bt_gatt: No buffer available to send notification

[Gabriele]

Hello,
As for my peripheral BLE NUS service under NCS 1.9.2, when I search and connect it from my Windows PC running nRF Connect for Desktop BLE Standalone 4.0.0, I found that the ATT MTU default value (23) is insufficient to allow the transmission of messages from peripheral to host.
On the peripheral console I get

    <wrn> bt_att: No ATT channel for MTU 23
    <wrn> bt_gatt: No buffer available to send notification

Then I have to navigate through the "Update MTU" setting and type a larger value.
Can this be fixed programmatically on the peripheral side ?
Alternatively, can the default setting be changed on the host application ?

[Hieu]

Hi Gabriele,

Can this be fixed programmatically on the peripheral side ?
Alternatively, can the default setting be changed on the host application ?

Yes, it can be done both in the peripheral side and in the central side.

In the NCS peripheral side, you can take a look at bt_gatt_exchange_mtu().

For the central side, I found that Android provides requestMtu().
For Windows, I found this thread with quite in-depth discussions and information.
For iOS, I found this thread which says at least in 2021, it is not possible to request MTU.

Please note that the maximum ATT MTU is also limited by several Kconfig settings in NCS. You can see more information in this DevZone Q&A:  Updating MTU Size for Tx and Rx with Android BLE Client 

Best regards,

Hieu

[Gabriele]

Hi Hieu,

calling bt_gatt_exchange_mtu right after connection and adjusting some constants in prj.conf do the job.

Thank you so much

[lcj]

What settings did you change in the prj.conf?

[Gabriele]

I've defined this

static void exchange_func(struct bt_conn *conn, uint8_t err,
    struct bt_gatt_exchange_params *params)
{
    if (!err) {
        /* According to 3.4.7.1 Handle Value Notification off the ATT protocol.
         * Maximum supported notification is ATT_MTU - 3 */
        uint32_t bt_max_send_len = bt_gatt_get_mtu(conn) - 3;
        LOG_INF("max send len is %d", bt_max_send_len);
    }
}

On connection event, I do this

/* maximize ATT MTU at peer side (CONFIG_BT_L2CAP_TX_MTU)*/
    exchange_params.func = exchange_func;
LOG_INF("sending ATT MTU to peer..");
rc = bt_gatt_exchange_mtu(current_conn, &exchange_params);
if (rc) {
    LOG_ERR("failed to negotiate maximum mtu with peer [%d]", rc);
}

In prj.conf, set this (or wahtever you want)

CONFIG_BT_L2CAP_TX_MTU=247