A point of clarification, which is that last I knew, Steam was still not P2P, excluding the voice portion of the application. They maintain(ed) their own content servers.
You might argue that this is an unimportant distinction, and if so, I will not disturb your ignorance. But I will tell you that the moment it goes P2P for content, I'll have a very unhappy son, because it will be banned. And NOT due to security concerns over players manipulating the content on their own systems prior to your download, although that alone would be a valid concern.
No, it's because you CANNOT turn off a P2P "server" and stop the traffic if it's overwhelming your connection, or causing you other problems. Right now, you CAN stop the Steam service, and because it's all "pull" technology from their content servers, the traffic stops.
But on every P2P system I've seen to date, in order to provide robustness while using a hacked together hodgepodge of whatnot that was never meant to be whole, the directory service as well as other nodes who have cached your IP address as a source do not drop the listing for an extended period of time, and other systems do not stop pounding on your connection until the cache clears, which is some times DAYS after you've disabled the P2P server.
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