Decompose the codec, document the session surface and record the review decisions

AGENTS.md

@@ -154,6 +154,20 @@ exactly 140.
 
 ## Decisions
 
+- **The published surface is frozen at what `src/index.ts` exports today.** `Session` is exported and
+  publicly constructible, which is why `SessionOptions` and `ReconnectOptions` are public too — that
+  is correct, not a leak, and it has been raised twice. The collaborators `session.ts` delegates to
+  (`Reassembler`, `PendingRequests`, `SendWindow`, `ReconnectLoop`, `LinkTimers`, `DlrMerger`,
+  `submitSms`) stay unpublished so they can be reshaped.
+- **The sub-3.4 optional-parameter rule is a predicate, not a chokepoint.** `acceptsOptionalParams()`
+  is consulted by the library's own senders; `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. The guarantee is "what this library sends honours the rule",
+  never "the session cannot send optional parameters to an old peer".
+- **Only the server feeds `peerInterfaceVersion`.** `acceptBind()` records what the peer declared;
+  the client never reads `sc_interface_version` out of its bind response, so a client session is
+  permissive. That is not a defect today — this library's ESME direction sends no TLVs at all — but
+  anyone adding a client-side TLV owes the other half of the feed.
 - **The library speaks SMPP 3.4 on the wire, and `defs/` keeps the 5.0 tables as a superset.**
   Maintainer's call, 2026-08-26: 3.4 is what SMSCs actually run, while the wider tables let the codec
   parse and build whatever a peer sends. The declared version is an option on both `client()` and

README.md

@@ -171,7 +171,7 @@ await smpp.close();      // stop listening and close every live session
 | `tls` | `false` | A `tls.TlsOptions` object with your certificate and key. |
 | `idleTimeout` | `40000` | Drop a peer that has been silent this long. |
 | `maxReassembly` | `1000` | Incomplete multipart messages held per session. |
-| `reassemblyTimeout` | `300000` | How long an incomplete multipart message is held. |
+| `reassemblyTimeout` | `300000` | How long a late segment can still join an incomplete message. |
 | `responseTimeout`, `maxOutstanding`, `log`, `signal` | as for the client | |
 
 ## Errors
@@ -218,6 +218,11 @@ const { err, pduObj } = await session.send({
 });
 ```
 
+`acceptsOptionalParams()` answers whether the peer declared SMPP 3.4 or later, which is the version
+at and above which the spec allows optional parameters to be sent to it; `peerInterfaceVersion` is
+the raw value it declared. The library's own senders consult the first before attaching a TLV — a
+`send()` you build yourself is passed through as written, so consult it too when you attach TLVs.
+
 ## Working with PDUs directly
 
 The codec is exported, synchronous, and never throws — handy for inspecting captured traffic:

eslint.config.js

@@ -39,13 +39,9 @@ export default tseslint.config(
 		rules: { 'max-lines': 'off' },
 	},
 	{
-		// The codec branches per wire type and per parameter; splitting it scatters the wire format
+		// ESLint counts every ?. and ?? in dlrFromPdu as a branch; the 19 is 26 lines of flat field resolution.
-		// across files instead. These two keep the ceiling they have today.
+		files: ['src/dlr.ts'],
-		files: ['src/dlr.ts', 'src/pdu.ts'],
+		rules: { complexity: ['error', 19] },
-		rules: {
-			complexity: ['error', 22],
-			'max-lines-per-function': ['error', { max: 75, skipBlankLines: true, skipComments: true }],
-		},
 	},
 	{
 		// ESC (0x1B) is the GSM 03.38 escape character, so it belongs in these patterns.

src/dlr.ts

@@ -120,6 +120,7 @@ export function parseReceipt(message: string): Receipt {
 export function dlrFromPdu(pduObj: PduObject): Dlr | undefined {
 	const message = pduObj.params.short_message;
 	const receipt = typeof message === 'string' ? parseReceipt(message) : undefined;
+	const receiptState = receiptStates[receipt?.stat?.toUpperCase() ?? ''];
 
 	const tlvState = pduObj.tlvs.message_state?.tagValue;
 	const tlvId = pduObj.tlvs.receipted_message_id?.tagValue;
@@ -132,13 +133,13 @@ export function dlrFromPdu(pduObj: PduObject): Dlr | undefined {
 
 	const statusMsg = typeof tlvState === 'number'
 		? constsById.MESSAGE_STATE?.[tlvState]
-		: receiptStates[receipt?.stat?.toUpperCase() ?? ''];
+		: receiptState;
 
 	if (statusMsg === undefined) return undefined;
 
 	const statusId = typeof tlvState === 'number'
 		? tlvState
-		: consts.MESSAGE_STATE[receiptStates[receipt?.stat?.toUpperCase() ?? ''] ?? 'UNKNOWN'];
+		: consts.MESSAGE_STATE[receiptState ?? 'UNKNOWN'];
 
 	return {
 		doneDate: receiptDate(receipt?.doneDate),

src/pdu.ts

@@ -1,7 +1,7 @@
-import type { CommandName, PduParams, PduParamsInput } from './defs/commands.ts';
+import type { CommandDefinition, CommandName, PduParams, PduParamsInput } from './defs/commands.ts';
 import type { ErrorName } from './defs/errors.ts';
 import type { ParamValue } from './defs/types.ts';
-import type { Result } from './result.ts';
+import type { Result, VoidResult } from './result.ts';
 import type { Tlv } from './defs/tlvs.ts';
 import { cmds, commandNameById, isCommandName } from './defs/commands.ts';
 import { consts } from './defs/constants.ts';
@@ -79,46 +79,31 @@ function tagIdOf(name: string, input: TlvInput): Result<{ tagId: number }> {
 	return { tagId };
 }
 
-function buildPdu(
+/** Encoding a string short_message also settles data_coding and sm_length. */
-	cmdName: CommandName,
+function resolveShortMessage(
-	cmdStatus: ErrorName,
-	seqNr: number,
 	params: Record<string, ParamValue | undefined>,
-	tlvs: Record<string, TlvInput> | undefined,
+): Record<string, ParamValue | undefined> {
-): Result<{ buffer: Buffer }> {
+	const message = params.short_message;
-	const definition = cmds[cmdName];
 
-	if (!definition) {
+	if (typeof message !== 'string') return { ...params };
-		return { err: new Error(`Invalid cmdName: ${JSON.stringify(cmdName)}`) };
-	}
 
-	if (!isErrorName(cmdStatus)) {
+	const dataCoding = params.data_coding;
-		return { err: new Error(`Invalid cmdStatus: ${JSON.stringify(cmdStatus)}`) };
+	const encoding = typeof dataCoding === 'number' ? encodingByDataCoding(dataCoding) : detect(message);
-	}
-
-	if (!Number.isInteger(seqNr) || seqNr < 0 || seqNr > maxSeqNr) {
-		return { err: new Error(`Invalid seqNr: ${JSON.stringify(seqNr)}`) };
-	}
-
-	const resolved = { ...params };
-	const message = resolved.short_message;
-
-	// A string short_message is encoded here, which also settles data_coding and sm_length.
-	if (typeof message === 'string') {
-		const dataCoding = resolved.data_coding;
-		const encoding = typeof dataCoding === 'number'
-			? encodingByDataCoding(dataCoding)
-			: detect(message);
 	const encoded = encodeMessage(message, encoding);
 
-		resolved.short_message = encoded.buffer;
+	return {
-		resolved.sm_length = encoded.buffer.length;
+		...params,
-
+		data_coding: typeof dataCoding === 'number' ? dataCoding : consts.ENCODING[encoded.encoding],
-		if (typeof dataCoding !== 'number') {
+		short_message: encoded.buffer,
-			resolved.data_coding = consts.ENCODING[encoded.encoding];
+		sm_length: encoded.buffer.length,
-		}
+	};
-	}
+}
 
+function writeParams(
+	definition: CommandDefinition,
+	resolved: Record<string, ParamValue | undefined>,
+	cmdName: CommandName,
+): Result<{ chunks: Buffer[] }> {
 	const chunks: Buffer[] = [];
 
 	for (const [name, type] of Object.entries(definition.params ?? {})) {
@@ -139,6 +124,12 @@ function buildPdu(
 		chunks.push(chunk);
 	}
 
+	return { chunks };
+}
+
+function writeTlvs(tlvs: Record<string, TlvInput> | undefined): Result<{ chunks: Buffer[] }> {
+	const chunks: Buffer[] = [];
+
 	for (const [name, tlv] of Object.entries(tlvs ?? {})) {
 		const tag = tagIdOf(name, tlv);
 
@@ -169,7 +160,39 @@ function buildPdu(
 		chunks.push(chunk);
 	}
 
-	const body = Buffer.concat(chunks);
+	return { chunks };
+}
+
+function buildPdu(
+	cmdName: CommandName,
+	cmdStatus: ErrorName,
+	seqNr: number,
+	params: Record<string, ParamValue | undefined>,
+	tlvs: Record<string, TlvInput> | undefined,
+): Result<{ buffer: Buffer }> {
+	const definition = cmds[cmdName];
+
+	if (!definition) {
+		return { err: new Error(`Invalid cmdName: ${JSON.stringify(cmdName)}`) };
+	}
+
+	if (!isErrorName(cmdStatus)) {
+		return { err: new Error(`Invalid cmdStatus: ${JSON.stringify(cmdStatus)}`) };
+	}
+
+	if (!Number.isInteger(seqNr) || seqNr < 0 || seqNr > maxSeqNr) {
+		return { err: new Error(`Invalid seqNr: ${JSON.stringify(seqNr)}`) };
+	}
+
+	const written = writeParams(definition, resolveShortMessage(params), cmdName);
+
+	if (written.err) return { err: written.err };
+
+	const writtenTlvs = writeTlvs(tlvs);
+
+	if (writtenTlvs.err) return { err: writtenTlvs.err };
+
+	const body = Buffer.concat([...written.chunks, ...writtenTlvs.chunks]);
 	const header = Buffer.alloc(16);
 
 	header.writeUInt32BE(body.length + 16, 0);
@@ -223,22 +246,11 @@ function parseTlvs(
 	return { offset, tlvs };
 }
 
-function parseOnce(pdu: Buffer, trailingNull: boolean): Result<{ aligned: boolean; pduObj: PduObject }> {
+function readParams(
-	const cmdLength = pdu.readUInt32BE(0);
+	cmdName: CommandName,
-	const cmdId = pdu.readUInt32BE(4);
+	pdu: Buffer,
-	const cmdName = commandNameById(cmdId);
+	trailingNull: boolean,
-
+): Result<{ offset: number; params: Record<string, ParamValue> }> {
-	if (!cmdName) {
-		return { err: new Error(`Unknown PDU command id: ${String(cmdId)}`) };
-	}
-
-	const cmdStatusId = pdu.readUInt32BE(8);
-	const seqNr = pdu.readUInt32BE(12);
-
-	if (seqNr > maxSeqNr) {
-		return { err: new Error(`Invalid seqNr, exceeds ${String(maxSeqNr)}: ${String(seqNr)}`) };
-	}
-
 	const params: Record<string, ParamValue> = {};
 	let offset = 16;
 
@@ -255,10 +267,34 @@ function parseOnce(pdu: Buffer, trailingNull: boolean): Result<{ aligned: boolea
 		if (name === 'short_message' && trailingNull) offset++;
 	}
 
-	const parsed = parseTlvs(pdu, offset, cmdLength);
+	return { offset, params };
+}
+
+function parseOnce(pdu: Buffer, trailingNull: boolean): Result<{ aligned: boolean; pduObj: PduObject }> {
+	const cmdLength = pdu.readUInt32BE(0);
+	const cmdId = pdu.readUInt32BE(4);
+	const cmdName = commandNameById(cmdId);
+
+	if (!cmdName) {
+		return { err: new Error(`Unknown PDU command id: ${String(cmdId)}`) };
+	}
+
+	const cmdStatusId = pdu.readUInt32BE(8);
+	const seqNr = pdu.readUInt32BE(12);
+
+	if (seqNr > maxSeqNr) {
+		return { err: new Error(`Invalid seqNr, exceeds ${String(maxSeqNr)}: ${String(seqNr)}`) };
+	}
+
+	const read = readParams(cmdName, pdu, trailingNull);
+
+	if (read.err) return { err: read.err };
+
+	const parsed = parseTlvs(pdu, read.offset, cmdLength);
 
 	if (parsed.err) return { err: parsed.err };
 
+	const params = read.params;
 	const message = params.short_message;
 	const esmClass = numberOr(params.esm_class, 0);
 
@@ -282,7 +318,7 @@ function parseOnce(pdu: Buffer, trailingNull: boolean): Result<{ aligned: boolea
 	};
 }
 
-export function pduToObj(pdu: Buffer): Result<{ pduObj: PduObject }> {
+function checkFraming(pdu: Buffer): VoidResult {
 	if (pdu.length < 16) {
 		return { err: new Error(`PDU is too short, minimum is 16 octets, got ${String(pdu.length)}`) };
 	}
@@ -297,6 +333,14 @@ export function pduToObj(pdu: Buffer): Result<{ pduObj: PduObject }> {
 		return { err: new Error(`cmd_length ${String(cmdLength)} exceeds the ${String(pdu.length)} octets given`) };
 	}
 
+	return {};
+}
+
+export function pduToObj(pdu: Buffer): Result<{ pduObj: PduObject }> {
+	const framing = checkFraming(pdu);
+
+	if (framing.err) return { err: framing.err };
+
 	const plain = parseOnce(pdu, false);
 
 	if (!plain.err && plain.aligned) return { pduObj: plain.pduObj };
@@ -311,6 +355,25 @@ export function pduToObj(pdu: Buffer): Result<{ pduObj: PduObject }> {
 	return { err: plain.err };
 }
 
+/** Fields the response shares with the request are echoed back unless the caller overrode them. */
+function echoParams(
+	respName: CommandName,
+	pdu: PduObject,
+	params: Record<string, ParamValue>,
+): Record<string, ParamValue> {
+	const respParams: Record<string, ParamValue> = { ...params };
+
+	for (const name of Object.keys(cmds[respName]?.params ?? {})) {
+		const value = pdu.params[name];
+
+		if (respParams[name] === undefined && value !== undefined) {
+			respParams[name] = value;
+		}
+	}
+
+	return respParams;
+}
+
 export function pduReturn(
 	pdu: Buffer | PduObject,
 	status: ErrorName = 'ESME_ROK',
@@ -329,16 +392,5 @@ export function pduReturn(
 		return { err: new Error(`"${pdu.cmdName}" has no response command`) };
 	}
 
-	const respParams: Record<string, ParamValue> = { ...params };
+	return buildPdu(respName, status, pdu.seqNr, echoParams(respName, pdu, params), tlvs);
-
-	// Fields the response shares with the request are echoed back unless the caller overrode them.
-	for (const name of Object.keys(cmds[respName]?.params ?? {})) {
-		const value = pdu.params[name];
-
-		if (respParams[name] === undefined && value !== undefined) {
-			respParams[name] = value;
-		}
-	}
-
-	return buildPdu(respName, status, pdu.seqNr, respParams, tlvs);
 }

src/server.ts

@@ -4,7 +4,7 @@ import type { Result } from './result.ts';
 import type { Server as NetServer, Socket } from 'node:net';
 import type { Server as TlsServer, TlsOptions } from 'node:tls';
 import { EventEmitter } from 'node:events';
-import { Session, bindCommands } from './session.ts';
+import { Session, bindCommands, defaultSystemId } from './session.ts';
 import { createServer as createNetServer } from 'node:net';
 import { createServer as createTlsServer } from 'node:tls';
 import { defaultInterfaceVersion } from './defs/constants.ts';
@@ -45,7 +45,7 @@ const defaults = {
 	idleTimeout: 40_000,
 	interfaceVersion: defaultInterfaceVersion,
 	port: 2775,
-	systemId: '',
+	systemId: defaultSystemId,
 };
 
 /** A listening SMPP server. Sessions arrive as `session` events; `close()` stops listening. */

src/session.ts

@@ -72,6 +72,8 @@ export type SessionOptions = {
 	systemId?: string | undefined;
 };
 
+export const defaultSystemId = '';
+
 const defaults = {
 	maxDelay: 30_000,
 	maxOutstanding: 10,
@@ -79,7 +81,7 @@ const defaults = {
 	minDelay: 1000,
 	reassemblyTimeout: 300_000,
 	responseTimeout: 30_000,
-	systemId: '',
+	systemId: defaultSystemId,
 };
 
 export const bindCommands: readonly string[] = [

test/dlr.test.ts

@@ -84,20 +84,23 @@ describe('dlrFromPdu()', () => {
 		assert.equal(dlr.doneDate?.toISOString(), '2025-08-25T14:31:00.000Z');
 	});
 
-	test('maps every spec status code back to its message state', () => {
+	test('maps every spec status code back to its message state and id', () => {
-		for (const [code, expected] of [
+		for (const [code, expected, statusId] of [
-			['DELIVRD', 'DELIVERED'],
+			['DELIVRD', 'DELIVERED', 2],
-			['UNDELIV', 'UNDELIVERABLE'],
+			['UNDELIV', 'UNDELIVERABLE', 5],
-			['EXPIRED', 'EXPIRED'],
+			['EXPIRED', 'EXPIRED', 3],
-			['DELETED', 'DELETED'],
+			['DELETED', 'DELETED', 4],
-			['ACCEPTD', 'ACCEPTED'],
+			['ACCEPTD', 'ACCEPTED', 6],
-			['REJECTD', 'REJECTED'],
+			['REJECTD', 'REJECTED', 8],
-			['ENROUTE', 'ENROUTE'],
+			['ENROUTE', 'ENROUTE', 1],
-			['UNKNOWN', 'UNKNOWN'],
+			['UNKNOWN', 'UNKNOWN', 7],
-		]) {
+			['delivrd', 'DELIVERED', 2],
-			const dlr = dlrFromPdu(deliverSm(`id:x stat:${String(code)} err:0`));
+		] as const) {
+			const dlr = dlrFromPdu(deliverSm(`id:x stat:${code} err:0`));
 
-			assert.equal(dlr?.statusMsg, expected);
+			assert.ok(dlr);
+			assert.equal(dlr.statusMsg, expected);
+			assert.equal(dlr.statusId, statusId);
 		}
 	});
 

test/pdu.test.ts

@@ -120,6 +120,21 @@ describe('encoding submit_sm', () => {
 		);
 	});
 
+	test('encodes a string short_message with the given data_coding instead of a detected one', () => {
+		const pdu = encode({
+			cmdName: 'submit_sm',
+			params: {
+				data_coding: 0x08,
+				destination_addr: '46709771337',
+				short_message: 'hi',
+				source_addr: '46701113311',
+			},
+			seqNr: 12,
+		});
+
+		assert.equal(pdu.subarray(-7).toString('hex'), '08000400680069');
+	});
+
 	test('keeps a UDH-carrying short_message as a buffer', () => {
 		const message = Buffer.concat([
 			Buffer.from('050003010101', 'hex'),

todo.md

@@ -77,8 +77,10 @@ Every defect listed in the AGENTS.md table has a regression test naming the beha
 - [ ] **`session.ts` is 459 lines.** The one seam left in it is a socket-to-PDU transport, which
       would move the deliberately public `sock` field out of `Session` or turn it into a getter —
       a public-surface change, so it waits for a decision.
-- [ ] **`buildPdu` and `dlrFromPdu` carry a complexity of 22.** `eslint.config.js` holds them at
+- [ ] **Group the session's collaborators under `src/session/`.** Only `session.ts` imports
-      that ceiling rather than below the repo-wide 10, so neither can grow but neither shrinks.
+      `reassembly`, `dlr-merger`, `send-window`, `link-timers`, `reconnect-loop`, `pending-requests`
+      and `send-sms`, so the directory would make that boundary visible. Do it on the next
+      extraction out of `session.ts`, not as a move of its own.
 - [ ] **`submit_multi` and the broadcast commands** encode and decode, but nothing exercises them
       end to end. The interop suite is the natural place.
 - [ ] **Move to TypeScript 7** once `typescript-eslint` supports it; `renovate.json` pins TypeScript