Suppose, for example, that the window has a size of 5 MSS, the sender sends 5 segments to the network, the fifth is lost. Actually the question: how does the recipient behave? It just doesn't acknowledge receipt of 4 segments and waits for the sender to shrink the window and send a smaller window? Or the recipient has some kind of timer by which he concludes that the segment is lost and sends an acknowledgment of 4 segments to the sender
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"waits for the sender to shrink the window" The receiver, not the sender sets the window, which is how many bytes the sender can send before getting permission to send more. The receiver sets the Window in the ACKs sent back to the sender.– Ron Maupin ♦Dec 9, 2020 at 16:17
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1 Answer
how does the recipient behave?
The recipient ACKs the first four segments = beginning of the fifth segment.
waits for the sender to shrink the window and send a smaller window?
The sender doesn't shrink the transmission window just because of a lost segment. The window shrinks when congestion is detected which is a bit more complex.
When the sender receives the ACK for the beginning of the fifth segment, the previous four are removed from the window, advancing it to segments 5-9. Depending on the exact algorithm and timing, the sender may then send those four segments right away (resending seg 5). Alternatively, it may select to wait a little while longer for the ACK for the fifth segment that might still be in transit (max 1 RTT), and failing that, send segments 5-9 when the short timer expires.
Or the recipient has some kind of timer by which he concludes that the segment is lost and sends an acknowledgment of 4 segments to the sender
Yes, the recipient likely uses cumulative acknowledgment (in contrast to ACKing each segment separately) and has a short timer for when these acknowledgments are sent (restarting the timer after each received segment while watching the window). Accordingly, there might be a short delay between reception of seg 4 and ACK of segs 1-4.
