5GC - How AMF will find the best UPF/SMF?

How AMF will find the best UPF/SMF ?
Still using DNS or replaced by NRF ?

How AMF will find old AMF ?

AMF will find suitable SMF based on few key parameters:

  • DNN
  • S-NSSAI
  • TA
    AMF can use either NRF (using NF Discovery Procedure) OR using local configuration to select SMF
    Further SMF can select UPF based on different set of input parameters such as DNN (+S-NSSAI), LADN etc. using local configuration or using NRF interworking.

For the Old AMF selection, new AMF can use 5G-GUTI provided by UE.

DNS is useful in some Edge configuration for the other applications, NRF can provide information depending upon services.
There may be preconfiguration of the SMF served by particular AMF which can be selected, details available in 23.501 & 502 specs

How UPF will register itself in NRF? Mainely, SMF will select UPF based on local config only.

UPF can be enhances to connect over SBI and further UPF can register itself with NRF. SMF then can utilize NRF discovery to select UPF.