A debugging system used for a data center in a network, the system comprising: a monitoring engine to monitor network traffic by collecting a monitored log from a network controller, the monitored log comprising a reference log of working states of an application and a problem log of non-working states of the application; a modeling engine to model an application signature of working states using the reference log, an application signature of non-working states using the problem log, an infrastructure signature and a task signature using the monitored log, wherein the infrastructure signature characterizes the physical infrastructure comprising physical components of the network, and wherein the task signature relates to an operational task in the data center; a debugging engine to detect a change between the application signature of working states and the application signature of non- working states, and to validate the change using the task signature, thereby representing a known change in the application signatures, and, if the change cannot be validated, representing an unknown change in the application signatures; wherein an unknown change in the application signatures is correlated to a known problem class, such as host failure, host performance problem, network disconnectivity, network bottleneck, by considering a dependency of the unknown change to a change in the infrastructure signature, based on a mapping of the application signature changes and related infrastructure signature changes to different problem classes; and a providing unit to correlate the problem class to physical components of the infrastructure signature for further troubleshooting purposes to provide troubleshooting information wherein the debugging engine further partitions the reference log into a plurality of time intervals, and computes the application signature of working states for the plurality of time intervals, and wherein the debugging engine further determines that the application signature of working states is a stable signature if the application signature for an application group does not change substantially across the plurality of time intervals and otherwise discards the application signature, wherein the network comprises an OpenFlow network and the traffic information is collected through at least one of PacketIn and PacketOut messages, wherein the application signature is modeled by at least one of connectivity graph, delay distribution, component interaction, partial correlation, and flow statistics. a monitoring engine to monitor network traffic by collecting a monitored log from a network controller, the monitored log comprising a reference log of working states of an application and a problem log of non-working states of the application; a modeling engine to model an application signature of working states using the reference log, an application signature of non-working states using the problem log, an infrastructure signature and a task signature using the monitored log, wherein the infrastructure signature characterizes the physical infrastructure comprising physical components of the network, and wherein the task signature relates to an operational task in the data center; a debugging engine to detect a change between the application signature of working states and the application signature of non- working states, and to validate the change using the task signature, thereby representing a known change in the application signatures, and, if the change cannot be validated, representing an unknown change in the application signatures; wherein an unknown change in the application signatures is correlated to a known problem class, such as host failure, host performance problem, network disconnectivity, network bottleneck, by considering a dependency of the unknown change to a change in the infrastructure signature, based on a mapping of the application signature changes and related infrastructure signature changes to different problem classes; and a providing unit to correlate the problem class to physical components of the infrastructure signature for further troubleshooting purposes to provide troubleshooting information wherein the debugging engine further partitions the reference log into a plurality of time intervals, and computes the application signature of working states for the plurality of time intervals, and wherein the debugging engine further determines that the application signature of working states is a stable signature if the application signature for an application group does not change substantially across the plurality of time intervals and otherwise discards the application signature,