Short Paper: What Peer Announcements Tell Us About the Size of the Bitcoin P2P Network

Bitcoin is based on a P2P network of which only a few quantities are publicly known. While the number of peers that disseminate transactions and blocks is relevant for the robustness of the network, only the number of reachable peers is so far being measured. However, there exists an unknown number of unreachable peers in the network, that is, peers that do not accept incoming connections but typically also disseminate transactions and blocks. We propose the Passive Announcement Listening (PAL) method that gives an estimate of the number of unreachable peers by observing peer announcements in addr messages. We use the PAL method to analyze data from a long-term measurement of the Bitcoin P2P network from 2015 to 2022. The PAL estimate shows that since 2018 the number of unreachable peers is at least three times higher than the number of reachable peers. An empirical validation indicates that about 76 % of all unreachable peers announce their address and the PAL approach finds about 94 % of these unreachable peers. Thus, we estimate the total number of unreachable peers in May 2022 to be around 34,000. We also report on a spam wave of addr messages that shows that peer announcements ‘leak’ even more information than the size of the network.