My Cisco 675 occasionally throws out the error message 

TCP        Alarm      MTU value returned by get_ip_mtu was zero^M

It invariably occurs in the midst of a bunch of alerts about someone trying
to connect to ports they shouldn't be playing with, so it appears to be a
sign that someone is up to no good, but that's all I know about it.

What exactly causes this?  My best guess is that it's a component of a
stealth attack ("Here's a packet.  Don't send a response larger than 0
bytes.") or possibly an attempt to detect/exploit a TCP/IP vulnerability, but
I don't want to just be guessing.

(Searches didn't turn up anything useful.  If there's a FAQ, HOWTO, FM, or
other document covering this in detail, a pointer to it would be great.)

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