CTI 2572 Technical Overview Manual
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the datagram may actually pass through many gateways before reaching the destination network.
IP has defined a comprehensive set of rules for processing datagrams. Among these
specifications are the Internet Control Message Protocol (ICMP).
Internet Control Message Protocol (ICMP)
As described above, IP uses a best-effort method to deliver datagrams to their destination. There
will be times IP cannot deliver a datagram to its destination (host system down, network
congestion, etc.). When this occurs, IP attempts to inform the sender that the datagram could not
be delivered. It accomplishes this by sending an error message to the original source in a
datagram. The format of this message is defined by ICMP. ICMP also defines the format of
certain network diagnostic messages.
5.3 Address Resolution Protocol (ARP)
Once the datagram has reached the destination network, the local gateway then sends the
datagram to the correct host. On Ethernet networks, the datagram is delivered in an Ethernet
frame addressed to the physical address of the Network Interface Card. Thus the gateway must
be able to translate the host portion of the IP address into a physical Ethernet address.
Fortunately, IP provides an elegantly simple way to handle this translation. If a physical address
for a particular IP host is not known, the gateway can use ARP (Address Resolution Protocol).
The gateway accomplishes this by broadcasting an ARP message to all nodes on the network.
The ARP message requests a reply from a station with the specific IP address. When the
applicable host responds, the gateway determines the physical address from the source address
contained in the Ethernet frame. Once a host has responded to an ARP message, all hosts and
gateways on the local network keep a cache of the address information for future use.
5.4 User Datagram Protocol (UDP)
IP routes datagrams from a source host computer to a destination host computer. However, most
host computers are capable of running multiple application programs or processes. Therefore,
additional information is needed to identify the source and destination processes within the host
computers. Information in the UDP header is used for this purpose.
