Section 2.3. Summary

2.3. Summary

Link-local addressing, like all of Bonjour/Zeroconf, has two important goals: simplicity and reliability. Simplicity is important because this technology is not only for powerful $1,000 computers; it is for all manner of emerging ultra-low-cost devices that will use IP networking. Reliability is important because this technology is not just for today's computer experts. This technology is for use by the general public, who have neither the knowledge nor the patience to struggle with all manner of arcane and inexplicable computer failures, and indeed they shouldn't have to. Global communication on the worldwide Internet is very powerful and very useful, but there are many ways that global connectivity can fail, so it's beneficial to have an alternative backup technology that can be relied upon to always work, no matter what. Sometimes, reduced functionalitycommunication only on the local linkis better than no communication at all.

Consequently, following the goals of simplicity and reliability, the steps for obtaining a link-local address are designed to be as straightforward and simple as possible. Choose a potential IP address in a reasonable way. Select from the allowable addresses in the 169.254/16 range. As Zeroconf is designed for situations in which fewer than 2% of the IP addresses have been assigned, your host will almost surely obtain an address within the first one or two tries. To increase this likelihood, your best strategy is to first try to reclaim an address you have previously successfully claimed. You can then use this address to seed your pseudorandom-number generator. After selecting an address, your host ARPs three times with the target address in the ARP set to the desired IP address. If there is no response during a reasonable time period, the address is claimed by sending a further ARP with the desired IP address entered as both the source and target address in the ARP broadcast. Once an address is selected and in use, the reliability requirement calls for ongoing vigilance, to handle the extremely rare (but possible) case where conflicts are not detected until later.