Network devices add annotation information to network packets as they travel through the network devices. The network devices may be switches, routers, bridges, hubs, or any other network device. The annotation information may be information specific to the network devices, as opposed to simply the kinds of information available at application servers that receive the network packets. As just a few examples, the annotation information may include switch buffer levels, routing delay, routing parameters affecting the packet, switch identifiers, power consumption, and heat, moisture, or other environmental data.