1. “Promoting the Use of End-to-End Congestion Control in the Internet”
Sally Floyd, Kevin Fall in Proceedings of IEEE/ACM Transactions on Networking, 1999
A Summary by Ashish Samant
CS577 – Spring 2005
1/14/2019
CS577 Spring 2005

2. Outline Need for end-to-end congestion control
Unfairness, Congestion Collapse
Per flow based scheduling Vs Congestion Control mechanisms at the router
Identifying candidate flows for regulation
Other incentives for flows
1/14/2019
CS577 Spring 2005

3. Introduction
End hosts/applications may not use end-to-end congestion control schemes.
Problem - This may lead to congestion collapse and unfairness, in times of congestion.
Solution – Isolate ill-behaving flows, use per-flow based queuing at routers.
This may not be sufficient !!
1/14/2019
CS577 Spring 2005

4. Introduction …. continued
Authors suggest -
Routers must support congestion control and regulate high-bandwidth flows.
Routers must regulate ‘best effort flows’ that
are TCP-Unfriendly,
unresponsive to congestion,
use disproportionate bandwidth.
1/14/2019
CS577 Spring 2005

5. Introduction …. continued
Unresponsive flows cause two problems
- Unfairness; well-behaved flows may suffer bandwidth starvation because unresponsive flows do not react to congestion.
- Congestion collapse; the scarce bandwidth of the network is consumed by packets from unresponsive flows, that will be discarded sooner or later.
1/14/2019
CS577 Spring 2005

6. Experimental Setup
1/14/2019
CS577 Spring 2005

7. Unfairness – 3 TCP, 1 UDP flow, FCFS
1/14/2019
CS577 Spring 2005

8. Fairness – 3 TCP, 1 UDP flow, WRR
1/14/2019
CS577 Spring 2005

9. Congestion Collapse – 3 TCP, 1 UDP flow, FCFS
1/14/2019
CS577 Spring 2005

10. Congestion Control – 3 TCP, 1 UDP flow, WRR
1/14/2019
CS577 Spring 2005

11. Congestion Control – 3 UDP, 1 TCP flow, WRR
1/14/2019
CS577 Spring 2005

12. Identifying non TCP-Friendly Flows
TCP Friendly Flow – arrival rate does not exceed that of any other TCP conformant flow.
Maximum sending rate for a TCP Friendly flow :
T - sending rate ; p - packet drop rate ; B – max packet size ; R – minimum RTT
Actual rates will be less than T.
1/14/2019
CS577 Spring 2005

13. Identifying non TCP-Friendly Flows
Limitations :
Inconsistencies in finding packet size, round trip time.
Measurements should be taken over a long interval of time.
Bursty packet drops.
Router Response :
Routers should ‘freely restrict’ the bandwidth of non
TCP – Friendly flows.
1/14/2019
CS577 Spring 2005

14. Identifying Unresponsive Flows
TCP Friendly test cannot be used at routers that are unable to determine packet sizes and RTTs.
If packet drop rates increase by x , the arrival rate should drop by √x .
When packet drop is constant, no flow will be identified as unresponsive.
1/14/2019
CS577 Spring 2005

15. Identifying Unresponsive Flows
Limitations :
Packet drop may be because of various reasons, hard for flows with variable demand.
Flows might be tempted to start with a higher initial bandwidth demand.
Response :
Actively regulate the bandwidth of unresponsive flows in times of congestion.
1/14/2019
CS577 Spring 2005

16. Identifying flows using disproportionate flows
Flows that require larger bandwidth than other flows that reduce their demand.
These might be TCP friendly but still be ‘disproportionate’.
Arrival rate <= log(3n) / n ; n = no of flows
Arrival rate <= c / √p ; p = pkt drop rate
c = some constant
1/14/2019
CS577 Spring 2005

17. Comments and Conclusion
Alternate approaches
- use schemes that are a mix of FCFS and per-flow based approach ( FCFS scheduling with differential dropping ).
- pricing incentives.
granularity of flows
- apply fairness tests to single/aggregate of flows.
min-max fairness measure
- need to look at the entire path, all the congested links.
1/14/2019
CS577 Spring 2005

18. Comments and Conclusion … continued
Breaking a TCP connection, increased local throughput but also increases global packet drop rate.
1/14/2019
CS577 Spring 2005

19. Derivation of TCP Friendly Rate
Once congestionWindow >= W ; a packet is dropped and the congestion window is halved.
As long as congestionWindow < W ; window is increased by 1, per RTT
W/2 + (W/2+1) + (W/2+2) + … W = 3/8*W2
=> per packet drop ; max 3/8*W2 packets are sent
=> max packet drops <= 1/(3/8*W2)
1/14/2019
CS577 Spring 2005

20. Derivation of TCP Friendly Rate … Continued
Max bytes transferred per cycle of steady state:
Total packets sent * Avg. packet Size
Avg Round Trip Time
( Total packets sent = 0.75*W )
= > (0.75 * W * B) / R
= >
1/14/2019
CS577 Spring 2005