1. A method performed by a classification system for classifying a packet, comprising: receiving, at the classification system, a packet destined for a recipient, the received packet comprising a plurality of included features; describing the packet as a feature vector, the feature vector comprising at least one feature that is algorithmically-generated at the classification system based on one or more of the plurality of included features or from previously received packets, wherein the at least one algorithmically-generated feature is represented by an undefined value that is an element of a range of the at least one algorithmically-generated feature, the undefined value indicating that at least one optional field is not present in the received packet; and mapping the feature vector to a feature space by employing one or more spatial access methods to classify the feature vector according to one or more classification rules, wherein the one or more spatial access methods have a non-linear complexity with respect to a number of the one or more classification rules, and wherein the one or more spatial access methods include one or more of an R-tree, an R+-tree, or an R*-tree whose leaf minimum-bounding rectangles are isomorphic with the one or more classification rules.