Protocols as Postal Centers

team lib

Most of the action that occurs between applications and hardware consists of taking messages, breaking them down, and stuffing them into envelopes as those messages move farther from the application and closer to the hardware. From the other direction from hardware to application protocols unpack envelopes and reassemble individual pieces to build complete messages. We hope that the resulting message is meaningful, but remember GIGO, that immutable law of computing garbage in, garbage out.

Exploring a post office analogy here may be useful. The post office handles anything that has an address on it and has sufficient postage to pay its way, as long as it conforms to the legal dimensions for a letter or a package. How is a letter delivered? It works something like this:

  1. You address a letter, stick on a stamp, and drop it in a mailbox.

  2. The mail carrier picks up the letter.

  3. The mail carrier delivers the letter to the local post office.

  4. Mail sorters check the zip code and route the letter.

  5. The letter goes to the post office that serves the destination zip code.

  6. Mail sorters check the street address and route the letter to the appropriate mail carrier.

  7. The mail carrier delivers the letter to the address of its recipient.

At least, that's the way it's supposed to work. The basic requirements for successful mail delivery are timely pickup, short transit time, and correct delivery. Factors that affect transit time and delivery are correct identification of and routing to the destination address, plus potential transportation delays between sender and receiver.

The similarity between networking protocols and the postal service lies in the capability to recognize addresses, route messages from senders to receivers, and provide delivery. The major difference is that the postal service, unlike networking protocols, doesn't care what's in the envelopes we send as long as they meet size , weight, and materials restrictions. Networking protocols are very unpostman-like in that they care a great deal about what's in the envelopes we send. One of the main jobs of a networking protocol is to divide the contents of our envelopes and put them into smaller envelopes for delivery.

For example, suppose that you want to copy a 10MB file from your computer to another machine on your network. The file consists of a spreadsheet with some charts and graphics that include a sales forecast for the next quarter, so you want it to arrive quickly and correctly.

To use the post office (or what net-heads call snail mail), you would copy the file to a floppy disk and mail it to its recipient. However, that's not fast enough for most computer users. Over the network, it gets there in about 30 seconds (compared to the days it would take via snail mail). As the file moves from your workstation to the other machine, it's chopped into lots of small packages and then reassembled into its original 10MB form upon receipt.

Size restriction that is, the biggest chunk of data that can move across the network in a single message is only one reason that network messages are broken up and put into multiple envelopes. Handling addresses is another reason. In the post office example, the post office cares about the destination zip code, whereas the delivering mail carrier cares only about the street address. Likewise, one protocol may care about only the name of the computer to which the file is to be shipped. However, at a lower level, the protocol needs to know where to direct the chunks of data moving from sender to receiver so that the file can be correctly reassembled upon delivery.

For senders, the protocol software spends most of its time taking things apart to send them accurately and completely. On the receiving end, the protocol software spends its time stripping off packaging information and putting things back together again. The sender and receiver also exchange information during this process to monitor the accuracy and effectiveness of their communications and to determine if and when delivery is complete. Protocols also keep track of the quality and usability of network links.

In short, a lot more communication and activity is involved in sending and receiving messages across a network than is required to route mail from the sending post office to the receiving one. But the mail analogy remains a pretty good explanation for how things work in general (ignoring the routine rooting around inside messages performed by protocols but not by the post office).

team lib


Windows Server 2003 for Dummies
Windows Server 2003 for Dummies
ISBN: 0764516337
EAN: 2147483647
Year: 2003
Pages: 195

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net