[7.460][DUPE][CONFIRMED] many IPS alerts saying TCP Timestamp is outside of PAWS window

what is this ?

2009:06:23-18:54:51 snort[4306]: id="2101" severity="warn" sys="SecureNet" sub="ips" name="Intrusion protection alert" action="alert" reason="TCP Timestamp is outside of PAWS window" group="0" srcip="218.213.238.230" dstip="192.168.1.175" proto="6" srcport="80" dstport="45395" sid="0" class="" priority="3" generator="129" msgid="1

There are too may entries with this

Thanks