code: def _send_stream_start(self, stream_id = None, stream_to = None):
          
          if self._output_state in ("open", "closed"):
              raise StreamError("Stream start already sent")
          if not self.language:
              self.language = self.settings["language"]
          if stream_to:
              stream_to = unicode(stream_to)
          elif self.peer and self.initiator:
              stream_to = unicode(self.peer)
          stream_from = None
          if self.me and (self.tls_established or not self.initiator):
              stream_from = unicode(self.me)
          if stream_id:
              self.stream_id = stream_id
          else:
              self.stream_id = None
          self.transport.send_stream_head(self.stanza_namespace,
                                          stream_from, stream_to,
                                      self.stream_id, language = self.language)
          self._output_state = "open"
docs: Send stream start tag.