+ \begin{subfigure}[t]{.2\textwidth}
+ \begin{tabular}{ll}
+ \toprule
+ \multicolumn{2}{c}{Request}\\
+ byte & meaning\\
+ \midrule
+ 1 & \texttt{'s'}\\
+ 2,3 & identifier\\
+ 4,5 & value\\
+ \midrule\midrule
+ \multicolumn{2}{c}{Response}\\
+ byte & meaning\\
+ \midrule
+ 1 & \texttt{'s'}\\
+ 2,3 & identifier\\
+ \bottomrule
+ \end{tabular}
+ \caption{Send an SDS specification}
+ \end{subfigure}
+ \quad%
+ \begin{subfigure}[t]{.2\textwidth}
+ \begin{tabular}{ll}
+ \toprule
+ \multicolumn{2}{c}{Response}\\
+ byte & value\\
+ \midrule
+ 1 & \texttt{'a'}\\
+ 2,3 & SDS id\\
+ \midrule\midrule
+ \multicolumn{2}{c}{Request}\\
+ byte & value\\
+ \midrule
+ 1 & \texttt{'a'}\\
+ 2,3 & SDS id\\
+ \bottomrule
+ \end{tabular}
+ \caption{Delete a SDS}
+ \end{subfigure}
+ \quad%
+ \begin{subfigure}[t]{.2\textwidth}
+ \begin{tabular}{ll}
+ \toprule
+ \multicolumn{2}{c}{Request}\\
+ byte & meaning\\
+ \midrule
+ 1 & \texttt{'u'}\\
+ 2,3 & sdsid\\
+ 4,5 & value\\
+ \midrule\midrule
+ \multicolumn{2}{c}{Response}\\
+ \bottomrule
+ \end{tabular}
+ \caption{SDS update}
+ \end{subfigure}
+ \quad%
+ \begin{subfigure}[t]{.2\textwidth}
+ \begin{tabular}{ll}
+ \toprule
+ \multicolumn{2}{c}{Response}\\
+ \midrule\midrule
+ \multicolumn{2}{c}{Request}\\
+ byte & value\\
+ \midrule
+ 1 & \texttt{'p'}\\
+ 2,3 & SDS id\\
+ 4,5 & value\\
+ \bottomrule
+ \end{tabular}
+ \caption{SDS publish}
+ \end{subfigure}
+ \caption{Message protocol for exchanging SDSs}