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Game Networking 101

Vol. 2, Issue 10
January 31, 2000 

 

Ok - so now we know how data gets to and from and about on the Internet. It all sounds cool and froody, so what's the problem? Why doesn't Quake play well then? Well, there are many potential problems. I'll list some - and by all means not all - here.

The routers.

Routers have a finite capacity. They can only examine and forward one packet at a time. The rest sit in a 'queue' waiting to be dealt with. Once the queue is full, any packet that gets submitted will be ignored. Welcome to the world of dropped packetsJ Actually, this is pretty rare, you have to have a real heavy load on a router before it does this, but it does happen. Another problem with queuing is that it takes time. It delays your packet before it's processed and adds to the round trip time it takes for your packet to get to its destination.

More often there are problems with the router itself, or it leads to a dead end. To explain, when a packet hits a router, its destination address (its destination IP) is examined, and the router compares it against its own route tables. These routes come in two flavors, static and dynamic, (at least they do now - older routers have only the static lists). Route Tables are basically a list of destination addresses it knows about. For instance router A gets a packet that wants to go to Router F. Router A can see routers B and C - but not router F. What does it do? It requests those routers that B and C can see (and in this case both know where router F is), and decides based on the info it receives which one to send it to. This info includes loads on the B & C routers, number of hops to get to Router F and so on. Since loads can vary second to second, the decision to send it via B or C can change second to second. Hence you can see how multiple paths can be used for packets going to the same destination. So what's the difference between static and dynamic trace routes? Static are routes that are 'programmed in' to the router. It KNOWS these routes exist, and expects them to be there at all times. Dynamic routes are those that it gets from other routers. This list is constantly changing and updating dependant on what routers are up, what routes are the fastest and what routers may be inoperable further up the chain. If a line goes down somewhere, or a router breaks down, most routers with only static lists don't know/care. They send it on, since they KNOW the route is supposed to be there, but once it gets to the next one, there is nowhere for it to go, because a line was down. Before dynamic trace routes were around, if a line went down between you and your destination, you could well be SOL. Obviously I've take some liberties with exactly how routers work and simplified it considerably, but this is more a layman's document than a programmers guide.

One of the worst things about all this is that there is nothing that you, as the user, can do about this. The Internet was designed to be robust, in real time, but not instant. It's a shame, but Quake wasn't on their minds at design time. :)

As an aside, you might be interested to know that with the new IP design that has 6 IP numbers instead of 4 (apparently we are going to run out of IP addresses by 2014). Some new addressing schemes include a 'preferred route' in the header for IP packets. This way the router itself won't be doing the decision-making, but letting the packet creator chose the route itself. At least this will gain consistency, and reduce lost packets & out of order packets, but at the risk of speed of transmission.

TCP/IP - UDP.

As we learned in the last section, you can't guarantee that packets are going to be delivered at all. Another draw back to this situation is packet ordering. You may transmit your packets in order, but they may end up going via different paths and encounter different delays in getting to their destination, with the practical upshot that they get there out of order. This is a problem, and there is not much that the hardware of the Internet can do about this. But a solution is in hand in the shape of Internet Protocols. We've all heard about TCP/IP but what does it mean? Well, it stands for Transmission Control Protocol / Internet Protocols. While we are talking about initials lets define UDP as well. That’s User Datagram Protocol. So we know that they stand for. Does this make everything clear? No. Ahhh.


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Credits: Illustration © 2000 Costas (Darkpath Images) and Andy, This article is © 2000 Jake Simpson. All other content is © 2000 loonyboi productions. Unauthorized reproduction is prohibited. So don't do it, or we'll give you really, really, bad lag.