host1      host2      host3     | |
            +-------+  +-------+  +-------+   | |
ssh 1 ---------\    |  |       |  | proxy |   |n|
ssh 2 --------------------\    |  |   arp |   |e|
ssh 3 --------------------------------\  \|   |t|
ppp ===== compress ======================^===== |
ssh 3 --------------------------------/   |   |w|
ssh 2 --------------------/    |  |       |   |o|
ssh 1 ---------/    |  |       |  |       |   |r|
            +-------+  +-------+  +-------+   |k|