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3.3: Selecting an
Appropriate QoS
Policy Model
© 2006 Cisco Systems, Inc. All rights reserved.
Objectives
 Describe 3 QoS models: best effort, IntServ and
Diffserv.
 Identify the strengths and weaknesses of each of the 3
QoS models.
 Describe the purpose and functionality of RSVP.
© 2006 Cisco Systems, Inc. All rights reserved.
Three QoS Models
Model
Characteristics
Best effort
No QoS is applied to packets. If it is not
important when or how packets arrive, the besteffort model is appropriate.
Integrated
Services
Applications signal to the network that the
applications require certain QoS parameters.
(IntServ)
Differentiated
Services
The network recognizes classes that require
QoS.
(DiffServ)
© 2006 Cisco Systems, Inc. All rights reserved.
Best-Effort Model
 Internet was initially based on a best-effort packet
delivery service.
 Best-effort is the default mode for all traffic.
 There is no differentiation among types of traffic.
 Best-effort model is similar to using standard mail—
“The mail will arrive when the mail arrives.”
 Benefits:
Highly scalable
No special mechanisms required
 Drawbacks:
No service guarantees
No service differentiation
© 2006 Cisco Systems, Inc. All rights reserved.
Integrated Services (IntServ) Model Operation
 Ensures guaranteed delivery and
predictable behavior of the network for
applications.
 Provides multiple service levels.
 RSVP is a signaling protocol to
reserve resources for specified QoS
parameters.
 The requested QoS parameters are
then linked to a packet stream.
 Streams are not established if the
required QoS parameters cannot be
met.
 Intelligent queuing mechanisms
needed to provide resource
reservation in terms of:
Guaranteed rate
Controlled load (low delay, high
throughput)
© 2006 Cisco Systems, Inc. All rights reserved.
IntServ Functions
Control Plane
Routing Selection
Admission Control
Reservation Setup
Reservation Table
Data Plane
Flow Identification
© 2006 Cisco Systems, Inc. All rights reserved.
Packet Scheduler
Benefits and Drawbacks of the IntServ Model
 Benefits:
Explicit resource admission control (end to end)
Per-request policy admission control (authorization object,
policy object)
Signaling of dynamic port numbers (for example, H.323)
 Drawbacks:
Continuous signaling because of stateful architecture
Flow-based approach not scalable to large implementations,
such as the public Internet
© 2006 Cisco Systems, Inc. All rights reserved.
Resource Reservation Protocol (RSVP)
 Is carried in IP—protocol ID
46
 Can use both TCP and UDP
port 3455
 Is a signaling protocol and
works with existing routing
protocols
 Requests QoS parameters
from all devices between the
source and destination
Sending
Host
RSVP
Tunnel
RSVP Receivers
 Provides divergent performance requirements for multimedia
applications:
Rate-sensitive traffic
Delay-sensitive traffic
© 2006 Cisco Systems, Inc. All rights reserved.
RSVP Daemon
Policy
Control
Admission
Control
RSVP
Daemon
Reservation
Routing
Data
© 2006 Cisco Systems, Inc. All rights reserved.
Packet
Classifier
Packet
Scheduler
Reservation Merging
R3
R5
R5
R4
R4
Sender
R2
R1
 R1, R2 and R3 all request the same reservation.
 The R2 and R3 request merges at R4.
 The R1 request merges with the combined R2 and R3 request at R5.
 RSVP reservation merging provides scalability.
© 2006 Cisco Systems, Inc. All rights reserved.
RSVP in Action
 RSVP sets up a path through the network with the requested QoS.
 RSVP is used for CAC in Cisco Unified CallManager 5.0.
© 2006 Cisco Systems, Inc. All rights reserved.
The Differentiated Services Model
 Overcomes many of the limitations best-effort and IntServ models
 Uses the soft QoS provisioned-QoS model rather than the hard QoS
signaled-QoS model
 Classifies flows into aggregates (classes) and provides appropriate QoS for
the classes
 Minimizes signaling and state maintenance requirements on each network
node
 Manages QoS characteristics on the basis of per-hop behavior (PHB)
 You choose the level of service for each traffic class
Edge
End Station
Edge
Interior
Edge
DiffServ Domain
© 2006 Cisco Systems, Inc. All rights reserved.
End Station
Self Check
1. Which of the QoS models is more scalable, yet still
provides QoS for sensitive traffic?
2. Which QoS model relies on RSVP?
3. What are some drawbacks of using IntServ for QoS?
4. What is admission control?
5. What are the drawbacks of using Diffserv?
© 2006 Cisco Systems, Inc. All rights reserved.
Summary
 Best effort QoS is appropriate where sensitive traffic
does not have to be services. When sensitive traffic
must be services, IntServ or Diffserv should be used to
provide QoS.
 IntServ uses RSVP to guarantee end to end services
for a traffic flow. RSVP has significant signaling
overhead and is not highly scalable.
 Diffserv uses classes to identify traffic and then
provides QoS to those classes. Diffserv is highly
scalable, but does not provide a service guarantee.
© 2006 Cisco Systems, Inc. All rights reserved.