3.7 Extensions for Dialog State Recovery in Case of Outages in SIP and other Network Elements on the Dialog Path

This section follows the product behavior described in endnote <27>.

To achieve reliability of message delivery between SIP endpoints, typical installations deploy sets of redundant SIP proxies and other network elements, such as firewalls or NAT devices, providing an alternate path to process and route traffic between endpoints in cases of unplanned or scheduled outages. However, as described in section 3.5 and section 3.6, both SIP and other network elements often maintain state information that they associate directly or indirectly, through indexing, with the SIP dialog state, and when the main SIP proxy or other network device goes out of service, the alternate, or redundant, element, which does not have the corresponding state, cannot continue processing or routing messages. This protocol defines extensions that allow SIP proxies to communicate to the endpoints that the SIP dialog state carried in the mid-dialog messages no longer has necessary information. It also provides a mechanism for endpoints to update, or recover, the dialog state without breaking the SIP dialog and associated media, such as audio or video, session.

A SIP endpoint can register with its SIP registrar via one or more SIP proxies, as specified in [RFC3261] and [MS-SIPREGE]. If the SIP registrar gets recycled because of unplanned or scheduled outages, the binding information associated with the SIP endpoint can be lost. In such a scenario, SIP message delivery to the endpoint is impacted until the client re-registers and recreates the registration binding. If the SIP endpoint tries to establish a new dialog with another SIP endpoint, mid-dialog messages are not deliverable until the SIP endpoint refreshes its registration binding. This protocol defines extensions that allow SIP registrars to communicate to the endpoints (5) that the SIP registration binding is no longer valid. It also provides a mechanism for endpoints (5) to update the registration binding without breaking any other SIP dialogs and associated media sessions that it is participating in.<28>