I have a question regarding fast retransmit. For example if packet#9 is lost. We send 3 ACK/NACK#9 after each other. But my question is, are we sending 3 acks or nacks in a row with fast retransmit? Because teachers and "experts" always tell me diffrently.

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Depends on your protocol. I guess you talk about TCP. In this case it is ACK, because there are only ACKs in TCP.

Fast retransmit, as well as as ACK/NACK as details of your specific network protocol. Therefore you could in principle combine them as you wish. Nevertheless some combinations make more sense than others and some parts are required if you use others (e.g. having more than one packet in flight between sender and receiver will force you to use something like sequence numbers).

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