Call Preservation

Last Updated : Mar 08, 2023 |

With the Call Preservation feature, the dialog context of the SIP user agent can survive a Session Manager failure even when the Session Manager context is lost. The dialog continues with end-to-end signaling of the intact user agent through an alternate Session Manager. The Call Preservation feature is available for SIP Routing Element (SRE) flows.

For the Call Preservation, a Session Manager Failover Group comprising a pair of Session Manager servers is associated with peer entities. The peer entities, such as Avaya SBC, use enhanced SIP timing and recovery techniques to provide signaling path continuity during Session Manager failure. When Avaya SBC detects that a Session Manager is unreachable, it uses the Failover Group Domain Name (FGDN) in the Session Manager through and Record-route headers to route the SIP traffic through the alternate Session Manager. The FGDN is a fully qualified domain name (FQDN) that resolves to an ordered set of Session Manager servers within a Session Manager Failover Group that provides a high availability SRE service. When the preferred Session Manager becomes unresponsive, the peer SIP entity uses the Session Manager Failover Group Domain resolution to identify and communicate with the alternate Session Manager.

The naming convention for the failover group is as follows:

  • Failover group name: Primary SM-Secondary SM

    For example, sm1–sm2.

  • Primary FGDN: Primary SM-Secondary SM.sip domain

    For example, sm1–sm2.example.com.

  • Secondary FGDN: Primary SM-Secondary SM-Identifier.sip domain

    For example, sm1–sm2–2.example.com.

  • Session Manager FQDN: SM.SM IP Domain

    For example, sm1.example.com.

The Session Manager failover group can contain two or more Session Manager member instances. The primary Session Manager carries the traffic for the failover group in normal conditions. For more information about administering the Call Preservation feature, refer Call Preservation Feature Description and Administration Guide.

To support the Call Preservation feature, Avaya SBC:

  • Maintains an affinity to the last preferred Session Manager in the failover group for every dialog.

  • Preserves the failover group target set in the dialog context. This caching in the dialog prevents unnecessary duplicate DNS queries.

  • Supports requests with an FGDN in the Via, Next Hop Route, or Record-Route headers.

  • Resets the TCP or TLS socket to the failed Session Manager if Avaya SBC detects that the preferred Session Manager is unreachable.

  • Supports Call Preservation on TCP and TLS transport types.

  • Changes the dialog-scoped affinity to the preferred Session Manager when the preferred Session Manager instance becomes unreachable. Avaya SBC reevaluates the affinity by selecting one of the following:

    • The alternate Avaya SBC with highest priority

    • The alternate Session Manager with highest priority, excluding any alternate Session Manager instances in the FG that are already unavailable.

  • Detects that a Session Manager is back in service and reachable within the interval configured in the Frequency field on the Server Configuration page in the Heartbeat tab.

    Important:

    Heartbeat configuration is mandatory for the Call Preservation feature. The heartbeat is used to detect the restored Session Manager.

    Supports provisional response reliability with a 100 rel message and sends PRACK to all received provisional responses.