TCP hole punching for Bitcoin nodes behind home NATs?

Efficiently managing the allocation of connection slots on a board is essential to prevent malicious actors from overwhelming the system. Typically, the number of available slots on each board is restricted. When all slots become occupied, connections are evicted according to a scoring mechanism. This score might be determined using a function such as hash(board IP | node IP | date), where preference is given to scores with more zero bits. This approach ensures that nodes with higher-quality scores are less likely to be displaced.

Honest participants in the network can calculate which boards they are most suited to connect with on any given day, optimizing their chances of remaining connected and avoiding eviction. Meanwhile, an attacker attempting to monopolize slots by connecting multiple IP addresses across several boards faces increasing difficulty, as honest nodes will eventually evict these connections over time. To gain more presence, the attacker must invest in acquiring additional IP addresses. Observers monitoring the boards can often identify those dominated by attackers, since the IPs associated with these boards typically yield lower-quality scores compared to those found on boards with predominantly honest participants.

There are privacy considerations in this design. If one has knowledge of which home node IPs are actively seeking connections, it becomes possible to deduce which nodes may be associated with specific boards. This scenario potentially exposes a set of home nodes currently establishing connections, although members of a board inherently have visibility into who else is present. As legitimate home nodes fill their available slots and subsequently reduce their participation on boards, there is a risk that attackers may find it easier to occupy the remaining space, further emphasizing the need for careful management and ongoing refinement of slot allocation strategies.