Cisco is beefing up its denial of service defences through the $39m cash purchase of Riverhead Networks.

Distributed Denial of Service (DDoS) attacks are designed to cripple websites by flooding their servers with maliciously-created, useless traffic.

Cisco is buying Riverhead to act as the belt to its braces, filling in the gaps missed by Cisco Security Agent, its host-based intrusion prevention software, which is meant to keep a website functioning under sustained hacker attack.

Riverhead's network-based threat prevention technology analyses traffic flows for protocol compliance and divergence from normal traffic patterns and behaviour. This info supplies the means to quickly detect and block malicious traffic without affecting legitimate business transactions.

Riverhead was founded in 2000 and has 44 employees. It competes against vendors such as TippingPoint, Mazu Networks, Top Layer and Arbor Networks. Arbor is backed financially by Cisco, but may have to consider a new exit strategy, now that its benefactor is engaged to a major rival.

The acquisition is subject to the usual closing conditions and is expected to close in Cisco's Q3 2004. Riverhead will then become part of Cisco's Internet switching business unit. ®

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