registrant_ip and modify nameserver

[Mark]

I'm looking at the api documentation, and it says that the way to 
change a nameserver is something like this:

'protocol' => 'XCP',
'action'        => 'modify',
'object'        => 'nameserver',
'registrant_ip' => '10.0.10.19',
'attributes'    => array(
  'name' => 'ns1.domaindirect.com',
  'new_name' => 'cns1.idirect.com'
)

but when I send this a get 'authorization error', also I guess this 
will change the nameserver info for all domains that have that 
registrant_ip, but what if I just want to change it for 1 domain?

one more thing is I'm not sure what cookie is supposed to be used 
for, is this an optional thing?

thanks,
- Mark