Mr. Mark Welton
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Quality of Service is deployed to prevent data
from saturating a link to the point that other
data cannot gain access to it
QoS allows certain types of traffic to be given
a higher priority than other traffic
Fundamental purpose of QoS is to determine
which traffic should be given priority
Recall:
 When the rate of bits being sent to a link
exceeds the line rate (called?)
 The interface on the router buffers as many
of the packets as possible
 Then the interface begins dropping packets
(called?)
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QoS helps to deal with congestion
QoS can not deal with continued saturation
It can get important traffic through a
saturated link
It will never turn a T1 into a DS3
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Transport Layer protocols behave differently
on congested networks
TCP - connection-oriented, sender/receiver
adapt to network conditions and can resend
lost packets
◦ TCP-based applications are generally not
sensitive to lost packets
◦ What are some examples?
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UDP – connectionless, no error checking, no
notification of loss packets
◦ UDP-based applications can be sensitive to lost
packets
◦ Examples???
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Queues
◦ FIFO – first in first out
◦ WFQ - Cisco proprietary, Weighted Fair Queuing
◦ CBWFQ – Class Based WFQ, creates classes of traffic
that each have a priority and each class queue uses
WFQ
◦ Priority - queues created and traffic is serviced in
highest priority queue first
◦ Low Latency queuing – a CBWFQ with strict priority
queue. Preferred method for voice networks
◦ Traffic Shaping – monitors traffic, and when a
configured threshold is met, packets are queued
until a point where we want them to be sent
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QoS Mechanics
Marking - deciding what priority a packet
should be and labeling it accordingly
Policing – the actions the router takes based
on how the packets are marked
Scheduling – the interface actually serving
the packets in the order determined by how
the marked packets are policed
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Two primary types of IP prioritization used at
Layer 3 (what device is implementing this?)
◦ IP precedence
 Defined in RFC 795
◦ Differential services
 Defined in RFC 2474
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Both use the type of service (TOS) field in a IP
packet
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Class of service (CoS) is a Layer-2 form of
QoS
CoS works under the same principles as IP
precedence
The values are stored in the 802.1P frame
header not the IP header
What does this mean???
CoS
IP precedence
DSCP
Name
0
0
0-7
Routing (Best
Effort)
1
1
8-15
Priority
2
2
16-23
Immediate
3
3
24-31
Flash
4
4
32-39
Flash-override
5
5
40-47
Critical
6
6
48-55
Internet
7
7
56-63
Network
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Traffic policing vs. Traffic-shaping
Traffic policy deals with what type of traffic
should given priority over other traffic
◦ If the traffic is not given priority and the link is
congested then it will be dropped
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Traffic-shaping will “delay” traffic and deliver
it “late”
◦ It will attempt to queue the traffic that would be
dropped and deliver it as the link is not congested
NOTE: I have over simplified this description to help make it easier to
understand
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Traffic policing vs. Traffic-shaping
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So what happens if I have a DS3 in
center and my branch office haves
frame relay network?
So what happens if I have a DS3 in
center and my branch office haves
MPLS network?
my data
a T1on a
my data
a T1on a
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A DS3 is 28 T1s
So a DS3 gives you more bandwidth?
In Frame relay each end of a DLCI is defines
with a bandwidth setting (throughput)
What does MPLS do?
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We increase the “frequency” that a bit can be
sent which increases the throughput
This is also true for other media
◦ CAT 5, CAT 5E, CAT 6, Fiber
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If we send the traffic at DS3 speed to the T1
the queue will fill and packets will be dropped
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If we “hold” the packets back at the DS3 and
send them at a speed the T1 can handle we
will not fill the queue and not drop packets
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Problems????
Possibly
◦ As long as we do not hold the packets too long
◦ Bufferbloat
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Time for you to do some work
What is bufferbloat tech talk
http://en.wikipedia.org/wiki/Bufferbloat
http://packetpushers.net/show-125bufferbloat-what-can-you-do-today-tosuffer-less-2/
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“QoS “Carves Up” a Link into Smaller Logical
Links”
◦ “If I set a policy to give 10% of a link to voice then I
have reserved this to only be used by voice traffic”
◦ WRONG – it will use this policy only during times of
congestion
◦ When congestion is not present the link will work as
normal
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“QoS Limits bandwidth”
◦ WRONG - QoS guarantees a queue will get priority
of some traffic over others during time of
congestion
◦ If a QoS setting sets FTP to 10% of the link if
congestion does not exist it will receive all the link
it asks for
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“QoS resolves a need for more bandwidth”
◦ WRONG - the best QoS can do is to prioritize
which packets should be sent first, which also
means it’s determining which packets can be
dropped!
◦ If you have a T1 and you’re trying to shove 20
Mbps through it, it doesn’t matter how you
prioritize and queue your packets, you’re going
to drop a lot of them
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“QoS prevents packets from being dropped”
◦ WRONG – What QoS will do for you is help you get
the important packets through so that only the less
important packets get dropped
◦ Traffic shaping can prevent packets from being
dropped (assuming certain buffers are not
similarly saturated), though this gets complicated