Tcp; Icmp; Other Protocols - Watchguard Firebox X5500E Reference Manual

• Gives abstraction of ports. A connection is made of its source and destination ports and its
source and destination IP addresses. In typical use, port numbers less than 1024 are saved for
well-known services (destinations). The client side can use ports higher than 1023 for the source
of the connection. But, this rule has many exceptions: NFS (port 2049) and Archie (port 1525) use
server ports at numbers higher than 1024. Some services use the same source and destination
port for server to server connections. Examples include DNS (53), NTP (123), syslog (514), and RIP
(520).

TCP

Transmission Control Protocol (TCP) enables two hosts to make a connection and send streams of data
to each other. TCP makes sure that the data that is sent gets to its destination. It also makes sure that
packets are put in the same sequence as when they were sent.
TCP manages connections with properties that control the condition of a connection. Three very
important properties of TCP packets are the SYN, ACK, and FIN bits. The SYN bit is set only on the first
packet sent in each direction for a given connection. The ACK bit is set when the other side gets the
data. The FIN bit is set when the source or destination closes the connection.

ICMP

The Internet Control Message Protocol (ICMP) is used most frequently to supply error information
about other services. It operates by using the same method as UDP. That is, ICMP does not use
connections and does not make sure that packets reach their destination. One dangerous ICMP packet
is the ICMP redirect packet, which can change routing information on the devices that receive it.

Other protocols

Most traffic on the Internet uses TCP, UDP, or ICMP protocols. Some other protocols are as follows:
IGMP (Internet Group Multicast Protocol)
A protocol used by a host on multicast access networks to notify a locally attached router to
which group the router belongs.
IPIP (IP-within-IP)
An encapsulation protocol that is used to assemble virtual networks on the Internet.
GGP (Gateway-Gateway Protocol
A routing protocol that is used between different systems.
GR
A protocol used for PPTP.
ES
An encryption protocol used for IPSec.
Reference Guide
Transfer Protocols
7