Each level of the OSI model can be attacked in a DDoS, and its still of Denial of Service attack. I.E.: You can hold down a pre-2009 windows server with as little as 10-15 packets per second, totally less then a kilobyte per second.

Yes if your have a 10/100/1000 nic and your getting 20Gb/s then yeah nothing's getting in. Or if you have a 10Gb/s router, then 20Gb/s will freeze the whole data center. But if you have a 100Gb/s router with a 4x bounded 10Gb/s nics. And your service is down, then it isn't your hardware, but your software. Your kernel, sockets.h, are still processing packets, you can still be attacked.