The communication service providers (CSP) would like the ability to take different actions depending on the severity of the error received from the PGW (S2B interface). If there is a temporary congestion in the network, a retry is appropriate.
The ePDG needs mapping of S2B to SWu error codes for communicating different error codes to device, enabling device to identify whether its temporary failure or permanent and can accordingly try connecting to the ePDG.
The ePDG continues to release the call while notifying the UE about the S2B error, however the UE based on error code shall take decision when to try connecting again.
For the mapping ePDG uses Notify Error Message type between 31 to 8191 from the range reserved for IANA or from the private range 8192 to 16383.