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Let the source has sent four TCP segments to Destination with the sequence numbers $50, \ 74, \ 97,\ 120$ ($1$st, $2$nd, $3$rd and $4$th segment) if the first and fourth segments arrive at destination successfully then the negative acknowledgement that destination sends to source is _______

Whenever receiver found that some packet is missing, then a nack packet is send from receiver to sender to tell them your packet is missing.

here also from packet with SN 56, receiver will calculate the next packet SN, which has to be 74. and when receiver will detect that there is no packet with SN = 74 then it send a nack with ack =74 to sender.
what if the packet with SN 97 is lost, then NACK should be sent for 97, isn't it? I question it is not mentioned which packet is get lost, so I think it could be 74 or 97. Please correct me if I'm wrong here.
@bhuv

it have lose both of the packets.

TCP only acknowledges bytes upto first missing byte in the stream(cumulative acknowledgements). Here given the segment with sequence number $74$ and $97$ is lost so the NAK send to the source is $74$ (the next expected sequence number).

http://www2.ic.uff.br/~michael/kr1999/3-transport/3_05-segment.html

How does the destination knows that the next missing byte is 74 only. It could be 51 also as there is no mention of amount of data bytes sent ??..plz ans.

if the first and fourth segments arrive at destination successfully

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