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.

In the mid-80s, the internet was subject to a phenomenon called “Congestion Collapse” that frequently made it unusable. Congestion Collapse (also known as “internet meltdown”) came about because the system that was designed into IP to notify systems of network congestion, “Source Quench” messages, didn’t work when the network was too congested to handle additional traffic. So an algorithm was developed - the Jacobson Algorithm - to slow down the rate at which TCP offered traffic to the network when it was evident that routers weren’t able to handle the traffic load. While Jacobson is highly inefficient – often causing network links to cycle between 50 per cent to 75 per cent of capacity – it does in fact lead to stability, a much more important goal than efficiency.

The ISPs' dilemma

ISPs which throttle users based on raw traffic volume (as the new Comcast system will do) are protected from the effects of the massive use of aggressive UDP inside their networks. And they should be, as these private networks aren’t internets in and of themselves. The damage is going to appear inside the core internet links connecting ISPs, which will become much less responsive to load management.

One rational response is to make UDP the prime candidate for packet discard. When five per cent of users consume half the network’s resources and block access to 75 per cent of its total capacity, it makes sense to target them for throttling. But such throttling will utterly destroy VoIP.

(Note: in principle, VoIP can be distinguished from P2P over UDP, but only by non-politically correct means such as Deep Packet Inspection. Nor is it consistent with the net neutrality laws proposed in the US and the EU forbidding discrimination based on protocol type, source, or destination.)

uTorrent’s net-killing feature is a slap in the face to the very regulators who’ve sanctioned ISPs in the name of this “innovative new application”: it bites the hand that’s fed it with immunity from rational management. And it also gives the lie to the internet Utopians who’ve claimed that internet users manage shared facilities so well that throttling isn’t necessary inside the network. Such idealism is simply a prescription for the return of internet meltdown, this time with a vengeance.

The internet evolved as a gentleman’s system in the comfortable confines of the ivory towers of academe, but now that it’s an essential part of daily life for more than a billion people, the time has come to get realistic about its management. Some of the people who use this system are spoiled children with no more concern for the greater good than junkies looking for their next fix. They can’t be allowed to spoil it for the rest of us, and the only practical means to prevent their doing so is to unleash effective management upon them.

The best way to ensure that uTP doesn’t kill the internet is to throttle it at the source, and any law that stands in the way of ISPs exercising that level of management is deadly to the internet. We can thank the uTorrent developers for reminding us of that salient fact.

Richard Bennett is a network architect and occasional activist in Silicon Valley. He wrote the first standard for Ethernet over twisted-pair wiring and contributed to the standards for Wi-Fi and the Ultra-Wideband wireless networks. His 11-year-old blog is at bennett.com.