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.

Answer 2

RFC1122 Requirements for Internet Hosts discusses this.

     2.3.2.1  ARP Cache Validation

        An implementation of the Address Resolution Protocol (ARP)
        [LINK:2] MUST provide a mechanism to flush out-of-date cache
        entries.  If this mechanism involves a timeout, it SHOULD be
        possible to configure the timeout value.

      ...

       DISCUSSION:
             The ARP specification [LINK:2] suggests but does not
             require a timeout mechanism to invalidate cache entries
             when hosts change their Ethernet addresses.  The
             prevalence of proxy ARP (see Section 2.4 of [INTRO:2])
             has significantly increased the likelihood that cache
             entries in hosts will become invalid, and therefore
             some ARP-cache invalidation mechanism is now required
             for hosts.  Even in the absence of proxy ARP, a long-
             period cache timeout is useful in order to
             automatically correct any bad ARP data that might have
             been cached.

Networks can be very dynamic; DHCP servers can assign the same IP address to different computers when old lease times expire (making current ARP data invalid), there can be IP conflicts that will never be noticed unless ARP requests are periodically made, etc.

It also provides a mechanism for checking if a host is still on the network. Imagine you're streaming a video over UDP to some IP address 192.168.0.5. If you cache the MAC address of that machine forever, you'll just keep spamming out UDP packets even if the host goes down. Doing an ARP request every now and then will stop the stream with a destination unreachable error because no one responded with a MAC for that IP.