Australia's hugely controversial ISP filtering plan received a lump of Christmas coal in its stocking with the release this week of a new report that points out the many difficulties with such a scheme. The current government's response is to make clear that the report was commissioned by the previous government—which apparently makes it a bit suspect. A live trial of the filtering system has been delayed into January, but it is still going ahead.

"The Government is aware of technical concerns raised in the report, and that is why we are conducting a pilot to put these claims to the test," said Senator Stephen Conroy, Minister for Broadband, Communications and the Digital Economy.

The lengthy report was commissioned by the Howard Government back in 2007 and was conducted by the Internet Industry Association. Not that the current government wants you to take the report's points too seriously; Conroy points out that "the report methodology was a literature review of existing studies as well as interviews and surveys. It involved no empirical testing of filtering technology."

The group did, however, conduct interviews with ISPs, content producers, filter vendors, telecommunications lawyers, and technical experts from Sydney University. The project team also "visited and interviewed organizations in countries that have implemented content filtering schemes." What emerges from the report is a sense of the difficulties that Australia would have to face if it moves forward with implementation.

Not least among them are the legal questions. The report identifies a huge list of potential problems, including:

Over-blocking and under-blocking content

Service degradation and the potential impact on existing service level agreements

Interception and hacking

Impairing freedom of expression

Privacy breaches

Contractual claims

Negligence

Misleading conduct

Breaching sale of goods legislation

Incorrect blocks, though they might prove uncommon, could prove costly. The report notes that a single blocked site could result in multiple legal claims, including "defamation, due to the grave implication that the owner has been involved in the distribution of illegal content" and "loss of revenue during the time site was blocked, possibly as a result of degradation of product or brand awareness, or direct loss of potential earnings."

The ISPs consulted for the report pointed out that the filtering scheme could be bypassed quite easily; VPN and HTTPS traffic would be harder to filter, for instance, and the proposed scheme doesn't appear to include e-mail, instant messaging, or P2P content. If it's not on the web, it won't be filtered.

The likely result? An arms race. "There is an evident risk therefore that web filtering will suffer from the [same] high ongoing costs that e-mail filtering does, as web content providers seek to get their content in front of as many users as possible," said one ISP. "For instance, some sites use dynamically generated URLs for each page of content which would make filtering of exact URLs effective only. Such a technique could be employed by such web content providers in order to stymie blacklist filtering."

There's also the question of the increased cost. In the UK, BT recently spent around £500,000 to implement its "CleanFeed" system. Australian ISPs said that increasing their service fees "even $1/month will be too much." These ISPs expect the government to cover any costs they incur implementing the program.

And then there's the question of performance, a key point of pride for ISPs. Most ISPs believe that any scheme would slow speeds, since pulling the destination IP addresses from every packet that passes through the network is a terrifically intensive job; matching those in realtime against a blacklist, then looking up whether the user in question has opted out of the second (optional) level of filtering, could take too much time.

The report quotes an unnamed analyst saying, "Broadband speeds in Australia are low and [ISPs] are concerned about any scheme which degrades existing performance, particularly as it relates to video streaming which has grown dramatically over the past 18 months. Speed degradation would seriously impact that growth."

Finally, the report takes a look at filtering schemes in European countries, many of which take steps to restrict access to child pornography. Nearly all such schemes are voluntary, though, and ISPs can implement them any way they wish. One odd approach was Sweden's, which uses a voluntary industry scheme but only addresses "commercial child pornography sites: i.e., sites that offer child abuse images for sale."

All such schemes were far more limited than Australia's plan, but they do show that filtering is possible. (If the government cared less about free speech and performance concerns, it could obviously adopt a system more like China's, though this isn't the sort of reputation Australia is looking to acquire.)

In any event, the government is going ahead with the scheme. A trial was supposed to begin this week; for reasons not made clear, the trial has now been pushed into January. The government is also signing up mobile operators for a future trial of their own.