htsim is a high-performance discrete event network simulator designed to evaluate congestion control algorithms and transport protocols at scale.
The tool addresses the need to study how congestion control behaves under realistic network conditions, particularly when many flows are multiplexed or when examining multipath transport scenarios. It achieves high performance through a discrete event simulation architecture, allowing researchers to model complex network topologies and protocol behaviors efficiently. The simulator supports a range of transport protocols and network models, including TCP variants, Multipath TCP, NDP, DCTCP, DCQCN, RoCE, PFC, Swift, and HPCC, with configurable switch models that support various forwarding methods.
Developers working on congestion control research, datacenter network design, or transport protocol development should consider htsim if they need to simulate large numbers of flows across diverse network topologies. The tool is particularly suited for academic research and protocol development where simulation speed and protocol flexibility matter more than real-world packet-level fidelity. It requires only a C++ compiler with no external dependencies, making it straightforward to build and deploy on Linux or macOS systems. The project includes example experiments in its repository to help new users get started, though documentation is limited to a wiki.
The project maintains active engagement with its codebase through regular updates to support emerging protocols and network models. Development includes incremental refinements to existing features such as switch forwarding capabilities and protocol implementations rather than wholesale architectural changes. The tool continues to receive contributions that extend its modeling capabilities for new transport protocols and datacenter scenarios.
