'Lookup user information about the specified address pair.
Return value should be a two-tuple of system name and username.
Acceptable values for the system name may be found online at::
U{http://www.iana.org/assignments/operating-system-names}
This method may also raise any IdentError subclass (or IdentError
itself) to indicate user information will not be provided for the
given query.
A Deferred may also be returned.
@param serverAddress: A two-tuple representing the server endpoint
of the address being queried.  The first element is a string holding
a dotted-quad IP address.  The second element is an integer
representing the port.
@param clientAddress: Like I{serverAddress}, but represents the
client endpoint of the address being queried.'

def lookup(self, serverAddress, clientAddress):

    raise IdentError()
