Massive distributed denial-of-service attacks against ISPs and their customers doubled in intensity over the past year, according to a new survey.

Distributed DoS attacks are now reaching 42Gbit/s in sustained intensity, up from 24Gbit/s last year and just 17Gbit/s the year prior to that, according to Arbor Networks' annual survey of ISPs from North America, Europe and Asia.

Some of the 66 global ISPs surveyed for the Worldwide Infrastructure Security Report are Arbor customers, some are not.

"This attack size is the largest we've ever seen," said Arbor CTO Rob Malan. He links the larger scale of distributed DoS attacks directly to ISPs' ever-larger network backbones as well as growth in higher-speed local connections such as cable networks.

Malan believes many of the massive distributive DoS attacks are intended to strike and flood both ISP infrastructure and customer web-facing applications. "These tricky requests tie up use of a database and web server. They're not necessarily a typical security attack where they're looking for a vulnerability," he says.

On the plus side, the majority of ISPs surveyed expressed confidence in detecting distributed DoS attacks using both commercial and open source tools.

Survey results showed "significant adoption of inline mitigation infrastructure and a migration away from less discriminate technologies like blocking all customer traffic (including legitimate traffic) via routing announcements."

The number of ISPs that use either source or destination-based access-control lists on routers as their primary attack-mitigation technique decreased from 47 percent to 30 percent this year, according to the report. "Many ISPs also report deploying walled-garden and quarantine infrastructure to combat botnets," the report states.

Still, it can take considerable time to fend off distributed DoS attacks.

Fifteen percent of respondents said it typically took 15 minutes or less to mitigate an attack. Another 15 percent said it took less than 20 minutes, and 14 percent said it took less than 30 minutes. It took an hour for 26 percent of respondents, and 30 percent typically needed more than an hour to mitigate a distributed DoS attack, even after it had been detected.

ISPs said the most vulnerable elements of their infrastructure are DNS services, routers, VoIP components and load balancers.

For the third year in a row, fewer ISPs are referring attacks to law enforcement. Roughly 58 percent of the surveyed ISPs said they had referred no incidents to law enforcement over the past year, compared with 50 percent the year before. Among the reasons cited for not reporting the attacks was the opinion that law enforcement had limited capabilities and the expectation that customers would report the attacks.

When asked which threats caused the biggest drain on operational resources, ISPs named spam. Distributed DoS took second place. Peer-to-peer network usage, and its impact on operations in terms of engagement with law enforcement and other entities, was also listed by about 5 percent of respondents.

Another issue the report highlighted is concern that ISPs' security products don't adequately address the IPv6 routing protocol, which is starting to show growth in usage.

"Last year IPv6 didn't register in scale, but now it's emerging as a concern on the security side," says Malan. "Attackers are going to try it or use it as a transport mechanism for botnets. IPv6 has become a problem on the operational side."

The Arbor survey doesn't tally how many distributed DoS attacks were reported by the surveyed ISPs, or where the attacks originated.