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

2. Lecture No. 37

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

4. Router-based Congestion Avoidance

5. DECbit: Destination Experiencing Congestion bit
Developed for Digital Network Architecture
Basic idea
One bit allocated in packet header
Any router experiencing congestion sets bit
Destination returns bit to source
Source adjusts rate based on bits
Note that responsibility is shared
Routers identify congestion
Hosts act to avoid congestion

6. DECbit (continued) Routers Calculate average queue length
Averaging interval spans last busy+idle cycles
busy: queue is non-empty
idle: queue is empty
Set DECbit when:
average queue length >= 1
Why 1?
Maximizes power function
Smaller values result in more idle time
Larger values result in more queuing delay

7. DECbit – Average Queue Length

8. DECbit (continued) Hosts
Count number of marked packets in last congestion window of packets
Increase congestion window by 1 if less than 50% of packets marked
Decrease congestion window by factor of 7/8
if greater than or equal to 50% of packets marked
Why 50%?
Maximizes power function
Resurfaced as Explicit Congestion Notification (ECN) in last few years, proposed TCP extension

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

10. Random Early Detection (RED) Gateways
Developed for use with TCP
Basic idea
Implicit rather than explicit notification
When a router is “almost” congested
Drop packets randomly
Responsibility is again shared
Router identifies, host acts
Relies on TCP’s response to dropped packets

11. Weighted Running Average Queue length

12. Random Early Detection (RED) Gateways
Hosts
Implement TCP congestion control
Back off when a packet is dropped
Routers calculate average queue length (as exponential moving average)
length = (A) measurement + (1 – A) length
Routers act based on average queue length
Below minimum length, keep new packets
Above maximum length, drop new packets
Between the two, drop randomly

13. RED Thresholds on a FIFO Queue

14. Drop Probability Function for RED

15. Random Early Detection (RED) Gateways
Router drop probability depends on two factors
Relation of average queue length to bounds
Number of packets (count) since
Last drop, or
Length outside random drop range

16. Random Early Detection (RED) Gateways
Router calculates
Base probability for drop:
baseProb = MaxP × (length – minThresh) /(maxThresh – minThresh)
Actual probability for drop
prob = baseProb / (1 – baseProb × count)

17. Random Early Detection (RED) Gateways
Parameter values
MaxP is typically 0.02 (drops roughly 1 in 50 packets at midpoint)
min (threshold) is typically max/2
Choosing parameters
Carefully tuned to maximize power function
Confirmed through simulation
But answer depends on accuracy of traffic model
May create oscillating behavior in the Internet

18. Tuning the RED Gateways
Drops roughly in proportion to flow bandwidths
Queue size changes
Due to difference in input/output rates
Not related absolute magnitude of those rates
min must allow reasonable link utilization
Difference between min and max thresholds
Large enough to absorb burstiness
In practice, can use max = 2 × min
Use penalty box for offenders!

19. Host-based Congestion Avoidance

20. TCP Vegas – Basic Idea Watch for signs of queue growth
In particular, difference between
Increasing congestion window
Stable throughput (presumably at maximum capacity)
Keep just enough “extra data” in the network
Time to react if bandwidth decreases
Data available if bandwidth increases

21. Trace of TCP Vegas Congestion Avoidance Mechanism

22. TCP Vegas - Implementation
Estimate un-congested RTT, baseRTT, as minimum measured RTT
Calculate expected throughput as congestion window / baseRTT
Measure throughput each RTT
Mark time of sending distinguished packet
Calculate data sent between send time and receipt of ACK

23. TCP Vegas - Implementation
Act to keep the difference between estimated and actual throughput in a specified range
Below min threshold, increase congestion window
Above max threshold, decrease congestion window
Additive decrease used only to avoid congestion
Want between 1 and 3 packets of extra data (used to pick min/max thresholds)

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

25. Quality of Service Outline Realtime Applications Integrated Services
Differentiated Services

26. Quality of Service
How “good” are late data and low-throughput channels?
It depends on the application. Do you care if...
your takes 1/2 hour to reach your friend?
you have to spend 1/2 hour to make a cheaper
plane reservation on the Web?
your call to Rescue-15 takes 1/2 hour to go through
your nifty new IP phone service?

27. Quality of Service High-throughput allows new classes of applications
Some are sensitive to network delay, e.g. voice, video
Called real-time applications
Control applications (e.g. factories)
More an issue of possible starvation (see below)
But fit within QoS framework

28. Quality of Service How to achieve timely delivery
When actual RTT small (less than 2/3) relative to acceptable delay, retransmit
When base RTT (no queuing delay) large (greater than 2) relative to acceptable delay, impossible
Otherwise possible, but not through retransmission

29. Quality of Service Within the United States, for example
Base RTT (no queuing delay) peaks around 75 ms
Actual RTT is often ms
Humans notice about 50 ms delay for voice
When in comparable regime, retransmission cannot satisfy, so...
Use erasure codes across packets (e.g. FEC) , or
Support delay preferences in the network, called quality of service, or QoS

30. QoS Granularity
Fine-grained: guarantees for individual flows, used in RSVP and ATM virtual circuits between applications
Coarse-grained: guarantees for flow, used in DIFFSERV and ATM VC’s between routers
IETF is standardizing extensions …

31. Real Time Applications
Require “deliver on time” assurances
Must come from inside the network
Example application (audio)
Sample voice once every 125µs
Each sample has a playback time
Packets experience variable delay in network
Add constant factor to playback time: playback point
Microphone
Speaker
Sampler
A D
converter
Buffer
D A