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RCA THOMSON SpeedTouchTM (Wireless) Business DSL Router 3.4 Differentiated Services Code Point (DSCP)

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Chapter 3

Basic QoS Concepts

E-NIT-CTC-20041213-0013v0.5

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3.4 Differentiated Services Code Point (DSCP)

Introduction A small bit-pattern, called the DS field, in each IP packet is used to mark the packets

that should receive a particular forwarding treatment. The DS field uses the space of

the former ToS byte in the IPv4 IP header and the traffic class byte in the IPv6

header. All network traffic inside of a domain receives a service that depends on the

traffic class that is specified in the DS field.

The structure of the DS field is shown below:

A six-bit field, known as the Differentiated Services Code Point (DSCP), in the DS

field specifies the PHB for a given flow of packets. The DSCP is composed of the six

most significant bits of the DS field. The two least significant bits of the DS field are

used for Explicit Congestion Notification (ECN) by DiffServ-capable nodes that

support ECN. The ECN field contains 2 bits, the ECT bit and the CE bit.

The ECT bit is set to 1 to advertise to the network that the node is an ECN capable

node.

The CE bit is set to 1 incase the node experiences congestion.

Per Hop Behaviour Routers look at the DSCP to select a per-hop behaviour, such as a queueing

algorithm and its parameters.

A PHB defines a DiffServ router’s externally observable forwarding behaviour (in

terms of buffer/bandwidth resource allocation) related to a BA. This is essentially

defined by the queuing/scheduling/buffer management in the forwarding path.

PHBs are implemented in DiffServ nodes by means of some buffer management and

packet scheduling mechanism. The PHB definition is not depending on the

mechanism that offers the service but in terms of behaviour characteristics relevant

to service provisioning policy.

For example, "voice" traffic might select a "strict" queuing algorithm with a

parameter of "place in top priority queue".

Standardized PHBs The following specific PHBs and recommended DSCPs for each PHB have been

standardized by the IETF:

Default PHB.

Expedited Forwarding PHB.

Class Selector (CS) PHB.

Assured Forwarding PHB.

Refer to RFC2474 for more information on the definition of the DS field.

7 6 5 4 3 2 1 0

DSCP ECN

Refer to RFC2475 for more information.

Contents

Main Page Page Page Contents Contents 4 6 Meters, queues and IPQoS......................................... 59 7 Scenario 1: Residential user.......................................85 Page Page About this IPQoS Configuration Guide About this IPQoS Configuration Guide ! Page Chapter 1 1 Document scope Page 2Introduction 2.1 What is Quality of Service? Definition Misunderstandings ! Page 2.2 Relative versus Guaranteed QoS Types of QoS 3 Basic QoS Concepts 3.1 Precedence and TOS Introduction First generation Precedence values Second generation 3.2 Differentiated Services Differentiated Services domain 3.3 Classification and conditioning principles Introduction classifier Traffic classification Traffic conditioning Page 3.4 Differentiated Services Code Point (DSCP) Introduction Per Hop Behaviour Standardized PHBs Page Assured Forwarding (AF) PHB Group: 4 IP QoS Framework Overview 4.1 Main Framework Components Graphical overview QoS Components 4.2 Resource Management Mapping to internal class 5 Packet Classification and Labelling 5.1 Classification Introduction Terminology 5.1.1 Order of classification rules Introduction Routing classification Sub-chains ! Page 5.2 Labels Introduction CLI Command groups ! Label command group Adding a label Label parameters Debug command group 5.2.1 Label parameters explained Introduction Classification Defclass Page Page 5.2.2 Using TOS, DSCP or Precedence Introduction Precedence TOSmarking TOS DSCP 5.2.3 Forwarding parameters. Introduction TTLoverwrite TTL Modify the label parameters Page 5.3 Rules 5.3.1 Rules parameters explained Introduction Srcintf Chain Index Srcip Dstip Serv Page The flush command 5.3.2 Rule debug commands Introduction Traceconfig Traceconfig result Stats 5.4 Chains Page 5.4.1 Define a relation between chains 5.5 Expressions Page Bridgeport Addr Precedence Tos Dscp ! Srcportend Dstport Icmptype Dstportend Page Page 6 Meters, queues and IPQoS 6.1 Meters and queues 6.2 The IPQoS command group Overview IPQoS command group EF command group meter command group Page Page Page MTU explained. EF stats command EF list command Line Speed 6.4 Meter command group Introduction Adding a meter 6.4.1 Meter config command Meter parameters meter config Label Meter config parameters explained Name Intf Droprate Markrate Burst Page Dscp Precedence Class Classification Meter delete command Meter list command Meter start command Meter stop command Meter flush command Meter stats command 6.4.2 Packet flow Illustration Classification Metering 6.5 Queue command group Introduction Queue config command 6.5.1 Queue config parameters explained Dest Ecnmarking Queue Propagate Data ACK1Data DataACK2 Example Maxpackets Respackets Maxbytes Resbytes Hold Markprob Queue list command Queue stats command Queue clear command 6.6 IPQoS Command group Introduction ipqos config command 6.6.1 Ipqos config parameters explained Introduction Dest Discard State ! Priority Realtimerate Burstsize Weight Maxpackets Maxbytes Ipqos list command 7 Scenario 1: Residential user. 7.1 Configuring labels and rules for VoIP. Introduction The web interface Labels Rules Expressions Page 7.2 Configuring labels and rules for DSCP. Introduction Labels Rules Expressions 7.3 Configuring labels and rules for Interactive traffic. Introduction Labels Page Expressions Page Page 8 Scenario 2: Business user with TOS marking. ! 8.1 Labels Page Page Page 8.2 Rules. Page Page Page Page Page Page Page Page 8.3 IPQoS per PVC Page Page Chapter 9 9 Scenario 3: Metering Page Reference List Reference List Page Abreviation List Abreviation List