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A community network often operates within the same ISP (Internet Service Provider) domain or the network is administered by a virtual organization spanning across multiple network domains with an established trust relationship. To counter DDoS (distributed denial-of-service) attacks in such a federated network environment, the routers can work cooperatively to raise early warning to avoid catastrophic damages. This paper proposes a collaborative architecture to detect DDoS flooding attacks. The scheme appeals, in particular, to protect networked resource centers that work as a collaboration Grid. By monitoring the distribution of suspicious traffic changes over a number of attack-transit routers, we developed a new Change-Aggregation Tree (CAT) mechanism to enable early detection of DDoS attacks on community networks. We want to detect flooding attacks as early as possible. Here, we report preliminary NS-2 simulation results on a singledomain ISP core network to prove the effectiveness of the new collaborative CAT architecture for DDoS defense. The simulated system achieved a detection rate as high as 95% with less than 1% of false positive alarms. Extensions of this architecture to cross-domain DDoS defense are discussed with further research challenges identified.