07 load

Date: 2026-09-06. Repo commit: ab93833. Host: AMD Ryzen 9 5950X, 8 vCPUs allotted, 31GB RAM, Alpine 6.18.38-0-virt kernel, Docker 29.6.2. Images: interop-smppload:2.5.3-49fb653 (smppload cloned at commit 49fb653, tag 2.5.3, built with erlang:27.3.4.17-alpine + a fresh rebar3 3.27.0 release replacing the commit's own pre-OTP-27 vendored one), interop-dumbclient:de0334b (vponomarev/libsmpp cloned at commit de0334b, built with golang:1.26.8-alpine3.23 on alpine:3.23.5), nicolaka/netshoot:v0.16 (capture sidecar and tshark), node:24.18.0-bookworm-slim (test runner, from the root compose.yaml).

Setup

smppload: builds, binds, then puts a corrupted PDU on the wire - blocked

Issue #8 (rebar3/BEAM load errors) is real but resolved in minutes: the commit's own vendored ./rebar3 escript predates OTP 27 and fails to load under it (please re-compile this module with an Erlang/OTP 27 compiler). Replacing it with a fresh rebar3 3.27.0 release before make escriptize fixes the build outright - no further patching needed, and git:// dependency URLs in rebar.config resolved fine once rewritten to https:// (a one-line git config --global url.insteadOf).

The built escript is not usable against any SMSC, though: every bind_transceiver it sends is two octets short of what its own command_length declares. Captured with a raw tshark sidecar against ukarim/smscsim:0.2.0 (independent of both this library and smppload's own logging):

002a000000090000000000000001736d7070636c69656e74310070617373776f7264000050010100

40 octets on the wire, but command_length (the first 4 octets, if the PDU were whole) would need to read 0000002a (42) for a system_id "smppclient1" / password "password" bind - the actual first two octets of that field are simply missing, so the wire instead starts 002a0000 (2,752,512) with command_id and everything after shifted two octets early. tshark's own SMPP dissector does not recognise the stream as SMPP at all (-Y smpp matches zero frames, though the raw capture has the SYN/ACK/PSH/FIN sequence and the 40-octet data frame) - a second, independent confirmation this is not merely a framing quirk our own codec is stricter about.

Traced as far as oserl's smpp_pdu_syntax:pack/2 (the trx_deadlock_fix_1 branch smppload's rebar.config pins), which builds the header as plain 32-bit bit-syntax (<<Len:32, CmdId:32, 0:32, SeqNum:32>>) - correct on inspection, so the corruption happens somewhere between that call and the socket write, not chased further given the time-box. Reproduced identically on three separate runs (byte-for-byte). Recorded as blocked; smppload.test.ts keeps a live reproducer asserting what our server does when it receives it (refuses the stream as unframeable - see Scenarios) rather than removing the peer. smpp-dumb-client covers S9, and substitutes for S6 and (partially) S8 - see below.

smpp-dumb-client: builds and interoperates cleanly; its own window bookkeeping stalls under sustained load

No build friction. Two binaries from the same pinned source: smpp-dumb-client (unmodified) and smpp-dumb-client-noping (its two enquireSender() call sites in smpp.go commented out at build time), the second built because every load tool built for this phase sends enquire_link on its own otherwise (smpp-dumb-client every 10s, unconditionally, not configurable) and smppload - the one peer that genuinely never does - is blocked, leaving S6 with no peer at all otherwise.

One integration snag, not a build one: smpp.remote in config.yml is fed straight into net.ParseIP (hdr.go) with no DNS resolution at all, so the compose service name cannot appear there directly. Fixed in the entrypoint: every conf/*.yml carries a NODE_HOST placeholder, resolved with getent hosts and substituted into a writable copy before the real binary starts.

Four one-shot scenarios share dumbclient-w2000's network namespace (network_mode: "service:dumbclient-w2000") - they are pure outbound clients with nothing of their own listening, so the only shared cost is a source IP, and one capture sidecar sees all four conversations with node:2775 the same way compose.kannel.yaml's does for its four bearerbox variants.

The long soak (below) surfaced a peer-side limit worth designing around rather than fighting: with a fast, immediate-response handler and a window of 100 - nothing our server should ever have trouble draining - the peer's own reported in-flight count (GetTrackQueueSize, read from len(TrackTX)) gets stuck pinned at the window within the first minute, and its log fills with Expired TX packet lines (libsmpp's hardcoded, non-configurable 7000ms TX_MAX_TIMEOUT_MS) - throughput drops from ~500/s to a trickle of tens per second, gated by how many tracked entries individually cross that 7s mark each second rather than by real responses being matched. Our own server-side counters (arrived/answered/peakOutstanding, tracked independently in dumbclient.test.ts) stay in lockstep throughout with a low peak - see Scenarios - which places the stall entirely on the peer's own window bookkeeping, not on anything our server did or failed to do. The soak test was redesigned around this: bounded by wall-clock (5 minutes) rather than a target count, asserting the invariants that matter regardless of how much the peer's own bug lets through (every arrival answered, nothing duplicated, memory shape), and reporting whatever throughput was actually reached rather than requiring a specific one.

One test-harness bug found and fixed between the two runs below, not a library defect: the S6 test's first version attached its session.on('close', ...) listener lazily inside the test body, after already waiting on the S9 assertions (which can run for the better part of a minute) - by the time the S6 test ran, the idle session had already closed, and an EventEmitter never replays a past event to a listener added after it fired. Fixed by attaching every session's close listener at session-creation time, recording it in the same per-scenario stats every other assertion reads.

Two runs of ./interop-tests/run.py dumbclient. Run 1 (the original 300,000-count soak) surfaced both the peer's TX-tracking stall and the S6 harness bug above; run 2, after both fixes, is the one reported below. smppload.test.ts passed on every run it was given (three, across the investigation above); its one scenario needs no repeat - a second run reproduces the identical corrupted PDU, adding nothing.

dumbclient run 2: frames 111300, bind_transceiver 4/4, enquire_link 12 (enquire_link_resp 9 - the
                   capture stops moments after the test does, catching some requests before their
                   response), submit_sm 62441, submit_sm_resp 48830, malformed 0, expert errors 0
smppload:          frames 0 (tshark's own SMPP dissector does not recognise the corrupted stream at all)

submit_sm_resp reads lower than submit_sm in the capture for the same reason enquire_link_resp does - the sidecar is stopped right after the test file's own after() hook finishes, which is itself moments after the last response goes out, so a handful of writes land after the capture stops seeing them. Not a lost response: submit_sm (62441) matches the sum of every session's own arrived exactly, and every session's own answered matches its arrived too (see Scenarios) - both counted independently, in the server process, of anything on the wire.

Throughput and memory

dumb-w500 and dumb-w2000 (S9) both ran to their full 20,000-message count in ~44s each, concurrently, against a handler serialised to answer roughly one message every 2ms (SLOW_HANDLER_DELAY_MS) - peakOutstanding read exactly 500 and exactly 2000, the two configured windows, confirming the peer never let more than its own window ride at once.

The soak (fast, immediate-response handler; window 100) reached 22,440 submit_sm over its fixed 300s observation window - about 75/s, well under the peer's own configured rate: 500 and under what our server can sustain (see Setup: smpp-dumb-client's own TX-tracking bookkeeping is the ceiling here, not our server - peakOutstanding stayed at 25 throughout). Sampled every 5s across the whole run (69 samples over 340s, all four scenarios combined): rss first=170MiB, min=124MiB, max=306MiB (during the two window runs' backlog), last=125MiB, heapUsed at the last sample 15MiB - back below its own starting point once the backlog drained, not merely flat. No monotonic trend in either direction.

Scenarios (PLAN.md)

Id	Result	Evidence
S6 (idleTimeout, no peer ever pings)	pass	`dumbclient.test.ts` "S6 - idle peer..." - dropped at idleTimeout, `linkTimers - closing an idle peer` logged, no response past the one owed
S8 (throughput, long messages, receipts)	blocked (smppload) / partial substitute	smppload's own scenario is blocked - see Setup. The soak below gives a genuine submit_sm/s figure without long messages or receipts, which `smpp-dumb-client` does not support (`research/esme-clients-and-validators.md` section B) - and is itself capped well below what our server can sustain by the peer's own TX-tracking stall, also see Setup
S9 (bounded window)	pass	`dumbclient.test.ts` "S9 - bounded window..." - 20,000/20,000 answered on both window 500 and window 2000, in arrival order, no duplicate ids, `peakOutstanding` exactly 500 and exactly 2000
Backpressure at the server	pass	Same run: `peakOutstanding` 2000 exceeds `maxHeldMessages` (1000, session-options.ts) and the eviction warning fires; window 500 (`peakOutstanding` 500) never does; memory sampled before/after the window runs (170MiB before, 306MiB after, 125MiB once the soak's own run had also settled)
Long soak	pass (run once at the redesigned, wall-clock-bounded shape - see Setup)	`dumbclient.test.ts` "Long soak" - 22,440 arrived, 22,440 answered, 0 duplicates, 0 unanswered errors, `close()` drains with no error
smppload bind corruption (not in PLAN.md - found this phase)	blocked	`smppload.test.ts` - our server refuses the unreadable stream instead of hanging

Defects in @larvit/smpp

None found. smppload.test.ts's own scenario is smppload's defect, not ours: our server's reaction (refusing the stream as unframeable, per the decision in the root AGENTS.md, "A stream this library cannot frame...") is the documented behaviour working exactly as designed against a peer that never gets as far as a readable PDU. The soak's throughput ceiling is the peer's own defect (see Setup) - our own arrived/answered/peakOutstanding counters stayed clean throughout every run.

Peer quirks

smppload's bind_transceiver is corrupted on the wire - see Setup. Not chased past oserl's pack/2 (which is correct on inspection) given the time-box.
smpp-dumb-client's window bookkeeping stalls under sustained load, throttling its own throughput far below what a promptly-answering server can sustain - see Setup. Its enquire_link interval (10s once bound as an ESME) is also hardcoded (smpp.go, enquireSender(10)), not exposed through config.yml at all - the no-ping binary built for S6 patches the call site out rather than configuring it.
smpp.remote takes a literal IP, never a hostname (net.ParseIP, no DNS resolution) - see Setup.

Open questions

Whether smppload's bind corruption is in oserl's gen_esme_session/smpp_session send path (not reached, given the time-box) or something specific to this build's dependency versions.
Whether smpp-dumb-client's stall is a sequence-number correlation bug (a response failing to match its TrackTX entry, falling back to the 7s expiry) or something else in its own window accounting - not chased past the observation in Setup, given the time-box and that the fault is clearly on the peer's side (our own counters stayed clean throughout).

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