ZyXEL ZyWALL USG 2000 User Guide - Page 711

CHAPTER 42 Services 42.1 Overview Use service objects to define TCP applications, UDP applications, and ICMP messages. You can also create service groups to refer to multiple service objects in other features. 42.1.1 What You Can Do in this Chapter • Use the Service screens (Section 42.2 on page 712) to view and configure the ZyWALL's list of services and their definitions. • Use the Service Group screens (Section 42.2 on page 712) to view and configure the ZyWALL's list of service groups. 42.1.2 What You Need to Know IP Protocols IP protocols are based on the eight-bit protocol field in the IP header. This field represents the next-level protocol that is sent in this packet. This section discusses three of the most common IP protocols. Computers use Transmission Control Protocol (TCP, IP protocol 6) and User Datagram Protocol (UDP, IP protocol 17) to exchange data with each other. TCP guarantees reliable delivery but is slower and more complex. Some uses are FTP, HTTP, SMTP, and TELNET. UDP is simpler and faster but is less reliable. Some uses are DHCP, DNS, RIP, and SNMP. TCP creates connections between computers to exchange data. Once the connection is established, the computers exchange data. If data arrives out of sequence or is missing, TCP puts it in sequence or waits for the data to be retransmitted. Then, the connection is terminated. In contrast, computers use UDP to send short messages to each other. There is no guarantee that the messages arrive in sequence or that the messages arrive at all. ZyWALL USG 2000 User's Guide 711