NRF_BLE_GATT_MAX_MTU_SIZE > 23 and backwards compatibility

[sijohans]

Hello,

I didn't find much information about how different settings for the MTU size behaves with older versions of BLE devices. I'm using the softdevice s132 4.0.2.

/*  Configure the maximum ATT MTU. */
memset(&ble_cfg, 0x00, sizeof(ble_cfg));
ble_cfg.conn_cfg.conn_cfg_tag                       = BLE_CONN_CFG_TAG;
ble_cfg.conn_cfg.params.gatt_conn_cfg.att_mtu       = NRF_BLE_GATT_MAX_MTU_SIZE; //247
err_code = sd_ble_cfg_set(BLE_CONN_CFG_GATT, &ble_cfg, ram_start);
APP_ERROR_CHECK(err_code);

How will this work with a device that supports max 23? My device is a peripheral. Will the central that connects, negotiate this to 23?

[run_ar]

The att mtu length will be negotiated. This is done with an Exchange MTU Request (sd_ble_gattc_exchange_mtu_request) and a Exchange MTU Response (sd_ble_gatts_exchange_mtu_reply). The default length will be used unless a longer MTU is negotiated.

[sijohans]

I posted a comment but it seemed to disappear. I still don't really understand the flow here. For instance, when connecting with an android device i get the following printout:

nrf_ble_gatt:DEBUG:Requesting to update ATT MTU to 247 bytes on connection 0x0.
nrf_ble_gatt:DEBUG:Requesting to update data length to 251 on connection 0x0.
nrf_ble_gatt:ERROR:sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.
nrf_ble_gatt:DEBUG:ATT MTU updated to 247 bytes on connection 0x0 (response).

Return code 0x13 seems to be NRF_ERROR_RESOURCES. The peripheral must support centrals that only support a MTU size of 23. But we really would like a bigger MTU size to increase the throughput.

The debug output seems to have more to do with data length update than MTU.