L22. What is congestion collapse, how can it be avoided? Is TCP or UDP a fair protocol?

Question 1

What is congestion collapse?

Answer 1

Congestion collapse is a condition where increasing load on a network results in a drastic reduction of useful throughput, causing the network to become essentially unusable.

Question 2

When does congestion collapse occur?

Answer 2

When the network is overwhelmed by a data packets, leading to excessive packet loss, subsequent retransmissions, and ultimately, a situation where the network spends more time dealing with these issues than actually delivering useful data.

Question 3

What causes congestion collapse?

Answer 3

Routers and switches become overwhelmed from too many packets being sent into the network too quickly, causing packet queues to overflow and packets to be dropped. This packet loss triggers retransmissions, which further increase the load on the network, creating a vicious cycle.

Question 4

What kind of networks are vulnerable to congestion collapse?

Answer 4

Networks that lack effective congestion control mechanisms.

Question 5

What are some exacerbating factors for congestion collapse?

Answer 5

1. Unregulated Traffic: Without proper control, sources can send data at rates exceeding network capacity.
2. Ineffective Congestion Control: Protocols that do not adequately respond to congestion signals contribute to the problem.
3. High Network Latency: Long round-trip times can delay the detection of congestion, leading to excessive data injection before corrective measures are taken.

Question 6

What prevents congestion collapse?

Answer 6

Congestion control mechanisms

Question 7

Name some congestion control mechanisms

Answer 7

1. Congestion control algorithms in the transport protocol, eg. TCP Reno and TCP Tahoe
2. AQM (Active Queue Managment) – techniques used at the network layer to preemptively drop packets before queues become full.
3. QoS (Quality of Service) mechanisms that prioritize critical traffic and ensure that important data gets through even during congestion.

Question 8

How do congestion control algorithms in TCP prevent congestion collapse?

Answer 8

Congestion control algorithms adjust the transmission rate based on network feedback, such as packet loss and delay. Techniques like slow start, congestion avoidance, fast retransmit, and fast recovery help TCP dynamically adapt to changing network conditions.

Question 9

How does AQM prevent congestion collapse?

Answer 9

It helps manage queue lengths by preemptively dropping packets before queues become full. This provides early congestion signals to the sender, prompting it to reduce its transmission rate.

Question 10

Is TCP or UDP a fairer protocol? Why?

Answer 10

TCP is generally considered more fair, because of its built-in congestion control mechanisms.

TCP adjusts its transmission rate based on network feedback, reducing its sending rate when packet loss is detected, which helps to alleviate congestion. This behavior ensures that TCP connections share network resources more equitably.

TCP also uses flow control to prevent overwhelming the receiver, further contributing to its fairness in resource allocation.

In contrast, UDP lacks these features. It sends data without considering the network state or the receiver’s capacity, which can lead to congestion and unfair resource usage. UDP does not adapt to network congestion, potentially exacerbating the problem.