3.3: Selecting an Appropriate QoS Policy Model

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3.3: Selecting an Appropriate QoS Policy Model


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.


Three QoS Models

Model CharacteristicsBest effort No QoS is applied to packets. If it is not

important when or how packets arrive, the best-effort model is appropriate.

Integrated Services

(IntServ)

Applications signal to the network that the applications require certain QoS parameters.

Differentiated Services

(DiffServ)

The network recognizes classes that require QoS.


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 scalableNo special mechanisms required

Drawbacks:No service guaranteesNo service differentiation


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 rateControlled load (low delay, high throughput)


IntServ Functions

Flow Identification Packet Scheduler

Data Plane

Routing Selection Admission Control

Reservation Setup

Control Plane

Reservation Table


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 architectureFlow-based approach not scalable to large implementations, such as the public Internet


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 Receivers

RSVP Tunnel

Provides divergent performance requirements for multimedia applications:

Rate-sensitive trafficDelay-sensitive traffic


RSVP Daemon

PolicyControl

AdmissionControl

Packet Classifier

PacketScheduler

Routing

RSVPDaemon

Reservation

Data


Reservation Merging

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.

R5 R4

R3

R5 R4

R1

R2Sender


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.


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

Edge

Interior

Edge

DiffServ Domain

End Station

End Station


Self Check1. 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?


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.

3.3: Selecting an Appropriate QoS Policy Model

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