sim is a protocol that lies at the bottom of the protocol graph. It simulates the networks and routers that make up an internetwork. Passing a message to the sim protocol is equivalent to putting it onto a network's physical wire. The
sim protocol then simulates the message's journey through the modeled network until it reaches a protocol subgraph representing the final destination host, at which time sim passes the message up to that host's protocol graph. sim also replies to ARP and RARP requests made by hosts. Special protocols, called driver protocols, are used between standard x-kernel protocols and the sim protocol. There are no arguments used with the sim protocol.