the user. Each layer creates a header and passes the data down to the next layer. (The arrows in
Figure 3-2, Step 1, denote the passing of data between layers.) Passing the data down to the next
layer implies that the lower layer needs to perform some services for the higher layer; to
perform these services, the lower layer adds some information in a header or trailer. For
example, the transport layer hands off its data and header; the network layer adds a header with
the correct destination network layer address so that the packet can be delivered to the other
computer.
instance, Layer 4 considers the Layer 5, 6, and 7 headers, along with the original user data, to
be one large data field.
perform their work, adding the appropriate header and trailer. The physical layer can use the
media to send a signal for physical transmission, as shown in Step 2 in Figure 3-2.
of Figure 3-2 shows an arrow pointing next to the computer (Step 4), signifying that the
received data is being processed as it goes up the protocol stack. In fact, thinking about what
each layer does in the OSI model can help you decide what information could be in each header.