+-----------------+              +-----------------+
 | Domain A        |              | Domain B        |
 |       +---------+              |                 |
 |       | ITR A.1 |---           |                 |
 |       +---------+    \         +---------+       |
 |                 |      ------->| ETR B   |       |
 |                 |      ------->|         |       |
 |       +---------+    /         |         |       |
 |       | ITR A.2 |---      -----| ITR B   |       |
 |       |         |       /      +---------+       |
 |       | ETR A.2 |<-----        |                 |
 |       +---------+              |                 |
 |                 |              |                 |
 +-----------------+              +-----------------+