allocated more or less linearly on the disk, decreasing listing time
even more.
+@cindex ack files
+@anchor{ACKFile}
+@item tx/ack/LYT64MWSNDK34CVYOO7TA6ZCJ3NWI2OUDBBMX2A4QWF34FIRY4DQ
+If tossing was performed with @option{-gen-ack} option, then for each
+generated @ref{ACK} packet there will be corresponding empty @file{ack/}
+file. ACK outbound packets needs to be deleted after the transmission,
+but since they are stored encrypted in the spool, there needs to be some
+kind of knowledge what packets are ACK ones. Both ACK and corresponding
+@file{ack/} files are removed by @command{@ref{nncp-rm} -ack} command.
+
@end table