Closed-loop congestion control (Part 2)

In this post, we will learn about the remaining techniques of closed-loop congestion control.
→Choke packet
→Implicit signalling
→Explicit signalling
     →Forward signalling
     →Backward signalling

Here is Part 1 of closed-loop congestion control if you haven't seen it yet:
https://sairohithkaranam.blogspot.com/2020/04/closed-loop-congestion-control-part-1.html

Choke packet:

      crux points :

1. The only difference between backpressure and choke packet techniques are-
   In backpressure, the source node eventually gets to know about congestion from the congested node.
   In the choke packet, the source node gets to know about congestion directly from the congested node via the choke packet.
2. Choke packet is a packet which informs about congestion in a network to source node from congested node directly.

Let's have a look at the diagram now.

Here you can see, the congested node(node-3) is directly informing the source node about congestion via choke packet.

Implicit signalling:

       crux points:

1. No communication between the congested node and source.
2. When the acknowledgement is not sent from the receiver side even after sending multiple packets, source guesses that congestion has occurred and slows down the data rate reducing congestion.

These are the crux points to memorize. Now, let's understand the actual mechanism.

• In the case of backpressure and choke packet techniques, the source node eventually or directly gets to know about congestion via other nodes or choke packet.
• But, In the case of implicit signalling technique, there will be no direct or indirect communication between the source node and the congested node.
  Then, how does the source gets to know about congestion?
• In potential TCP connection, when a packet is received by the destination, the destination sends an acknowledgement to the source showing the sign that packet has received.
• When congestion has occurred in a network, data can't be transmitted smoothly, right?
• When data packets can't be transmitted to the destination, there will be no acknowledgement from destination to source even after sending multiple packets.
• Finally, source guesses that something is wrong within the network and slows down the data rate reducing congestion.

Explicit signalling:
        
       crux points:

1. The only difference between the choke packet and explicit signalling is
   In the case of the choke packet technique, a choke packet is sent from congested node to source to inform about congestion.
   Whereas, In the case of explicit signalling, a packet with a signal is sent to either source or destination to inform about congestion.
2. Explicit signalling can occur either in forwarding or backward direction.
   Forward signalling:
   There is a bit set along with the packet as a signal which warns the destination about congestion. In forward signalling, the signal is sent in the same direction of congestion.
   Backward signalling:
   There is a bit set along with the packet as a signal which warns the source about congestion. In backward signalling, a signal is sent in the opposite direction of congestion.

These crux points are your strength!

Cheers! We've learnt all the techniques of closed-loop congestion control.