The document discusses security issues in cloud computing environments, particularly distributed denial-of-service (DDoS) attacks. It provides background on cloud computing and defines DDoS attacks. The document then reviews different approaches in existing literature to address the problem of tracing the source of DDoS attacks, including using packet marking to probabilistically or deterministically mark packets with router addresses as they are traversed. Flexible packet marking schemes and the use of tables to reconstruct attack paths from marked packets are also discussed.