1. A system comprising a network of devices configured to send and receive packets over the network, the devices including:sending devices configured to send packets over network paths within the network;
annotating devices configured to annotate selected packets by inserting state information in the selected packets as the selected packets traverse through the annotating devices;
reflecting devices configured to reflect certain packets of the selected packets that were annotated by the annotating devices by sending copies of the certain packets back to particular sending devices from which the certain packets were respectively sent, or back to collection devices associated with the particular sending devices, the reflecting devices also sending the certain packets on to respective destinations identified by the certain packets;
the collection devices, configured to collect the reflected packets and record metrics based on the state information that was inserted into the reflected packets by the annotating devices;
action devices, configured to reconfigure one or more settings affecting traffic flow on the network based on the metrics.