\node[draw,circle,label=below:$i_0$,fill,outer sep=0.1cm, inner sep=0pt, minimum size=0.1cm] (i0) at (-1, 0) {};
\node[draw,circle,label=below:$i_1$,fill,outer sep=0.1cm, inner sep=0pt, minimum size=0.1cm] (i1) at (1, 0) {};

\draw (i0) -- (i1) node [midway, above] (seg) {seg};
% \draw[-] (i0) -- (i1);