Grand Prix Garage - GPL

The workings of GPL Online - Part 1

(Below is a general discussion of the topic. See the part 2 for all details, especially when you host races.)

The limitations of the Internet

When the Internet is used as a medium to race online against each other, there are limitations in terms of bandwidth and latency. The available bandwidth, the amount of data that can be transmitted between participating computers, is limited. Latency, the time it takes to transmit data from one computer to another and back, can be high and irregular at times.

GPL copes with these limitations in the following ways:

• In order to reduce bandwidth requirements, not all cars are shown to all clients. Only the server can see all other cars. Using the default core.ini settings, clients will see 4 cars in front and 1 car behind. This can lead to cars seemingly disappearing and appearing on the track as positions change.

• In order to account for latency, GPL predicts where a car has travelled in the time it took the data describing that car to reach the client. This can lead to slightly different positions being shown for the same car on its own client, on the server, and on the other clients where it is shown.

The consequences for racing

In normal, moderate conditions, good racing is possible despite the limitations of the Internet. There are a number of situations however, where these limitations play an important role in the events:

• Start of the race. At the start of the race, when all cars are close together, the limitation of seeing 4 cars in front and 1 behind makes that you can't see cars that are close by. Especially when there is a crash in front of you and you pass the crashed cars, suddenly other cars may appear in front of you (and they may not be moving as fast as you...). Also, a car may suddenly appear close behind you when it passes the car that was behind you until that point.
  My online grids can help avoid big crashes at the start.

• Being lapped. When you get lapped, or when you leave the pits to qualify, and two or more cars are lapping you, you won't see or hear the second car until the first one has passed you. The second car can suddenly appear very closeby.

• Braking. When a car brakes, the computer signals this to the server and from there to the other clients. Due to latency, the other computers won't know of this for some time (typically 100 to 500 ms, maybe more). So on the other systems, the braking takes place later, and the car is further ahead than on its own system. This goes the other way around as well: other cars seem to be further ahead on the local system than on their own when they brake. The result is that when one car chases another, under braking the car behind sees a greater distance to the car in front than the car in front sees to the car behind. So when the car behind brakes as late as one would normally do when racing against the ai cars, he will hit the car in front. Maybe not on his own system, but on the front cars system, where the distance is less, he will! When the car in front crashes, it may take out the car behind as well. So the lesson is to keep a bit more distance when racing online, or take an alternative line.

The view of the car in front is in black, the view of the car behind is in red. To the car in front, there is less distance between the two than to the car behind. The effect of the prediction mechanism is shown exaggerated by the broken straight lines.

• Together through a corner. When going side by side through a corner, an effect similar to what happens under braking may occur. Because prediction assumes a straight line, each car will be more to the outside on a remote system than on its own system. This too works both ways, with the result that the car on the inside will see more space between him and the other car than the car on the outside will see. It is therefore possible that the car on the outside experiences a hit by the car on the inside, while the car on the inside sees no hit at all. He will be able to see the outside car act according to the hit, but as far as he can tell, it was not him that caused it. The lesson is be careful when you're on the outside of a corner.

The view of the car with the inside line is in black, the view of the car with the outside line is in red. To the car with the inside line there is a greater distance between the two than to the car with the outside line. Again the effect of the prediction mechanism is shown exaggerated by the broken straight lines.

• Bad connection. The predictions GPL makes about other cars are corrected all the time when more recent data arrives. Normally the corrections are so numerous and small that they go mostly unnoticed and the motion seems to be smooth and continuous. When prediction has to cover a greater gap, due to a bad spell in the connection, the corrections will be greater. The cars will behave jerky, jump across the track (warping) or even disappear completely an reappear later (winking out). When you see only one car behave like this, it's probably his connection to the server that has gone bad. When you see all cars jerking, warping or winking out, including the car of the server, it's probably your own connction that has gone bad, or the server has a problem, either on its machine or its connection to the Internet.

• Watching an accident. During an accident, a car's velocity and direction change fast and abruptly. This is a problem for the prediction mechanism. As a result, the car will appear to move about wildly and it will often be shown in a place where it is not, appear to be stuck half in the ground or an object, and so on. So be prepared for the car to warp at any moment, hopefully not in your way.

• Overlapping cars. When a predicted position of a car is corrected, it may be placed in a position where it overlaps with the local car. When this happens, a very violent accident may occur, that has no relation to the speed of the cars at that time. For an artificial example, see my expoding grid. The better the connections of all clients to the server, the less likely these violent collision are to occur.

More details on bandwidth, latency and the related core.ini parameters can be found in part 2.