SMTP is used for transferring mail between hosts over TCP$.$ A TCP connection is set up between a user agent and a server program. The server listens on TCP port $25$ for incoming connection requests. The user end of the connection is on a TCP port number above $1023.$ Suppose you want to configure a packet filter rule set allowing inbound and outbound SMTP traffic.

$\backslash$table$[[$Rule$,$Direction,Src Addr,Dest Addr,Protocol,Dest Port,Action$],[$A$,$In$,$External,Internal,TCP$,25,$Permit$],[$B$,$Out,Internal,External,TCP$,>1023,$Permit$],[$C$,$Out,Internal,External,TCP$,25,$Permit$],[$D$,$In$,$External,Internal,TCP$,>1023,$Permit$],[$E$,$Either,Any,Any,Any,Any,Deny$]]$

Someone from the outside world with IP address $10\mathrm{.}1\mathrm{.}2\mathrm{.}3($External$)$ attempts to open a connection from port $5150$ on a remote host to the Web proxy server on port $8080$ on one of your local hosts $172\mathrm{.}16\mathrm{.}3\mathrm{.}4($Internal$)$ in order to carry out an attack. Typical packets are as follows

$\backslash$table$[[$Packet$,$Direction,Sre Addr,Dest Addr,Protocol,Dest Port,Action$],[5,$In$,10\mathrm{.}1\mathrm{.}2\mathrm{.}3,172\mathrm{.}16\mathrm{.}3\mathrm{.}4,$TCP$,8080,?$