Introduction to QoS

QoS features provide improved and more predictable network service by implementing different feature services.

• Supporting guaranteed bandwidth
• Improving loss characteristics
• Avoiding and managing network congestion
• Shaping network traffic
• Setting traffic priorities across the network

QoS Design

For effectively implementing QoS features, it is mandatory to implement QoS throughout a network. The following three components are necessary to deliver QoS across a heterogeneous network:

• QoS within a single network element, which includes queuing, scheduling, and traffic shaping features.
• QoS signaling techniques for coordinating QoS for end-to-end delivery between network elements.
• QoS policing and management functions to control and administer end-to-end traffic across a network.

There is a need to appropriately select QoS techniques for different network elements, as per different operations they perform in a Network. For example, appropriate QoS features will be selected for edge and backbone routers in a given IP network.
Major QoS functions for edge routers:

• Packet classification and marking
• Admission control
• Configuration management

Major QoS Functions for backbone routers:

• Congestion management
• Congestion avoidance

Need for QoS

Need and requirements for implementing QoS varies from one network to other. However, the most compelling requirements of QoS implementation arises to effectively coexist multiple mission-critical applications over a single network. Effective QoS implementation will ensure following practical needs:

• Costly WAN media is used efficiently by mission-critical applications that are most important to your business.
• Necessary bandwidth and minimum delays required by time-sensitive multimedia and voice applications ensured on throughout the network.
• All other applications sharing the network get their fair service without interfering with mission-critical traffic.

Cisco QoS Models

Cisco IOS supports different QoS service models based on target applications and network design.

Best-Effort Service

For best-effort service, the network delivers data if it can, without any assurance of reliability, delay bounds, or throughput. It is implementation of first-in, first-out (FIFO) queuing.

Integrated Service

Integrated service is a multiple service model. In this model the application requests a specific kind of service from the network before it sends data. The request is made by explicit signaling. The network performs admission control on the basis of information from the application and available network resources. It also commits to meeting the QoS requirements of the application as long as the traffic remains within the profile specifications. The network fulfills its commitment by maintaining per-flow state and then performing packet classification, policing, and intelligent queuing based on that state. Cisco IOS QoS includes following features for integrated service model:

• Resource Reservation Protocol (RSVP) used by applications to signal their QoS requirements to the router.
• Intelligent queuing mechanisms like weighted fair queuing (WFQ) and Weighted Random Early Detection (WRED) to provide "Guaranteed rate service" and "Controlled load service".

Differentiated Service
Differentiated service is also a multiple service model. It does not require an application using differentiated service to explicitly signal the router before sending data. In this model, the network tries to deliver a particular kind of service based on the QoS specified by each individual packet, for example, using the IP Precedence bit or source and destination addresses in each IP packet. The network uses the QoS specification to classify, mark, shape, and police traffic and to perform intelligent queuing.

This model is more appropriate for aggregate flows because it performs a relatively les s refined traffic classification. In Cisco IOS following features support this model:

• Committed access rate (CAR), which performs metering and policing of traffic, providing bandwidth management.
• Intelligent queuing schemes such as WRED and WFQ and their equivalent features on the Versatile Interface Processor (VIP), which are distributed WRED (DWRED) and distributed WFQ.

QoS Model Selection Criteria

Selection criteria of QoS mechanism / model will depend on following:

• Type of application
• Level of capability to allocate resources
• Cost-benefit analysis for implementing and deploying a QOS model