Distributed denial-of-service (DDoS) attacks are steadily increasing in size and speed, creating new problems for enterprise defenses, according to a study published today.

Arbor Networks' first quarter ATLAS report, which measures the size and speed of DDoS attacks, says the average size of a DDoS attack continues to grow at about 20 percent a year. The average attack during Q1 was about 1.77 Gbps, up from about 1.48 Gbps in 2012.

While the recent attack on Spamhaus set new records at more than 300 Gbps, the average attack is still less than 2 Gbps, according to Arbor Networks. Still, the proportion of attacks that are in the 2 Gbps to 10 Gbps range has grown from 15 percent to 21.5 percent, the study says.

In fact, in Q1 alone, Arbor has already seen about 74 percent of the total number number of attacks exceeding 10 Gbps as it saw in all of 2012, the report says. "Attacks above 10 [Gbps] and even 20 Gbps now occur multiple times per day somewhere in the world," the security firm says.

An average attack is more than enough to overwhelm most enterprise defenses, experts say.

"A common response by many administrators to the challenges of DDoS is the belief that their firewall and IPS infrastructure will protect them from attack. Unfortunately, this is not true," says Richard Martinez, enterprise network security analyst at Frost & Sullivan. "Firewalls and IPS devices, while critical to network protection, are not adequate to protect against complex DDoS attacks."

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