I have a question about ALOHA protocol acknowledgement messages.

If host A sends a message to host E, and E sends an acknowledgement back to host A, but then it collides with a message being sent from another host, say, host C, then both messages end up being garbled. Does that mean host A will have to send his original message again, even though host E has already received it?


Host A, not receiving an ACK within a reasonable amount of time must assume that the original message was lost due to a collision. So, yes, it will re-transmit the original message.

There is a PDF that explains how this works in Section 3:

If a sending node does not see on the downlink within a reasonable time an ACK for a frame it sent up, it assumes that a collision occurred and re-transmits the frame after some random delay. Thus ALOHA depends on the ability of a node to detect or learn that a collision has occurred.

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