2. Contents
• What is CONGESTION
• What causes Congestion
• Approach to control Congestion
• Three components of Congestion control mechanism
3. What is CONGESTION in network
When a link or node carries data more than it can handle
results in congestion.
4. What causes the congestion in the network?
Two main scenario:
Two senders and a router with infinite buffers:
Let assume two system connected via a router which has
infinite buffer to catch hold all the packets sent by the sender system.
If the router capacity of sending packets to destination is R/2
and the sender sends the packets in greater than R/2 rate that causes
congestion in the network.
5. Two senders and a router with a finite buffer:
Let assume two system connected via a router which has finite buffer
to catch hold the packets sent by the sender system.
If router capacity of sending packets to destination is R/2 and the
sender sends the packets in greater than R/2 rate results in congestion maybe
due to insufficient space to hold packets that results in loss of packets that
intern results in retransmission of packets that causes congestion in the
network
6. What is approach taken by the TCP to control
congestion?
Sender have to limit the rate at which it sends the traffic into the
connection as the function of perceived network congestion.
If a TCP sender perceives that there is little congestion on the path
between itself and the destination, then the TCP sender increases its send
rate .If the sender perceives that there is congestion along the path, then
the sender reduces its send rate.
8. Three components of congestion control mechanism:
1.Slow start
2.Congestion avoidance
3.Fast recovery
9. Slow Start:
o The CWND is set to 1 MSS every time ack is received the CWND is doubled
o Because of CWND starts at slowly by setting 1 MSS it is called as Slow start
and results in exponential growth.
o This stage stops when there is a loss event indicated by timeout. Then sets the
CWND to 1 and sets the SSTHRESH to CWND / 2 and starts the slow start
process again. when CWND is greater than SSTHRESH it enters Congestion
avoidance stage.
10. Congestion Avoidance:
o On entry to this stage the CNWD is approximately half the value when
congestion was last encountered.
o Rather than doubling the CWND value this algorithm increases the CWND
by MSS * (MSS/CWND).
o This stage ends when there is timeout.
o And loss event can be triggers by triple duplicate ACK but it has to treated
differently than timeout loss event hence CWND is set to CWND / 2 + 3
MSS and SSTHRESH is set to CWND / 2.
o Then Fast recovery stage starts.
