Packet Scheduling andQoS

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Packet Scheduling and QoS

EECS 489 Computer Networkshttp://www.eecs.umich.edu/~zmao/eecs489

Z. Morley MaoThursday Oct 14, 2004

Acknowledgement: Some slides taken from Kurose&Ross and Katz&Stoica

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Packet Scheduling

§ Decide when and what packet to send on output link- Usually implemented at output interface of a router

1

2

Scheduler

flow 1

flow 2

flow n

Classifier

Buffer management

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Goals of Packet Scheduling

§ Provide per flow/aggregate QoS guarantees in terms of delay and bandwidth

§ Provide per flow/aggregate protection§ Flow/Aggregate identified by a subset of following fields in

the packet header - source/destination IP address (32 bits)- source/destination port number (16 bits)- protocol type (8 bits)- type of service (8 bits)

§ Examples: - All packets from machine A to machine B- All packets from UMich- All packets between UMich and MIT - All TCP packets from EECS-UMich

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Outline

Ø QoS guarantees§ Link sharing§ Service curve (short)

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Recap: Token Bucket and Arrival Curve

§ Parameters- r – average rate, i.e., rate at which tokens fill the bucket- b – bucket depth- R – maximum link capacity or peak rate (optional parameter)

§ A bit is transmitted only when there is an available token§ Arrival curve – maximum number of bits transmitted within an

interval of time of size t

r bps

b bits

<= R bps

regulatortime

bits

b*R/(R-r)

slope R

slope r

Arrival curve

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How Is the Token Bucket Used?

§ Can be enforced by - End-hosts (e.g., cable modems)- Routers (e.g., ingress routers in a Diffserv domain)

§ Can be used to characterize the traffic sent by an end-host

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Source Traffic Characterization

§ Arrival curve – maximum amount of bits transmitted during an interval of time ?t

§ Use token bucket to bound the arrival curve

?t

bits

Arrival curve

time

bps

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Source Traffic Characterization: Example

§ Arrival curve – maximum amount of bits transmitted during an interval of time ?t

§ Use token bucket to bound the arrival curve

bitsArrival curve

time

bps

0 1 2 3 4 5

1

2

1 2 3 4 5

1

2

3

4

(R=2,b=1,r=1)

?t

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QoS Guarantees: Per-hop Reservation

§ End-host: specify- the arrival rate characterized by token-bucket with parameters (b,r,R)- the maximum maximum admissible delay D

§ Router: allocate bandwidth ra and buffer space Ba such that - no packet is dropped- no packet experiences a delay larger than D

bits

b*R/(R-r)

slope rArrival curve

DBa

slope ra

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Packet Scheduling

§ Make sure that at any time the flow receives at least the allocated rate ra

§ The canonical example of such scheduler: Weighted Fair Queueing (WFQ)

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Recap: Fair Queueing

§ Implements max-min fairness: each flow receives min(ri, f) , where

- ri – flow arrival rate- f – link fair rate (see next slide)

§ Weighted Fair Queueing (WFQ) – associate a weight with each flow

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Fair Rate Computation: Example 1

§ If link congested, compute f such that

Cfri

i =∑ ),min(

862

44

2

f = 4: min(8, 4) = 4min(6, 4) = 4min(2, 4) = 2

10

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Fair Rate Computation: Example 2

§ Associate a weight wi with each flow i§ If link congested, compute f such that

Cwfr ii

i =×∑ ),min(

862

44

2

f = 2: min(8, 2*3) = 6min(6, 2*1) = 2min(2, 2*1) = 2

10(w1 = 3)

(w2 = 1)

(w3 = 1)

If Sk wk <= C, flow i is guaranteed to be allocated a rate >= wi

Flow i is guaranteed to be allocated a rate >= wi*C/(Sk wk)

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Fluid Flow System

§ Flows can be served one bit at a time§ WFQ can be implemented using bit-by-bit weighted

round robin- During each round from each flow that has data to send, send a

number of bits equal to the flow’s weight

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Fluid Flow System: Example 1

1 2 3 4 5

1 2 3 4

1 2 31 2

43 4

55 6

5 6Flow 2(arrival traffic)

Flow 1(arrival traffic)

Servicein fluid flow

system

time

time

time (ms)

10

10

Packet inter-arrival time (ms)

50500Flow 2

1001000Flow 1

Rate (Kbps)

Packet Size (bits)100 KbpsFlow 1 (w1 = 1)

Flow 2 (w2 = 1)

0 10 20 30 40 50 60 70 80

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Fluid Flow System: Example 2

0 152 104 6 8

5 1 1 11 1

§ Red flow has packets backlogged between time 0 and 10

- Backlogged flow à flow’s queue not empty

§ Other flows have packets continuously backlogged

§ All packets have the same size

flows

link

weights

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Implementation In Packet System

§ Packet (Real) system: packet transmission cannot be preempted. Why?

§ Solution: serve packets in the order in which they would have finished being transmitted in the fluid flow system

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Packet System: Example 1

1 2 1 3 2 3 4 4 55 6Packetsystem time

1 2 31 2

43 4

55 6

Servicein fluid flow

system time (ms)

§ Select the first packet that finishes in the fluid flow system

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Packet System: Example 2

0 2 104 6 8

0 2 104 6 8

§ Select the first packet that finishes in the fluid flow system

Servicein fluid flow

system

Packetsystem

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Implementation Challenge

§ Need to compute the finish time of a packet in the fluid flow system…

§ … but the finish time may change as new packets arrive!

§ Need to update the finish times of all packets that are in service in the fluid flow system when a new packet arrives

- But this is very expensive; a high speed router may need to handle hundred of thousands of flows!

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Example

§ Four flows, each with weight 1

Flow 1

time

time

e

time

time

Flow 2

Flow 3

Flow 4

0 1 2 3

Finish times computed at time 0

time

time

Finish times re-computed at time e

0 1 2 3 4

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Solution: Virtual Time

§ Key Observation: while the finish times of packets may change when a new packet arrives, the order in which packets finish doesn’t!

- Only the order is important for scheduling

§ Solution: instead of the packet finish time maintain the number of rounds needed to send the remaining bits of the packet (virtual finishing time)

- Virtual finishing time doesn’t change when the packet arrives

§ System virtual time – index of the round in the bit-by-bit round robin scheme

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System Virtual Time: V(t)§ Measure service, instead of time§ V(t) slope – normalized rate at which every backlogged flow receives

service in the fluid flow system- C – link capacity- N(t) – total weight of backlogged flows in fluid flow system at time t

1 23

1 24

3 45

5 6Servicein fluid flow

systemtime

time

V(t)

)()(

tNC

ttV

=∂

∂

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System Virtual Time (V(t)): Example 1

§ V(t) increases inversely proportionally to the sum of the weights of the backlogged flows

1 2 31 2

43 4

55 6

Flow 2 (w2 = 1)

Flow 1 (w1 = 1)

time

time

C

C/2V(t)

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System Virtual Time: Example

0 4 128 16

w1 = 4

w2 = 1

w3 = 1

w4 = 1w5 = 1

C/4

C/8C/4V(t)

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Fair Queueing Implementation

§ Define- - virtual finishing time of packet k of flow i- - arrival time of packet k of flow i- - length of packet k of flow i

- wi – weight of flow i

§ The finishing time of packet k+1 of flow i is

kiL

kiak

iF

111 )),(max( +++ += ki

ki

ki

ki LFaVF / wi

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Properties of WFQ

§ Guarantee that any packet is transmitted within packet_lengt/link_capacity of its transmission time in the fluid flow system

- Can be used to provide guaranteed services

§ Achieve max-min fair allocation- Can be used to protect well-behaved flows against

malicious flows

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Outline

§ QoS guaranteesØ Link sharing§ Service curve (short)

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Hierarchical Link Sharing

§ Resource contention/sharing at different levels

§ Resource management policies should be set at different levels, by different entities

- Resource owner- Service providers- Organizations- Applications

Link

Provider 1

seminarvideo

Math

Stanford.Berkeley

Provider 2

WEB

155 Mbps

50 Mbps50 Mbps

10 Mbps20 Mbps

100 Mbps 55 Mbps

Campus

seminaraudio

EECS

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Hierarchical WFQ (H-WFQ) Example

4 1

1 11 1

§ Red session has packets backlogged at time 5

§ Other sessions have packets continuously backlogged

5

0 10 20

10

1

First red packet arrives at 5 …and it is served at 7.5

Servicein fluid flow

system

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Packet Approximation of H-WFQ

§ Idea 1- Select packet finishing first in

H-WFQ assuming there are no future arrivals

- Problem:• Finish order in system

dependent on future arrivals

• Virtual time implementation won’t work

§ Idea 2- Use a hierarchy of WFQ to

approximate H-WFQ

6 4

321

WFQ WFQ WFQ

WFQ WFQ

WFQ

10

Packetized H-WFQFluid Flow H-WFQ

6 4

321

WFQ WFQ WFQ

WFQ WFQ

WFQ

10

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Problems with Idea 1

§ The order of the 4th blue packet finish time and of the first green packet finish time changes as a result of a red packet arrival

Make decision here

Blue packet finish first

Green packet finish first4 1

1 11 15

10

1

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Problem with Idea 2

§ A packet on the second level can miss its deadline (finish time)

4 1

1 11 15

10

1

First red packet arrives at 5 …but it is served at 11 !

First level packet schedule

Second level packet schedule

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Solution

§ Hierarchical-WFQ with a better implementation of WFQ, called Worst-Case Weighted Fair Queueing (WF2Q)

§ Main idea of WF2Q- Consider for scheduling only eligible packets- Eligible packet at time t: a packet that has started being

serviced in the fluid flow system at time t

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Example

Fluid-Flow System

WFQ (smallest finish time first)

WF2Q (smallest eligible finish time first)

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Hierarchical-WF2Q Example

§ In WF2Q, all packets meet their deadlines modulo time to transmit a packet (at the line speed) at each level

4 1

1 11 15

10

1

First red packet arrives at 5 ..and it is served at 7

First level packet schedule

Second level packet schedule

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Outline

§ QoS guarantees§ Link sharingØ Service curve (short)

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Bandwidth-Delay Coupling

§ Assume- An arrival curve specified by token bucket (R, r, b) - A reservation request (rmin, D), where rmin – minimum bandwidth, D – maximum delay

§ WFQ can be inefficient: to satisfy delay D, the allocated rate ra may need to be much higher than r !

bits

b*R/(R-r)

slope rArrival curve

slope ra

D

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Example

§ End-host: request a worst-case delay D=10ms for a flow characterized by token bucket (R=1Mbps, b=10Kb, r=100Kbps)

§ Router: allocate rate ra, where

§ Thus, the router needs to allocate to the flow more than 5 times its average rate r !

ra = b*R / (D*(R-r) + b) = 0.52Mbps

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Solution: Service Curve

§ Generalize the service allocated to a flow- Assume a flow that is idle at time s and it is backlogged

during the interval (s, t)- Service curve: the minimum service received by the flow

during the interval (s, t)

t

S (t) = service curve

Arrival Curve

D

B

bits

WFQ implements a linear service curve

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Linear Service Curves: Example

t

bits

t

bits

t

Arrival traffic

t

bits

t t

bitsbits

bits

Service curves

Arrival process

Service

VideoFTP

t

Video packets have to wait after ftp packets

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Non-Linear Service Curves: Example

t

bits

t

bits

t

Arrival traffic

t

bits

t t

bitsbits

bits

Service curves

Arrival process

Service

t

Video FTP

Video packets transmittedas soon as they arrive

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What You Need to Know

§ Basic concepts- Arrival & service curve- Token-bucket specification- System virtual time / finish virtual time- WFQ properties - Link sharing requirements and challenges- Bandwidth-delay coupling problem

§ Mechanisms: - WFQ implementation in the fluid flow & packet system

§ You don’t need to know- Details of WF2Q- How service curve works