CKSN clarification

[Max]

Hello.

CKSN - Ciphering Key Sequence Number is associated with Kc and is a bit trickier than
trivial counter.

According to GSM 04.08 (p. 278 in v5.3.0 in my case) it's comprised as follows:
* 3 bits - actual key sequence (value 111 seems to be reserved)
* 1 bit - always 0
* 3 bits - CKSN IEI

Could you help me to understand how to properly generate it:
- the last bit (8th) is not specified - does it mean it's always 0?
- what is CKSN IEI and how do I calculate\use it?
- am I right that 3-bit key seq. value is simple counter that I increment with each
Kc generation?
- what do I do when key seq. reach 6 - start over with 0 again?

And of course I'm interested if CKSN-related code is available as part of libosmogsm
or some other open source project: the code is always helpful in understanding and I
don't have to reinvent the wheel :)

best regards,
Max.