The Shadow Simulator

What is Shadow?

Shadow is a discrete-event network simulator that directly executes real application code, enabling you to simulate distributed systems with thousands of network-connected processes in realistic and scalable private network experiments using your laptop, desktop, or server running Linux.

Shadow experiments can be scientifically controlled and deterministically replicated, making it easier for you to reproduce bugs and eliminate confounding factors in your experiments.

How Does Shadow Work?

Shadow directly executes real applications:

  • Shadow directly executes unmodified, real application code using native OS (Linux) processes.
  • Shadow co-opts the native processes into a discrete-event simulation by interposing at the system call API.
  • The necessary system calls are emulated such that the applications need not be aware that they are running in a Shadow simulation.

Shadow connects the applications in a simulated network:

  • Shadow constructs a private, virtual network through which the managed processes can communicate.
  • Shadow internally implements simulated versions of common network protocols (e.g., TCP and UDP).
  • Shadow internally models network routing characteristics (e.g., path latency and packet loss) using a configurable network graph.

Why is Shadow Needed?

Network emulators (e.g., mininet) run real application code on top of real OS kernels in real time, but are non-determinsitic and have limited scalability: time distortion can occur if emulated processes exceed an unknown computational threshold, leading to undefined behavior.

Network simulators (e.g., ns-3) offer more experimental control and scalability, but have limited application-layer realism because they run application abstractions in place of real application code.

Shadow offers a novel, hybrid emulation/simulation architecture: it directly executes real applications as native OS processes in order to faithfully reproduce application-layer behavior while also co-opting the processes into a high-performance network simulation that can scale to large distributed systems with hundreds of thousands of processes.


Shadow implements over 150 functions from the system call API, but does not yet fully support all API features. Although applications that make basic use of the supported system calls should work out of the box, those that use more complex features or functions (e.g., fork()) may not yet function correctly when running in Shadow. Extending support for the API is a work-in-progress.

That being said, we are particularly motivated to run large-scale Tor Network simulations. This use-case is already fairly well-supported and we are eager to continue extending support for it.

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