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 have another clue: 

The "long disconnect" only happens when we try to disconnect within 30s of starting a new connection. 

If the device has been connected for more than 30s, it will disconnect immediately when the disconnect is initiated.

If the device has been connected for less than 30s when the disconnect is initiated, it will disconnect 31 seconds after the initial connection, no matter whether the disconnect was initiated 10s, 15s, 20s, etc after the initial connection. 

Again, the below is the only thing in my code that seems to use a 30s delay: 

#define NEXT_CONN_PARAMS_UPDATE_DELAY    APP_TIMER_TICKS(30000)                     /**< Time between each call to sd_ble_gap_conn_param_update after the first call (30 seconds). */

But changing this parameter does not seem to affect the long disconnect. 

So, why might this be happening?