Optimizing Converged Cisco Networks (ONT)

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Optimizing Converged Cisco Networks (ONT)

Module 4: Implement the DiffServ QoS Model


Module 4: Implement the DiffServ QoS Model

Lesson 4.1: Introducing Classification and Marking


Objectives Describe the classification and marking for QoS.

Explain the relationship between IP Precedence and DSCP.

Describe the standard Per Hop Behavior (PHB) groups and their characteristics.

Explain how a service class is used to implement QoS policies.

Describe a trust boundary and the guidelines used to establish this boundary.


Classification Classification is the process of identifying and

categorizing traffic into classes, typically based upon:Incoming interface

IP precedence

DSCP

Source or destination address

Application

Without classification, all packets are treated the same.

Classification should take place as close to the source as possible.


Marking Marking is the QoS feature component that “colors” a

packet (frame) so it can be identified and distinguished from other packets (frames) in QoS treatment.

Commonly used markers:Link layer:

CoS (ISL, 802.1p)

MPLS EXP bits

Frame Relay

Network layer:

DSCP

IP precedence


Classification and Marking in the LAN with IEEE 802.1Q

IEEE 802.1p user priority field is also called CoS.

IEEE 802.1p supports up to eight CoSs.

IEEE 802.1p focuses on support for QoS over LANs and 802.1Q ports.

IEEE 802.1p is preserved through the LAN, not end to end.


Classification and Marking in the Enterprise


DiffServ Model Describes services associated with traffic classes,

rather than traffic flows.

Complex traffic classification and conditioning is performed at the network edge.

No per-flow state in the core.

The goal of the DiffServ model is scalability.

Interoperability with non-DiffServ-compliant nodes.

Incremental deployment.


Classification ToolsIP Precedence and DiffServ Code Points

IPv4: three most significant bits of ToS byte are called IP Precedence (IPP)—other bits unused

DiffServ: six most significant bits of ToS byte are called DiffServ Code Point (DSCP)—remaining two bits used for flow control

DSCP is backward-compatible with IP precedence

7 6 5 4 3 2 1 0

ID Offset TTL Proto FCS IP SA IP DA DataLenVersion Length

ToSByte

DiffServ Code Point (DSCP) IP ECN

IPv4 Packet

IP Precedence UnusedStandard IPv4

DiffServ Extensions


IP ToS Byte and DS Field Inside the IP Header


IP Precedence and DSCP Compatibility

Compatibility with current IP precedence usage (RFC 1812)

Differentiates probability of timely forwarding:

(xyz000) >= (abc000) if xyz > abc

That is, if a packet has DSCP value of 011000, it has a greater probability of timely forwarding than a packet with DSCP value of 001000.


Per-Hop Behaviors

DSCP selects PHB throughout the network:Default PHB (FIFO, tail drop)

Class-selector PHB (IP precedence)

EF PHB

AF PHB


Standard PHB Groups


Expedited Forwarding (EF) PHB

EF PHB:Ensures a minimum departure rate

Guarantees bandwidth—class guaranteed an amount of bandwidth with prioritized forwarding

Polices bandwidth—class not allowed to exceed the guaranteed amount (excess traffic is dropped)

DSCP value of 101110: Looks like IP precedence 5 to non-DiffServ-compliant devices:

Bits 5 to 7: 101 = 5 (same 3 bits are used for IP precedence)

Bits 3 and 4: 11 = No drop probability

Bit 2: Just 0


Assured Forwarding (AF) PHB

AF PHB:Guarantees bandwidth

Allows access to extra bandwidth, if available

Four standard classes: AF1, AF2, AF3, and AF4

DSCP value range of aaadd0:aaa is a binary value of the class

dd is drop probability


AF PHB Values

Each AF class uses three DSCP values.

Each AF class is independently forwarded with its guaranteed bandwidth.

Congestion avoidance is used within each class to prevent congestion within the class.


Mapping CoS to Network Layer QoS


QoS Service Class A QoS service class is a logical grouping of packets

that are to receive a similar level of applied quality.

A QoS service class can be:A single user (such as MAC address or IP address)

A department, customer (such as subnet or interface)

An application (such as port numbers or URL)

A network destination (such as tunnel interface or VPN)


Implementing QoS Policy Using a QoS Service Class


QoS Service Class Guidelines Profile applications to their basic network requirements.

Do not over engineer provisioning; use no more than four to five traffic classes for data traffic:

Voice applications: VoIP

Mission-critical applications: Oracle, SAP, SNA

Interactive applications: Telnet, TN3270

Bulk applications: FTP, TFTP

Best-effort applications: E-mail, web

Scavenger applications: Nonorganizational streaming and video applications (Kazaa, Yahoo)

Do not assign more than three applications to mission-critical or transactional classes.

Use proactive policies before reactive (policing) policies.

Seek executive endorsement of relative ranking of application priority prior to rolling out QoS policies for data.


Classification and Marking DesignQoS Baseline Marking Recommendations

ApplicationL3 Classification

DSCPPHBIPP CoS

Transactional Data 18AF212 2

Call Signaling 24CS3*3 3

Streaming Video 32CS44 4

Video Conferencing 34AF414 4

Voice 46EF5 5

Network Management 16CS22 2

L2

Bulk Data 10AF111 1

Scavenger 8CS11 1

Routing 48CS66 6

Mission-Critical Data 26AF31*3 3

Best Effort 000 0


How Many Classes of Service Do I Need?

4/5 Class Model

Scavenger

Critical Data

Call Signaling

Realtime

8 Class Model

Critical Data

Video

Call Signaling

Best Effort

Voice

Bulk Data

Network Control

Scavenger

11 Class Model

Network Management

Call Signaling

Streaming Video

Transactional Data

Interactive-Video

Voice

Best Effort

IP Routing

Mission-Critical Data

Scavenger

Bulk Data

Time

Best Effort


Trust Boundaries: Classify Where?

For scalability, classification should be enabled as close to the edge as possible, depending on the capabilities of the device at:

Endpoint or end system

Access layer

Distribution layer


Trust Boundaries: Mark Where?

For scalability, marking should be done as close to the source as possible.


Self Check

1. Which PHB would be used for voice traffic?

2. How many bits are used for IP Precedence? For DSCP?

3. Which PHB can allow access to extra bandwidth if it is available?

4. How is CDP used to establish trust boundaries?


Summary Classification, marking, and queuing are critical

functions of any successful QoS implementation.

Classification allows network devices to identify traffic as belonging to a specific class with the specific QoS requirements determined by an administrative QoS policy.

The DiffServ model uses classes to describe services offered to network traffic, rather than traffic flows.

DiffServ uses DSCP to establish Per Hop Behaviors (PHBs) to classify and service traffic.


Q and A


Resources DiffServ -- The Scalable End-to-End QoS Model

http://www.cisco.com/en/US/partner/products/ps6610/products_white_paper09186a00800a3e2f.shtml

Quality of Service - The Differentiated Services Modelhttp://www.cisco.com/en/US/partner/products/ps6610/products_data_sheet0900aecd8031b36d.html


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