IP quality of service

IP Quality of Service


Objectives
 Upon completing this module, you will be able to:





Describe the need for IP QoS
Describe the Integrated Services model
Describe the Differentiated Services model
Describe the building blocks of IP QoS mechanisms (classification,
marking, metering, policing, shaping, dropping, forwarding, queuing)

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Introduction


Objectives
 Upon completing this lesson, you will be able to:
 Describe different types of applications and services that have
special resource requirements
 List the network components that affect the throughput, delay,
and jitter in IP networks
 List the benefits of deploying QoS mechanisms in IP networks
 Describe typical enterprise and service provider networks and
their QoS-related requirements

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Why IP QoS?






Application X is slow.
Video broadcast occasionally stalls.
Phone calls over IP are no better than over satellite.
Phone calls can have very bad voice quality.
ATMs (the money-dispensing type) are
nonresponsive.

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Because ...
 Application X is slow!
(not enough bandwidth)
 Video broadcast occasionally stalls!
(delay temporarily increases – jitter)
 Phone calls over IP are no better than over satellite! (too
much delay)

 Phone calls can have very bad voice quality!
(too many phone calls – admission control)
 ATMs (the money-dispensing type) are nonresponsive!
(too many drops)

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What Causes ...
 Lack of bandwidth?: Multiple flows are contesting for
a limited amount of bandwidth.
 Too much delay?: Packets have to traverse many
network devices and links.
 Variable delay?: Sometimes there is a lot of other
traffic, which results in more delay.
 Drops?: Packets have to be dropped when a link is
congested.

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Available Bandwidth

IP

IP

IP

IP

512 kbps
256 kbps
10 Mbps

100 Mbps

BWmax = min(10M, 256k, 512k, 100M)=256 kbps
BWavail = BWmax /Flows



Maximum available bandwidth equals the bandwidth of the
weakest link.
Multiple flows are competing for the same bandwidth, resulting in much less
bandwidth being available to one single application.
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End-to-End Delay

IP

IP

Propagation
Delay (P1)
Processing and
Queuing Delay (Q1)

IP

Propagation
Delay (P2)
Processing and
Queuing Delay (Q2)

IP

Propagation
Delay (P3)

Propagation
Delay (P4)

Processing and
Queuing Delay (Q3)

Delay = P1 + Q1 + P2 + Q2 + P3 + Q3 + P4 = X ms
 End-to-end delay equals a sum of all propagation, processing, and
queuing delays in the path.
 Propagation delay is fixed; processing and queuing delays are
unpredictable in best-effort networks.
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Processing, Queuing, and
Propagation Delay

IP

IP

Processing Delay

IP

Bandwidth

Forwarding

IP

Queuing Delay
Propagation Delay

 Processing delay is the time it takes for a router to take the packet from an input
interface and put it into the output queue of the output interface.
 Queuing delay is the time a packet resides in the output queue of a router.
 Propagation or serialization delay is the time it takes to transmit a packet.

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Packet Loss

Forwarding

IP

IP

IP

IP

IP

Tail-drop

 Tail-drops occur when the output queue is full. These are the most common
drops which happen when a link is congested.
 There are also many other types of drops (input queue drop, ignore, overrun, no
buffer, etc), which are not as common and which may require a hardware
upgrade. These drops are usually a result of router congestion.
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How to Increase Available
Bandwidth?
TCP Header Compression
RTP Header Compression
cTCP Data

Compress
the Headers
IP

TCP

FIFO queuing
Fancy
Queuing

Data
Compress
the Payload

Compressed Packet

Stacker
Predictor

Priority Queuing (PQ)
Custom Queuing (CQ)
Modified Deficit Round Robin (MDRR)
Class-Based Weighted Fair Queing (CBWFQ)

 Upgrade the link—the best solution but also the most expensive.

• Take some bandwidth from less important applications.
• Compress the payload of Layer 2 frames.
• Compress the header of IP packets.
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How to Reduce Delay?
TCP Header Compression
RTP Header Compression
cRTP Data

Compress
the Headers
IP

UDP RTP

FIFO queuing
Fancy
Queuing

Data

Compress
the Payload

Compressed Packet

Stacker
Predictor

Priority Queuing (PQ)
Custom Queuing (CQ)
Strict Priority MDRR
IP RTP Prioritization
Class-Based Low-Latency Queuing (CBLLQ)

 Upgrade the link—the best solution but also the most expensive.

• Forward the important packets first.
• Compress the payload of Layer-2 frames (it takes time).
• Compress the header of IP packets.
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How to Prevent Packet Loss?

Weighted Random Early Detection (WRED)

IP

Data

Dropper

Fancy
Queuing
FIFO queuing

Custom Queuing (CQ)
Modified Deficit Round Robin (MDRR)
Class-Based Weighted Fair Queuing (CBWFQ)

 Upgrade the link—the best solution but also the most expensive.

• Guarantee enough bandwidth to sensitive packets.
• Prevent congestion by randomly dropping less important packets
before congestion occurs.
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Which Applications Have Which QoS
Requirements?
Throughput

Delay

Loss

Jitter

Interactive

(e.g., Telnet)

Low

Low

Low

Not
Important

Batch (e.g.,
FTP)

High

Not
Important

Low

Not
Important

Fragile (e.g,.
SNA)

Low

Low

None

Not
Important

Voice

Low

Low and
Predictable

Low

Low

Video

High

Low and
Predictable

Low

Low

 Enterprise networks are typically focused on providing QoS to
applications.

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Which Services Can Be
Implemented in a Network?
Throughput

Delay

Loss

Jitter

Gold

Guaranteed

Low

Low

Low

Silver

Guaranteed

No

Guarantee

No
Guarantee

No
Guarantee

Bronze

Guaranteed
Limited

No
Guarantee

No
Guarantee

No
Guarantee

No
Guarantee

No
Guarantee

No
Guarantee

No
Guarantee

Best Effort

 Service provider networks typically offer services based on
source and destination addresses.
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How Can QoS Be Applied?
 Best effort—no QoS is applied to packets (default
behavior)
 Integrated Services model—applications signal to the
network that they require special QoS
 Differentiated Services model—the network
recognizes classes that require special QoS

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Summary
 Upon completing this lesson, you should be able
to:
 Describe different types of applications and services that have

special resource requirements
 List the network components that affect the throughput, delay,
and jitter in IP networks
 List the benefits of deploying QoS mechanisms in IP networks
 Describe typical enterprise and service provider networks and
their QoS-related requirements

Inner Mongolia University


Review Questions
1.What are the relevant parameters that define quality
of service?
2.What can be done to give more bandwidth to an
application?
3.What can be done to reduce delay?
4.What can be done to prevent packet loss?
5.Name the two QoS models.

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Integrated Services
Model


Objectives
 Upon completing this lesson, you will be
able to:
 Describe the IntServ model

 List the key benefits and drawbacks of the IntServ
model
 List some implementations that are based on the
IntServ model
 Describe the need for Common Open Policy Service
(COPS)

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Integrated Services
 The Internet was initially based on a
best-effort packet delivery service.
 Today's Internet carries many more different
applications than 20 years ago.
 Some applications have special bandwidth and delay
requirements.
 The Integrated Services model (RFC1633) was
introduced to guarantee predictable network behavior
for these applications.

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IntServ Building Blocks
Local
Admission
Control

request

request

reply

reserve

Policy Enforcement
Point (PEP)

request

reserve

Local
Admission
Control

Remote Admission
Control

request

request

reserve

reserve

Policy Decision
Point (PDP)

 Resource reservation is used to identify an application (flow) and signal
if there are enough available resources for it.
 Admission control is used to determine if the application (flow) can get
the requested resources.
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Reservation and Admission
Protocols
 The Resource Reservation Protocol (RSVP) was
developed to communicate resource needs between
hosts and network devices (RFCs 2205 to 2215).
 Common Open Policy Service (COPS) was
developed to offload admission control to a central
policy server (RFCs 2748 to 2753).

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RSVP-Enabled Applications

 RSVP is typically used by applications carrying voice
or video over IP networks (initiated by a host).

 RSVP with extensions is also used by MPLS Traffic
Engineering to establish MPLS/TE tunnels (initiated
by a router).

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