Consider the figure below in which TCP a sender and receiver communicate over a

Question

Consider the figure below in which TCP a sender and receiver communicate over a connection in which the sender-to-receiver segments may be lost. The TCP sender sends initial window of five segments at t-1,2,3,4,5 respectively. Suppose the initial value of the sender-to-receiver sequence number is 118 and the first five segments each contain 506 bytes. The delay between the sender and the receiver is 7 time units, and so the first segment arrives at the receiver at t-8. As shown in the figure, two of the five segmentis) are lost between the sender and the receiver Seg 1 g 3 t 5 t-9 t-10 t-11 t-12 Give the sequence numbers associated with each of the five segments sent by the sender List the sequence of acknowledgments transmitted by the TCP receiver in response to the receipt of the segments actually received. In particular, give the value in the acknowledgment field of each receiver-to- sender acknowledgment, and give a brief explanation as to why that particular acknowledgment number value is being used. You can answer the previous questions by filling the table below: Sender-to- Receiver sent Time segment Sender-to-receiver segmentTime segment received, sequence number field valueand ACK segment sent Receiver-to-sender ACK field value Seg #1 Seg #2 Seg #3 Seg #4 Seg #4

Explanation / Answer

In segment the numbering of each byte of data is being done and sequence number of any segment is nothing but the number allocated to the first byte of data in that segment.

so,sequence number of first segment = 118

byte number of last byte in first segment = x ( let)

then x - 118 + 1 = 506

x = 506 + 117 = 623

sequence number of second segment = 623 + 1 = 624

last byte number in second segment = 506 + 623 = 1129

sequence number of third segment = 1129 + 1 = 1130

last byte number in third segment = 506 + 1129 = 1635

sequence number of fourth segment = 1635 + 1 = 1636

last byte number of fourth segment = 506 + 1635 = 2141

sequence number of fifth segment = 2141 + 1 = 2142

now,TCP always gives acknowledge with the number which is supposed to sequence number of next segment in line or in order.TCP accepts segments which have arrived out of order but it always send acknowledge with number which is supposed to be sequence number of next segment in line or in order.
here when it receives segment 2 successfully then acknowledgement is sent to sender with number 1130(sequence number of third segment).but third segment get lost.

when fifth segment arrives out of order , receiver accepts that but send Acknowledgement with number = sequence number of third segment = 1130

upon receiving that , it sends the segment third again.when third is received successfully then it sends the acknowledgement with number = sequence number of fourth segment = 1636.generally sender send the segments which were lost during transmission after receiving 3 duplicate acknowledgement from the receiver.

upon receiving this acknowledgement sender send again this fourth segment to receiver.

after successfully receiving that receiver send a Acknowledgement with number = sequence number of sixth segment = 2143

now for lost segments:-

segment number 5 is received at t = 12 , so it send ACK with number = 1130 at t = 12

sender receives it at t = 12 + 7 = 19 and after that it send segment #3 right away at t = 19

so,segment is received at t = 19+7 = 26 and after receiving that receiver send an ACK with number = sequence number of 4th segment = 1636 at t = 26

sender receives this ACK at t = 26+7 = 33,so,sender sends 4th segment at t= 33 + 7 = 40 and then sends ACK with number = sequence number of 6th segment = 2143

Sender-to-

Receiver

Time

Segment

Sent

sender-to-receiver segment

sequence number field value

Time segment received,

and ACK segment sent

Seg#3

Sender-to-

Receiver

Time

Segment

Sent

sender-to-receiver segment

sequence number field value

Time segment received,

and ACK segment sent

Receiver-to-sender ACK Field value seg #1 1 118 8 624 Seg#2 2 624 9 1130

Seg#3

3 1130 26 1636 Seg#4 4 1636 33 2143 Seg#5 5 2142 12 1130

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