DDoS attacks originally targeted Spamhaus on 16th March. Spamhaus engaged the services of CloudFlare (http://blog.cloudflare.com/) who were able to mitigate the initial attacks successfully. The attacks then escalated between 19th and 21st March exhausting the capabilities of CloudFlare. The attacks also moved on to target next-hop addresses at IX’s around the world (AMS-IX, DEC-IC, HK-IX, Equinix and LINX) causing congestion and a perceived Internet slow down in some geographies. ISPs around the world have worked to deploy filters to mitigate the impact of the attacks.
This the largest known DDoS attack to date by a significant margin. The previous largest reported (and verified) attacks were at around 100Gb/sec. However, this is not the only example of a large (damaging) DNS reflection / amplification attack to have taken place this year.