article1138.asp.htm

一个网络游戏程序编写的例子

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<P ALIGN="center"><SPAN CLASS="title">Networking for Games 101</SPAN>
<BR><SPAN CLASS="author">by <A HREF="javascript:if(confirm('http://www2.ravensoft.com/users/jakeworld/  \n\n这个文件不能通过 Teleport Pro 取回, 因为 它被访问于一个域或在它的起始地址边界外部的路径上.  \n\n你想从服务器打开它吗?'))window.location='http://www2.ravensoft.com/users/jakeworld/'" tppabs="http://www2.ravensoft.com/users/jakeworld/">Jake Simpson</A>

<H1>A brief history of the Internet</H1>
<P>Before we begin this journey into the inner workings of networking in games, it's important to define some terms, and get some background on the Internet and how it works. This is of inestimable help later when trying to explain why certain things are done the way they are when coding for the net, plus, if you're anything like me, it's just plain interesting.</P>
<P>I'm going to go over some history of the Internet, with some simple examples of how it works, without getting too technical. This is not 
intended to be a programming reference document, more an enlightenment of what others are talking about when they talk about latency, pings, TCP/IP and so on. I will avoid those areas of the net that aren't directly related to games, since there is no reason to bore the pants off anyone more than is strictly necessary. This is not a 'how to' document, but more a FYI type of thing. There is nothing in this about making your home system better over the Internet, but more an explanation of why so many Internet games have networking troubles, and where they come from.</P>

<H1>So let's begin</H1>
<P>The Internet as everyone knows it came about from a much smaller network called ARPANET that the Pentagon created a) because America was on a science kick in the 60's and wanted to get a head start in burgeoning industries and b) because the Pentagon wanted to use and keep tabs on the expensive mainframes it was funding at places like MIT and UCLA without having to use multiple remote terminals.</P>
<P>To cut a long story short, the pentagon put out a contract to tender that would link multiple mainframes together, for use in real time. This would mean that one man at one terminal should be able to access multiple machines, share data and run programs on different machines.</P>
<P>From this contract, the concept of packet switching and routers was born. Now everyone bandies those words 'routers' and 'packet switching' around, but what do they actually mean? Well, first up, lets dissolve one common misconception. Many people use the phone system as an example when discussing the Internet. &quot;It's like phone system&quot; they say &quot;You have an IP address that's like a telephone number&quot;. Well, not really. A better one would be to use the post office as an example. Imagine that when you send a file from one computer to another it's like a letter being sent. It first goes to your post office, where it is examined, and it's decided if it's intended for someone that has an address that post office serves. If it's not, then it's forwarded to another post office for examination again. Eventually it will arrive at a post office that says, &quot;Oh, I know where the post office that this letter is intended for is located&quot; and it's forwarded directly to the correct one, which then sends it on to the intended recipient. Long winded but you get the idea. Well, a router is effectively a post office. It sorts files that come in and decides what to do with them and where to send them. This is very different from the phone system where you end up with a direct link between you and who you are calling. With routers, there is no direct link. Incidentally, there is a common myth that states that the original APRANET and by default, the Internet, was designed to withstand a nuclear war, so that if one machine was taken out, then others would still be able to communicate, since there was no one route that everyone depended on. Having researched this, there is no actual proof that this was ever an original requirement. It would be able to withstand loosing a large portion of it's connecting machines, but that would appear to be more of a side benefit than an original requirement.</P>
<P>Anyway, back to packet switching. When you send a file over the Internet, you don't actually send the whole thing in one big chunk. It's broken up into small packets - like postcards if you want to continue the post office simile - and each one is transmitted one after the other. The beauty of this is that the routers can handle many, many of these little packets, without ever having to know what's in them (or indeed, the order they are transmitted in). So your packets get mixed in with someone else's, and the data stream gets maximum efficiency. All your machine has to do is create the little packets, number them, so they get re-assembled on the other end in the correct order, and send them out to the router. Of course they need an address too. That's where IP's come in. An IP is a unique address for your machine on the Internet. It's a 4-digit number, all of which are between 0 and 255. For instance 204.57.198.32. All those www.whatever.com are actually converted into IP addresses when packets are exchanged with another machine on the net. Sometimes these are specific and constant on one machine, more often than not they are dynamically allocated by the host system. Every time you log onto you service provider, they send you an IP address they have free from a range that's been allocated to them. For instance your ISP may have the range 204.198.32. 0 to 255, which gives them 256 possible IP addresses. 256 people can all be using the system at once, but no more than that. When you log in, the system looks to see what IP's are free, and sends you one. That way more than 256 people can be on the books for this Host, but only 256 can use it at once.</P>
<P>The alternative to this would be the phone system approach, which would mean creating dedicated routers that would reserve an entire line for you to send data to and from the other computer, but that would not get used most of the time, especially if you are doing stuff like typing in real time. You may think you are a fast typist, but in the time between a message going to and from your machine to another, the network could have transmitted War and Peace several times. A good simile that I heard used would be "like reserving the entire Interstate road system to drive a car from Washington DC to LA". You would never dream of doing that, instead you share it with other car drivers. Just like on the Internet. Maybe that dumb 'super highway' label thing has some merit after all<FONT FACE="Wingdings">J</FONT></P>
<P>I'm sure you can see how the sharing lines with others, and breaking messages into small packets is the most efficient use of network time and data streams. The same system is in use today as was originally designed for the ARPANET way back when. Why? Cos it works real well<FONT FACE="Wingdings">J</FONT></P>