Lecture5 - IP QoS

8/3/2019 Lecture5 - IP QoS

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Prasad Kularatne



Objective

We will define and understand the motivation behindimplementation of Quality of Service in IP Networks

We will discuss the concepts and implementation of IPQuality of Service

QoS implementation

Understand the concepts of classification, marking, handling

of congestion and traffic conditioning i.e. policing andshaping

Understand the DiffServ architecture

Deployment



What is IP QoS?

A set of techniques to manage

How fast do I need to transfer the data? [rate/Bandwidth]

How long will it take for the data packets to arrive at the destination?[latency/Delay]

Whether or not data packets arrive at the destination with differing

delays? [jitter/Delay variation]

How much of data packets will be dropped in transit? [loss]

in an IP Network



How to identify different traffic types?

BandwidthOver-subscriptionPacket dropsPackets in error

Latency Speed of lightSerialization delay Forwarding process

Jitter

Over-subscriptionOutput Buffering

LossCongestion

What is therequired

bandwidth?

How latency sensitive the

traffic is?

Howtolerant thetraffic is for

packet loss?

How jitter

sensitive thetraffic is?

How to define thecharacteristics of differenttraffic types



Traffic Characteristics

Source: Introduction to QoS tools and Design, Cisco Systems Inc.



Motivation for QoS

Best effort is not good enough, why?

Converged Networks: different traffic types ride on thesame network

This is good: why? Different traffic profiles




Why do we need QoS?

Networks are converging… No separate networks for telephony, video and data anymore

Efforts are under way to converge Local Area Networks (LAN) andStorage Area Networks (SAN’s) – Converged Enhanced Ethernet

Limited bandwidth and buffer space existing in thenetworks Over-subscribed links

More traffic types competing for the same set of resources

Without QoS? Best Effort - unpredictable



Why implement QoS? [Application view]

Different applications generate different traffic types(classes) with distinct characteristics

Application <-> Traffic types (traffic class)

Different traffic classes have different demands forbandwidth, delay, jitter and losses

I want more bandwidth!, I cannot withstand jitter!, I

cannot tolerate losses! etc.

Conclusion: Different applications require differenttreatment by the network for better performance



Why implement QoS? [Business view]

Different applications have different businesspriorities at different times

1. Web browsing can tolerate delays2. I do not want scavenger traffic in my network3. During morning I should be able to issue invoices to my

customers as fast as possible4. My ERP should give fast response during production

hours5. I want my nightly network backup to run faster?6. I am OK to have good video quality in my evening

conferences with partners



So, expectations from QoS are…

Mechanism to classify different traffic types

Framework to let the external networks know about the

classification

Mechanisms to define and propagate a set of rules on

how to treat these different classes

Implementation of differential treatment mechanisms



When is QoS most effective?

Time

Time

100%

Time

LinkUtilization • Link over-provisioned

• May not be cost effective

• No QoS is required.

• May provide a safety net

100%

LinkUtilization

100%

LinkUtilization

• Transient congestion

• QoS most useful

• Link highly over-subscribed

• QoS somewhat useful but morebandwidth required



QoS implementation

First classify traffic into distinct classes

Based on traffic profile

Based on business priorities

Tell the external networks about your classification Through signaling (IntServ)

Through marking of each packet (DiffServ)

Apply the QoS policy Congestion Management and Congestion Avoidance

Traffic conditioning

Drop packets belonging to traffic class X (Police traffic)

Delay packets belonging to traffic class Y (Shape traffic)



QoS implementation


Handle congestionClassify & MarkConform to

traffic contract



Classification

Based on a marker if marked at the source

E.g. IP telephones marks DSCP code point at source

Based on IP Access Lists Source and Destination IP or IP ranges

Destination port number (type of application layerservice)

Network Based Application Recognition ( NBAR)

Does deep packet inspection on the first packet of a flow

Cisco proprietary



Marking

Layer-2

User priority bits in the IEEE 802.1q tag field in theEthernet header

Inherent in architecture: ATM MPLS

Inferred from 3-bit EXP field in the MPLS header

Layer-3

InterServ: Implementation is RSVP (Resource ReservationProtocol).

Make an end-to-end reservation before transfer

DiffServ: Via the Type of Service (TOS) field in IP header



Congestion Management

Controls the network congestion once it occurs

While honoring the QoS policy

Sort the traffic into multiple queues decide how toservice each of these queues based on QoS policy

Priority Queuing (PQ)

Custom Queuing (CQ)

Weighted Fair Queuing (WFQ)



Congestion Avoidance

Monitors and anticipates network congestion and triesto avoid it by dropping packets

While honoring the QoS policy

Implement algorithms to drop low priority packets andallow high priority packets to flow

WRED (Weighted Random Early Detection) algorithmis the widely used algorithm in deciding which packetsto drop



Traffic Conditioning

Attempts to condition the traffic behavior of the outputlink so that is falls in line with the QoS policy Detect misbehaviors and drop (policing) or delay (shaping) packets

from those misbehaving traffic flows

Policing attempts to control the traffic bursts by droppingselected packets to ensure certain traffic types get thedemanded QoS parameters as per the QoS policy

Shaping limits transmission rates (delays via buffering) of low priority traffic types in an attempt to smooth out theoutgoing traffic flow



Policing vs. Shaping

Rate limitingwithout buffering

Packets will bedropped

Rate limiting with

buffering Packets will be

delayed or may drop




Differentiated Services (DiffServ)

Classification via a set of bits in the Type of Service (TOS)Field in the IP header Each class of traffic with same bit setting in the header (a traffic

aggregate) will be treated same

DiffServ term: Behavior Aggregate

Two important components DSCP: The bit setting in the first six bits in the TOS field which

indicates the desired behavior within the network Apply the desired behavior for each behavior aggregate in the core

DSCP is an index to a set of pre-defined Per Hop Behaviors (PHB)

PHB’s are derived by an ISP based on its SLA with its customers



Differentiated Services Code Point

Bits in the IP header that distinguish behavior

aggregates are called DiffServ Code Point (DSCP) 6-bits used




DiffServ architecture

DiffServ domain: Your Intranet or an ISP’s managed domain

Ingress Routers provide the entrance to a DiffServ domain

Looks at traffic flows (e.g. FTP traffic for customer #1) Police and/or shape traffic

Write/Re-write the DSCP in the IP header

Core routers

Only look at traffic aggregates (Collection of f lows with same DSCP)

For each DSCP look up and apply the relevant Per Hop Behavior (PHB)

Edge:worries about

each flowCore:worries about

an aggregate

of flows



DiffServ Routers

Classifier Meter Policer/

ShaperMarker

DiffServ EdgeRouter

ExtractDSCP

Localconditions

PHBPHB

PHBPHB

Select PHB

Packettreatment

DiffServ Core

Router



Sample Per Hop Behaviors (PHB’s)




General QoS Design principles

What is my organization objective ?

Protect voice/video traffic

Interactive response for my ERP is business-critical

Carefully select as few applications as possible inbusiness-critical category

How many classes do I need to fully meet my organizational objectives

More the classes more closely can I achieve my objectives

Executive engagement for the defined QoS objectivesbefore design



How many classes do I need?

Deploying Cisco QoS for Enterprise Networks, Cisco System Inc.



Deployment methodology

Start with what business objective am I to achieve viaQoS?

Business priorities <-> Treatment of different traffic classes

What service-levels does my business demand fromeach traffic class?

Design and test QoS policies

Role out the tested design Continuous Monitoring to ensure business priorities

are adhered to

Lecture5 - IP QoS

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