Network Management Quality of Service I

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Network Management

Quality of Service I

Patrick J. Stockreisser

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Lecture Outline

Basic Network Management (Recap)

Introduction to QoS

Packet Switched Networks (Recap)

Common QoS approaches

Best Effort Service

(Integrated Service)

(Differentiated Service)

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Network Management

Network management means different things

to different people.

Network management is the execution of a

set of functions required for controlling,

planning, allocating, deploying, coordinating,

and monitoring the resources of a network.

Generally it is a service that employs a

variety of tools, applications, and devices to

assist human network managers in

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History

Early 1980s

 Great expansion in the area of network deployment.

 Cost benefits

 Productivity Gains

By mid-1980s,

 Growing difficulties in management of the networks

 A result of deploying many different (and sometimes incompatible) network technologies

By late-1990s

 Wireless technologies emerge (standards agreed)

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Network Management

Automation

The problems associated with network expansion

affect both day-to-day network operation

management and strategic network growth planning.

Each new network technology requires its own set of

experts.

In the early 1980s, the staffing requirements alone for

managing large, heterogeneous networks created a

crisis for many organizations.

An urgent need arose for automated network

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Some Network Management

Functions

Security:

Ensuring that the network is protected from

unauthorized users.

Performance:

Eliminating bottlenecks in the network.

Reliability:

Making sure the network is reliable to users and

responding to hardware and software malfunctions.

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Quality of Service

In computer networking, the traffic engineering

term Quality of Service (QoS) refers to

control mechanisms that can provide different

priority to different users or data flows, or

guarantee a certain level of performance to a

data flow in accordance with requests from

the application program.

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An Analogy for

Quality of Service

Consider two competing cargo airlines (A and B)

operating out of New York JFK airport with a service

to London Heathrow.

 Both airlines have the same aircraft, both airlines charge the

same rate per package shipped

 Both airlines offer seven flights a week.

There is little to differentiate the service offered by

these two companies.

These competing airlines offer the same throughput

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Same Service? Same QoS?

Imagine this:

 Airline A offers one flight per day for each day of the week

 Airline B offers all of its seven flights on a single day of the week.

So while both airlines provide the same throughput capacity over the weekly period, they differ greatly in the actual service provided.

 Depending on your delivery needs these different airline service models

will succeed or fail in quite dramatic fashion.

 Eg: an online real-time mail order business  Eg: stock supply for a for a large warehouse

 Obviously, the business requirements for delivery define which service

model works best.

 Real-time (daily) demand needs a very regular and consistent service

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Some other QoS definitions

“QoS is described in terms of a set of user perceived

characteristics of the performance of a service. It is

expressed in user-understandable language and

manifests itself as a number of parameters, all of

which have either a subjective or objective values”

“QoS refers to the capability of a network to provide

better service to selected network traffic over various

technologies”

“QoS is the collective effect of service performance

which determine the degree of satisfaction of a user

of the service”

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The simplest of networks

Packets are sent between nodes.

Network

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Network Basics

 Computers running on the

Internet communicate to each other using either the

Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP)

 When you write programs that

communicate over a network, you are programming at the application layer. Typically, you don't need to concern yourself with the TCP and UDP layers.

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Packet Review

 A network breaks a message into parts of a certain size in bytes.  These are the packets

 Each packet carries the information that will help it get to its

destination:

 the sender's IP address,

 the intended receiver's IP address,

 something that tells the network how many packets this e-mail

message has been broken into and

 the number of this particular packet.

 The packets carry the data in the protocols that the Internet

uses:

 Transmission Control Protocol/Internet Protocol (TCP/IP).

 Each packet contains part of the body of your message. A typical

packet contains perhaps 1,000 or 1,500 bytes.

(Packet) (frame) (block) (cell)

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Packet Sending

Each packet is then sent off to its destination

by the best available route

A route that might be taken by all the other

packets in the message or by none of the

other packets in the message.

This makes the network more efficient.

Load balancing

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Packet Structure

 Most packets are split into three parts:

1. Header - The header contains instructions about the data carried by the packet. These

instructions may include:

Length of packet (some networks have fixed-length packets, while others rely on the header to

contain this information)

Synchronization (a few bits that help the packet match up to the network) Packet number (which packet this is in a sequence of packets)

Protocol (on networks that carry multiple types of information, the protocol defines what type of

packet is being transmitted: e-mail, Web page, streaming video)

Destination address (where the packet is going) Originating address (where the packet came from)

2. Payload - Also called the body or data of a packet. This is the actual data that the packet is

delivering to the destination. If a packet is fixed-length, then the payload may be padded with blank information to make it the right size.

3. Trailer - The trailer, sometimes called the footer, typically contains a couple of bits that tell the

receiving device that it has reached the end of the packet. It may also have some type of error checking. The most common error checking used in packets is Cyclic Redundancy Check (CRC). CRC is pretty neat. Here is how it works in certain computer networks: It takes the sum of all the 1s in the payload and adds them together. The result is stored as a hexadecimal value in the trailer. The receiving device adds up the 1s in the payload and compares the result to the value stored in the trailer. If the values match, the packet is good. But if the values do not match, the receiving device sends a request to the originating device to resend the packet.

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Network Problems

Dropped packets

The routers might fail to deliver (drop) some packets if they arrive when their buffers are already full. Some, none, or all of the packets might be dropped, depending on the state of the network, and it is impossible to determine what will happen in advance. The receiving application must ask for this information to be

retransmitted, possibly causing severe delays in the overall transmission.

Delay

It might take a long time for a packet to reach its destination, because it gets held up in long queues, or takes a less direct route to avoid congestion. Alternatively, it might follow a fast, direct route. Thus delay is very unpredictable.

Jitter

Packets from source will reach the destination with different delays. This variation in delay is known as jitter and can seriously affect the quality of streaming audio and/or video.

Out-of-order delivery

When a collection of related packets is routed through the Internet, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order to the one with which they were sent. This problem necessitates special additional protocols responsible for

rearranging out-of-order packets to an isochronous state once they reach their destination. This is

especially important for video and VoIP streams where quality is dramatically impacted by both latency or lack of isochronicity.

Error

Sometimes packets are misdirected, or combined together, or corrupted, while en route. The receiver has to detect this and, just as if the packet was dropped, ask the sender to repeat itself.

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Internet Problems

The Internet today does not make any promises

about QoS an application will receive.

 An application will receive whatever level of performance

(e.g. end-to-end packet delay and loss) that the network is able to provide at that moment.

Delay-sensitive multimedia applications cannot

request any special treatment.

 All packets are treated equal at the routers, including

delay-sensitive audio and video packets.

Network congestion (or interfering traffic) can

severally limit the performance of an application

 (especially audio-video streaming, Multimedia, VoIP and

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QoS Question

Hence, what new architectural components

can be added to the Internet architecture to

shield an application from such congestion

and thus make high-quality networked

multimedia applications a reality?

Guarantees for:

high bandwidth

low latency

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Common QoS Approaches

Best Effort Services

(no guarantees)

The network does not provide any guarantees that

data is delivered or that a user is given a guaranteed

quality of service level or a certain priority.

Integrated Services

(resource reservation)

The network resources are assigned according to the

application QoS request and subject to the bandwidth

management policy

Differentiated Services

(prioritisation)

Network traffic is classified and network elements

give preferential treatment to classifications identified

having more demanding requirements.

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Best Effort

 In a best effort network all users obtain best effort service  Each service obtains variable bit rate and delay.

 Removing features such as recovery of lost or corrupted data and

pre allocation of resources, the network operates more efficiently, and the network nodes are inexpensive.

 Application sends data whenever it feels like, as much as it feels like

without requiring any permission.

 Network elements try their best to deliver the packets to the

destination without any bounds on delay, latency, jitter, etc.

 Network elements can give up to deliver without informing either the

sender or the receiver.

 Conventional IP routers only provide best-effort service. The

simplicity of routers is a key factor why IP has been much more successful than more complex protocols such as X.25 and ATM.

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Best-Effort

Post Office Analogy

 The post office service delivers letters using a best effort delivery

approach.

 The delivery of a certain letter is not scheduled in advance

 no resources are pre allocated by the post office

 The postman will make his "best effort" to try to deliver a

message but may be delayed if:

 All of a sudden, too many letters arrive at the post office  The postal address is incomplete

 The postman’s van breaks down

 The sender is not informed if a letter has been delivered

successfully.

 However, the sender can pay extra for a delivery confirmation

receipt,

 This requires that the carrier get a signature from the recipient to

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Scenario Example:

Consider two hosts H1, H2

sending packets via router R1

to R2, and subsequently to

hosts H3 and H4

Let us assume the LAN speeds

are significantly higher than 1.5

Mbps, and focus on the output

queue of router R1;

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Packet Delay

• Packet delay and packet loss will occur if the

aggregate sending rate of the H1 and H2 exceeds 1.5

Mbps.

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QoS: Four Principles

To tackle the problems which may arise in such

scenarios we will look at 4 QoS Principles:

Packet Classification

Isolation Scheduling and Policing

High Resource Utilisation

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Another Problem

 A 1 Mbps audio application (e.g. a

CD-quality audio call) shares the 1.5 Mbps link between R1 and R2 with an FTP application that is

transferring a file from H2 to H4

 In the best-effort Internet, the audio

and FTP packets are mixed in the output queue R1 and (typically) transmitted in a first-in-first-out (FIFO) order

 Burst of packets from FTP source

could potentially fill up the queue, causing IP audio packets to be

excessively delayed or lost to buffer overflow at R1

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Packet Marking

A solution is to give priority to audio packets,

as FTP does not have timing constraints –

hence the notion of distinguishing the types

of packets via Traffic Class field in IPv6.

Principle 1:

Packet marking allows a router to distinguish among

packets belonging to different classes or traffic.

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Another Scenario

Imagine the FTP user has

purchased “platinum service”

(i.e. high priced) Internet

access from its ISP, while the

audio user has purchased a

cheap, low-budget Internet

service.

Should the cheap user’s audio

packets be given priority over

FTP packets in this case?

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Packet Classification

 A more reasonable solution is to distinguish packets on the basis of

the sender’s IP address.

 More generally, we see that it is necessary for a router to classify

packets according to some criteria.

 A router must be able to distinguish between packets according to a

“policy” decision

 One way to achieve this is through marking the packets, however,

this does not mandate that a certain QoS will be given.

Principle 1 (new):

Packet classification allows router to distinguish among

packets belonging to different classes of traffic.

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Another Scenario

Suppose the outer knows it should give priority to

packets from the 1 Mbps audio application

 Since outgoing link is 1.5 Mbps, FTP packets receive lower

priority, they will still, on average, receive 0.5 Mbps of transmission service

Suppose audio applications starts transmitting at

greater than 1.5 Mbps (link capacity)

 This may lead to starvation of FTP packets

Similarly for multiple audio applications sharing a link

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Flow Isolation

There is a need for a degree of isolation

among flows, in order to protect one flow

from another misbehaving flow

Principle 2:

It is desirable to provide a degree of isolation among

traffic flows, so that one flow is not adversely affected by

another misbehaving flow

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Policing

 Policing Mechanism: A monitoring

(policing) mechanism put in place to ensure that traffic flows meet some pre-defined criteria

 If a policed application misbehaves, the

policing mechanism will take some action e.g., drop or delay packets that are in

violation of the criteria so that the traffic actually entering the network conforms to the criteria

 Packet classification and marking

mechanism (principle 1) and the policing mechanism (principle 2) are co-located at the “edge” of the network, either in the end system, or at an edge route.

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Bandwidth Enforcement

Traffic isolation can also be

achieved by the link level protocol

providing fixed bandwidth to each

application flow

 Audio - 1 Mbps  FTP - 0.5 Mbps

Here, audio and FTP flows see a

logical link with capacity 1.0 and

0.5 Mbps, respectively

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Enforcement Issues

 When bandwidth is enforced, a given flow cannot use bandwidth

not being used by another application (it can only use a maximum of its own limit)

 For example, if the audio flow goes silent (e.g., if the speaker

pauses and generates no audio packets), the FTP flow would still not be able to transmit more than 0.5 Mbps over the R1-to-R2 link – this is clearly wasteful

Principle 3:

While providing isolation among flows, it is desirable to

use resources (e.g., link bandwidth and buffers) as

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Another Example

 Consider two competing applications

transmitting at 1 Mbps, with a link capacity (R1-to-R2) of 1.5 Mbps

 In this case the combined rate for the two

applications is 2 Mbps (higher than link capacity)

 No marking, isolation or classification will

help solve this problem

 Each app gets 0.75 Mbps of link (half of

link capacity)

 Each app gets 25% packet loss

 This quality is unacceptable

 So its better not to transmit any packet at

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QoS Guarantees

The network should provide the minimum

quality of service to enable an application to

run, or block the application – example

call-blocking on a telephone network (where

end-to-end quality of service is necessary)

Hence, in the previous case either the

minimum QoS is guaranteed, or the

application is stopped, as it would not be

usable.

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QoS Requirements

 Implicit with the need to provide a guaranteed QoS to a flow is

the need for the flow to declare its QoS requirements

 This process of having a flow declare its QoS requirement, and

then having the network either accept the flow (at the required QoS) or block the flow (because the resources needed to meet the declared QoS requirements can not be provided) is referred to as the call admission process

Principle 4:

A call admission process is needed in which flows declare

their QoS requirements and are then either admitted to

the network (at the required QoS) or blocked from the

network (if required QoS can not be provided by the

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Scheduling and Policing

Mechanisms

 Packets from various sources are multiplexed together and

queue for transmission at the output buffer of a link

 A Link Scheduling Policy determines how these packets are then

selected for transmission

 The plays an important role in providing QoS guarantees

 First-In-First-Out: Packets arriving at link output queue are

buffered if link is busy transmitting. If not sufficient buffering space, then invoke a Packet Discarding Policy.

 In FIFO policy, packet departure from buffer is based on time of

arrival – first packet to arrive is the first to leave

 Packet Discarding Policy: determines whether packets will be

dropped (lost) when queue is full – can be based on removing already buffered packets

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Scheduling Priority

and Round Robin

 Priority: packets arriving at output link are classified into one of two

more priority classes – priority value is based on information carried in packet header (such as the Traffic Class field in IPv6)

 A different queue is maintained for each priority class – with the

highest priority queue given preference when transmitting packets.

 Round Robin: assumes existence of queues, but a round robin

scheduler alternated service between classes. A “work-conserving” scheduler based on the round-robin strategy will keep the link busy, always checking for low priority (class) packets when the high

priority (class) queue is empty

 Weighted Fair Queuing (WFQ): similar to round robin, except that

each class may receive a differentiated amount of service in any interval of time. Each class i is assigned a weight wi

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Weighted Fair Queuing

In WFQ, during any interval of time, if there are class i

packets to send, class i will be guaranteed to receive a

fraction of service equal to:

w

i

/Σw

i

where the sum in the denominator is taken over all

classes that also have packets queued for transmission

Alternatively, we can say that with WFQ queues, a link

with transmission rate R, class i will always achieve a

throughput of at least

R x w

i

/ Σ w

i

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Policing

 Policing is used to regulate the rate at which packets can be

inserted into a network

 An important part of a QoS architecture

 Three important policing criteria:

Average rate: limit the long-term average rate (packets per time

period) at which a flow’s packets can be sent into the network. Must determine time interval over which the average value is calculated. For instance, average rate of 100 packets/sec is more constrained than 6000 packets/minute

Peak rate: constrains the maximum number of packets that can

be sent over relatively short period of time (compared to average rate)

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Leaky Bucket

 A Leaky Bucket algorithm can be used to

characterise these policing limits

It consists of a “bucket” that can hold up to b

tokens – which determines the burst size

New tokens added to bucket at r tokens/sec

 if bucket is full, a newly generated token is

ignored

 Max number of packets that can enter the

network within any time interval t, is rt+b

 Can use multiple Leaky buckets in series

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Leaky Bucket Analogy

The task: to maintain a high level of channel utilization in the router, while restricting the dropping probability to 1% or below

The task: to maintain a high water level in the bucket, while restricting the overflow rate to 1% or below

Dropped Packets (terminated in the middle of a packet flow, service very annoying*)

Water drops overflowing

Hand over packets (switching from a neighbouring wireless port)

Water from an extra pipe

Blocked Packets (terminated before the service starts)

Water drops prevented from entering the bucket by the tap

Packets Sending Completed Water drops leaking from the bottom

New Packet to Send Water drops from the tap

Bandwidth Capacity Bucket size

Wireless networks Leaky bucket

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Lecture Review

In this lecture we have:

Looked at the primary principles behind QoS

Looked at the existing best effort approach

A brief look at scheduling and policing

techniques

Round Robin

Leaky Bucket

Figure

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References

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