Subject: RE: [mobile-ip] Question of FA obtaining MAC addresses

From: "Lorang, Martin" <[email-masked]>
Date: Mon, 5 Nov 2001 09:15:15 +0100

-- Sharif wrote --

One of the "major changes" listed in rfc2002-bis is that the foreign
agent
is not permitted to broadcast an ARP Request on all its local links, in
order to retreive the MAC address of a mobile node.

However, in RFC2002, I did not see any text that prohibits this action.

-- End Sharif

Unfortunately, I can't answer Your question but only add another one:

_Why_ is the foreign agent not allowed anymore to use broadcast ARP? 

As far as I can see, draft-...-rfc2002-bis-08.txt does not give any
reasons for this deviation from RFC2002. If I missed something (it's
extremely hard to search in the Mobile IP e-mail list archive), please
let me know.

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From: Bjorn Andersson <[email-masked]>
Date: Mon, 5 Nov 2001 10:57:55 +0200

> extremely hard to search in the Mobile IP e-mail list archive), please

I have personally come across one practical problem when deploying a
foreign agent in a network that I did not completly administer. In this
network the gateway responded to ARP requests for all IP addresses
that did not match those of the local network. When a mobile node
came to the foreign network, and the FA wanted to reply to the MN,
the gateway answered to the ARP request, since the address of the MN
was not one from the local network. The MN never got the packet.

I don't know how common it is to use proxy ARP in this way.

Another thing is that using ARP consumes bandwidth and is redundant,
since all needed information can be obtained from the MIP protocol
messages.

The authors maybe have other reasons?

  Bjorn

-- 
Bjorn Andersson <[email-masked]>                       +358 50 341 2556
Lifix Systems Oy <http://www.lifix.fi/>                 PGP id 5AFC144B
Yliopistonkatu 5, 3rd floor; FIN-00100 Helsinki

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From: "Carl Williams" <[email-masked]>
Date: Mon, 5 Nov 2001 01:15:19 -0800

Based on interoperability testing at various MIP bake-offs, what we
learned was that some routers are configured to automatically proxy
ARP for any address not belonging to the local network.  The presence
of such routers can interfere with the operation of the Mobile IP agent.
A foreign agent, as described in RFC2002, sends out an ARP request
to determine the hardware address for a visiting mobile node.  If the
proxy-ARPing router responds before the mobile node, forwarding
loops can occur and cuasing the mobile node to be unreachable.

This input from MIP bake-offs was presented to Charlie Perkins who
then made the appropriate changes to 2002bis to specify that 
the FA not ARPing for a mobile node's hardware address.

An implementation could pick up that information from the 
ethernet header of an incomping registration request.

Just some historical information regarding why the 2002bis change
is there.

Carl