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RFC 5227 / IPV4 CONFLICT DETECTION

Announce, refresh stale mappings and answer peers

04

Successful probing leads to two broadcast ARP Announcements. The first establishes when ordinary use may begin; an existing peer cache entry can be refreshed from the sender mapping.

DIAGRAMRead RFC section 2.3 ↗
RFC 5227 · § 2.3EXAMPLE
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Announce, refresh stale mappings and answer peersAfter successful probing and the final wait: broadcast Announcement1refreshes an existing stale peer cache entry; use may begin now. Announcement2is sent2seconds later while normal use may continue. Separate later peer Probe targets the configured address and receives an ordinary ARP Reply. Announcements are Requests; universal broadcast Replies are not recommended.Separate normal reply to a peer ProbeCurrent userProbing peerProbe for our addressordinary ARP ReplySender IP zero is not an ongoing claimMUST answer this nonconflicting Request
03
Answer a later nonconflicting Probe.

Separate ongoing case: a peer sends a Probe for192.0.2.10. As the current user, answer with an ordinary ARP Reply. A target-IP match alone is not an ongoing conflict when sender IP is zero.

Source for this drawing: RFC 5227 · Read RFC section 2.3 ↗

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A message needs an address.

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RFC 826 ARP RFC 2131 DHCPv4 RFC 791 IPv4

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KEY POINT

Use may start after Announcement 1; Announcement 2follows two seconds later.

  • A host implementing ACD MUST send2 Announcements separated by2 seconds. Sender and target IP equal its selected address; opcode remains ARP Request.
  • The host may begin legitimate use immediately after the first Announcement. Sending the second can proceed asynchronously alongside ordinary networking.
  • For its whole period of address use, the host MUST answer nonconflicting ARP Requests for that address, including Probes with zero sender IP (§2.5).
  • Broadcast ARP Replies are NOT RECOMMENDED for general use and SHOULD NOT be used universally (§2.6). Do not confuse mandatory broadcast Announcements with optional broadcast Replies.
DETAILS

The cache illustration starts with an existing stale mapping; it does not claim every unsolicited Announcement creates a new entry in every peer. ARP packet reception remains as in826. Configuring-agent notification can go to a person, DHCP (Dynamic Host Configuration Protocol) client/server, or other software.

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