Transcript Lecture 4 - Introducing QOS

QOS
Lecture 4 - Introducing QOS
© 2006 Cisco Systems, Inc. All rights reserved.
Objectives
 Explain why converged networks require QoS.
 Identify the major quality issues with converged
networks.
 Calculate available bandwidth given multiple flows.
 Describe mechanisms designed to use bandwidth more
efficiently.
 Describe types of delay.
 Identify ways to reduce the impact of delay on quality.
 Describe packet loss and ways to prevent or reduce
packet loss in the network.
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Traditional Nonconverged Network
 Traditional data traffic characteristics:
Bursty data flow
FIFO access
Not overly time-sensitive; delays OK
Brief outages are survivable
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Converged Network Realities
 Converged network realities:
Constant small-packet voice flow competes
with bursty data flow.
Critical traffic must have priority.
Voice and video are time-sensitive.
Brief outages are not acceptable.
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Converged Network Quality Issues
 Lack of bandwidth: Multiple flows compete for a limited
amount of bandwidth.
 End-to-end delay (fixed and variable): Packets have to
traverse many network devices and links; this travel
adds up to the overall delay.
 Variation of delay (jitter): Sometimes there is a lot of
other traffic, which results in varied and increased
delay.
 Packet loss: Packets may have to be dropped when a
link is congested.
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Measuring Available Bandwidth
 The maximum available bandwidth is the bandwidth of the slowest link.
 Multiple flows are competing for the same bandwidth, resulting in much less
bandwidth being available to one single application.
 A lack in bandwidth can have performance impacts on network applications.
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Increasing Available Bandwidth




Upgrade the link (the best but also the most expensive solution).
Improve QoS with advanced queuing mechanisms to forward the important packets first.
Compress the payload of Layer 2 frames (takes time).
Compress IP packet headers.
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Using Available Bandwidth Efficiently
Voice
1
1
• LLQ
• RTP header
compression
(Highest)
Data
2
2
3
3
3
4
4
4
(High)
Data
(Medium)
Data
Voice
4
4
3
2
1
1
Data
• CBWFQ
• TCP header
compression
(Low)
 Using advanced queuing and header compression mechanisms,
the available bandwidth can be used more efficiently:
Voice: LLQ and RTP header compression
Interactive traffic: CBWFQ and TCP header compression
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Types of Delay
 Processing delay: The time it takes for a router to take the packet from an input
interface, examine the packet, and put the packet into the output queue of the
output interface.
 Queuing delay: The time a packet resides in the output queue of a router.
 Serialization delay: The time it takes to place the “bits on the wire.”
 Propagation delay: The time it takes for the packet to cross the link from one end to
the other.
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The Impact of Delay and Jitter on Quality
 End-to-end delay: The sum of all propagation, processing,
serialization, and queuing delays in the path
 Jitter: The variation in the delay.
 In best-effort networks, propagation and serialization delays are fixed,
while processing and queuing delays are unpredictable.
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Ways to Reduce Delay





Upgrade the link (the best solution but also the most expensive).
Forward the important packets first.
Enable reprioritization of important packets.
Compress the payload of Layer 2 frames (takes time).
Compress IP packet headers.
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Reducing Delay in a Network
 Customer routers perform:
TCP/RTP header compression
LLQ
Prioritization
 ISP routers perform:
Reprioritization according to the QoS policy
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The Impacts of Packet Loss
 Telephone call: “I cannot understand you. Your voice is breaking up.”
 Teleconferencing: “The picture is very jerky. Voice is not synchronized.”
 Publishing company: “This file is corrupted.”
 Call center: “Please hold while my screen refreshes.”
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Types of Packet Drops
 Tail drops occur when the output queue is full. Tail drops are common
and happen when a link is congested.
 Other types of drops, usually resulting from router congestion, include
input drop, ignore, overrun, and frame errors. These errors can often
be solved with hardware upgrades.
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Ways to Prevent Packet Loss
 Upgrade the link (the best solution but also the most expensive).
 Guarantee enough bandwidth for sensitive packets.
 Prevent congestion by randomly dropping less important packets
before congestion occurs.
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Traffic Rate
Policing
Traffic
Traffic
Traffic Policing and Traffic Shaping
Time
Traffic Rate
Shaping
Time
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Traffic
Traffic
Time
Traffic Rate
Traffic Rate
Time
Reducing Packet Loss in a Network
 Problem: Interface congestion causes TCP and voice packet
drops, resulting in slowing FTP traffic and jerky speech quality.
 Conclusion: Congestion avoidance and queuing can help.
 Solution: Use WRED and LLQ.
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Summary
 Converged networks carry different types of traffic over
a shared infrastructure. This creates the need to
differentiate traffic and give priority to time-sensitive
traffic.
 Various mechanisms exist that help to maximize the
use of the available bandwidth, including queuing
techniques and compression mechanisms.
 All networks experience delay. Delay can effect time
sensitive traffic such as voice and video.
 Without proper provisioning and management,
networks can experience packet loss. Packet loss is
especially important with voice and video, as no
resending of lost packets can occur.
© 2006 Cisco Systems, Inc. All rights reserved.
3.2: Implementing
QoS
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Objectives
 Describe the need for QoS as it relates to various types
of network traffic.
 Identify QoS mechanisms.
 Describe the steps used to implement QoS.
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What Is Quality of Service?
Two Perspectives
 The user perspective
Users perceive that their applications are
performing properly
Voice, video, and data
 The network manager perspective
Need to manage bandwidth allocations
to deliver the desired application
performance
Control delay, jitter, and
packet loss
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Different Types of Traffic Have Different
Needs
 Real-time applications especially
sensitive to QoS
Application
Examples
Delay
Jitter
Packet
Loss
Interactive Voice
and Video
Y
Y
Y
Streaming Video
N
Y
Y
Transactional/
Interactive
Y
N
N
Bulk Data
Email
File Transfer
N
N
N
Interactive voice
Videoconferencing
 Causes of degraded performance
Congestion losses
Sensitivity to
QoS Metrics
Variable queuing delays
 The QoS challenge
Manage bandwidth allocations to
deliver the desired application
performance
Control delay, jitter, and packet
loss
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Need to manage
bandwidth allocations
Cisco IOS QoS Tools
 Congestion management:
PQ
CQ
WFQ
CBWFQ
 Queue management
WRED
 Link efficiency
Link fragmentation and interleave
RTP and CRTP
 Traffic shaping and traffic policing
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QoS Toolbox
Priority Queuing
PQ puts data into four levels of queues: high, medium,
normal, and low.
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Priority Queuing
 Priority output queuing allows a network administrator
to define four priorities of traffic---high, normal, medium,
and low---on a given interface.
 As traffic comes into the router, it is assigned to one of
the four output queues.
 Packets on the highest-priority queue are transmitted
first.
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Priority Queuing ctd..
 When that queue empties, traffic on the next highestpriority queue is transmitted, and so on.
 This mechanism assures that during congestion, the
highest-priority data does not get delayed by lowerpriority traffic.
 However, if the traffic sent to a given interface exceeds
the bandwidth of that interface, lower-priority traffic can
experience significant delays.
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Custom Queuing
CQ handles traffic by assigning a specified amount of queue space to each class of
packet and then servicing up to 17 queues in a round-robin fashion.
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Custom Queuing
 Custom queuing allows a customer to reserve a
percentage of bandwidth for specified protocols.
 Customers can define up to 10 output queues for
normal data and an additional queue for system
messages such as LAN keepalive messages (routing
packets are not assigned to the system queue).
 The routers service each queue sequentially,
transmitting a configurable percentage of traffic on each
queue before moving on to the next one.
 Custom Queuing guarantees that mission-critical data
is always assigned a certain percentage of the
bandwidth, but also assures predictable throughput for
other traffic.
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Custom Queuing …
 To provide this feature, routers determine how many
bytes should be transmitted from each queue, based on
the interface speed and the configured percentage.
 When the calculated byte count from a given queue has
been transmitted, the router completes transmission of
the current packet and moves on to the next queue,
servicing each queue in a round-robin fashion.
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Weighted Fair Queuing
•WFQ makes the transfer rates and interarrival periods of active high-volume conversations much
more predictable.
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Weighted Fair Queuing (Flow based)
Flow-Based WFQ: Creating Fairness Among Flows
 For situations in which it is desirable to provide
consistent response time to heavy and light network
users alike without adding excessive bandwidth, the
solution is flow-based WFQ (commonly referred to as
just WFQ).
 It is a flow-based queuing algorithm that creates bitwise fairness by allowing each queue to be serviced
fairly in terms of byte count.
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WFQ ….
 For example, if queue 1 has 100-byte packets and
queue 2 has 50-byte packets, the WFQ algorithm will
take two packets from queue 2 for every one packet
from queue 1.
 This makes service fair for each queue: 100 bytes each
time the queue is serviced.
 WFQ ensures that queues do not starve for bandwidth
and that traffic gets predictable service.
 Low-volume traffic streams that comprise the majority
of traffic, receive increased service, transmitting the
same number of bytes as high-volume streams.
 This behavior results in what appears to be preferential
treatment for low-volume traffic, when in actuality it is
creating fairness.
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Weighted Random Early Detection
•WRED provides a method that stochastically discards packets if congestion begins to increase.
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© 2006 Cisco Systems, Inc. All rights reserved.
Implementing QoS
Step 1: Identify types of traffic and
their requirements.
Step 2: Divide traffic into classes.
Step 3: Define QoS policies for
each class.
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Step 1: Identify Types of Traffic and Their
Requirements
 Network audit: Identify traffic on the network.
 Business audit: Determine how important each type of
traffic is for business.
 Service levels required: Determine required response
time.
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Step 2: Define Traffic Classes
Scavenger
Class
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Less than
Best Effort
Step 3: Define QoS Policy
 A QoS policy is a
network-wide definition of
the specific levels of QoS
that are assigned to
different classes of
network traffic.
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Quality of Service Operations
How Do QoS Tools Work?
Classification
and Marking
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Queuing and
(Selective) Dropping
Post-Queuing
Operations
Self Check
1. What types of applications are particularly sensitive to
QoS issues?
2. What is WFQ? How is it different than FIFO?
3. What are the 3 basic steps involved in implementing
QoS?
4. What is Scavenger Class?
© 2006 Cisco Systems, Inc. All rights reserved.
Summary
 QoS is important to both the end user and the network
administrator. End users experience lack of QoS as
poor voice quality, dropped calls or outages.
 Network traffic differs in its ability to handle delay, jitter
and packet loss. Traffic sensitive to these issues
requires priority treatment. QoS measures can provide
priority to sensitive traffic, while still providing services
to more resilient traffic.
 Implementing QoS involves 3 basic steps: identify the
types of traffic on your network, divide the traffic into
classes, and define a QoS policy for each traffic class.
© 2006 Cisco Systems, Inc. All rights reserved.