Infiniband and most other specialized protocols have data-link-layer reliable messaging. The downside of this is that a congested switch cannot drop packets, so

Infiniband and most other specialized protocols have data-link-layer reliable messaging. The downside of this is that a congested switch cannot drop packets, so you end up backpressuring your network to death unless the people writing software really know what they're doing. Google was not able to make this work at scale.