RFC 1868 (rfc1868) - Page 3 of 4
ARP Extension - UNARP
Alternative Format: Original Text Document
RFC 1868 UNARP November 1995
3. The Solution
When a server, as described above, disconnects from a remote host for
which it has responded to a Proxy ARP, it broadcasts an UNARP. An
UNARP is an unsolicited ARP Reply with the following field values:
Hardware Address Space as appropriate
Protocol Address Space 0x800 (IP)
Hardware Address Length 0 (see Backwards Compatibility)
Protocol Address Length 4 (length of an IP address)
Opcode 2 (Reply)
Source Hardware Address Not Included
Source Protocol Address IP address of detaching host
Target Hardware Address Not Included
Target Protocol Address 255.255.255.255 (IP broadcast)
NOTE: this is a 16-byte packet (not including MAC header)
On receiving an UNARP, a node deletes the ARP cache entry associated
with the IP address.
It is not strictly necessary that a server keep state information
about whether or not it has actually sent a Proxy ARP Reply; it would
be sufficient if a server always sends an UNARP when a remote host
disconnects.
Of course, there is no reason why a host which gracefully detaches
from a LAN cannot also send an UNARP for itself. This would be
especially useful if, upon re-attaching, it might have a different
hardware address.
4. Backwards Compatibility
The modifications to support UNARP are trivial, so there is every
expectation that it will be widely supported. Of course, there will
be a period of time during which nodes which support UNARP will
coexist with nodes which do not support UNARP. To prevent
unenlightened nodes from adding spurious ARP cache entries with
hardware addresses of zero, UNARP packets specify a hardware address
length of zero. This should be rejected by nodes which do not
support UNARP. As a consequence of this, the source and target
hardware address fields do not exist in UNARP packets (as previously
described).
It is recommended that implementors include a configuration switch to
disable UNARP in the event that some vendor's ARP implementation
might take offense at the abbreviated UNARP packet format.
Malkin Experimental