name in/ up/ above/ message? header? out dn below upnm out up above no no up out up above yes no dnnm out dn below no no dnlm out dn below no yes dn out dn below yes yes upnm in up below no no uplm in up below no yes up in up below yes yes dnnm in dn above no no dn in dn above yes no
Table 1: The 10 standard event handlers.
In the standard configuration, each layer has 10 handlers. A handler is uniquely specified by a set of characteristics: whether it is an incoming or outgoing handler, a handler for up events or down events, a handler for communication with the layer above or for the layer below, whether it has an associated message, and whether it has an associated header. See table 1 for a enumeration of the 10 handlers. Of the 10 handlers, 5 are outgoing and 5 are incoming; 5 are up event handlers and 5 are down event handlers; 4 are for event communication with the layer below and 6 are for event communication with the layer above. These are depicted in fig 5. The names of the handlers have two parts. The first specifies the sort of event the handler is called with (``up'' or ``dn''). The second specifies the sort of message that is associated with the event and may be either ``'' (nothing, the default case), ``lm'' (for local message), or ``nm'' (for no message), which correspond to:![]()
Figure 5: Diagram of the 10 standard event handlers. Note that the ABOVE layer has a similar interface above it as the BELOW layer. Likewise with the interface beneath the BELOW layer.