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

2. Lecture No. 34

3. Where we are now … Understand how to Today’s topic
Build a network on one physical medium
Connect networks together (with switches)
Implement a reliable byte stream on Internet
Implement a UDP/TCP connection/channel
Address network heterogeneity
Address global scale
End-to-end issues and common protocols
Today’s topic
Congestion control

4. Congestion Control Outline Queuing Discipline Reacting to Congestion
Avoiding Congestion

5. Air Travel Planning a vacation for your semester break ?
Try a trip to scenic Northern Areas
Its a mere 3 hops from VU-Islamabad
Flight
was
overbooked
Islamabad Airport
Peshawer VU
Sakardu

6. Congestion Control Reading: Peterson and Davie, Ch. 6
Basics: problem, terminology, approaches, metrics
Solutions
Router-based: queuing disciplines
Host-based: TCP congestion control
Congestion avoidance
DECbit
RED gateways
Quality of service

7. Congestion Control Basics
Problem
Demand for network resources can grow beyond the resources available
Want to provide “fair” amount to each user
Examples
Bandwidth between Islamabad and Peshawer
Bandwidth in a network link
Buffers in a queue

8. Range of Solutions
Congestion control: cure congestion when it happens
Congestion avoidance: stay away from congestion
Resource allocation: prevent congestion from occurring

9. Model of Network
Packet-switched internetwork (or network)
Connectionless flows (logical channels/connections) between hosts

10. State Maintained in Routers
Hard state: state used to support routing
Soft state:
Information about flows
Helps control congestion
Not necessary for correct routing

11. Congestion Control Basics
Router role
Controls forwarding and dropping policies
Can send feedback to source
Host role
Monitors network conditions
Adjusts accordingly
Routing vs congestion
Effective adaptive routing schemes can sometimes help congestion
But not always

12. Finite Buffering Capacity per Output
Customer Service
Standard check-in lines x a
angry mob eagerly awaiting opportunity to address underpaid customer service representative
1x6 Switch
you are turned away at door

13. Congestion Control Taxonomy

14. Congestion Control Taxonomy
Why is explicit per-flow congestion feedback from routers rarely used in practice?
Too many sources to track
Millions of flows may fan in to one router
Can’t send feedback to all of them
Adds complexity to router
Need to track more state
Certainly can’t track state for all sources

15. Congestion Control Taxonomy
Wastes bandwidth: network already congested, not the time to generate more traffic
Can’t force the sources (hosts) to use feedback
Sol: use stochastic methods to select fairly between flows, or classify flows into categories at edge routers

16. Why Consider Rate Control ?
Buffer space (window size) is an intrinsic physical quantity
Can provide rate control with window control
Only need estimate of RTT
Answer:
Want rate control when granularity of averaging must be smaller than RTT

17. Criticism of Resource Allocation
Example: you have 10 Gbps total bandwidth
Case 1: reserve whatever you want
Users’ line of thought: On average, I don’t need much bandwidth, but when my personal web crawler goes to work, I need at least 100 Mbps, so I’ll reserve that much.
Result: 100 users consume all bandwidth, all others get 0
Case 2: fair/equitable reservations
35,000 students + 5,000 faculty and staff
Each user gets 250 kbps, almost 5x a modem! (watch that crawler crawl...)

18. Back to Air Travel Analogy
Daily Islamabad to Peshawer flight, ~200 seats
Case 1: reserve whatever you want
200 of us get seats. I’m Gold...are you ?
Case 2: fair/equitable reservations
2,000 possible customers
0.099 seats per customer per flight
Disclaimer: the passenger assumes all risks and damages related to unsuccessful reassembly in Peshawer !!!

19. Congestion Control Metrics
How do we evaluate solutions ?
Effectiveness (shown as power in the figure below)
fairness

20. Congestion Control Metrics: What’s Fair ?
Give flow A 1/3 of the link bandwidth ?
Globally “fair”
A uses 2 links at 1/3 bandwidth
B and C use 1 link at 2/3 bandwidth
Give flow A 1/2 of the link bandwidth ?
Locally “fair” (fair on each link)
Also globally “fair”: maximizes minimum flow

21. Power Measure for a Sliding Window
Bottleneck link with capacity C
Round-trip time RTT