I worked for a couple of networking companies back in the day, and used to see these things all the time - it's a great way to display any kind of message protocols, and there's a nice tool to do it (I used to do them by hand). The mscgen program accepts a simple text description and generates diagrams in PNG, SVG, EPS (or image maps). The output, of course, can be run through my inverse video script to render it in the HaD.io theme. I used the PNG output option then converted the colors for the diagram above.

Here's the description file for this example diagram:

msc {
    arcgradient = "8";

    a [label = "Meter"], b [label = "Adapter"], c [label = "Application"];

    a=>b [label = "value1"];
    a=>b [label = "value2"];
    a=>b [label = "value3"];
    c=>b [label = "read"];
    b->b [label = "flush & wait"];
    a=>b [label = "value4"];
    b>>c [label = "value4"];
}

There are all sorts of neat formatting options and colors and stuff you can use, too.

I was able to just 'sudo apt-get install mscgen' to install the program.