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Fair QueuingFair Queuing

Hamed KhanmirzaPrinciples of NetworkUniversity of Tehran

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Fairness

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Fairness Goals• Allocate resources fairly

• Isolate users– Router does not send explicit feedback to source– Still needs e2e congestion control

• Still achieve statistical muxing– One flow can fill entire pipe if no contenders– Work conserving scheduler never idles link if it has a

packet

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What is Fairness?• MAX-MIN Fairness

– No user receives more than its share– No other scheme satisfy this condition which has higher min allocation

Ui = MIN (Ui,Ri)-

• What does “user” means?– Per source of packet

• Restrict for example File Servers– Per receiver

• Malicious user can reduce receiver bandwidth– Per host process

• Encourage user to start several communication simultaneously– Source-Destination pair

• Allows source consume unlimited resources for different destinations

• However the last one seems the best trade-off

iUU

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Queuing Algorithm for Fairness

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Queuing Algorithms• Controls:

– Order of sending packets– Buffer usage

• Queuing Algorithms do not directly affect congestion, must be combined with intelligent queue manager

• Why not simple round robin?– Variable packet length can get more service by sending

bigger packets– Unfair instantaneous service rate

• What if arrive just before/after packet departs?

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Implementing MAX-MIN Fairness• Generalized processor sharing (GPS)

– Fluid fairness– Bitwise round robin among all queues– We assume we have a server which can service N bits

simultaneously from N flows.– Generally we have a variable keeps amount of service that

each flow has received since last busy period– Objective is equalizing this amount named as “potential”– Approaches differs in way updating potentials

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Implementing MAX-MIN Fairness• One way is defining “System Potential (Virtual Time)”

function– When a flow goes idle, after activation its potential is set

equal to value of system potential– Different approaches have different functions to maintain

System Potential

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GPS

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Packet by Packet GPS• In packet system GPS is not feasible

– Though, scheduler can not update potentials of all flows at the same rate

• One of the well known approaches is WFQ (Weighted Fair Queuing)

• Single flow: clock ticks when a bit is transmitted. For packet i:– Pi = length, Ai = arrival time, Si = begin transmit time, Fi =

finish transmit time– Fi = Si+Pi = max (Fi-1, Ai) + Pi

• Multiple flows:– Send packet with smallest Fi

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WFQ (Weighted Fair Queuing)

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1010

1010

Si Fi

q

iii WLSF

),( 1 ii FAiMAXS

Wq = 10

Wq = 1

Wq = 1

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Delay Allocation• Reduce delay for flows using less than fair share

– Advance finish times for sources whose queues drain temporarily

• Schedule based on Bi instead of Fi

– Fi = Pi + max (Fi-1, Ai) Bi = Pi + max (Fi-1, Ai - )

– If Ai < Fi-1, conversation is active and has no effect

– If Ai > Fi-1, conversation is inactive and determines how much history to take into account

• Infrequent senders do better when history is used

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PP GPS• Problems

– Needs Sorting at least among active sessions– System Potential Problem

• How we should store? With Time ? How about for variable bit rate servers?

• When we must update System Potential?– How we must assign newly arrived packet potential during a packet is

sending• System Potential is always increasing function of time

– If busy time lasts too long, System Potential maybe overflow– Is It fair enough??

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WFQ & GPS• Suppose a system from start

– Session 1 has 11 packets with weight 0.5– 10 other sessions has weight 0.05– All packets has 1 unit length– Our system services 1 packet at one time unit

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WFQ & GPS

WFQ

GPS

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WFQ & GPS• Impact on Link Sharing & Real Time Service

– Much larger delay bounds• Impact on Feedback-Based Algorithms

– Oscillation of RTT

The Most Exact Approximation : WF2Q

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WF2Q• selects packet among Eligible ones no from all of the

packets in all sessions (SEFF Policy).

• Eligible means set of packets which has started receiving service in GPS system at time t.

• We have one Start and Finish time for a session not for each packet

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WF2Q

GPS

WF2Q

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WF2Q

WFQ

WF2Q

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WF2Q & WF2Q+• Yes, we achieved better granulity but

– Now we have to sort twice !– How we should compute Virtual Time !– We should run GPS system in background !

• WF2Q+ : Has exactly the same properties with lighter complexity

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22th

itBiQWFQWFi

QWFStVtV

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Other Famous Variations• Self Clocked FQ (SCFQ)

– Calculating Virtual Time• V(t) is the finish time of the packet being serviced or was serviced

• Start Time FQ (STFQ)– Calculating Virtual Time

• During busy period, V(t) is the Si of the packet being serviced• At the end of busy period V(t) is Fi of the last packet was serviced

– SSF (Smallest Start time First)

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Other Simple Approaches• FIFO• Strict Priority

– Starvation– Just for important flow or determined rate flows

• RR– Simply do round robin– Do not consider packet lengths

• DRR (Deficit Round Robin)– Each Queue has a “quantum” based on its weight and a “Deficit

Counter”– In each round can send quantum bytes and the Deficit counter keeps

count of sent bytes and remained quantum– If the queue is empty Deficit counter will be set to zero

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Deficit Round Robin

50020064700

100

5121024

100100100

Quantum = 1500

DC = 700

DC = 500

DC = 400

√

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Deficit Round Robin

50020064700

5121024

Quantum = 1500

DC = 700

DC = 100

DC = 400

√

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Deficit Round Robin

50020064700

1024

Quantum = 1500

DC = 188

DC = 0

DC = 400

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Deficit Round Robin

50020064700

1024

Quantum = 1500

DC = 1688

DC = 0

DC = 1900

√

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DRR++

50020064700

100

5121024

100100100

QBE = 1500

DC = 700

DC = 500

DC = 400

QLC = 1000

500250

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DRR++• Advantages

– Reduces delay comparing with DRR– Reduces burst comparing with PQ

• BUT …– Sensitive to transient bursts

– Hard to configure (QLC)• Esp. for TCP traffic in the core!

– worst-case condition configuration