Models Of QOS

Quality of Service Models

In this post, we will see Models of QOS.A QOS model, also called a level of service, describes a set of end-to-end QOS capabilities. End-to-end QOS is the ability of the network to deliver service required by specific network traffic from one end of the network to another.

Cisco IOS QOS software supports three types of service models: best effort, integrated, and differentiated services.

This section describes these service models:

•          Best-Effort Service
•          Integrated Service
•          Differentiated Service

The features in Cisco IOS QOS software address the requirements for these service models.

Best-Effort Service

Best effort is a single service model in which an application sends data whenever it must, in any quantity, and without requesting permission or first informing the network. For best-effort service, the network delivers data if it can, without any assurance of reliability, delay bounds, or throughput.

The Cisco IOS QOS feature that implements best-effort service is first-in, first-out (FIFO) queuing. Best-effort service is suitable for a wide range of networked applications such as FTP, Exchange,etc.

Integrated Service

Integrated service is a multiple service model that can accommodate multiple QOS requirements. In this model the application requests a specific kind of service from the network before sending data. The request is made by explicit signalling; the application informs the network of its traffic profile and requests a particular kind of service that can encompass its bandwidth and delay requirements. The application is expected to send data only after it gets a confirmation from the network. It is also expected to send data that lies within its described traffic profile.

The network performs admission control, based on 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.

Differentiated Service

Differentiated service is a multiple service model that can satisfy differing QOS requirements. However, unlike the integrated service model, an application using differentiated service does not explicitly signal the router before sending data.

For differentiated service, the network tries to deliver a particular kind of service based on the QOS specified by each packet.

This specification can occur in different ways, for example, using the IP Precedence bit settings in IP packets or source and destination addresses.

These features that support the differentiated service model:

·         Committed access rate (CAR), which performs packet classification through IP Precedence and QoS group settings. CAR 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 VIP-Distributed WRED and VIP-Distributed WFQ. These features can be used with CAR to deliver differentiated services.

QOS Techniques

QOS Tools/Techniques:

In previous post we discussed about overview of QOS. Now we will see, what are tools/techniques in QOS. How we are going to do this ?

QOS provides us some tools/techniques: 

• Classification and Marking
• Congestion Avoidance
• Policing and Shaping
• Queuing

Classification: Classification means identifying applications and grouping of applications. Performed by Access-list and NBAR (Cisco proprietary)

Marking: Marking means coloring/tagging identified groups as per requirement, So that it can be recognized in network with same marking and whatever the priority is assigned that color/tag it will be catered to that group in full network. Performed by IP Precedence and DSCP.

Suppose I identified a group of application (http) then I will assign a tag to that group (IP Precedence 5). That means HTTP is having 5 as marking and 5 means best in IP Precedence. It results that HTTP will be passed first from interface. But we need to configure a service policy and then apply that service policy to interface.

Here is mentioned example:

Router#conf t

Router(config)#class-map match-all WEBTRAFFIC  --- Class-map is used to classify the traffic

Router(config-cmap)#match protocol http               --- Identified HTTP traffic by NBAR

Router(config-cmap)#exi

Router(config)#policy-map QOS                                  --- Created Policy

Router(config-pmap)#class WEBTRAFFIC                  --- Match class to assign marking

Router(config-pmap-c)#set ip precedence 5             --- Marked with IP Precedence 5   

Router(config-pmap-c)#exi

Router(config-pmap)#exi

Router(config)#

Router(config)#int fa0/0

Router(config-if)#service-policy output QOS          --- Policy map applied on WAN Interface.

Router(config-if)#exi

Router(config)#exi

Router#

Congestion Avoidance:

Congestion avoidance techniques monitor network traffic loads in an effort to anticipate and avoid congestion at common network and internetwork bottlenecks before it becomes a problem.

 These techniques are designed to provide preferential treatment for premium (priority) class traffic under congestion situations while concurrently maximizing network throughput and capacity utilization and minimizing packet loss and delay.

WRED and RED are the Cisco IOS QOS congestion avoidance features.

Policing and Shaping:
The following diagram illustrates the key difference. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate, excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.

Shaping implies the existence of a queue and of sufficient memory to buffer delayed packets, while policing does not. Queuing is an outbound concept; packets going out an interface get queued and can be shaped. Only policing can be applied to inbound traffic on an interface. Ensure that you have sufficient memory when enabling shaping. In addition, shaping requires a scheduling function for later transmission of any delayed packets. This scheduling function allows you to organize the shaping queue into different queues. Examples of scheduling functions are Class Based Weighted Fair Queuing (CBWFQ) and Low Latency Queuing (LLQ).

Queuing:

You can set a queuing technique on a device's interface to manage how packets are queued to be sent through the interface. Some queuing techniques use the packet marking, while others ignore them.

Queuing techniques are primarily used for managing traffic congestion on an interface, that is, they determine the priority in which to send packets when there is more data than can be sent immediately:

·         Class-Based QOS Queuing: Multiple-Action, Class-Based Policies

·         Fair Queuing (FQ): Flow-Based Queuing

·         Priority Queuing (PQ): Basic Traffic Prioritization on Routers

·         Custom Queuing (CQ): Advanced Traffic Prioritization on Routers

·         Weighted Fair Queuing (WFQ): Intelligent Traffic Prioritization on Routers

·         First In, First Out (FIFO) Queuing: Basic Store and Forward on Routers

Ref: Cisco.com