ARP

The ARP is intended to solve the problem derived from the fact that the
Internet is a virtual network working on physical networks [8]. When a datagram
must travel from one machine to another (to the destination host or to
a gateway) located in the same physical network, the physical layer that exists
below the Internet encapsulates the datagram with the frame of the network
itself, which must include the physical address of the machine. The problem
consists in that this physical address can be unknown and must be obtained
from the known IP address.
There are two basic methods to obtain the physical address.
• Direct mapping. Direct mapping, the most trivial way, allows the
physical address to be found from the IP address. Direct mapping
can be used when the physical address fulfils one of the following
two conditions: either it can be chosen freely or it is sufficiently
small. In the first case, it is enough to assign each machine a physical
address that coincides with the part of the IP address that identifies
the host. In this case, the determination of the physical address is
trivial. In the second case, it is enough to have a simple function that
maps each IP address to a physical address.
• Dynamic mapping. Dynamic mapping requires a specific protocol
such as the ARP to find out the physical address. When the physical
address is fixed, or when it is made up of a large number of bits,
direct mapping is unviable. This is the case of Ethernet networks, in
which the address of a machine is made up of 48 bits fixed by each
manufacturer and cannot be modified. A simple solution is to have a
table with the correspondences among addresses, but it must be
modified every time a new machine is added to the network. The
ARP protocol provides a mechanism to resolve this problem in a
dynamic way with the only condition that the network has the
capacity of broadcasting The ARP basically works by sending all of the machines in the physical
network a datagram with a special format including the IP address. All of the
machines will receive the message, but only the one whose IP address coincides
will send a reply with its physical address. From this moment on, the
physical address of the host is known.
This initial method can have some improvements. The first consists of
maintaining a cache memory with the last physical addresses obtained. Thus,
when a physical address is needed, the cache memory is consulted first, sending
the ARP message only if there is no correspondence. Another improvement
is based on the fact that if a machine A needs the physical address of B,
it is likely that B will need the address of A in the near future. It is enough for
A to include its physical address in its request message to B. Furthermore, as
all of the machines in the network receive this request, they can take advantage
by storing the physical address of A in their corresponding caches for
future communications.