Split the session, gate optional parameters on the peer version, own the SMPP version in defs

src/send-window.ts

@@ -0,0 +1,32 @@
+/** Caps how many requests are on the wire at once; anything past the limit waits its turn. */
+export class SendWindow {
+	private readonly limit: number;
+	private readonly waiting: (() => void)[] = [];
+	private inFlight = 0;
+
+	constructor(limit: number) {
+		this.limit = limit;
+	}
+
+	acquire(): Promise<void> {
+		if (this.inFlight < this.limit) {
+			this.inFlight++;
+
+			return Promise.resolve();
+		}
+
+		return new Promise<void>(resolve => this.waiting.push(resolve));
+	}
+
+	release(): void {
+		const next = this.waiting.shift();
+
+		if (next) {
+			next();
+
+			return;
+		}
+
+		this.inFlight--;
+	}
+}