This question is related to EPS Fallback with Redirection.
Does EPC/MME inform 5GC/AMF of the EPS Fallback success in case of redirection?
If yes, does AMF send an acknowledgement message to the CU-CP, “EPS Fallback Success”?
I cannot find anything in 3GPP specs that I have read yet !
Any 3GPP guide that I may have missed?
Redirection isn’t fully controlled compared to handover procedure where old and new connecting parties negotiate and gracefully move session. Once the UE is informed (RRC Redirect) to go to other technology, existing connection is terminated and no success/failure is exchanged between controllers (MME and AMF in this case).
Right.
Then what would be the way for me to make sure that the remaining data at the CU PDCP layer is to the new eNB/EPS when the UE moves as a result of fallback?
I was assuming that the AMF would receive some form of confirmation from the AMF, and consequently AMF would pass that message to CU-CP via NGAP:UE Context Release Command. After which the CU-UP would be allowed to forward the data the new connecting EPC/eNB.
There is no success message for redirection procedure.
There is a release with the cause “other” with Redirect Info.
If N26 interface is available between MME and AMF, then UE perform TAU after reselecting LTE carrier.
To get this TAU success MME require UE context from AMF.
So UE context request and response messages are exchanged between MME and AMF.
This gives some idea about EPS FB success at 5G Core level.
But there is no communication about this to gNB.
if N26 interface is not available, then TAU is rejected and fresh attach is perform.
In this case even 5G core is not involved.
I have more generic theory about this:
Redirection isn’t fully controlled compared to handover procedure where old and new connecting parties negotiate and gracefully move session.
Once the UE is informed (RRC Redirect) to go to other technology, existing connection is terminated and no success/failure is exchanged between controllers (MME and AMF in this case).
But TAU require UE context.
May be your scenario is not TAU but fresh attach in LTE?
No, no information is provided.
Since this is somewhat open loop handover and not controlled.