Introduces the development and characteristic of the firewall,analyses the working principle of the stateful-inspection firewall.The data structure of the state table and rule table is built.Considering the security and performance of the firewall,pre￣sents the structural design for TCP,UDP,ICMP,ARP,which is the main protocol in the TCP/IP stack.Method of TCP sequence number check,UDP virtual connection,ICMP package inspection ensure the security and high performance of the network.

State detection is the main stream of the current firewall technologies. This paper introduces the principle of state detection firewall.The flowchart of state detection and the model of state transition for the TCP packets are presented.A method of sequence number scope check and dynamic timeout management ensures the system security.A hash algorithm is adopted to manage the state table,and the design is implemented on FPGA.The experiment shows that the design can work well in a Gigabit network environment.