Question

Congestion control is done by

• A. Network layer
• B. Physical layer
• C. Presentation layer
• D. Application layer

Answer 1

Application layer, Presentation layer and Session layer are out of league as they deal with the user level problems.

Physical layer's main functionality is to encode and decode data bits for transportation over physical medium.

So the problem boils down to Network, Transport and Data Link Layer.

Important function of Network Layer is to provide switching and routing of datagrams because of this no flow control mechanism is implemented in this layer.

Now, both Transport Layer and Data Link Layer contains flow control mechanisms but then again Data Link Layer deals with hop to hop connection. This makes DLL incapable of controlling congestion in whole network.

The only option remaining is Transport Layer. Moreover if you look at the TCP header, it contains a field named Window Size which is used for controlling congestion.

Comments

TRansport is right but the window size has specific use and its used for flow control (and not congestion control).

Answer 2

It is done by Transport layer but not in option.