digraph ladder {
  ranksep=".5"; nodesep="1"; splines="line";

  node [shape=point fontsize=10];
  edge [dir=none fontsize=10];

  // Column labels
  a [shape=Square label="Contact A"];
  w [shape=Square label="Notification Server"];
  b [shape=Square label="Contact B"];

  // Draw the 3 column headings, no line
  { rank=same; edge[style=invis] a -> w -> b   }

  // Draw the columns
  a -> a1 [style=dotted weight=1000];
  w -> w1 [style=dotted];
  b -> b1 [style=dotted weight=1000];

  a1 -> a2 -> a3 -> a4 -> a5 [weight=1000];
  a3 [xlabel="Contact A wants\nto send a message\nto Contact B" weight=1000];
  w1 -> w2 -> w3 -> w4 -> w5 [weight=1000];
  b1 -> b2 -> b3 -> b4 -> b5 [weight=1000];

  // Now each step in the ladder
  { rank=same; a1 -> w1 [dir=forward label="Requests topic\nfor notifications\nto Contact B"]  }
  { rank=same; a2 -> w2 [dir=back label="Replies with its Whisper topic"]  }
  { rank=same;
    w3 [shape=none style=invis width=0 height=0 label=""]
    a3 -> w3;
    w3 -> b3 [dir=forward label="Sends message\nwith agreed topic"]  }
  { rank=same; a4 -> w4 [dir=forward label="Sends message using topic received"]  }
 }