This structure describe event descriptor to fully identify a SIP event.
Events are the only way for a lower layer object to inform something to higher layer objects. Normally this is achieved by means of callback, i.e. the higher layer objects register a callback to handle the event on the lower layer objects.
This event descriptor is used for example by transactions, to inform endpoint about events, and by transports, to inform endpoint about unexpected transport error.
Forward declaration for events (sip_event.h).