MME retains UEs in the ECM-CONNECTED during SCTP failures, or during SCTP recovery triggered from eNodeB for an association.
An SCTP PDU is sent to the SCTP stack to process the received chunk. On completion, the SCTP stack indicates an association restart to the MME manager after re-establishing an SCTP association.
On the arrival of S1-Setup Request, SCTP stack sends a data indication
to the MME manager, which sends the data indication to S1-AP stack for further
S1-AP stack maintains a peer control block consisting of a peer state
and a peer association state. A peer state update will be sent to SESSMGR when
peer state changes from normal state to setup in progress state and another
update for setup in-progress state to normal state. S1-AP stack sends an update
to its peer MME DEMUX and SESSMGR (Session Manager) and sends a data indication
to the MMEMGR, indicating the SCTP peer state change during the
re-establishment of a broken association, by S1-Setup Request message and
S1-Setup Response message.
On receiving a peer state update from the S1-AP stack for an eNodeB
association, SESSMGR sends a release indication to all UEs connected through
that eNodeB association. SESSMGR moves all the ECM-CONNECTED state UEs to
ECM-IDLE state by sending Release Access Bearer Request message to SGW.
The purpose of a peer state update request from S1-AP stack to SESSMGR is to inform peer state change to SESSMGR, and force the SESSMGR to move the ECM-CONNECTED state UEs to ECM-IDLE state by sending Release Access Bearer Request message to SGW.
With the UE Context Retention feature, the S1-AP stack does not send an update to the SESSMGR when a UE Retention Information IE is received in the S1-Setup Request message. This is because the S1-Setup Request message after a restart of SCTP association only changes the peer state (Normal to Set up in-progress) when eNodeB detects SCTP association failure before MME detects.
S1-AP stack can check S1-AP PDU to check presence of UE Retention
Information IE in S1-Setup Request message, and need not to send peer state
change update to SESSMGR if MME supports UE retention feature and UE Retention
Information IE is present in S1-Setup Request message. MMEMGR shall include UE
Retention Information IE in S1-Setup Response message if UE Retention
Information IE received in S1-Setup Request message and UE retention feature
enabled in MME service.
During inbound SCTP Abort or SCTP Shutdown, MME starts the retention
timer when it detects SCTP association failure.