Fundamentals of Quality of Service

Gopan Govindan

QoS

Introduction

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Definition metrics

Characteristics of network that define QoS

• Bandwidth

• Latency

• Jitter

• Packet loss

The “Model”

DiffServ

• Each packet in the network contains a marking that indicates the required QoS

• Each network device applies QoS based upon this marking

Signaling QoS Requirements

Markers

• ToS – RFC 791

• CoS – QoS at MAC level

• DSCP – RFC 2474

ToS

Bits that matter

CoS

Bits that matter

* Push of packets with a specific CoS has dependency on Switch

Bits that matter

DSCP

Treatment

PHB

• Default PHB – RFC 2474 – DSCP 0 – Traditional best-effort service (FIFO)

• Class-selector PHB – RFC 2474 – for backward compatibility with IP precedence – xxx000

• Assured Forwarding PHB – RFC 2597 - 4 AF classes - AF1, AF2, AF3, AF4

• Expedited Forwarding PHB – RFC 2598 – place packets to priority queue – DSCP 46

• Voice Admit PHB – RFC 5865 -  identical characteristics to the EF PHB – also admitted by the network using CAC procedure – DSCP 44

The “FLOW”

QoS process

Configuration…

Class map

Class map

To classify traffic of our interest.

Policy map

Defines a series of actions (functions) that we wish to apply to a set of classified inbound traffic.

Configuration…

Mathematics ;)

Minor algebra

Class map

• AR• CIR• Bc• Be• Tc

Lets relate….

Class map

• AR - Access Rate - full line speed in bps, maximum transmit value - 128000 in our example

• CIR - Committed Information Rate - target value for transmission, normal transmit value

• Bc - Normal Burst - Equals CIR/Tc

• Be - Excess Burst - Equals or can be upto (AR/Tc)-Bc

• Tc will vary. 1/8 sec in data and 1/100 in voice.

Lets relate….

Class map

AR/Tc = Bc+Be Bc = CIR/Tc

Before entering the show…

Class map

Queuing

• FIFO Queuing

• WFQ – Fair-queue (#packets) (#dynamic) (#reserved)

• CBWFQ

• Priority Queues – implemented as heap

Queue Management (Discard)

• Tail drop

• RED

• WRED

Algorithms

Traffic Shaping

• Leaky Bucket

• Token Bucket

The baggage scanner !

Class map

Ready for a better QoS ?