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Consider the GBN protocol with sender and receiver window size as 5.Suppose client sends data 0,1,2,3,4 and only data packet 2 is lost and all ACKs are lost. What will be the content of receiver and sender window. Before Senders timeout value expires ?


A. Sender: 01234  ,  Receiver : 01234

B. Sender: 23456  ,  Receiver : 01234

C. Sender: 23456  ,  Receiver : 23456

D. Sender: 01234  ,  Receiver : 23456
asked in Computer Networks by Loyal (6.6k points)
edited by | 439 views
shouldn't the sequence number run from = 0 to 9
Available sequence no = sender + receiver = 5 + 5 = 10 ??

Na462  those are not sequence number..In this question no role of sequence number

this are data packets only which is numbered  0 1 2  3 4

because in this question total sequence number  =  5 +1 = 6

0 to 5 sequences we use

but answer given  Sender: 01234  ,  Receiver : 23456

it means 6 is the data packet number

WHy total sequence number is 5 + 1 ?? receiver WS is not 1 here na.

And data packet number ???? Packet is represented by a sequence number na
ouuuh sorry sorry  I read it wrong
So... what should it be ?
the question is wrong probably


shouldn't the sequence number run from = 0 to 9
Available sequence no = sender + receiver = 5 + 5 = 10 ??

there is no restriction on usage of sequence no.

generally 5+5=10 ==> 4 bits for representating sequence number is sufficient, it doesn't mean we should use 4 bits only

in the above solution, they take no.of bits for sequence no is 5. ( in directly we are wasting the sequence numbers. )

In go back n the situation why the receiver window size is 1 is that the receiver will strictly accept those packets which should be next in sequence thats why if say 2,3,4 are sent and 2 is lost it wont accept other packets.

But it doesnt mean that the receiver window size cant be greater then 1 , if its greater then 1 (See below its from tanenbaum)

if you continue the reading the last para to the next page, you will understand it is Selective Repeat.


only data packet 2 is lost

This statement is telling data is lost or ACK are lost ?

If data is lost. Then there is no point of ack right?
After 2nd packet loss, sender will go back 5 packets

So, sender window can contain 2,3,4,5,6

and receiver hasnot yet able to send any ACK

So, it still contains 0,1,2,3,4

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