fix(mtu): Fix mtu negotiation

Author: Marco Crespi

lib/bindings/hci/Peripheral.js

@@ -4,6 +4,9 @@ import { HciAdapter } from './Adapter';
 import { HciGattRemote } from './gatt';
 import { Hci, Signaling } from './misc';
 
+// 512 bytes is max char size + 1 byte att opcode + 2 bytes handle + 2 bytes offset for long writes
+const DEFAULT_MTU = 517;
+
 export class HciPeripheral extends Peripheral {
 	public adapter: HciAdapter;
 
@@ -25,8 +28,7 @@ export class HciPeripheral extends Peripheral {
 		this.signaling = new Signaling(this.hci, this.handle);
 
 		this.gatt = new HciGattRemote(this, hci, handle);
-		await this.gatt.exchangeMtu(256);
-		this.mtuExchanged = true;
+		this.mtuExchanged = false;
 
 		this._state = 'connected';
 	}
@@ -57,10 +59,10 @@ export class HciPeripheral extends Peripheral {
 			throw new Error(`Peripheral is not connected`);
 		}
 
-		/*if (!this.mtuExchanged) {
-			await this.gatt.exchangeMtu(requestMtu || 256);
+		if (!this.mtuExchanged) {
+			await this.gatt.exchangeMtu(requestMtu || DEFAULT_MTU);
 			this.mtuExchanged = true;
-		}*/
+		}
 
 		return this.gatt;
 	}

lib/bindings/hci/Peripheral.js.map

@@ -3,12 +3,15 @@ import { HciAdapter } from '../../Adapter';
 import { Hci } from '../../misc';
 import * as CONST from '../Constants';
 
+// 512 bytes is max char size + 1 byte att opcode + 2 bytes handle + 2 bytes offset for long writes
+const DEFAULT_MAX_MTU = 517;
+
 export class HciGattLocal extends GattLocal {
 	private hci: Hci;
 
 	private negotiatedMtus: Map<number, number>;
 
-	public constructor(adapter: HciAdapter, hci: Hci, maxMtu: number = 256) {
+	public constructor(adapter: HciAdapter, hci: Hci, maxMtu: number = DEFAULT_MAX_MTU) {
 		super(adapter, maxMtu);
 
 		this.hci = hci;
@@ -98,12 +101,8 @@ export class HciGattLocal extends GattLocal {
 	private handleMtuRequest(_handle: number, _cid: number, request: Buffer) {
 		let mtu = request.readUInt16LE(1);
 
-		if (mtu < 23) {
-			mtu = 23;
-		} else if (mtu > this.maxMtu) {
-			mtu = this.maxMtu;
-		}
-
+		mtu = Math.max(23, Math.min(mtu, this.maxMtu));
+		console.log('mtu negotiated', mtu);
 		this.negotiatedMtus.set(_handle, mtu);
 
 		const response = Buffer.alloc(3);

lib/bindings/hci/gatt/local/Gatt.js

@@ -71,7 +71,7 @@ const main = async () => {
 
 	console.log('Starting advertisement...');
 
-	await adapter.startAdvertising(NAME);
+	await adapter.startAdvertising(NAME, ['48ee0000bf49460ca3d77ec7a512a4ce']);
 
 	console.log(adapter.address);