https://blog.gongquiz.com/2021/04/21/osi-7%ea%b3%84%ec%b8%b5-%ec%a4%91-%ec%a2%85%ec%a0%90-%ed%98%b8%ec%8a%a4%ed%8a%b8-%ec%82%ac%ec%9d%b4%ec%9d%98-%eb%8d%b0%ec%9d%b4%ed%84%b0-%ec%a0%84%ec%86%a1%ec%9d%84-%eb%8b%a4%eb%a3%a8%eb%8a%94/