Making Things Talk

326 MAKING THINGS TALK

Network Identification

So far, you’ve identified network devices computationally by their address. For devices

on the Internet, you’ve seen both IP addresses and MAC addresses. Bluetooth and

802.15.4 devices have standard addresses as well. The address of a device doesn’t tell

you anything about what the device is or what it does.

Recall the networked air quality project in Chapter 4.

The microcontroller made a request via HTTP and the

PHP script sent back a response. Because you already

knew the microcontroller’s capabilities, you could send

a response that was short enough for it to process effi-

ciently, and format it in a way that made it easy to read.

But what if that same PHP script had to respond to HTTP

requests from an XPort, a desktop browser like Safari

or Internet Explorer, and a mobile phone browser? How

would it know how to format the information?

Most net communications protocols include a basic

exchange of information about the sender’s and receiver’s

identity and capabilities as part of the initial header

messages. You can use these to your advantage when

designing network systems like the ones you’ve seen

here. There’s not room here to discuss this concept

comprehensively, but following are two examples that

use HTTP and mail.

HTTP Environment Variables

When a server-side program, such as a PHP script,

receives an HTTP request, it has access to ...