\begin{tikzpicture}
\draw [thick] (0,0) coordinate [label=below:A] (A) --  ++(4,0) coordinate [label=below:B] (B) ;
\draw [bend right, distance=1cm] (A) to node [midway,fill=white] {$r_{AB}$} (B);
\draw (A) -- node [midway,left] {$r_{A1}$} ++(70:2) node [circle, fill,inner sep=2pt, label=left:1] (e1) {};
\draw (B) -- node [midway,right] {$r_{B2}$}++(110:3) node [circle, fill,inner sep=2pt, label=right:2] (e2) {};
\draw (B) -- node [midway,above] {$r_{B1}$} (e1) {};
\draw (e1) --  node [midway,above] {$r_{12}$} (e2);
\end{tikzpicture}