MTU negotiation problem

[dingari]

I'm facing a rather strange problem with an nRF52832 board I'm developing for. I'm using SDK 15.2 and S132 v6.0.0.

Sometimes, during the connection/negotiation phase, my device seems to get stuck in a weird state and can't send notifications. I've examined my logs and found out that when it works, I receive BLE_GATTC_EVT_EXCHANGE_MTU_RSP, and when it gets stuck, there is no such event received.

This is an example of a failed connection attempt:

<info> app: BLE_GAP_EVT_CONNECTED, conn handle: 1, role: 1
<debug> app: max_conn_interval: 11 ms
<debug> app: min_conn_interval: 11 ms
<debug> app: slave_latency: 0 connection events
<debug> app: conn_sup_timeout: 2000 ms
<debug> nrf_ble_gatt: Data length updated to 64 on connection 0x1.
<debug> nrf_ble_gatt: max_rx_octets: 64
<debug> nrf_ble_gatt: max_tx_octets: 64
<debug> nrf_ble_gatt: max_rx_time: 2120
<debug> nrf_ble_gatt: max_tx_time: 2120
<debug> app: BLE EVENT: 0x0024
<debug> nrf_ble_gatt: Peer on connection 0x1 requested an ATT MTU of 515 bytes.
<debug> nrf_ble_gatt: Updating ATT MTU to 64 bytes (desired: 64) on connection 0x1.
<debug> app: BLE EVENT: 0x0055
<debug> app: BLE EVENT: 0x0050
<debug> app: BLE_GATTS_EVT_WRITE, handle: 0x0017, len: 2
<error> app: Failed to notify, error: NRF_ERROR_INVALID_STATE (0x0008)

The central I'm connecting to is a Mac. This happens in the logs on the central:

default	12:11:12.512164+0000	bluetoothd	Sending MTU Request - MTU:515 Transport:2
default	12:11:12.555573+0000	bluetoothd	Received MTU Response - Response:64 Requested:515 Transport:2

This is an example of a successful attempt:

<info> app: BLE_GAP_EVT_CONNECTED, conn handle: 1, role: 1
<debug> app: max_conn_interval: 11 ms
<debug> app: min_conn_interval: 11 ms
<debug> app: slave_latency: 0 connection events
<debug> app: conn_sup_timeout: 2000 ms
<debug> nrf_ble_gatt: Data length updated to 64 on connection 0x1.
<debug> nrf_ble_gatt: max_rx_octets: 64
<debug> nrf_ble_gatt: max_tx_octets: 64
<debug> nrf_ble_gatt: max_rx_time: 2120
<debug> nrf_ble_gatt: max_tx_time: 2120
<debug> app: BLE EVENT: 0x0024
<debug> nrf_ble_gatt: Peer on connection 0x1 requested an ATT MTU of 515 bytes.
<debug> nrf_ble_gatt: Updating ATT MTU to 64 bytes (desired: 64) on connection 0x1.
<debug> app: BLE EVENT: 0x0055
<debug> nrf_ble_gatt: ATT MTU updated to 64 bytes on connection 0x1 (response).
<debug> app: BLE EVENT: 0x003A
<debug> app: BLE EVENT: 0x0050

And the central:

default	12:14:35.229238+0000	bluetoothd	Sending MTU Request - MTU:515 Transport:2
default	12:14:35.260755+0000	bluetoothd	Received MTU Request - MTU:64 Transport:2
default	12:14:35.264938+0000	bluetoothd	Received MTU Response - Response:64 Requested:515 Transport:2

Is there anything obvious that could be missing in some parameters/negotiation steps?

[Simonr]

Hi

I think we're still misunderstanding one another. The MTU request is not required by the Bluetooth specification, but we have opted to add this MTU request in our on_connected_evt. on_connected_evt is not called before the BLE_GAP_CONNECTED event, but rather called when this event is triggered (l497 in nrf_ble_gatt.c)

void nrf_ble_gatt_on_ble_evt(ble_evt_t const * p_ble_evt, void * p_context)
{
    nrf_ble_gatt_t * p_gatt      = (nrf_ble_gatt_t *)p_context;
    uint16_t         conn_handle = p_ble_evt->evt.common_evt.conn_handle;

    if (conn_handle >= NRF_BLE_GATT_LINK_COUNT)
    {
        return;
    }

    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GAP_EVT_CONNECTED:
            on_connected_evt(p_gatt, p_ble_evt);
            break;

        case BLE_GAP_EVT_DISCONNECTED:
            on_disconnected_evt(p_gatt, p_ble_evt);
            break;

        case BLE_GATTC_EVT_EXCHANGE_MTU_RSP:
            on_exchange_mtu_rsp_evt(p_gatt, p_ble_evt);
            break;

        case BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST:
            on_exchange_mtu_request_evt(p_gatt, p_ble_evt);
            break;

#if !defined (S112)
        case BLE_GAP_EVT_DATA_LENGTH_UPDATE:
            on_data_length_update_evt(p_gatt, p_ble_evt);
            break;

        case BLE_GAP_EVT_DATA_LENGTH_UPDATE_REQUEST:
            on_data_length_update_request_evt(p_gatt, p_ble_evt);
            break;
#endif // !defined (S112)

        default:
            break;
    }

Best regards,

Simon

[dingari]

Hi Simon,

I understand. I just meant that the GATT module receives BLE_GAP_CONNECTED before my application.

Don't you think there's an imposed problem if both peripheral and central simultaneously initiate an MTU update request? If I don't request the MTU update from the peripheral, I never get in the strange state where my notifications fail with NRF_ERROR_INVALID_STATE.