$ ssh -o 'ProxyCommand ssh remA mconnect -r -p 22 remB' remB

##</code><code>##

$ ssh -M -S ~/.ssh/mux_socket -o 'ProxyCommand ssh remA mconnect -r -p 22 remB' -N remB
$ ssh -S ~/.ssh/mux_socket remB COMMAND1
$ ssh -S ~/.ssh/mux_socket remB COMMAND2
...