ZyXEL Communications ZyWALL 1000 User Manual

ZyWALL USG 1000 User’s Guide

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 

 for related information on these screens.

See 

 for a list of commonly-used services.

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.

Use service objects to define IP protocols.

• TCP applications