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36 votes
36 votes

A firewall is to be configured to allow hosts in a private network to freely open TCP connections and send packets on open connections. However, it will only allow external hosts to send packets on existing open TCP connections or connections that are being opened (by internal hosts) but not allow them to open TCP connections to hosts in the private network. To achieve this the minimum capability of the firewall should be that of

  1. A combinational circuit
  2. A finite automaton
  3. A pushdown automaton with one stack
  4. A pushdown automaton with two stacks

3 Answers

Best answer
63 votes
63 votes
  1. A combinational circuit$\Rightarrow$ Not possible, because we need memory in Firewall, Combinational ckt has none.
     
  2. A finite automaton$\Rightarrow$ We need infinite memory, there is no upper limit on Number of TCP ckt so Not this.
     
  3. A pushdown automaton with one stack$\Rightarrow$ Stack is infinite. Suppose we have $2$ connections, we have pushed details of those on stack we can not access the details of connection which was pushed first, without popping it off. So, Big NO.
     
  4. A pushdown automaton with two stacks$\Rightarrow$ This is TM. It can do everything our normal computer can do so Yes. A firewall can be created out of TM.

Correct Answer: $D$

edited by
5 votes
5 votes
It should be D as it is equal to turing machine and since we need to keep track of all the open connections as they should only be connected with outside world(one to one matching) a PDA won't suffice..
0 votes
0 votes
pushdown automata with two stacks which is the Turing machine.
Turing machine can do everything as the normal computer can do, so firewall can be created by the TM.
Answer:

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