smpp-js/docs/decisions.md

# Decisions

The standing decisions for `@larvit/smpp`, grouped by what each one constrains. A decision is
written down only when it cannot be put better as a goal — [AGENTS.md](../AGENTS.md) carries that
rule and an index of the titles below.

## The public surface

- **`Session` is publicly constructible, which is what makes `SessionOptions` and `ReconnectOptions`
  public too.** Raised twice as a leak; it is not one. The collaborators `session.ts` delegates to
  (`Reassembler`, `PendingRequests`, `SendWindow`, `ReconnectLoop`, `LinkTimers`, `LinkGate`,
  `DlrMerger`, `PduTransport`, `submitSms`) stay unpublished so they can be reshaped.

- **`acceptsOptionalParams()` and `bindAllows()` are predicates, not chokepoints.** The library's own
  senders consult them; `session.send({ tlvs })` is passed through as written, because silently
  stripping a caller's explicit TLVs off a deliberately public low-level surface would be worse than
  sending them. Only `submit_sm`, `deliver_sm` and `data_sm` are policed by bind direction — the
  three the library dispatches by it, of which it sends the first two.

- **Both emitters re-declare their listener methods to accept a promise.** Maintainer's call,
  2026-08-27: `EventEmitter` types every listener as void-returning, so the
  `session.on('sms', async sms => …)` README documents reads as a misused promise in any strict
  consumer. `declare on: …` and its six siblings re-type the inherited methods to return `unknown`,
  which emits nothing and needs no cast; overriding them as real methods cannot work, because the
  `super.on()` call needs one. The cost is that a subclass can no longer reach those seven through
  `super` — re-declaring them the same way is its way out. `unknown` rather than
  `void | Promise<void>` because a listener may return anything: `session.on('close', () =>
  set.delete(session))` returns a boolean. This also settles what the drain can wait on: a listener's
  own promise would be the better completion signal, and reaching it needs `listeners()`, which
  cannot be re-declared the same way — Node types it invariantly enough that widening `void` to
  `unknown` is `TS2416`. Re-probed 2026-09-01; `sendResp()` stays the signal.

- **`PduRefusedError` is exported, and `sessionError` names it in the event's type.** Maintainer's
  call, 2026-09-05, from a product review: one event carries both a PDU the peer malformed and the
  session's own failure, and `instanceof` is the only way to separate them that hard rule 4 allows —
  without the class as a value an application is left string-matching `err.message`. Goal 8 is paid by
  exporting the discriminant and the struct it carries and nothing else: `PduHeader` is named because
  an application that logs or forwards a header wants a name for it, `PduRefusalReason` is not
  because `reason` is compared against string literals, and an accessor
  (`PduRefusedError['header']`) names either one where a signature wants it. The payload union
  enforces nothing — a subclass narrows out of `Error` either way — and is there so the event's own
  type names what to narrow to, which is also what makes it a half-truth if a second `Error` subclass
  ever reaches this event without joining it. Rejected: a `SessionError` alias for that union, a
  third name for a type that is structurally `Error`. Rejected: a separate `pduRefused` event, which
  splits the failure channel so an application that wants every failure listens twice and an existing
  listener silently stops seeing refusals. Rejected: coalescing or rate-limiting them, which re-opens
  the standing decision that `sessionError` carries every failure, never coalesced or suppressed —
  the filtering belongs where the application is, since only it knows which peer is routinely sloppy.
  Rejected: an error code on a plain `Error`, which reads back off an `unknown` property only through
  a cast and types nothing it carries. Accepted: a second copy of the package installed alongside
  this one defeats `instanceof`, where `err.name` still reads `PduRefusedError`.

- **`bitCount()`, `encodeMessage()` and `splitMessage()` keep their total signatures, because
  `EncodingName` is what keeps an alphabet with no codec away from them.** Maintainer's call,
  2026-09-09, from the architecture review of [#95](https://github.com/larvit/larvitsmpp/pull/95):
  all three index `encodings` by name and would throw on one it has no codec for, which hard rule 1
  forbids. That PR left no such name to pass — `encodings` is a `Record<EncodingName, Encoding>`, so
  every member of the union has a codec and one added without a codec, or without a segment budget,
  fails to compile in four places. What was missing is the door for a caller holding a name at
  runtime: `Object.hasOwn(encodings, x)` is the only test the published surface offered and it
  narrows nothing, so `isEncodingName()` is exported beside `isCommandName()` and `isErrorName()`,
  which serve their own tables that way. Rejected: a `Result` signature on all three, which costs
  every typed consumer a narrow forever — goal 8, and the tag is the last cheap chance to spend it —
  to guard a state the compiler refuses. Where the domain really is open the check is already there:
  `sendSms()` widens `encoding`, `messagingMode` and the two time options to `unknown` and refuses
  each by name, which is what a caller without types gets. `smppTime.encode()` is where that reasoning lands the other way and is recorded
  under [The wire](#the-wire): `Date | number | string` is not a closed set, so it is a `Result`.

- **A segment the SMSC took and named no id for is `undefined` in `smsIds`, not an empty string.**
  Maintainer's call, 2026-09-12: `paramText()` resolves an absent `message_id` and one a peer wrote
  empty to the same `''`, which `string[]` then presented as an id — taking `smsIds[0]`, or keying a
  correlation table by the array, compiled and then misbehaved, and one message's empty entry
  collides with another's. Telesign names an id for the first segment of a concatenated submit only,
  so it is a documented operator's shape rather than a hypothesis. Nothing else moves:
  `parseSegmentId('')` matched nothing, so `DlrMerger` already abandoned such a send and `undefined`
  reaches that same refusal. `expect()` takes the wider type rather than a filtered `string[]`
  because the arity is what `idNumbering()` refuses on: filtering `['a-1', undefined, 'a-3']` leaves
  a numbering that spells out a whole message, and merges one that was never whole. `dlrFromPdu()`
  reads an id through `nonEmptyText()`, so no receipt could ever have matched an empty entry — and
  that reading stays separate from this one rather than sharing a helper, since it must leave a
  Buffer-valued `receipted_message_id` unresolved for `messageType()` to read the PDU as unmarked.
  What settles it here is the resolved text rather than the parameter, because `writeParams()`
  substitutes the field's own default: a peer that omits `message_id` and one that writes it empty
  build the same octets, leaving a raw-parameter test nothing to tell apart. Rejected: keeping `''`
  and documenting it, which leaves the published type promising what the value does not keep — goal
  2, a wrong answer about what the peer named. Rejected: dropping the unnamed entries, which breaks
  the positional correspondence with `pduObjs` that README promises and loses which segment a PDU
  belongs to. Rejected: `{ id?: string; pduObj: PduObject }[]`, which makes that positional promise
  structural where today the compiler cannot check it; deferred to the next breaking release, the
  first place two documented fields may become one. Accepted: every consumer reading `smsIds`
  narrows, including the majority whose SMSC names every id; indexing narrows too, except for the
  consumer who sets `noUncheckedIndexedAccess`, which typed `smsIds[0]` as `string | undefined`
  already.

## The wire

- **The declared interface version is an option on both `client()` and `server()`, and is not the
  optional-parameter threshold.** That threshold is fixed at 0x34 by the spec, so an implementation
  that must declare 5.0 throughout can, without moving it.

- **A peer that declared no version is pre-3.4, and `undefined` means no bind yet.** `acceptBind()`
  records what the ESME declared and the client's `bind()` records the `sc_interface_version` the
  SMSC answered with; a peer that declared nothing is recorded as `undeclaredInterfaceVersion` (0x00)
  and sent no optional parameters, which is how the spec reads an absent `sc_interface_version`.

- **`esm_class` decides what a `deliver_sm` is, and the body is read only when it names nothing.**
  The two types the MC writes about a message we submitted — `MC_DELIVERY_RECEIPT` (0x04) and
  `INTERMEDIATE_DELIVERY` (0x20) — are reports whatever the body parses to, so one in a format
  `dlrFromPdu()` cannot read reaches `dlr` with `smsId` undefined instead of arriving as an inbound
  SMS. The three the far-end SME writes (0x08, 0x10, 0x18) are messages and their bodies are not
  scraped: Kannel reads 0x08 as report-bearing and this does not, because a delivery acknowledgement
  is the handset's word about a message, not the network's. A message type of 0 or one of the ten
  reserved keeps the scrape, and a non-empty `receipted_message_id` TLV marks a report on the same
  footing. A report this library recognises never reaches the reassembler, so an SMSC that splits one
  across segments gets a `dlr` per segment rather than one merged report. The `message_state` TLV is
  authoritative only where it names a state in the table — SMPP reserves 0x80-0xFF for
  MC-vendor-specific values, so an unnameable one keeps its raw `statusId` and leaves `statusMsg` to
  the body.

- **A body is read from `message_payload` where `short_message` carries none, and `short_message`
  wins where a peer filled both.** Maintainer's call, 2026-09-06, from the Jasmin interoperability
  phase: SMPP 3.4 5.3.2.32 makes the TLV the alternative for a body the mandatory field cannot
  carry, several SMSCs use it, and Jasmin relays one faithfully — reading `short_message` alone
  handed the application an empty message
  ([interop-tests/findings/03-jasmin.md](../interop-tests/findings/03-jasmin.md)). `messageOctets()` is
  the single answer to where a body is, so the message path, the reassembler and `dlrFromPdu()`
  cannot disagree about it, and `esm_class` still says whether that body starts with a UDH wherever
  it was carried, which leaves concatenation reading exactly as before. Filling both contradicts the
  spec's own instruction to leave `sm_length` zero, and taking the mandatory field there keeps the
  rule purely additive: no PDU that parsed before reads differently now. Rejected: preferring the
  TLV, which re-reads every message a peer echoes into both. Rejected: refusing a PDU carrying both,
  which discards a message that is almost certainly present twice over, where goal 3 keeps the
  traffic.

- **A segment's concatenation is read from its UDH, or from the `sar_*` TLVs where it declares none,
  and each spelling groups in a reference space of its own.** Maintainer's call, 2026-09-06, from
  the Jasmin and Java-client interoperability phases: SMPP 3.4 5.3.2.31-5.3.2.33 make
  `sar_msg_ref_num`/`sar_total_segments`/`sar_segment_seqnum` the other way to say what a UDH says,
  Jasmin documents it as its own segmentation and jsmpp writes it, and reading the UDH alone handed
  the application one `sms` per fragment
  ([interop-tests/findings/05-java-clients.md](../interop-tests/findings/05-java-clients.md)).
  `concatOf()` is the single answer to how a PDU says it is a segment, as `messageOctets()` is to
  where a body is, and both are exported for the same reason: an application on the low-level
  surfaces would otherwise rewrite the read this fixed. It carries the spelling beside the
  reference, so the key is two tokens the reassembler joins and interprets neither of, and so the
  refusal can name the field the peer got wrong — `ESME_RINVESMCLASS` for a UDH, `ESME_RINVTLVVAL`
  for the TLVs, whose segment's `esm_class` is 0x00 and correct. Keying them together instead would
  assemble two of a peer's messages into one, since a UDH reference is 8 bits and `sar_msg_ref_num`
  is 16 and neither counts the other's messages; the two UDH widths share a space because they are
  one sender's counter in one layer, where a `sar_*` reference is another layer's. A UDH that names
  the concatenation wins over the TLVs — one carrying only a port leaves them to say — which keeps
  the change additive for every message that reassembled before, and leaves the library nothing to
  guess where the two disagree. Rejected: preferring the TLVs, which regroups every message a
  gateway derived them from. Rejected: comparing the parts and reporting a disagreement: the
  references are not comparable at all, and where the parts are, the UDH is still what the message
  is assembled by, so the report would name a failure the application cannot act on. Accepted: a
  peer that switches spelling mid-message now has two groups that expire rather than fragments that
  arrive, which goal 2 prefers to a message assembled from two counters. Receive-only: `sendSms()`
  goes on writing a UDH with an 8-bit reference, where a send-side `sar_*` would be a second
  spelling of one message whose only difference is which peers accept it.

- **`sendSms()` takes the messaging mode by name, and it is the only part of `esm_class` a caller
  writes.** Maintainer's call, 2026-09-06, closing target 5 of the interoperability plan: every peer
  the suite ran took the 0x40 this library sends on a concatenated segment, but Route Mobile and
  Kaleyra both document `esm_class` 0x43 for one, and a caller facing either had to hand-build every
  segment through `send()` — giving up the split, the per-segment ids, the send window and the
  receipt merge, which is what goal 8 means by beating "the application can do this itself". The four
  modes of SMPP 3.4 5.2.12 are a `MESSAGING_MODE` constant group and the option takes one of their
  names, so 0x43 is a composition this library makes rather than a value a caller states, and the UDH
  indicator a segment carrying a header needs cannot be cleared by anything the option can express.
  It takes three of those four: 2.10.3 carries transaction mode on `data_sm` alone, and none goes out
  of here, so `FORWARD` stays in the group that mirrors the spec table and `sendSms()` refuses it by
  that reason rather than as an unknown name — a mode this library cannot deliver is a promise goal 8
  will not let it make. `DATAGRAM` with `dlr: true` is refused on the same footing: 2.10.2 defines the
  report away, so arming `DlrMerger` for one is goal 2's wrong answer, where the mode alone and a
  report under any other mode both go out untouched. Those three names left `ESM_CLASS`, where they
  had `constsById.ESM_CLASS` read 0x03 as a whole `esm_class`. Rejected: a raw `esmClass` number,
  which is exactly that clearable state and would need refusing bit by bit to be safe. Rejected:
  taking a number beside a name, two spellings of one goal — which is why a value naming no mode is
  refused, by name, before a segment goes out. Rejected: a session-level default with a per-send
  override; an operator's requirement is a property of the link, but the library can verify nothing
  the caller's own options object does not, and shipping both buys a precedence rule to document and
  test for that. `SMSC_DEFAULT` is named so pinning the default deliberately is sayable.

- **An inbound `data_sm` stands in for whichever of `submit_sm` and `deliver_sm` its direction makes
  it, and none goes out.** Maintainer's call, 2026-09-06, from the Jasmin interoperability phase:
  SMPP 3.4 4.7.1 makes it a peer of both that always carries its body in `message_payload`, and
  Jasmin's `[dlr-thrower] dlr_pdu = data_sm` throws real receipts on it, which `ESME_RINVCMDID`
  dropped with nothing reported to the application at all. Every command but this one names its own
  direction, which is why the bind gate and the dispatch never had to be told which end of the link
  they are on; `linkEnd` is that fact, and it decides both. At the ESME end an inbound one is a
  delivery, so `esm_class` classifies it as it classifies a `deliver_sm`; at the SMSC end it is a
  submission and is read as one, because a report about a message this end never sent is goal 2's
  wrong answer whatever `esm_class` a peer wrote on it. A concatenated one is answered segment by
  segment either way, and 4.7.2 gives `data_sm_resp` a `message_id` where 4.6.2 leaves
  `deliver_sm_resp`'s unused, so the answer carries one. `linkEnd` is a field beside `boundAs`
  rather than a `SessionOptions` entry, so the code that knows which end this is writes it and
  nothing else can contradict what the session then binds as. Rejected: grouping the command with
  `deliver_sm` in the gate, which refuses a transmitter-bound ESME's legitimate submission, and with
  `submit_sm`, which refuses the receiver-bound delivery this was fixed for. Rejected: sending one —
  `send()` reaches the command raw, and an option choosing which command a message goes out on would
  be a second spelling of `sendSms()` whose only difference is which peers accept it.

- **A receipt's body is read as octets, and its own `data_coding` never says how.** Maintainer's
  call, 2026-09-05 via the SMPPSim interop run: SMPPSim copies the reported message's `data_coding`
  onto a receipt whose body it always writes as plain text, and Melrose Labs documents the same
  echo, so decoding by that field turns an Appendix B receipt into UCS-2 garbage — total loss
  against the many peers that send no TLVs to fall back on. `dlrFromPdu()` reads
  `PduObject.shortMessageOctets` through Latin-1, the one codec that maps every octet to a
  character, so the fixed fields parse whatever the PDU claims; the codec keeps both spellings
  because a message needs the text and a receipt needs the octets. Rejected: honouring `data_coding`
  where the octets yield no field, which reads one body two ways for the sake of a peer writing a
  UCS-2 receipt body that no researched SMSC is — that peer's receipt yields no fields at all here,
  which goal 2 reports as undetermined rather than guessed. An inbound message is untouched: nothing
  but `data_coding` can say how a message was written.

- **A message class is read where GSM 03.38 puts it, `flash` is class 0 alone, and a flash message
  with no alphabet to carry it is refused.** Maintainer's call, 2026-09-09, closing the last target
  of the interoperability plan: `sms.flash` was `(data_coding & 0xF0) === 0x10`, which called the
  ME-, SIM- and TE-specific classes immediate display and missed the 0xF0 group entirely — the only
  one SMPP 3.4 5.2.19 names, since it marks 0x0F to 0xBF reserved and hands 0xF0 to 0xFF to GSM
  03.38, and the one SMPPSim demonstrated
  ([interop-tests/findings/02-smppsim.md](../interop-tests/findings/02-smppsim.md), C17).
  `messageClassOf()` is the single answer to whether a `data_coding` carries a class and which, as
  `concatOf()` is to how a PDU says it is a segment: 03.38 section 4 puts the class in bits 1-0,
  carried where bit 4 says so in every group below 0x80 and always in the 0xF0 group, and
  `encodingByDataCoding()` reads the alphabet off that same test rather than repeating the group
  masks beside it. It is exported for the reason `concatOf()` is — an application that needs a class
  other than 0 would otherwise rewrite the read this fixed. Rejected: a `messageClass` field on the
  `sms` event, which pays goal 8 for three classes nothing here acts on, where the boolean the
  application already had covers the one it does. Compressed text is out of scope and stays out —
  nothing here implements 3GPP TS 23.042, so a compressed body reaches the application as whatever
  its declared alphabet makes of it — but bit 5 does not move the class bits, so 0x30 is read as
  class 0 rather than special-cased into a wrong answer; 01xx is read for the same reason, 03.38
  coding it exactly as 00xx. Rejected: reading only the two groups the defect named, which needs an
  extra test to produce a wrong answer for a class the spec puts in plain sight. Accepted: the
  alphabet is read only where a class is, so 0x58 is UCS2 while 0x48 — the same alphabet with the
  class bit clear — stays ASCII, because below 0x10 SMPP's flat table contradicts 03.38 and wins
  (0x03 is Latin-1 there, GSM 7-bit here) and a class is the only evidence a peer below 0x80 is
  spelling 03.38 at all. Send-side: `flash`
  is that class, so it goes out as 0x18 beside UCS2 and 0x10 beside GSM 7-bit, while
  `encoding: 'LATIN1'` beside it is refused before a segment goes out, the way a messaging mode this
  library cannot deliver is — 03.38's class groups hold GSM 7-bit, 8-bit data and UCS2, and Latin-1 is
  SMPP's own flat-table alphabet, so the pair has no spelling. Rejected: 0x10 with Latin-1 octets,
  which declares an alphabet the body is not in; rejected: 0x14, 8-bit data, which is not text to
  the handset that would display it; rejected: promoting it to UCS2, which overrides the one option
  the caller wrote in order to override a choice. Rejected with them: `encoding: 'FLASH'`, which
  named `data_coding` 0x10 among the alphabets and so reached the class through the option that
  chooses a charset — a second spelling of `flash: true` that also flattened every non-GSM character
  to a space on the way. It leaves `EncodingName`, which is now exactly the three codecs `detect()`
  and `encodingByDataCoding()` return, and `encoding` is checked by name like `messagingMode` so a
  caller without types gets a refusal rather than a throw out of the codec table. `consts.ENCODING`
  keeps its `FLASH` entry: the low-level surface reaches raw constants, and nothing reads that group
  as an alphabet any more. Accepted: `flash` is now false for `data_coding` 0x11 to 0x13, which no
  peer means as immediate display.

- **A report is final unless its `esm_class` or its state says otherwise, and only `ENROUTE` and
  `SCHEDULED` say otherwise.** SMPP 3.4 Appendix B lists every other receipt state as final,
  `UNKNOWN` and `ACCEPTED` included, so a peer writing `ACCEPTD` for a carrier-accepted step is taken
  at its word. Rejected: reading `UNKNOWN` as non-final, which leaves a peer whose receipt body this
  library cannot read with no `messageDlr` at all — goal 2 wants that reported as undetermined, not
  withheld. Both spellings resolve into `Dlr.intermediate` at the boundary rather than being read a
  second time in `DlrMerger`, so the library cannot answer the application one way and conclude the
  other. Not every peer marks a transient report 0x20 — an ordinary receipt carrying `stat:ENROUTE`
  is common — so the state test is what the marker test cannot replace. `message_state` 0 is 5.0's
  `SCHEDULED` and undefined in 3.4; a peer that writes it is read as transient rather than as saying
  nothing, maintainer's call, 2026-09-03, since the codec refuses a zero-length integer TLV and so an
  absent one cannot land there.

- **A `stat:` an operator spells outside Appendix B is read as the state it names, and the two
  researched ones are `FAILED` and CM.com's `DELIVERD`.** Maintainer's call, 2026-09-08, from the
  operator-fixture phase: Kaleyra and Route Mobile both document `FAILED` in that field as a terminal
  delivery failure, and the research attributes it to Vonage as well; CM.com's own code table prints
  `DELIVERD` — eight characters — beside six correct ones. Both were left at `statusMsg: UNKNOWN` —
  the same answer a receipt really saying `stat:UNKNOWN` gets, so an application could not tell an
  operator's "it failed" from its "I do not know", nor a delivered message from one whose state
  could not be read; and `DlrMerger` ranks `UNKNOWN` below `EXPIRED`, reporting a multipart send
  carrying a failed segment as expired. They join `receiptStates` alone: `receiptCodes` goes on
  writing the seven characters 3.4 defines, so nothing this library sends gains either spelling. Rejected: a `FAILED` member of `MESSAGE_STATE`, which
  is 3.4's own numbered table — the code has no number there, so one would have to be invented, and
  every consumer's switch would grow a case no `message_state` TLV can carry. Rejected: leaving it
  `UNKNOWN` and sending the application to `dlr.receipt.stat` for the state, which reports a terminal
  failure as undetermined and leaves the merge ranking it below `EXPIRED`. Rejected: reading the
  numeric status tables Syniverse and Route Mobile publish beside it, which are vendor fields of
  their own rather than the seven characters `stat:` holds. Accepted: all three of those operators
  document `FAILED` and `UNDELIV` as separate codes, and both now resolve to `UNDELIVERABLE` — an
  application that must tell them apart reads `dlr.receipt.stat`, which carries what the SMSC wrote.
  Accepted: an unmarked `deliver_sm` whose body says one of them now reaches the application as a
  report where it used to arrive as an inbound message, which is what every code already in the table
  does. What decides a spelling is whether the corpus in `test/operator-receipts.test.ts` can cite the
  page it is printed on and no other code could be meant, which is why `DELIVERD` is read and a
  spelling nobody publishes is not: a mapping that costs nothing where an operator's own docs merely
  contain a typo saves an application everything where they do not.

- **A transient state goes out as an intermediate delivery notification (0x20), every other state as
  a delivery receipt (0x04).** Appendix B makes a receipt's `stat` the message's final status, so
  0x04 over `ENROUTE` emits the two disagreeing spellings of finality the reading side above has to
  reconcile, and goal 3 has our own senders write the marker 3.4 defines. `sendDlr()` takes the list
  from `transientStates` in `dlr.ts`, the same one the reader uses, so the two cannot drift.
  Rejected: 0x04 for every state, for the sake of a peer that classifies on the marker — the cost
  accepted here is that such a peer stops recognising a transient report as a report at all and hands
  its application receipt text as an inbound message, where under 0x04 it would have read the state
  from `stat:` and been right. A transient state also carries `err:000`, since a message still on its
  way has not failed.

- **A refused PDU is answered from its header, and any 32-bit `sequence_number` is echoed as it
  arrived.** Maintainer's call, 2026-09-05 via the interop plan. The header of a framed PDU always
  parses, so it carries the answer SMPP 3.4 4.3 asks for, with the status 3.4 names for the part
  that would not parse. Rejected: nacking a refused *response*, whose sequence number is one of
  ours — the `generic_nack` would land in the peer's own numbering and nack a request of the peer's
  we never saw, so a refused response is written back nothing and settles the request it names
  instead. An unknown command id with the response bit set takes that branch too: a peer echoing a
  sequence number of ours is answering something, and settling it reaches the undetermined outcome
  `responseTimeout` would have reached anyway, sooner. Rejected: clamping a sequence number outside 4.7.1's 0x00000001–0x7FFFFFFF into range
  before answering, which correlates with nothing at the peer — stacks write the field as a plain
  uint32 (ukarim/smscsim signs every unprompted `deliver_sm` with a raw `rand.Int()`), so goal 3
  keeps that traffic and `PendingRequests.nextSeqNr()`, the only thing that invents one, is what
  holds our own sends inside the spec.

- **The optional parameters run to `command_length` exactly, and the only slack tolerated is one
  NULL octet where a peer padded `short_message`.** Maintainer's call, 2026-09-06, from the
  Java-client interoperability phase: accepting any parse that merely did not error answered
  `ESME_ROK` to a `deliver_sm` whose three trailing octets were never read, dropping the
  `receipted_message_id` that makes a receipt a receipt
  ([interop-tests/findings/05-java-clients.md](../interop-tests/findings/05-java-clients.md)). Goal 2
  settles it against goal 3: octets this codec cannot name are a PDU it did not read, so a region
  that does not end on `command_length` — the padded read included — is refused with the `tlvs`
  reason and `ESME_RINVTLVSTREAM` a truncated TLV value already gets. What the rule costs is paid
  once, in `readCstring()`: a trailing C-Octet String a peer left out entirely consumes no octet,
  where reporting the terminator it never sent puts every later offset past the declared end and
  refuses a bind, and every bodyless response, that used to parse. That composes, so a run of them
  at the tail all read empty — `outbind` is the only command with two, and an absent field and an
  empty one say the same thing, so goal 2 is not at stake even there. Rejected: keeping the tolerance
  for the one to three trailing octets too few to hold a TLV header, which no researched peer sends
  and which cannot be told apart from the truncated tail this fixes. Rejected: refusing it as
  `body`/`ESME_RINVCMDLEN`, which names the mandatory fields — the part the peer got right.

- **`smsIdFormat` names a notation per place, and normalisation never reaches inside a `<base>-<n>`
  id.** An SMSC may answer `submit_sm_resp` in hex and write the receipt's `id:` in decimal, so one
  transform over both sides cannot make them equal. `submitResp` covers the `receipted_message_id`
  TLV too, which SMPP 3.4 5.3.2.26 defines as the id the `submit_sm_resp` carried: naming one
  notation for whichever id a receipt yields would break the peer that sends both. Omitting a place
  is what leaving it alone means, so there is no `raw` notation, and a caller-supplied formatter is
  refused because it would make the promise that the two ids are comparable unverifiable — `onRequest`
  and the PDU on the `dlr` event are the escape hatches. A `<base>-<n>` id parses as no number and so
  reaches `expect()` and `collect()` unchanged, which is what keeps `DlrMerger` working; normalising
  the base instead would break that pair. The option is on `client()` only, since a `server()` session
  writes both ids itself.

- **A concatenated segment is budgeted at 134 octets, which is 153 septets where the SMSC packs them
  and 134 octets of anything it does not.** Maintainer's call, 2026-09-09, from the architecture
  review of [#95](https://github.com/larvit/larvitsmpp/pull/95): `segmentUnits` handed 153 to
  everything but UCS2, so a long `encoding: 'LATIN1'` message went out as segments of 153 octets plus
  a 6-octet UDH — 159 on the air where GSM 03.40 carries 140, which no SMSC can deliver. Goal 1 owns
  it. There is one budget, 140 less the UDH, and the alphabet decides only what it is counted in, so
  Latin-1 and UCS2 both take those 134 octets — 134 characters and 67 — and it is GSM 7-bit's 153
  that is the odd number rather than the other way round. `Record<EncodingName, number>` is what makes
  a fourth alphabet state its own. Rejected: 134 for GSM 7-bit too, which is the mistake
  [GSM 7-bit is sent unpacked](../AGENTS.md#gsm-7-bit-is-sent-unpacked) exists to stop. Accepted: a Latin-1 message past the 140 characters
  one SMS holds now costs more segments than it did, and `smsIds` is that much longer.

- **An alphabet the caller named has to carry the message, and a time the format cannot express is
  refused, both before a segment goes out.** Maintainer's call, 2026-09-09, from the architecture and
  stability reviews of [#96](https://github.com/larvit/larvitsmpp/pull/96): `encoding: 'LATIN1'` on
  `あいう` put `42 44 46` — `"BDF"` — on the wire and returned success, `encoding: 'ASCII'`
  flattened every character outside 03.38 to a space, and `validityPeriod: new Date('nope')` wrote
  `NaNNaNNaNNaNNaNNaNNaN00+` into the PDU. Goal 2 owns all three: bytes that do not say what the
  caller asked, reported as sent. `unencodable()` is the single answer to whether an alphabet can
  carry a message, as `messageClassOf()` is to whether a `data_coding` carries a class, and it asks
  the codec — `decode(encode(c)) === c` per code point — rather than restating the tables beside it,
  so the guard cannot drift from what the encoder writes for any one character, and a fourth
  alphabet answers by having a codec at all. It is exported for the reason `concatOf()` is: a caller
  composing a `submit_sm` through `send()` and `encodeMessage()` would otherwise rewrite the read
  this fixed. `match()` cannot be that answer — it doubles as the auto-selection policy `detect()`
  reads, where LATIN1 is hardcoded false so nothing picks it, and using it would refuse the 8-bit
  binary body Latin-1 is kept for. The guard is
  on the named branch alone, so an unspecified send is untouched: every alphabet `detect()` returns
  carries every character it was picked for, over the whole code point range. `smppTime.encode()`
  returns a `Result`, where the three encoding helpers stayed total: that argument was that
  `EncodingName` is a closed set the compiler guards, and `Date | number | string` is not — an
  invalid `Date` and `NaN` inhabit it, which hard rule 1 makes a result "wherever the types admit
  one", and `decode()` has been fallible for the same reason since it was written. Rejected:
  transcoding to UCS2, which overrides the one option the caller wrote in order to override a choice
  — the same reason a flash Latin-1 message is refused rather than promoted, and an operator that
  accepts only `data_coding` 0x03 would be handed something it never agreed to take. Rejected:
  guarding `sendSms()` alone and leaving `smppTime.encode()` writing `NaN`s, which leaves this
  library's own published helper composing the garbage the guard exists to stop. Rejected: a
  `holds()` member beside `match()` on `Encoding`, a second per-alphabet table to keep in step with
  the codec. Rejected: validating the `string` spelling of a time, which is a stamp the caller
  formatted for a peer whose format is theirs to name, its width included, where SMPP 3.4 gives the
  field 1 or 17 octets. Accepted: a second count past 99d 23:59:59 is refused rather than clamped to
  it, a negative one and `Infinity` with it — clamping `86400 * 365` reported success for a year and
  put 99 days on the wire, the wrong answer about what happened that the rest of this bullet exists
  to remove. The ceiling is this encoder's rather than the format's: 3.4's `YYMMDDhhmmss000R`
  carries years and months, which `decode()` reads back, and no fixed number of seconds is either
  one, so spelling a second count in days and below is where the guess would go — which is why the
  too-long refusal names the `Date` that reaches every instant the absolute form holds, and the
  negative one names nothing, there being no period to reach. Rejected: documenting the clamp, which
  leaves the caller told a true thing and still sent the wrong period. Accepted: GSM's 0x1B is an
  extension prefix rather than a character, so a bare ESC beside one of the ten extension bases is
  the one input a per-character reading passes and the encoder then writes as the extended character
  — the only composition in any of the three codecs, and not a character a message is written in.

- **A string body is written in the alphabet its own `data_coding` names, and one that alphabet
  cannot carry is refused by the codec — `message_payload` on the same terms as `short_message`.**
  Maintainer's call, 2026-09-09, from the architecture review of
  [#97](https://github.com/larvit/larvitsmpp/pull/97): `objToPdu()` took the codec off the caller's
  own `data_coding` and encoded with it whatever the text was, so `data_coding` 3 beside `あいう`
  returned `42 44 46` — `"BDF"` — reported as built, while a string `message_payload` was cut to its
  low octets whatever `data_coding` said. Goal 2 owns it, as it owns the `sendSms()` guard above.
  The line falls at the string: a `Buffer` is octets the caller already chose and goes out as given
  under any `data_coding`, which is what keeps goal 8's escape hatch open — the raw UDH, 8-bit binary
  and deliberately malformed bodies `interop-tests/` builds are all still buildable — and a string
  with no `data_coding` is untouched, detection carrying every character it was picked for. The
  guard is `unencodable()` again rather than a second reading, and `unencodableText()` is the
  character, its code point and its index said once for both refusals — unexported where
  `unencodable()` is published, since wording `{ char, index }` into a sentence rewrites no read a
  caller would get wrong, where asking the codec is, and publishing it would freeze this library's
  error prose as API for an application whose own refusal should read like itself. Goal 8, from the
  architecture review of [#99](https://github.com/larvit/larvitsmpp/pull/99), 2026-09-09. It is
  reached through
  `encodeBody()` in `message.ts`, which is where the `data_coding`-to-text pair already lives:
  `encodeBody(text, dataCoding)` is `decodeMessage(buffer, dataCoding)`'s mirror and resolves the
  alphabet through the same `encodingByDataCoding()`. `send()` and `sendReturn()` inherit it,
  since both build through `buildPdu()`; `sendSms()` does not, and keeps its own guard, because
  `splitMessage()` hands the codec a Buffer with nothing left to refuse and the index a segment
  could name is not the one in the message. The TLV is encoded rather than merely checked because
  `data_coding` names the alphabet of the body wherever it is carried — that is how
  `messageOctets()` and `decodeMessage()` read one back, and a `data_sm` has nowhere else to put one
  — so refusing what Latin-1 cannot hold while still writing UCS-2 text as Latin-1 octets would
  close half of it. `short_message` settles the `data_coding` wherever it carries octets at all, the
  order `messageOctets()` reads the two in, so the alphabet a PDU declares is the one its body will
  be read under — and a `short_message` on a command whose table declares none is ignored here as
  `writeParams()` ignores it, so an empty one, an absent one and one the wire cannot carry are the
  same input rather than three. A `data_coding` on a command that declares no such field is honoured
  the other way round, since it is `replace_sm`'s only way to name the alphabet its octets are in.
  Every entry carrying the payload tag is resolved, by tag id rather
  than by record key, since `tagIdOf()` lets a caller name it anything and a spelling that escaped
  the guard would be a second spelling that disagrees about correctness. Rejected: refusing a string
  `message_payload` outright and demanding
  octets, which contradicts `short_message` on the same PDU. Rejected: guarding every string-valued
  field against `data_coding`, which says nothing about them — a text field on the wire has an
  alphabet of its own.

- **A GSM 03.38 message declares `data_coding` 0x00, and an inbound 0x01 is still read as GSM.**
  Maintainer's call, 2026-09-09: `dataCodingFor()` and `encodeBody()` both resolved an alphabet
  through `consts.ENCODING`, so `encoding: 'ASCII'` went out as 0x01 — SMPP 3.4 5.2.19's *IA5 (CCITT
  T.50)/ASCII* — while the codec writes GSM 03.38, where `$` is 0x02 and `@` is 0x00 against IA5's
  STX and NUL. Goal 1 owns it, and this library's own reader hid it by resolving both codings to the
  same codec. `dataCodingByEncoding` is the single answer to which coding an alphabet is written
  under, as `unencodable()` is to whether one can carry a message: the mirror of
  `encodingByDataCoding()`, and reached by both the `sendSms()` path and `encodeBody()`'s detected
  one rather than each spelling the map again, which is what `sendDlr()` inherits it through. It is
  exported for the reason `unencodable()` is — a caller pairing `encodeMessage()`'s octets with a
  `data_coding` of its own had only `consts.ENCODING` to reach for, which is the trap. 0x00 is the
  *SMSC's* default alphabet rather than 03.38 by name, so it is a convention rather than a guarantee;
  it is also what every peer in `interop-tests/` submits under and what LINK Mobility, Route Mobile
  and Telesign all publish 03.38 as, where 0x01 names a different alphabet from the one written and
  so is wrong whatever the peer makes of it. Reading is untouched, goal 3: those same three map 0x01
  to 03.38 too, and Kaleyra and Route Mobile publish that value as known to cause problems, so no
  researched peer means IA5 by it. The two tables agree over most of the printable range and part at
  0x00-0x09, 0x0B-0x0C, 0x0E-0x1A, 0x1C-0x1F, 0x24, 0x40, 0x5B-0x60 and 0x7B-0x7F — line feed,
  carriage return and escape are common to both — which is where a peer that did mean IA5 is
  misread. Accepted with it: `consts.ENCODING` loses its `ASCII` alias and keeps `IA5`, the two
  names 5.2.19 gives 0x01, because that alias was the only name the two tables shared at different
  values and so the only one a reader could carry from the option's vocabulary into SMPP's flat
  table; `constsById.ENCODING[0x01]` already read `IA5`, so nothing moves but the forward name.
  Rejected: moving `consts.ENCODING.ASCII` to 0x00, which would make that table contradict the
  section it exists to spell — the group is SMPP's flat `data_coding` table, not the `encoding`
  option's vocabulary, the distinction the `FLASH` removal already drew. Rejected: reading 0x01 as
  Latin-1, the closest codec here to IA5, which mojibakes every peer that means GSM for one nothing
  researched has found. Accepted: a message already in flight is unmoved — both codings resolve to
  the same codec, `messageClassOf()` finds no class in either, and `Reassembler` groups on the
  concatenation reference rather than on `data_coding` — so a receipt or a segment that crossed the
  change reads exactly as it did.

- **Every text field on the wire is latin1, and what the field cannot carry is refused rather than
  truncated.** Maintainer's call, 2026-09-21, the refusals from the security and stability passes on
  [#16](https://gitea.larvit.se/larvit/smpp-js/pulls/16). 3.4 calls these fields ASCII, so goal 3
  settles the read alone — its generous clause is scoped to reading, and its sender clause is strict.
  Goal 1 settles the write, being 3.4 as SMSCs actually run it: an operator routing an alphanumeric
  sender through the upper half is traffic to keep, and Node's `ascii` write already put those octets
  on the wire, so naming the write latin1 makes the round trip idempotent and no peer sees a change.
  Goal 2 settles the two latin1 refusals, each a `size()` that would have agreed with a `write()`
  that put something else on the wire: a character past `U+00FF` written as its low octet, and a caller's
  own `U+0000`, which a mandatory field's reader takes as the end of the field. Goal 4 settles them
  twice over: for one character in every 256 that low octet is `0x00`, and the PDU went out malformed
  on the operator's parser. `wantText()` and `wantCstringText()` are the two places that decide the
  refusals; every read and write spells `latin1` itself. Rejected: reading latin1 and leaving the write spelled ASCII, which leaves two halves agreeing
  only by accident. Rejected: refusing the upper half on send
  to stay strict to 3.4's ASCII, which would be a new restriction taking away traffic this library
  already sends and operators already accept, on no defect. Rejected: refusing `U+0000` in every
  text field, which would buy one spelling by taking a legitimate octet away from the
  length-prefixed Octet String, whose length octet is what ends it.

## The session's life

- **A close arriving after our own `unbind` is a clean unbind, not an error.** Maintainer's call,
  2026-08-26: most SMSCs drop the socket instead of answering, so the documented shutdown would
  otherwise always report a failure. It does mask a socket that died mid-unbind for an unrelated
  reason, which is accepted — the peer sees the same TCP close either way.

- **`close` means the session is over, and a drop the loop will retry is `disconnected`.**
  Maintainer's call, 2026-08-31: without the split, an application that opens a replacement client on
  `close` ends up holding two binds on one account. `teardown()` picks the event by whether the
  reconnect loop is still live, and `end()` stops that loop before tearing down, so every deliberate
  shutdown emits `close`. A retry that opens a socket and then loses it clears `closed` through
  `attach()`, which is why a second drop emits again.

- **An answer belongs to the link the message arrived on; a receipt does not.** Maintainer's call,
  2026-09-01. Rejected: answering on the new link, which succeeds and reports `{}` for a response
  that correlates with nothing — goal 2's wrong answer. Accepted: a receipt sent after a refused
  response names an id the peer has no record of.

- **`reconnect` takes `{ minDelay, maxDelay }` to retune and `false` to turn off**, so absent means
  on and there is one spelling for each. Only `client()` reconnects — a `server()` session is a
  connection the peer opened, and nothing at this end can reopen it. The retry timer is `unref()`'d,
  so a process with nothing else left to do still exits between attempts.

- **Coming up is not proof a link works, so only one that outlasted `maxDelay` resets the backoff.**
  An unreadable stream is found after the bind returns, so resetting on connect gave a link that died
  on arrival a fresh `minDelay` every cycle — one TCP connect and bind per second, forever. A drop
  after a healthy link still retries at `minDelay`.

- **`reconnect: { fromStart: true }` puts the first connect and bind through that same loop, and
  `client()` then resolves only once it is bound.** Maintainer's call, 2026-09-05: an application
  started before its SMSC is up otherwise writes that retry itself, around the one this library
  already owns. A field on `reconnect` rather than an option of its own, so the combination that
  would contradict `false` cannot be written at all — `false` carries no fields — and a top-level
  `fromStart` is refused by name rather than ignored. Nothing but the caller's `signal` ends the
  wait: a bound of its own would be a second spelling of a deadline the caller already writes with
  that signal, and giving up after one is what the default does. A bind the SMSC refuses is
  retried like any other failure — rejected: giving up on `ESME_RINVPASWD` and `ESME_RBINDFAIL`,
  which would have the initial attempts and a rebind disagree about what a refused bind means, and
  gives up on the operator whose provisioning lands a minute later; the backoff is what bounds the
  rate goal 4 cares about. The attempts before the first link report nothing, because the session
  running one has not reached the application: `disconnected` would have no listener and `close`
  would be a lie. Its wait is the one retry timer that is not `unref()`'d, for the reason
  `LinkGate`'s hold is not — it is awaited with no other handle, so a process whose only work is
  `client()` would exit unbound.

- **`connectTimeout` defaults to 10 s, bounds the whole connect including the TLS handshake, and
  `false` is the one way to turn it off.** Maintainer's call, 2026-09-20, serving goal 5: a connect
  that never returns is one `reconnect` cannot retry, because the operating system holds the attempt
  for around 130 s at Linux's default `tcp_syn_retries` and nothing above it is counting — and no
  application chooses that, so it is a default rather than an option that switches on what the caller
  obviously wanted. Accepted: the far end sees roughly four times the SYNs against a dead host, one
  per ~40 s rather than one per ~160 s, which goal 4 tolerates because the backoff still caps the
  rate. `false` spells the operating system's wait, as it does for `reconnect`, and `0` is refused
  naming it, so one spelling reaches each result. What expires is reported as the ordinary connect
  failure, so the loop retries it like any other, and the message names the peer and whether the TCP
  connect or the TLS handshake stalled — different faults, different answers. It settles on
  `secureConnect` for a TLS socket, so a peer that accepts and then says nothing is bounded the same
  way a black-holed SYN is. Rejected: shipping the option with no default, which left goal 5's "an
  option does not switch on the thing the caller obviously wanted" unmet, and would have cost a
  second breaking minor plus a reversal of the `0` spelling to correct later. Rejected:
  `socket.setTimeout()`, an idle timeout that goes on arming once the link is up. Rejected: bounding
  it with `responseTimeout`, which names the wait for an answer on a link that already exists and
  would retune both at once. `server()` shares the checker and ignores the option, as it already
  ignores `reconnect` — nothing at that end connects out.

- **A stream this library cannot frame is a dead link; one PDU it cannot parse is not.**
  Maintainer's call, 2026-08-31, narrowed 2026-09-05 via the interop plan: a `command_length` below
  16 or above `maxPduLength` leaves nothing that can say where the next PDU starts, so it tears the
  link down through `teardown()` and the reconnect loop retries it on a fresh socket with a fresh
  framer. Every other codec failure honoured `command_length`, so the stream is still in sync and
  the next PDU starts where it says — tearing the link down there cost one peer half its receipts
  and its MO to a reconnect loop (`interop-tests/findings/01-smscsim.md`), and left the peer waiting
  for answers it was owed. `sessionError` carries every failure of either kind, never coalesced or
  suppressed, so a peer that only ever sends garbage is visible in the log rather than silent.

- **A deliberate shutdown drains; an unusable link and an abort do not.** `close()` and `unbind()`
  wait on the send window rather than the pending map — the map misses a segment still queued behind
  a full window, and finishing a half-sent multipart message is the point. The window counts slots,
  never outcomes, and empties on a drop too, where `teardown()` settles everything the link was
  carrying, which is why `drain()` reads `closed` before it reads the count. A stream the framer or
  the codec cannot read takes `teardown()` instead, and `close({ signal })` on an aborted signal and
  a peer's own `unbind` take `end()`: nothing on a dead link can answer, an abort means stop now, and
  a peer that has declared itself finished will not answer what it still owes, so draining any of the
  three would only hold a socket open for the timeout. `unbind()` sends its own PDU through
  `request()` past both the window and the drain gate, because it must go out either way.
  `shutdownTimeout` stays a session option rather than a `close()` argument: `server()` builds
  sessions on the caller's behalf, so the option is the only composition point. `SmppServer.close()`
  reports each session's unfinished drain through `serverError`, because its own result says nothing
  but that the listener stopped.

- **`sendSms()` puts every segment of a message on the wire together.** Goal 6: a long message costs
  one round trip rather than one per segment. Rejected: sending each segment once the last is
  answered, which a receiver waiting for the whole message before answering would deadlock.

- **Every segment of a concatenated message is answered as it arrives, so `sendResp()` on one is the
  application's own signal rather than the peer's answer.** Maintainer's call, 2026-09-06, from the
  Jasmin interoperability phase: Jasmin dispatches one `submit_sm` per connector at a time and will
  not send segment 2 until segment 1 is answered, so holding a group unanswered until it was whole
  deadlocked every multi-segment message against a production gateway
  ([interop-tests/findings/03-jasmin.md](../interop-tests/findings/03-jasmin.md)). Goal 1 has the answer
  a real SMSC gives — one `message_id` per `submit_sm`, immediately — so the group's id base is
  generated when it opens and each segment is answered `<base>-<n>`, the notation `sms-id.ts` owns
  and `DlrMerger` reads back. The id is therefore fixed by the first segment, which is why an `smsId`
  or a refusing `status` passed to `sendResp()` on such a message is an error rather than a silent
  no-op. `answeredOnArrival` is on `Sms` because nothing the application can compute says it, and the
  discriminant a reader would reach for instead is wrong. A message `sendResp()` still answers itself is
  untouched, and is where a caller-chosen id and a refusal live; `onRequest` is the escape hatch for
  an application that must refuse a PDU the `sms` event could not have shown it yet. `collect()`
  answers every segment it will not carry rather than leaving it unanswered, which is the same stall
  in miniature: the field that numbered it where the segment belongs to no group, the retry status
  where the segment's own arrival overran the octet cap, since a peer told that still holds it. Rejected:
  answering every segment but the one that completes the group, which leaves the peer holding some
  segments accepted and one refused with nothing in SMPP to retract the rest, and still cannot honour
  a caller's `smsId` on the segments already gone. Rejected: a hook that mints the id per segment,
  which asks the application to name a message it cannot read yet — what it wants is `sms.smsId`
  afterwards. Rejected: an option to keep the old behaviour, a second spelling whose only
  distinguishing feature is that it deadlocks. Accepted: a group given up on — expired, evicted, or
  dropped with the link — is traffic the peer will not send again, so each one reaches `sessionError`
  as well as the log. Rejected there: an exported `MessageLostError` carrying the group, on the
  `PduRefusedError` pattern — no `sms` ever fired for that group, so there is nothing in it the
  application could act on, and goal 8 does not buy a second exported class to make a count
  distinguishable. Accepted: a completing segment whose own answer the socket would not carry still
  reaches the application, because the message is whole and correct and the failed answer is on
  `sessionError` — a peer that re-sends after the drop is the smaller risk than dropping a message
  in hand. The answer goes out before the `sms` event either way, so a listener's own receipt can
  never precede the acceptance of the message it reports on.

- **`server()` composes the application's `onRequest` after its own bind handling, and offers it
  every request that handling did not answer.** Maintainer's call, 2026-09-06, from a product review
  of the multipart change: `server()` filled the session's only `onRequest` slot, so the escape hatch
  the error above names was reachable only by hand-wiring a `Session` over a raw socket, giving up
  bind acceptance, `authenticate`, the session set and the drain `close()` runs over it — which is
  what goal 8 means by beating "the application can do this itself". What the library verifies is the
  ordering rather than the hook's honesty about answering: the hook is consulted only for a non-bind
  request on a session already bound, so no bind — a second one on a live session included — and
  nothing a peer sends before one can be intercepted however the hook is written. One
  `OnRequest` type on both option bags, because a second contract under one name is two spellings of
  one goal; widened to accept a plain boolean, as `authenticate` already is, so an observing hook need
  not be `async`. Nothing of ours is written for a request whose hook failed, the same on both
  surfaces: the library cannot tell one that failed before answering from one that failed after, so
  goal 2 reports the outcome as undetermined rather than guessing, and the peer's own
  `responseTimeout` is what settles it — the answer `authenticate` failing already takes. A hook that
  throws or rejects reaches `sessionError` on the way; one that never settles reaches nothing at all,
  and is visible only as the request that was never answered. That takes the keepalive with it, since
  a hook broken across the board leaves `enquire_link` unanswered and the peer drops the link — the
  back-pressure wanted, because an application that cannot serve a link should not hold one.
  `sessionError` rather than `serverError` because the
  failure belongs to one session's request, and that channel already carries every failure of one.
  The hook is consulted before the bind-direction gate, so it sees a `submit_sm` a receiver-bound
  peer may not send; first refusal means first, and one it declines still gets `ESME_RINVBNDSTS`.
  Nothing is held for a request the hook answered: `HeldMessages` is opened by the `sms` event the
  hook skipped, so the drain waits on none of it. `OnRequest` stays unexported where
  `AuthenticateInput` is exported, because that hook's argument is a shape this library invents and
  this one's are two types already published. Rejected: consulting the hook first, which puts
  bind and authentication inside the application's reach for nothing. Rejected: a narrower hook
  returning a status for the library to write, which makes the answer verifiable but pays a second
  contract under a second name for it, and could not express what the session-level hook already
  does — answer a bind, a vendor command, a `data_sm` — leaving that error naming something only
  half the surface can do. Rejected: falling the request through to the built-in handling on a
  failure, which reads as the answer the peer would have had with no hook — true only of a hook
  that failed before answering, where one that failed after put a second response on the peer's own
  sequence number, goal 1's wire violation. Rejected with it: recording what the hook wrote so the
  fall-through could be gated on it, which buys a fail-open path with state and an internal contract
  no other collaborator needs.

- **The drain waits on the messages the application holds, and `sendResp()` is what says it is done
  with one.** Maintainer's call, 2026-09-01: waiting on the send window alone tore a server session
  down while the application was still answering a `submit_sm`, so the peer timed out and re-sent —
  the duplicate goal 2 forbids, in the direction the window already covers. No completion signal was
  added to the `sms` event: `sendResp()` is what an application already calls when it is done with a
  message, so it is the one the drain waits for. Counting every inbound request until `sendReturn()`
  answered it was rejected: an `onRequest` that deliberately answers nothing would then cost a full
  `shutdownTimeout` on every close. The response reaching the wire ends the wait, so a `sendResp()`
  the library refused or the socket would not carry leaves `close()` still reporting the message the
  peer is owed.

- **The drain's wait on the application ignores `shutdownTimeout: 0`.** Waiting forever is safe for
  the peer, whose every request is bounded by `responseTimeout` unless the caller set that to 0 as
  well, and unsafe for the application, which nothing bounds — `close()` is what you reach for when
  the application is stuck, so it may not block on the application coming unstuck. That half falls
  back to `responseTimeout`, the same answer the link gate's hold already takes — and to that
  option's default where it is 0 as well, since neither option is an answer about the application.

- **What the application holds unanswered is capped on constants, and a message past the cap is
  refused.** A bound the application cannot raise is the point: an application that answers nothing
  would otherwise grow it for the life of the link, which goal 4 forbids. Reassembly's `maxOctets`
  is an option because it bounds what the peer sends; this bounds what the application leaves
  unanswered. Maintainer's call, 2026-09-26: a message arriving past the cap is refused with the
  retry status, so the peer keeps it (goal 2). Rejected: dropping the oldest to make room, which
  frees nothing while the application still holds its `Sms`, and stops the drain waiting for a
  message the peer is owed. Rejected: pausing the socket, which also stalls every answer and
  `enquire_link` on the link. Reaching the bound is a log line and nothing more (goal 8): an event
  or a public count would be surface for what the application already knows, since it is the one
  not answering. A message held past its timeout is still dropped, so `close()` can
  report fewer unanswered than there were — accepted, because the alternative is holding what
  nothing will answer.

- **A store at its bound answers `ESME_RTHROTTLED` to a `submit_sm` and `ESME_RX_T_APPN` to a
  `deliver_sm`.** Maintainer's call, 2026-09-26, for reassembly and held messages alike, so "keep it
  and retry" has one spelling per direction. `ESME_RTHROTTLED` asks the sender to slow down, which
  is what the peer outrunning us needs, and operators send it (Vonage, LINK Mobility, Route Mobile,
  Jasmin), so clients built against them meet it (goal 1). Rejected: `ESME_RMSGQFUL`, which names an
  exhausted queue and no rate. `ESME_RTHROTTLED` is the SMSC's to send, so an ESME answers with
  SMPP 3.4's temporary receiver error, the one an SMSC retries on (goal 3).

- **A reconnect keeps the delivery-receipt merges; everything else the link held is dropped.**
  `onDelivery()` answers each receipt before the group it belongs to is complete, and `teardown()`
  runs on every path — an idle timeout and a failed rebind, not only `close()` — so clearing the
  merges there loses receipts no peer has a reason to send again. They are cleared where the session
  is over instead. Inbound segments stay in `teardown()`: a concatenation reference is the
  peer's own counter, so a half-arrived group kept across a drop would take a later message's
  segments as readily as the rest of its own, and goal 2 will not hand the application a message
  assembled that way. What goes there is traffic already answered, which is why each group reaches
  `sessionError` like every other one given up on.

- **A message id base is merged at most once.** A receipt carries nothing but `<base>-<n>`, so a
  straggler for a message whose group is gone cannot be told from a receipt for a later message the
  peer handed the same ids — an SMSC whose id counter restarts with its process is the realistic
  case. `DlrMerger` remembers the bases it has finished with, capped and expiring exactly like the
  groups, and refuses to open one a second time: the later message gets no `messageDlr`, and an
  earlier one whose receipts are still arriving is dropped rather than left to collect the later
  one's. Every segment still reaches the application as a `dlr`. `expect()` ignores a lone id, so a
  single-part message never claims a base.

- **A send that never reached the socket waits for the next link; one that did is counted, not
  resent.** Maintainer's call, 2026-09-01: re-queueing everything unanswered would resend a
  `submit_sm` the SMSC accepted and answered into a dead socket, which is delivered and billed twice,
  while a request that never left this process can be lost for free. `attempt()` therefore wraps all
  three ways a written request can fail in `UnansweredError`; counting only the dropped-link case, as
  the first cut did, would have called the commonest one safe to resend. A count rather than a
  boolean because `sendSms()` aggregates segments into one `err` slot, and required rather than
  optional so every construction site answers. `UnansweredError` stays unexported: `unanswered` is
  the one spelling on the public surface. The hold is bounded by `responseTimeout` rather than an
  option of its own — that is already the answer to how long one request may wait — and its clock
  starts when the send is issued rather than when it first finds the gate shut, so one budget covers
  every hold a single call makes. That timer is the one here that is not `unref()`'d: a held request
  is awaited with the socket already destroyed, so an unref'd one lets a process whose only remaining
  work is that send exit without settling it.

- **A send queued for a send-window slot is bounded by the caller's `signal`, and by nothing else.**
  Maintainer's call, 2026-09-06, from a review of PR #71: the hold above observes the signal and the
  `acquire()` on the next line did not, so a caller that aborted while the window was full waited for
  a slot it no longer wanted — at `responseTimeout: 0` for as long as the peer stayed quiet, which is
  the deadline the README sends the caller to that signal for. Goal 4 is not re-opened by an
  unbounded wait here: the queue is the application's own backlog, unbounded in depth as well as in
  time because capping it would refuse a send the application asked for, and nothing in it keeps the
  peer waiting — which is what separates it from the inbound stores capped on constants. Rejected:
  having `release()` skip a waiter whose signal already fired, which leaves the departed waiter in
  the queue where `unfinished()` still counts it and the drain waits on it; the waiter leaves as it
  settles instead. Rejected: bounding this wait by `responseTimeout` as the hold is bounded — a full
  window is this end's own concurrency draining as the peer answers rather than a link going nowhere,
  and that bound would fail a message with more segments than `maxOutstanding` partway through
  against a slow peer. The failure is a plain `Error` rather than `UnansweredError`, the same answer
  an abort at the gate already gives. The drain half needs nothing: `close({ signal })` already hands the signal to
  `window.idle()`, and `unbind()` taking none is the shape README states.

- **The gate decides whether a link can carry a request, and a bind is what makes it one.**
  Maintainer's call, 2026-09-01: `attach()` clears `closed` the moment a socket is handed over, one
  round trip before the bind is answered, so gating on `closed` let a send arriving in that window go
  out unbound and come back `ESME_RINVBNDSTS`. `LinkGate` owns the answer instead — `shut(returning)`
  on every teardown, `open()` only once `comeBackUp()` has a bound link — and
  `OutgoingRequests.linkDown()` reads it rather than `closed`. The bind itself cannot wait for what it
  creates, so `pastDrain()` lets the three bind commands past the gate and the window, the same door
  `unbind()` takes through `now()`. The gate is told what happened and never reads back into the
  session: a collaborator that has to ask does not own its decision, which is how the first cut ended
  up answering the same question two different ways at admit and at release. For the same reason the
  retry in `pastDrain()` asks `gate.isUp()` rather than `linkDown()`, which also reads the socket — a
  condition that loops on something the gate does not gate on spins against a gate that admits it
  straight back. `LinkGate.returning` is a copy of `retrying()` taken at teardown, and stays true
  only because nothing stops the reconnect loop without `emitClose()` following it: `drain()` and
  `end()` are the only callers of `stop()`. A third caller has to shut the gate itself.


## Internals and tests

- **A listener that rejects is routed by Node's `captureRejections`, not by hand-dispatching.** Both
  emitters construct with `captureRejections: true` and implement
  `[EventEmitter.captureRejectionSymbol]`, which lands a rejected `async` listener on `sessionError`
  or `serverError` beside the synchronous guard in `emit()`. Dispatching `rawListeners()` from
  `emit()` instead needs a cast to call them with the event's argument tuple, which hard rule 4
  forbids. A rejection reason is `unknown` and `String()` throws on a null-prototype object, so both
  handlers normalise through `errorFrom()` rather than inline — a route out of the handler would land
  on a bare `process.nextTick` with nothing to catch it.

- **The four-line abort dance is copied across `LinkGate`, `IdleWaiters`, `PendingRequests` and
  `SendWindow` rather than extracted.** Architecture review, 2026-09-06: pre-check `aborted`, attach
  `{ once: true }`, detach on settle, leave the registry. What differs at each site is the registry
  and what settling means — a FIFO handing over a slot, a set released together, a map keyed by
  sequence number, a count recomputed at settle — so a shared `Waiters<T>` fits two of the four and
  is a shallower module than the copies. Extract it once a fifth appears.

- **`SmppLog` is a five-method contract this library declares, not a dependency.** `debug`, `error`,
  `info`, `verbose` and `warn` are what the code actually calls, so an application can satisfy it
  with an object literal. `@larvit/log` implements it structurally and stays a devDependency, where
  `test/tls.test.ts` passing a real `Log` as the server's logger keeps that compatibility compiled.

- **The TLS tests build their own self-signed certificate in DER** (`test/tls.test.ts`) instead of
  adding a devDependency or shelling out to openssl. Maintainer's call, 2026-08-26: the dev image
  `node:24.18.0-bookworm-slim` ships no openssl binary, so a shelled-out fixture would pass in CI and
  fail on every developer machine, and a committed key leaks in a public repository. Valid while the
  dev image has no openssl.

- **`src/` stays flat until a module has to move for another reason.** Architecture review,
  2026-09-06: the grouping the [file map](../AGENTS.md#architecture) already implies — `wire/` for `pdu*` and `defs`,
  `link/` for `link-*`, `reconnect-*`, `pdu-transport` and `send-window`, `messages/` for `sms*`,
  `dlr*`, `message*`, `reassembly` and `udh` — rewrites every import for no change to
  `dist/index.js`, the one published entry. Valid while that map is what a reader navigates by.

- **`test/` stays flat too, and a file there is named for the question it answers rather than for the
  module it covers.** Architecture review, 2026-09-08, at 18 test files: what keeps that count honest
  is the naming rule rather than a tree — `operator-receipts.test.ts` holds a corpus defined by where
  it came from, cutting across four modules, where filing it by module would enter each new operator
  twice. A split also has to be made twice, since `test` and `test:compiled` each carry a path of
  their own. The four files that are not tests are the exception the rule needs stated:
  `dummy-smsc.ts`, `raw-pdus.ts`, `reference-smpp.d.ts` and `teardown.ts` answer no question and are
  named for what they hold.

- **CI tests on Linux only; `src/` keeps off what is known to break on macOS or Windows.** Maintainer's
  call, 2026-09-14. Nothing verifies either platform, so the code avoids what is known to differ there:
  shelling out, a path joined by hand, a signal Windows does not deliver, a Unix socket or a file mode.
  That binds what `dist/` runs; the container tooling, `interop-tests/` and the `package.json` scripts
  run on Linux by goal 10. Rejected: macOS and Windows runners, on GitHub's mirror or as Gitea
  host-mode runners on a Windows VM and a Mac.

- **GitHub mirrors Gitea without pruning, and a ref deleted on Gitea is deleted on GitHub by a run of
  its own.** Maintainer's call, 2026-09-14; valid while nothing deploys from GitHub.
  `.gitea/workflows/mirror.yaml` never prunes, and `mirror-delete.yaml` runs once per deleted ref. A
  delete run that fails or outlives Gitea's queue timeout, or a push run that cloned before the
  delete, leaves the ref on GitHub until the delete run is re-run. Accepted: a stale ref there is
  harmless, and refs only GitHub has must survive.