host1      host2      host3
            +-------+  +-------+  +-------+
ssh 1 ----------\   |  |       |  |       |
ssh 2 ---------------------\   |  |       |
ssh 3 ================================>   |
ssh 2 ---------------------/   |  |       |
ssh 1 ----------/   |  |       |  |       |
            +-------+  +-------+  +-------+