They combine the aspects of other three
types of firewalls. This firewall keeps track of all packets associated with a
specific communication session. A typical communication session between two
computers will consists a several thousand packets, each of which is identified
by a unique source and destination address and a sequence number that allows
all of the packets to be reassembled into the correct data file at destination
computer. Each packet of data is checked to ensure that it belongs to proper
session. Any packets that are not part of an existing session are rejected. In
addition to checking and validating the communication session ensuring that all packets belong to the proper session, these are further
screens the packets at the application layer also. Filtering at the s/w application port level provides an
additional layer of control for the network administrator to ensure that only authorized
transactions are allowed through the firewall. These firewalls close off ports
until connection to the specified port is requested.
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