MTU exchange seems to fail, nRFConnect keeps reporting 23

[Remco Poelstra]

Hi,

I'm using nRF Connect to connect to the secure bootloader. In the debug log I see that an MTU of 185 is requested and 183 is replied. I also see this in the Wireshark log.

Strangely though, nRF Connect still reports a MTU of 23.

I've a similar problem with the actual firmware, but I thought it's best to initially solve the problem using your (more stable) code.At least for the firmware there have been times that other MTU sizes were reported by nRF Connect.

What am I missing? How can I make sure the MTU size is properly negotiated.

Thanks in advance.

Kind regards,

Remco Poelstra

[Remco Poelstra]

I should probably add that the central is an iPhone SE.

[Amanda Hsieh]

Hi Remco, 

What is the SDK version you're using?

What are the values of NRF_SDH_BLE_GAP_DATA_LENGTH and NRF_SDH_BLE_GATT_MAX_MTU_SIZE in the sdk_config.h?

Could you also provide the nRF Connect log?

-Amanda H.

[Remco Poelstra]

Hi,

Thanks for your answer.

I'm using SDK 16.0.0, from the 7.0.1 HomeKitADK

The defines:

#define NRF_SDH_BLE_GAP_DATA_LENGTH 27

#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247

The log is attached. I created it under macOS, as I can't find any button like you've shown on the iPhone nRF app.2020-06-05T09_35_36.256Z-log kopie.txt

[Amanda Hsieh]

Hi, 

iOS will for some reason accept a smaller MTU size if the peripheral initiates the request first. You need to do the DLE request first and wait for the MTU request from IOS. You can refer to https://devzone.nordicsemi.com/f/nordic-q-a/44825/ios-mtu-size-why-only-185-bytes

-Amanda H.

[Remco Poelstra]

Hi,

I think my problem is not similar as from the post you mention. I do not initiate an MTU request (with `sd_ble_gattc_exchange_mtu_request()`) and my MTU is also not limited to 185, but to 23, even though the communication seems to suggest that 185/183 is negotiated.

[Amanda Hsieh]

Hi Remco, 

It seems like the MTU size might be reported incorrectly in the nRF Connect app. Maybe ask him to connect to the \nRF5_SDK_16.0.0_98a08e2\examples\ble_peripheral\ble_app_uart and see if gives the same result .

-Amanda H.

[Remco Poelstra]

Hi,

The ble_app_uart properly reports as MTU of 185 in nRF connect on the iPhone.

[Remco Poelstra]

Hi,

Amanda Hsieh said:

You need to do the DLE request first and wait for the MTU request from IOS.

It seems it's the other way around. The ble_uart example actively asks for a MTU and get's a 'BLE_GATTC_EVT_EXCHANGE_MTU_RSP' event before the DLE event. While the bootloader only gets a 'BLE_GATTS_EVT_EXCHANGE_MTU_REQUEST'. 

Is there a way I can verify (maybe by inspecting packets) whether the MTU exchange was successful?

Is there a difference between me asking an MTU update vs. the telephone asking the MTU update? 

[Amanda Hsieh]

Hi Remco, 

We can reproduce the issue with iPhone 8. I have reported to the internal that the MTU is updated but that it's not correctly shown in the app. Thanks for the report. 

-Amanda H.