Certificate             ACME/STAR
   User                    Server
   |     Retrieve cert     |                     [...]
   |---------------------->|                      |
   |                       +------.              /
   |                       |      |             /
   |                       | Automatic renewal :
   |                       |      |             \
   |                       |<-----'              \
   |     Retrieve cert     |                      |
   |---------------------->|            short validity period
   |                       |                      |
   |                       +------.              /
   |                       |      |             /
   |                       | Automatic renewal :
   |                       |      |             \
   |                       |<-----'              \
   |     Retrieve cert     |                      |
   |---------------------->|            short validity period
   |                       |                      |
   |                       +------.              /
   |                       |      |             /
   |                       | Automatic renewal :
   |                       |      |             \
   |                       |<-----'              \
   |                       |                      |
   |         [...]         |                    [...]