Wait, probe three times, and limit retries
02A host checks before beginning to use an address, including after relevant link changes. Random timing reduces synchronized probes; silence only means no conflict has been observed yet.
Separate failure case: at least10 conflicts on this interface trigger a maximum of one new-address attempt per60 seconds. Each permitted attempt still has its own probing sequence.
Source for this drawing: RFC 5227 · Read RFC section 2.1.1 ↗
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.
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These pictures are analogies to help you learn. The linked RFC explains the actual rules.
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“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
Listen from the start through the two-second wait after the third Probe.
- For this RFC’s normal profile, the initial delay is uniform 0–1 second, followed by3 Probes with independent uniform 1–2 second gaps. These are protocol constants, not user-adjustable timeout knobs.
- After the last Probe, continue conflict checking for ANNOUNCE_WAIT=2 seconds. The example0.5,2.0,3.5second Probe times are one permitted schedule, not mandated values.
- After10 or more conflicts on an interface, a host MUST limit new-address attempts to at most one per60 seconds. This includes conflicts during probing and later operation; it limits attempts, not individual Probe packets.
- A host implementing ACD MUST probe before use but MUST NOT repeat probing periodically as a matter of course (§2.1). Ongoing checking is passive. Sections1.3/2.2 discuss other network technologies and later timing specifications.
Times are relative seconds on a schematic timeline. No real timer or randomness source runs here. Shorter-timeout standards are not invented, and absence of an observed reply does not authenticate an address holder.
Read the RFC section ↗