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This arrangement allows communication to be conceptually simple. Applications that want
to talk HTTP only need to know about HTTP and not all the other layers. But this simpli-
city doesn't come without cost. Each layer needs to add some information to what's be-
ing transmitted—this information generally can be referred to as headers , and you'll learn
more about them in the next section.
D.2. The overhead of headers
Figure D.2 focuses on exactly what's going on at the interchange between the layers, when
data needs to be passed from an application over HTTP and down the network stack. At
each stage, a small amount of information is added to allow the receiving layer to under-
stand what the data is and what it's for.
Figure D.2. Each layer adds header information and passes the data down the stack.
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