The basic interprocess communication mechanism provided by Off is the Portal. A Portal can be seen like a ``distributed interrupt line'' or a ``network-wide gate''. Portals have the basic utility of interrupts, i.e. they can be invoked transparently to make a handler perform some task.
Portals do not implement buffering and do not specify whether synchronous messages, asynchronous messages or RPCs are to be used. The mechanisms and policies used to locate and to invoke portals over the network are left up to the user. User provided transport and location protocols are used to extend the portal mechanism transparently over the network. In this way traps, interrupts, exception and user messages can be accessed and adapted in the whole system.