The report stated that a subnetwork of Bitcoin nodes is largely responsible for reaching consensus and communicating with miners and that a “vast majority of nodes do not meaningfully contribute to the health of the network.
The authors also found vulnerabilities in the infrastructure, based on the fact that Bitcoin protocol traffic is unencrypted and 60% of the network traffic traverses only three ISPs. Trail of Bits is a cybersecurity research and consulting firm that was engaged by DARPA to develop the report.