Missing UL NAS RegistrationComplete

[vulcanfan]

Hi, Just running the default registration, auth, security and PDUEstablishment procedure that the ue does on start-up, but I see no UL NAS RegistrationComplete message even though a new GUTI as been allocated by the core. I would expect it piggybacked onto the UL InitialContextSetupResponse message (although I don't think that in itself is mandatory).

I don't know enough about the code base to trace it through the gnb, so not sure where it may be being dropped. Any thoughts?

Many thanks

[aligungr]

Hi @vulcanfan

RegistrationComplete is sent in the following 3 cases:

1. GUTI included in the RegistrationAccept message
2. Configured NSSAI included in the msg.
3. NSSCI indication received in the msg.

If none of the conditions is satisfied, then UE does not send a Registration Complete message. (Otherwise there might be a bug in the UE.)

A PCAP file might be helpful to analyze the situation.

Thanks

[vulcanfan]

Hi,

Yes, adding some debug in src/ue/nas/mm/register.cpp shows that the flag 'sendComplete' is set correctly, as the GUTI condition is met. So the UE may believe it's sending the RegistrationComplete, but it doesn't appear in a Wireshark on the N2/NGAP interface to the core. I've attached the Wireshark here.

Maybe the gNB is blocking UL NAS messages whilst it is in the InitialContextSetupRequest/Response cycle?

MissingRegComplete.zip

Thanks

[aligungr]

Please see the packet 17 has two NGAP messages. Registration Complete is included in the first NGAP PDU. However Wireshark only considers the last one while showing packet info column.

The main reason is that SCTP messages can contain more than one NGAP PDUs, (and it's not deterministic)

[vulcanfan]

Ah, great, my bad!

Thanks very much :-)