As per Relevance of the word solution, we have this rfc below:
Network Working Group G.
Request For Comments: 1868 Xylogics, Inc
Category: Experimental November 1995
ARP Extension -
Status of this
This memo defines an Experimental Protocol for the
community. This memo does not specify an Internet standard of
kind. Discussion and suggestions for improvement are requested
Distribution of this memo is unlimited
The Address Resolution Protocol allows an IP node to determine
hardware (datalink) address of a neighboring node on a
network. The protocol depends on timers to age away old ARP entries
This document specifies a trivial modification to the ARP mechanism
not the packet format, which allows a node to announce that it
leaving the network and that all other nodes should modify their
tables accordingly
Thanks to James Carlson/Xylogics for reviewing this document
proposing the backwards compatibility mechanism
1.
The primary purpose of the Address Resolution Protocol, as defined
[1], is to determine a node's hardware address based on its
address (protocol address in ARPspeak). The ARP
specifically states that nodes should not periodically
their existence for two reasons: first, this would generate a lot
network traffic and table maintenance overhead; second, it is
unlikely that all nodes will need to communicate to all other nodes
Since a node does not advertise its existence, neither does
advertise its imminent departure. This is not a serious
since most ARP implementations maintain timers to age away
entries, and departing nodes seldom depart gracefully in any case
Over time, an additional use has been found for ARP: Proxy ARP
While there are those who believe Proxy ARP is an evil thing, it
serve a purpose; that is, it allows for communication in ways
considered in the original IP architecture. For example,
dial-in hosts to connect to a network without consuming a
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RFC 1868 UNARP November 1995
amount of the IP address space (i.e., all of the hosts
addresses on the same subnet, even though they are not
attached to the physical network associated with that subnet address
It is this use of Proxy ARP which produces the problem addressed
this document
2. The
Consider the following topology
+--------+
| Host A |
+--------+
|
========================================
| |
+--------+ +--------+
| CS1 | comm. servers | CS2 |
+--------+ +--------+
| | | |
+-+ +-+ +-+ +-+
| | | | modems | | | |
+-+ +-+ +-+ +-+
Assume that all of the modems are on the same rotary; that is, when
remote host dials in, it may be assigned a modem on either of
communication servers. Further assume that all of the remote hosts
IP addresses have the same subnet address as the servers and Host A
this in order to conserve address space
To begin, a remote host dials into CS1 and attempts to
with Host A. Host A will assume, based on the subnet mask, that
remote host is actually attached to the LAN and will issue an
Request to determine its hardware address. Naturally, the
host will not hear this request. CS1, knowing this, will respond
the remote host's place with its own hardware address. Host A,
receiving the ARP Reply, will then communicate with the remote host
transparently through CS1. So far everything is just fine
Now, the remote host disconnects and, before Host A can age its
cache, reconnects through CS2. Herein lies the problem.
Host A attempts to send a packet to the remote host, it will send
to CS1 because it cannot know that its ARP cache entry is invalid
If, when the remote host disconnects, the server to which it
attached could inform other nodes on the LAN that the
address/hardware address mapping was no longer valid, the
would not occur
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RFC 1868 UNARP November 1995
3. The
When a server, as described above, disconnects from a remote host
which it has responded to a Proxy ARP, it broadcasts an UNARP.
UNARP is an unsolicited ARP Reply with the following field values
Hardware Address Space as
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
Source Protocol Address IP address of detaching
Target Hardware Address Not
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
with the IP address
It is not strictly necessary that a server keep state
about whether or not it has actually sent a Proxy ARP Reply; it
be sufficient if a server always sends an UNARP when a remote
disconnects
Of course, there is no reason why a host which gracefully
from a LAN cannot also send an UNARP for itself. This would
especially useful if, upon re-attaching, it might have a
hardware address
4. Backwards
The modifications to support UNARP are trivial, so there is
expectation that it will be widely supported. Of course, there
be a period of time during which nodes which support UNARP
coexist with nodes which do not support UNARP. To
unenlightened nodes from adding spurious ARP cache entries
hardware addresses of zero, UNARP packets specify a hardware
length of zero. This should be rejected by nodes which do
support UNARP. As a consequence of this, the source and
hardware address fields do not exist in UNARP packets (as
described).
It is recommended that implementors include a configuration switch
disable UNARP in the event that some vendor's ARP
might take offense at the abbreviated UNARP packet format
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RFC 1868 UNARP November 1995
5. Security
Security issues are not discussed in this memo
[1] Plummer, D., "An Ethernet Address Resolution Protocol", STD 37,
RFC 826, MIT, November 1982.
Author's
Gary Scott
Xylogics, Inc
53 Third
Burlington, MA 01803
Phone: (617) 272-8140
EMail: gmalkin@xylogics.
Malkin Experimental [Page 4]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
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