Are you interested in contributing to a widely-used performance evaluation tool for computer networking research? ns-3 is a *discrete-event, packet-level network simulator* with an emphasis on networking research and education. Users of ns-3 can construct simulations of computer networks using models of traffic generators, protocols such as TCP/IP, and devices and channels such as Wi-Fi and LTE, and analyze or visualize the results. Simulation plays a vital role in the research and education process, because of the ability for simulations to obtain reproducible results (particularly for wireless protocol design), scale to large networks, and study systems that have not yet been implemented. A particular emphasis in ns-3 is a high degree of realism in the models (including frameworks for using real application and kernel code) and integration of the tool with virtual machine environments and testbeds. Very large scale simulations are possible; simulations of hundreds of millions of nodes have been published. ns-3 has been in development since 2005 and has been making regular releases since June 2008. The simulator is written in C++, with bindings for Python scripting, and uses the CMake build system. We use a GPLv2 licensing model and heavily use mailing lists and Zulip chat, but typically not other social media.