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What Is P2P ... And What Isn't

by Clay Shirky

After a year or so of attempting to describe the revolution in file sharing and related technologies, we have finally settled on a label for what's happening: peer-to-peer.

Somehow, though, this label hasn't clarified things. Taken literally, servers talking to one another are peer-to-peer. The game Doom is peer-to-peer. There are even people applying the label to e-mail and telephones. Meanwhile, Napster, which jump-started the conversation, is not peer-to-peer in the strictest sense, because it uses a centralized server to store pointers and resolve addresses.

If we treat peer-to-peer as a literal definition for what's happening, then we have a phrase that describes Doom but not Napster, and suggests that Alexander Graham Bell was a peer-to-peer engineer but Shawn Fanning is not.

This literal approach to peer-to-peer is plainly not helping us understand what makes P2P important. Merely having computers act as peers on the Internet is hardly novel, so the fact of peer-to-peer architecture can't be the explanation for the recent changes in Internet use.

What has changed is what the nodes of these P2P systems are -- Internet-connected PCs, which had been formerly relegated to being nothing but clients -- and where these nodes are -- at the edges of the Internet, cut off from the DNS system because they have no fixed IP address.

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Resource-centric addressing for unstable environments

P2P is a class of applications that takes advantage of resources -- storage, cycles, content, human presence -- available at the edges of the Internet. Because accessing these decentralized resources means operating in an environment of unstable connectivity and unpredictable IP addresses, P2P nodes must operate outside the DNS system and have significant or total autonomy from central servers.

That's it. That's what makes P2P distinctive.

Note that this isn't what makes P2P important. It's not the problem designers of P2P systems set out to solve -- they wanted to create ways of aggregating cycles, or sharing files, or chatting. But it's a problem they all had to solve to get where they wanted to go.

What makes Napster and ICQ and Popular Power and Freenet and AIMster and Groove similar is that they are all leveraging previously unused resources, by tolerating and even working with the variable connectivity of the hundreds of millions of devices that have been connected to the edges of the Internet in the last few years.

One could argue that the need for P2P designers to solve connectivity problems is little more than an accident of history, but improving the way computers connect to one another was the rationale behind IP addresses, and before that DNS, and before that TCP, and before that the net itself. The internet is made of such frozen accidents.

P2P is as P2P does

Up until 1994, the whole Internet had one model of connectivity. Machines were assumed to be always on, always connected, and assigned permanent IP addresses. The DNS system was designed for this environment, where a change in IP address was assumed to be abnormal and rare, and could take days to propagate through the system.

With the invention of Mosaic, another model began to spread. To run a Web browser, a PC needed to be connected to the Internet over a modem, with its own IP address. This created a second class of connectivity, because PCs would enter and leave the network cloud frequently and unpredictably.

Furthermore, because there were not enough IP addresses available to handle the sudden demand caused by Mosaic, ISPs began to assign IP addresses dynamically, giving each PC a different, possibly masked, IP address with each new session. This instability prevented PCs from having DNS entries, and therefore prevented PC users from hosting any data or net-facing applications locally.

For a few years, treating PCs as dumb but expensive clients worked well. PCs had never been designed to be part of the fabric of the Internet, and in the early days of the Web, the toy hardware and operating systems of the average PC made it an adequate life-support system for a browser, but good for little else.

Over time, though, as hardware and software improved, the unused resources that existed behind this veil of second-class connectivity started to look like something worth getting at. At a conservative estimate, the world's Net-connected PCs presently host an aggregate ten billion Mhz of processing power and ten thousand terabytes of storage, assuming only 100 million PCs among the net's 300 million users, and only a 100 Mhz chip and 100 Mb drive on the average PC.

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