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; logic which when executed by the one or more processors is operable to cause: receiving over the network an application-layer message comprising one or more of the packets; forwarding the application-layer message toward a destination endpoint and concurrently copying the application-layer message without disrupting the forwarding; using the copied application-layer message, discovering one or more applications or services that are using the network; using the copied application-layer message, identifying one or more network-layer condition metrics, and identifying one or more application-layer condition metrics; determining, based on the identified network-layer condition metrics and the application-layer condition metrics, whether one or more conditions of a service level agreement are violated; in response to determining a violation, performing one or more responsive operations on one or more network elements.