TCP Congestion Control

Prerequisites – Basic Congestion control knowledge

TCP congestion control is a method used by the TCP protocol to manage data flow over a network and prevent congestion. TCP uses a congestion window and congestion policy that avoids congestion. Previously, we assumed that only the receiver could dictate the sender’s window size. We ignored another entity here, the network. If the network cannot deliver the data as fast as it is created by the sender, it must tell the sender to slow down. In other words, in addition to the receiver, the network is a second entity that determines the size of the sender’s window 

Congestion Policy in TCP

1. Slow Start Phase: Starts slow increment is exponential to the threshold.
2. Congestion Avoidance Phase: After reaching the threshold increment is by 1.
3. Congestion Detection Phase: The sender goes back to the Slow start phase or the Congestion avoidance phase.

Slow Start Phase

Exponential increment: In this phase after every RTT the congestion window size increments exponentially. 

Example:- If the initial congestion window size is 1 segment, and the first segment is successfully acknowledged, the congestion window size becomes 2 segments. If the next transmission is also acknowledged, the congestion window size doubles to 4 segments. This exponential growth continues as long as all segments are successfully acknowledged.

Initially cwnd = 1
After 1 RTT, cwnd = 2^(1) = 2
2 RTT, cwnd = 2^(2) = 4
3 RTT, cwnd = 2^(3) = 8

Congestion Avoidance Phase

Additive increment: This phase starts after the threshold value also denoted as ssthresh. The size of cwnd(congestion window) increases additive. After each RTT cwnd = cwnd + 1.

Example:- if the congestion window size is 20 segments and all 20 segments are successfully acknowledged within an RTT, the congestion window size would be increased to 21 segments in the next RTT. If all 21 segments are again successfully acknowledged, the congestion window size would be increased to 22 segments, and so on.

Initially cwnd = i
After 1 RTT, cwnd = i+1
2 RTT, cwnd = i+2
3 RTT, cwnd = i+3

Congestion Detection Phase

Multiplicative decrement: If congestion occurs, the congestion window size is decreased. The only way a sender can guess that congestion has happened is the need to retransmit a segment. Retransmission is needed to recover a missing packet that is assumed to have been dropped by a router due to congestion. Retransmission can occur in one of two cases: when the RTO timer times out or when three duplicate ACKs are received.

Case 1: Retransmission due to Timeout – In this case, the congestion possibility is high.

(a) ssthresh is reduced to half of the current window size.
(b) set cwnd = 1
(c) start with the slow start phase again.

Case 2: Retransmission due to 3 Acknowledgement Duplicates – The congestion possibility is less.

(a) ssthresh value reduces to half of the current window size.
(b) set cwnd= ssthresh
(c) start with congestion avoidance phase

Example

Assume a TCP protocol experiencing the behavior of slow start. At the 5th transmission round with a threshold (ssthresh) value of 32 goes into the congestion avoidance phase and continues till the 10th transmission. At the 10th transmission round, 3 duplicate ACKs are received by the receiver and entered into additive increase mode. Timeout occurs at the 16th transmission round. Plot the transmission round (time) vs congestion window size of TCP segments.

GATE CS Corner Questions 

Practicing the following questions will help you test your knowledge. All questions have been asked in GATE in previous years or in GATE Mock Tests. It is highly recommended that you practice them.

1. GATE CS 2008, Question 56
2. GATE CS 2012, Question 65
3. GATE CS 2014 (Set 1), Question 65
4. GATE IT 2005, Question 73