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A. routing B. flow-control C. congestion control D. All of the above

My guess is flow-control only because routing and congestion is part of the network layer. The network layer is the third later and it provides data routing paths for network communications. I'm still not sure if I am right though.

Manoj Selvakumar
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the answer to "is definitelly NOT is A."

routing

routing is done by network layer

before i go in the next ones here is disabmiguity warning. B. and C. are performed by TCP which is transport layer. if the application is using a custom transport protocol on top of UDP, i.e., RTP for transmitting multimedia, then, depending on the definition, it won't be transport layer. But B is definitelly not done by network layer and C may be assisted.

flow control

ensures that the sender is not overloading the receiver with too much data to send. in TCP receiver sends the sender its window, i.e., the number of packets the receiver is ready to receive

congestion control

the goal of congestion control is for the senders not to overflow the network. in Internet, congestion control is responsibility of senders. TCP is required to implement congestion control. Non-TCP applications are required to implement TCP-friendly congestion control (i.e., one that coexists with TCP). The algorithm usually tries to estimate available bandwidth based on some heuristic.

Routers can assist in performing congestion control in form of active queue management and explicit congestion notification. However, both of them can only work, if TCP can react to what they do.

A "very long time ago" there was in-network congestion control in form of source quench messages. It didn't work. Approaches, that rely on senders seem to work better.

Effie
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