DDoS mitigation layer

Design a DDoS-mitigation layer protecting a global platform from volumetric (L3/L4 floods), protocol, and application-layer (L7) attacks, capable of absorbing multi-terabit attacks while keeping p99 latency for legitimate users under 50ms. It must distinguish attack traffic from flash-crowd legitimate spikes, automatically engage scrubbing, and not blackhole real users. Cover the network architecture, detection, the mitigation pipeline, and the legitimate-vs-attack discrimination trade-off.