1. Advanced Computer Networks
CS716
Advanced Computer Networks
By Dr. Amir Qayyum 1

2. Lecture No. 36

3. Congestion Control Basics: problem, terminology, approaches, metrics
Solutions
Router-based: queuing disciplines
Host-based: TCP congestion control
Congestion avoidance
DECbit
RED gateways
Quality of service

4. Host Solutions: TCP Congestion Control
Host has very little information
Assumes best-effort network
Acts independently of other hosts
Host infers congestion
From synchronization feedback
e.g. dropped packet timeouts, duplicate ACK’s
Loss on wired lines rarely due to transmission error
Host acts
Reduce transmission rate below congestion threshold
Continuously monitor network for signs of congestion

5. TCP Congestion Control
Add notion of congestion window
Effective sliding window is smaller of
Remote advertised window (flow control)
Local congestion window (congestion control)
Changes in congestion window size
Slow increases to absorb new bandwidth
Quick decreases to eliminate congestion

6. TCP Congestion Control Strategy
Self-clocking
Send data only when outstanding data ACK’d
Equivalent to send window limitation mentioned
Growth
Add one Maximum Segment Size (MSS) per congestion window of data ACK’d
It’s really done this way, at least in Linux: see tcp_cong_avoid in tcp_input.c. Actually, every ack for new data is treated as an MSS ACK’d… any problems with that?
Known as additive increase

7. TCP Congestion Control Strategy
Decrease
Cut window in half when timeout occurs
In practice, set window = window/2
Known as multiplicative decrease
Additive Increase, Multiplicative Decrease (AIMD)

8. TCP Congestion Control
AIMD sawtooth trace

9. TCP Congestion Control
AIMD fairness for two competing flows
Slope of 1 for additive increase
Proportional decrease (towards origin) for multiplicative decrease

10. Initialization of Congestion Window
Congestion window should start small
Avoid congestion due to new connections
Start at 1 MSS, reset to 1 MSS with each timeout (note that timeouts are coarse-grained, ~1/2 seconds)
Known as slow start

11. Initialization of Congestion Window
To make up for slow start, ramp up quickly
Maintain threshold window size
Use multiplicative increase
When congestion window is smaller than threshold
Double window for each window ACK’d

12. Initialization of Congestion Window
Threshold value
Initially set to maximum window size
Set to 1/2 of current window on timeout
In practice, increase congestion window by 1 MSS for each ACK of new data (or N bytes for N bytes)

13. TCP Congestion Control
Coarse-grained timeouts lead to idle periods …
Solution: fast retransmission
Send ACK for each segment received
When duplicate ACK’s received
Resend lost segment immediately
Do not wait for timeout
In practice, retransmit on 3rd duplicate
Fast recovery
When fast retransmission occurs, skip slow start
Congestion window becomes 1/2 of the previous
Start additive increase immediately

14. TCP Congestion Window Trace

15. Congestion Avoidance

16. Flight Planning for Your Air Travel
Planning a vacation for your semester break ?
Trying for a trip to scenic Northern Areas
No way to go (route is too congested)
Delay your program till next semester
Flights
are
FULL
Islamabad Airport
Peshawer
CASE
Sakardu

17. Congestion Avoidance Control vs Avoidance
Control: minimize impact of congestion when it occurs
Avoidance: avoid producing congestion
In terms of operating point limits: