Flow Rate Fairness

Many slides are borrowed from Bob Briscoehttp://www.bobbriscoe.net/projects/refb/

Presented by: Yang GuanMarch 25, 2010CISC 856: TCP/IP & Upper Layer Protocols

Resource Sharing in the Internet

The Internet is based on a simple premise: Sharing communication links are more

efficient than dedicated channels The primary sharing algorithm is built

into Transmission Control Protocol (TCP) TCP provides mechanisms to guard

people how to share Internet capacity politely

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How TCP shares the Internet

The protocol allows you to seem to be polite TCP constantly increases transmission

rate if it can Until it sees some sign of congestion,

TCP politely reduces bit rate

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TCP-Friendliness

TCP is even used as a standard For applications that do not utilize

TCP in transport layer, they are called TCP-friendly if they consume about the same data rate as TCP does.

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Does TCP make the world perfect?

The answer is of course NO! Methods to circumvent TCP-

friendliness rules: Running multiple TCP sessions Running each TCP session for long time

It is really the application software that determines how to share the Internet fairly

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What does TCP overlook?

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Fig 1. TCP overlooks users’ activity over time [4]

What does TCP overlook? (cont’d)

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Fig 2. TCP overlooks multiple TCP instances [4]

Rethink: What is fair?

Equal flow rate? It is not about

how much a TCP consumes

It is about how much a TCP can affect others

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Rethink: What is fair? (cont’d)

How to measure the effect on others? Congestion volume: the amount of data

that is sent during network congestion

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Rethink: What is fair? (cont’d)

04/21/23 10Fig 3. Different TCP sharing schemes [4]

Rethink: What is fair? (cont’d)

Fair is faster: Light browsing goes blisteringly faster Heavy downloading is not obviously

prolonged

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Problems with TCP Congestion is only detected and

managed solely by computers at the edge

ISPs cannot set congestion limits The few ruin the life of the many Massive capacity is required But poor incentive to invest in

capacity

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A New TCP Routine

Parameterize TCP with weight Behave like 12 TCP flows, or Behave like 0.25 of a TCP flow

The key is High weights for light interactive usage

(web surfing) Low weights for heavy usage (movie

downloads)

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A New TCP Routine (cont’d)

Whenever congestion happens Higher weighted TCP goes much faster Lower weighted TCP expands back to

fast rate afterwards

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A New TCP Routine (cont’d)

On today’s Internet, the balance of weights is the wrong way around

How to persuade people to reasonably choose weights? We should limit people by the effects

they have on others—the incremental cost of their usage

Congestion volume: the volume of data sent during congestion

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A New TCP Routine (cont’d)

Solution: ISPs provide a monthly congestion-

volume allowance (CVA) High weights TCPs consumes CVA while

low weights ones doesn’t Heavy usage does not consume CVA

since weights are set to be low Light intensive usage does not consume

too much CVA due to short lifetime

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congestion policer – one example: per-user policer

non-interactive long flows(e.g. P2P, ftp)

interactive short flows(e.g. Web, IM)

overdraftcongestionvolumeallowance

NA NBR1S1

Making Congestion Visible to Network Layer

Why? Healthy supply of bandwidth Reroute data around congested links Costumers draw down the limited

allowances if congestion can not be avoided.

Currently, only the router that drops a packet knows the drop

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8 6 3579

First Step: ECN

Explicit Congestion Notification Standardized into TCP/IP in 2001 ECN allows end-to-end notification of

network congestion without dropping packets[3]

Routers set CE (Congestion Experience) bit when the average queue length exceeds configured threshold levels.

Receivers feedback congestion information back to senders

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Flag

Service Type

First Step: ECN (cont’d)

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Fig 4. IPv4 header format

VER HLEN Total length

Identification Fragmentation offset

TTL Protocol Header checksum

Source IP address

Destination IP address

First Step: ECN (cont’d)

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NA NB RS

8 6 4 2357

8642 3 5 7 9

feedback

Fig 5. ECN mechanism [5]

Second Step: re-feedback

04/21/23 22Fig 6. Re-feedback mechanism [4]

Conclusion

Ready to be implemented as ECN has been included into TCP/IP

Sticks to the Internet e2e principle Makes congestion visible to the

networks in the middle

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References[1] Bob Briscoe (BT), Illustrations by QuickHoney, A Fairer, Faster Internet Protocol, IEEE Spectrum, Dec 2008 pp38-43[2] B. Briscoe. Flow rate fairness: Dismantling a religion. Computer Communications Review, 37(2):63–74, Apr. 2007.[3] Explicit Congestion Notification, http://en.wikipedia.org/wiki/Explicit_Congestion_Notification[4] Bob Briscoe, Internet: Fairer is Faster, BT White Paper[5] Bob Briscoe, et al, Policing congestion response in an internetwork using re-feedback, Sigcomm 2005

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Questions

Thanks

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