TCP negative ACK

Question

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

Comments

74........

Answer 1

In TCP data transmission , the following point has to be kept in mind :

Transmission of segments may be out of order but acknowledgement will be in order always.

So here the byte numbers 1st segment   =  (50 - 73)

byte numbers of the 2nd segment           = (74 - 96)

byte numbers of the 3rd segment           = (97 - 119)

byte numbers of the 4th segment           = 120 onwards

In the question it is mentioned that only 1st and 4th packets are transmitted successfully..This means , as we have acknowledgement number mechanism in order only , so the first byte which is not transmitted successfully will be reported by receiver to sender in NAK .. 

Here such byte belongs to 2nd segment .Hence first byte number of 2nd segment = 74

Hence this is the required negative ack sent by receiver to sender.