ZyXEL ZyWALL USG 2000 User Guide - Page 712

Chapter 42 Services Both TCP and UDP use ports to identify the source and destination. Each port is a 16-bit number. Some port numbers have been standardized and are used by lowlevel system processes; many others have no particular meaning. Unlike TCP and UDP, Internet Control Message Protocol (ICMP, IP protocol 1) is mainly used to send error messages or to investigate problems. For example, ICMP is used to send the response if a computer cannot be reached. Another use is ping. ICMP does not guarantee delivery, but networks often treat ICMP messages differently, sometimes looking at the message itself to decide where to send it. Service Objects and Service Groups Use service objects to define IP protocols. • TCP applications • UDP applications • ICMP messages • user-defined services (for other types of IP protocols) These objects are used in policy routes, firewall rules, and IDP profiles. Use service groups when you want to create the same rule for several services, instead of creating separate rules for each service. Service groups may consist of services and other service groups. The sequence of members in the service group is not important. Finding Out More • See Section 6.6 on page 114 for related information on these screens. • See Appendix B on page 959 for a list of commonly-used services. 42.2 The Service Summary Screen The Service summary screen provides a summary of all services and their definitions. In addition, this screen allows you to add, edit, and remove services. To access this screen, log in to the Web Configurator, and click Configuration > Object > Service > Service. Click a column's heading cell to sort the table 712 ZyWALL USG 2000 User's Guide