Abstract: Described is a technology by which an engine parses data based upon modules arranged in a tree-like model structure. Only those modules that meet a condition with respect to the data are invoked for processing the data. Each child module specifies a parent module and specifies a condition for when the parent is to invoke the child module. As a module processes the data, if a child module's specified condition is met, it invokes the corresponding child module, (which in turn may invoke a lower child if its condition is met, and so on). When the data corresponds to protocols, the model facilitates protocol layering. A top level parent may represent one protocol (e.g., TCP), a child beneath may represent a lower-layer protocol (e.g., HTTP), whose children may handle certain types of HTTP commands, or correspond to a signature that the child module is programmed to detect.