Computer Networks Multiple Choice Questions on “Congestion Control”.
1. Two broad categories of congestion control are
a) Open-loop and Closed-loop
b) Open-control and Closed-control
c) Active control and Passive control
d) Active loop and Passive loop
Answer: a
Clarification: Open loop congestion control techniques are used to prevent congestion before it even happens by enforcing certain policies. Closed loop congestion control techniques are used to treat congestion after it has happened.
2. In open-loop control, policies are applied to __________
a) Remove after congestion occurs
b) Remove after sometime
c) Prevent before congestion occurs
d) Prevent before sending packets
Answer: c
Clarification: Open loop congestion control techniques are used to prevent congestion before it even happens by enforcing certain policies. Retransmission policy, window policy and acknowledgement policy are some policies that might be enforced.
3. Retransmission of packets must not be done when _______
a) Packet is lost
b) Packet is corrupted
c) Packet is needed
d) Packet is error-free
Answer: d
Clarification: Retransmission refers to the sender having to resend the packet to the receiver. It needs to be done only when some anomaly occurs with the packet like when the packet is lost or corrupted.
4. In Go-Back-N window, when the timer of the packet times out, several packets have to be resent even some may have arrived safe. Whereas in Selective Repeat window, the sender resends ___________
a) Packet which are not lost
b) Only those packets which are lost or corrupted
c) Packet from starting
d) All the packets
Answer: b
Clarification: In Selective Repeat, the sender side uses a searching algorithm to find the packets which need to be retransmitted based on the negative acknowledgements received and then resends only those packets thus saving bandwidth.
5. Discarding policy is mainly done by _______
a) Sender
b) Receiver
c) Router
d) Switch
Answer: c
Clarification: The discarding policy adopted by the routers mainly states that the routers discard sensitive or corrupted packets that it receives, thus controlling the integrity of the packet flow. The discarding policy is adopted as an open loop congestion control technique.
6. Closed-Loop control mechanisms try to _________
a) Remove after congestion occurs
b) Remove after sometime
c) Prevent before congestion occurs
d) Prevent before sending packets
Answer: a
Clarification: In closed loop congestion control, methods are implemented to remove congestion after it occurs. Some of the methods used are backpressure and choke packet.
7. The technique in which a congested node stops receiving data from the immediate upstream node or nodes is called as _______
a) Admission policy
b) Backpressure
c) Forward signaling
d) Backward signaling
Answer: b
Clarification: In this closed loop congestion control technique, the congested node propagates in the opposite direction of the data flow to inform the predecessor node to reduce the flow of packets. This is why this technique is called a node-to-node congestion control technique.
8. Backpressure technique can be applied only to _______
a) Congestion networks
b) Closed circuit networks
c) Open circuit networks
d) Virtual circuit networks
Answer: d
Clarification: In Virtual circuit networks, each node knows the upstream node from which a flow data is coming. So, it makes possible for the congested node to track the source of the congestion and then inform that node to reduce the flow to remove congestion.
9. The packet sent by a node to the source to inform it of congestion is called _______
a) Explicit
b) Discard
c) Choke
d) Backpressure
Answer: c
Clarification: Choke packet is sent by a node to the source to inform it of congestion. Two choke packet techniques can be used for the operation called hop-by-hop choke packet and source choke packet.
10. In the slow-start algorithm, the size of the congestion window increases __________ until it reaches a threshold.
a) exponentially
b) additively
c) multiplicatively
d) suddenly
Answer: a
Clarification: In slow-start algorithm, the size of the congestion window increases exponentially until it reaches a threshold. When it reaches the threshold, it stops increasing and continues sending packets through the threshold window thus preventing congestion.
11. In the congestion avoidance algorithm, the size of the congestion window increases ____________ until congestion is detected.
a) exponentially
b) additively
c) multiplicatively
d) suddenly
Answer: b
Clarification: In the congestion avoidance algorithm, the size of the congestion window increases additively until congestion is detected. Once congestion is detected, the size of congestion window is decreased once and then the packets are transmitted to achieve congestion avoidance.