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Re: TCP receive window set to 0; DoS or not?
- From: Fernando Gont
- Date: Tue Sep 26 06:36:11 2006
At 21:55 08/09/2006, Jim Shankland wrote:
FYI, this issue was raised at the IETF TCPM WG mailing-list a month
ago or so. The OP argued to reduce the amount of time for which a
peer could advertise a 0 window.
Travis Hassloch <email@example.com> writes:
> The part where it becomes a DoS is when they tie up all the listeners
> on a socket (e.g. apache), and nothing happens for several minutes until
> their connections time out. Whether intentional or not, it does have
> a negative effect.
Ah, that makes sense. I was assuming a deliberate attack, which is
not actually implicit in the term "DoS". A deliberate denial of
service is not made easier by shrinking the window. But an implementation
that advertises a 0 window in lieu of sending FIN or RST can certainly
deny service inadvertently by tying up resources that should have been
However, the problem is that if the goalis to perform a DoS attack,
the attacker could advertise a 1-byte window (or ay other small
window). Or he could advertise a 0-window for some time (less than
the "threshold" the OP proposed), then increase the window to, say,
one segment, and then go back to advertising a 0 window.
The OP had suggested seeing this behaviour tying up all system
resources, hence leading to the attacked system to not be able to
service legitimate systems.
There seemed to be agreement as at the TCPM WG that yu should handle
these scenarios at the application layer.
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