congestion control

Question

Suppose , a sender sends a packet and it does not receive an acknowledgement . The timer at the sender side runs out . In which cases the sender resends the packets and in which cases it does not ?

I know this : If the sender does not receive acknowledgement frame the sender will resend the frames and it will add congestion .

But in a book I read that : In acknowledgement policy , if the receiver does not acknowledge every packet , then it may reduce congestion . How is it possible ? How will the sender come to know that the packet is successfully delivered ?

Answer 1

You didn't read the full sentence. Here it is from Data Communications and Networking 5th edition by Behrouz A. Forouzan Page 526:

Acknowledgment Policy The acknowledgment policy imposed by the receiver may also affect congestion. If the receiver does not acknowledge every packet it receives, it may slow down the sender and help prevent congestion. Several approaches are used in this case. A receiver may send an acknowledgment only if it has a packet to be sent or a special timer expires. A receiver may decide to acknowledge only N packets at a time.

I highlighted the important part. The receiver can be "smart" about sending acknowledgements and send them when it also has data to send, instead of sending the acknowledgement by itself. In other words, instead of the receiver saying "I received your last packet," it says "Here is my data to you, and I also received your last packet" The receiver sends one message instead of two. If the receiver has no data to send, then it sends the acknowledgement anyway when the timer expires.