Days
Days: A focused Rust simulator for packet-level studies of congestion, queues, and topologies. Ranked #26 of 30 in Network Simulation Software by our editors (6.5/10); pricing: Open source; best for data-center congestion-model researchers.
At a glance
- Editor score6.5 / 10
- PricingOpen source
- Best forData-center congestion-model researchers
- Paid fromFree
- Facts checked20 Sep 2026
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
Our verdict on Days
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.
Your product? Our editors can test Days in depth and publish a review, labelled as paid · from $149/mo →
Days pricing
Days fact sheet
| Free plan | Not verified |
|---|---|
| Paid from | Free |
| Simulation model | Discrete-event |
| Real-time emulation | Not verified |
| Hardware-in-the-loop | Not verified |
| Deployment options | Not verified |
| Documented node limit | Not verified |
| Automation API | Yes |
| Deployment | Self-hosted |
| Support | Docs |
| Built for | Solo, Small business, Mid-market, Enterprise (editorial estimate) |
| Pricing | Open source |
| Website | days.sh |
| Facts checked | 20 Sep 2026 |
Alternatives to Days
- GNS3A free, broad multivendor lab for simulating, emulating, and connecting real networks.9.4
- Cisco Modeling LabsA Cisco-focused virtual lab with broad automation, integration, and deployment options.9.2
- EVE-NGA broad emulation platform for large, collaborative multivendor network labs.9.0
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Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026
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