advanced computer networking congestion control for high bandwidth-delay product environments (xcp...

Advanced Computer Advanced Computer NetworkingNetworking

Congestion Control for High Bandwidth-Delay Product

Environments(XCP Algorithm)

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Trends in the Future Trends in the Future InternetInternet

•High Bandwidth–Gigabit Links

•High Latency–Satellite–Wireless

As we will find out…these spell bad news for TCP!

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TCP congestion control performs poorly TCP congestion control performs poorly as bandwidth or delay increasesas bandwidth or delay increases

Round Trip Delay (sec)

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Bottleneck Bandwidth (Mb/s)

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Because TCP lacks fast response

• Spare bandwidth is available TCP increases by 1 pkt/RTT even if spare bandwidth is huge• When a TCP starts, it increases exponentially Too many drops Flows ramp up by 1 pkt/RTT, taking forever to grab the large bandwidth

Because TCP lacks fast response

• Spare bandwidth is available TCP increases by 1 pkt/RTT even if spare bandwidth is huge• When a TCP starts, it increases exponentially Too many drops Flows ramp up by 1 pkt/RTT, taking forever to grab the large bandwidth

50 flows in both directionsBuffer = BW x Delay

RTT = 80 ms

50 flows in both directionsBuffer = BW x Delay

BW = 155 Mb/s

33 behnam shafagatybehnam shafagaty

Efficiency and FairnessEfficiency and Fairness

• Efficiency of a link involves only the aggregate traffic’s behavior

• Fairness is the relative throughput of flows sharing a link.

• Can have efficiency but not fairness• Coupled in TCP since the same

control low is used for both, uses AIMD (additive increase multiplicative decrease).


What If We Could Do It OverWhat If We Could Do It Over??

• If you could build a new congestion control architecture, what would it look like?

• Points of Observation– Packet loss is a poor signal of congestion

• Congestion is not a binary variable!• We want precise congestion feedback

– Aggressiveness of sources should adjust to delay• As delay increase, rate change should be slower


Points of Observations Points of Observations ContCont..– Needs to be independent of number of flows

• Number of flows at AQM is not constant therefore it cannot be fast enough to adapt to changes

– De-coupling of efficiency and fairness• Done with both an efficiency controller and a

fairness controller• Simplifies design and provides framework for

differential bandwidth allocations• Use MIMD for the efficiency (quickly get BW)• Use AIMD for fairness


Proposed Solution:

Decouple Congestion Control from Fairness

High Utilization; Small Queues; Few Drops

Bandwidth Allocation Policy


Proposed Solution:

Decouple Congestion Control from Fairness

Example: In TCP, Additive-Increase Multiplicative-Decrease (AIMD) controls both

Coupled because a single mechanism controls both

How does decoupling solve the problem?

1. To control congestion: use MIMD which shows fast response

2. To control fairness: use AIMD which converges to fairness


Characteristics of Our Characteristics of Our SolutionSolution

1. Improved Congestion Control (in high bandwidth-delay & conventional environments):

• Small queues

• Almost no drops

2. Improved Fairness

3. Scalable (no per-flow state)

4. Flexible bandwidth allocation: min-max fairness, proportional fairness, differential bandwidth allocation,…


XCP: An eXCP: An eXXplicit plicit CControl ontrol PProtocolrotocol

1. Congestion Controller2. Fairness Controller


Feedback

Round Trip Time

Congestion Window

Congestion Header

Feedback

Round Trip Time

Congestion Window

How does XCP WorkHow does XCP Work??

Feedback = + 0.1 packet


Feedback = + 0.1 packet

Round Trip Time

Congestion Window

Feedback = - 0.3 packet



Congestion Window = Congestion Window + Feedback

Routers compute feedback without any per-flow state Routers compute feedback without any per-flow state


XCP extends ECN


How Does an XCP Router Compute How Does an XCP Router Compute the Feedbackthe Feedback??

Congestion Controller

Fairness ControllerGoal: Divides between flows to converge to fairnessLooks at a flow’s state in Congestion Header

Algorithm:If > 0 Divide equally between flowsIf < 0 Divide between flows proportionally to their current rates

MIMD AIMD

Goal: Matches input traffic to link capacity & drains the queueLooks at aggregate traffic & queue

Algorithm:Aggregate traffic changes by ~ Spare Bandwidth ~ - Queue SizeSo, = davg Spare - Queue 1414 behnam shafagatybehnam shafagaty

= davg Spare - Queue

224

0 2 and

Theorem: System converges to optimal utilization (i.e., stable) for any link bandwidth, delay, number of sources if:

(Proof based on Nyquist Criterion)

Getting the devil out of the detailsGetting the devil out of the details ……

Congestion Controller Fairness Controller

No Parameter Tuning

No Parameter Tuning

Algorithm:If > 0 Divide equally between flowsIf < 0 Divide between flows proportionally to their current rates

Need to estimate number of flows N

Tinpkts pktpkt RTTCwndTN

)/(1

RTTpkt : Round Trip Time in header

Cwndpkt : Congestion Window in headerT: Counting Interval

No Per-Flow State

No Per-Flow State


ImplementationImplementation

Implementation uses few multiplications & additions per packet

Practical!

XCP can co-exist with TCP and can be deployed gradually

Gradual Deployment

Liars?• Policing agents at edges of the network or statistical monitoring• Easier to detect than in TCP


PerformancePerformance


BottleneckS1

S2

R1, R2, …, Rn

Sn

Subset of Results

Similar behavior over:


XCP Remains Efficient as XCP Remains Efficient as Bandwidth or Delay IncreasesBandwidth or Delay Increases

Bottleneck Bandwidth (Mb/s)

Utilization as a function of Bandwidth

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Round Trip Delay (sec)

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Utilization as a function of Delay


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XCP Remains Efficient as XCP Remains Efficient as Bandwidth or Delay IncreasesBandwidth or Delay Increases

Bottleneck Bandwidth (Mb/s) Round Trip Delay (sec)

Utilization as a function of Delay

XCP increases proportionally to spare bandwidth

and chosen to make XCP robust to delay

Utilization as a function of Bandwidth


XCP Shows Faster Response XCP Shows Faster Response than TCPthan TCP

XCP shows fast response!XCP shows fast response!

Start 40 Flows

Start 40 Flows

Stop the 40 Flows

Stop the 40 Flows


XCP Deals Well with Short Web-Like FlowsXCP Deals Well with Short Web-Like Flows

Arrivals of Short Flows/sec

Average Utilization

Average Queue

Drops


XCP is Fairer than TCPXCP is Fairer than TCP

Flow ID

Different RTTSame RTT

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Flow ID

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(RTT is 40 ms 330 ms )


XCP SummaryXCP Summary•XCP

–Outperforms TCP–Efficient for any bandwidth

–Efficient for any delay–Scalable

•Benefits of Decoupling–Use MIMD for congestion control which

can grab/release large bandwidth quickly–Use AIMD for fairness which converges to

fair bandwidth allocation


NS Code & More Information at:NS Code & More Information at:

http://ana.lcs.mit.edu/dina/XCPhttp://ana.lcs.mit.edu/dina/XCP


advanced computer networking congestion control for high bandwidth-delay product environments (xcp...

Documents

tcp congestion control

fairness efficiency

behnam shafagaty slide

fairness controller

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improved congestion

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