Subject: Re: [Tsvwg] Re: Last Call: Robust ECN Signaling with Nonces to Experimental

From: [email-masked]
Date: Wed, 8 Jan 2003 11:42:18 -0600

Hi Mark, all,

> This case is not "cheating".  Everything is still congestion
> responsive in this case.  The cheating that the nonce prevents is
> when the receiver can conceal a congestion indication from the
> network.

You are right. I don't know what I was thinking while making that
argument. Apologies.

> This is FUD.  The nonce is *much* better than 50%.  You have a 50%
> shot at guessing the nonce right on every ACK.  But, if you guess
> wrong once then the sender is on to you -- so, the probability of
> guessing right is 0.5^N (where N is the number of ACKs
> transmitted).  So, clearly as the connection progresses and N
> grows large the probability of keeping your game going gets pretty
> small pretty quickly.

This is just half of the story. The other half is that the
probability of an attacker being successful in perpetrating 
a DoS attack is (1-0.5^n). But an attacker does not have 
to send N ACKs, total of four ACKs messages for 2 different TCP 
segments will suffice (the sequence of messages
 
(time=0, ACK-1, Nounce=0)
(time=0, ACK-1, Nounce=1)
(time=1, ACK-2, Nounce=1)
(time=1, ACK-2, Nounce=0), 

is all that an attacker has to send to trigger the sender's 
policy--assuming FIFO queues.)

> (Actually, when we run out I am sure that the first thing that
> will happen will be someone generating an RFC defining a TCP "more
> bits" option.)

Sure. The other option is to let the 4 bits in the TCP be the MSbs
of DOFF, and let the Nounce be an option.

The reason we differ on nounces is probably because I am too
pessimistic about DoS attacks, and the draft is too pessimistic
about cheating receivers and middle boxes.

To cheat an attacker needs the source code of the OS, technical 
expertise of the internal working of the OS etc. On the other hand, 
to do a DoS attack, all one needs is a script that can snoop and 
construct TCP ACKs. I don't know which of the two is more dangerous 
in a realistic scenario. 

--yogesh
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From: Mark Allman <[email-masked]>
Date: Wed, 08 Jan 2003 12:53:29 -0500

> > > A cheating TCP receiver is a malicious user (if not a third
> > > party), and 1 bit nounce can only protect in 50% of the cases.
> > > That's why I said the security is weak--it's weak even for the
> > > problem it's trying to solve--not because it cannot protect
> > > TCP from all possible attacks. I know people have difficulty
> > > getting good security with even 2048 bits or more...
> > 
> This is just half of the story. 

I have requoted the statement you made (at the top) that I responded
to.  I was considering only the case when we were considering
*receivers* (not third parties) lying.

I stick by the above argument.  The case of someone snooping traffic
and hosing the data rate by lying is as you outlined (not quoted).
But, that was not the comment I responded to.  We agree that
eavesdroppers can exploit ECN and the ECN nonce as a mechanism for
DoS.  I simply wanted to apply pushback to your statement that the
nonce does not offer good protection for the problem it purports to
solve.  In my opinion, it offers good protection for its intent.

allman

--
Mark Allman -- BBN/NASA GRC -- http://roland.grc.nasa.gov/~mallman/
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From: Neil Spring <[email-masked]>
Date: Wed, 8 Jan 2003 11:18:30 -0800

On Wed, Jan 08, 2003 at 12:53:29PM -0500, Mark Allman wrote:
> We agree that eavesdroppers can exploit ECN and the ECN
> nonce as a mechanism for DoS.

Can either of you explain this statement a little bit more?

If an evesdropper is able to spoof an acknowledgement with a
faulty nonce, as described in Yogesh's prior mail, it would
seem just as capable of spoofing a RST to tear down the
connection in one packet (not probabilistically four).
While the nonce doesn't solve the problem of spoofing
evesdroppers, I don't yet see how the nonce increases the
potential for misbehavior in the network.

-neil