Network | TCP Congestion Control

Congestion control algorithm:1. Castian, multiplicative sex minus; 2. slow start,3. Responding to time-out events;

The overall process is as follows:

Slow Start Arrival threshold Value - Additive (Windows + 1 MSS), this stage is called congestion Avoidance (CA) ->3 a redundant ack the threshold and window are reduced to half ( multiplicative minus ), and then the increment (CA)--fast retransmit, and wait for confirmation, this stage is called fast recovery , if not confirmed, it is treated as a timeout event ;

Time-Out (event), fast retransmission, TCP will go back to the original state, into the slow start;

TCP performs a fast retransmission in the event of packet loss and timeout.

When the TCP receiver receives a segment that has a sequence number that is greater than the next expected, it detects an interval in the data stream, which means that there is a loss of the packet segment. It sends a redundant ACK and repeats the acknowledgment, and if the TCP sender receives 3 redundant ACK of the same data, it considers that the segment of the message following the message segment that has been confirmed 3 times has been lost.

Example (Theft chart):

Assuming that the maximum window is 32 segments, the threshold is 16 segments (half of the maximum window), and at the slow start stage, the window size increases exponentially from 1 until it reaches the threshold value.
When the threshold is reached, the congestion avoidance (additive increase) process allows the window size to grow linearly to know that the timer reaches or reaches the maximum window size.
On the way, when the window is 20 o'clock, the timer arrives. At this time Enter the multiplicative reduction process, set the threshold to half the size of the current window, and when the timer is reached, the current window size is 10, so the threshold is now 10.
TCP enters the slow start again, and the window is set to a size of 1, and when the new threshold (10) is reached, TCP enters the additive increase phase.
When the window size is 12 o'clock, 3 ACK events occur, again into the multiplicative reduction process, the threshold is set to 6, then TCP enters the additive increase phase, which is maintained to another timer or another 3 ACK events occur.

Network | TCP Congestion Control