Which protocol ensures that mobile hosts can participate in transactions with…

In a distributed transaction, a commit protocol coordinates all participating nodes so that they either all apply or all undo the transaction's updates together. The classic two-phase commit protocol does this through a prepare/vote phase followed by a commit/abort phase, but it assumes every participant stays reachable throughout both phases -- if a participant becomes unreachable, the protocol stalls (blocks) until that participant returns. Mobile hosts routinely go offline (weak signal, power-saving, hand-off between cells), so a commit scheme built for mobile clients needs a way to keep the transaction moving even while the mobile host itself is unreachable.

For this reason, mobile transaction-processing research introduced proxy-based commit protocols. Instead of having the mobile host itself act as a full participant in the commit exchange, a fixed, always-reachable proxy (a base-station or fixed-network representative) is assigned to stand in for the mobile host during the protocol. The proxy holds the mobile host's vote and state information and exchanges prepare and decision messages with the coordinator on its behalf. Because the proxy, not the intermittently-connected device, is the one the coordinator waits on, the transaction can proceed through the commit exchange with only minimal blocking even while the mobile host is disconnected; the mobile host simply reconciles with its proxy once it reconnects.

Contrasting the other options by what they actually are:

• Two-phase commit: the baseline protocol that coordinates a vote phase and a decision phase across all participants directly; it has no built-in mechanism for a participant that goes offline mid-protocol, so an unreachable participant stalls the round.

• Presumed commit: an optimization layered onto standard two-phase commit that lets the coordinator skip force-writing certain log records by assuming a commit outcome by default; it changes logging and recovery overhead, not how disconnected participants are represented.

• None of these: not applicable here, since a named protocol among the options, proxy-based commit, is exactly the mechanism designed for this purpose.