  Several blockchain consensus protocols proposed to use of Directed Acyclic
Graphs (DAGs) to solve the limited processing throughput of traditional
single-chain Proof-of-Work (PoW) blockchains. Many such protocols utilize a
random transaction selection (RTS) strategy (e.g., PHANTOM, GHOSTDAG, SPECTRE,
Inclusive, and Prism) to avoid transaction duplicates across parallel blocks in
DAG and thus maximize the network throughput. However, previous research has
not rigorously examined incentive-oriented greedy behaviors when transaction
selection deviates from the protocol. In this work, we first perform a generic
game-theoretic analysis abstracting several DAG-based blockchain protocols that
use the RTS strategy, and we prove that such a strategy does not constitute a
Nash equilibrium, which is contradictory to the proof in the Inclusive paper.
Next, we develop a blockchain simulator that extends existing open-source tools
to support multiple chains and explore incentive-based deviations from the
protocol. We perform simulations with ten miners to confirm our conclusion from
the game-theoretic analysis. The simulations confirm that greedy actors who do
not follow the RTS strategy can profit more than honest miners and harm the
processing throughput of the protocol because duplicate transactions are
included in more than one block of different chains. We show that this effect
is indirectly proportional to the network propagation delay. Finally, we show
that greedy miners are incentivized to form a shared mining pool to increase
their profits. This undermines the decentralization and degrades the design of
the protocols in question. To further support our claims, we execute more
complex experiments on a realistic Bitcoin-like network with more than 7000
nodes.
