IPv6 - MCAST Institute of Information & Communications Technology

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26 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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QoS


What is QoS?


QoS

provides standards and mechanisms that ensure high
-
quality performance for critical applications.


Traditionally, all network traffic was
Best effort


All applications treated equally


If the network cannot perform according to required level,
applications simply bear the effect of it


The concept is to make best use of the resources available


This is of particular importance when considering the costs
of the resources


Particularly WAN connections


Also on internal communications

Bernard Borg

2

QoS

How traffic is affected


VOICE traffic


Voice is a time
-
sensitive application


Irregular data rates or high delay makes voice conversations incomprehensible


VIDEO traffic


Video is bandwidth
-
sensitive


Every stream of video consumes a large amount of bandwidth


Remote Desktops


If there are delays, remote desktop connections will have poor response times


Business critical applications


Applications that are vital to the company operations


Example: a purchasing system which is accessed over the WAN link



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QoS

What QoS does


QoS is used to control how applications use bandwidth on a network


Applications that have higher requirements are given preferential
treatment


The emphasis is on ensuring that the critical data always gets to
destination and within a required time.


Examples


Voice, video, business critical data


It is about controlling network congestion.


It would be convenient to buy enough bandwidth to provide for all
requirements, but if we can’t, QoS is required



It’s job is to ensure that bandwidth is used
correctly

by various
applications

Bernard Borg

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QoS

3 Problems that occur on the network


Insufficient Bandwidth


Delay


Packet Loss

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QoS

BANDWIDTH


When considering end
-
to
-
end bandwidth, the maximum
bandwidth that you have available is the bandwidth of your
slowest link


Normally the WAN link is the place where bandwidth is an issue


Bandwidth can also be limited at uplinks of switches or routers
on the internal network or at its perimeter


It is a point of aggregation of multiple connections



Effects of low bandwidth


Low bandwidth affects performance of most types of traffic


In particular, video is very sensitive to bandwidth availability because of
its very large
filesize

Bernard Borg

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QoS

Delay (1)


Every transmission has a measurable end
-
to
-
end delay


Also called
Latency


We have already considered the following types of delay


Processing delay


The actual amount of time that it takes a router to move a packet from the interface
it comes in on to the output queue


Queuing delay


How long a packet has to sit in the output queue


Serialization delay


T
he amount of time it takes to actually put a frame on the physical wire to be sent


Propagation delay


T
he amount of time it takes for the packet to cross the link from one end to the other

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7

QoS

Delay (2)


The first type of delay is related directly to the router


The CPU power, the RAM, the switching mode


The queuing delay depends on the number of packets and
size of packets being transferred


It is also dependent on the bandwidth of the interface


The serialisation delay is a factor of the bandwidth


It is characteristically inversely proportional to the bandwidth of
the link


The last delay is related to the medium


In fibre it is extremely low, in copper it is reasonably low.


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8

QoS

Delay & Jitter


Another aspect of delay is Jitter


This refers to the
variation in delay


The packets arrive at destination with significant
differences in delay


A longer delay that is constant has less effect than a
delay that is shorter but has a high rate of jitter


Jitter causes significant problems in voice and video
transmissions

Bernard Borg

9

QoS

Sender

t

Receiver

t

Packet Loss


Packet loss happens for several reasons, but only one is
common in a network

tail dropping.


Tail dropping happens when an output interface queue is
full and it cannot accept an incoming packet


So the incoming packet is dropped.


Called tail
-
dropping because data is usually dropped at the tail
-
end


Packets also can be dropped for other reasons, for
example, as the result of a frame error or a frame with an
incorrect CRC


Much less common.

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QoS

3 measures that can be taken


Increase bandwidth


Priority Forwarding


Compression

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11

QoS

Increase Bandwidth


This is absolutely the best way to prevent degradation of
application performance


However, increasing bandwidth will always be a less
favourite option because of the costs involved


The router connecting to the WAN link has to be powerful
enough to handle the traffic


this reduces the processing delay


Upgrading a WAN link will also reduce the queuing and
transmission (serialisation) delay


This will reduce congestion and the likelihood of packet loss


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12

QoS

Priority Forwarding (1)


This is considered as a more direct QoS solution


Priority forwarding is a method which uses queuing in
such a way that traffic of particular type is given
priority over other traffic


The default, basic type of queuing is FIFO (first in first
out)


FIFO has the effect of smoothing jitter


FIFO doesn’t make any difference between different types of
traffic

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13

QoS

Priority Forwarding (2)


Other types of queuing provide techniques that treat certain
kinds of traffic with preference


LLQ


Low Latency Queuing,


WFQ
-

Weighted Fair Queuing,


WRR


Weighted Round Robin


As a result these types of traffic receive the bandwidth they
need


Another effect of priority forwarding is reduction of delay for
the prioritised traffic


The main issue with this use of queuing is that uncontrolled use
of it can heavily degrade the performance of other low priority
streams

Bernard Borg

14

QoS

Compression


Compression method either remove data or represent a
larger amount of data by smaller volume ‘code data’


However compression increases delay because of the time
and processing power it takes to perform the compression


Header compression can be very effective where most of
the packets carry small amounts of data.


The header/payload ratio justifies its use


Payload compression is most effective if the compression
is used from end to end


Header compression is most effective when applied hop
by hop


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QoS

A more detailed look at QoS

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QoS

Steps in Implementing QoS


Traffic Identification


Identify different types of traffic and the technical
requirements of each of them


Traffic Classification


Divide the traffic into classes, which will be used to group up
traffic with similar requirements


Assigning QoS mechanisms policies


Different kinds of mechanisms can be configured as
required within QoS policies

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17

QoS

Identification of Traffic types


This step of the process is about gathering information on which to
base decisions about separate kinds of traffic


The process requires an understanding of the


User perspective


what is and is not an issue from the user perspective

Example: a user will be heavily frustrated by poor voice calls but not affected significantly by a 3
second delay in accessing a file over the network


Business perspective


requirements from a business perspective are not
the same as from a technical perspective

Example: from a technical perspective all HTTP traffic is considered the same BUT from a business
perspective, traffic to the internal web servers is tremendously more important than casual web
browsing


There are software and hardware tools which can be used to analyse
the patterns of specific types of traffic on the network in order to
come to an informed decision


E.g. Cisco provide Network Based Application Recognition

(NBAR)

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18

QoS

Classifying Traffic


Traffic which has been identified (mentioned in previous slide) is then
categorized into different classes.


In order to define classes, various parameters can be used to
determine what traffic belongs and what does not.


The following descriptors can be used


The interface that is being used


VLAN ID


MPLS labels


IPv4 Type of Service or IPv6 traffic class or flow labels


Specific protocols


After the packet has been classified, the packet is ready for Quality of
Service (
QoS
) handling on the

network

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QoS

Example of Traffic Class organisation


A company might set up its classes in the following manner

1.
Voice


The top priority is assigned to VoIP

2.
Business critical data


Includes a few applications that are critical to the function of the
company

3.
Data services


Includes access and transactions with databases

4.
Best effort


This includes Email and HTTP data, which is not as sensitive to delay as
other traffic from higher classes


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QoS

QoS mechanisms


Once classification is done, QoS makes use of the
following types of mechanisms to provide the desired
level of service


Traffic Marking


Traffic Policing


Traffic Shaping


Congestion avoidance

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QoS

Traffic Marking


Individual packets are marked by using specific bits within the
headers.


There are various ways. We will consider 2 categories


Marking at the Data Link Layer


Marking contained in 3 bits of the header of 802.1q VLAN header


MPLS uses 3 bits in the EXP (experimental) field to mark priority


Marking at the Network layer


The Type of Service (TOS) field in IP is used to mark packets


The original method, IP Precedence, used the first 3 bits of the octet


Eventually DSCP replaced it, which uses the first six bits of the octet


In all cases, the bits are used to mark the packets in order to
represent a higher or lower priority


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QoS

Traffic Conditioning


Traffic conditioning is used to control the rates at which the data is sent.


The bandwidth for different types of traffic can be set


The mechanisms of traffic conditioning uses the traffic's classification in
order to treat traffic accordingly


By comparing the actual traffic rate to a configured policy rate, the flow of
traffic can be controlled.


This also reduces congestion


The two primary means of implementing traffic conditioning are:


Policing


Shaping


Each of the mechanisms accomplishes its task differently and can be
implemented in different places.


Bernard Borg

23

QoS

Traffic Policing


An administrator configures the limits that are considered to be
acceptable


Any traffic that goes beyond the set limits is dropped.


Alternatively it can be re
-
marked for lower priority transmission


The traffic is forwarded (instead of being dropped), but at a lower priority
rate


Policing can be configured inbound and outbound on an interface


It does not make use of queuing or buffering


The advantage of this method is that it reduces the delay that is
normally caused by queuing


On the other hand, when packets are dropped, there is an increase in
TCP re
-
transmissions

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QoS

Traffic Shaping


The concept of traffic shaping is to control bursts of traffic by
buffering packets


So any excess traffic is delayed


Shaping smoothes out the traffic and creates a more constant flow of
data, thus avoiding or managing congestion on the network


Once again the limits have to be configured.


It can only be configured outbound on an interface


Shaping mechanisms do not re
-
mark the traffic


It does not drop packets and so it avoids the re
-
transmission problem
of traffic policing


The downside is that delays are introduced




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QoS

Comparing Traffic Policing and Traffic Shaping

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QoS

Source: cisco.com

Congestion avoidance


As already mentioned earlier on, when an interface receives more traffic
than the router can cope with, tail dropping occurs


To prevent congestion and tail dropping, methods are used to


Two methods are commonly used


Random early detection (RED)


The administrator sets a threshold and when the queue fills to the given threshold, it
randomly drops packets


This method does not drop traffic based on what type it belongs to


Weighted random early detection (WRED)


This uses actual QoS factors


This method uses the markings on IP (based on the bits in the
ToS

field) to select
which packets to drop


Since it depends on the packet’s classification, lower priority packets stand a greater
probability of being dropped

Bernard Borg

27

QoS

Final considerations about QoS


Management
-
software packages are a must to avoid
complex configuration challenges.


In fact many companies specialise in providing software that
will facilitate configuration of
QoS
.


Some of them also monitor the effect of
QoS

configurations
and help to make adjustments to refine the policies


QoS

can also create political problems as battles arise
over who gets the good
QoS

and who controls it.

Bernard Borg

28

QoS

Conclusion


As we have seen, QoS provides a wide number of
improvements for network traffic


Support guaranteed bandwidth


Improve loss characteristics


Avoid or manage network congestion


Shape network traffic


Set traffic priorities across network


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QoS

Bernard Borg

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QoS