Android Ble library MTU change on peripheral

RE: Android Ble library MTU change on peripheral

Hieu — Thu, 02 Mar 2023 17:34:52 GMT

Hi Jean Baptiste,

I see, it looks like the MTU negotiation was indeed successful.

To have the MTU value updated correctly within a BleManager, you have two approaches:

If you setup a BleServerManager, the BleServerManager will receive the necessary callback and update the MTU value automatically for you.
Here is an example for BleServerManager initialization: link.
The BleServerManager must also be added to each BleManager instance; refer to: link.
In your android.bluetooth.BluetoothGattServerCallback#onMtuChanged(), you must call BleManager#overrideMtu().

Please try one of the two options and let me know if it helps you.

RE: Android Ble library MTU change on peripheral

Jean-Baptiste — Thu, 02 Mar 2023 07:33:03 GMT

Hi Hieu,

thank you for your answer,

the MTU request and update is reflected in both logs of the app and the nrf :

RTT traces :

...
00> [12:00:37] <debug> nrf_ble_gatt: Requesting to update ATT MTU to 245 bytes on connection 0x0.
00> [12:00:37] <debug> nrf_ble_gatt: Updating data length to 249 on connection 0x0.

...
00> [12:00:38] <debug> nrf_ble_gatt: ATT MTU updated to 245 bytes on connection 0x0 (response).
00> [12:00:38] <info> BLE: The ATT_MTU size was updated to 245 bytes.
...

APP log :

Sniffer :

Then from the sniffer trace, we can observe that the transaction from the nrf to the app ( indications or notifications ) use correctly the new MTU, but the app using the nordic android library still considers that the mtu has not changed and continue splitting the long packets in small chunks.

RE: Android Ble library MTU change on peripheral

Hieu — Wed, 01 Mar 2023 19:39:10 GMT

Hi Jean-Baptiste,

I am asking our app team regarding your question about the design decision around the tracking of the MTU value. Looks to me that the API expects you to call overrideMtu() on the relevant MTU callbacks.

Meanwhile, I would like to check if MTU negotiation was actually successful between the devices.

What are the logs on both the Android side and the nRF52840 side concerning the MTU exchange? Is the result reflected in your sniffer trace?

On the nRF52840 device, if your application is using the nrf_ble_gatt library, you can enable log level debug for that library and see the result of the negotiation.
If the application is handling the negotiation on its own, then you can log the event data of the SoftDevice event BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST.

Hieu