nndb.el requires tcp

[Eli Zaretskii]

lisp/gnus/nndb.el has this fragment near line 60:

    (eval-and-compile
      (unless (fboundp 'open-network-stream)
        (require 'tcp)))

What is the reason for the conditional requrement, and what is the tcp
package it wants?

The reason I ask is that open-network-stream is not fboundp in the
MS-DOS port, so nndb.el cannot be byte-compiled, since the tcp package
is not there as well.