ARP Timeouts. Why fixed periodic?

Question

This one\'s been bugging me for years.

Basic question: Is there some reason ARP has to be implemented with fixed timeouts on ARP cache entries?

Answer 1

It originated in distrust of routing protocols, especially in the non-Ethernet world (especially MIT's CHAOS networks). Chris Moon, one of the early "ARPAnauts" was quoted specifically about this in the original ARP RFC.

You can, of course, keep the other guys' ARP caches from timing out by proactively broadcasting your own ARP announcements. Most Ethernet layers will accept gratuitous ARP responses into their caches without trying to correlate them to ARP requests they have previously sent.