TCP Congestion Control

Question 1

What is the difference between congestion window and the advertised window in TCP?

Answer 1

The congestion window (CWND) is the sender will limit how much data can be sent before waiting for an acknowledgment which is based on network congestion, and the advertised window (RWND) is when the receiver will indicate how much buffer space the receiver has available.

Question 2

What is the difference between Congestion Control vs Flow Control?

Answer 2

Congestion control keeps the senders from sending too much data into the network because there will be a lack of resources at one point, but can be avoided all together if the network allocates resources effectively. Flow control keeps a fast sender from overrunning a slow receiver.

Question 3

What is additive increase?

Answer 3

Additive increase is when the congestion window needs to be increased and is incremented when ACK’s are received from last packets and the sender will gradually increase the congestion window until packet loss occurs, then window size will be reduced. (Increased linearly)

Question 4

What is multiplicative decrease?

Answer 4

TCP interprets timeouts as congestion, the congestion window is halved if a timeout happens (decreasing exponentially)

Question 5

What is TCP slow start?

Answer 5

Where TCP starts with a small congestion window and increases exponentially until it reaches a threshold. When connection is dead or timed out slow start is used and many packets are usually lost during the initial start period, and TCP also learns how much network bandwidth is available.

Question 6

What is fast retransmit?

Answer 6

Enhanced TCP congestion control, triggers a retransmission of a dropped packet since it’s faster than waiting for a timeout. If 3 duplicate acknowledgments, TCP will retransmit the lost segment

Question 7

What is fast recovery?

Answer 7

After receiving 3 duplicate ACKs, fast recovery is used to avoid a slow start.