The Power ofExplicit Congestion Notification

Aleksandar KuzmanovicNorthwestern University

http://www.cs.northwestern.edu/~akuzma/

2 A. Kuzmanovic The Power of ECN

MotivationRecent measurements [PF01,MPF04]:– 2000: 1.1% Web servers support ECN– 2004: the percent increased to 2.1%– Not a single packet was marked in the network

2 10 0 2 20 0 2 30 0 24 00 year2 000

50%

1 00 %E C N d ep loym ent

year 2396(Sigcom m # 411)

3 A. Kuzmanovic The Power of ECN

Background

S e rve rC lie nt Marke r/D rop pe r

F C F S sche dule rA Q M R oute r

Active Queue Management (AQM):– Simultaneously achieves high throughput and low average delay– AQM algorithms can mark (instead of drop) packets– The router and both endpoints have to be ECN-enabled

4 A. Kuzmanovic The Power of ECN

Negotiating ECN Capabilities (I)Client initiates ECN-capable communication by setting appropriate bits in the TCP SYN packet’s TCP header

S erve rC lient

T C P S YN

5 A. Kuzmanovic The Power of ECN

Negotiating ECN Capabilities (II)An ECN-capable server replies by setting appropriate

bits in the SYN ACK packet’s TCP header

S erve rC lient

T C P S YN

S YN A C K

Once the SYN ACK packet arrives, ECN negotiation is completed

6 A. Kuzmanovic The Power of ECN

Barriers to Adoption of ECN

S erve rC lient

T C P S YN

rese t

route r

"B roken"f irewall

“Broken” firewalls and load balancers incorrectly reset TCP flows attempting to negotiate ECN – The problem addressed in RFC 3360

Consequences are devastating New incentives?

7 A. Kuzmanovic The Power of ECN

ECN and TCP’s Control Packets

S erve rC lient

T C P S YN

S YN A C K

HT T P R E Q

TCP SYN and SYN ACK packets are dropped during congestionCan significantly reduce end-to-end performance– RTO = 3 sec (+6 sec, +12 sec, etc.)

Marking SYN packets?

8 A. Kuzmanovic The Power of ECN

Marking TCP SYN Packets?

S erve rC lie nt

T C P S YN

TCP SYN packets:– Security problems

SYN ACK packets:– No security obstacles– More relevant

• Congestion likely to happen from servers to clients

9 A. Kuzmanovic The Power of ECN

Marking SYN ACK Packets?

S erve rC lient

T C P S YN

S YN A C K

TCP SYN packets:– Security problems

SYN ACK packets (ECN+): – No security obstacles– More relevant

• Congestion likely to happen from servers to clients

10 A. Kuzmanovic The Power of ECN

Deployment Requirements

Security– No novel security holes

Performance improvements– Necessary to provide incentives to all involved parties

Incremental deployability– What level of deployment is needed to achieve the

above improvements?– What happens to those who do not apply the change?

11 A. Kuzmanovic The Power of ECN

Simulation Scenario

Client pool S erver poo l

1 Gbps 1 Gbps

100/622/1 ,000M bps

S e rve r

S e rve r

S e rve r

C lient

C lient

C lient

reques ts

responsesA QM

90% o bjectsdo wnlo aded inless than 0.5 sec

Light and persistent congestion from servers to clientsWeb and general traffic mixesAQM algorithms: Random Early Detection (RED) (others in the paper)

12 A. Kuzmanovic The Power of ECN

Outdated Implementation

m in_ th m ax_ th AverageQueueLength

m ax_ p

1 00 %

Drop/mark rate

RED (1993) – “This notification can consist of dropping or marking a packet.”

RFC 3168 (2001)– Guidelines for setting ECN with RED

Older RED versions still present (e.g., Linux)

RED’s dropping/marking rate as a function of the queue length

13 A. Kuzmanovic The Power of ECN

Dropping RED

m in_ th m ax_ th AverageQueueLength

m ax_ p

10 0 %

Drop/mark rate

op e rating p o int

Reduced performance due to congestion

14 A. Kuzmanovic The Power of ECN

Add ECN

m in_ th m ax_ th AverageQueueLength

m ax_ p

100 %

Drop/mark rate

op e rating p o int

All SYN packets are dropped

Outdated implementation can cause drastic performance degradations

15 A. Kuzmanovic The Power of ECN

Add ECN+

m in_ th m ax_ th AverageQueueLength

m ax_p

100%

Drop/mark rate

op erating po int

ECN+ systematically improves throughput and responsetimes of all investigated AQM schemes

SYN ACK packets are NOT dropped

16 A. Kuzmanovic The Power of ECN

Incremental DeployabilityScenario

C lient poo l S erver poo lS e rve r

S e rve r

S e rve r

C lie nt

C lient

C lient

E C N +at servers

E C Nat routers

x% c lients : E C N(100-x)% : no E C N

17 A. Kuzmanovic The Power of ECN

5% Deployment

Ins tant g ains fo rE C N-enab ledc lie nts

9 5% no E C N

5% E C N

18 A. Kuzmanovic The Power of ECN

50% Deployment

50 % E C N

50 % no E C N

G rad ual deg rad ationfo r c lie nts no tap p lying E C N

19 A. Kuzmanovic The Power of ECN

95% Deployment

95% E C N

5% no E C N

P erfo rm ancenecessarilydeg raded

20 A. Kuzmanovic The Power of ECN

Testbed Experiments

S erver pool100 M bps

10 M bps

S erve r

S e rve r

S e rve r

C lie nt

reques ts

responses (15 M bps)

router

E CN no E CNE CNE CN +

21 A. Kuzmanovic The Power of ECN

ECN and Flash Crowds

R E D , no E C N

R E D , E C N

R E D , E C N +

A verage R espo nse T im e

T hro ughput(% o f capacity)

26 sec

4.5 sec

0.5 sec

44%

56%

99%

Reasonable performancedespite huge congestion

22 A. Kuzmanovic The Power of ECN

Conclusions

Security– No novel security holes

Incremental deployability– Instant benefits for clients applying the change – Gradual degradation for those not applying the change

Incentives– Providers, clients, and servers

Implementation– Wrong or outdated implementation can significantly reduce

deployment and performance