Internet Core Protocols: The Definitive Guide

Inverse ARP (InARP)

The design goal for ARP was to allow an IP system to locate the hardware address associated with the (known) IP address of another system on a local network. Inverse ARP—which is documented in RFC 2390—works in the exact opposite manner, instead allowing a system to locate the IP address associated with a known data-link address. This feature is necessary when the devices are known to each other at the data-link layer, but the IP addresses of those systems are not known, which will prevent the two systems from being able to communicate with each other using IP. This is a common scenario on networks that share data-link addresses across different physical networks, such as Frame Relay and ATM.

Frame Relay networks are somewhat different from traditional networks in that the devices themselves do not have hardware addresses. Instead, the devices have addresses for each of the circuits that they are connected to (known as “data-link connection identifiers,” or DLCI addresses). Whenever a device needs to send data to another device, it sends the data to the DLCI address that is associated with the specific circuit that the destination device is available through.

From IP’s point of view, if a device wants to send IP packets to another device on the Frame Relay network, then the sender must have an ARP cache of sorts that maps destination IP addresses to specific circuit identifiers, allowing the sender to transmit data for a known IP address through the correct ...