Long time to disconnect

[nordev]

Hi, 

Our NRF52840 (running 14.2) often takes a very long time to disconnect, from when a disconnection is initiated by the phone. It can take 30s, sometimes even more, for the disconnect to happen. Sometimes the disconnect happens immediately like it should, but I would say that it is more common for the disconnect to take a long time than for the disconnect to be normal. 

I installed the NRF Sniffer and have done a capture, but I am not really sure what to make of the result. 

The below was a bad disconnect. Here, the highlighted line was about when the disconnect was initiated on the phone (image 1). Then I got "Empty PDU" for a long time. The empty PDU continues for a long time, with only one "encrypted packet decrypted incorrectly" in between, then in the second image, the highlighted line shows when the disconnect actually happens. There is ~13 sec in between the initiation and the actual disconnect.

By contrast, this was one of the times that I got a normal disconnection. 

The above was done with our custom app. The issue also happens when using the NRF Connect App for iOS. For reference, below is the capture when the disconnect was initiated by the NRF app:

I pressed the disconnect button around the time of this highlighted line: 

There is no command of any kind here though; the closest command is this one (but I don't know if they're even related, this is just the first command I found when scrolling up):

The disconnect comes what looks like 23 seconds later: 

My questions:

(MAIN QUESTION) How can I find out why it is taking a long time to disconnect? Note that it happens with different phones, but we have only been able to test with iPhones so far.

AUXILLIARY QUESTIONS: 

1. What are control opcodes and why do they only show up in the test using the NRF app, but not our custom app? Should we do something to enable these opcodes? 

2. What is empty PDU? 

3. What is "bad MIC"?

4. Where do I learn about how to interpret the captured packets? I read this but it doesn't really explain what I should be looking for.

Thanks!

[nordev]

I will send it to you via private message.

As a further update, I noticed that the long disconnect does not happen with Android devices, which all disconnect immediately. It only happens with iOS devices. I tried with multiple Android devices and multiple iOS devices.

This would seem to suggest that maybe it is a phone issue, but the iOS devices disconnect just fine when I use one of the Nordic example apps. So it is maybe something to do with how the iOS BLE stack interacts with the softdevice?

(I know Apple has particular requirements for the connection parameters, but I followed all the parameters in section 36.6 here, and the issue persisted. Values I used are below)

#define MIN_CONN_INTERVAL                MSEC_TO_UNITS(15, UNIT_1_25_MS)            /**< Minimum acceptable connection interval (0.1 seconds). */
#define MAX_CONN_INTERVAL                MSEC_TO_UNITS(500, UNIT_1_25_MS)          /**< Maximum acceptable connection interval (0.2 second). */
#define SLAVE_LATENCY                    0                                          /**< Slave latency. */
#define CONN_SUP_TIMEOUT                 MSEC_TO_UNITS(5000, UNIT_10_MS)