Subject: [MOBILE-IP] Draft rfc2002-bis: a question

From: Sufatrio <[email-masked]>
Date: Fri, 5 Nov 1999 23:40:49 +0800

Hello,

I have a question on draft rfc2002-bis
regarding its specification of the Identification field.

On the use of nonce-style anti-replay protection,
Section 5.6.2 says the following in its last paragraph:
"If a registration message is rejected because of an invalid nonce,
 the Reply always provides the mobile node with a new nonce to
 be used in the next registration.  Thus the nonce protocol is
 self-synchronizing."

Though it sounds OK, however, there might be a little problem
when an Attacker replays a previous Request to HA.

In this case, according to the specification above,
HA will send a rejected Reply with a new nonce included.
While HA now expects this new nonce in MN's next registration,
the real MN, which is not expecting any Reply, will discard the Reply.
MN thus has no knowledge about HA's new nonce.
As a result, another registration round will be uselessly performed,
should later legitimate MN initiate a Request.

I would suggest some changes in the specification above, that a new
nonce should not be generated upon receipt of Request with invalid nonce.
But rather, HA must keep sending the same expected-nonce
until it receives a Request with that correct nonce
(though this Request may be rejected for other reasons).

Thanks,

Sufatrio.