CKN: $30560$

$3.($a$)$ A TCP sender$,$ called Ois$$n$,$ and a TCP receiver, called Niamh, communicate over a

connection, in which segment loss is possible. Oisin's goal is to send ten segments to

Niamh. Oisin sends an initial window of five segments at times t $=1,2,3,4$ and $5$

respectively.

The initial value of the sequence number used by Oisin is $69.$ Every segment sent to the

Niamh contains $975$ bytes.

The delay between Oisin and Niamh is $7$ time units. For example, the first segment

arrives at Niamh's TCP at time t $=8$ units. The ACK corresponding to this segment

arrives at time t $=15.$

Three of the five segments are lost between Oisin and Niamh. None of the ACKs are

lost. No timeouts occur, and any out of order segments received are thrown out.

Oisin

t$=1$

t$=2$

t$=3$

t$=4$

t$=5$

Niamh

Oisin segment

Oisin segment

Oisig segment

Oisin segment

Oisin segment

Ack sek

Ack seg

t$=9$

t $=10$

t$=11$

t $=12$

t$=1$

t $=16$

t$=17$

t$=18$

t $=19$

Figure $6$: TCP Sender and Receiver.

$($i$)$ What are the sequence numbers of the segments sent at times t $=1,2,3,4$ and

t $=5?$

$($ii$)$ What are the values of the ACK numbers sent at times t $=8,9,10,11$ and t $=12?$

$($If the corresponding segment is lost, respond by writing $"$x$")$

$($iii$)$ What are the sequence numbers of the segments sent at times t $=15,16,17,18$

and t $=19?($If and ACK never arrives, give $"$x$"$ as your answer$)$

$[10$ Marks$]$