Go Back N Protocol

'Go-Back-N' Protocol:

The main theme of this protocol, sender sends multiple packets (Such As 1, 2, 3, 4, 5).
1
2
3
4
5
6
7
8
9
10

Receiver will give ACK for every packet sequentially. ACK1, ACK2, ACK3, ACK4, ACK5. Then sender will slide its window another 5 packets and send packets 6, 7, 8, 9, 10
5
6
7
8
9
10
11
12
13
14

In these ways GBN slide window, so GBN itself referred as a Sliding-window protocol.
Now we will see how sender and receiver send and receive data

Problem Studies 1:
If receiver gets 6, 7, 8, 10 packets and misses packet 9 then receiver will send ACK6, ACK7, ACK8
Then sender will slide its window another 5 packets and send packets 11, 12, 13 because sender knows he sent 9 and 10
8
9
10
11
12
13
14
15
16
17

But receiver end will discard packet 11, 12, 13 and will send no ACK. Sender will wait for a time and then send again packets 9, 10, 11, 12, 14. Receiver will send ACK sequentially for all packets.
Sender will slide its window from 14- 19 and will send 5 packets.

Problem Studies 2:
If receiver gets 6, 7, 8, 9, 10 packets then receiver will send ACK6, ACK7, ACK8, ACK9, ACK10
But ACK9 lost. Then sender will slide its window another 5 packets and send packets 9, 10, 11, 12, 13
8
9
10
11
12
13
14
15
16
17


There are some problem in GBN that if 1-100 packet send and packet 3 missing in this protocol sender have to retransmit again 3-100 packets. That is a wasting time. Cause receiver has no buffer to store. If there is buffer receiver can store 4-100 data and sender then has to retransmit only packet 3.
So there is another protocol which is doing this. This protocol is Selective Repeat Protocol.


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