L4S3

L4S3 is a modular, cross-platform software framework designed for simulating and prototyping networked systems and edge computing deployments. It aims to enable researchers and developers to model Layer 4 behaviors, test new congestion control ideas, and validate protocol designs in a controlled environment. The framework emphasizes modularity, performance, and reproducibility, with a focus on making experiments easy to reproduce.

The architecture comprises a core event-driven engine, a plugin system for network protocols and hardware models, and language bindings for Python, C++, and Rust. It supports both discrete-event simulation and real-time emulation, and includes a library of reference protocol implementations along with a telemetry subsystem for collecting metrics. The modular design allows users to swap components such as schedulers, transport stacks, and link models without altering application code.

L4S3 began as an open-source project in 2021 developed by the L4S3 Consortium. Version 3, the current release, introduces deeper modularization, an enhanced scheduler, and improved signaling mechanisms inspired by low-latency networking concepts. Community governance relies on a merit-based contributor model and a permissive open-source license.

In practice, L4S3 is used in graduate courses to illustrate networking concepts, in research to prototype latency-sensitive protocols, and by startups for rapid edge-service prototyping. Its emphasis on reproducibility and portability facilitates collaboration across institutions and vendors, contributing to shared benchmarks and methodologies for evaluating layer-4 behavior.

The project notes limitations such as the gap between simulation and real-world deployment, and ongoing work focuses on scaling simulations, better real-time fidelity, and hardware-in-the-loop integration.