McDermott, J. "The B2/C3 problem: How big buffers overcome covert channel 
cynicism in trusted database systems." Proc. 8th Annual IFIP Conference 
on Database Security. Bad Salzdetfurth, Germany, August 1994.
<a href="1994mcdermott-ifip.ps">
 PostScript</a>

<blockquote> 
We present a mechanism for communication from low to high
security classes that allows partial acknowledgments and flow control
without introducing covert channels. By restricting our mechanism to
the problem of maintaining mutual consistency in replicated
architecture database systems, we overcome the negative general
results in this problem area. A queueing theory model shows that big
buffers can be practical mechanisms for real database systems.
</blockquote>