\begin{tikzpicture}
\draw[semithick] (0, -0.5) node[below right] {$D$} -- node[midway, left] {$\ell$} (0, 0.5) node[above left] {$C$} -- node[midway, above] {$1$} (-4, 0.5) node[left] {$A$} to[out=-30, in=-150] node[midway, below] {$L$} (0, -0.5) -- node[midway, below] {$1$} (4, -0.5) node[right] {$B$} to[out=150, in=30] node[midway, above] {$L$} (0, 0.5);
\fill (4, -0.5) circle[radius=0.1];
\fill (0, -0.5) circle[radius=0.1];
\fill (0,  0.5) circle[radius=0.1];
\fill (-4, 0.5) circle[radius=0.1];
\end{tikzpicture}