Discussion Question: Is there any hope of organizing structure of network Or at least our discussion of networks?
Interestingly enough, this notion of relying on lower-layer services is prevalent in many other forms of communication. For example, consider ordinary postal mail. When you write a letter, you include envelope information such as the destination address and the return address with the letter. The letter, along with the address information, can be considered a PDU at the highest layer of the protocol stack. You then drop the PDU in a mailbox. At this point, the letter is out of your hands. The postal service may then add some of its own internal information onto your letter, essentially adding a header to your letter. For example, in the United States a barcode is often printed on your letter. Once you drop your envelope into a mailbox, you rely on the postal service to deliver the letter to the correct destination in a timely manner. For example, you don't worry about whether a postal truck will breakdown while carrying the letter. Instead the postal service takes care of this, presumably with well-defined plans to recover from such failures. Furthermore, within the postal service itself there are layers, and the protocols atone layer rely on and use the services of the layer below.
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