Subject: RE: [Mip4] ARP with MIPv4

From: [email-masked]
Date: Tue, 20 Sep 2005 16:23:03 +0200

Hi Alexandru,

thanks for your detailed answer. See some clarifications of my initial
question below.

RFC 3344 states:
----------------
While the mobile node is away from home, it MUST NOT transmit any
broadcast ARP Request or ARP Reply messages.  Finally, while the
mobile node is away from home, it MUST NOT reply to ARP Requests in
which the target IP address is its own home address, unless the ARP
Request is unicast by a foreign agent with which the mobile node is
attempting to register or a foreign agent with which the mobile node
has an unexpired registration.
----------------
So my question was "Why is a MN *not* allowed to transmit any broadcast
ARP Request or ARP Reply messages while away from home?"

>There may be a risk of address collision when two Mobile Nodes of
>different HAs have the same home address from the private space
>(10.x.x.x) and visit the same FA.

Let's call the two MNs A and B. If a third node C (of the same network
as B) attaches to the same FA (where A and B are already attached to), a
broadcast ARP Request of A would in general make C update its ARP cache
to (IP@ of B, L2@ of A). Thus, when C returns to its home network, it
would not be able to send packets to B until the ARP cache entry
expires.

So is this *the* reason why a MN is not allowed to transmit broadcast
ARP Request or ARP Reply messages while away from home?

Rajendra

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---

From: Alexandru Petrescu <[email-masked]>
Date: Tue, 20 Sep 2005 17:57:47 +0200

[email-masked] wrote:

> Hi Alexandru,
> 
> thanks for your detailed answer. See some clarifications of my 
> initial question below.
> 
> RFC 3344 states: ---------------- While the mobile node is away from
>  home, it MUST NOT transmit any broadcast ARP Request or ARP Reply 
> messages.  Finally, while the mobile node is away from home, it MUST
>  NOT reply to ARP Requests in which the target IP address is its own
>  home address, unless the ARP Request is unicast by a foreign agent 
> with which the mobile node is attempting to register or a foreign 
> agent with which the mobile node has an unexpired registration. 
> ---------------- So my question was "Why is a MN *not* allowed to 
> transmit any broadcast ARP Request or ARP Reply messages while away 
> from home?"

Sorry, it seems I'm once again revolving around some confusion.  I guess
the RFC contributors' intention was to avoid that Home Address-MAC
Address pairs get into the ARP tables of any nodes other than FA
present on the FA wireless link.  Which is a good thing in a way (nodes
under FA won't fill ARP tables with Home Addresses intrinsically not
topological correct at that place) and less good in other ways (e.g.
MN-to-MN packets under same FA must get through that FA,
while optimally they'd be direct).

> Let's call the two MNs A and B. If a third node C (of the same 
> network as B) attaches to the same FA (where A and B are already 
> attached to), a broadcast ARP Request of A would in general make C 
> update its ARP cache to (IP@ of B, L2@ of A). Thus, when C returns to
>  its home network, it would not be able to send packets to B until 
> the ARP cache entry expires.
> 

I guess so, yes, in some way that may be the reason why MN not allowed
to broadcast Requests or Replies.

However I think that _if_ private Home Addresses are used the ARP
behaviour as specified by 3344 still can't deal with the fact that FA
itself may become confused by two different MNs having same Home Address
on its wireless link and two different MAC addresses.

Whether this is an issue in practice is another matter.  Probably Mobile
IPv4 is deployed within a single closed system where a unique authority
assigns Home Addresses to all MNs and makes sure they're unique.  Within
this closed system the FA won't accept the visit of other MNs of other
systems, because probably they don't have the right keys.

If however this becomes an issue in practice then maybe the FA could use
the visitor list info to distinguish between two colliding Home
Addresses of MNs on its "mobility" link.

Alex