3GPP 23.501 v20.2.0 — the document's own text
5.21.1.2 NGAP UE-TNLA-binding
Taught in 27. Overload, energy, deployment and the rest (The 5G system architecture, in depth).
While a UE is in CM-Connected state the 5G-AN node shall maintain the same NGAP UE-TNLA-binding (i.e. use the same TNL association and same NGAP association for the UE) unless explicitly changed or released by the AMF.
An AMF shall be able to update the NGAP UE-TNLA-binding (i.e. change the TNL association for the UE) in CM-CONNECTED state at any time. The NGAP UE-TNLA-binding can also be updated when a UE-specific NGAP message initiated by AMF is received via a new TNL association.
An AMF shall be able to update the NGAP UE-TNLA-binding (i.e. change the TNL association for the UE) in response to an N2 message received from the 5G-AN by triangular redirection (e.g. by responding to the 5G-AN node using a different TNL association).
An AMF shall be able to command the 5G-AN node to release the NGAP UE-TNLA-binding for a UE in CM-CONNECTED state while maintaining N3 (user-plane connectivity) for the UE at any time.