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