Paper:
This paper considers multi-agent reinforcement learning (MARL) in networked system control. Specifically, each agent learns a decentralized control policy based on local observations and messages from connected neighbors. We formulate such a networked MARL (NMARL) problem as a spatiotemporal Markov decision process and introduce a spatial discount factor to stabilize the training of each local agent. Further, we propose a new differentiable communication protocol, called NeurComm, to reduce information loss and non-stationarity in NMARL. Based on experiments in realistic NMARL scenarios of adaptive traffic signal control and cooperative adaptive cruise control, an appropriate spatial discount factor effectively enhances the learning curves of non-communicative MARL algorithms, while NeurComm outperforms existing communication protocols in both learning efficiency and control performance.

Review:
This paper is concerned with network multi-agent RL (N-MARL), where agents need to update their policy based on messages obtained only from neighboring nodes. This is done under sensible restrictions on the state transition distribution, which can be claimed to hold true in realistic networked settings. The authors argue that introducing a spatial discount factor (along a temporal one), where neighboring nodes have a small distance, stabilizes learning. Also, they provide a way of learning a networked communication protocol. Experiments are done on somewhat realistic simulations of traffic.
I am not an expert in this type of distributed MARL, and what the SotA there is. What is proposed here makes sense to me, even though it also does not look very surprising to me. Spatial discounting seems to make sense. The authors do not comment on the relationship between the d_{ij} (or how to choose them) with the neighborhood graph, I suppose that neighbors have short distances, while non-neighbors have not. The NeurComm protocol is a bit vague to me, and I cannot say how it relates to other proposals. I find it a bit strange (unless I misunderstood something) that complete state and policy information is transmitted to every neighbor, and on top some latent vector. This sounds expensive to me, surely these communication channels are limited? It'd be more interesting to consider bandwidth limitations here. Without knowing related work on other protocols in detail, I suspect they probably work less well than NeurComm, simply because they allow for smaller messages only (f.ex., whereas they talk about "policy fingerprints", they seem to submit the complete policy parameters to neighbors -- that is not just a fingerprint). Maybe I am wrong, but if so, the paper does not explain things properly (there is some encoding and decoding going on, but just to propagate the hidden states at each node).
To me as an outsider, this looks like interesting work with well-simulated experiments. Of course, these are simulated and allow for no real-world conclusions, but then much of RL work does neither. It is just I'd be hard pressed to say what is really surprising in this paper, and what could be learned from it.