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c67bddd2b4
In case an external HSS is used, and the NAM field is set to 0 (PACKET_ONLY),
Open5GS MME will only respond with an "EPS_ONLY" attach accept. This behavior
causes a lot of UEs (mainly phones) to disconnect after 1-2 seconds without
further signalling.
To resolve this, a new flag is introduced:
```
global:
parameter:
fake_csfb: true
```
If this flag is set to 'ture', the MME will respond with a combined EPS/IMSI
attach accept even if the HSS NAM field is set to "PACKET_ONLY", or if the
MME has no SGs connection towards a CS core.
By default this flag is false, thus not modifying the original behavior.
Note: some commercial core network vendors do include the LAI part in a
"fake" combined EPS/IMSI attach accept message. As that field is optional, and
testing also indicates that it is not needed, this patch does not implement it.