In any sort of ARQ, when is a packet considered delivered? Is it when ACK is sent or when ACK is received?

I would think it's when it's sent, as it has been received on the receiver end. However, it's not completely confirmed yet so the sender doesn't know it's delivered.


1 Answer 1


I assume you mean TCP segments. Packets don't get ACKs; IP is connectionless.

Whether it is considered as having been received is a matter of perspective. Certainly, the receiver is happy to consider the TCP segment as received as soon as it gets it. On the other hand, the sender is not satisfied that any of the segments sent in a window have been received until it receives an ACK. If its timer expires before an ACK is received, it is obligated to resend the segments in the window without an ACK.

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