What is the primary function of packet filters used in screening routers?

The primary function of packet filters in screening routers is to manage both network and transport layer traffic. Packet filters inspect the header information of packets to make decisions about whether to allow or block them based on predetermined rules. This operation occurs at layers 3 (Network Layer) and 4 (Transport Layer) of the OSI model, where routing decisions are made based on IP addresses and port numbers.

By regulating traffic at these layers, packet filters can effectively govern what type of traffic is permitted into or out of a network. For example, a packet filter may allow traffic to port 80 (HTTP) while blocking traffic to port 25 (SMTP) based on an organization's security policy. This function does not require an understanding of the content of the packet (which would be layer 7 functionality) but focuses instead on the characteristics of the traffic as defined by its IP and transport layer information.

The other options address functions that packet filters do not perform. For instance, analyzing traffic exclusively at layer 7 pertains more to application layer inspection and is characteristic of application layer firewalls or intrusion detection systems, which look at the content of the data rather than just the packet headers. Similarly, establishing user permissions on data packets leans towards access control measures within the