Gamers, VoIP and video conference users beware. The leading BitTorrent software authors have declared war on you - and any users wanting to wring high performance out of their networks. A key design change in the P2P application promises to make the headaches faced by ISPs so far look like a party game. So what's happened, and why does it matter?

Upset about Bell Canada’s system for allocating bandwidth fairly among internet users, the developers of the uTorrent P2P application have decided to make the UDP protocol the default transport protocol for file transfers. BitTorrent implementations have long used UDP to exchange tracker information – the addresses of the computers where files could be found – but the new release uses it in preference to TCP for the actual transfer of files. The implications of this change are enormous.

As BitTorrent implementations follow uTorrent’s lead – and they will, since uTorrent is owned by BitTorrent Inc, and is regarded as the canonical implementation – the burden of reducing network load during periods of congestion will shift to the remaining TCP uses, the most important of which are web browsing and video streaming.

By most estimates, P2P accounts for close to half of internet traffic today. When this traffic is immune to congestion control, the remaining half will stumble along at roughly a quarter of the bandwidth it has available today: half the raw bandwidth, used with half efficiency, by 95% of internet users. Oops.

When your internet bandwidth is divided by four, you’re going to notice. Even the downloading fiends who haunt the message boards at Broadband Reports can see this, as several have noted:

Is bypassing TCP congestion control a good thing for the users of the network? Why should one persons [sic] non-interactive file sharing generating a dozen to a hundred streams be more important than my interactive VoIP call or gaming experience? Using it as a feature, maybe, but enabling this behavior by default is just wrong and will lead to continuing counter, counter measures and more justification for caps.

But this insight isn’t shared by downloaders in general, most of whom have a sense of entitlement where their etiquette gene should be.

What's UDP and why does this matter?

UDP was intended for real-time data transfers such as VoIP that typically move small amounts of data with a low tolerance for delay. By most estimates, UDP traffic amounts to less than two per cent of all internet traffic. Bulk data transfers are supposed to use TCP, in large part because it shoulders the burden of congestion control for the internet’s end-to-end layer. There’s no doubt that P2P file transfers are the epitome of delay-tolerant, bulk data applications.

The internet is only a stable system because application developers are gentlemanly with regard to the amount of traffic they shove onto the network. But it hasn’t always been so.