 +--------+      +--------+     +---------+
 | alice  |      |  bob   |     |  carl   |
 |        |      |        |     |         |
 +--------+      +--------+     +---------+
      |               |              |     
      |-----hello---->|              |     
      |               |              |     
      |<----jello-----|              |     
      |               |              |     
      |--------------bye------------>|     
      |               |              |     
