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 the 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 (our last release was ns-3.29 in January 2019). The tool is in wide use; we provide statistics about the project on our web site (under the About/Statistics page), but in summary, we have a users mailing list (Google Groups forum) of over 8000 members as of January 2018, averaging roughly 700 posts per month. Our developers' list has over 1500 subscribers, and the code base has a total of 220 authors and around 10 active maintainers. ns-3 is operated as an open source project, originally funded with financial backing from three NSF grants and from the French government (and via help from Google Summer of Code), but with most current contributions coming from interested researchers and students worldwide. We use a GPLv2 licensing model and heavily use mailing lists, and chat for code springs, but typically not other social media.