Knowledge Checks

Congestion control versus flow control.

Consider the five images below. Indicate which of these images suggest the need for flow control (the others would suggest the need for congestion control).

A crowd of people

A glass overflowing

A penguin crowd

Car traffic

A talking head

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1/5

Two congested senders.

Consider the figure below, which shows the application-to-application throughput achieved when two senders are competing at a shared bottleneck link. Suppose that when the overall arrival rate, lambda_in' (for each sender) is close to R/2, the throughput to the application layer (at each receiver), lambda_out, is equal to 0.8 * lambda_in'.

What fraction of the packets transmitted at the sender are retransmissions?

.50

.80

.20

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2/5

Network-assisted or end-end congestion control?

Which of the following actions are used in network-assisted congestion control (say versus end-end congestion control) to signal congestion. Check all that apply.

A router marks a field in the datagram header at a congested router.

The transport-layer receiver informs sender of the size of its (transport-payer receiver) receive window.

A router drops a packet at a congested router.

A datagram experiences delay at a congested network router, which is then measured by the sender and used to decrease the sending rate.

A router sends an ICMP message to a host telling it to slow down its sending rate.

The sender decreases its sending rate in response to a measured increase in the RTT.

A sender decreases its sending rate in response to packet loss detected via its transport-layer ACKing.

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3/5

Network-assisted or end-end congestion control (2)?

Which of the following actions are associated with end-end congestion control (say versus network-assisted congestion control). Check all that apply.

A router marks a field in the datagram header at a congested router.

A router drops a packet at a congested router, which causes the transport-layer sender to infer that there is congestion due to the missing ACK for the lost packet.

The transport-layer receiver informs sender of the size of its (transport-payer receiver) receive window.

A router sends an ICMP message to a host telling it to slow down its sending rate.

The transport-layer sender decreases its sending rate in response to a measured increase in the RTT.

A sender decreases its sending rate in response to packet loss detected via its transport-layer ACKing.

A datagram experiences delay at a congested network router, which is then measured by the sender and used to decrease the sending rate.

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4/5

Different approaches towards congestion control.

Use the pulldown menu to match a congestion control approach to how the sender detects congestion.

Question List:

The sender infers segment loss from the absence of an ACK from the receiver.

Bits are set at a congested router in a sender-to-receiver datagram, and bits are in the returned to the sender in a receiver-to sender ACK, to indicate congestion to the sender.

The sender measures RTTs and uses the current RTT measurement to infer the level of congestion.

Answer List:

network-assisted
end-end
delay-based

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5/5

Knowledge checks

Congestion control versus flow control.

Two congested senders.

Network-assisted or end-end congestion control?

Network-assisted or end-end congestion control (2)?

Different approaches towards congestion control.

Question List:

Answer List: