In an embodiment, an out-of-order, reliable, end-to-end protocol is provided that can enable direct user-level data placement and atomic operations between nodes of a multi-node network. The protocol may be optimized for low-loss environments such as High Performance Computing (HPC) applications, and may enable loss detection and de-duplication of packets through the use of a robust window state manager at a target node. A multi-node network implementing the protocol may have increased system reliability, packet throughput, and increased tolerance for adaptively routed traffic, while still allowing atomic operations to be idempotently applied directly to a user memory location.