答案1
代码可以更漂亮,但结果就是你想要的。
\documentclass[border=2cm,tikz]{standalone}
\begin{document}
\begin{tikzpicture}[
%\tikzset{every picture/.style=thick}
every node/.append style={very thick,rounded corners=0.1mm}
]
\node[draw,rectangle] (User) at (0,0) {User};
\node[draw,rectangle] (Server) at (3,0) {Server};
\node[draw=blue!50,rectangle,thick] (Network) at (0,-1) {Network Attach};
\draw [very thick] (User) -- (Network)--(0,-5);
\draw [very thick] (Server)--++(0,-5);
\draw [->,very thick] (0,-2)--node [auto] {HTTP GET}++(3,0);
\draw [<-,very thick] (0,-2.5)--node [auto] {200 OK}++(3,0);
\draw [->,very thick] (0,-3.5)--node [auto] {HTTP POST}++(3,0);
\draw [<-,very thick] (0,-4)--node [auto] {200 OK}++(3,0);
\end{tikzpicture}
\end{document}