any paper on how layer3/4 and layer 7 attacks defended? appreciate any pointer
A great resource is The Akamai Blog - Akamai .
If you are looking for some detailed latest attacks you can look for blogs like: When Things Attack - The Akamai Blog
or 620+ Gbps Attack - Post Mortem - The Akamai Blog
For general knowledge - you can look at OWASP: Top 10 2013-Top 10 - OWASP, they explain how the biggest attacks look like. There is also a lot of great trainings out there on Network and transport Layer Security. Google is your friend
For additional info, i highly recommend reading our State Of the Internet Security reports. You can get those here: Global State of the Internet Security & DDoS Attack Reports | Akamai
Hope this helps!
Very simple starting point, but not detailed:
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