S-GW Restoration
helps in handling the S-GW failure in the EPC network. It allows affected PDNs
that fail due to S-GW to be restored by selecting another S-GW to serve the
affected PDNs. This avoids unnecessary flooding of signaling for PDN cleanup.
The P-GW maintains
the sessions in case path failure is detected or if S-GW restart is detected
during recovery IE on GTP-C signaling. The P-GW will ensure that any dropped
packets in this scenario are not charged. The P-GW also rejects any bearer
additions or modification requests received for the PDN connection maintained
after the S-GW failure detection. This occurs until the PDN is restored.
Once the session has
been restored by the MME and the P-GW receives a Modify Bearer Request from the
restarted S-GW or a different S-GW, then the P-GW continues forwarding any
received downlink data and start charging them.
When a subscriber is
in S-GW restoration phase, all RARs (expect for Session Termination) reject the
PCEF. The P-GW rejects all internal updates which can trigger CCR-U towards the
PCRF. The P-GW triggers a CCR-U with AN-GW changes for the PDNs that are
restored if the S-GW has changed on restoration.
The MME/S4-SGSN is
locally configured to know that the P-GW in the same PLMN supports the S-GW
restoration feature. When this feature is enabled at the P-GW, it supports it
for all S-GWs/MMEs.
Important:
Only MME/S4-SGSN
triggered S-GW restoration procedure will be supported.
S-GW restoration
detection based on GTP-U path failure shall not be considered for this release.
GTP-C path failure detection should be enabled for enabling this feature.
S-GW restoration
detection based on GTP-U path failure shall not be considered for this release.
GTP-C path failure detection should be enabled for enabling this feature.
The P-GW Restart
Notification may also be used to signal that the peer P-GW has failed and not
restarted. In this case, the P-GW Restart Notification contains a cause value:
P-GW not responding. While sending the PRN, the S-GW includes the cause with
this new cause value depending on the echo response.