Each flow or connection is identifiable by the set of parameters that makes it unique; two of these parameters are the network addresses of the two end-points. For protocols that support multiplexing of multiple flows or connections to/from a single network address - UDP and TCP both support this - the remaining parameters are the identifiers used to do the multiplexing. For both UDP and TCP, this identifier is a port-number, whose scope is local to the end-point, and is therefore usually different at each end-point for a given flow/connection.

Normally, only one of the two port-numbers involved will be known a priori - this will be the documented port-number used for a specific service at the server end (for example, port 80 for an HTTP service); the other is dynamically chosen from the available pool of unused port numbers at the client end.

Therefore the filter criteria can only specify that known port-number; the other port-number can only be determined by inspection of the IP packet payloads, discovering which protocol is being carried, and using knowledge of the protocol to extract the port-number.

This information must then be stored, and held for a duration not less than the duration that communications occur over the flow or connection. This information defines a session, and is stored in the session-table. The key point of the session table entry is that it will then cause return traffic to be allowed, and sent to the correct place. Without the session table entry, the FB2500 would have no way of knowing that the return traffic is part of an allowed (by firewalling rules) session, and it would likely be dropped due to firewalling.

The overall process of analysing packet payloads and maintaining the session-table is referred to as session-tracking.

Session-tracking is necessary to be able to implement firewalling using the kind of rules you might expect to specify - for example:

"allow TCP connection to port 80 on IP address 10.1.2.3, from any IP address" (note source port number not specified)

Session-tracking will therefore be present in a firewall, but not required in a router.

The contents of the session-table can be viewed in the web user interface by clicking "Sessions" in the "Status" menu. You will normally see two entries per session, one with a green background and one with a yellow background. These two 'entries' are the forward and reverse details of the session.