TCP: Why do we need flow control and congestion control

Question

I've briefly read up on both flow control and congestion control and I cannot understand why we need them both and not just congestion control.

Cannot we use just use congestion control and include the receiver as the last node? It seems to me that flow control could just be a special case of congestion control.

I've looked at this question, I think OP are having the same question as me, but question is not formulated well enough thus the answer are not complete.

Assume we are sending 100 packets but only the first five have been ACK-ed. How do the sender know if it's the network or the receiver that are slow? And should it actually matter? The network with the receiver is not capable of receiving fast enough so the sender should slow down.

What is the problem with an implementation going something like this?

if (acked() / sent() < THRESHOLD){
    transmission_rate(0.5);
}else{
    transmission_rate(SOME_RATE); // where SOME_RATE > 0
}

Is there something flow control does that congestion control does not that requires a flow control in the TCP protocol?

Answer 1

I think the author here explained flow-control reasonably well.

In essence, you are right, there is some overlap between the congestion control and the flow control. However, they both function in a different way. For example, the network might be big enough and both are needed.

Imagine, connecting a raspberry pi to a supercomputer with a gigabit network. Now, this example is silly but would serve for illustration purposes. The sender (the super-computer) and the network might be well able to support high speeds, but if the pi has a low receive buffer, there will not be much point to send fast. For this reason the receiver will advertise its send window, so the sender know how fast the other side can process data. This way it will limit the cases where it is sending data over the network successfully, but the receiver cannot process and needs to drop it. In general sending less bytes over the network is a good idea because it will help prevent possible congestion.