Quality Of Service - An Introduction€¦ · (and largely predictable) delay, and elastic traffic that can usually be queued without difficulty Cisco Networking Academy ... Quality

1Cisco Networking Academy Conference 2007 - QOS

Quality of Service An Introduction

Mike Weaver


Session Objectives

The new CCNP-ONT course introduces the concept of providing a Quality of Service framework in a converged IP (packet switched) networkThis session aims to introduce you to some of the fundamental components associated with this technology, so that..at the completion of this session, you will be able to:-

Explain the necessity for Quality of Service (QoS) in converged networks and describe and define some of the fundamental terms associated with incorporating QoS (e.g. bandwidth, delay, packet loss, queues, queuing, etc.)


Quality of Service

Beyond best effort what is QoS?

QoS is a term used to refer to technologies used to minimise some of the negative effects associated with congestion in a packet switched networkPacket switched networks rely on queue (or buffer) availability at all ingress and egress points of network connected devices to smooth the bursty nature of computer generated trafficThese buffers and the associated queues were the essential components of packet networks that allowed a far better performance for this type of traffic than traditional circuit switched networks


Quality of Service

Dealing with congested networks the problems

However, if these networks experience congestion at any node, they must queue traffic packets until there is sufficient capacity to allow onward transmission, leading to variable delays in deliveryIn early switched networks, all traffic was treated equally, regardless of its degree of elasticityDevice buffers are finite, leading to packet loss if a buffer (queue) is full and cannot accept further packetsIn addition, queue length grows exponentially as the packet arrival rate approaches the transmission rate


Quality of Service

Dealing with congested networks the solutions

There are two fundamental techniques that can be used to attempt to alleviate these congestion problems in a converged IP network

1. Over provision the network. If there is no congestion, then essentially there are no

problems!

2. Deploy QoS in the network, differentiating between inelastic traffic which must be delivered with minimal (and largely predictable) delay, and elastic traffic that can usually be queued without difficulty


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A joint approach

In reality, both approaches over provisioning and configuration of network devices to incorporate QoS techniques are often combined in modern networksQoS should generally be deployed in those parts of the network that are permanently or occasionally overloadedIt is common that peripheral parts of a network are overloaded, and the backbone is over-provisioned


Quality of Service

Delay in a perfect network

1 2 3 4 5 6

1 2 3 4 5 6

d1 d2 d3

t1

t1

t2

t2

time

time

d1=d2=d3


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Delay in a practical network

1 2 3 4 5 6

1 2 34 6

d1 d2 d3

T1

T1'

time

time

d6

d1 d2 d3 d6T1 T1'


Review Questions

The Practical Network

Why are the delays different?What are some of the causes of packet loss?What effects will overall delay have on a traffic flow? Voice call? Streaming media? FTP traffic? Telnet traffic?What about variations in delay within the same flow?How can this variation be handled?What is delay a function of?


Quality of Service

End to end packet delivery delay

It can be seen that total packet delivery delay is a random valueThere will always be a minimum delay due to various physical characteristics of the network signal propagation, serialisation etc the End to End Delay


Quality of Service

End to End Packet Delivery Delay

`

Processing and Queuing

Delay Q1


Delay Q2


Delay Q3

Propagation and Serialisation Delay

P2


P4


P3


P1

End/End Delay = (P1 + Q1 + P2 + Q2 + P3 + Q3 + P4) ms


Quality of Service

Delay of packet delivery

There will also be a variable component due to additional buffering; queuing, etc and the amount of congestion within the networkTo analyse the delay, we need to create a distribution histogram a method that will allow us to compute the delay variability within the transmission of a very large number of packets ( )


Quality of Service

Delay histogram Delay Distribution

0

50

100

150

200

250

0-24 25 -34

35 -44

45 -54

55 -64

65 -74

75 -84

85 -94

Delay (mS)

# of

pac

kets

The greater the number of packets measured, the more precisely the histogram reflects network delay

For example, using this data allows us to state that the probability of packets having a delay < 54mS is 60%


Quality of Service

Delay distribution density function

End to end delay for a typical connection smoothed over an infinite number of

packets


Review Questions

Delay characteristics

Are delay characteristics important in an under-provisioned network that includes both elastic and inelastic traffic?If so, which characteristics?Why does this need to be a statistical analysis?


Quality of Service

BandwidthTransmission rates can be measured between any two interface elements in a networkHowever, the network bandwidth is always equal to the path that has minimum bandwidthTo increase bandwidth, it is necessary to deal with the slowest elements, or bottlenecks


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QoS Techniques

Now that we are aware of some of the factors to be considered in analysing network congestion, we can investigate how congestion problems can best be minimised for traffic that requires predictable characteristics.

..also called inelastic trafficProviding a known, or predefined, specific quality of service to inelastic packet streams is an essential tool in modern converged networks


Quality of Service

QoS Support

In most converged networks, there will be multiple information flows at any given point in timeEach flow will need servicing according to some prerequisite QoS requirementAs a minimum, each flow will be serviced by two queues at each network device

A switch process queue (input buffer)An output interface queue (output buffer)

To ensure that the required QoS can be achieved, it is essential that the utilisation coefficient of each resource serving the specific flow does not exceed the predefined value


Quality of Service

Maintaining utilisation < 90% for elastic traffic is considered congestion control and has been refined over many yearsTo achieve a lower utilisation for inelastic traffic requires a separate queue for each resource at the output

Utilisation vs delay

0 0.2 0.4 0.6 0.8 1

Ws

We

0.9


Quality of Service

Queuing Models

Queue(Waiting Area) Link (Server)

Departures

Arrivals

Single FIFO Queue

Arrivals

High Priority Queue(Waiting Area)

Low Priority Queue(Waiting Area)

Link (Server)

Departures

Classify

Priority Queuing Model


Quality of Service

First In First Out (FIFO) queue

This is the traditional queue or buffer also sometimes called First Come First Served (FCFS)Packets are placed into a single common queue in the order they arrive and retrieved in the same orderThis has generally been the default queuing algorithm used in packet switching devices, but it affords no opportunity to differentiate between different types of information flowAll traffic types are treated equally


Quality of Service

Priority QueuingA priority queuing mechanism divides traffic into a small number of classes, and assigns some priority characteristic to each class before placing the assigned packets in separate queuesEach traffic class (queue) can then be treated according to this characteristicThe actual mechanism used to classify the traffic is a separate processIf there are packets in the higher priority queues then they are serviced before the lower priority queues using a scheduling algorithm


Quality of Service

Priority Queuing

If the traffic flow assigned a high priority is a small component of the overall traffic say a single VoIPconversation this approach works wellIt will nearly always provide the high priority flow the bandwidth it requires with minimal delayLower priority (elastic) traffic will hardly be impacted, as the delay whilst the (relatively small) VoIP flow is serviced is largely transparent


Quality of Service

Priority Queuing

High


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Weighted Queuing aka Custom Queuing

An alternative to using a priority queue is to weight the queues such that they guarantee some minimum percentage of the available bandwidth to different classes of traffic when there is network congestionThis is similar to priority queuing except each queue is allocated a percentage of available bandwidth rather than a specific priority


Quality of Service

Weighted queuing


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Weighted queuing

The queues are serviced in a round robin fashionwithin a specific time periodAssume the servicing cycle is 1s. Then the queues in the diagram would be activated for 150ms; 150ms; 400ms and 300msIf the output interface b/w is 100Mb/s, then 15Mb of data would be retrieved from the first queue during its service interval; 40Mb from the third queue etcEach queue receives guaranteed minimum b/w, which in most cases is more acceptable than suppressing low priority traffic


Quality of Service

Weighted queuing

Note that since the data is retrieved as packets, and not as bits, the actual value of allocated bandwidth will varyThe variation will be a function of the queue cycle time a longer time means that the actual bandwidth tends towards the theoretical valuebut longer cycle times mean longer delays in transmission


Quality of Service

Weighted queuing

When a weighted queuing algorithm is used, the utilisation coefficient for a traffic class significantly influences the delay and delay variation for that class of packetsFor example, if 10% of the bandwidth of 100Mb/s is allocated to the queue, and the average rate of flow is 3Mb/s, then the utilisation coefficient is 3/10 = 0.3Delays would be insignificant at such a value. However, if the flow was 9Mb/s, the queue would grow significantly!


Quality of Service

Weighted Fair Queuing

A modified approach to weighted queuing is to weight each queue equally, thus ensuring that each queue is treated equally or fairlyNote that in both methods, if a queue is empty at its scheduled retrieval time, then it is omitted and the time available is distributed between all other queues according to each queues weightingFlow based weighted fair queuing is one of Ciscos premier queuing techniques * and is the default on most serial interfaces at or below E1* CCNP-ONT Curriculum 3.2.2


Quality of Service

Priority vs Weighted Queuing

Priority queuing ensure minimum delays for traffic of the highest priorityIt does not provide any guarantees for lower priority traffic flowsWeighted queuing guarantees a mean traffic rate, but doesnt provide any guarantees about delayHowever, all flows get a guaranteed rate


Quality of Service

Hybrid queuing

To attempt to find some middle ground between the two extremes of priority vs weighted queuing, hybrid schemes have been developedThe most popular scheme uses a single priority queue for the highest priority traffic, and weights each of the remaining queues (Cisco call this class based weighted fair queuing or CBWFQ)The priority queue is used for real-time traffic (i.e. voice) and the remaining queues for other traffic of varying degrees of elasticity according to needObviously, there must be a mechanism to limit priority traffic from consuming all available bandwidth!


Questions

Queuing

If a router is managing two flows, one with packets of 500 bytes and the other with packets of 1000 bytes, what is the effect on bandwidth of a simple fair queue?What can be done to change this?Would a system that incorporates fair queuing have a longer overall delay than one that didnt? Why?


Quality of Service

Queue management

Using queuing algorithms is in response to congestion that is already apparent on the network congestion control mechanismsHowever, QoS techniques also incorporate methods that attempt to predict and prevent network congestion congestion avoidance mechanismsThe aim of congestion avoidance is to prevent congestion, since it is better to transmit data at a lower rate without loss, than at a higher rate and lose packets during periods of congestion


Quality of Service

Queue management

Such mechanisms can be viewed as control processesthat act on an inherently unstable (open loop) system, attempting to achieve a degree of stabilityThe primary Cisco QoS tool used for congestion avoidance is known as Weighted Random Early Detection (WRED)


Quality of Service

Weighted Random Early Detection

WRED is a process by which packets are discarded randomly and earlier (i.e. before they would have been if the queue buffer became full) as congestion begins to increase at a network bottleneckThe dropping of packets will cause transport protocols such as TCP (but NOT UDP) to throttle back the rate at which packets are launched into the network, (hopefully!) leading to an easing of the congestionThe weighting in this process refers to preferential traffic handling for high priority packets, allowing lower priority packets to be discarded first


Quality of Service

Classification

If packets or more correctly, flows are to be identified for placing in different queues, then it is necessary to have a method of classification


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Questions

How much QoS is there currently on the Internet?What is the major issue with QoS on a public network?How should traffic be classified?How do we handle UDP avoidance?What is best to drop voice packets or ftp packets?


Quality of Service

QoS Summary

This presentation has looked at some of the fundamental components and issues associated with QoSThe techniques described are primarily ones that are used on a hop by hop basisThere are other methods not discussed here that attempt to provide guarantees from end to endHowever, remember that currently! the Internet is a best effort network so there is no guarantee the mechanisms will work across public links


Quality of Service

QoS Summary

And a timely last word on the arguments between those who suggest that more Internet bandwidth is the answer vs. those who suggest that traffic should be differentiated.


Quality of Service

QoS Summary

We dont need no reservationWe dont need admission controlAll applications must be adaptiveThe Net works fine, so leave it aloneHey! Professor! Leave the Net alone!

All we want is just flat rate pricing for allAll we want is just flat rate pricing for all..

We dont need no traffic managementOverprovision bandwidth for allThe only true god is TCP/IPThe Net isnt broken, so leave it aloneHey! Professor! Leave the Net alone!

All we want is just flat rate pricing for allAll we want is just flat rate pricing for all.

(With thanks to Keshav, S; An Engineering Approach to Computer Networking; Addison-Wesley; 1997 and sung to Pink Floyds The Wall)


Quality of Service

Questions?

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Quality Of Service - An Introduction€¦ · (and largely predictable) delay, and elastic traffic that can usually be queued without difficulty Cisco Networking Academy ... Quality