Dreamweaver MX 2004: Your Connection to the Internet - TCP/IP
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A key component of the infrastructure of the Internet is the communications protocol over which it operates. Actually a suite of protocols, known as TCP/IP, is the several-tiered method by which data is packaged and sent across the wires that connect the world’s computers together. It is made up of the Transmission Control Protocol and the Internet Protocol.
Internet Protocol Although IP comes after TCP in the name of the protocol, IP is the communications core that makes the Internet work. You have likely heard of IP addresses, those sets of numbers separated by dots that are assigned to each host computer and domain on the Net (for example, 208.43.51.78). The Internet Protocol utilizes that numbering scheme to determine the path it should take across the routers and hosts that make up the Internet to reach the destination it is intended for. When you make a connection to a computer somewhere out on the Web, you are, in reality, connecting to any number of other computers and routers that forward your request in the most efficient way they can determine, given the millisecond they have to think about it. If you are interested to see how your requests are being routed, you can use the tracert (trace route) utility from a command prompt on your computer to see the connections, or hops, your request makes as it travels to the destination you provide (see Figure 1-1).
Figure 1-1 Running tracert to www.communitymx.com
returns a number of stops along the way.
Now, given that this is the way the Internet works, with each request you make being forwarded through a number of stops, consider what must happen when you download a large Web page, or even a 15MB program from a shareware site. Without the IP protocol, it might be necessary for the entire file to be copied to each node along the way until it reaches your computer. That could involve as many as 30 or more copies of the same file, depending on where you and your destination site are located. Thanks to IP, your request and the response for the computer at the other end can be split up into small packets of data that travel easily across the network, following the most efficient path each of them can find.
NOTE A number of things can affect the route a packet takes across the Internet, includingbottlenecks and outages along the way. The ability of IP to dynamically route aroundthese problems is a key factor in the stability of this kind of distributed network. Consider how you might get a group of 12 friends across town to the movies. Unless one of you owns a bus, you would probably split up into three cars and head off for the theater. Perhaps one of the drivers likes the expressway, another knows a “shortcut,” and the third doesn’t have enough change for the tolls and popcorn and takes the normal route. Three cars are all taking different routes to the same destination, and they are all liable to encounter things along the way that might speed their travel (such as no line at the toll booth) or slow them down (such as a wreck along the shortcut). Although each car leaves one after the other, there is no guarantee in what order or in what timeframe they might arrive at the theatre. One car might not even make it at all.
To make the point more clearly, suppose you order a book from Barnes and Noble, and, instead of shipping you the entire book at once, they send the individual pages by different carriers with no page numbers. When and if you receive all the pages (and how would you know if you did?), you would be hard-pressed to get them back together in an order that’s useful. This is pretty close to what happens to a file that is being transmitted across a network by the IP protocol alone. IP needs some help to make sure that things end up where they belong. That help comes from the Transmission Control Protocol.
Transmission Control Protocol The Transmission Control Protocol (TCP) is like the big stack of envelopes the shipping clerk at Barnes and Noble would use to send you all the pages of the book you ordered. Each envelope would be numbered in order and would indicate the total number of envelopes in the sequence—for example, 36 of 1,008. Each envelope would also give some indication of what was on the page inside so you could make sure you got the right one. Using this scheme, you could receive all the envelopes, put them back in order, and make sure you received what the store meant to send you. Then you could call Barnes and Noble and yell at them for sending you a book in such a stupid way.
But that’s the way it has to work on the Web. Each packet created by the IP protocol is packaged up, numbered, and labeled so the receiving computer knows what to do with it. If the receiving computer is missing any packets, it knows to send back for them from the sending computer. Also, the TCP information indicates what the packet should contain so the receiving computer can identify corrupted data.
Together, the two protocols within TCP/IP provide the communications basis on which the Internet is built. But it can really only handle the connection between the computers over which the requests and responses of information are sent. Those actual messages are handled by the Hypertext Transfer Protocol.
 This chapter is from Dreamweaver MX 2004: A Beginner's Guide, by Tom Muck and Ray West (McGraw-Hill/Osborne, 2004, ISBN: 0-07-222996-9). Check it out at your favorite bookstore today.
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