Past exam of the computer science course of the University of Oxford / 2015 / Computer networks / 2 / c

Dynamic network address and port translation has an important soft state aspect: activity refreshes a binding's lifetime, and inactivity eventually expires it without a reliably delivered close message. This is especially useful for UDP, which supplies no connection-closing exchange. Even for TCP, the router cannot assume that it will observe every closing packet.

However, an expiring entry is not necessarily harmlessly disposable. Suppose the binding 10.0.0.2:49152 to 198.51.100.7:62001 disappears while the remote endpoint is still using it. A reply to port 62001 no longer reveals the private destination. New outbound traffic might obtain a different public transport port, which does not reconstruct the old connection as viewed by the remote endpoint. Losing the binding can therefore break the exchange even though both endpoints remain alive. This conflicts with fate-sharing when indispensable connection information resides in a separate router.

Static address mappings and configured port forwarding are hard state; a fixed translation rule need not even require a dynamic per-exchange table. ThusDescribing all network address translation information as fully reconstructible soft state would overlook both this recovery limitation and the static configuration.