In a rather startling revelation, the UK's National Infrastructure Coordination Centre has discovered a major flaw in TCP/IP that could allow hackers to crash vulnerable routers and severely disrupt Internet traffic. The problem stems from the fact that it's far easier to reset TCP/IP sessions using spoofed packets than previously thought.
In short, the exact sequence number need not be predicted as most TCP/IP implementations will accept any sequence number that falls in a particular range! This makes predicting the sequence number much easier, and thus spoofing RST packets is possible. Vendors such as Cisco are rushing patches.