\begin{sequencediagram}
  \newthread{a}{:Machine A}
  \newthread[gray]{b}{:Machine B}

  \mess[1]{a}{}{b}
  \node[anchor=east] (t0) at (mess from) {$t_0$};
  \node[anchor=west] (t1) at (mess to) {$t_1$};
  \prelevel
  \mess[1]{b}{}{a}
  \node[anchor=east] (t2) at (mess to) {$t_2$};

  \path (t0.east) |- coordinate(t12) (t1);
  \draw[dashed] (t1) -- (t12);
  \node[anchor=south west] at (t12) {$\Delta{}t_1$};
  \node[anchor=north west] at (t12) {$\Delta{}t_2$};
\end{sequencediagram}