Before we discuss the TCP and IP protocols further, we present a broader picture of how data is transmitted over the Internet by drawing an analogy to the service provided by a courier company.
Imagine that we want to send some hand-drawn illustrations from our office in Australia, to the office U.S.A. We would put our drawings into an envelope addressed to our editor and a courier would carry the envelope back to the courier company's city office. At the courier's city office our envelope would be sorted from the locally bound envelopes and packed into an air freight bag for Los Angeles and then on to Boston. A similar process would happen, but in reverse, once the bag was unloaded from the plane in Boston. Not knowing exactly where Cambridge is, the envelope may be put on another plane, a train, or a donkey. Eventually, our drawings arrive on editor desk. This detail isn't important to us, because the courier company is providing a door-to-door service.
Our courier analogy demonstrates a message service over heterogeneous transport technologies. The details on the envelope are understood by all courier companies regardless of how they operate. At each point in the network of courier offices, someone reads the details and makes a decision about where the envelope should go next, and how. The Internet is many networks interconnected and a set of protocols-just like the addresses and serial numbers on the envelope-that provide an end-to-end service over the heterogeneous transport technologies.
Our analogy fails to demonstrate one other network characteristic. The set of drawings make up one message as far as we and our editor are concerned. If it were not for privacy expectations, our courier company could have opened the envelope and repackaged each sheet of paper into individual envelopes and sent some by air via Sydney, some via Auckland, and even some by sea. No doubt these separate messages would not arrive at the courier's office at Cambridge in order-some might not arrive at all and would have to be sent again-but as long as there was information on each envelope that related them, the original message could be reassembled. The courier's Cambridge office would have to hold on to messages that arrived out of order, decide when to ask for missing envelopes to be resent, then reassemble them into the one envelope and deliver the original message as if nothing had happened. Of course, if a courier company did this, they would go out of business, but this is what happens when applications such as web browsers and servers send a message on the Internet.