Flow Control in Networks

Delays in Networks

1. Transmission delay
2. Propagation delay
3. Queueing delay
4. Processing delay

Transmission delay

Equal to $Data to be transferred \over Bandwidth$

Propagation delay

Equal to $Distance \over Velocity$

Total time taken to send a packet from source to destination is $Transmission delay + Propagation delay$

Queueing delay

At destination once the packets are arrived they will be placed in buffer and processed one by one, So the amount of time packet sits in buffer is queueing delay

Processing delay

Taking packet and one by one from buffer in destination side and processing it is called processing delay

In most of the cases Queueing delay and Processing delay is considered as negligible

Stop and Wait Method

Sender will send a packet and after receiver comsumes that packet and sends acknowledgement, Then sender will send next packet

Possible scenarios and their solutions

1. If sender sends packet that got lost in the middle then sender will wait for ack and receiver will wait for packet (Deadlock) - Solution is timeout at sender side, If within the timeout sender doesnot get ack it will resend the packet again (ARQ - Automatic Repeat Request)
2. If ack is lost then after timeout sender will resend packet again, but at receiver we should know that its the same packet that got resent by sender - Solution is sequence number of packets
3. If ack is delayed, sender is going to send packet and again and receiver is going to send the ack again, If sender sends next packet then ack can come for previous packet - Solution is sequence number in ack's

An improvement for stop and wait method is to use sliding window protocol

Sliding window protocol

Instead of sender waiting till the ack comes, it will send certain number of packets(based on window size) by the time ack of packet 1 will be back and sender is going to send next packet

Sliding window protocol is implemented by two protocols

1. GBN (Go Back N)
2. SR (Selective Repeat)

Go Back N

1. Sender window size is N
2. Receiver window size is 1
3. Cumulative acknowledgement using ack timer

Minimum number of distinct sequence numbers required is N+1 In general it will be sender window size + receiver window size

Since receiver window size is 1, If one packet timeout happens then entire window will be replayed again

If the packet is corrupted receiver will discard the packet silently

Selective Repeat

1. Receiver window size is equal to Sender window size

Since receiver window size is more, it can keep track of all packets even if one packet is lost. So from sender side we don't need to send entire window, we can just replay the selected timed out packet

If the packet is corrupted receiver is going to send negative ack because of which sender will retransmit packet before timeout

Credits

1. Computer Networking Lectures