Network administrators hate peer-to-peer - when they're not using it themselves to download MP3s, that is - because not only is it hard to block, but it has a tendency to aggressively consume whatever bandwidth is available, to the detriment of everything else on the network.

Recent statistics from CacheLogic, which runs a global monitoring network called Streamsight, show that P2P overtook the web as the largest consumer of Internet bandwidth during 2002. By the end of 2004, P2P accounted for 60 percent of all Internet traffic.

This may come as little surprise to those who already monitor the traffic flowing over to their company's connection to the Internet. What you might not realise is that the problem could be about to get worse, and somewhat counter-intuitively, it's because of legal file sharing, not illegal.

"As P2P becomes more used for legal content, it becomes harder for ISPs because they can't just traffic-shape P2P down," says Andrew Parker, CacheLogic's CTO. "The one big metric they have is subscriber numbers - they don't want to do anything to upset their subscribers and cause churn."

The same applies to companies and other user organisations, he says: various companies can sell you boxes to block P2P or limit the bandwidth available to it. However, P2P is now gaining legitimate business uses, which makes it awkward to simply block. So if you don't figure out what P2P could do for your organisation and deal with it appropriately, you could be in for trouble.

How it works
P2P works because users who have already downloaded a file then make it available for others to download. It also breaks each file into sections, so you can download the same file from different sources simultaneously.

This aggregation makes tremendous bandwidth available, plus if one host goes offline, the file can still be downloaded from others. Popular P2P networks include BitTorrrent, eDonkey and Gnutella, Parker says.

He adds that businesses want to use P2P because it can save their bandwidth by spreading the load - it means that would-be downloaders are not limited to whatever upstream bandwidth the software company can afford.

Already, some 10 to 20 percent of P2P traffic is compressed data and various Unix file types, according to CacheLogic. Several Linux distros can be downloaded via BitTorrent, for example, although thus far it's mainly freeware rather than enterprise versions. Microsoft is showing interest in using it for software distribution too - it has a P2P research project underway, called Avalanche.

P2P for legal media
It's not just software - media companies too see P2P as the next wave of cheap distribution technology. Parker points to the BBC's interactive media player, iMP, which is being developed to avoid centralised downloads. And there are P2P applications as well, such as Skype.

"One reason people are looking at P2P is because it gets them away from CDN - content distribution networks," Parker adds.

Some bandwidth can be saved by caching P2P content within the network, just as web content is cached locally - this is one of the areas where CacheLogic, which installs monitoring boxes free of charge on ISP networks and feeds the resulting data back to the host ISPs, makes its money.

"It's very complex - P2P is very large objects, very long sessions, undocumented protocols, it's very demanding," Parker says. "Your average copy of Squid or Microsoft ISA is not capable of dealing with P2P."

He adds that P2P is also hard to track: "Network monitoring tools are typically port-based. One thing that software like Skype does is masquerade as other traffic to evade controls - it's port-agile and protocol-agile."

Plus, by making things more readily available, P2P actually increases demand. It also causes problems for service providers because it hammers the upstream and downstream bandwidth equally, whereas most connections are asymmetrical - and adding symmetry to fix that could mean a complete network overhaul.

But as Parker points out, P2P is now a fact and it's moving into business. Fighting it won't work, as the saner heads in the music and video industry are realising - all that the MPAA and RIAA crackdowns achieved is displacement from one P2P network to another.

And anyway, as it acquires legal uses, how do you tell if it's a bootleg MP3 or a Microsoft service pack coming down?