Choose a conflict response for an address already in use
05Monitoring continues for the entire time an address is in use. A conflicting sender can lead to giving up, a limited defense, or special behavior for infrastructure deliberately configured to retain its address.
Separate configured infrastructure case: it retains the address, logs/reports the conflict with controlled notifications, and MUST NOT send another defensive Announcement within10 seconds. With no recent conflict it MUST record the time and send one; suppression is not surrender.
Source for this drawing: RFC 5227 · Read RFC section 2.4 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
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Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
Ordinary defense and deliberately fixed-address defense have different second-conflict rules.
- An ARP Request or Reply with our configured sender IP but hardware unlike any of our interfaces is an ongoing conflict. The host MUST respond using one of the three options in§2.4.
- Option(a): MAY cease use immediately and notify the configuring agent. Hosts SHOULD actively try to reset existing connections before abandoning the address.
- Option(b): if there is reason to retain the address and no conflict within the previous10 seconds, MAY record the time and send one defensive Announcement. A second conflict within10 seconds means MUST cease use and report the error.
- Option(c), only if deliberately configured never to relinquish the address: MAY defend indefinitely; log/inform an administrator with controlled notifications. With no recent conflict, MUST record the time and send one Announcement; within10 seconds MUST NOT send another defensive Announcement. It continues using the address.
- ACD inherits ARP spoofing vulnerabilities (§5). Neither a Probe, an Announcement nor a defensive reply authenticates a peer. Passive monitoring is not periodic re-probing.
Each response example is labelled separately. The ten-second gate is not a general rule that every device must surrender: option(c) is a deliberate infrastructure configuration. The teaching decision helper has no real interface, timers, connection resets or defense state.
Read the RFC section ↗