The process of passing data down the stack using
PDUs is called data encapsulation. Encapsulation
works as follows: When a layer receives a PDU
from the layer above it, it encapsulates the PDU
with a header and trailer and then passes the PDU
down to the next layer. The control information
that is added to the PDU is read by the peer layer
on the remote device. Think of this as like putting
a letter in an envelope, which has the destination
address on it. The envelope is then put in a mailbag
with a zip code on it. The bag is then placed in
large box with a city name on it. The box is then
put on a plane for transport to the city.