downtime. IMPs could not afford to be dependent on a local host computer or host-site personnel; they should be able to continue operating and routing network traffic whether or not a host was running. The subnetwork also had to continue functioning when individual IMPs were down for service. This idea that maintaining reliability should be incumbent on the subnetwork, not the hosts, was a key principle. Roberts and others believed the IMPs should also attend to such tasks as route selection and acknowledgment of receipt.
