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C
H A P T E R
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. See
5.5 on page 122
36.1 Services Overview
See
Appendix C on page 703
36.1.1 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.
36.1.2 Service Objects and Service Groups
Use service objects to define IP protocols.
• TCP applications
ZyWALL USG 1000 User's Guide
for related information on these screens.
for a list of commonly-used services.
36
Services
Section
521
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