You can use these links :

http://www.infosecurity-magazine.com/news/ddos-ers-ramp-up-app-layer-attacks/

http://www.academia.edu/25354963/Application_Layer_DDoS_Attack_Defense_Framework_for_Smart_City_using_SDN

Dear Nikhil Tripathi,

In first place you should look more to DDoS attacks rather on DoS. Disrupting transactions and access to databases are also very common denial of service attacks. I think that  Rezaeipour Davood have put two good links about it. Look also for DDoS mitigation.

Good luck to your R&D.

1. volume based attack

2. Protocol Attacks

3. Application layer attacks

dns amplification attack ?