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

From: "Shahrier, Sharif M." <[email-masked]>
Date: Mon, 5 Nov 2001 09:07:44 -0500

Hello Carl,

	It is explicitly stated in 2002bis that the FA is not allowed to
proxy ARP the links attached to its  router.

However, the HA is not prohibited from proxy ARP'ing its links to determine
the MN's MAC address when its at home.

I don't know why it is different for the 2 cases.

I thought that it is still possible to use de-registration messages from the
MN for the HA to learn its MAC address.

Perhaps the contributors to 2002 & 2002bis can explain why the two cases are
different.

-Sharif.

-----Original Message-----
From: Carl Williams [mailto:[email-masked]]
Sent: Monday, November 05, 2001 4:15 AM
To: [email-masked]
Subject: Re: [mobile-ip] Question of FA obtaining MAC addresses

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

----- Original Message ----- 
From: "Lorang, Martin" <[email-masked]>
To: <[email-masked]>
Sent: Monday, November 05, 2001 12:15 AM
Subject: RE: [mobile-ip] Question of FA obtaining MAC addresses

> -- 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.
>

---

From: Ashish Mehta <[email-masked]>
Date: Mon, 05 Nov 2001 23:30:18 -0800

Sharif,

some questions below for clarification...

>"Shahrier, Sharif M." wrote:
> 
> Hello Carl,
> 

I haven't seen proxy ARP being used with reference to the FA
in 2002bis. Did you mean broadcast ARP instead ?

> the MN's MAC address when its at home.

The HA uses proxy ARP to send ARP replies on behalf of the MN
(and not to determine the MN's MAC address). Or am I misunderstanding
what you said ? As far as I can tell, the HA needs to know
the MN's MAC address only during de-registration when it (the HA)
sends out a gratutious ARP (as described in section 4.6 of 2002bis)

> I don't know why it is different for the 2 cases.

If you meant broadcast ARP, then as already pointed out,
the FA broadcast ARP'ing could cause forwarding loops.

ashish

---

From: Charlie Perkins <[email-masked]>
Date: Wed, 07 Nov 2001 17:08:08 -0800

Hello Sharif,

The two cases are different, because we have protocol to tell
the home agent when the mobile node is there.  Unless we adopt
Route Optimization for IPv4 (which would be a great idea!) there
is no protocol by which a mobile node can tell a previous foreign
agent that it is no longer in that visited network.

Thus there is no protocol by which a foreign agent can maintain
a consistent and correct ARP cache.

Regards,
Charlie P.