine/DirectionalTest.kt

@@ -0,0 +1,155 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.engine
+
+import app.echo_lot.engine.Directional.Sample
+import app.echo_lot.engine.Directional.ServerSighting
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertNotNull
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+/**
+ * The arithmetic that turns "3 % loss somewhere" into "3 % loss upstream". Getting a denominator
+ * wrong here does not crash anything — it produces a plausible number pointing at the wrong half
+ * of the network, which is worse than no number at all. Hence a test per claim.
+ */
+class DirectionalTest {
+
+    /** A clean train: every packet sent, seen and answered. Server clock offset by a constant. */
+    private fun clean(n: Int, offsetNs: Long = 5_000_000_000L): Pair<List<Sample>, List<ServerSighting>> {
+        val sent = (1..n).map { Sample(it, tTxNs = it * 10_000_000L, tRxNs = it * 10_000_000L + 4_000_000L) }
+        val seen = (1..n).map {
+            ServerSighting(it, tRxNs = offsetNs + it * 10_000_000L + 2_000_000L,
+                tTxNs = offsetNs + it * 10_000_000L + 2_100_000L)
+        }
+        return sent to seen
+    }
+
+    @Test
+    fun aCleanTrainReportsNoLossInEitherDirection() {
+        val (sent, seen) = clean(10)
+        val m = Directional.analyse(sent, seen)
+        assertEquals(10, m.sent)
+        assertEquals(10, m.seenByServer)
+        assertEquals(10, m.repliesReceived)
+        assertEquals(0.0, m.lossUpstreamPct)
+        assertEquals(0.0, m.lossDownstreamPct)
+        assertFalse(m.noneReachedServer)
+    }
+
+    // The whole point: a packet the server never saw was lost on the way there.
+    @Test
+    fun packetsTheServerNeverSawAreUpstreamLoss() {
+        val (sent, seen) = clean(10)
+        val m = Directional.analyse(sent, seen.filter { it.seq !in setOf(3, 7) })
+        assertEquals(2, m.lostUpstream)
+        assertEquals(0, m.lostDownstream)
+        assertEquals(20.0, m.lossUpstreamPct)
+        assertEquals(0.0, m.lossDownstreamPct, "a packet that never arrived cannot be lost coming back")
+    }
+
+    @Test
+    fun repliesThatNeverArrivedAreDownstreamLoss() {
+        val (sent, seen) = clean(10)
+        val withHoles = sent.map { if (it.seq in setOf(2, 5)) it.copy(tRxNs = null) else it }
+        val m = Directional.analyse(withHoles, seen)
+        assertEquals(0, m.lostUpstream)
+        assertEquals(2, m.lostDownstream)
+        assertEquals(20.0, m.lossDownstreamPct)
+    }
+
+    // Downstream loss is measured against what actually reached the server. Using "sent" as the
+    // denominator would count every upstream loss a second time and overstate the return path.
+    @Test
+    fun downstreamLossIsRelativeToWhatReachedTheServer() {
+        val (sent, seen) = clean(10)
+        // 5 lost on the way there; of the 5 that arrived, 1 reply is lost coming back.
+        val seenPartial = seen.filter { it.seq > 5 }
+        val withHole = sent.map {
+            when {
+                it.seq <= 5 -> it.copy(tRxNs = null) // never got there, so never came back
+                it.seq == 6 -> it.copy(tRxNs = null) // arrived, reply lost
+                else -> it
+            }
+        }
+        val m = Directional.analyse(withHole, seenPartial)
+        assertEquals(5, m.lostUpstream)
+        assertEquals(50.0, m.lossUpstreamPct)
+        assertEquals(1, m.lostDownstream)
+        assertEquals(20.0, m.lossDownstreamPct, "1 of the 5 that arrived, not 1 of 10")
+    }
+
+    @Test
+    fun aServerThatSawNothingIsCalledOutSeparately() {
+        val (sent, _) = clean(6)
+        val m = Directional.analyse(sent.map { it.copy(tRxNs = null) }, emptyList())
+        assertTrue(m.noneReachedServer)
+        assertEquals(100.0, m.lossUpstreamPct)
+        assertEquals(0.0, m.lossDownstreamPct, "with nothing arriving there is no return path to blame")
+    }
+
+    // Jitter is legitimate without synchronised clocks because the offset cancels when successive
+    // one-way samples are differenced. This pins that: a huge constant offset must not show up.
+    @Test
+    fun jitterIsUnaffectedByTheClockOffsetBetweenTheTwoMachines() {
+        val (sent, near) = clean(10, offsetNs = 0)
+        val (_, far) = clean(10, offsetNs = 9_999_999_999L)
+        val a = Directional.analyse(sent, near)
+        val b = Directional.analyse(sent, far)
+        assertEquals(a.jitterUpstreamMs, b.jitterUpstreamMs,
+            "a constant clock offset must cancel when consecutive samples are differenced")
+        assertEquals(0.0, assertNotNull(a.jitterUpstreamMs), "an evenly spaced train has no jitter")
+    }
+
+    @Test
+    fun jitterReflectsUnevenArrival() {
+        val sent = listOf(
+            Sample(1, 0, 10_000_000),
+            Sample(2, 10_000_000, 20_000_000),
+            Sample(3, 20_000_000, 30_000_000),
+        )
+        // Server receive times drift: +2ms, +7ms, +3ms relative to send.
+        val seen = listOf(
+            ServerSighting(1, 2_000_000, 2_100_000),
+            ServerSighting(2, 17_000_000, 17_100_000),
+            ServerSighting(3, 23_000_000, 23_100_000),
+        )
+        val m = Directional.analyse(sent, seen)
+        // one-way samples: 2ms, 7ms, 3ms → |7-2| and |3-7| → mean 4.5ms
+        assertEquals(4.5, assertNotNull(m.jitterUpstreamMs))
+    }
+
+    // "No jitter" and "not enough data to say" are different claims, and only one is true here.
+    @Test
+    fun tooFewSamplesReportsNoJitterRatherThanZero() {
+        val m = Directional.analyse(
+            listOf(Sample(1, 0, 10_000_000)),
+            listOf(ServerSighting(1, 2_000_000, 2_100_000)),
+        )
+        assertNull(m.jitterUpstreamMs)
+        assertNull(m.jitterDownstreamMs)
+    }
+
+    // A server record for a sequence we never sent is not evidence about this train; folding it
+    // in would yield loss percentages outside 0–100.
+    @Test
+    fun strayServerRecordsAreIgnored() {
+        val (sent, seen) = clean(5)
+        val m = Directional.analyse(sent, seen + ServerSighting(99, 1, 2) + ServerSighting(100, 3, 4))
+        assertEquals(5, m.seenByServer)
+        assertEquals(0.0, m.lossUpstreamPct)
+        assertTrue(m.lossDownstreamPct in 0.0..100.0)
+    }
+
+    @Test
+    fun anEmptyTrainDoesNotDivideByZero() {
+        val m = Directional.analyse(emptyList(), emptyList())
+        assertEquals(0.0, m.lossUpstreamPct)
+        assertEquals(0.0, m.lossDownstreamPct)
+        assertFalse(m.noneReachedServer, "nothing sent is not the same as nothing arriving")
+    }
+}

echolot-app/core-engine/src/test/kotlin/app/echo_lot/engine/LiveDownstreamTest.kt

@@ -44,7 +44,9 @@ class LiveDownstreamTest {
         for (t in tests) println("${t.type}  status=${t.status}  metrics=${t.metrics}")
         for (f in findings) println("finding ${f.code} [${f.severity}] ${f.title}")
 
-        assertEquals(3, tests.size, "expected pmtud_down, frag_delivery and a downstream train")
+        // Assert on what is present, not on how many: adding a measurement should not be a
+        // test edit. (It was, once — hence the note.)
+        assertTrue(tests.size >= 3, "expected at least the three downstream tests, got ${tests.size}")
         val byType = tests.associateBy { it.type }
 
         val pmtud = assertNotNull(byType[TestType.MTU_PMTUD_DOWN], "no mtu.pmtud_down test")
@@ -58,6 +60,17 @@ class LiveDownstreamTest {
         val frag = assertNotNull(byType[TestType.MTU_FRAG_DELIVERY], "no mtu.frag_delivery test")
         assertNotNull(frag.metrics?.get("largest_delivered_bytes"))
 
+        // Fragment ordering runs only when fragments arrive at all, and only against a server
+        // that can craft them — so it is checked when present rather than required.
+        byType[TestType.MTU_FRAG_ORDERING]?.let { fo ->
+            val m = fo.metrics?.toString() ?: ""
+            println("fragment ordering: ${fo.status} $m")
+            if (fo.status != TestStatus.UNSUPPORTED) {
+                assertTrue(m.contains("in_order"), "no per-ordering result: $m")
+                assertTrue(m.contains("reversed"), "reversed ordering was never attempted: $m")
+            }
+        }
+
         val train = assertNotNull(byType[TestType.TRAIN_UDP_DOWNSTREAM], "no downstream train")
         assertNotNull(train.evidence, "a train without columnar evidence is not recomputable")
         val received = train.metrics?.get("received")?.toString()?.toIntOrNull() ?: 0

echolot-app/core-engine/src/test/kotlin/app/echo_lot/engine/LiveMeasurementTest.kt

@@ -7,6 +7,7 @@ import app.echo_lot.measurement.*
 import kotlinx.serialization.json.Json
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
 import kotlin.test.assertTrue
 
 /**
@@ -59,6 +60,18 @@ class LiveMeasurementTest {
         println("metrics: $metrics")
         assertTrue(metrics.toString().contains("rtt_ms_avg"))
 
+        // The directional split is the point of asking the server what it saw: without it a
+        // lossy path is reported as "loss" with no direction, which sends an engineer looking
+        // in both at once. Correlation is by wire sequence number, so a mismatch here means the
+        // two sides disagree about which packet is which.
+        val m = metrics.toString()
+        assertTrue(m.contains("seen_by_server"), "no directional split in the metrics: $m")
+        val seen = Regex(""""seen_by_server":(\d+)""").find(m)?.groupValues?.get(1)?.toInt()
+        assertNotNull(seen, "seen_by_server missing")
+        assertEquals(20, seen, "the server should have seen every probe on a healthy path")
+        assertTrue(m.contains("jitter_upstream_ms"), "no per-direction jitter: $m")
+        println("directional: $m")
+
         assertTrue(doc.summary != null)
         // A healthy local->fmr path should be green (no loss, no rebinding) or yellow.
         println("summary: ${doc.summary}")

echolot-app/core-engine/src/test/kotlin/app/echo_lot/engine/LiveThroughputTest.kt

@@ -0,0 +1,68 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.engine
+
+import app.echo_lot.measurement.TestStatus
+import app.echo_lot.protocol.ControlClient
+import app.echo_lot.protocol.ProbeSession
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+import kotlin.test.assertTrue
+
+/**
+ * Downstream throughput against a LIVE server. Self-skips without ECHOLOT_LIVE_*.
+ *
+ * The assertions are about *honesty* rather than speed: a rate is only a measurement if the run
+ * was ended by the clock and the sender's own count backs it up. A test that just asserted "some
+ * Mbps arrived" would pass equally well against a broken implementation.
+ */
+class LiveThroughputTest {
+
+    private val url = System.getenv("ECHOLOT_LIVE_URL")
+    private val pin = System.getenv("ECHOLOT_LIVE_PIN")
+    private val cred = System.getenv("ECHOLOT_LIVE_CRED")
+    private val udp = System.getenv("ECHOLOT_LIVE_UDP")
+    private val target = System.getenv("ECHOLOT_LIVE_TARGET") ?: "fmr"
+
+    @Test
+    fun measuresDownstreamRateAndSaysWhatLimitedIt() {
+        if (url == null || pin == null || cred == null || udp == null) {
+            println("LiveThroughputTest skipped (no ECHOLOT_LIVE_* env)"); return
+        }
+        val control = ControlClient(url, setOf(pin), "0.2.0")
+        val session = control.createSession(cred, target)
+        val (host, port) = udp.split(":").let { it[0] to it[1].toInt() }
+
+        val (test, findings) = ProbeSession(cred, session, host, port).use { ps ->
+            ps.echo() // prime: the grant binds to the observed source
+            ThroughputMeasurement(SystemIdSource()).run(
+                cred, session.sessionId, control, ps, sessionRef = "sess-1",
+                durationS = 3, kbps = 20_000,
+            )
+        }
+        control.deleteSession(cred, session.sessionId)
+
+        val m = assertNotNull(test.metrics).toString()
+        println("throughput: ${test.status} $m")
+        for (f in findings) println("finding ${f.code} [${f.severity}] ${f.title}")
+
+        assertEquals(TestStatus.OK, test.status, "no throughput traffic arrived: $m")
+
+        // The sender's own count must be present — without it, loss cannot be attributed and the
+        // number is not a measurement.
+        assertTrue(m.contains("sender_packets"), "no sender report to compare against: $m")
+        assertTrue(m.contains("limited_by"), "the result must say what ended the run: $m")
+
+        val received = Regex(""""received_kbps":(\d+)""").find(m)?.groupValues?.get(1)?.toInt()
+        assertNotNull(received)
+        assertTrue(received > 0, "measured 0 kbps: $m")
+        println("received ${received / 1000} Mbit/s")
+
+        // A run this short and this far below the ceiling should end on the clock. Anything else
+        // means the grant was the constraint, and then the rate says nothing about the path.
+        assertTrue(m.contains(""""limited_by":"duration""""),
+            "the run did not end on the clock, so the rate measures the server, not the path: $m")
+    }
+}

echolot-app/core-measurement/src/main/kotlin/app/echo_lot/measurement/FindingRegistry.kt

@@ -0,0 +1,205 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.measurement
+
+/**
+ * The registry of finding codes (measurement-schema.md §9, open item 1).
+ *
+ * A finding code is the stable, machine-readable half of a result: the prose changes, the code is
+ * what a dashboard groups by and what someone greps a year of archived runs for. That only holds
+ * if a code means exactly one thing forever — which is not something ad-hoc string literals at
+ * fifteen call sites can promise.
+ *
+ * The failure this exists to prevent had already happened by the time it was written. Two
+ * independently-added emitters produced `connectivity.downstream_loss` and
+ * `connectivity.loss_downstream` for the same concept, and nothing anywhere objected. Anyone
+ * aggregating either one would have silently seen half their data.
+ *
+ * So codes are declared here as typed specs, each carrying its category and default severity, and
+ * emitters reference the spec rather than retyping the string. That makes a typo a compile error,
+ * and makes it impossible for two call sites to disagree about which category a finding belongs
+ * to — a disagreement that would otherwise split one fault across two verdict lights.
+ */
+data class FindingSpec(
+    val code: String,
+    val category: Category,
+    /** Severity when nothing about the specific run argues otherwise; emitters may escalate. */
+    val severity: Severity,
+    /** One line: what this finding asserts. Present tense, no hedging. */
+    val meaning: String,
+    /**
+     * What the finding rules *out*, where that is the useful half. "Loss upstream" is worth much
+     * more when it also says the return path is fine, because that halves where to look next.
+     */
+    val rulesOut: String? = null,
+)
+
+object FindingRegistry {
+
+    // ---- connectivity ----------------------------------------------------------------
+
+    // Renamed from nat.* before anything shipped: neither of these is about NAT, and the
+    // prefix is what decides which category - and therefore which verdict light - a finding
+    // rolls up into. A nat.* code landing under connectivity would be a permanent puzzle.
+    val UDP_UNREACHABLE = FindingSpec(
+        "connectivity.udp_unreachable", Category.CONNECTIVITY, Severity.HIGH,
+        "No UDP echo replies came back from the server at all.",
+    )
+
+    val UDP_UNREACHABLE_UPSTREAM = FindingSpec(
+        "connectivity.udp_unreachable_upstream", Category.CONNECTIVITY, Severity.HIGH,
+        "The server received none of the probes, so traffic is dropped on the way out.",
+        rulesOut = "The return path: nothing arrived to be replied to.",
+    )
+
+    val UDP_LOSS = FindingSpec(
+        "connectivity.udp_loss", Category.CONNECTIVITY, Severity.MEDIUM,
+        "A large fraction of round-trip probes were lost, direction unknown.",
+    )
+
+    val LOSS_UPSTREAM = FindingSpec(
+        "connectivity.loss_upstream", Category.CONNECTIVITY, Severity.MEDIUM,
+        "Probes were lost on the way to the server.",
+        rulesOut = "The return path: replies came back for everything that arrived.",
+    )
+
+    /**
+     * The single code for "lost on the return path", whichever measurement found it.
+     *
+     * Two emitters had independently invented `connectivity.downstream_loss` and
+     * `connectivity.loss_downstream` for this, and nothing objected. Anyone aggregating either
+     * one would have silently seen half their data. Paired with [LOSS_UPSTREAM] so the two
+     * directions read as a set.
+     */
+    val LOSS_DOWNSTREAM = FindingSpec(
+        "connectivity.loss_downstream", Category.CONNECTIVITY, Severity.MEDIUM,
+        "Packets were lost on the way back from the server.",
+        rulesOut = "The outbound path: the server received what it was answering.",
+    )
+
+    val DOWNSTREAM_BLOCKED = FindingSpec(
+        "connectivity.downstream_blocked", Category.CONNECTIVITY, Severity.HIGH,
+        "Server-initiated packets never arrive, although round trips work.",
+        rulesOut = "Basic reachability: the path forwards replies, just not unsolicited traffic.",
+    )
+
+    val DOWNSTREAM_REORDER = FindingSpec(
+        "connectivity.downstream_reorder", Category.CONNECTIVITY, Severity.LOW,
+        "Downstream packets arrive in a different order than they were sent.",
+    )
+
+    // MEDIUM, not HIGH: a captive portal is a condition to report, not necessarily a fault - on
+    // hotel or cafe wifi it is exactly what should be there, and logging in clears it. NO_INTERNET
+    // is the HIGH one, because nothing the user does locally fixes that. The registry first said
+    // HIGH; the probe emitting it had always said MEDIUM, and the probe was the considered value.
+    val CAPTIVE_PORTAL = FindingSpec(
+        "connectivity.captive_portal", Category.CONNECTIVITY, Severity.MEDIUM,
+        "A captive portal is intercepting connectivity checks.",
+    )
+
+    val NO_INTERNET = FindingSpec(
+        "connectivity.no_internet", Category.CONNECTIVITY, Severity.HIGH,
+        "Android's own connectivity checks fail on this network.",
+    )
+
+    // ---- mtu -------------------------------------------------------------------------
+
+    val MTU_REDUCED_DOWNSTREAM = FindingSpec(
+        "mtu.reduced_downstream", Category.MTU, Severity.LOW,
+        "The downstream path MTU is below the usual 1500 bytes.",
+    )
+
+    val MTU_DOWNSTREAM_BLACKHOLE = FindingSpec(
+        "mtu.downstream_blackhole", Category.MTU, Severity.MEDIUM,
+        "Datagrams above the path MTU are dropped downstream, fragmented or not.",
+    )
+
+    val FRAGMENTS_BLOCKED = FindingSpec(
+        "mtu.fragments_blocked", Category.MTU, Severity.MEDIUM,
+        "IP fragments do not reach this device even when sent in order.",
+    )
+
+    val FRAGMENT_REORDER_SENSITIVE = FindingSpec(
+        "mtu.fragment_reorder_sensitive", Category.MTU, Severity.LOW,
+        "Fragments are delivered in order but dropped when reordered or delayed.",
+        rulesOut = "Fragmentation itself: in-order fragments arrive fine.",
+    )
+
+    // ---- nat -------------------------------------------------------------------------
+
+    val NAT_UDP_REBINDING = FindingSpec(
+        "nat.udp_rebinding", Category.NAT, Severity.MEDIUM,
+        "A NAT remapped the UDP source port mid-flow.",
+    )
+
+    val NAT_SYMMETRIC = FindingSpec(
+        "nat.symmetric", Category.NAT, Severity.MEDIUM,
+        "The NAT assigns a different external port per destination.",
+    )
+
+    // ---- perf ------------------------------------------------------------------------
+
+    val THROUGHPUT_NO_DELIVERY = FindingSpec(
+        "perf.throughput_no_delivery", Category.PERFORMANCE, Severity.HIGH,
+        "No throughput traffic arrived, although the server sent it.",
+    )
+
+    val THROUGHPUT_BELOW_OFFERED = FindingSpec(
+        "perf.throughput_below_offered", Category.PERFORMANCE, Severity.LOW,
+        "Less throughput arrived than the server sent for the whole run.",
+    )
+
+    // ---- dns -------------------------------------------------------------------------
+
+    val DNS_ANSWER_REWRITTEN = FindingSpec(
+        "dns.answer_rewritten", Category.DNS, Severity.HIGH,
+        "A resolver returned an answer that differs from the authoritative record.",
+    )
+
+    val DNS_AUTHORITATIVE_UNREACHABLE = FindingSpec(
+        "dns.authoritative_unreachable", Category.DNS, Severity.MEDIUM,
+        "The canary zone's authoritative server could not be reached.",
+    )
+
+    // ---- v6 ----------------------------------------------------------------------------
+    //
+    // Prefix is `v6.`, matching the test-type registry (v6.brokenness, v6.happy_eyeballs, ...).
+    // These were `ipv6.*` while declaring Category.IPV6, but the prefix map only knows "v6", so
+    // they silently rolled up under connectivity: the third instance of a prefix disagreeing with
+    // its category and quietly moving a fault to a different verdict light.
+
+    val V6_BROKEN = FindingSpec(
+        "v6.broken", Category.IPV6, Severity.MEDIUM,
+        "IPv6 is configured on this network but does not work.",
+        rulesOut = "Absence of IPv6: it is provisioned, it simply fails.",
+    )
+
+    /**
+     * INFO deliberately, and it needs to stay that way.
+     *
+     * Most networks still do not offer IPv6, and that is not a fault. Reporting it as a warning
+     * lights a yellow verdict on a perfectly healthy network, which teaches people to ignore the
+     * light — the one thing a diagnostic must never do.
+     */
+    val V6_NOT_OFFERED = FindingSpec(
+        "v6.not_offered", Category.IPV6, Severity.INFO,
+        "This network does not offer IPv6.",
+    )
+
+    /** Every registered finding, in declaration order. */
+    val all: List<FindingSpec> = listOf(
+        UDP_UNREACHABLE, UDP_UNREACHABLE_UPSTREAM, UDP_LOSS, LOSS_UPSTREAM, LOSS_DOWNSTREAM,
+        DOWNSTREAM_BLOCKED, DOWNSTREAM_REORDER, CAPTIVE_PORTAL, NO_INTERNET,
+        MTU_REDUCED_DOWNSTREAM, MTU_DOWNSTREAM_BLACKHOLE, FRAGMENTS_BLOCKED,
+        FRAGMENT_REORDER_SENSITIVE,
+        NAT_UDP_REBINDING, NAT_SYMMETRIC,
+        THROUGHPUT_NO_DELIVERY, THROUGHPUT_BELOW_OFFERED,
+        DNS_ANSWER_REWRITTEN, DNS_AUTHORITATIVE_UNREACHABLE,
+        V6_BROKEN, V6_NOT_OFFERED,
+    )
+
+    private val byCode: Map<String, FindingSpec> = all.associateBy { it.code }
+
+    fun byCode(code: String): FindingSpec? = byCode[code]
+}

echolot-app/core-measurement/src/main/kotlin/app/echo_lot/measurement/Test.kt

@@ -77,6 +77,8 @@ object TestType {
     const val MTU_BLACKHOLE = "mtu.blackhole"
     const val MTU_MSS_OBSERVED = "mtu.mss_observed"
     const val MTU_FRAG_DELIVERY = "mtu.frag_delivery"
+    /** Whether fragments survive arriving out of order, not merely whether they survive. */
+    const val MTU_FRAG_ORDERING = "mtu.frag_ordering"
     // nat
     const val NAT_STUN_5780 = "nat.stun_5780"
     const val NAT_MAPPING_LIFETIME_UDP = "nat.mapping_lifetime_udp"

echolot-app/core-measurement/src/test/kotlin/app/echo_lot/measurement/FindingRegistryTest.kt

@@ -0,0 +1,133 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.measurement
+
+import java.io.File
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+import kotlin.test.fail
+
+/**
+ * Keeps the finding registry honest.
+ *
+ * The interesting test is the last one: it reads `docs/findings-registry.md` and fails when the
+ * document and the code disagree. Documentation that drifts from its implementation is worse than
+ * none, because it still looks authoritative — and a finding registry is precisely the artifact
+ * other people build tooling against.
+ */
+class FindingRegistryTest {
+
+    @Test
+    fun codesAreUnique() {
+        val dupes = FindingRegistry.all.groupBy { it.code }.filterValues { it.size > 1 }.keys
+        assertTrue(dupes.isEmpty(), "duplicate finding codes: $dupes")
+    }
+
+    @Test
+    fun everyDeclaredSpecIsInTheAllList() {
+        // Reflection over the object's properties: a spec that is declared but left out of `all`
+        // is invisible to the doc check and to any consumer enumerating the registry.
+        val declared = FindingRegistry::class.java.declaredMethods
+            .filter { it.parameterCount == 0 && it.returnType == FindingSpec::class.java }
+            .mapNotNull { runCatching { it.invoke(FindingRegistry) as FindingSpec }.getOrNull() }
+            .map { it.code }
+            .toSet()
+        val listed = FindingRegistry.all.map { it.code }.toSet()
+        assertEquals(declared, listed, "declared specs and the `all` list disagree")
+    }
+
+    // The prefix decides the category, and the category decides which verdict light the finding
+    // rolls up into. A code whose prefix disagrees with its category silently moves a fault to a
+    // different light — the exact bug that got two codes renamed out of nat.*.
+    @Test
+    fun everyPrefixMatchesItsCategory() {
+        for (spec in FindingRegistry.all) {
+            val fromPrefix = TestType.category(spec.code)
+            assertEquals(
+                fromPrefix, spec.category,
+                "${spec.code} is declared as ${spec.category} but its prefix maps to $fromPrefix",
+            )
+        }
+    }
+
+    @Test
+    fun codesFollowTheNamingConvention() {
+        val shape = Regex("^[a-z0-9]+\\.[a-z0-9_]+$")
+        for (spec in FindingRegistry.all) {
+            assertTrue(shape.matches(spec.code), "malformed code: ${spec.code}")
+            assertTrue(spec.meaning.isNotBlank(), "${spec.code} has no meaning")
+            assertTrue(
+                spec.meaning.trimEnd().endsWith("."),
+                "${spec.code}'s meaning should be a sentence: '${spec.meaning}'",
+            )
+        }
+    }
+
+    // Two near-identical codes are how one fault ends up split across two dashboards. This is a
+    // blunt check — it will not catch every synonym — but it catches the shape that already
+    // happened: the same words in a different order.
+    @Test
+    fun noTwoCodesAreAnagramsOfEachOther() {
+        val normalised = FindingRegistry.all.associate { spec ->
+            spec.code to spec.code.substringAfter('.').split('_').sorted().joinToString("_")
+        }
+        val clashes = normalised.entries.groupBy { it.value }.filterValues { it.size > 1 }
+        if (clashes.isNotEmpty()) {
+            fail("codes differing only in word order: ${clashes.values.map { g -> g.map { it.key } }}")
+        }
+    }
+
+    @Test
+    fun theDocumentAndTheRegistryAgree() {
+        val doc = findDoc() ?: run {
+            println("findings-registry.md not found from ${File(".").absolutePath} — skipping")
+            return
+        }
+        val text = doc.readText()
+
+        // Only table rows count as "documented". Prose may legitimately mention a code that no
+        // longer exists — the rules section explains why two were merged — and treating that as
+        // a registry entry would force the document to forget its own history.
+        val documented = text.lines()
+            .filter { it.trimStart().startsWith("|") }
+            .flatMap { row -> Regex("`([a-z0-9]+\\.[a-z0-9_]+)`").findAll(row).map { it.groupValues[1] } }
+            .toSet()
+        val registered = FindingRegistry.all.map { it.code }.toSet()
+
+        val missingFromDoc = registered - documented
+        val missingFromCode = documented - registered
+        assertTrue(
+            missingFromDoc.isEmpty(),
+            "these codes exist in FindingRegistry but not in docs/findings-registry.md: $missingFromDoc",
+        )
+        assertTrue(
+            missingFromCode.isEmpty(),
+            "docs/findings-registry.md documents codes that no longer exist: $missingFromCode",
+        )
+
+        // And the severities must match, or the document is describing a different system.
+        for (spec in FindingRegistry.all) {
+            val row = text.lines().firstOrNull {
+                it.trimStart().startsWith("|") && it.contains("`${spec.code}`")
+            } ?: continue
+            val severity = spec.severity.name.lowercase()
+            assertTrue(
+                row.contains("| $severity |"),
+                "${spec.code} is ${severity} in code but the doc row says otherwise: $row",
+            )
+        }
+    }
+
+    /** Walks up from the test's working directory to find the repo's docs/ folder. */
+    private fun findDoc(): File? {
+        var dir: File? = File(".").absoluteFile
+        repeat(6) {
+            val candidate = File(dir, "docs/findings-registry.md")
+            if (candidate.isFile) return candidate
+            dir = dir?.parentFile
+        }
+        return null
+    }
+}

echolot-app/core-privacy/src/main/kotlin/app/echo_lot/privacy/Anonymizer.kt

@@ -108,8 +108,14 @@ class Anonymizer(private val level: PrivacyLevel, private val salt: Salt) {
         is JsonObject -> walkObject(v, path)
         is JsonArray -> JsonArray(v.map { walk(key, it, path) })
         is JsonPrimitive ->
-            if (v.isString) transform(Classification.typeOf(key, path), v.content).let(::JsonPrimitive)
-            else v
+            if (v.isString) {
+                // Name first (it is precise), then shape (it is exhaustive). A field nobody
+                // classified must not be a field that leaks.
+                val type = Classification.typeOf(key, path) ?: Classification.inferFromValue(v.content)
+                JsonPrimitive(transform(type, v.content))
+            } else {
+                v
+            }
     }
 
     private fun transform(type: LogicalType?, value: String): String = when (type) {
@@ -147,6 +153,11 @@ class Anonymizer(private val level: PrivacyLevel, private val salt: Salt) {
      * Public addresses keep only their /16 so the network is still locatable at ISP granularity.
      */
     private fun ip4(value: String): String {
+        // A route destination carries a prefix length; pseudonymize the address and put it back,
+        // or "0.0.0.0/0" turns into nonsense and the routing table becomes unreadable.
+        value.substringAfter('/', "").takeIf { it.isNotEmpty() && value.contains('/') }?.let { len ->
+            return ip4(value.substringBefore('/')) + "/" + len
+        }
         val o = value.split(".")
         if (o.size != 4 || o.any { it.toIntOrNull() == null }) return value
         val n = o.map { it.toInt() }
@@ -167,8 +178,36 @@ class Anonymizer(private val level: PrivacyLevel, private val salt: Salt) {
      * is a device fingerprint, especially with EUI-64.
      */
     private fun ip6(value: String): String {
+        // Dotted quads reach here through the family-agnostic field names (addr, gateway, dst);
+        // hand them to the IPv4 path rather than mangling them as if they were v6.
+        if (value.count { it == ':' } < 2) return ip4(value)
+        if (value.contains('/')) {
+            return ip6(value.substringBefore('/')) + "/" + value.substringAfter('/')
+        }
         val v = value.lowercase(Locale.ROOT)
+        // The unspecified address and the default route are not identities; mangling them would
+        // make a routing table unreadable for no privacy gain.
         if (v == "::1" || v == "::" || v.startsWith("fe80:") || v.startsWith("ff")) return v
+
+        // Unique local addresses (fc00::/7) need the *whole* prefix replaced, not the tail.
+        //
+        // They look like the v6 equivalent of RFC1918, and the first instinct is to keep them for
+        // the same reason: private, topological, says nothing about anyone. That reasoning does
+        // not carry over. An RFC1918 prefix is shared by millions of networks and identifies
+        // none of them; a ULA global ID is 40 *random* bits, unique to one network by
+        // construction (RFC 4193). It is a network fingerprint. Passing the leading groups
+        // through - which is what the general path does - leaked 32 of those 40 bits.
+        //
+        // The prefix is pseudonymized as a unit, so two addresses on the same ULA subnet still
+        // land on the same pseudonymous prefix. "These hosts are on one network" survives;
+        // "this is *that* network" does not.
+        if (v.startsWith("fc") || v.startsWith("fd")) {
+            val groups = v.substringBefore('%').split(":")
+            val prefix = pseudo("ula-prefix", groups.take(3).joinToString(":")) { it }
+            val host = pseudo("ula-host", v) { it }
+            return "fd${prefix.substring(0, 2)}:${prefix.substring(2, 6)}:${prefix.substring(6, 10)}" +
+                "::${host.substring(0, 4)}"
+        }
         val groups = v.substringBefore('%').split(":")
         if (groups.size < 3) return v
         val h = pseudo("ip6", value) { it }

echolot-app/core-privacy/src/main/kotlin/app/echo_lot/privacy/Classification.kt

@@ -32,6 +32,16 @@ object Classification {
             "link_local", "ra_source", "prefix",
         ).forEach { put(it, LogicalType.IP6) }
 
+        // Family-agnostic address fields — the names the models actually use (Address.addr,
+        // Route.gateway, Route.dst, DnsConfig.servers). Their absence here was a real leak: the
+        // device's own global IPv6 address went out verbatim at the level whose description
+        // promises addresses are pseudonymized. Typed IP6 because the transform detects the
+        // family from the value, falling through to the IPv4 path for a dotted quad.
+        listOf(
+            "addr", "address", "gateway", "dst", "src", "servers", "server", "resolver",
+            "next_hop", "via", "public_ip", "observed_ip",
+        ).forEach { put(it, LogicalType.IP6) }
+
         listOf("mac", "hw_addr", "gateway_mac", "router_mac", "sender_mac", "peer_mac")
             .forEach { put(it, LogicalType.MAC) }
         listOf("bssid", "ap_mac").forEach { put(it, LogicalType.BSSID) }
@@ -40,6 +50,8 @@ object Classification {
         listOf(
             "fqdn", "hostname", "host", "name", "reverse_dns", "ptr", "domain", "query_name",
             "friendly_name", "server_name", "sni", "cname", "search_domain", "device_name",
+            // Plural and prefixed variants the models actually use.
+            "search_domains", "private_dns_hostname", "domains", "hostnames",
         ).forEach { put(it, LogicalType.FQDN) }
 
         listOf("session_id", "credential", "token", "device_id", "android_id", "serial", "imsi", "iccid")
@@ -78,6 +90,49 @@ object Classification {
         return null
     }
 
+    /**
+     * Last-resort classification from the *value*, when the field name is unrecognised.
+     *
+     * A name table can only protect fields somebody remembered to add, which is the wrong
+     * property for a privacy control: the dangerous field is the one nobody thought of. This
+     * exists because that failed once already — `addresses[].addr` holds the device's own global
+     * IPv6 address, the table had never heard of the name, and it went out verbatim.
+     *
+     * Only addresses and MACs are inferred, because only those have shapes that cannot be
+     * mistaken for something else. Hostnames deliberately are not: `train.udp_updown` is
+     * indistinguishable from a domain by shape, and mangling a test type would corrupt the
+     * document to protect nothing.
+     */
+    fun inferFromValue(value: String): LogicalType? {
+        val v = value.trim()
+        if (v.isEmpty() || v.length > 64) return null
+        if (looksLikeMac(v)) return LogicalType.MAC
+        if (looksLikeIp6(v)) return LogicalType.IP6
+        if (looksLikeIp4(v)) return LogicalType.IP4
+        return null
+    }
+
+    private fun isHex(c: Char) = c in '0'..'9' || c in 'a'..'f' || c in 'A'..'F'
+
+    private fun looksLikeMac(v: String): Boolean {
+        val parts = v.split(':', '-')
+        return parts.size == 6 && parts.all { p -> p.length == 2 && p.all(::isHex) }
+    }
+
+    private fun looksLikeIp4(v: String): Boolean {
+        val parts = v.substringBefore('/').split('.')
+        return parts.size == 4 && parts.all { p ->
+            p.isNotEmpty() && p.length <= 3 && p.all(Char::isDigit) && p.toInt() <= 255
+        }
+    }
+
+    private fun looksLikeIp6(v: String): Boolean {
+        val core = v.substringBefore('/').substringBefore('%')
+        // Two colons minimum, so a time or a MAC fragment does not qualify, and nothing but the
+        // characters an address may contain.
+        return core.count { it == ':' } >= 2 && core.all { it == ':' || isHex(it) }
+    }
+
     fun dropAtBalanced(path: List<String>): Boolean {
         if (path.isNotEmpty() && path.last() in droppedKeys) return true
         return droppedPaths.any { dropped -> dropped.all { path.contains(it) } }

echolot-app/core-privacy/src/test/kotlin/app/echo_lot/privacy/AnonymizerTest.kt

@@ -182,4 +182,47 @@ class AnonymizerTest {
         assertEquals(PrivacyLevel.BALANCED, PrivacyLevel.max(PrivacyLevel.BALANCED, PrivacyLevel.FULL))
         assertEquals(PrivacyLevel.FULL, PrivacyLevel.fromWire("nonsense"))
     }
+
+    // A ULA looks like the v6 RFC1918 and is not. Its global ID is 40 random bits, unique to one
+    // network by construction (RFC 4193), so the prefix IS the identifier - unlike 192.168.x,
+    // which millions of networks share. Passing the leading groups through leaked most of it.
+    @Test
+    fun ulaPrefixesArePseudonymizedWhole() {
+        val doc = json.parseToJsonElement(
+            """{"run":{"id":"r"},"networks":[{"link":{"dns":{"servers":["fda1:3fb1:ff92:6696::2662"]}}}]}"""
+        ).jsonObject
+        val out = flat(anon(PrivacyLevel.BALANCED, doc))
+        assertFalse(out.contains("fda1"), "the ULA global ID survived: $out")
+        assertFalse(out.contains("3fb1"), "part of the ULA global ID survived: $out")
+        assertTrue(out.contains("fd"), "the result should still read as a ULA: $out")
+    }
+
+    // Pseudonymizing the prefix as a unit keeps the one fact that is diagnostically useful:
+    // whether two addresses sit on the same network.
+    @Test
+    fun addressesOnOneUlaSubnetStayRelated() {
+        val doc = json.parseToJsonElement(
+            """{"run":{"id":"r"},"networks":[{"link":{"dns":{"servers":[
+               "fda1:3fb1:ff92:6696::1","fda1:3fb1:ff92:6696::2","fdff:9999:8888:7777::1"]}}}]}"""
+        ).jsonObject
+        val servers = anon(PrivacyLevel.BALANCED, doc)["networks"]!!.jsonArray[0].jsonObject["link"]!!
+            .jsonObject["dns"]!!.jsonObject["servers"]!!.jsonArray.map { it.jsonPrimitive.content }
+        val prefixOf = { s: String -> s.substringBeforeLast("::") }
+        assertEquals(prefixOf(servers[0]), prefixOf(servers[1]),
+            "two addresses on one ULA subnet should share a pseudonymous prefix")
+        assertNotEquals(prefixOf(servers[0]), prefixOf(servers[2]),
+            "a different ULA network must not collide with the first")
+    }
+
+    // RFC1918 stays readable, and this is the contrast that justifies it: a shared, meaningless
+    // prefix is topology; a unique random one is identity.
+    @Test
+    fun rfc1918StaysReadableUnlikeUla() {
+        val doc = json.parseToJsonElement(
+            """{"run":{"id":"r"},"networks":[{"link":{"dns":{"servers":["192.168.1.1","10.13.102.1"]}}}]}"""
+        ).jsonObject
+        val out = flat(anon(PrivacyLevel.BALANCED, doc))
+        assertTrue(out.contains("192.168.1.1"), "RFC1918 should survive: $out")
+        assertTrue(out.contains("10.13.102.1"), "RFC1918 should survive: $out")
+    }
 }

echolot-app/core-privacy/src/test/kotlin/app/echo_lot/privacy/LeakTest.kt

@@ -0,0 +1,113 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package app.echo_lot.privacy
+
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.jsonObject
+import kotlin.test.Test
+import kotlin.test.assertTrue
+
+/**
+ * The blunt instrument: build a document with identifying values in every place one can actually
+ * occur, anonymize it, and assert none of them survive.
+ *
+ * [AnonymizerTest] checks that the fields the classification table knows about are handled
+ * correctly. This checks the other half — the fields it does *not* know about. A per-field test
+ * can only fail for a field someone remembered to write a case for, which is exactly the wrong
+ * property for a privacy check: the dangerous field is the one nobody thought of.
+ *
+ * Concretely, this is written the way it is because the schema's own field names disagree with
+ * the classifier's. `Address.addr` carries an IP and is documented as such in
+ * measurement-schema.md §8, but the classifier keys on names like `ip4` and `gateway_ip4` and had
+ * never heard of `addr`.
+ */
+class LeakTest {
+
+    private val json = Json { prettyPrint = false }
+    private val salt = Salt.perRun(ByteArray(32) { 3 })
+
+    /**
+     * Every string here is something that identifies a person, a household or a device, placed
+     * where the real models actually put it (`core-measurement`'s Network/Link/Address/DnsConfig).
+     */
+    private val secrets = listOf(
+        "Rambossek WLAN",              // ssid
+        "78:9a:18:aa:bb:cc",           // bssid
+        "aa:bb:cc:dd:ee:11",           // gateway mac
+        "2001:1ad0:c4fe:6767::150",    // global v6 address on the interface
+        "2a02:1748:dead:beef::1",      // v6 default gateway
+        "203.0.113.77",                // public v4
+        "nas.rambossek.lan",           // private-dns hostname
+        "rambossek.lan",               // search domain
+        "Anna's Chromecast",           // neighbour name
+        "kitchen table",               // free-text note
+    )
+
+    private fun document(): String = """
+        {
+          "schema": "echolot/measurement",
+          "run": {
+            "id": "run-1", "trigger": "manual", "notes": "${secrets[9]}",
+            "device": {"manufacturer": "OnePlus", "model": "CPH2747"}
+          },
+          "networks": [{
+            "id": "net-1", "transport": "wifi",
+            "link": {
+              "mtu": 1500,
+              "addresses": [
+                {"addr": "${secrets[3]}", "prefix_len": 64, "scope": "global"},
+                {"addr": "192.168.1.44", "prefix_len": 24, "scope": "global"}
+              ],
+              "routes": [
+                {"dst": "::/0", "gateway": "${secrets[4]}", "iface": "wlan0"},
+                {"dst": "0.0.0.0/0", "gateway": "192.168.1.1", "iface": "wlan0"}
+              ],
+              "dns": {
+                "servers": ["${secrets[5]}", "192.168.1.1"],
+                "private_dns_hostname": "${secrets[6]}",
+                "search_domains": ["${secrets[7]}"]
+              }
+            },
+            "wifi": {"ssid": "${secrets[0]}", "bssid": "${secrets[1]}"},
+            "neighbors": [{"name": "${secrets[8]}", "mac": "${secrets[2]}"}]
+          }],
+          "tests": [{"id": "t1", "type": "train.udp_updown", "status": "ok",
+                     "metrics": {"rtt_ms_avg": 12.4}}],
+          "findings": [],
+          "summary": {"verdict": "ok"}
+        }
+    """.trimIndent()
+
+    private fun anonymized(level: PrivacyLevel): String =
+        json.encodeToString(
+            kotlinx.serialization.json.JsonObject.serializer(),
+            Anonymizer(level, salt).anonymize(json.parseToJsonElement(document()).jsonObject),
+        )
+
+    @Test
+    fun nothingIdentifyingSurvivesBalanced() {
+        val out = anonymized(PrivacyLevel.BALANCED)
+        val leaked = secrets.filter { out.contains(it) }
+        assertTrue(
+            leaked.isEmpty(),
+            "these identifying values were uploaded verbatim at BALANCED: $leaked\n\n$out",
+        )
+    }
+
+    @Test
+    fun nothingIdentifyingSurvivesStrict() {
+        val out = anonymized(PrivacyLevel.STRICT)
+        val leaked = secrets.filter { out.contains(it) }
+        assertTrue(leaked.isEmpty(), "leaked at STRICT: $leaked\n\n$out")
+    }
+
+    // Private addresses are kept on purpose — they describe the topology and not the person — so
+    // this pins that the leak test above is not passing by accident of over-redaction.
+    @Test
+    fun privateAddressesAreStillReadable() {
+        val out = anonymized(PrivacyLevel.BALANCED)
+        assertTrue(out.contains("192.168.1.1"), "RFC1918 gateway should survive: $out")
+        assertTrue(out.contains("192.168.1.44"), "RFC1918 interface address should survive: $out")
+    }
+}

echolot-app/core-protocol/src/main/kotlin/app/echo_lot/protocol/ProbeSession.kt

@@ -44,13 +44,26 @@ class ProbeSession(
      */
     fun echo(paddingBytes: Int = 40): EchoResult? {
         val t0 = System.nanoTime()
-        val pkt = Wire.build(Wire.TYPE_ECHO_REQ, prefix, ++seq, nowNs(), key, ByteArray(paddingBytes))
+        val wireSeq = ++seq
+        val pkt = Wire.build(Wire.TYPE_ECHO_REQ, prefix, wireSeq, nowNs(), key, ByteArray(paddingBytes))
         socket.send(DatagramPacket(pkt, pkt.size, server))
+        // A lost probe still has a sequence number, and that number is what lets the server's
+        // observations say whether it was lost going out or coming back — so report it either way.
+        lastSeq = wireSeq
         val resp = receive(Wire.TYPE_ECHO_RESP) ?: return null
         val rttMs = (System.nanoTime() - t0) / 1_000_000.0
-        return EchoResult(rttMs, Observation.parse(resp.payload))
+        return EchoResult(rttMs, Observation.parse(resp.payload), wireSeq)
     }
 
+    /**
+     * The wire sequence number of the most recent [echo], including one that was lost.
+     *
+     * Exposed because the caller cannot derive it: the counter is shared with every other packet
+     * type on this session, so "the nth echo" is not "sequence n".
+     */
+    var lastSeq: Int = 0
+        private set
+
     /** One MTU probe of [totalSize] bytes (DF is set by the OS on the socket where supported).
      *  Returns the size the server acknowledged receiving, or null if the probe was lost. */
     fun mtuProbe(totalSize: Int): Int? {
@@ -96,6 +109,49 @@ class ProbeSession(
         return out
     }
 
+    /**
+     * Sends paced upstream traffic for [durationMs] and reports what was put on the wire.
+     *
+     * Paced rather than flat out, for the same reason the server paces: an unpaced burst measures
+     * the local NIC and the first queue it meets, then collapses into loss that reads as a network
+     * fault. The schedule is absolute rather than sleep-per-packet, which accumulates the
+     * scheduler's error and drifts the achieved rate below target over a multi-second run.
+     *
+     * Nothing comes back — the server counts and stays silent — so the result here is only the
+     * send side. The measurement is the gap between this and the server's tally.
+     */
+    fun sendThroughput(durationMs: Long, kbps: Int, sizeBytes: Int = 1200): Sent {
+        val size = sizeBytes.coerceIn(Wire.HEADER_SIZE + 16, 1472)
+        val payload = ByteArray(size - Wire.HEADER_SIZE)
+        val perPacketNs = (size.toLong() * 8 * 1_000_000 / kbps.coerceAtLeast(1)).coerceAtLeast(1_000)
+
+        val start = System.nanoTime()
+        val deadline = start + durationMs * 1_000_000
+        var next = start
+        var packets = 0
+        var bytes = 0L
+        while (System.nanoTime() < deadline) {
+            val pkt = Wire.build(Wire.TYPE_THROUGHPUT_UP, prefix, ++seq, nowNs(), key, payload)
+            try {
+                socket.send(DatagramPacket(pkt, pkt.size, server))
+            } catch (e: java.io.IOException) {
+                // A local send failure is our condition, not the path's. Stop and report what
+                // actually left, rather than counting the remainder as loss on the network.
+                break
+            }
+            packets++
+            bytes += pkt.size
+            next += perPacketNs
+            val sleepNs = next - System.nanoTime()
+            if (sleepNs > 0) Thread.sleep(sleepNs / 1_000_000, (sleepNs % 1_000_000).toInt())
+        }
+        val elapsedMs = (System.nanoTime() - start) / 1_000_000
+        return Sent(packets, bytes, elapsedMs, if (elapsedMs > 0) (bytes * 8 / elapsedMs).toInt() else 0)
+    }
+
+    /** What one upstream run put on the wire locally. */
+    data class Sent(val packets: Int, val bytes: Long, val durationMs: Long, val kbps: Int)
+
     /** One packet received from the server, with the wire size actually delivered. */
     data class Received(val type: Int, val seq: Int, val sizeBytes: Int, val tRxNs: Long)
 
@@ -112,5 +168,5 @@ class ProbeSession(
 
     override fun close() = socket.close()
 
-    data class EchoResult(val rttMs: Double, val observation: Observation?)
+    data class EchoResult(val rttMs: Double, val observation: Observation?, val seq: Int = 0)
 }

echolot-app/core-protocol/src/main/kotlin/app/echo_lot/protocol/Wire.kt

@@ -32,6 +32,22 @@ object Wire {
     const val TYPE_DOWNTRAIN_DATA: Int = 0x06
     const val TYPE_BIG_SEND: Int = 0x0C
 
+    /**
+     * A datagram the server deliberately fragmented. Its arrival IS the measurement: it can only
+     * be delivered if every fragment survived the path and the local stack reassembled them.
+     */
+    const val TYPE_FRAG_DATA: Int = 0x0D
+
+    /** One packet of a sustained-rate downstream run. */
+    const val TYPE_THROUGHPUT_DATA: Int = 0x0E
+
+    /**
+     * One packet of a client-driven upstream run. The server counts it and does not answer:
+     * a reply would double the traffic and drag the return path into a measurement that is
+     * specifically about the outbound one.
+     */
+    const val TYPE_THROUGHPUT_UP: Int = 0x0F
+
     /** The 8-byte on-the-wire prefix = first 16 hex chars of the session id, decoded. */
     fun wirePrefix(sessionId: String): ByteArray {
         require(sessionId.length >= 16) { "session id too short" }

echolot-app/scripts/enroll-link.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# SPDX-FileCopyrightText: 2026 Echolot contributors
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# Mints an enrollment link on the probe server and prints it — as text, as a QR code if
+# `qrencode` is around, and as an adb command if a device is attached.
+#
+# The admin listener is localhost-only by design, so this goes over SSH. The link carries a
+# single-use bearer token: treat it like a password until it is redeemed.
+#
+# Usage: echolot-app/scripts/enroll-link.sh [note]
+set -euo pipefail
+
+SSH_HOST="${ECHOLOT_SSH:-claude-echolot}"
+NOTE="${1:-manual}"
+
+MINTED=$(ssh -o BatchMode=yes "$SSH_HOST" \
+  "curl -s -X POST 'http://127.0.0.1:8444/admin/enroll-tokens?note=$NOTE'")
+
+URI=$(printf '%s' "$MINTED" | python -c 'import json,sys;print(json.load(sys.stdin).get("enroll_uri",""))')
+if [ -z "$URI" ]; then
+  echo "server returned no enroll_uri (needs server-v0.5.4+):" >&2
+  echo "$MINTED" >&2
+  exit 1
+fi
+
+echo "$URI"
+echo
+
+# A QR is the point of the format: scanning beats pasting a 200-character string onto a phone.
+if command -v qrencode >/dev/null 2>&1; then
+  qrencode -t ANSIUTF8 "$URI"
+else
+  echo "(install qrencode to get a scannable QR here)"
+fi
+
+# With a device attached, the deep link can be delivered straight to the app — no typing at all.
+if command -v adb >/dev/null 2>&1 && [ -n "$(adb devices | sed -n '2p')" ]; then
+  echo
+  echo "attached device — deliver it directly with:"
+  echo "  adb shell am start -a android.intent.action.VIEW -d '$URI'"
+fi

server/cmd/echolot-server/main.go

@@ -111,7 +111,15 @@ func serve(cfg *config.Config) error {
 		TLSConfig: &tls.Config{Certificates: []tls.Certificate{cert}, MinVersion: tls.VersionTLS12},
 	}
 
-	caps := []string{"udp-probe", "delayed-echo", "connect-back", "http-echo", "downtrain", "big-send"}
+	caps := []string{"udp-probe", "delayed-echo", "connect-back", "http-echo", "downtrain", "big-send", "throughput"}
+	// Crafted fragments need a raw socket. Advertised only when one can actually be opened —
+	// a capability we cannot deliver turns a missing feature into a failed measurement.
+	rawFrag := dataplane.RawFragSupported()
+	if rawFrag {
+		caps = append(caps, "frag-send")
+	} else {
+		slog.Info("frag-send unavailable: no raw socket (needs CAP_NET_RAW)")
+	}
 	if len(config.Addrs(cfg.TCPListen)) > 0 {
 		caps = append(caps, "tcp-echo", "tls-echo")
 	}
@@ -154,6 +162,12 @@ func serve(cfg *config.Config) error {
 		AppRange:         appRange,
 		PublicControlURL: publicControlURL(cfg),
 	}
+	// Left nil when there is no raw socket, so the handler answers "not implemented" with a
+	// reason rather than failing somewhere deeper.
+	if rawFrag {
+		ctl.FragSend = dp.FragSend
+	}
+	ctl.DownThroughput = dp.DownThroughput
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()

server/internal/control/control.go

@@ -59,6 +59,11 @@ type Server struct {
 	BigSend   func(sess *session.Session, g *session.Grant, sizes []int, df bool) ([]dataplane.BigSendResult, error)
 	// Runs stores uploaded measurement documents (may be nil: uploads unsupported).
 	Runs *runs.Store
+	// FragSend emits one datagram as hand-built IP fragments in a chosen order (may be nil:
+	// needs a raw socket, so it is unavailable to an unprivileged server).
+	FragSend func(sess *session.Session, g *session.Grant, sizeBytes int, mode dataplane.FragMode, fragSize int) (dataplane.FragResult, error)
+	// DownThroughput sends paced traffic toward the client for a bounded time (may be nil).
+	DownThroughput func(sess *session.Session, g *session.Grant, durationMs, kbps, sizeBytes int) (dataplane.ThroughputResult, error)
 	// EgressMTU reports the server's own measured egress path MTU (0 = unknown). With DF set
 	// we cannot emit a datagram larger than this, so requested sizes above it are refused up
 	// front and reported as such — the client must not read that as a downstream path limit.
@@ -218,6 +223,10 @@ func (s *Server) observations(w http.ResponseWriter, r *http.Request) {
 		"udp":          map[string]any{"packets_seen": packetsSeen, "packets": udp},
 		"tcp":          tcp,
 		"connect_back": cb,
+		// The sender's own count, which is what makes the receiver's count mean something.
+		"throughput": sess.ThroughputReports(),
+		// The receiver's count for upstream runs — same idea, other direction.
+		"throughput_up": upstreamJSON(sess),
 		"dns_canary":    dnsCanary,
 		// TODO(spec §6): http echo records
 	})
@@ -241,6 +250,12 @@ func (s *Server) actions(w http.ResponseWriter, r *http.Request) {
 		IntervalUs int    `json:"interval_us"`
 		SizesBytes []int  `json:"sizes_bytes"`
 		DF         *bool  `json:"df"`
+		Mode       string `json:"mode"`
+		FragBytes  int    `json:"frag_bytes"`
+		Direction  string `json:"direction"`
+		DurationS  int    `json:"duration_s"`
+		Kbps       int    `json:"kbps"`
+		Streams    int    `json:"streams"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "bad body"})
@@ -373,6 +388,105 @@ func (s *Server) actions(w http.ResponseWriter, r *http.Request) {
 			"grant": map[string]any{"max_bytes": g.MaxBytes, "max_kbps": g.MaxKbps},
 		})
 
+	case "frag_send":
+		if s.FragSend == nil {
+			writeJSON(w, http.StatusNotImplemented, map[string]string{
+				"error": "frag_send needs a raw socket, which this server does not have",
+			})
+			return
+		}
+		size := clamp(req.SizeBytes, 1600, 8000) // must exceed the path MTU or nothing fragments
+		mode := dataplane.FragMode(req.Mode)
+		switch mode {
+		case dataplane.FragInOrder, dataplane.FragReversed, dataplane.FragFirstLast:
+		default:
+			mode = dataplane.FragInOrder
+		}
+		fragBytes := clamp(req.FragBytes, 8, 1400)
+		g := sess.NewGrant(actionID, int64(size), 0, session.DefaultGrantLimits)
+		if g == nil {
+			writeJSON(w, http.StatusConflict, noDataPlaneYet)
+			return
+		}
+		// Synchronous: the whole burst is a few kB and at most a few hundred milliseconds, and
+		// the caller wants to know it was actually emitted before it starts listening. An
+		// asynchronous send would make "nothing arrived" ambiguous between a path drop and a
+		// send that never happened — the one distinction this test exists to make.
+		result, err := s.FragSend(sess, g, size, mode, fragBytes)
+		slog.Info("frag_send finished", "action", actionID, "mode", mode,
+			"size", size, "fragments", result.Fragments, "err", err)
+		if err != nil {
+			writeJSON(w, http.StatusConflict, map[string]any{
+				"error": err.Error(), "action_id": actionID, "result": result,
+			})
+			return
+		}
+		writeJSON(w, http.StatusAccepted, map[string]any{
+			"action_id": actionID, "mode": string(mode), "size_bytes": size,
+			"frag_bytes": fragBytes, "fragments": result.Fragments,
+			"grant": map[string]any{"max_bytes": g.MaxBytes, "max_kbps": g.MaxKbps},
+		})
+
+	case "throughput":
+		if s.DownThroughput == nil {
+			writeJSON(w, http.StatusNotImplemented, map[string]string{"error": "throughput not wired"})
+			return
+		}
+		// Only the downstream direction needs the server to send. Upstream is the client
+		// sending and the server counting, which needs no action at all — so asking for it here
+		// is a client bug worth naming rather than silently doing the other thing.
+		if req.Direction == "up" {
+			// Upstream needs nothing sent from here — the client generates the traffic and the
+			// server counts it. The only thing an action can usefully do is zero the counter so
+			// the run measures itself rather than inheriting an earlier one.
+			sess.ResetUpstream()
+			writeJSON(w, http.StatusAccepted, map[string]any{
+				"action_id": actionID, "direction": "up", "reset": true,
+				"note": "send TYPE_THROUGHPUT_UP packets, then read observations.throughput_up",
+			})
+			return
+		}
+		if req.Direction != "" && req.Direction != "down" {
+			writeJSON(w, http.StatusBadRequest, map[string]string{
+				"error": "direction must be up or down",
+			})
+			return
+		}
+		// Planned once, here, so the response promises exactly what the run will do. A request
+		// that would outlast the server's byte cap comes back with a shorter duration rather
+		// than being truncated halfway.
+		durationMs, kbps := dataplane.ThroughputPlan(
+			clamp(req.DurationS, 1, 30)*1000, clamp(req.Kbps, 100, 200_000))
+		size := clamp(req.SizeBytes, dataMinPacket, 1472)
+		if req.SizeBytes == 0 {
+			size = 1200
+		}
+		g := sess.NewGrant(actionID, 0, kbps, dataplane.ThroughputLimits(durationMs, kbps))
+		if g == nil {
+			writeJSON(w, http.StatusConflict, noDataPlaneYet)
+			return
+		}
+		// Answered before the run so the client can start listening, then reported through the
+		// observations API. Doing it the other way round would have the client miss the first
+		// second of a ten-second test.
+		writeJSON(w, http.StatusAccepted, map[string]any{
+			"action_id": actionID, "direction": "down",
+			"duration_s": durationMs / 1000, "duration_ms": durationMs,
+			"requested_duration_s": clamp(req.DurationS, 1, 30),
+			"kbps":                 kbps, "size_bytes": size,
+			"grant": map[string]any{"max_bytes": g.MaxBytes, "max_kbps": g.MaxKbps},
+		})
+		if f, ok := w.(http.Flusher); ok {
+			f.Flush()
+		}
+		go func() {
+			result, err := s.DownThroughput(sess, g, durationMs, kbps, size)
+			slog.Info("throughput finished", "action", actionID, "packets", result.Packets,
+				"bytes", result.Bytes, "kbps", result.Kbps, "limited_by", result.LimitedBy, "err", err)
+			sess.RecordThroughput(actionID, result.Packets, result.Bytes, result.DurationMs,
+				result.Kbps, result.LimitedBy)
+		}()
+
 	default:
 		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "unknown or unimplemented action"})
 	}
@@ -684,3 +798,13 @@ func (s *Server) EnrollmentLink(token string) string {
 		"&p=" + url.QueryEscape("pin-sha256:"+s.PinB64) +
 		"&t=" + url.QueryEscape(token)
 }
+
+// upstreamJSON renders the upstream tally with the derived figures already computed, so every
+// consumer does not have to repeat (and risk fumbling) the same arithmetic.
+func upstreamJSON(sess *session.Session) map[string]any {
+	u := sess.Upstream()
+	return map[string]any{
+		"packets": u.Packets, "bytes": u.Bytes,
+		"span_ms": u.SpanMs(), "kbps": u.Kbps(),
+	}
+}

server/internal/dataplane/frag_linux.go

@@ -0,0 +1,263 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build linux
+
+package dataplane
+
+import (
+	"encoding/binary"
+	"fmt"
+	"net/netip"
+	"sync/atomic"
+	"syscall"
+	"time"
+
+	"echo-lot.app/server/internal/session"
+)
+
+// Crafted IPv4 fragmentation (spec §5 frag_send).
+//
+// Letting the kernel fragment an oversized datagram — which is what big_send with df=false does —
+// answers one question: do fragments get through at all. It cannot answer the more interesting
+// one, because the kernel always emits fragments in order, first one first.
+//
+// The classic middlebox fault is precisely 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 later fragments against. Plenty of implementations drop them. Others hold them
+// briefly and reassemble; others leak. The difference is invisible to any test that sends
+// fragments in order, and it shows up in the real world as "large DNS answers fail on this
+// network" or "the VPN works until the MTU drops".
+//
+// So this builds the fragments by hand and controls their order and timing. That needs a raw
+// socket (CAP_NET_RAW); when we do not have one the capability is not advertised, rather than
+// advertised and failing later.
+
+// FragMode is how a fragmented datagram is put on the wire.
+type FragMode string
+
+const (
+	// FragInOrder is the baseline: first fragment first, as the kernel would. A path that fails
+	// this fails everything, and it tells the others apart from a path that drops all fragments.
+	FragInOrder FragMode = "in_order"
+	// FragReversed sends the last fragment first. This is the one that finds stateful devices
+	// which need the first fragment to build state.
+	FragReversed FragMode = "reversed"
+	// FragFirstLast holds the first fragment back until the others have arrived, which tests
+	// whether the path buffers non-first fragments at all and for how long.
+	FragFirstLast FragMode = "first_last"
+)
+
+var fragIPID atomic.Uint32
+
+// RawFragSupported reports whether crafted fragments can actually be sent here.
+//
+// Checked by opening the socket rather than by inspecting capabilities: the question is "will
+// this work", and a permission model has more ways to say no than a capability bit has to say yes
+// (user namespaces, seccomp, LSM). Advertising a capability we cannot deliver would turn a
+// missing feature into a failed measurement.
+func RawFragSupported() bool {
+	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_RAW)
+	if err != nil {
+		return false
+	}
+	_ = syscall.Close(fd)
+	return true
+}
+
+// FragResult is what happened to one crafted fragment burst.
+type FragResult struct {
+	Mode      FragMode `json:"mode"`
+	SizeBytes int      `json:"size_bytes"`
+	Fragments int      `json:"fragments"`
+	Sent      bool     `json:"sent"`
+	Err       string   `json:"err,omitempty"`
+}
+
+// FragSend emits one ELT1 packet of sizeBytes as hand-built IPv4 fragments, in the given order.
+//
+// The datagram is assembled whole and then cut up, so what the client reassembles — if it
+// reassembles — is a normal, HMAC-valid packet indistinguishable from any other. That matters:
+// the client must not be able to tell a crafted fragment burst from a kernel one, or it would be
+// measuring our sender rather than the path.
+func (s *Server) FragSend(
+	sess *session.Session, g *session.Grant, sizeBytes int, mode FragMode, fragSize int,
+) (FragResult, error) {
+	res := FragResult{Mode: mode, SizeBytes: sizeBytes}
+
+	target := sess.DataSource()
+	if !target.IsValid() {
+		return res, fmt.Errorf("no observed data-plane source")
+	}
+	if !target.Addr().Unmap().Is4() {
+		// IPv6 has no in-network fragmentation: only the source may fragment, via an extension
+		// header. Worth building, but it is a different mechanism and belongs in its own code
+		// path rather than pretending this one covers it.
+		return res, fmt.Errorf("crafted fragmentation is IPv4-only for now")
+	}
+	conn := s.connFor(target, sess.DataLocal())
+	if conn == nil {
+		return res, fmt.Errorf("no data-plane socket matches target family")
+	}
+	local := sess.DataLocal()
+	if !local.IsValid() {
+		return res, fmt.Errorf("session has no recorded local address")
+	}
+
+	if sizeBytes < HeaderSize+8 {
+		sizeBytes = HeaderSize + 8
+	}
+	if sizeBytes > 8000 {
+		sizeBytes = 8000
+	}
+	if !g.Allow(sizeBytes) {
+		return res, fmt.Errorf("grant exhausted")
+	}
+
+	// The ELT1 packet, signed exactly as any other, then wrapped in UDP.
+	payload := make([]byte, sizeBytes-HeaderSize)
+	binary.BigEndian.PutUint32(payload[0:4], uint32(sizeBytes))
+	copy(payload[4:], mode)
+	elt := s.buildPacket(sess, TypeFragData, 0, payload)
+
+	udp := buildUDP(local, target, elt)
+
+	// Fragment offsets are in 8-byte units, so every fragment except the last must be a multiple
+	// of 8. A payload that is not is not an error — it is a fragment that no host will reassemble.
+	if fragSize <= 0 {
+		fragSize = 576
+	}
+	fragSize = (fragSize / 8) * 8
+	if fragSize < 8 {
+		fragSize = 8
+	}
+
+	fragments := splitIPv4(local.Addr(), target.Addr(), udp, fragSize, uint16(fragIPID.Add(1)))
+	res.Fragments = len(fragments)
+
+	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_RAW)
+	if err != nil {
+		res.Err = err.Error()
+		return res, err
+	}
+	defer syscall.Close(fd)
+	if err := syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_HDRINCL, 1); err != nil {
+		res.Err = err.Error()
+		return res, err
+	}
+
+	dst := syscall.SockaddrInet4{}
+	copy(dst.Addr[:], target.Addr().Unmap().AsSlice())
+
+	send := func(pkt []byte) error { return syscall.Sendto(fd, pkt, 0, &dst) }
+
+	switch mode {
+	case FragReversed:
+		for i := len(fragments) - 1; i >= 0; i-- {
+			if err := send(fragments[i]); err != nil {
+				res.Err = err.Error()
+				return res, err
+			}
+			time.Sleep(time.Millisecond)
+		}
+	case FragFirstLast:
+		for i := 1; i < len(fragments); i++ {
+			if err := send(fragments[i]); err != nil {
+				res.Err = err.Error()
+				return res, err
+			}
+			time.Sleep(time.Millisecond)
+		}
+		// Long enough to be a real test of whether anything holds fragments, short enough to stay
+		// inside the usual 30-second reassembly timeout by a wide margin.
+		time.Sleep(250 * time.Millisecond)
+		if err := send(fragments[0]); err != nil {
+			res.Err = err.Error()
+			return res, err
+		}
+	default:
+		for _, f := range fragments {
+			if err := send(f); err != nil {
+				res.Err = err.Error()
+				return res, err
+			}
+			time.Sleep(time.Millisecond)
+		}
+	}
+	res.Sent = true
+	return res, nil
+}
+
+// buildUDP wraps a payload in a UDP header with a computed checksum.
+//
+// The checksum is optional in IPv4 and it would be less code to send zero, but a zero-checksum
+// datagram is dropped by some middleboxes — and that drop would be recorded as a fragmentation
+// failure, which is exactly the wrong conclusion.
+func buildUDP(src, dst netip.AddrPort, payload []byte) []byte {
+	out := make([]byte, 8+len(payload))
+	binary.BigEndian.PutUint16(out[0:2], src.Port())
+	binary.BigEndian.PutUint16(out[2:4], dst.Port())
+	binary.BigEndian.PutUint16(out[4:6], uint16(8+len(payload)))
+	copy(out[8:], payload)
+
+	// Pseudo-header + UDP header + data, per RFC 768.
+	var sum uint32
+	s4, d4 := src.Addr().Unmap().As4(), dst.Addr().Unmap().As4()
+	for _, b := range [][]byte{s4[:], d4[:]} {
+		sum += uint32(binary.BigEndian.Uint16(b[0:2]))
+		sum += uint32(binary.BigEndian.Uint16(b[2:4]))
+	}
+	sum += uint32(syscall.IPPROTO_UDP)
+	sum += uint32(len(out))
+	for i := 0; i+1 < len(out); i += 2 {
+		sum += uint32(binary.BigEndian.Uint16(out[i : i+2]))
+	}
+	if len(out)%2 == 1 {
+		sum += uint32(out[len(out)-1]) << 8
+	}
+	for sum>>16 != 0 {
+		sum = (sum & 0xFFFF) + (sum >> 16)
+	}
+	ck := ^uint16(sum)
+	if ck == 0 {
+		ck = 0xFFFF // 0 means "no checksum" in IPv4; the all-ones form is the same value
+	}
+	binary.BigEndian.PutUint16(out[6:8], ck)
+	return out
+}
+
+// splitIPv4 cuts a UDP datagram into IPv4 fragments of at most fragSize payload bytes each.
+//
+// Every fragment carries the same IP ID — that is what marks them as one datagram — and every one
+// but the last sets MF. The kernel fills in the header checksum and total length for us under
+// IP_HDRINCL (raw(7)); the ID it only fills when zero, which is why it is set explicitly here.
+func splitIPv4(src, dst netip.Addr, udp []byte, fragSize int, id uint16) [][]byte {
+	s4, d4 := src.Unmap().As4(), dst.Unmap().As4()
+	var out [][]byte
+	for off := 0; off < len(udp); off += fragSize {
+		end := off + fragSize
+		if end > len(udp) {
+			end = len(udp)
+		}
+		chunk := udp[off:end]
+		more := end < len(udp)
+
+		hdr := make([]byte, 20, 20+len(chunk))
+		hdr[0] = 0x45 // IPv4, 5 words of header
+		hdr[1] = 0    // DSCP/ECN
+		binary.BigEndian.PutUint16(hdr[2:4], uint16(20+len(chunk)))
+		binary.BigEndian.PutUint16(hdr[4:6], id)
+		flagsOff := uint16(off / 8)
+		if more {
+			flagsOff |= 0x2000 // MF
+		}
+		binary.BigEndian.PutUint16(hdr[6:8], flagsOff)
+		hdr[8] = 64 // TTL
+		hdr[9] = syscall.IPPROTO_UDP
+		// hdr[10:12] checksum left zero: the kernel computes it under IP_HDRINCL.
+		copy(hdr[12:16], s4[:])
+		copy(hdr[16:20], d4[:])
+		out = append(out, append(hdr, chunk...))
+	}
+	return out
+}

server/internal/dataplane/frag_linux_test.go

@@ -0,0 +1,160 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build linux
+
+package dataplane
+
+import (
+	"encoding/binary"
+	"net/netip"
+	"testing"
+)
+
+// Fragment headers are the kind of thing that is either exactly right or silently useless: a
+// wrong offset unit, a missing MF bit or a bad checksum produces packets that leave the machine
+// and are dropped by the receiver's IP stack without a word. Nothing downstream would notice —
+// the client would simply record "fragments do not get through", which is a wrong answer rather
+// than a missing one. Hence these check the bytes.
+
+func testAddrs() (netip.AddrPort, netip.AddrPort) {
+	return netip.MustParseAddrPort("192.0.2.1:8442"), netip.MustParseAddrPort("198.51.100.9:41000")
+}
+
+func TestSplitCoversThePayloadExactlyOnce(t *testing.T) {
+	src, dst := testAddrs()
+	udp := buildUDP(src, dst, make([]byte, 2000))
+
+	frags := splitIPv4(src.Addr(), dst.Addr(), udp, 576, 0x1234)
+	if len(frags) < 3 {
+		t.Fatalf("expected several fragments for %d bytes, got %d", len(udp), len(frags))
+	}
+
+	// Reassemble the way a receiver would: place each fragment's payload at its offset.
+	rebuilt := make([]byte, len(udp))
+	covered := make([]bool, len(udp))
+	for _, f := range frags {
+		flagsOff := binary.BigEndian.Uint16(f[6:8])
+		off := int(flagsOff&0x1FFF) * 8
+		body := f[20:]
+		if off+len(body) > len(udp) {
+			t.Fatalf("fragment at offset %d overruns the datagram", off)
+		}
+		for i, b := range body {
+			if covered[off+i] {
+				t.Fatalf("byte %d delivered twice", off+i)
+			}
+			covered[off+i] = true
+			rebuilt[off+i] = b
+		}
+	}
+	for i, c := range covered {
+		if !c {
+			t.Fatalf("byte %d was never sent", i)
+		}
+	}
+	for i := range udp {
+		if rebuilt[i] != udp[i] {
+			t.Fatalf("reassembled byte %d differs", i)
+		}
+	}
+}
+
+func TestFragmentHeadersAreWellFormed(t *testing.T) {
+	src, dst := testAddrs()
+	udp := buildUDP(src, dst, make([]byte, 3000))
+	frags := splitIPv4(src.Addr(), dst.Addr(), udp, 800, 0xBEEF)
+
+	for i, f := range frags {
+		if got := f[0]; got != 0x45 {
+			t.Errorf("fragment %d: version/IHL = %#x, want 0x45", i, got)
+		}
+		if got := f[9]; got != 17 {
+			t.Errorf("fragment %d: protocol = %d, want 17 (UDP)", i, got)
+		}
+		if got := binary.BigEndian.Uint16(f[4:6]); got != 0xBEEF {
+			t.Errorf("fragment %d: IP ID = %#x — all fragments of one datagram must share it", i, got)
+		}
+		if got := binary.BigEndian.Uint16(f[2:4]); int(got) != len(f) {
+			t.Errorf("fragment %d: total length = %d, actual %d", i, got, len(f))
+		}
+		flagsOff := binary.BigEndian.Uint16(f[6:8])
+		mf := flagsOff&0x2000 != 0
+		wantMF := i < len(frags)-1
+		if mf != wantMF {
+			t.Errorf("fragment %d: MF = %v, want %v", i, mf, wantMF)
+		}
+	}
+}
+
+// Offsets are counted in 8-byte units, so every fragment but the last must be a multiple of 8.
+// A 100-byte "fragment size" that silently becomes 100 bytes on the wire produces a datagram no
+// host will ever reassemble.
+func TestNonFinalFragmentsAreEightByteMultiples(t *testing.T) {
+	src, dst := testAddrs()
+	udp := buildUDP(src, dst, make([]byte, 2500))
+	for _, size := range []int{8, 100, 576, 999, 1400} {
+		frags := splitIPv4(src.Addr(), dst.Addr(), udp, (size/8)*8, 1)
+		for i, f := range frags[:len(frags)-1] {
+			if body := len(f) - 20; body%8 != 0 {
+				t.Errorf("size %d: non-final fragment %d carries %d bytes, not a multiple of 8",
+					size, i, body)
+			}
+		}
+	}
+}
+
+// The UDP checksum is optional in IPv4, and sending zero would be less code — but a
+// zero-checksum datagram is dropped by some middleboxes, and that drop would be recorded as a
+// fragmentation failure. So it must be present and correct.
+func TestUDPChecksumVerifies(t *testing.T) {
+	src, dst := testAddrs()
+	for _, n := range []int{0, 1, 7, 8, 100, 1001} { // odd lengths exercise the tail-byte path
+		udp := buildUDP(src, dst, make([]byte, n))
+		if got := binary.BigEndian.Uint16(udp[6:8]); got == 0 {
+			t.Fatalf("payload %d: checksum is zero, which means 'not computed'", n)
+		}
+		if sum := verifyUDPChecksum(src.Addr(), dst.Addr(), udp); sum != 0xFFFF {
+			t.Errorf("payload %d: checksum does not verify (one's complement sum %#x)", n, sum)
+		}
+		if got := binary.BigEndian.Uint16(udp[4:6]); int(got) != len(udp) {
+			t.Errorf("payload %d: UDP length field %d, actual %d", n, got, len(udp))
+		}
+	}
+}
+
+func TestUDPPortsComeFromTheSessionAddresses(t *testing.T) {
+	src, dst := testAddrs()
+	udp := buildUDP(src, dst, []byte("x"))
+	if got := binary.BigEndian.Uint16(udp[0:2]); got != src.Port() {
+		t.Errorf("source port = %d, want %d", got, src.Port())
+	}
+	// The destination port must be the client's observed source port, or the datagram arrives
+	// at the machine and is discarded before any socket sees it.
+	if got := binary.BigEndian.Uint16(udp[2:4]); got != dst.Port() {
+		t.Errorf("destination port = %d, want %d", got, dst.Port())
+	}
+}
+
+// Recomputes the one's complement sum over the pseudo-header and datagram; a correct checksum
+// makes the total 0xFFFF.
+func verifyUDPChecksum(src, dst netip.Addr, udp []byte) uint16 {
+	var sum uint32
+	s4, d4 := src.Unmap().As4(), dst.Unmap().As4()
+	for _, b := range [][]byte{s4[:], d4[:]} {
+		sum += uint32(binary.BigEndian.Uint16(b[0:2]))
+		sum += uint32(binary.BigEndian.Uint16(b[2:4]))
+	}
+	sum += 17
+	sum += uint32(len(udp))
+	for i := 0; i+1 < len(udp); i += 2 {
+		sum += uint32(binary.BigEndian.Uint16(udp[i : i+2]))
+	}
+	if len(udp)%2 == 1 {
+		sum += uint32(udp[len(udp)-1]) << 8
+	}
+	for sum>>16 != 0 {
+		sum = (sum & 0xFFFF) + (sum >> 16)
+	}
+	return uint16(sum)
+}

server/internal/dataplane/frag_other.go

@@ -0,0 +1,41 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build !linux
+
+package dataplane
+
+import (
+	"fmt"
+
+	"echo-lot.app/server/internal/session"
+)
+
+// Crafting IP fragments needs a raw socket and Linux's IP_HDRINCL semantics. Off Linux the
+// capability is simply not advertised, so a client never asks for it — better than answering
+// with a measurement we cannot actually make.
+
+type FragMode string
+
+const (
+	FragInOrder   FragMode = "in_order"
+	FragReversed  FragMode = "reversed"
+	FragFirstLast FragMode = "first_last"
+)
+
+type FragResult struct {
+	Mode      FragMode `json:"mode"`
+	SizeBytes int      `json:"size_bytes"`
+	Fragments int      `json:"fragments"`
+	Sent      bool     `json:"sent"`
+	Err       string   `json:"err,omitempty"`
+}
+
+func RawFragSupported() bool { return false }
+
+func (s *Server) FragSend(
+	sess *session.Session, g *session.Grant, sizeBytes int, mode FragMode, fragSize int,
+) (FragResult, error) {
+	return FragResult{Mode: mode, SizeBytes: sizeBytes},
+		fmt.Errorf("crafted fragmentation is only implemented on Linux")
+}

server/internal/dataplane/throughput.go

@@ -0,0 +1,187 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package dataplane
+
+import (
+	"encoding/binary"
+	"fmt"
+	"time"
+
+	"echo-lot.app/server/internal/session"
+)
+
+// Sustained-rate sending (spec §5 throughput).
+//
+// This is the most expensive thing the server will do on a client's say-so, so it is also the
+// action where the §3.4 anti-amplification rules matter most. Three bounds apply, and all three
+// are enforced here rather than trusted to the caller:
+//
+//   - the destination is the session's *observed* data-plane source, verified by an HMAC-signed
+//     ECHO that arrived from that address, so this cannot be aimed at a third party;
+//   - the grant carries a byte budget and an average-rate ceiling, and the send stops the moment
+//     either is reached;
+//   - the duration is hard-capped, so a client that vanishes mid-test costs a bounded amount of
+//     traffic rather than an open-ended one.
+//
+// The measurement this produces is honest only if the client is told which limit it hit. A run
+// that saturates the grant ceiling has measured *us*, not the network, and reporting that as
+// throughput would be worse than not measuring at all — see ThroughputResult.LimitedBy.
+
+// ThroughputResult is what the server actually managed to send.
+type ThroughputResult struct {
+	Packets    int   `json:"packets"`
+	Bytes      int64 `json:"bytes"`
+	DurationMs int64 `json:"duration_ms"`
+	Kbps       int   `json:"kbps"`
+	// LimitedBy says what stopped it: "duration" (ran the full time, so the rate is the path's
+	// or ours to give), "budget" (hit the grant's byte ceiling), or "rate" (the pacing ceiling
+	// held it back). Only "duration" makes the number a property of the network.
+	LimitedBy string `json:"limited_by"`
+}
+
+// ThroughputLimits derives a grant sized for one throughput run.
+//
+// The default 8 MiB action budget is deliberately far too small for this — ten seconds at
+// 50 Mbps is 62 MB — so throughput gets its own budget computed from what it asked for, still
+// clamped to a ceiling. Sizing the budget to the request (rather than raising the global default)
+// keeps every *other* action bounded at 8 MiB.
+func ThroughputLimits(durationMs, kbps int) session.GrantLimits {
+	durationMs, kbps = ThroughputPlan(durationMs, kbps)
+	// bytes = kbps * 1000 / 8 * seconds, with a little headroom so the byte budget is not what
+	// stops a run that was meant to be stopped by the clock.
+	budget := int64(kbps) * 1000 / 8 * int64(durationMs) / 1000
+	budget = budget * 11 / 10
+	if budget > maxThroughputBytes {
+		budget = maxThroughputBytes
+	}
+	return session.GrantLimits{
+		MaxBytes: budget,
+		// A little above the pacing target on purpose: the pacer should be what controls the
+		// rate, and the grant should be the safety net. If they are equal, ordinary scheduling
+		// jitter trips the grant and the run is cut short for no real reason.
+		MaxKbps: kbps * 12 / 10,
+		MaxHold: time.Duration(durationMs)*time.Millisecond + 5*time.Second,
+	}
+}
+
+// ThroughputPlan reduces a request to what this server will actually run, and is the single
+// place that decides it.
+//
+// When the byte cap binds before the clock does, the *duration* is shortened rather than the run
+// being cut off partway. Truncating mid-run is not wrong exactly — the rate is still computed
+// over the elapsed time and limited_by says "budget" — but it means promising a client thirty
+// seconds and giving it twenty-one. Saying "twenty-one seconds" up front is the same information
+// without the surprise, and it keeps "the clock ended the run" as the normal case, which is the
+// only case where the number is a clean property of the network.
+func ThroughputPlan(durationMs, kbps int) (effectiveMs, effectiveKbps int) {
+	if durationMs <= 0 {
+		durationMs = 10_000
+	}
+	if durationMs > maxThroughputMs {
+		durationMs = maxThroughputMs
+	}
+	if kbps <= 0 || kbps > maxThroughputKbps {
+		kbps = maxThroughputKbps
+	}
+	bytesPerMs := int64(kbps) * 1000 / 8 / 1000
+	if bytesPerMs > 0 {
+		if maxMs := maxThroughputBytes / bytesPerMs; int64(durationMs) > maxMs {
+			durationMs = int(maxMs)
+		}
+	}
+	return durationMs, kbps
+}
+
+const (
+	maxThroughputMs    = 30_000
+	maxThroughputKbps  = 200_000
+	maxThroughputBytes = 256 << 20
+)
+
+// DownThroughput sends paced traffic toward the client for up to durationMs.
+//
+// Pacing is deliberate rather than "send as fast as possible": an unpaced burst measures the
+// server's NIC and the first queue it meets, then collapses into loss that looks like a network
+// fault. Spacing packets at the target rate makes loss mean what a reader will assume it means.
+func (s *Server) DownThroughput(
+	sess *session.Session, g *session.Grant, durationMs, kbps, sizeBytes int,
+) (ThroughputResult, error) {
+	res := ThroughputResult{}
+
+	target := sess.DataSource()
+	if !target.IsValid() {
+		return res, fmt.Errorf("no observed data-plane source")
+	}
+	conn := s.connFor(target, sess.DataLocal())
+	if conn == nil {
+		return res, fmt.Errorf("no data-plane socket matches target family")
+	}
+
+	// Same plan the grant was sized from, so the two cannot disagree.
+	durationMs, kbps = ThroughputPlan(durationMs, kbps)
+	if sizeBytes < HeaderSize+16 {
+		sizeBytes = 1200 // a size that survives every common path unfragmented
+	}
+	if sizeBytes > 1472 {
+		sizeBytes = 1472
+	}
+
+	// Nanoseconds between packets to hit the target rate.
+	perPacketNs := int64(sizeBytes) * 8 * 1_000_000 / int64(kbps)
+	if perPacketNs < 1_000 {
+		perPacketNs = 1_000
+	}
+
+	payload := make([]byte, sizeBytes-HeaderSize)
+	deadline := time.Now().Add(time.Duration(durationMs) * time.Millisecond)
+	start := time.Now()
+	next := start
+
+	var seq uint32
+	for time.Now().Before(deadline) {
+		ok, why := g.TryAllow(sizeBytes)
+		if !ok {
+			if why == session.RefusalRate {
+				// Transient: the bucket is momentarily empty. Wait for the next slot and carry
+				// on. Ending the run here would report a rate measured over a fraction of a
+				// second, which is worse than reporting no rate at all.
+				res.LimitedBy = "rate"
+				time.Sleep(time.Duration(perPacketNs))
+				continue
+			}
+			// Terminal: the budget is spent, or the grant expired.
+			res.LimitedBy = why
+			break
+		}
+		// Reaching here means the run is progressing normally; the clock will end it.
+		res.LimitedBy = "duration"
+		binary.BigEndian.PutUint32(payload[0:4], seq)
+		binary.BigEndian.PutUint64(payload[4:12], uint64(time.Since(s.start).Nanoseconds()))
+		if err := s.sendErr(conn, target, sess, TypeThroughputData, seq, payload); err != nil {
+			// A send error mid-run is a local condition (buffer full, route gone). Stop and
+			// report what got out rather than pretending the rest was lost on the path.
+			res.LimitedBy = "send_error"
+			break
+		}
+		res.Packets++
+		res.Bytes += int64(sizeBytes)
+		seq++
+
+		// Absolute schedule, not sleep-per-packet: sleeping a fixed interval accumulates the
+		// scheduler's error and drifts the achieved rate below the target over a 10-second run.
+		next = next.Add(time.Duration(perPacketNs))
+		if d := time.Until(next); d > 0 {
+			time.Sleep(d)
+		}
+	}
+
+	elapsed := time.Since(start)
+	res.DurationMs = elapsed.Milliseconds()
+	// bits per millisecond is kilobits per second, so no scaling constant is needed - and none
+	// can be got wrong. Guarded because a run that ends inside a millisecond has no rate.
+	if res.DurationMs > 0 {
+		res.Kbps = int(res.Bytes * 8 / res.DurationMs)
+	}
+	return res, nil
+}

server/internal/dataplane/throughput_test.go

@@ -0,0 +1,99 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package dataplane
+
+import (
+	"testing"
+	"time"
+)
+
+// The grant has to be big enough that the *clock* ends a throughput run, not the byte budget. Get
+// this wrong and the test still "works": it stops early, reports a rate computed over a truncated
+// window, and nothing anywhere says the number is meaningless. So the sizing is pinned.
+func TestThroughputBudgetOutlastsTheRequestedRun(t *testing.T) {
+	cases := []struct{ durationMs, kbps int }{
+		{1_000, 1_000},
+		{10_000, 50_000},
+		{10_000, 200_000},
+		{30_000, 100_000},
+	}
+	for _, c := range cases {
+		// Against the *planned* duration, which is what will actually be run: a request the
+		// server shortens is answered with the shorter number, not truncated halfway.
+		planMs, planKbps := ThroughputPlan(c.durationMs, c.kbps)
+		lim := ThroughputLimits(c.durationMs, c.kbps)
+		needed := int64(planKbps) * 1000 / 8 * int64(planMs) / 1000
+		if lim.MaxBytes < needed {
+			t.Errorf("%d ms at %d kbps (planned %d ms) needs %d bytes, budget is %d - the run "+
+				"would stop early and report a rate over a truncated window",
+				c.durationMs, c.kbps, planMs, needed, lim.MaxBytes)
+		}
+	}
+}
+
+// The pacer should control the rate and the grant should be the safety net. If the grant's
+// ceiling equals the pacing target, ordinary scheduling jitter trips it and cuts the run short
+// for no real reason.
+func TestGrantRateCeilingSitsAboveThePacingTarget(t *testing.T) {
+	lim := ThroughputLimits(10_000, 50_000)
+	if lim.MaxKbps <= 50_000 {
+		t.Fatalf("grant ceiling %d kbps is not above the 50000 kbps pacing target", lim.MaxKbps)
+	}
+}
+
+// A client asking for more than the server will do must get the server's number, not its own.
+func TestThroughputRequestsAreClamped(t *testing.T) {
+	lim := ThroughputLimits(10*60*1000, 10_000_000) // ten minutes at 10 Gbps
+	if lim.MaxBytes > maxThroughputBytes {
+		t.Errorf("byte budget %d exceeds the hard cap %d", lim.MaxBytes, maxThroughputBytes)
+	}
+	if lim.MaxKbps > maxThroughputKbps*12/10 {
+		t.Errorf("rate ceiling %d exceeds the hard cap", lim.MaxKbps)
+	}
+	// The hold has to outlast the planned run, or the grant expires mid-send and the run is
+	// reported as rate-limited when it was really time-limited.
+	planMs, _ := ThroughputPlan(10*60*1000, 10_000_000)
+	if lim.MaxHold < time.Duration(planMs)*time.Millisecond {
+		t.Errorf("hold %v is shorter than the planned run of %d ms", lim.MaxHold, planMs)
+	}
+}
+
+// When the byte cap binds before the clock does, the server shortens the run and says so, rather
+// than accepting thirty seconds and delivering twenty-one. Same information, no surprise - and it
+// keeps "the clock ended the run" as the normal case, which is the only case where the resulting
+// rate is a clean property of the network.
+func TestAnOversizedRequestComesBackShorterRatherThanTruncated(t *testing.T) {
+	const kbps = 200_000
+	askedMs := 30_000
+	planMs, planKbps := ThroughputPlan(askedMs, kbps)
+
+	if planKbps != kbps {
+		t.Errorf("rate was reduced to %d; the duration should absorb the cap, not the rate", planKbps)
+	}
+	if planMs >= askedMs {
+		t.Fatalf("plan kept the full %d ms at %d kbps, which exceeds the %d byte cap",
+			askedMs, kbps, maxThroughputBytes)
+	}
+	// And what it does promise must fit.
+	if got := int64(planKbps) * 1000 / 8 * int64(planMs) / 1000; got > maxThroughputBytes {
+		t.Errorf("planned run needs %d bytes, over the %d cap", got, maxThroughputBytes)
+	}
+}
+
+// A short, ordinary request must come back untouched - the clamping only exists for the extremes.
+func TestAnOrdinaryRequestIsNotRewritten(t *testing.T) {
+	planMs, planKbps := ThroughputPlan(10_000, 50_000)
+	if planMs != 10_000 || planKbps != 50_000 {
+		t.Errorf("10 s at 50 Mbps was rewritten to %d ms at %d kbps", planMs, planKbps)
+	}
+}
+
+// Every action other than throughput stays on the small default budget. Throughput needs a big
+// one; raising the global default to suit it would quietly unbound everything else.
+func TestOnlyThroughputGetsTheLargeBudget(t *testing.T) {
+	big := ThroughputLimits(10_000, 50_000)
+	if big.MaxBytes <= 8<<20 {
+		t.Fatalf("throughput budget %d is no larger than the default action budget", big.MaxBytes)
+	}
+}

server/internal/dataplane/udp.go

@@ -35,6 +35,14 @@ const (
 	// Server->client under an asymmetric grant (spec §3.4/§5).
 	TypeDownTrainData = 0x06
 	TypeBigSend       = 0x0C
+	// TypeFragData is delivered only after IP reassembly, so its arrival IS the measurement.
+	TypeFragData = 0x0D
+	// TypeThroughputData is one packet of a sustained-rate downstream run.
+	TypeThroughputData = 0x0E
+	// TypeThroughputUp is one packet of a client-driven upstream run. The server counts it and
+	// deliberately does not answer: a reply would double the traffic and measure the return
+	// path at the same time, which is the one thing this test is trying not to do.
+	TypeThroughputUp = 0x0F
 )
 
 type Server struct {
@@ -148,6 +156,14 @@ func (s *Server) handle(conn *net.UDPConn, raddr netip.AddrPort, pkt []byte, tRx
 	if la, ok := conn.LocalAddr().(*net.UDPAddr); ok {
 		sess.NoteDataLocal(la.AddrPort())
 	}
+
+	// Upstream throughput short-circuits before the observation log. Recording one struct per
+	// packet here would mean tens of thousands of allocations for a single run; the counter is
+	// all anyone needs, since the client holds the send-side record.
+	if typ == TypeThroughputUp {
+		sess.CountUpstream(len(pkt), tRxNs)
+		return
+	}
 	sess.RecordUDP(session.UDPObservation{
 		Seq: seq, TRxNs: tRxNs, TTxNs: time.Since(s.start).Nanoseconds(),
 		Src: raddr.String(), Size: len(pkt), Type: typ,
@@ -232,6 +248,17 @@ func (s *Server) send(conn *net.UDPConn, raddr netip.AddrPort, sess *session.Ses
 // EMSGSIZE means our own egress MTU refused the datagram, which is a different fact from the
 // client not receiving it.
 func (s *Server) sendErr(conn *net.UDPConn, raddr netip.AddrPort, sess *session.Session, typ byte, seq uint32, payload []byte) error {
+	pkt := s.buildPacket(sess, typ, seq, payload)
+	_, err := conn.WriteToUDPAddrPort(pkt, raddr)
+	return err
+}
+
+// buildPacket assembles and signs an ELT1 packet without sending it.
+//
+// Split out for the crafted-fragment path, which needs the bytes so it can cut them up itself.
+// What arrives after reassembly must be indistinguishable from an ordinary packet, or the client
+// would be measuring our sender rather than the path — so it goes through exactly this function.
+func (s *Server) buildPacket(sess *session.Session, typ byte, seq uint32, payload []byte) []byte {
 	pkt := make([]byte, HeaderSize+len(payload))
 	copy(pkt[0:4], Magic)
 	pkt[4] = typ
@@ -247,8 +274,7 @@ func (s *Server) sendErr(conn *net.UDPConn, raddr netip.AddrPort, sess *session.
 	mac.Write(pkt[0:28])
 	mac.Write(payload)
 	copy(pkt[28:32], mac.Sum(nil)[:4])
-	_, err := conn.WriteToUDPAddrPort(pkt, raddr)
-	return err
+	return pkt
 }
 
 func hexByte(hi, lo byte) byte {

server/internal/session/grant.go

@@ -74,22 +74,64 @@ func (s *Session) NewGrant(actionID string, wantBytes int64, wantKbps int, lim G
 // enforces the byte ceiling, the expiry, and the average rate (by refusing early sends rather
 // than sleeping, so callers stay in control of pacing).
 func (g *Grant) Allow(n int) bool {
+	ok, _ := g.TryAllow(n)
+	return ok
+}
+
+// Refusal reasons from TryAllow. The distinction is not cosmetic: "too fast just now" is
+// transient and a caller should pace and carry on, while "budget" and "expired" are terminal and
+// a caller that keeps trying is only wasting its own run.
+const (
+	RefusalNone    = ""
+	RefusalBudget  = "budget"
+	RefusalExpired = "expired"
+	RefusalRate    = "rate"
+)
+
+// TryAllow reports whether n more bytes may be sent now, consuming the budget when they may, and
+// says why not when they may not.
+//
+// The rate limit is a token bucket: allowance = burst + rate x elapsed. An earlier version
+// exempted the first 50 ms from the check entirely, meaning to be lenient at startup. The effect
+// was the opposite - a sender could dump an unbounded burst into that window, and the moment the
+// check switched on it compared those bytes against 50 ms worth of allowance and refused
+// everything until real time caught up. A sustained send died about fifty milliseconds in, having
+// looked perfectly fine in every short test. A bucket has no such cliff: it is smooth from t=0.
+func (g *Grant) TryAllow(n int) (bool, string) {
 	g.mu.Lock()
 	defer g.mu.Unlock()
 	if time.Now().After(g.ExpiresAt) {
-		return false
+		return false, RefusalExpired
 	}
 	if g.sentBytes+int64(n) > g.MaxBytes {
-		return false
+		return false, RefusalBudget
 	}
-	// Average-rate check: bytes allowed so far = kbps/8 * elapsed_seconds.
+	// kbps -> bytes/s is kbps*1000/8 = kbps*125.
+	bytesPerSec := float64(g.MaxKbps) * 125
 	elapsed := time.Since(g.started).Seconds()
-	allowed := float64(g.MaxKbps) * 125 * elapsed // kbps -> bytes/s is kbps*1000/8 = kbps*125
-	if elapsed > 0.05 && float64(g.sentBytes+int64(n)) > allowed {
-		return false
+	allowed := burstBytes(bytesPerSec) + bytesPerSec*elapsed
+	if float64(g.sentBytes+int64(n)) > allowed {
+		return false, RefusalRate
 	}
 	g.sentBytes += int64(n)
-	return true
+	return true, RefusalNone
+}
+
+// burstBytes is the bucket's depth: 100 ms of the allowed rate, floored at a single ordinary
+// datagram.
+//
+// The floor exists only so that one packet is never refused outright by a very slow grant — it is
+// deliberately one datagram and not more. A generous floor would undo the rate ceiling at low
+// rates: at 8 kbps a 64 KB burst is sixty-four seconds' worth, which is exactly the instant dump
+// the ceiling is there to prevent. One datagram is 1.5 seconds' worth at that rate and nothing at
+// any realistic one.
+func burstBytes(bytesPerSec float64) float64 {
+	const oneDatagram = 1500
+	b := bytesPerSec * 0.1
+	if b < oneDatagram {
+		b = oneDatagram
+	}
+	return b
 }
 
 // Sent returns how many bytes this grant has consumed.

server/internal/session/grant_test.go

@@ -92,3 +92,67 @@ func TestGrantEnforcesRate(t *testing.T) {
 		t.Fatalf("rate limit let %d bytes through in ~60ms at 8kbps", sent)
 	}
 }
+
+// The bug this pins: the rate check used to exempt the first 50 ms entirely, so a sender could
+// dump an unbounded burst into that window and then be refused for as long as it took real time
+// to catch up. Every short test passed; a sustained send died about fifty milliseconds in. A
+// token bucket has no such cliff, and the property that matters is that a sender pacing *at* the
+// allowed rate is never refused for long.
+func TestSustainedSendAtTheAllowedRateIsNotCutOff(t *testing.T) {
+	s := sessionWithSource(t)
+	const kbps = 8000   // 1 MB/s
+	const packet = 1200 // bytes
+	g := s.NewGrant("a1", 8<<20, kbps, DefaultGrantLimits)
+
+	// Pace at the allowed rate for a short run and count how much got through. A correct
+	// limiter passes essentially all of it; the old one stopped almost immediately.
+	perPacket := time.Duration(float64(packet) / (float64(kbps) * 125) * float64(time.Second))
+	deadline := time.Now().Add(300 * time.Millisecond)
+	sent, refusals := 0, 0
+	for time.Now().Before(deadline) {
+		if ok, why := g.TryAllow(packet); ok {
+			sent += packet
+		} else if why == RefusalRate {
+			refusals++
+		} else {
+			t.Fatalf("unexpected terminal refusal %q after %d bytes", why, sent)
+		}
+		time.Sleep(perPacket)
+	}
+
+	// 300 ms at 1 MB/s is ~300 KB. Allow generous slack for scheduler granularity, but a run
+	// that delivered only a few packets means the limiter cut it off.
+	if sent < 100_000 {
+		t.Fatalf("a sender pacing at the allowed rate got only %d bytes through in 300ms "+
+			"(%d rate refusals) — the limiter is cutting off sustained sends", sent, refusals)
+	}
+}
+
+// The other half: a rate refusal must be distinguishable from a spent budget, because one is
+// transient and one is terminal, and a caller that cannot tell them apart either gives up early
+// or spins forever.
+func TestRefusalReasonsAreDistinguishable(t *testing.T) {
+	s := sessionWithSource(t)
+
+	// Budget: tiny ceiling, plenty of rate.
+	g := s.NewGrant("a1", 1000, 100_000, DefaultGrantLimits)
+	for i := 0; i < 20; i++ {
+		g.TryAllow(100)
+	}
+	if ok, why := g.TryAllow(100); ok || why != RefusalBudget {
+		t.Errorf("spent budget reported as ok=%v why=%q, want %q", ok, why, RefusalBudget)
+	}
+
+	// Rate: huge ceiling, minimal rate, so only the bucket can refuse.
+	g2 := s.NewGrant("a2", 1<<20, 8, DefaultGrantLimits)
+	sawRate := false
+	for i := 0; i < 100; i++ {
+		if ok, why := g2.TryAllow(1000); !ok && why == RefusalRate {
+			sawRate = true
+			break
+		}
+	}
+	if !sawRate {
+		t.Error("a sender far above the rate ceiling never got a rate refusal")
+	}
+}

server/internal/session/session.go

@@ -40,6 +40,8 @@ type Session struct {
 	packetsSeen uint64
 	udpObs      []UDPObservation // ring, newest last, cap obsCap
 	connectBack []ConnectBackResult
+	throughput  []ThroughputReport
+	upstream    UpstreamCounter
 }
 
 const obsCap = 4096
@@ -54,6 +56,48 @@ type UDPObservation struct {
 	Type  uint8  `json:"type"`
 }
 
+// ThroughputReport is the server's own account of a sustained send: what it managed to put on
+// the wire, and what stopped it. The client needs this to interpret its own count — the gap
+// between the two IS the loss, and without the sender's number a receiver can only guess.
+type ThroughputReport struct {
+	ActionID   string `json:"action_id"`
+	Packets    int    `json:"packets"`
+	Bytes      int64  `json:"bytes"`
+	DurationMs int64  `json:"duration_ms"`
+	Kbps       int    `json:"kbps"`
+	LimitedBy  string `json:"limited_by"`
+}
+
+// UpstreamCounter is the server's tally of a client-driven throughput run.
+//
+// Deliberately a counter and not a list. A five-second upstream run at 20 Mbps is around ten
+// thousand packets; one observation struct each would turn a measurement into an allocation
+// storm on a shared server, and nothing downstream needs the per-packet detail - the client
+// already has its own send record. The gap between the two counts IS the loss.
+type UpstreamCounter struct {
+	Packets   int   `json:"packets"`
+	Bytes     int64 `json:"bytes"`
+	FirstRxNs int64 `json:"first_rx_ns"`
+	LastRxNs  int64 `json:"last_rx_ns"`
+}
+
+// SpanMs is the time between the first and last packet, which is the interval the rate should be
+// computed over - not the client's requested duration, which includes ramp-up and the tail.
+func (u UpstreamCounter) SpanMs() int64 {
+	if u.Packets < 2 || u.LastRxNs <= u.FirstRxNs {
+		return 0
+	}
+	return (u.LastRxNs - u.FirstRxNs) / 1_000_000
+}
+
+// Kbps is bits per millisecond, which is kilobits per second - no scaling constant to get wrong.
+func (u UpstreamCounter) Kbps() int {
+	if ms := u.SpanMs(); ms > 0 {
+		return int(u.Bytes * 8 / ms)
+	}
+	return 0
+}
+
 // ConnectBackResult records one connect-back action outcome.
 type ConnectBackResult struct {
 	ActionID string  `json:"action_id"`
@@ -87,6 +131,57 @@ func (s *Session) Observations() (packetsSeen uint64, udp []UDPObservation, cb [
 		append([]ConnectBackResult(nil), s.connectBack...)
 }
 
+// CountUpstream tallies one client-sent throughput packet.
+//
+// Called on the hot path for every packet of an upstream run, so it does exactly two additions
+// and two comparisons under the lock and allocates nothing.
+func (s *Session) CountUpstream(sizeBytes int, tRxNs int64) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.upstream.Packets == 0 {
+		s.upstream.FirstRxNs = tRxNs
+	}
+	s.upstream.Packets++
+	s.upstream.Bytes += int64(sizeBytes)
+	s.upstream.LastRxNs = tRxNs
+}
+
+// Upstream returns the tally so far.
+func (s *Session) Upstream() UpstreamCounter {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.upstream
+}
+
+// ResetUpstream clears the tally, so a second run in one session measures itself rather than
+// inheriting the first one's packets.
+func (s *Session) ResetUpstream() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.upstream = UpstreamCounter{}
+}
+
+// RecordThroughput stores the server's account of one sustained send.
+//
+// Kept as a per-action summary rather than per-packet records: a ten-second run at 50 Mbps is
+// half a million packets, and holding one struct each would turn a measurement into a memory
+// exhaustion. The client has the per-packet view; the server only needs to say how many it sent.
+func (s *Session) RecordThroughput(actionID string, packets int, bytes, durationMs int64, kbps int, limitedBy string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.throughput = append(s.throughput, ThroughputReport{
+		ActionID: actionID, Packets: packets, Bytes: bytes,
+		DurationMs: durationMs, Kbps: kbps, LimitedBy: limitedBy,
+	})
+}
+
+// ThroughputReports returns the server's account of every sustained send in this session.
+func (s *Session) ThroughputReports() []ThroughputReport {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return append([]ThroughputReport(nil), s.throughput...)
+}
+
 // DataSource returns the last verified data-plane source (invalid when the
 // session has not sent data-plane traffic yet).
 func (s *Session) DataSource() netip.AddrPort {