NXC Series User’s Guide
293
CHAPTER 23
Services
23.1 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.
23.1.1 What You Can Do in this Chapter
• The Service screens (Section 23.2 on page 294) display and configure the NXC’s list of services and
their definitions.
• The Service Group screens (Section 23.2 on page 294) display and configure the NXC’s list of service
groups.
23.1.2 What You Need to Know
The following terms and concepts may help as you read this chapter.
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.
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