When it becomes necessary to temporarily remove an Oracle Communications Session Border Controller (SBC) from active service, and make it available only for administrative purposes, the user issues a set-system-state offline ACLI command. The SBC begins a graceful shutdown. The shutdown is graceful in that active calls and registrations are not affected, but new calls and registrations are rejected except as discussed below. When the user issues the command, the SBC goes into becoming offline mode. Once there are no active SIP sessions and no active SIP registrations in the system, the SBC transitions to offline mode. If the SBC is a member of an Oracle Communications Subscriber-Aware Load Balancer (Subscriber-Aware Load Balancer) Cluster, the offline status is communicated to the Subscriber-Aware Load Balancer when the user issues the set-system-state offline command, and the Subscriber-Aware Load Balancer excludes the offline SBC in future endpoint (re)balancing algorithms.

A version of this SBC graceful shutdown procedure exists in SBC releases previous to S-CZ7.3.0, but the procedure is enhanced for this and future releases. Previous versions only looked at active SIP sessions (calls), without monitoring active SIP registrations, and did not attempt to manipulate the period of time that active calls and registrations lingered on the SBC. The problem with this approach was that in the interval between setting the SBC to offline mode, and the subscriber registrations expiring, any inactive subscriber was essentially unreachable. With some carriers setting registration expiry timers to an hour or more (or 30 minutes in between registration refresh), this may have resulted in significant periods of unreachability. With this release, the new sip-config parameter retry-after-upon-offline is used to minimize the amount of time active calls and registrations keep the SBC from going completely offline.

The SBC side of this graceful shutdown procedure is followed with or without the SBC being a member of an Subscriber-Aware Load Balancer cluster. The graceful shutdown procedure is limited only to SIP calls and registrations.