Summary

  • RFC 4429 made a new IPv6 address usable while Duplicate Address Detection was still running, but marked it Optimistic and withheld actions that could overwrite a legitimate neighbor's reachability state.
  • Early traffic could pass through a known router; collision evidence still forced deconfiguration, and later RFC 7527 refined how a node distinguishes a reflected probe from a real duplicate.

An address between silence and ownership

Ordinary Duplicate Address Detection gave a newly configured IPv6 address a clear legal status: Tentative. Until the link had been probed for another claimant, the address was not available for normal communication. That caution was easy to explain, but on a device changing its point of attachment the waiting interval could become visible as broken continuity.

RFC 4429 did not abolish the test or declare that collisions were harmless. It created an intermediate state. An Optimistic address was assigned to the interface and available for use while DAD continued. For source selection, implementations were told to treat it like a Deprecated address: usable, but subordinate to another suitable address. Successful completion would later promote it into the ordinary Preferred or Deprecated lifecycle.

That distinction is the architecture of the change. The host gained enough authority to communicate, not enough authority to behave as if local ownership were settled.

The configuration choice was conditional. A host should not make the address Optimistic unless it already knew the router's link-layer address and could support the constrained forwarding path. If it could not learn or use that information without breaking the restrictions, the address stayed Tentative and ordinary DAD supplied the delay.

The powers it could not yet exercise

Neighbor Discovery is where an unproven claim could injure the rightful owner. A normal advertisement can replace a neighbor-cache entry, and a solicitation can carry the sender's link-layer address into other machines' state. RFC 4429 therefore constrained those exact operations.

The Optimistic node could not source a Neighbor Solicitation from the address under test. A Router Solicitation from that address could not include a Source Link-Layer Address Option. Any Neighbor Advertisement sent for the Optimistic address had to clear the Override flag. These rules did not make a collision impossible; they reduced the chance that the provisional claimant would displace working reachability information for the existing node.

If the host already knew its router's link-layer address, it could send ordinary traffic through that router. When it did not know a local neighbor's address, it could not perform address resolution from the Optimistic source. It instead forwarded through the default router in hope of a redirect, or waited. If even the router could not be reached without violating the restrictions, the address remained Tentative and the optimization disappeared.

Collision remained a revocation event

The DAD probe still used the unspecified source address. If the Optimistic node received a solicitation from the unspecified address for the same target, that was duplicate evidence: it did not defend the address, and it deconfigured it. A unicast solicitation was different; the node could answer, but only with Override cleared.

Packets might already have been sent before a collision surfaced. They could be misdirected, and connections could fail. RFC 4429 consequently recommended the optimization only where the identifier was likely to be unique, such as one derived from randomness, a well-distributed hash or DHCPv6, and not for manually entered addresses. The protocol bounded the damage of uncertainty; it did not erase uncertainty.

When the probe itself lied

RFC 7527 later exposed another evidentiary problem. A circuit or device could loop a DAD solicitation back to its sender. Without a way to recognize its own packet, the node could interpret the reflection as a second claimant and leave IPv6 in a failed state.

Enhanced DAD attached a fresh nonce to each probe and retained that value locally. A returned probe bearing the same nonce could be classified as a reflection rather than ordinary evidence of a duplicate, allowing probing to continue. This repair sharpened the meaning of one observation; it did not turn an Optimistic address into proven ownership.

The historical lesson is narrower than “use addresses sooner.” RFC 4429 separated usefulness from authority. It let a host begin sending when the successful case was likely, while making the provisional state visible in the exact protocol actions most capable of disturbing someone else.

Sources