Days review
A focused Rust simulator for packet-level studies of congestion, queues, and topologies.
Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026
Days is an open-source, process-based discrete-event network simulator written in Rust. It is aimed at researchers modeling data-center networks at packet level, with network elements represented as concurrent coroutines that exchange packets through message passing. Experiments are configured with TOML files, while supported building blocks include switches, queues, transport protocols, routing, packet drops, link-layer features, and collective communication operations such as broadcast and all-reduce.
Its strongest technical depth is congestion and queueing analysis. Days supports FIFO, DRR, WRR, WFQ, and Virtual Clock disciplines, alongside TCP Reno, CUBIC, and BBR models. Optional DCQCN and PFC/802.1Qbb models extend the scope toward data-center traffic behavior. FatTree and 2D/3D Torus topologies provide defined environments for comparative experiments, and CSV reports can cover sources, sinks, switches, and optional protocol traces. Single-threaded and multi-threaded runtimes give researchers a choice of execution models for their simulations.
Days follows an open-source model and does not present commercial pricing or paid tiers. That makes it a focused option for research workflows built around configurable experiments and structured output rather than a packaged commercial platform. The product is presented as a research preview, and its documented emphasis is on data-center-oriented topologies, packet-level behavior, queue disciplines, transport protocols, and collective communication. Choose Days when those capabilities match the study design; choose a broader alternative when you need a more general-purpose simulator or a commercial deployment model.
Days pros and cons
- Where it wins
- Models FatTree and 2D/3D Torus network topologies
- Includes multiple queue disciplines and TCP congestion models
- Exports structured CSV reports with single- and multithreaded runtimes
- Where it doesn't
- Focused primarily on data-center-oriented network scenarios
- Presented as a research preview rather than a commercial product
- Requires TOML-configured experiments and Rust-based tooling
Days fact sheet, pricing and score →
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