1. A data processing apparatus, comprising: a plurality of network interfaces that are coupled to a data network for receiving one or more packets therefrom and sending one or more packets thereto; one or more processors; a switching system coupled to the one or more processors and packet forwarding logic, wherein the switching system and packet forwarding logic are configured to receive packets on a first network interface, determine a second network interface on which to send the packets, and to send the packets on the second network interface; a computer-readable storage medium recorded with one or more message classification rules and a mapping of message classification values to network-level QoS values, wherein each of the message classification rules specifies one or more message attributes and one of the message classification values; application QoS logic which when executed by the one or more processors is operable to cause: receiving an application-layer message; matching one or more attributes of the application-layer message to the message classification rules; determining a message classification of the application-layer message based on the matching; selecting one of the network-level QoS values using the mapping and based on the determined message classification; marking a network-level header of the application-layer message using the selected QoS value.