A filter is a 1:1 node whose function is to connect one node to another, filtering messages between them at high speed. It may use local store as a message pool. Messages will be filtered according to a policy, part of which is determined by the filter module used and part by a configuration file of some sort.

Filters will normally be positioned at network (lattice) boundaries. The filter will enforce the message filtering policy of the network which controls it. Because a filter is a 1:1 device, there will normally be a router on each of its interfaces in this application.

A filter has much the same functionality as an Internet firewall and is best understood as such.

Throughout this document, the term filter will always mean an XLattice filter as described in this section, except where otherwise qualified.