20120112_cht_tl_教育訓練 day7

© 2011 Juniper Networks, Inc. All rights reserved. | www.juniper.net

中華電信研究所網路技術基礎訓練課程

Day 7

CoS/QoS(Class of Service)/(Quality of Service)

Johnson [email protected] Jan.12 , 2012


CoS Overview

© 2011 Juniper Networks, Inc. All rights reserved. www.juniper.net | 3

Class of Service Overview

What is CoS?• Ability to treat packets differently as they transit a

network device• End-to-end mechanism

• Does not pertain to only a single network device

Why CoS?• Convergence of voice and data networks• Differentiation between applications• Guaranteed bandwidth


CoS Defined


CoS for Converged Networks

Identify traffic and its requirements

Divide traffic into classes

Define CoS policies for each class


CoS is Unidirecrional

CoS configuration is unidirectional• You must explicitly configure settings in both

directions

CoS Domain

Traffic flow

C1

C2

PE1

PE2 P1

P2fe-1/1/1

fe-1/1/1 fe-2/0/1fe-2/0/2

fe-0/0/3

fe-0/0/2

fe-1/0/2

fe-3/0/2

fe-1/0/0

fe-0/0/0

ingress

egress

Traffic flow

ingress

egress


A Historic Perspective

Circuit-switched networks• Designed around service levels needed for

telephony• Connection-oriented; one user per connection

• Suitable for real-time, loss-sensitive applications• Low (fixed) delay• Blocking of new connections during congestion

CoS is not required in the historic environment• The network is purpose-built to support

application (telephony) requirements

VoiceVoice

PSTN


Network Advances

Packet-switched networks• Developed to optimize efficiency for machine-to-

machine communications• Multiple users share a connection• Unexpected delays and loss during congestion

CoS is still not applicable• The network is purpose-built to support

application requirements• Applications do not require CoS

(Expand bandwidth to solve congestion issue when needed)

Data


Network Convergence Drives CoS

Convergence drives the need for CoS• Multiple applications supported over a common

network infrastructure• Traffic from specific applications must be recognized

and treated accordingly• Special handling is necessary to ensure that unique

applications perform as expected in the face of congestion or queuing delays

• User bandwidth usage must be controlled• IP is the convergence technology of choice

Video

Data

Voice


Definition of CoS Parameters

CoS parameters• Bandwidth: End-to-end information carrying

capacity• Delay: End-to-end delay for information delivery• Jitter(Delay variation): Variation in end-to-end

delays caused by packet queuing• Loss: Percentage of packets not delivered, usually

related to congestion Network CoS parameters affect a user’s

perception of performance


Three Models for Quality of Service

• Best-Effort: No QoS is applied to packets. • IntServ: Applications signal to the network that they require special QoS.

• DiffServ: The network recognizes classes that require special QoS.


Best-Effort Model

It will get there when it gets there.

• Internet initially based on a best-effort packet delivery service

• The default mode for all traffic

• No differentiation between types of traffic

• Like using standard mail


IntServ Model

• Some applications have special bandwidth or delay requirements or both

• IntServ introduced to guarantee a predictable behavior of the network for these applications

• Guaranteed delivery: no other traffic can use reserved bandwidth

• Like having your own private courier plane

It will be there by 10:30 a.m.


DiffServ Model

• Network traffic identified by class

• Network QoS policy enforces differentiated treatment of traffic classes

• You choose level of service for each traffic class

• Like using a package delivery service

Do you want overnight delivery?

Do you want two-day air delivery?

Do you want three- to seven-day ground delivery?


A Brief History of IP CoS—IntServ

Integrated Services(InterServ):• IETF’s first attempt at extending IP for other than

best-effort services• RSVP signaling used to describe specific CoS

requirements to the network(Host-to-Host)• Routers reserve resources across the network• Resembled a circuit-switched call setup

• Never deployed…• Scalability issues


A Brief History of IP CoSType-of-Service Field

Type-of-Service(TOS) byte in the IP header• Defined in RFC 791• IP Precedence(0~7) field to prioritize discards

• Industry supported precedence bits to minimize loss of network-control packets

IP Precedence Reserved

Bits0 1 2 4 5 6 73

D T R

MSB LSB


DiffServ architecture

DiffServ architecture:• Defined in RFCs 2474 and 2475• Redefined the IPv4 ToS field to support a 6-bit

DiffServ Code Point (DSCP)• DiffServ has no signaling component

• Operates on hop-by-hop basis

------------------------------------ DSCP ------------------------------------

------- ECN -------

0 1 2 4 5 6 73 BitsMSB LSB


DiffServ Terminology (1 of 2)

Key DiffServ terms:• IP Precedence

• 3-bit value (0~7)• Original IPv4 ToS byte

• DSCP• 6-bit value (0~63)• This is the CoS value for a packet

• Behavior Aggregate (BA)• Classification based on DSCP• Packets with a common DSCP belong to the same BA


DSCP Encoding

• DiffServ field: The IP version 4 header ToS octet or the IPv6 traffic class octet, when interpreted in conformance with the definition given in RFC 2474

• DSCP: The first six bits of the DiffServ field, used to select a PHB (forwarding and queuing method)


DiffServ Terminology (2 of 2)

Key DiffServ terms (contd.):• Per-Hop Behavior (PHB)

• Forwarding treatment associated with a given BA• Packets with the same DSCP value have the same PHB

• PHB group• A set of one or more PHBs with related forwarding

behavior • Example: assured forwarding (AF) is a PHB group,

consisting of multiple PHBs : AF1, AF2, AF3, and AF4


Per-Hop Behavior

Per Hop Behavior (PHB) • description of the externally observable forwarding

behavior of a DS node applied to a the set of packets with the same DSCP

• PHB may be defined in terms of • DS nodes resources priority relative to other PHBs• observable traffic characteristics (delay, loss, …)

PHBs are defined as “black box”• does NOT mandate particular implementation

mechanisms !

Different boxes implement PHBs in different ways which are optimised for each platform


Per-Hop Behaviors

DSCP selects PHB throughout the network.• Default (FIFO, Tail drop)• EF (Expedited Forwarding)• AF (Assured Forwarding)• CS (Class-Selector) mapping to IPP(IP

Precedence)


Per-Hop Behaviors (Cont.)

• Each AF class uses three DSCP values.• Each AF class is independently forwarded with

its guaranteed bandwidth.


Standardized DiffServ PHBs (1 of 2) Expedited Forwarding(EF)

• Designed to provide for low loss, low delay, and low jitter services

• Example: Voice• Recommended code point: 101110(usually

mapped to IPP(5)) Assured Forwarding(AF)

• Primarily concerned with controlling packet loss• Four classes: AF1, AF2, AF3, and AF4• Each class supports three drop probabilities; for

example, AF11 (low), AF12 (medium), and AF13 (high)AF 11/12/13

AF 21/22/23

AF 31/32/33

AF 41/42/43

Low 001010 010010 011010 100010

Medium 001100 010100 011100 100100

High 001110 010110 011110 100110


Standardized DiffServ PHBs (2 of 2)

Class Selector(CS)• Provide IP precedence compatibility• Typically used for network control traffic

Best effort is not specifically defined• Best effort is the default PHB


EF PHB Definition

RFC 2598 Expedited Forwarding PHB, June 99

EF PHB can be used to build a low loss, low latency, low jitter, assured bandwidth, end-to-end service – targets VoIP, Virtual Leased Lines– Assured traffic sees no (or very small)

queues/delay– Constraint: at every transit node, the

aggregate’s max arrival rate is less than the aggregate min departure rate

An example of how EF can be implemented is a Priority Queue (with rate limit).

Recommended DSCP=101110 (46)


AF PHB Group Definition

RFC2597, Assured Forwarding PHB Group, June 99

Assured Forwarding (AF) PHB Group is meant to offer different levels of forwarding assurances for IP packets received from a customer DS domain

Olympic Service (Gold, Silver, Bronze)• gold (C1) >= silver (C2) >= bronze (C3)

No quantifiable timing requirements!• delay or delay variation


Recommended DSCPs

IANA maintains a list of recommended DSCPs• Based on RFC recommendations for defined PHBs

Name DSCP

CS0 000000 (0)

CS1 001000 (8)

CS2 010000 (16)

CS3 011000 (24)

CS4 100000 (32)

CS5 101000 (40)

CS6 110000 (48)

CS7 111000 (56)

Name DSCP

AF11 001010 (10)

AF12 001100 (12)

AF13 001110 (14)

AF21 010010 (18)

AF22 010100 (20)

AF23 010110 (22)

AF31 011010 (26)

AF32 011100 (28)

AF33 011110 (30)

AF41 100010 (34)

AF42 100100 (36)

AF43 100110 (38)

EF 101110 (46)


• IEEE specification

• 802.1P user priority field

• Supports up to 8 classes of service

• Focuses on support for QoS over LANs and 802.1Q ports

Classification and Marking at the Data-Link Layer—Ethernet 802.1Q Class of Service


• Frame Relay DTE devices can set the DE bit of a frame so that if the network becomes congested, Frame Relay devices will discard frames with the DE bit set before discarding those that do not have the DE bit set.

• Preserved throughout the Frame Relay network.

Classification and Marking at the Data-Link Layer—Frame Relay/ATM QoS

• The CLP bit indicates that the cell should be discarded if it encounters congestion as it moves through the network.

• Preserved throughout the ATM network.


• MPLS uses a 32-bit label field (shim header) which is inserted between Layer 2 and Layer 3 headers (frame mode).

• Supports up to 8 classes of service.

• The IP precedence or DSCP field is not directly visible to MPLS label switch routers.

• In most casen, routers will copy the three most significant bits of the DSCP or the IP precedence of the IP packet to the EXP field.

• Preserved throughout the MPLS network.

Classification and Marking at the Data-Link Layer—MPLS Experimental Bits


Comparison

Best-Effort Diffserv Intserv

Service •Connectivity•No isolation•No guarantees

•Per aggregation isolation•Per aggregation guarantee

•Per flow isolation•Per flow guarantee

Service Scope

End-to-end Domain End-to-end

Complexity No set-up Long term setup Per flow setup

Scalability •Highly scalable•(nodes maintain only routing state)

Scalable (edge routers maintains per aggregate state; core routers per class state)

Not scalable (each router maintains per flow state)


CoS Processing Stages


What is Class of Service ?

CoS provides mechanisms for categorizing traffic and meeting performance requirements within a network

Note: CoS does not make a network faster or reduce congestion!

Packet B Packet CPacket A

Voice

Packet A

Video

Packet B

Data

Packet C


CoS components:• Traffic classification• Policing• Queuing• Scheduling• Rewrite rules

CoS Components


ShaperScheduler

Overview of CoS Processing

Forwarding Class

BA Classifier

Policing (Ingress)

Rewrite

Marker

Multifield

Classifier

Fabric

Ingress

Egress

Forwarding Policy

Policing (Egress)

Multifield Classifier

Loss Priority

RED


Meeting Performance Requirements

CoS meets a network’s performance requirements by:• Prioritizing latency-sensitive traffic such as VoIP• Controlling congestion to ensure service level

agreement(SLA) maintenance• Allocating bandwidth for different classes of traffic

Devices should treat traffic consistently throughout the entire network

Voice

Data

VoIP

Data

VoIP

Data

Voice

Data


Forwarding Classes

Forwarding classes:• Identify traffic that should receive common

treatment• Used to assign traffic to output queues

Packet B Packet CPacket A

Voice

Packet A

Video

Packet B

Data

Packet C

Queue 2

Queue 1

Queue 0

Forwarding Class Output Queue


Loss Priority

Loss priority:• Identifies the priority a system should give to

dropping a packet• Used to select the drop profile used in the

RED(Random Early Detection) process

If congestion exists…

Drop first

Data

VoIP

Voice

Data


JUNOS CoS Defaults

4 Queues Assigned to 4 Forwarding classes(up to 8 Queues/16 Forwarding classes)

In a default configuration, input BA classification is performed by the ipprec-compatibility table

Queue

Forwarding Class PriorityTransmit

RateDrop

Profile

0 Best-effort (BE) Low 95% Tail drop

1 Expedited-Forwarding (EF)

– –

2 Assured-Forwarding (AF)

– –

3 Network-Control (NC) Low 5% Tail drop


Typical CoS Processing Stages

Egress

Policing orRate Limit

Classify

Class 0

Class 1

Class 2

Class 3

Class n

IngressTo

Fabric

Scheduling and

Prioritization

Queue 0

(Class 0)

Queue 1(Class 1)

Queue 2(Class 2)

Queue 3

(Class 3)

HeaderRewrite

From Fabric

Input Processing

Output Processing

Congestion Control (WRED)

The order of stages can vary slightly across Junos

devices.


Traffic Classification

Classifiers map traffic to a forwarding class at ingress• Can match on existing CoS values

• BA classification• Can match on protocol, port, addresses, and so

forth• Multifield classification

• Support for IP precedence, DSCP (IPv4 and IPv6), MPLS EXP, and IEEE 802.1p

Classifier

Basic Service(BE)

Premium Service(EF)

Control Traffic(NC)

Packet APacket BPacket C

NC: Network control class


Policing

Policing limits traffic volume and burstiness• Enforces and protects CoS SLAs• Excess traffic can be marked or discarded• Functions at ingress, egress, or both

Policer within MF Classifier

InterfacePolicer

InterfacePolicer

Ingress Interface

Egress Interface

MF: Multifield


Policing vs Shaping

• These mechanisms must classify packets before policing or shaping the traffic rate.

• Shaping queues excess packets to stay within the desired traffic rate.

• Policing typically drops or marks excess traffic to stay within a traffic rate limit.


Policing vs. Shaping

• Out-of-profile packets are dropped

• Dropping causes TCP retransmits

• Less buffer usage (shaping requires an additional shaping queuing system)

• Out-of-profile packets are queued until a buffer gets full

• Buffering minimizes TCP retransmits

• Shaping supports interaction with Frame Relay congestion indication


CoS and Forwarding Policy

Policy can select the forwarding next hop for traffic associated with a particular forwarding class• CoS-Based Forwarding (CBF)

BE packetEF packetBE pack

et

EF packet

CBF in place at R2 for the BE

forwarding class

R1 R2 R4

R3


Schedulers

Schedulers define the prioritization properties of forwarding classes (queues):• Transmission rate

• Guaranteed and maximum rates

• Queue priority• Support for five priority levels

• Delay buffer• Storage space for traffic bursts

• Congestion management and avoidance• Support for RED for equal, random dropping of traffic• Support for WRED for weighted, preferred dropping of

traffic


First In First Out(FIFO)

• First packet in is first packet out• Simplest of all• One queue• All individual queues are FIFO


Priority Queuing(PQ)

• Uses multiple queues• Allows prioritization• Always empties first

queue before going to the next queue:– Empty Queue 1– If Queue 1 empty, then

dispatch one packet from Queue 2

– If both Queue 1 and Queue 2 empty, then dispatch one packet from Queue 3

• Queues 2 and 3 may “starve”


Round Robin

• Uses multiple queues• No prioritization• Dispatches one packet from each queue in each round– One packet from Queue 1– One packet from Queue 2– One packet from Queue 3– Then repeat


Weighted Round Robin(WRR)

• Allows prioritization• Assign a “weight” to each queue

• Dispatches packets from each queue proportionally to an assigned weight:– Dispatch up to 4 from

Queue 1– Dispatch up to 2 from

Queue 2– Dispatch 1 from Queue 3– Go back to Queue 1


RED Modes

• RED has three modes:• No drop: When the average queue size is

between 0 and the minimum threshold• Random drop: When the average queue size is

between the minimum and the maximum threshold

• Full drop (tail drop): When the average queue size is at maximum threshold or above

• Random drop should prevent congestion (prevent tail drops)


Weighted RED

Packet

Dro

p

Pro

bab

ilit

y

Queue Length

“Slope” is adjustable

(Random drop)

Queue Max

Packet

Dro

p

Pro

bab

ilit

y

Queue Length Queue Max

Packet

Dro

p

Pro

bab

ilit

y

Queue Length

Standard Service

Queue Max

WithoutRED

WithRED

WithWRED

Premium Service

Std. Min.

Prem. Min.


Standard Premium

Weighted Random Early Detection(WRED)

Upon congestion, packets from lower precedence are selectively discarded first

Minimize the congestion impact on higher precedence services


TCP Traffic Before RED

• TCP synchronization prevents average link utilization close to the link bandwidth.

• Tail drops cause TCP sessions to go into slow-start.


TCP Traffic After RED

• Average link utilization is much closer to link bandwidth.

• Random drops cause TCP sessions to reduce window sizes.


Scheduling Overview

Components of scheduling:• Priority• Transmission rate• Buffer size• RED configuration

Defines the order in which packets transmitDefines the storage and dropping of packets

Scheduler Map

Queue 0 Queue 1 Queue 2 Queue 3

Dat

aD

at

aD

at

aD

at

a


Queue Priority

Queues receive service according to their assigned priority; common priorities include:• Strict-High(might starve low priority queue)• High• Medium high• Medium low• Low

Sch

ed

ule

r M

ap

Queue 0(L)

Queue 1(ML)

Queue 2(MH)

Queue 3(H)

462715

15

7

2

46

Data

Serviced first

Serviced last

Queue 4(SH)

3

3


Rewrite Markers

The packet header rewrite sets CoS values for outbound traffic• Can be used by BA classification in downstream

nodes• Support for IP precedence, DSCP (IPv4 and IPv6),

MPLS EXP, and IEEE 802.1p

Packet

DSCP = 0001001

The inbound classifier assigns a packet to

forwarding class

Packet

DSCP = 000000

Rewrite sets the packet’s DSCP coding based on the forwarding

class


Review of CoS Processing

ShaperScheduler

Forwarding Class

BA Classifi

er

Policing (Ingres

s)

Rewrite

Marker

Multifield

Classifier

Fabric

Ingress

Egress

Forwarding

Policy

Policing

(Egress)

Multifield

Classifier

Loss Priority

RED

Reference:JUNOS CoS Configurtion Guide (JUNOS 11.4)

http://www.juniper.net/techpubs/en_US/junos/information-products/topic-collections/config-guide-cos/config-guide-cos.pdf





20120112_cht_tl_教育訓練 day7

Documents

average queue

key diffserv

standardized

require special

packet drop

de bit set

rate limit

link bandwidth