3.3: Selecting an Appropriate QoS Policy Model

1. 3.3: Selecting an Appropriate QoS Policy Model

2. 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.

3. Three QoS Models

4. 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

5. 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)

6. Control Plane Routing Selection Admission Control Reservation Setup Reservation Table Data Plane Flow Identification Packet Scheduler IntServ Functions

7. 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

8. 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 Resource Reservation Protocol (RSVP) • Provides divergent performance requirements for multimedia applications: • Rate-sensitive traffic • Delay-sensitive traffic

9. Policy Control Admission Control RSVP Daemon Reservation Routing Packet Classifier Data Packet Scheduler RSVP Daemon

10. R3 R5 R4 R5 R4 Sender R2 R1 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.

11. 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.

12. Edge End Station Edge Interior Edge DiffServ Domain End Station 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

13. Self Check • Which of the QoS models is more scalable, yet still provides QoS for sensitive traffic? • Which QoS model relies on RSVP? • What are some drawbacks of using IntServ for QoS? • What is admission control? • What are the drawbacks of using Diffserv?

14. 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.