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[AMF] Do not send Deregistration Event to UDM when UE deregisters
According to TS 23.502, 4.2.2.2.2, AMF sends Registration event to UDM in the following cases: - If the AMF has changed since the last Registration procedure, or - if the UE provides a SUPI which doesn't refer to a valid context in the AMF, - or if the UE registers to the same AMF it has already registered to a non- 3GPP access (i.e. the UE is registered over a non-3GPP access and initiates this Registration procedure to add a 3GPP access). In case that UE re-registers to the network with a GUTI, it bypasses authentication check to the AUSF. In this case, AMF does not send Registration event to UDM. Consequently, when UE deregisters again, AMF would send a Deregistration Event to a UDM, which does not have a context for it. 3GPP standard does not say when AMF sends Deregistration Event to UDM, only that it is optional. These (De-)Registration events are for (de-)registering AMF to the UDM for serving the UE. And not for (de-)registering UE itself for purpose of tracking when UE is registered on the network. This partially reverts commit 7be7029
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