https://www.itworld.co.kr/news/173069
https://www.affdu.com/ko/best-open-source-zoom-alternatives.html
https://www.bloter.net/newsView/blt201603160002
https://progsoft.net/ko/software/slack
https://rocket.chat/
https://github.com/RocketChat/Rocket.Chat
https://mattermost.org/
https://github.com/mattermost/mattermost-server