tramp over an HP-UX hop

[Andrea Crotti]

I'm trying to use tramp to connect through another machine,
so I set up the proxies alist as

(add-to-list 'tramp-default-proxies-alist
      '("final" nil "/ssh:middle:"))

where I have a .ssh/config entry for middle in my local machine and 
another entry in .ssh/config
for final in the middle machine.

I can't get it working however, and if I look in the tramp buffer I see 
that it connected to the middle machine
but it can't reach the final machine.

The doubt is that the machine in the middle is just too dumb to support 
it, it's an:

HP-UX sn41168b B.11.23 U 9000/800

What are the requirements in general to get the multi-hop working correctly?