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ZyWALL 110/310/1100 Series User’s Guide

390

H A P T E R

2 9

Services

29.1 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.

29.1.1 What You Can Do in this Chapter

• Use the Service screens (

Section 29.2 on page 391

) to view and configure the ZyWALL’s list of

services and their definitions.

• Use the Service Group screens (

Section 29.2 on page 391

) to view and configure the ZyWALL’s

list of service groups.

29.1.2 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 re-transmitted. 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.

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 low-level 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.