At aminimum, an event notification service exports two functions that together define what is usually referred to as the publish/subscribe protocol. Interested parties specify the events in which they are interested by means of the func- tion subscribe. Objects of interest publish notifications via the function publish. SIENA extends the publish/subscribe protocol with an additional interface func- tion called advertise, which an object of interest uses to advertise the notifica- tions it publishes. SIENA also adds the functions unsubscribe and unadvertise. Subscriptions can be matched repeatedly until they are canceled by a call to unsubscribe. Advertisements remain in effect until they are canceled by a call to unadvertise. Table I.
