No. 24 of 29 ·Network Simulation Software

Days

Linux

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Freedom report

No signalScore 5.0

Free tierNo free tier on record
Open codeNo open-source code on record
Runs widely1 of 6 device platforms
DocumentedThinly documented

Days is ranked #24 of 29 in network simulation software on Freedom251. It runs on Linux.

Compared on network simulation software

Simulation model: discrete-event
Automation API: Yes

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The record

Product: Days is an exact discrete-event packet network simulator.
Execution: It offers Scalar, multicore CPU, Apple Metal, and NVIDIA CUDA backends that aim to produce byte-identical complete simulation states.
Parallel correctness: The site describes five Lean-proved theorems about the executor’s scheduling model and says backend conformance is checked against the serial result.
Network models: It models fat-tree, torus, dragonfly, and custom graph topologies, along with TCP Reno and CUBIC, rate-paced sources, DCQCN, and CNP.
Switch features: Supported switch mechanisms include FIFO, strict priority, DRR, WRR, exact-rational WFQ, TailDrop, ECN thresholds, RED, RED with ECN, and PFC.
Collectives: It supports ring all-reduce and all-gather over TCP, with delay-only compute stages.
GPU limitation: RED admission, collectives, and compute stages are supported only on Scalar and CPU; Metal and CUDA reject those scenarios during validation.
Installation: The quick start builds and runs Days from the repository with Cargo; GPU backends require enabling their Cargo feature, and CUDA builds require the CUDA 13 toolkit.
Configuration: Simulations are configured in a TOML file passed to the command-line program.
Outputs: Days can write periodic CSV reports for source, sink, and switch statistics, with additional reports and event traces gated by features.
License: The project is licensed AGPL-3.0-only.
Intended users: The site describes using Days to debug simulations, reproduce regressions across backends, and compare other simulators’ approximations against an exact reference.

Where it ranks on Freedom251

Best Network Simulation Software in 2026#24 of 29

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Sources

days.sh· checked 4 Oct 2026
days.sh/docs/configuration/overview· checked 4 Oct 2026
days.sh/docs/quickstart· checked 4 Oct 2026