Reliable data transfers are one of the primary concerns in computer networking. This service department lies in the hands of TCP. Their major flow control protocols – Stop and Wait, Go Back N, and Selective Repeat.
- Stop and Wait ARQ –
The sender sends the packet and waits for the acknowledgement of the packet. Once the acknowledgement reaches the sender, it transmits the next packet in a row. If the acknowledgement does not reach the sender before the specified time, known as the timeout, the sender sends the same packet again.
- Go Back N ARQ –
The sender sends N packets which are equal to the window size. Once the entire window is sent, the sender then waits for a cumulative acknowledgement to send more packets. On the receiver end, it receives only in-order packets and discards out-of-order packets. As in case of packet loss, the entire window would be re-transmitted.
- Selective Repeat ARQ–
The sender sends packets of window size N and the receiver acknowledges all packets whether they were received in order or not. In this case, the receiver maintains a buffer to contain out-of-order packets and sorts them. The sender selectively re-transmits the lost packet and moves the window forward.
Differences:
| Properties |
Stop and Wait ARQ |
Go Back N ARQ |
Selective Repeat ARQ |
| Sender window size |
1 |
N |
N |
| Receiver Window size |
1 |
1 |
N |
| Minimum Sequence number |
2 |
N+1 |
2N |
| Efficiency |
1/(1+2*a) |
N/(1+2*a) |
N/(1+2*a) |
| Type of Acknowledgement |
Individual |
Cumulative |
Individual |
| Supported order at the Receiving end |
In-order delivery only |
In-order delivery only |
Out-of-order delivery as well |
| Number of retransmissions in case of packet drop |
1 |
N |
1 |
| Transmission Type |
Half duplex |
Full duplex |
Full duplex |
| Implementation difficulty |
Low |
Moderate |
Complex |
Where,
- a = Ratio of Propagation delay and Transmission delay,
- At N=1, Go Back N is effectively reduced to Stop and Wait,
- As Go Back N acknowledges the packed cumulatively, it rejects out-of-order packets,
- As Selective Repeat supports receiving out-of-order packets (it sorts the window after receiving the packets), it uses Independent Acknowledgement to acknowledge the packets.
Reference –
Book – Computer Networks by Tanenbaum
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Last Updated :
27 Jun, 2023
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