$6.$ TCP Protocols and Denial$-$of$-$Service Attacks: A common exploit for a hacker is to attempt to bring down a popular service, such as Yahoo, by doing a denial$-$of$-$service $($DOS$)$ attack. A simple DOS attack that can be understood using the simple TCP model of Fig. A$\mathrm{.}1$ is TCP Syn$-$Flooding. In this attack, the hacker sends a number of SYN packets to the chosen destination D $($e$.$g$.,$ Yahoo$)$ using randomly chosen source addresses.