Determining these for a new_from_fd() socket has the following problems:
protocol() depends on SO_PROTOCOL, and socktype() on SO_TYPE, not
implemented on all systems.
sockdomain() depends on sockname(), which is documented as
unimplemented for AF_UNIX sockets on HP-UX.
I'm not sure that detail is useful in the documentation.
=back
+=head1 LIMITATIONS
+
+On some systems, for an IO::Socket object created with new_from_fd(),
+or created with accept() from such an object, the protocol(),
+sockdomain() and socktype() methods may return undef.
+
=head1 SEE ALSO
L<Socket>, L<IO::Handle>, L<IO::Socket::INET>, L<IO::Socket::UNIX>