Subject: Re: Flow Label

From: "Steven M. Bellovin" <[email-masked]>
Date: Thu, 10 Jan 2002 12:26:08 -0500

In message <[email-masked]>, Gerard.Gastaud@alc
atel.fr writes:

>
>
>
>and the user may refuse to pay because it idid not ask for the flow label
that the malicious entity overwrote

An enemy who is overwriting flow labels could generate fake packets 
with arbitrary flow labels.  It's strictly easier -- instead of 
deleting and reinserting packets, you just generate them, with any 
fields you want.

If the routers can't cryptographically verify every flow labeled-packet
-- and they can't do that in any rational fashion, I suspect -- then 
the only other choice is border control.  Your border routers -- 
including the peering routers, if necessary -- have to check that 
incoming packets are, in some sense, "legal".  In particular, if you're 
going to charge someone extra for such services, you have to ensure 
that the right party sent the packets.  (This creates an interesting 
problem at peering links -- what do you do with packets that have a 
legal flow label for the peer, but not for you?)

		--Steve Bellovin, http://www.research.att.com/~smb
		Full text of "Firewalls" book now at http://www.wilyhacker.com

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