The Wikipedia article on TCP indicates that the IP packets transporting TCP segments can sometimes go lost, and that TCP "requests retransmission of lost data".
What exactly are the rules for requesting retransmission of lost data? At what time frequency are the retransmission requests performed? Is there an upper bound on the number? Is there functionality for the client to indicate to the server to forget about the whole TCP segment for which part went missing when the IP packet went missing?
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There's no fixed time for retransmission. Simple implementations estimate the RTT (round-trip-time) and if no ACK to send data has been received in 2x that time then they re-send.
They then double the wait-time and re-send once more if again there is no reply. Rinse. Repeat.
More sophisticated systems make better estimates of how long it should take for the ACK as well as guesses about exactly which data has been lost.
The bottom-line is that there is no hard-and-fast rule about exactly when to retransmit. It's up to the implementation. All retransmissions are triggered solely by the sender based on lack of response from the receiver.
TCP never drops data so no, there is no way to indicate a server should forget about some segment.
Source: Stackoverflow.com