net2o has a very low bandwidth acknowledge protocol (one ack for 32 packets, the ack is a small packet, 64 or at most 128 bytes, while the data is 32k). And net2o's flow control relies on the receiver to signal correct time stamps.
So a malicious receiver can just spoof some answers and drive the sender to create a lot of traffic. To prevent acknowledge spoofing, we require the receiver to compute a "cookie" for every packet transmitted — this cookie is something that proves he has received and correctly decrypted the packet, but the cookie itself is actually never sent around. We use Keccak's hidden state to create this cookie — reduced to a 64 bit number (this is more than sufficient — anything an attacker can create is bandwidth). We xor all cookies of one acknowledge lump together.
A malicious receiver who creates excessive traffic now will not receive the packets anymore, which prevents him from creating a legit acknowledge.