Flow Control vs Congestion Control

[life24]

Hello,
What is difference between these?
Each of ways are in data link layer?

 

[Gryz]

Flow control is an algorithm between two devices. To make sure the sender does not send more/faster than the receiver can receive. It can be as simple as the receiver sending control-S and control-Q to pause the sender.

Congestion control is an algorithm in a network with multiple devices. The goal is to make sure not one single sender can use all the bandwidth, and starve all the other senders in the network. The limitating factor with congestion is not the receiver, but the network that is in-between senders and receivers.

Flow-control and congestion-control are very related. But they are not the same.

Let's look at TCP.
When two devices communicate over TCP, the receiver can do flow-control by changing the value of the advertised receive-window. If the receiver sets the window to 0, it basically means that the sender needs to stop sending new data. Until the receiver send another TCP-segment with a larger receive-window (larger than 0).

Another mechanism for flow-control with TCP are the acknowledgments. When a sender starts sending TCP-segments, he will receive acks from the receiver. The sender can look at the pace/speed of the incoming acks to make an estimate how fast he can keep sending.

Congestion-control with TCP works differently. Routers in the middle of the network, that experience congestion, will start dropping packets. The sender will notice that, because he gets no more, or less acks back. When a sender sees that, he must start sending TCP-packets at a slower pace. So the dropping of packets is a mechanism to achieve congestion-control in TCP/IP-networks.

Flow control can happen on any layer. Congestion control is in layer 3 and 4.

Hope this helps.

 

[fiberst]

Flow control is controlled by the receiving side. It ensures that the sender only sends what the receiver can handle. Think of a situation where someone with a fast fiber connection might be sending to someone on dialup or something similar. The sender would have the ability to send packets very quickly, but that would be useless to the receiver on dialup, so they would need a way to throttle what the sending side can send. Flow control deals with the mechanisms available to ensure that this communication goes smoothly.

Congestion control is a method of ensuring that everyone across a network has a "fair" amount of access to network resources, at any given time. In a mixed-network environment, everyone needs to be able to assume the same general level of performance. A common scenario to help understand this is an office LAN. You have a number of LAN segments in an office all doing their thing within the LAN, but then they may all need to go out over a WAN link that is slower than the constituent LAN segments. Picture having 100mb connections within the LAN that ultimately go out through a 5mb WAN link. Some kind of congestion control would need to be in place there to ensure there are no issues across the greater network.​