When a wanda machine is reset, it needs to find out who it is, and how to boot. It does so using the rarp protocol. The format of rarp messages is described elsewhere in this collection. When a machine sends a rarp request it places in the message a unique identifier which it has read from its hardware. For some wanda machines this will be their Ethernet address, whilst for Fairisle port controllers this is their Fairisle Serial Number. This is a unique number which identifies each port controller. A rarp server is a server which, on receiving a rarp request, returns to the requester a message indicating its msnl address, and the msnl address of the host from which the requester should boot. It also supplies its own msnl address. The requester, on receiving a reply, assigns its msnl address from the information provided and then attempts to boot from the boot server at the supplied address.
Rarp servers are available for both unix and wanda. The msnl connection management daemon (see elsewhere in this collection) runs a rarp service if it is set up to do so. In addition, the master port on each Fairisle switch runs, by default, a rarp server which will respond to rarp requests from the other ports on the switch, when they boot. By convention a rarp server should only run on a wanda machine if that machine is also a gateway, and the gateway can route boot requests from the rarping machine to the boot server.
Each rarp server, on starting, loads a rarp table. For wanda, the rarp server uses a rarp table which is supplied in its hostinfo. For Fairisle switches, the rarp table is automatically constructed by the switch configuration tool sc (see below). For unix rarp servers, the rarp table is supplied to the msnl-master when it initialises. The rarp table specifies for each type of rarp, for each registered host, the msnl address of that host and the address of the boot server for that host. Two types of rarp requests are recognized: Ethernet-based rarp requests and FSN-based. For a Fairisle switch the rarp daemon responds with an address not based on the FSN number of the requester, but on the position of the requester in the switch. Likewise for LINKRARP the reply is based on the topology of the network, not the serial number of the board. The rarper has no idea that this is occurring but always puts its known information in place.