https://blog.gongquiz.com/2021/09/13/osi-7%ea%b3%84%ec%b8%b5%ec%9d%98-%ea%b0%81-%ea%b3%84%ec%b8%b5%eb%b3%84-%ea%b8%b0%eb%8a%a5-%ec%a4%91-%ec%a6%9d%ec%a0%90%ea%b0%84%ec%9d%98-%ec%98%a4%eb%a5%98%ec%88%98%ec%a0%95%ea%b3%bc-%ed%9d%90/