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Tech riderRev. 20 Sept 2026

Days

Ch 295.3of 29 Network Simulation Software
  1. 1Runs onLinux
  2. 2CostsNot stated by the maker
  3. 3Simulation modeldiscrete-event
  4. 4Automation APIYes
3 lines stated Written from the maker's own pages: days.sh
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The Days homepage

Overview

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

Compared on network simulation software

Simulation model
discrete-eventdays.sh
Automation API
Yesdays.sh

Facts

Product
Days is an exact discrete-event packet network simulator.days.sh · 4 Oct 2026
Execution
It offers Scalar, multicore CPU, Apple Metal, and NVIDIA CUDA backends that aim to produce byte-identical complete simulation states.days.sh · 4 Oct 2026
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.days.sh · 4 Oct 2026
Network models
It models fat-tree, torus, dragonfly, and custom graph topologies, along with TCP Reno and CUBIC, rate-paced sources, DCQCN, and CNP.days.sh · 4 Oct 2026
Switch features
Supported switch mechanisms include FIFO, strict priority, DRR, WRR, exact-rational WFQ, TailDrop, ECN thresholds, RED, RED with ECN, and PFC.days.sh · 4 Oct 2026
Collectives
It supports ring all-reduce and all-gather over TCP, with delay-only compute stages.days.sh · 4 Oct 2026
GPU limitation
RED admission, collectives, and compute stages are supported only on Scalar and CPU; Metal and CUDA reject those scenarios during validation.days.sh · 4 Oct 2026
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.days.sh · 4 Oct 2026
Configuration
Simulations are configured in a TOML file passed to the command-line program.days.sh · 4 Oct 2026
Outputs
Days can write periodic CSV reports for source, sink, and switch statistics, with additional reports and event traces gated by features.days.sh · 4 Oct 2026
License
The project is licensed AGPL-3.0-only.days.sh · 4 Oct 2026
Intended users
The site describes using Days to debug simulations, reproduce regressions across backends, and compare other simulators’ approximations against an exact reference.days.sh · 4 Oct 2026

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Where it ranks on Specifiction

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Sources