IMS: How an LTE UE deregisters from the IMS network?

Question

What signalling messages are exchanged between LTE and IMS nodes for the IMS de-registration ?

Answer 1

There are two scenarios what I know as following:
1. UE initiated IMS de-registration.
2. Network initiated IMS de-registration.

In first case, UE sends SIP register request message to P-CSCF with the value of expires = 0. P-CSCF forwards SIP register request message to I-CSCF and I-CSCF updates the information to HSS by exchanging UAR and UAA.
After that I-CSCF forwards the same SIP register to S-CSCF node which exchanges SAR/SAA messages with the HSS and also notify to application server. Since state is changed, S-CSCF node sends notify message to P-CSCF and P-CSCF forwards the message to UE. UE acknowledges with 200 OK.
Once the application server confirms with 200 OK for the notify message sent from S-CSCF, S-CSCF sends notify message to P-CSCF node. P-CSCF initiates session termination procedure towards PCRF node.

In second case, when UE switch off the mobile, bearer release is detected by PCRF to P-CSCF node. P-CSCF itself prepares and send SIP register request message to I-CSCF by setting expires = 0. Further signalling is same as above but nothing will exchange with UE since bearers have already been deleted.

Comments

Well explained.

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In which scenarios would a UE send a de-registration? I'm seeing many dropped calls in my network with the cause code "de-registering".