UE: Derive Serving Network Name from SIB1 PLMN

Aaron van Diepenrequested to merge
nr-ue-roaming-fix into develop

This MR modifies the nrUE to use the Serving Network Name derived from data communicated by the Serving Network instead of data stored in the UICC. Previously, the Serving Network Name (SNN) used during authentication was constructed from the IMSI, implicitly assuming the UE was attached to its home network. In roaming scenarios, this produced an incorrect SNN, causing authentication and key derivation failures.

Changes:

  • Extract the MCC and MNC from the SIB1 PLMN identity communicated by the serving network.
  • Store these values in the UE NAS context in order to propogate them from RCC to NAS.
  • The NAS key derivation functions (transferRES, derive_kausf, derive_kseaf) now build the SNN based on the MCC/MNC broadcast in SIB1.

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