TCP Congestion Control: Phases, Strategies, and Key Mechanisms

What are the classic phases of TCP congestion control?

Slow start, congestion avoidance, and fast recovery.

What triggers the transition from slow start to congestion avoidance in TCP?

The transition occurs when the congestion window (cwnd) reaches a threshold.

What is the behavior of cwnd during the slow start phase?

Cwnd grows exponentially.

What is the behavior of cwnd during the congestion avoidance phase?

Cwnd grows linearly.

What happens during fast recovery after receiving three duplicate ACKs?

The segment is retransmitted, ssthresh is set to cwnd/2, and cwnd is set to ssthresh + 3 MSS.

Why is a triple duplicate ACK considered better than a timeout?

It indicates that some data is being delivered, even if it is out of order.

What is the additive-increase, multiplicative-decrease (AIMD) strategy?

It is a strategy used in the congestion avoidance phase where the sending rate is increased additively and decreased multiplicatively.

What is TCP CUBIC's approach to congestion control?

It quickly increases the sending rate after no packet loss and cautiously probes for additional bandwidth as it approaches the pre-loss sending rate.

What is Explicit Congestion Notification (ECN)?

A method of network-assisted congestion control where routers set ECN bits in the IP header to signal congestion.

How does TCP Vegas differ from traditional TCP?

TCP Vegas reduces the sending rate as delays increase to avoid packet loss.

What is the bandwidth-delay product (BDP)?

The optimal operating point for network performance, calculated as bandwidth multiplied by propagation delay.

What is BBR in TCP congestion control?

BBR (Bottleneck Bandwidth and Round-trip time) estimates the BDP to set its sending rate without waiting for losses.

What happens to cwnd on timeout in TCP?

Cwnd is reduced to 1 MSS, and ssthresh is set to cwnd/2, returning to slow start.

What is the significance of the 'sawtooth' pattern in TCP congestion control?

It represents the cyclical nature of cwnd growth and reduction during congestion control phases.

Why might delay-based congestion control be preferable?

It can avoid packet loss by responding to increasing delays, which may indicate congestion.

What is the role of the receiver in ECN?

The receiver sets the ECE (ECN Echo) flag in the TCP header to signal congestion back to the sender.

What is the primary goal of TCP congestion control?

To manage network traffic effectively and prevent congestion while maximizing throughput.

What is the relationship between TCP Reno and TCP Vegas?

They may compete for bandwidth, with TCP Reno typically having a larger cwnd than TCP Vegas.

What is the impact of bufferbloat on network performance?

Bufferbloat can lead to increased delays and reduced throughput, making delay-based control methods more effective.

What does 'cwnd' stand for in TCP?

Congestion Window, which controls the amount of data that can be sent without receiving an acknowledgment.

What is the purpose of the CWR flag in TCP?

The CWR (Congestion Window Reduced) flag is set by the sender to indicate that it has reduced its sending rate due to congestion.