Closer look at P2P technology

The internet was invented to allow people to easily share files and information. But ever since the original Napster launched in 1999, file sharing has had a bad name.

This is due to the fact that people quickly realised they could easily find and download files that they weren’t supposed to, such as illegally copied music mp3s.

But public demand for the capabilities unleashed by Napsters’s peer-to-peer technology (P2P) spawned a whole host of ‘me-too’ products and protocols, including Kazaa, Grokster and Gnutella.

Many of these have now gone to the wall; the Napster brand was reborn as a (non-P2P) legitimate music download service, Kazaa seems to be in virtual hibernation and Grokster was infamously closed down by the US Supreme Court.

But there’s one open source P2P technology – called Bittorrent – that has survived and prospered, even though it’s just as open to abuse as any file-sharing technology. Invented in 2001 by programmer Bram Cohen, it was developed as a way to efficiently distribute large files without the need for a dedicated file server. This reason alone helped it to become a popular – and legal – way to make Linux distributions available for download.

You might consider it a niche application, but an awful lot of people are using it – more than 150 million according to www.bittorent.com. Statistics published by Cachelogic (www.cachelogic.com) in 2004 showed that Bittorrent traffic accounted for more than 35 per cent of all web traffic worldwide. Bittorrent.com itself is currently ranked in the top 2,000 websites according to the website monitoring service Alexa.

Smart moves
In 2005 Bittorrent Inc. (the company that Cohen and partner Ashwin Navin created in 2004) neatly distanced itself from the shady side of P2P by doing a deal with the Motion Picture Association of America (MPAA), and agreeing to remove any illicit copyrighted works from Bittorrent’s commercial site.

Since then, Bittorrent.com has started offering paid-for legal TV and movie downloads. In 2006 it signed a breakthrough deal for online movie distribution with Warner Brothers. It now seems to be going from strength to strength, featuring video, music, TV and game content from top media brands such as 20th Century Fox, MTV, Paramount and Eidos.

Bittorrent, the technology, remains open source and is available for anyone to develop an application around, which is another reason it’s remained so popular, But how does it work, and just what makes it so suited to sending digital movies or huge Iso files around the world? To answer this, first we need to step back and look at some of the basics of P2P technology.

Peer pressure
There are two main ways to share a file between lots of users. The traditional and familiar client-server method is to put the file on a central server and allow multiple clients (PCs) to access it directly.

There’s no need for any communications between the clients – all they need to do is talk to the server. But this means that the server has to be able to cope with delivering multiple copies of the file to lots of clients simultaneously, otherwise the server becomes a bottleneck. For sharing via the relatively low-bandwidth internet this can obviously be a major problem, leading to high costs and congestion at the server.

A peer-to-peer architecture simply does away with the central server and allows any PC connected to the network (a peer) to act as both a client and a server. Peers can then communicate directly with each other to obtain the files they need. This gives several benefits, the most important being data redundancy and no need for massive bandwidth on any one peer machine. A peer can limit peer connections or file downloads depending on how much bandwidth it has available.

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