If no reply is received, it means that the MAC address is no longer in use and the sending host will delete the corresponding ARP table entry. If the destination host is still using that MAC address, it will reply with an ARP reply message. In order to verify if the MAC address is still valid, a unicast ARP request is sent using the MAC address recorded in the ARP table. One of the mechanisms is called “ unicast poll” and is described as: “ Actively poll the remote host by periodically sending a point-to-point ARP Request to it, and delete the entry if no ARP Reply is received from N successive polls“.įollowing the example shown in the previous capture, the sending host has an entry in its ARP table that matches the IP 192.168.18.31 with the MAC address 60:f2:62:0f:50:a5.
Section 2.3.2.1 describe four mechanisms for flushing out-of-date ARP entries. What is, then, the purpose of a unicast ARP request if the destination MAC address is already known? I found the answer of this question in the RFC 1122 – “Requirements for Internet Hosts – Communication Layers”.
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