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sages have been sent, which have been received, when to consider a message lost and re-
peat it, and what order they should all be in when they arrive. It's only once you get above
TCP that you hit protocols like HTTP, which was designed specifically for passing web
pages around.
When writing a web server it's not necessary to consider how to communicate with dif-
ferent types of network hardware. It doesn't need different methods for sending messages
across Ethernet or Wi-Fi. All it needs to know is how to describe HTTP requests to the TCP
layer of the local network stack.
Figure D.1 shows this arrangement in pictorial form.
Figure D.1. A network stack. Conceptually, each layer communicates directly with its counterpart on another
computer. In reality, the data flow is down the stack, across the physical wires, and back up the other stack.
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