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Hard state persists until explicitly changed or deleted. The distinction from soft state concerns maintenance and recovery rules rather than simply whether the storage is volatile.

Soft state is information maintained by refreshes and allowed to expire when those refreshes cease. It can be reconstructed from continuing communication with the participants that still need it. Hard state instead persists until explicitly changed or removed; a lost deletion or failed participant can therefore leave stale information indefinitely. This distinction concerns the maintenance rules, rather than simply whether the information is in volatile memory.

For example, a router can keep IP multicast membership information for a local network. Periodic membership queries and reports refresh that information; if no member reports within the prescribed interval, the membership expires. A host that crashes without unsubscribing is eventually forgotten. A lost report need not remove membership immediately, because repeated reports and a suitable expiry interval tolerate occasional loss. If the router loses its membership information, subsequent queries and reports rebuild it.^[ref]

This is attractive in Internet components because packet loss, host crashes and router restarts need not be repaired by a perfectly delivered sequence of explicit deletions. Refreshes repair some omissions, and expiry removes abandoned entries and releases resources. Components can recover local state without retaining an indefinitely accurate history of everyone else's actions. Soft state does have a cost: refresh traffic, timer storage and temporary stale or missing information, with a tradeoff between rapid expiry and tolerance of delayed refreshes. Its central benefit is

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.