1. A networking device, comprising: one or more processors; an interface configured to receive and transmit data from and to a network; policy data configured to specify, for one or more application-layer messages, a session key that uniquely identifies an application session associated with the application-layer messages; logic encoded in one or more media for execution and when executed operable to receive a particular application-layer message through the interface, generate a particular session key for the particular application-layer message based on the policy data, and provide the particular session key to a message processing function; wherein the policy data comprises a regular expression configured to match two or more data fields in a payload of the particular application-layer message, and wherein the logic is operable to generate the particular session key for the particular application-layer message using a result of matching the regular expression to the data fields by extracting and combining data from the two or more data fields that match the regular expression and generating the particular session key from the combined data.