Characteristics of State Engines

State engines have the following characteristics:

• Connection management--Each state engine understands the connection management of a particular protocol or set of protocols. State engines can be general, such as the tcp state engine, which allows a simple TCP connection, or specific, such as the ftp state engine, which understands the FTP protocol and parses FTP PORT and PASV commands.

• Precedence level--Each state engine has a precedence level. A service with multiple state engines (that is, a service group) is internally ordered by state engine. This order is given by the order in the ss_stateengine default list.

• Discriminator value--Each state engine has a discriminator value. This value is used to bind the state engine to a particular service. Examples are a port number for TCP and UDP services, an RPC program number for RPC services, or an icmp type for ICMP services.

• Parameters--Each state engine has a set of parameters. These parameters have a default value that can be overridden when the service is defined to modify the behavior of the state engine.