QoS Principle

7/27/2019 QoS Principle

1/103

HUAWEI TECHNOLOGIES CO., LTD.

www.huawei.com

HUAWEI Confidential

For internal use only

QoS Principle

ISSUE 1.0


2/103

HUAWEI TECHNOLOGIES CO., LTD. HUAWEI Confidentialpage2

By taking this course, we can

master the following topics:

Basic QoS concepts

Traffic policing & traffic shaping

Congestion control

Congestion avoidance


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Chapter 1 Basic QoS ConceptsCha

pter 1 Basic QoS Concepts

Chapter 2 Packet Classification & Marking

Chapter 3 Traffic Policing & Shaping

Chapter 4 Congestion Control & Avoidance


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Basic QoS Concepts

QoS stands for the quality of service. It refers to the expected

quality of subscriber service in case of packet loss, delay, jitter

and bandwidth during network communication.

Targets of IP QoS:

Avoid and control the IP network congestion.

Reduce the packet loss rate of the IP packet.

Adjust the traffic over the IP network.

Provide private bandwidth for specific subscribers or services.

Support real-time services on the IP network.


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Bandwidth Limit

IP

IP IP

IP IP

IP IP

IP

10 Mbps

10 Mbps

256 kbps

256 kbps512 kbps

512 kbps

100 Mbps

100 Mbps

BWmax= min(10M, 256k, 512k, 100M)=256 kbps

BWavail= BWmax/Flows


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End-to-End Delay

IP

IP

Propagation

Delay (P1)

Propagation

Delay (P1)

Processing and

Queuing Delay (Q1)

Processing and

Queuing Delay (Q1)

IP

IP IP

IP IP

IP

Propagation

Delay (P2)

Propagation

Delay (P2)

Processing and

Queuing Delay (Q2)

Processing and

Queuing Delay (Q2)

Propagation

Delay (P3)

Propagation

Delay (P3)

Processing and

Queuing Delay (Q3)

Processing and

Queuing Delay (Q3)

Delay = P1 + Q1 + P2 + Q2 + P3 + Q3 + P4 = X ms

Propagation

Delay (P4)

Propagation

Delay (P4)


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Delay

Processing delay

Queuing delay

Transit delay

IPIP IPIPIPIPIPIP

Forwarding

Processing Delay Queuing DelayTransit Delay

Bandwidth


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Packet Loss

Tail-drop occurs when the transmitting queue is overlong. It is themost common discarding way in case of link congestion.

There are many other discarding ways such as input queue drop,ignore, overrun, no buffer, and so on. These ways are used in caseof router congestion.

IPIP

Forwarding

IPIPIPIPIPIPIPIP

Tail-dropTail-drop


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How To Reduce Delay?

Expand the link bandwidth.

FIFO queuingIP TCP Data Fancy Queuing

Forward critical packets preferentially.

Compressthe Headers

cTCP Data

Compress the header of the IP packet.

Compressthe Payload

Compressed Packet

Compress the valid load of the Layer2 frame.

Priority Queuing (PQ)Custom Queuing (CQ)

Modified Deficit Round Robin (MDRR)Class-Based Weighted Fair Queing (CBWFQ)

Priority Queuing (PQ)Custom Queuing (CQ)

Modified Deficit Round Robin (MDRR)Class-Based Weighted Fair Queing (CBWFQ)

StackerPredictor

StackerPredictor

TCP Header CompressionRTP Header CompressionTCP Header CompressionRTP Header Compression


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How To Reduce the Packet Loss Rate?

Expand the link bandwidth.

FIFO queuingIPIP DataData Fancy Queuing

Provide adequate bandwidth for delay-sensitive services.

Dropper

Use the random discard mechanism to prevent congestion.

Custom Queuing (CQ)Modified Deficit Round Robin (MDRR)

Class-Based Weighted Fair Queuing (CBWFQ)

Custom Queuing (CQ)Modified Deficit Round Robin (MDRR)

Class-Based Weighted Fair Queuing (CBWFQ)

Weighted Random Early Detection (WRED)Weighted Random Early Detection (WRED)


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Mapping Between Services and QoSRequirements

Throughput Delay Loss Jitter

Interactive (e.g.,Telnet)

Batch (e.g.,

FTP)

Fragile (e.g,.SNA)

Voice

Low Low

Not important

Not important

High

Low

Low

Not important

None Not important

Low LowLow Low andpredictable

Low Low

Video Low LowHighLow and

predictable


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Different Effects of Various Services

Throughput Delay Loss Jitter

Gold

Silver

Bronze

Best Effort

Guaranteed Low

No guarantee

Low

Guaranteed

Low

No guarantee No guarantee

No guarantee No guarantee

No guarantee No guaranteeNo guarantee No guarantee

Guaranteedlimited

No guarantee


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Three IP QoS Models

Best-Effort: It is the default service model of Internet. Best-Effort

uses the first-in-first-out (FIFO) queue.

IntServ: In this model, the service applies to the network for

specific QoS service by sending signaling. Within the range of

traffic parameters, the network reserves resources to meet therequirement.

DiffServ: In case of network congestion, DiffServ controls traffic

based on services and forwards traffic to solve the congestion

according to the committed QoS levels of various services.


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Best-Effort

Best-Effort is a single and simple service model.

Application programs can transmit any packet in any case if they

are permitted or inform the network of the transmission.

Network tries everything to transmit the packets but does not

guarantee the delay and reliability.

Best-Effort service is the default service model of Internet. It uses

FIFO technology.


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IntServ

Provide controllable and end-to-end service for applications.

Network unit supports the QoS control mechanism.

Application programs apply to the network for specific QoS

service.

Signaling protocols place the QoS request on network.

Resource Reservation Protocol (RSVP) is the mainly used

signaling protocol.


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IntServ

Resource reservation

Admission control

requestrequest requestrequest requestrequest requestrequest

reservereservereservereservereservereservereservereserve

Local

Admission

Control

Local

Admission

Control

Local

Admission

Control

Local

Admission

Control

Policy Decision

Point (PDP)

Policy Enforcement

Point (PEP)

r

equest

r

equest

rep

ly

rep

ly

Remote Admission

Control

Remote Admission

Control


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Related Protocols

RSVP is a protocol for reserving resources. The receiver initiates RSVP to set

up the resource reservation for the multicast and unicast data traffic. RSVP is

used between the host and the network device (FRCs 2205 to 2215).

Common Open Policy Service (COPS) is a simple protocol at the application

layer. COPS uses the query/response mode. This protocol applies to the policy

exchange between the policy server and the policy client. In the COPS protocol,

policy server is also the policy decision point (PDP), such as SPS in SIG. Policy

client is also the policy enforcement point (PEP), such as the MA5200F. (RFCs

2748 to 2753)


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RSVP Application

RSVP is used to transmit voice or video data on the IP network through

application programs. In this case, the host serves as the initiator.

RSVP applies to the MPLS traffic engineering and sets up the MPLS/TE tunnel.In this case, the router serves as the initiator.


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IntServ Application

RSVP

Class of Service

or

Best Effort


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IntServ Application End-to-End

All Routers

WFQ applied per flowbased on RSVP requests

RSVP


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IntServ DiffServ Integration

Precedence

Classifier

Premium

Standard

Ingress Router RSVP protocol

Mapped to classes

Passed through to egress Backbone WRED applied based

on class

Egress Router RSVP protocol

sent on to destination WFQ applied to

manage egress flow

RSVP RSVP

WRED


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IntServ Advantages & Disadvantages

+ RSVP advantages

Simple to control (end-to-end)

Dynamic signaling port (such as H.323)

RSVP disadvantages

No expandability

Transmission problems of successive signaling caused by the network structure

configured with diversified attribute.


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Common Open Policy Service

Compared with RSVP, COPS has the following advantages:

Manage services in an integrated manner.

authorize and access the RSVP traffic in an integrated manner.

QoS-based RSVP solution has more expandability.


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DiffServ Model Structure

DiffServ

network

Subscriber

network

Traffic

control

SLA/TCA

Inner

node

Boundary

node

Boundary

node

Inner

nodeClassify services and

adjust traffic on the edge

of network.

- Service classification

. Based on the DS field

. Based on other features- Traffic adjustment

. Measure

. Mark

. Discard

. Shape

Various DS fields can have different PHB to provide diversified policies based on services.

Cross-regional services are offered between these DS fields by balancing SLA and TCA.

. Service Level Agreement (SLA): It refers to the expected quality of the service traffictransmitted over networks.

. Traffic Control Agreement (TCA): It is an agreement on the service classification rules,

service models, and the related processing.

PHB determines the policy

of the services provided

by the DS filed. DS nodes

forward packets according

to the PHB attribute.

Subscriber

network

Boundary

node

DiffServ

network

Boundary

node


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Differentiated Services Model

The Differentiated Services model describes the relationship

between the service and the traffic classification.

Classification and shaping of integrated traffic is completed on the

edge router.

No per-flow/per-application status exists at the core layer.

The core layer completes only simple per-hop behaviors during

the traffic aggregation.

Its aims at retractility.


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DSCP Field: 6 bits Unused: 2 bits

Former ToS Byte = New DS Field

Packet Header

DS code point : The DS node chooses corresponding PHB according to the DSCP value.

DS field: It contains the TOS of the IPV4 packet header or the traffic type field of IPV6

(defined by RFC2474).


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DSCP Code

Three combinations

xxxxx0: common

xxxx11: pilot or for local use

xxxx01: pilot or for local use (It can be also used as the formal

one)

Default DSCP: 000000

Default PHB: FIFO, tail-drop


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Chapter 1 Basic QoS Concepts

Chapter 2 Packet Classification & MarkingChapter 2 Packet Classification & Marking




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Packet Classification & Marking

Packet classification and marking is the foundation for

implementing the QoS service.

Packet classification uses the ACL and IP priority technologies.

Forward packets to other modules to process or mark (coloring)

based on the classification result for the classification on the core

network.

ACL , IP priority


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Chapter 3 Traffic Policing & ShapingChapter 3 Traffic Policing & Shaping



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Traffic Shaping and Policing

Classifier Marker Dropper

Meter

TrafficStream


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Application of Traffic Shaping orPolicing

Low-SpeedLink

High-SpeedLink

No congestion on theegress.

Queues and WREDdo not work.

No congestion on theegress.

Queues and WREDdo not work.

Congestion occurs onlinks because no

packet is intelligentlydiscarded at layer 2.

Congestion occurs onlinks because no

packet is intelligentlydiscarded at layer 2.

ServerFarm

WAN

Internet

F

astEthe

rne

t

256 kbps

64 kbps

128 kbps

The accessresources are limited.

The accessresources are limited.

Use TDM on a singlephysical link orlease links.

Use TDM on a singlephysical link orlease links.


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Traffic Shaping vs. Traffic Policing

Traffic Shaping

Traffic shaping does not discard packets.

Traffic shaping displays the congestion of frame relay communication.

Traffic Policing

Traffic policing supports the packet marking.

Traffic policing does not require Buffer to be added, whereas traffic

shaping requires additional queue system.


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How Does the Router Measure the TrafficSpeed?

Use the token bucket mechanism to record the arrival rate of packets.

The token bucket functions only on the condition of new packets.

The conforming traffic and exceeding traffic are returned.

Bandwidth

Time

Link Bandwidth

Rate Limit

Exceeding Traffic

Conforming Traffic


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700700200200

Token Bucket

500 bytes500 bytes 500 bytesConform Action


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200200

Token Bucket (cont.)

300 bytes300 bytes Exceed Action

300bytes


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Token Bucket

Bcrefers to the normal burst traffic.

Berefers to the excess burst traffic.

Bc + Be

Bc of tokens is added

every Tc [ms]

Tc= Bc / CIR

Time

LinkUtilization

Tc

2*Tc

3*Tc

4*Tc

5*Tc

Bc Bc Bc Bc Bc Bc

Link BW

Average BW(CIR)

Be


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Traffic Shaping & Policing Mechanism

Shaping

Generic traffic shaping (GTS)

Frame Relay traffic shaping (FRTS)

Class-based traffic shaping

Policing

Committed access rate (CAR)

Class-based traffic policing


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GTS

Traffic

Stream

Classifier Marker Shaper

Dropper

Meter


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GTS Model

Classifier

Classifier

Classifier

No

No

NoPhysical Interface

Queue(s)

ShapingWFQ

Yes

Yes

Yes

ShapingWFQ

ShapingWFQ

No

No

No

Yes

Yes

Yes

Forwarder


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GTS Implementation

Shaping

Queue(WFQ)

Shaping

Queue(WFQ)

SoftwareQueue(FIFO, PQ,

CQ, WFQ, ...)

SoftwareQueue(FIFO, PQ,

CQ, WFQ, ...)

Hardware

Queue(FIFO)

Hardware

Queue(FIFO)

Transmitpacketsbased on theconfiguredtraffic.


Transmitpacketsbased on theline rate.




If the hardware queue is

empty skip over it

directly.


empty skip over it

directly.

Software queue does not work if the configured shaping rate ismuch less than the link bandwidth.


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GTSExample #1

ISP expects to provide a service for subscribers. With this service, subscribers

can use the whole E1 line for 30s in case of burst, but the normal rate is 256

kbps.

GTS parameters

Bit rate: 256,000 output rate is 256,000 bps

Burst size32,000 the number of bits sent in 125 ms

Excess burst size: 61,440,000 = 2,048,000 x 30


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Core

Customer

GTSExample #1 (cont.)

interface ethernet0/0

qos gts cir 256000 cbs 32000 ebs 61440000

!

interface serial1/0


interface ethernet0/0


!

interface serial1/0


WAN


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Core

Customer

GTSExample #2

Subscribers hope that the WEB traffic does not exceed

64 kbps.

WAN

interface ethernet 0/0

qos gts acl 101 cir 64000

interface serial 1/0


!access-list 101 permit tcp any any eq www

interface ethernet 0/0




!access-list 101 permit tcp any any eq www


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CAR


Meter

Inboundor

Outbound


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CAR Can Be Configured Both on the Egress andIngress.

Inbound Classifier Marker Dropper

Meter

Outbound


Meter

Forwarding

Queuing


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CAR Implementation

Software queue does not work if the configured CAR value is

much less than the link bandwidth.

SoftwareQueue

(FIFO, PQ,CQ, WFQ, ...)

SoftwareQueue

(FIFO, PQ,CQ, WFQ, ...)

HardwareQueue

(FIFO)

HardwareQueue

(FIFO)

CARCAR








empty skip over it

directly.


empty skip over it

directly.


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CAR Mechanism

Class 1?Class 1?

Class 2?Class 2?

Class n?Class n?

CARCAR

CARCAR

CARCAR

continue

continue

transmit

transmit

transmit

drop

drop

drop

Output Queueor

Forward

The CAR mechanism has three behaviors:TransmitContinueDrop


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CAR Mechanism

MeterMeter

Conforms?Conforms?

Set IP Precedence?Set IP Precedence?

Set DSCP?Set DSCP?

Set MPLS Experimental?Set MPLS Experimental?

Set QoS group?Set QoS group?

Mark?Mark?

Transmit?Transmit?Yes / No

Set IP PrecedenceSet IP Precedence

Set DSCPSet DSCP

Set MPLS ExperimentalSet MPLS Experimental

Set QoS GroupSet QoS Group

Continue?Continue?

Drop?Drop?

Yes

Yes

Yes

No

No

Forwardor

Enqueue

Go toNext

CAR Command

Marking depends on the configured conform andexceed parameters.

Yes

Yes

Yes

Yes


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CAR: LimitingExample #1

ISP connects all customers through the 2M leased link (or

ADSL) by using CAR to limit the communication traffic.

Provide various services according to the customer demand.


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CAR: LimitingExample #1 (cont.)

ISPCustomer

Customer

2Mbps

2Mbps

Customer

2Mbp

s

NAP

Internet


qos car input cir 256000 cbs 4000 ebs 96000

green pass red discard

qos car output cir 256000 cbs 4000 ebs 96000








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CAR: Limiting and Marking Example#2

The WEB traffic is limited as 512 kbps and with high priority.

The priority of excess traffic is the same as that of normal traffic.

All traffic cannot exceed 256 kbps and should have the priority 0.

Excess traffic is discarded.

Normal burst traffic is 16000 bytes.

Extra burst traffic is 24000 bytes.


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CAR: Limiting and MarkingExample #2 (cont.)

ISPCustomer

2 Mbps

NAP

Internet


qos car input acl 101 cir 256000 cbs 4000 ebs 96000

green remark-prec-pass 1 red continue


green remark-prec-pass 0 red discard

qos car output acl 101 cir 512000 cbs 64000 ebs 128000




!

access-list 101 permit tcp any any eq www

access-list 101 permit tcp any eq www any


qos car input acl 101 cir 256000 cbs 4000 ebs 96000




qos car output acl 101 cir 512000 cbs 64000 ebs 128000




!

access-list 101 permit tcp any any eq www

access-list 101 permit tcp any eq www any


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Chapter 4 Congestion Control & AvoidanceChapter 4 Congestion Control & Avoidance


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Congestion Control

In case of network congestion, guarantee that the packets with different

priorities can get different QoS, including delay and bandwidth.

Place the packets with different priorities into different queues. Various

queues are guaranteed with different scheduling priorities, discarding

probabilities, and bandwidth.

Algorithm

First In First Out (FIFO)

Priority Queue (PQ)

Custom Queue (CQ)

Weighted Fair Queuing (WFQ)

Inbound

Outbound


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FIFO Advantages & Disadvantages

+ Advantages FIFO is simple and fast.

Support many devices manufactured by different vendors.

Support all HUAWEI VRP versions.

All switching channels support FIFO.

Disadvantages

Cannot assign bandwidth fairly.

Some traffic occupies bandwidth alone and make other traffic is discarded.

Cause jitter and some burst traffic fill in all queues.


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PQ

PQ uses 4 FIFO queues.

Priority Queuing SystemPriority Queuing System

HardwareQueuing System


High?High? Queue 1Queue 1

Pre-emptiveScheduler

Pre-emptiveScheduler InterfaceInterface

Forwarded Packets

Hardware QHardware Q

Tail-dropTail-drop

Medium?Medium? Queue 2Queue 2Tail-dropTail-drop

Normal?Normal? Queue 3Queue 3Tail-dropTail-drop

Low?Low? Queue 4Queue 4Tail-dropTail-drop


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PQ Classification

PQ classification supports the following options (for IP):

Source interface

IP access list (standard and extended)

Fragments

TCP source/destination port ID

UDP source/destination port ID


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PQ Classification

PQ also supports other protocol options:

Other protocol access lists

Packet length (longer or shorter than the specified length)

The following protocols support PQ:

IPX

CLNS DECnet

AppleTalk

VINES

DLSw


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PQ Insertion Policy

Each queue has its maximum capacity.

Before packets are placed into queues, the router queues these packets if

the queue is not full. Each class uses tail-drop.


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PQ Scheduling

Packet inHIGH

queue?

Packet inHIGH

queue?

Packet inMEDIUMqueue?

Packet inMEDIUMqueue?

Packet inNORMALqueue?

Packet in

NORMALqueue?

Packet inLOW

queue?

Packet inLOW

queue?


Yes

Yes

Yes

Yes

No

No

No

No

Dispatch packetand start checking the

HIGH queue again

Dispatch packetand start checking the

HIGH queue again


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PQ Advantages & Disadvantages

+ Advantages

Transmit the packets with high priority at short delay.

Support devices manufactured by different vendors.


Disadvantages

Each PQ class has the disadvantages of FIFO.

The queue with low priority is discarded when congestion

occurs in the queue with high priority.

Configure the classification manually on each hop.


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CQ

Custom Queuing SystemCustom Queuing System



Class 1?Class 1? Queue 1Queue 1

Round-Robin

Scheduler

Round-Robin

SchedulerInterfaceInterface

Forwarded Packets


Tail DropTail Drop

Class 2?Class 2? Queue 2Queue 2Tail DropTail Drop


CQ uses 16 FIFO queues for subscriber queues.


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CQ Flowchart



HardwareQueuing SystemClass 1?

Class 1? Queue 1Queue 1

Round-Robin

Scheduler

Round-Robin

Scheduler

Inter-faceInter-face

Forwarded Packets


Tail DropTail Drop






Custom queuing has Queue 0 forsystem and link-level messagesthat use pre-emptive scheduling.

Custom queuing has Queue 0 forsystem and link-level messagesthat use pre-emptive scheduling.

Queue 1 is the lowestcustom queue that is

serviced by the round-robin scheduler.

Queue 1 is the lowestcustom queue that is



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CQ Flowchart



HardwareQueuing SystemClass 1?

Class 1? Queue 1Queue 1

Round-Robin

Scheduler

Round-Robin

Scheduler

Inter-faceInter-face

Forwarded Packets


Tail DropTail Drop



Custom queues can beconfigured to use the pre-

emptive scheduler.

Queue 2 is now the lowestcustom queue that is






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CQ Classification

CQ classification covers the following options (for IP):

Source interface

IP access list (standard and extended)

Packet length (longer or shorter than the specified length)

Fragments

TCP source/destination port ID

UDP source/destination port ID

CQ classification is the same as the PQ classification.


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CQ Insertion Policy

Each queue has its maximum capacity.

Before packets are placed into queues, the router queues

these packets if the queue is not full. Each class uses tail-

drop.


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CQ Scheduling

CQ uses the round-robin scheduling.

Each queue uses the round-robin scheduling to transmit packets

according to the pre-configured threshold.

Packet inQueue N?

Packet inQueue N?


Yes

No

DispatchPacket

DispatchPacket

Is Queue Nover the

threshold?

Is Queue Nover the

threshold?

No

Next Queue(increase N)

Next Queue(increase N)

Yes


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CQ Scheduling

The threshold parameterbyte-count defines the maximum byte number in

each forwarding for queues.

The router allows to send out a complete packet, even though the byte

number exceeds the threshold.

150014991500

Threshold (byte-count) = 3000Threshold (byte-count) = 3000

Up to 4499 bytes can be forwarded in oneround in the worst case.

Up to 4499 bytes can be forwarded in oneround in the worst case.


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CQ Advantages & Disadvantages

+ Advantages

Guarantee the service for all queues, and no traffic is discarded.



Disadvantages

Each CQ class has the disadvantages of FIFO.

Configure the classification manually on each hop.

Bandwidth is assigned inaccurately.

Scheduling causes jitter.


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WFQ

Weighted Fair Queuing (WFQ) uses the traffic FIFO queue.

Weighted Fair Queuing SystemWeighted Fair Queuing System



Flow 1?Flow 1? Queue 1Queue 1

WFQScheduler

WFQScheduler InterfaceInterface

Forwarded Packets


Flow 2?Flow 2?

Queue 2Queue 2

Flow N?Flow N? Queue NQueue N

WFQ dropWFQ drop

WFQ dropWFQ drop

WFQ dropWFQ drop


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WFQ Tool

Parameter

Classification mechanism

Weighted fairness Each queue uses the improved tail-drop.


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WFQ Classification

IP TCP PayloadIP TCP Payload

Src.Addr.

Dest.Addr.

Protocol ToS Src.Port

Dest.Port

Hash AlgorithmHash Algorithm

#queue(index of the queue)

The packets of the same traffic are terminated in the same

queue.

The TOS field is the only changeable parameter. Changing thisparameter enables the packets of the same traffic to beterminated in different queues.

WFQ classification uses the

following parameters: Source IP address Destination IP address Source TCP/UDP port Destination TCP/UDP port Transport protocol Type of service (ToS)

WFQ classification uses the

following parameters: Source IP address Destination IP address Source TCP/UDP port Destination TCP/UDP port Transport protocol Type of service (ToS)

The HASH algorithm is used to provide

queue indexes when packets enter the

queue.

The HASH algorithm is used to provide

queue indexes when packets enter the

queue.

WFQ I ti & Di di


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WFQ Insertion & DiscardingMechanism

Hold-queue out limit (HQO) restricts the total number of packets that can be

placed in all queues.

Congestive discard threshold(CDT) limits the number of packets that can be

placed in each queue.

N is an SN given to the classified packets before the decision that whether the

packets enter the queues or are discarded.

N>CDT?N>CDT?N>HQO?N>HQO?

WorstFinishTime?

WorstFinishTime?

WorstFinishTime?

WorstFinishTime?

EnqueuePacket

EnqueuePacketNth PacketNth Packet

Drop the packet with theworst finish time (old) andenqueue the Nth packet

(new).

Drop the packet with theworst finish time (old) andenqueue the Nth packet

(new).

No No

Yes

Yes Yes

No

No

Yes

New

Old


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Case Study

WFQ can contain 10 packets at maximum (hold-queue limit =

10).

The early packet discarding (for the invasive packet to

bandwidth) should be enabled when eight packets arrive. And

eight packets is the threshold for discarding packets in case of

congestion.

C St d


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Case Study:Interface Congestion

HQO reaches its upper threshold 10. Newly arrived packets are discarded as

the TDM system places them at the end.

C St d


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Case Study:Interface Congestion

HQO reaches its upper threshold 10. Newly arrived packets are not discarded

as the TDM system does not place them at the end. Instead, the packets

placed at the end are discarded.

C St d


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Case Study:Traffic Congestion

CDT exceeds its upper limit 8. Newly arrived packets are discarded for the TDM

system places them at the end.

C St d


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Case Study:Traffic Congestion

CDT exceeds its upper limit 8. Newly arrived packets enter the queue as they

are not placed at the end.


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WFQ Discarding Mechanism

The packets that are placed in the empty sub-queue will not

be discarded forever.

The packet priority does not affect the discarding mechanism.


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WFQ Scheduling

Each packet are marked with the finish time in the TDM system.

WFQ scheduling mechanism sends the packets marked with the earliest

finish time out of the TDM system.

refer to the On the Efficient Implementation of Fair Queuing, written by Keshav and Berkeley in

1994.


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Weight

Flow with P=001

Flow with P=000

WFQ System (Real-Size Packets)

1

1

2

23

Flow with P=001

Flow with P=000

WFQ System (Virtual-Size Packets)

1

1

2

23

Precedence-1 packets

appear half the realsize.

Hardware FIFO Queue

123 12

34

3

Precedence-1 flow getstwice as muchbandwidth as

Precedence-0 flow.

Virtual Packet Size = Real Packet Size / (IP Precedence + 1)

Mapping Between IP Priority and


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Mapping Between IP Priority andWeight

1024 (virtual IP Precedence)

32 (virtual IP Precedence)

7

6

5

4

3

2

1

0

IP Precedence

4 (RSVP)

128 (PAK-Priority)

512

585

682

819

1024

1365

2048

4096

Weight


85/103


WFQ Advantages & Disadvantages

+ Advantages Be simple to configure.

Guarantee the transmission of all traffic.

Preferentially discard the packets that is invasive to bandwidth.



Disadvantages

Each WFQ class has all disadvantages of FIFO.

Multiple traffic is terminated in a queue.

Cannot configure classification.

Cannot guarantee a certain bandwidth.

Performance is damaged by complex classification and schedulingmechanisms.

Comparison Between Several Queues


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Comparison Between Several Queues

Weighted Fair Queuing Priority Queuing Custom Queuing

No queue lists

Low-volume trafficgiven priority

Conversationdispatching

Interactive trafficgets priority

Works well on speedsup to 2 Mbps

Enabled by default

4 queues

High-priority queueserviced first

Packet-by-packetdispatching

Critical traffic getsthrough

Designed forlow-bandwidth links

Must configure

16 queues

Round-robin service

Threshold dispatching

Proportional allocationof bandwidth

Designed formedium-speed links

Must configure


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Disadvantages of Tail-Drop

Simple tail dropping has significant disadvantages:

TCP synchronization

TCP starvation

High delay and jitter

Packet discarding without distinguishing them

Have no better way to provide the feedback of TCP traffic


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TCP Synchronization

Multiple TCP sessions begin at different time.

The TCP slide window increases.

Tail-drop causes the packet of multiple sessions lost at the same time.

Multiple TCP sessions restart at the same time (synchronization).

Flow A

Flow B

Flow C

Average linkuse


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TCP Starvation, Delay, and Jitter

Too long queue causes delay.

A large amount of invasive traffic causes other traffic discarded.

Frequent buffer changes cause jitter.

Discard packets without distinguishing them.

Prec.0

Prec.0

Prec.0

Prec.0

Prec.0

Prec.0

Prec.0

Prec.0

Prec.3

Prec.3 Queue

Packet of theinvasive traffic

Prec.3

Packet of thestaving traffic

Delay

If the interface congestion persists,

traffic suffers delay for a long time.

Prec.3

Prec.3

TCP has no

feedback

mechanism

when multiple

packets are

discarded.

Tail-drop is

regardless of

the IP priority.


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Random Early Detection

Random early detection (RED) discards packets at random before the queueis full.

Packets discarded by RED increase gradually with the queue expansion.

Influence given by RED :

The speed of the TCP session slows down and approaches to the linkbandwidth.

Compared with the maximum queue length, the average queue length isshortened.

The IP priority can be used to preferentially discard the packet that has lowpriority and is invasive to bandwidth.


91/103


RED Profile

Average

Queue

Size

DropProbability

10%

100%

20 40

MinimumThreshold

MaximumThreshold

MaximumDrop

Probability

No drop Random drop Full drop


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RED Modes

RED has the following modes:

No drop: It functions when the average queue length is between 0

and the lower threshold.

Random drop: It functions when the average queue length is

between the upper and the lower thresholds.

Full drop/tail drop: It functions when the average queue length is

equal to or bigger than the upper threshold.

RED can prevent congestion and tail-drop.


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Before RED

The TCP synchronization prevents the value of average link use from

approaching to the link bandwidth.

Tail drops slows down the TCP session start.

Flow A

Flow B

Flow C

Average linkuse


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After RED

The value of average link use gets close to the link bandwidth.

Random drops reduces the value of slide windows for the TCP session.

Average linkuse

Flow A

Flow B

Flow C


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WRED

Compared with RED, WRED introduces weight. Various weights

have different discarding policy.

Each discarding policy contains three RED parameters:

Lower threshold

Upper threshold

Upper discarding probability

Weight can be

IP Precedence (8 profiles)

DSCP (64 profiles)

At present, the WRED weight can be divided according to DSCP andthe IP priority. The probability of discarding the packet of low priority

is higher than that of the packet of high priority.


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Different RED Parameters

WRED profiles can be manually set. WRED has 8 default value sets for IP priority-based WRED.

WRED has 64 default value sets for DSCP-based WRED.

Average

Queue

Size

DropProbability

10%

100%

20 4010

Mapping Between the IP Priority and RED


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Mapping Between the IP Priority and REDParameters

AverageQueueSize

DropProbability

10%

100%

20 40

RSVP0 1 2 3 4 5 6 7

22 24 26 28 31 33 35 37

IP Precedence

DSCP-based WRED


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DSCP based WRED

(Expedited Forwarding)

Average

Queue

Size

Drop

Probability

10%

100%

20 40

EF

36

DSCP-based WRED


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DSCP based WRED

(Assured Forwarding)

Average

Queue

Size

Drop

Probability

10%

100%

20 40

Assured Forwarding High Drop

3224 28

Assured Forwarding Medium Drop

Assured Forwarding Low Drop

WRED M d l


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HUAWEI TECHNOLOGIES CO., LTD. HUAWEI Confidential page

WRED Model

IP PacketIP Packet WREDWRED

Calculating the

average queuelength

Calculating the

average queuelength

FIFO QueueFIFO Queue

Select the

WRED

profile

Select the

WRED

profile

Current

Queue

Size

IP Precedence

or

DSCP

Minimum Threshold

Maximum Threshold

Queue

Full?

Queue

Full?

No

Yes

Tail DropRandom Drop

WRED P li


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WRED Policy

IPIPPriorityPriority MeaningMeaning

00 High-dropHigh-drop,, bestbest--effort trafficeffort traffic

Low-dropLow-drop,, best-effort trafficbest-effort traffic11

33 Premium traffic in the contractPremium traffic in the contract

22 Premium traffic outside of the contractPremium traffic outside of the contract

44 UnusedUnused

55 VoiceVoice overover IPIP

66 Routing protocol trafficRouting protocol traffic

77 Routing protocol trafficRouting protocol traffic

WRED E l


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WRED Examples

P

acketDiscard

Probability

Average

Queue Size

0.1

RSVP1

51

02

02

530

35

37

Priority 2

Priority 0

Priority 3

Priority 1

VoIP

Routing


103/103

Thank you

www.huawei.com

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