Ip qos principles
1 / 133

IP QoS Principles - PowerPoint PPT Presentation

  • Uploaded on

IP QoS Principles. Theory and Practice Dimitrios Kalogeras. Agenda. Introduction – History – Background QoS Metrics QoS Architecture QoS Architecture Components Applications in Cisco Routers. A Bit of History.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about ' IP QoS Principles' - tallis

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Ip qos principles

IP QoS Principles

Theory and Practice

Dimitrios Kalogeras


  • Introduction – History – Background

  • QoS Metrics

  • QoS Architecture

  • QoS Architecture Components

  • Applications in Cisco Routers

A bit of history
A Bit of History

  • The Internet, originally designed for U. S. government use, offered only one service level: Best Effort.

    • No guarantees of transit time or delivery

    • Rudimentary prioritization was available, but it was rarely used.

  • Commercialization began in early 1990’s

    • Private (intranet) networks using Internet technology appeared.

    • Commercial users began paying directly for Internet use.

    • Commerce sites tried to attract customers by using graphics.

    • Industry used the Internet and intranets for internal, shared communications that combined previously-separate, specialized networks -- each with its own specific technical requirements.

    • New technologies (voice over the Internet, etc.) appeared, designed to capitalize on inexpensive Internet technologies.

The demands on modern networks
The Demands on Modern Networks

  • Network flexibility is becoming central to enterprise strategy

    • Rapidly-changing business functions no longer carried out in stable ways, in unchanging locations, or for long time-periods

    • Network-enabled applications often crucial for meeting new market opportunities, but there’s no time to custom-build a network

  • Traffic is bursty

  • Interactive voice, video applications have stringent bandwidth and latency demands

  • Multiple application networks are being combined into consolidated corporate utility networks

    • Bandwidth contention as critical transaction traffic is squeezed by web browsing, file transfers, or other low-priority or bulk traffic

    • Latency problems as interactive voice and video are squeezed by transaction, web browsing, file transfer, and bulk traffic

Qos background
QoS Background

  • Video Streaming Services

  • Video Conferencing

  • VoIP

  • Legacy SNA / DLSw

QoS development inspired by new types of applications in IP environment:


  • Quality of Service (QoS) classifies network traffic and then ensures that some of it receives special handling.

    • May track each individual dataflow (sender:receiver) separately.

    • May include attempts to provide better error rates, lower network transit time (latency), and decreased latency variation (jitter).

  • Differentiated Class of Service (CoS) is a simpler alternative to QoS.

    • Doesn't try to distinguish among individual dataflows; instead, uses simpler methods to classify packets into one of a few categories.

    • All packets within a particular category are then handled in the same way, with the same quality parameters.

  • Policy-Based Networking provides end-to-end control.

    • The rules for access and for management of network resources are stored as policies and are managed by a policy server.

Statistical behavior random arrival
Statistical Behavior: Random Arrival

  • In random arrival, the time that each packet arrives is completely independent of the time that any other packet arrives.

    • If the true situation is that arrivals tend to be evenly spaced, then random arrival calculations will overestimate the queuing delay.

    • If the true situation is that arrivals are bunched in groups (typical of data flows, such as packets and acknowledgements), then random arrival calculations will underestimate the queuing delay.

  • Our intuition is usually misleading when we think of random processes.

    • We tend to assume that queue size increases linearly as the number of customers increases.

    • But, with random arrival, there is a drastic increase in queue size as the customer arrival rate approaches 80% of the theoretical server capacity. There’s no way to store the capacity that is unused by late customers, but early customers increase the queue.

Random arrival and intuition
Random Arrival and Intuition

  • The surprising increase in queue length is best shown by a graph:

Random arrival vs self similar
Random Arrival vs. Self-Similar

  • Although random arrival is very convenient mathematically (it’s relatively simple to do random arrival calculations), it has been shown that much data traffic is self-similar.

    • Ethernet and Internet traffic flows, in particular, are self-similar.

    • The rate of initial connections is still random, however.

  • Self-similar traffic shows the same pattern regardless of changes in scale.

    • Fractal geometry (e.g., a coastline) is an example.

  • Self-similar traffic has a heavy tail.

    • The probabilities of extremely large values (e.g., file lengths of a gigabyte or more) don’t decrease as rapidly, as they would with random distributions of file lengths.

    • This matches real data traffic behaviours.

      • Long file downloads mixed with short acknowledgements

      • Compressed video with action scenes mixed with static scenes

Implications of self similar behaviour
Implications of Self-Similar Behaviour

  • “If high levels of utilization are required, drastically larger buffers are needed for self-similar traffic than would be predicted based on classical queuing analysis [i.e., assuming random behaviour].” [Stallings]

    • Combining self-similar traffic streams doesn’t quickly result in smoother traffic patterns; it’s only at the highest levels of aggregation that random-arrival statistics can be used.

Qos metrics what are we trying to control



The path as perceived by a packet!



QoS Metrics: What are we trying to control?

  • Four metrics are used to describe a packet’s transmission through a network – Bandwidth, Delay, Jitter, and Loss

  • Using a pipe analogy, then for each packet:

    • Bandwidth is the perceived width of the pipe

    • Delay is the perceived length of the pipe

    • Jitter is the perceived variation in the length of the pipe

    • Loss is the perceived leakiness if the pipe

Qos metrics bandwidth

100 Mb/s


10 Mb/s

2 Mb/s Maximum Bandwidth

QoS Metrics – Bandwidth

The amount of bandwidth available to a packet is affected by:

  • The slowest link found in the transmission path

  • The amount of congestion experienced at each hop – TCP slow-start and windowing

  • The forwarding speed of the devices in the path

  • The queuing priority given to the packet flow

Qos metrics delay
QoS Metrics – Delay

The amount of delay experienced by a packet is the sum of the:

  • Fixed Propagation Delays

    • Bounded by the speed of light and the path distance

  • Fixed Serialization Delays

    • The time required to physically place a packet onto a transmission medium

  • Variable Switching Delays

    • The time required by each forwarding engine to resolve the next-hop address and egress interface for a packet

  • Variable Queuing Delays

    • The time required by each switching engine to queue a packet for transmission

Qos metrics jitter
QoS Metrics – Jitter

The amount of Jitter experienced by a packet is affected by:

  • Serialization delays on low-speed interfaces

  • Variations in queue-depth due to congestion

  • Variations in queue cycle-times induced by the service architectures – First-Come, First-Served, for example

~214ms Serialization Delay for a 1500-byte packet at 56Kb/s

60B every 20ms

60B every 214ms

60B every 214ms


1500 Bytes of Data



1500 Bytes of Data



1500 Bytes of Data


10 Mbps Ethernet

10 Mbps Ethernet

56 Kbps WAN

Qos metrics loss
QoS Metrics – Loss

The amount of loss experienced by a packet flow is affected by:

  • Buffer exhaustion due to congestion caused by oversubscription or rate-decoupling

  • Intentional packet drops due to congestion control mechanism such as Random Early Discard






Buffer Exhaustion

Qos architecture models
QoS Architecture Models

  • Best Effort Service

  • Integrated Service

  • Differentiated Service

Qos implementation models

No State

Aggregated State

Per-Flow State

1. Best Effort

2. IntServ/RSVP

3. DiffServ

4. RSVP+DiffServ+MPLS

QoS Implementation Models

Best effort service
Best Effort Service

What exactly IP does:

  • All packets treated equally

  • Unpredictable bandwidth

  • Unpredictable delay and jitter

Integrated services intserv





Integrated Services (IntServ)

  • The Integrated Services (IntServ) model builds upon Resource Reservation Protocol (RSVP)

  • Reservations are made per simplex flow

  • Applications request reservations for network resources which are granted or denied based on resource availability

  • Senders specify the resource requirements via a PATH message that is routed to the receiver

  • Receivers reserve the resources with a RESV message that follows the reverse path

Intserv components

Control Plane

Routing Selection

Admission Control

Reservation Setup

Reservation Table

Data Plane

Flow Identification

Packet Scheduler

IntServ – Components

The Integrated Services Model can be divided into two parts – the Control and Data Planes

Intserv components1
IntServ – Components

Control Plane

  • Route Selection – Identifies the route to follow for the reservation (typically provided by the IGP processes)

  • Reservation Setup – Installs the reservation state along the selected path

  • Admission Control – Ensures that resources are available before allowing a reservation

    Data Plane

  • Flow Identification – Identifies the packets that belong to a given reservation (using the packet’s 5-Tuple)

  • Packet Scheduling – Enforces the reservations by queuing and scheduling packets for transmission

Intserv service models
IntServ – Service Models

Applications using IntServ can request two basic service-types:

  • Guaranteed Service

    • Provides guaranteed bandwidth and queuing delays end-to-end, similar to a virtual-circuit

    • Applications can expect hard-bounded bandwidth and delay

  • Controlled-Load Service

    • Provides a Better-than-Best-Effort service, similar to a lightly-loaded network of the required bandwidth

    • Applications can expect little to zero packet loss, and little to zero queuing delay

      These services are mapped into policies that are applied via CB-WFQ, LLQ, or MDRR

Intserv scaling issues
IntServ – Scaling Issues

  • IntServ routers need to examine every packet to identify and classify the microflows using the 5-tuple

  • IntServ routers must maintain a token-bucket per microflow

  • Guaranteed Service requires the creation of a queue for each microflow

  • Data structures must be created and maintained for each reservation

Differentiated services diffserv
Differentiated Services (DiffServ)

  • The DiffServ Model specifies an approach that offers a service better than Best-Effort and more scalable than IntServ

  • Traffic is classified into one of five forwarding classes at the edge of a DiffServ network

  • Forwarding classes are encoded in the Differentiated Services Codepoint (DSCP) field of each packet’s IP header

  • DiffServ routers apply pre-provisioned Per-Hop Behaviors (PHBs) to packets according to the encoded forwarding class











Diffserv compared to intserv
DiffServ – Compared to IntServ

  • DiffServ allocates resources to aggregated rather than to individual flows

  • DiffServ moves the classification, policing, and marking functions to the boundary nodes – the core simply forwards based on aggregate class

  • DiffServ defines Per-Hop forwarding behaviors, not end-to-end services

  • DiffServ guarantees are based on provisioning, not reservations

  • The DiffServ focus is on individual domains, rather than end-to-end deployments

Diffsrv the ds field rfc 2474
DiffSrv – The DS Field (RFC 2474)

  • The DS field is composed of the 6 high-order bits of the IP ToS field

  • The DS field is functionally similar to the IPv4 TOS and IPv6 Traffic Class fields

  • The DS field is divided into three pools:

    • nnnnn0 – Standards Use

    • nnnn11 – Experimental / Local Use

    • nnnn01 – Experimental / Local Use, possible Standards Use

  • Class Selector Codepoints occupy the high-order bits (nnn000) and map to the IPv4 Precedence bits



DS field

Diffsrv forwarding classes
DiffSrv – Forwarding Classes

The DS Field can encode:

  • Eight Class Selector Codepoints compatible with legacy systems (CS0-7)

  • An Expedited Forwarding (EF) Class

  • Four Assured Forwarding Classes, each with three Drop Precedence (AFxy, where x=1-4, and y=1-3)

  • Packets in a higher AF Classes have a higher transmit priority

  • Packets with a higher Drop Precedence are more likely to be dropped

Diffserv per hop behaviours
DiffServ – Per-Hop Behaviours

  • A Per-Hop Behaviour (PHB) is an observable forwarding behaviour of a DS node applied to all packets with the same DSCP

  • PHBs do NOT mandate any specific implementation mechanisms

  • The EF PHB should provide a low-loss, low-delay, low-jitter, assured bandwidth service

  • The AF PHBs should provide increasing levels or service (higher bandwidth) for increasing AF levels

  • The Default PHB (CS0) should be equivalent to Best-Effort Service

  • Packets within a given PHB should not be re-ordered

Diffserv boundary nodes









DiffServ – Boundary Nodes

DiffServ Boundary Nodes are responsible for classifying and conditioning packets as they enter a given DiffServ Domain

  • Classifier Examine each packet and assign a Forwarding Class

  • Marker Set the DS Field to match the Forwarding Class

  • Meter Measure the traffic flow and compare it to the traffic profile

  • Remarker Remark (lower) the DS Field for out-of-profile traffic

  • Shaper Shape the traffic to match the traffic profile

  • Dropper Drop out of profile traffic

Diffserv summary

DiffServ Domain

Classification / Conditioning







DiffServ – Summary

The trouble with diffserv
The Trouble with DiffServ

  • As currently formulated, DiffServ is strong on simplicity and weak on guarantees

  • Virtual wire using EF is OK, but how much can be deployed?

  • DiffServ has no topology-aware admission control mechanism

Rsvp diffserv integration


No State

Per-Flow State

RSVP + DiffServ

Best Effort



Aggregated State

Firm Guarantees

Admission Control

RSVP-DiffServ Integration

The best of both worlds – Aggregated RSVP integrated with DiffServ

But – given the presence of a DiffServ domain in a network, how do we support RSVP End-to-End?

Rsvp diffserv integration how
RSVP-DiffServ Integration – How?

  • Routers at edge of a DS cloud perform microflow classification, policing, and marking

    • Guaranteed Load set to the EF, Controlled load set to AFx, and Best Effort set to CS0

    • Service Model to Forwarding Class mapping is arbitrary

  • RSVP signaling is used in both the IntServ and DiffServ regions for admission control

  • The DiffServ core makes and manages aggregate reservations for the DS Forwarding Classes based on the RSVP microflow reservations

  • The core then schedules and forwards packets based only on the DS Field

Rsvp diffserv integration1
RSVP-DiffServ Integration

Border Routers implement per-flow classification, policing, and marking

The DiffServ region aggregates the flows into DS Forwarding Classes

DiffServ Region

RSVP Signaling is propagated End-to End

The IntServ regions contain Guaranteed or Controlled Load Microflows

Rsvp diffserv integration summary
RSVP-DiffServ Integration – Summary

  • The forwarding plane is still DiffServ

  • We now make a small number of aggregated reservations from ingress to egress

  • Microflow RSVP messages are carried across the DiffServ cloud

  • Aggregate reservations are dynamically adjusted to cover all microflows

  • RSVP flow-classifiers and per-flow queues are eliminated in the core

  • Scalability is improved – only the RSVP flow states are necessary – Tested to 10K flows

Qos architecture components
QoS Architecture Components

  • Classification

  • Coloring

  • Admission Control

  • Traffic Shaping/Policing

  • Congestion Management

  • Congestion Avoidance

  • Signaling

Traffic classification
Traffic Classification

  • Most fundamental QoS building block

  • The component of a QoS feature that recognizes and distinguishes between different traffic streams

  • Without classification, all packets are treated the same

Traffic classification admission control issues
Traffic Classification/Admission Control Issues

  • Always performed at the network perimeter

  • Makes traffic conform to the internal network policy

  • Marks packets with special flags (colors)

  • Colors used afterwards inside the network for QoS management

Classification admission control scheme








Classification/Admission Control Scheme

Classification criteria
Classification Criteria

  • IP header fields

  • TCP/UDP header fields

  • Routing information

  • Packet Content (NBAR)i.e. HTTP, HTTPS, FTP, Napster etc.

Traffic coloring options
Traffic Coloring Options

  • IP Precedence

  • DSCP

  • QoS Group

  • 802.1p CoS


  • Frame Relay DE

Type of service rfc791
Type-of-Service (RFC791)








ToS Field

Total Length





Dscp diffserv code point
DSCPDiffserv Code Point

DSCP (6 bits)


Classification mechanisms
Classification mechanisms

  • MQC ( Modular Qos Command Line Interface)

  • CAR ( Commited Access Rate)

Modular qos cli
Modular QoS CLI

Modular QoS CLI (MQC)

  • Command syntax introduced in 12.0(5)T

  • Reduces configuration steps and time

  • Uniform CLI across all main Cisco IOS-based platforms

  • Uniform CLI structure for all QoS features

Basic mqc commands


class-map [match-any | match-all] class-name

  • 1.Create Class Map - a traffic class( match access list, input interface, IP Prec, DSCP, protocol (NBAR) src/dst MAC address, mpls exp).


policy-map policy-map-name

  • 2. Create Policy Map (Service Policy) - Associate a class map with one or more QoS policies(bandwidth, police, queue-limit, random detect, shape, set prec, set DSCP, set mpls exp).


service-policy {input | output} policy-map-name

  • 3. Attach Service Policy- Associate the policy map with an input or output interface.

Basic MQC Commands

Basic mqc commands1
Basic MQC Commands

  • 1. Create Class Map – defines traffic selection criteria

Router(config)# class-map class1

Router(config-cmap)# match ip precedence 5

Router(config-cmap)# exit

  • 2. Create Policy Map- associates classes with actions

Router(config)# policy-map policy1

Router(config-pmap)# class class1

Router(config-pmap-c)# set mpls experimental 5

Router(config-pmap-c)# bandwidth 3000

Router(config-pmap-c)# queue-limit 30

Router(config-pmap)# exit

  • 3. Attach Service Policy – enforces policy to interfaces

Router(config)# interface e1/1

Router(config-if)# service-policy output policy1

Router(config-if)# exit

Classification configuring sample
Classification Configuring Sample

IOS 12.1(5)T

MQC based

class-map match-all premium

match access-group name premium


class-map match-any trash

match protocol napster

match protocol fasttrack


policy-map classify

class premium

set ip precedence priority

class trash

police 64000 conform-action set-prec-transmit 1 excess-action drop


ip access-list extended premium

permit tcp host any eq telnet


interface serial 2/1

ip unnumbered loopback 0

service-policy input classify

Traffic class definitions

QoS policy definition

ACL definition

QoS Policy attachedto interface

Classification configuring sample1
Classification Configuring Sample

ip cef


interface serial 2/1

ip unnumbered loopback 0

rate-limit input access-group 100 64000 8000 8000 conform-action set-prec-transmit 1 exceed-action set-prec-transmit 0


access-list 100 permit tcp host any eq http

CAR based

CAR definition

ACL definition

Classification configuring sample2
Classification Configuring Sample

route-map classify permit 10

match ip address 100

set ip precedence flash


route-map classify permit 20

match ip next-hop 1

set ip precedence priority


interface serial 2/1

ip unnumbered loopback 0

ip policy route-map classify


access-list 1 permit

access-list 100 permit tcp host any eq http

Route-map based

Route-map definitions

Route-map attachedto interface

ACL definitions

Shaping policing

  • Used to assign more predictive behavior to traffic

  • Uses Token Bucket model

Token bucket model




Overflow Tokens






Token Bucket Model

Token Bucket main parameters:

  • Token Arrival Rate - v

  • Bucket Depth - Bc

  • Time Interval – tc

  • Link Capacity - C

Token Bucket characterizes traffic source

tc = Bc/v

Token bucket model1
Token Bucket Model

  • Bucket is being filled with tokens at a rate v token/sec.

  • When bucket is full all the excess tokens are discarded.

  • When packet of size L arrives, bucket is checked for availability of corresponding amount of tokens.

  • If several packets arrive back-to-back and there are sufficient tokens to serve them all, they are accepted at peak rate (usually physical link speed).

  • If enough tokens available, packet is optionally colored and accepted to the network and corresponding amount of tokens is subtracted from the bucket.

  • If not enough tokens, special action on packet is performed.

Token bucket model2
Token Bucket Model

Actions performed on nonconforming packets:

  • Dropped (Policing)

  • Delayed in queue either FIFO or WFQ (Shaping)

  • Colored/Recolored

Token bucket model3
Token Bucket Model

Bucket depth variation effect:

  • Bc = 0 Constant Bit Rate (CBR)

  • Bc No Regulation

    Bucket depth is characteristic of traffic burstiness

    Maximum number of bytes transmitted over period of time t:

    A(t)max = Bc+v·t

Excess burst be cisco implementation
Excess Burst (Be)Cisco Implementation

GTS ( Generic Traffic Shaping)

If during previous tcn-1 interval bucket Bc was not depleted (there is no congestion), in the next interval tcnBc+Be bytes are available for burst.

In frame relay implementations packets admitted via Be tokens are marked with DE bit.

Excess burst be cisco implementation1
Excess Burst (Be)Cisco Implementation

CBTS (Class Based Traffic Shaping)

allows higher throughput in uncongested environment up to peak rate calculated as vPeak = vCIR(1+Be/Bc)

Peak rate can be set up manually.

Excess burst be cisco implementation2
Excess Burst (Be)Cisco Implementation


allows RED like behavior:

  • traffic fitting into Bc always conforms

  • traffic fitting into Be conforms with probability proportional to amount of tokens left in the bucket

  • traffic not fitting into Be always exceedsCAR uses the following parameters:

  • t – time period since the last packet arrival

  • Current Debt (Dcur) – Amount of debt during current time interval

  • Compound Debt (Dcomp) – Sum of all Dcur since the last drop

  • Actual Debt (Dact) – Amount of tokens currently borrowed

Excess burst be cisco implementation3
Excess Burst (Be)Cisco Implementation

Packet of lengthL arrived

CAR Algorithm



Bccur – L > 0

Bccur = Bccur – L


Dcur = L - Bccur

Bccur = 0

Dcomp = Dcomp + Dcur

Dact = Dact + Dcur




Dact > Be



Dcomp = 0

Dcomp > Be


Shaping configuration sample
Shaping Configuration Sample

GTS Based

interface serial 2/1

ip unnumbered loopback 0

traffic-shape rate 64000 8000 1000 256


interface serial 2/2

ip unnumbered loopback 0

traffic-shape group 100 64000 8000 8000 512


access-list 100 permit tcp host any eq http

Shaper Definitions

ACL definition

Shaper can be only used to control egress traffic flow!

Policing configuration sample
Policing Configuration Sample

IOS 12.0(5)T

CAR Based

ip cef

interface serial 2/1

ip unnumbered loopback 0

rate-limit output access-group 100 64000 8000 16000 conform-action transmit excess-action drop


interface serial 2/2

ip unnumbered loopback 0

rate-limit input 128000 16000 32000 conform-action transmit excess-action drop


access-list 100 permit tcp host any eq http

CAR Definitions

ACL definition

Policer can be used to control ingress traffic flow!

Shaping policing configuration sample
Shaping/Policing Configuration Sample

IOS 12.1(5)T

MQI Based

class-map match-all policed

match protocol http

class-map match-all shaped

match access-group name ftp-downloads


policy-map bad-boy

class policed

police 64000 8000 8000 conform-action transmit exceed-action drop

class shaped

shape average 128000


interface serial 2/1

ip unnumbered loopback 0

service-policy output bad-boy


ip access-list extended ftp-downloads

permit tcp any eq ftp-data any

Class definitions

QoS policy definition

QoS Policy attachedto interface

ACL definition

Car policing problem
CAR Policing Problem

Why cannot my traffic reach CIR value?

Cause: Improper setting of Bc and Be values

CAR is aggressive, as drops excessive packets and the lost data needs to be retransmitted by upper layers (mainly TCP) after timeout. This also causes TCP to shrink its window reducing flow throughput.

Cisco Systems recommends the following settings:

Bc = 1.5xCIR/8

Be = 2xBc


  • Traffic burst may temporarily exceed interface capacity

  • Without queuing this excess traffic will be lost

  • Queuing allows bursty traffic to be transmitted without drops

  • Queuing strategy defines order in which packets are transmitted through egress interface

  • Queuing introduced additional delay which signals to adaptive flows (like TCP) to back off their throughput

Queuing algorithms
Queuing Algorithms

  • FIFO

  • Priority (Absolute)

  • Weighted Round Robin (WRR)

  • Fair


  • Simplest queuing method with the least CPU overhead

  • No congestion control

  • Transmits packets in the order of arrival

  • High volume traffic can suppress interactive flows

  • Default queuing for interfaces > 2Mbps (i.e. Ethernet)


FIFO average queue depth dependence on load

Absolute priority queuing
Absolute Priority Queuing

  • Generic Priority Queuing

  • Custom Queuing

  • RTP Priority Queuing

  • Low Latency Queuing (LLQ)

Simplest qos algorithm priority queuing
Simplest QoS Algorithm: Priority Queuing

  • Stated requirement:

    • “If <application> has traffic waiting, send it next”

  • Commonly implemented

    • Defined behavior of IP precedence

Priority queuing implementation approach
Priority Queuing Implementation Approach

  • Identify interesting traffic

    • Access lists

  • Place traffic in various queues

  • Dequeue in order of queue precedence

Priority queuing pq
Priority Queuing (PQ)

  • Interface Hardware

  • Ethernet

  • Frame Relay

  • ATM

  • Serial Link

  • Etc.


Traffic Destined for Interface




Transmit Queue

Output Line


Q Length Defined by Q Limit

Absolute Priority Scheduling

Interface Buffer Resources

  • Classification by:

  • Protocol (IP, IPX, AppleTalk, SNA, DecNet, Bridge, etc.)

  • Incoming Interface (EO, SO, S1, etc.)

Priority queuing scheme
Priority Queuing Scheme





High Empty?

Medium Empty?

Normal Empty?

Low Empty?





Send packet from High

Send Packet from Medium

Send Packet from Normal

Send Packet from Low

Generic pq drawbacks
Generic PQ Drawbacks

  • Needs thorough admission control

  • No upper limit for each priority level

  • High risk of low priority queues` starvation effect

Generic pq configuration sample
Generic PQ Configuration Sample

priority-list 1 protocol ip high tcp telnet

priority-list 1 protocol ip high list 100

priority-list 1 protocol ip medium lt 1000

priority-list 1 interface ethernet 0/0 medium

priority-list 1 default low


interface serial 2/1

ip unnumbered loopback 0

priority-group 1


access-list 100 permit tcp host any eq http

PQ Definition

PQ Attachedto Interface

ACL definition

Custom Queuing (CQ)

(Weighted Round Robin)

  • Interface Hardware

  • Ethernet

  • Frame Relay

  • ATM

  • Serial Link

  • Etc.



Traffic Destined for Interface


Transmit Queue

Output Line




Up to 16

Link Utilization Ratio

Weighted RoundRobin Scheduling

(byte count)

Q Length Deferred by Queue Limit

  • Classification by:

  • Protocol (IP, IPX, AppleTalk, SNA, DecNet, Bridge, etc.)

  • Incoming interface(EO, SO, S1, etc.)

Interface Buffer Resources

Allocate Proportion of Link Bandwidth)

Wrr drawbacks
WRR Drawbacks

  • Unpredictable jitter

  • Fairness significantly depends on MTU and TCP window size

  • Complex calculations to achieve desired traffic proportions

Cq byte count calculus
CQ Byte-count Calculus

Distribute bandwidth to 3 queues with proportion x:y:z and packet sizes qx, qy, qz.

  • Calculate ax=x/qx, ay=y/qy, az=z/qz.

  • Normalize and round ax, ay, az. ax’= round(ax/min(ax, ay, az)); ay’= round(ay/min(ax, ay, az)); az’= round(az/min(ax, ay, az)).

  • Convert obtained packet proportion into byte countbcx = ax’·qx; bcy = ay’·qy; bcz = az’·qz.

  • Actual bandwidth share of i-th queue can be calculated with the following formula:

  • For better approximation obtained byte-counts can be multiplied by some positive whole number.

Starting with IOS 12.1 CQ employs Deficit Round Robin algorithm and there is no need in such byte-count tuning.

Cq configuration sample
CQ Configuration Sample

queue-list 1 protocol ip 1 tcp telnet

queue-list 1 protocol ip 2 list 100

queue-list 1 protocol ip 3 udp 53

queue-list 1 interface ethernet 0/0 4

queue-list 1 queue 1 byte-count 3000

queue-list 1 queue 2 byte-count 4500

queue-list 1 queue 3 byte-count 3000

queue-list 1 queue 4 byte-count 1500

queue-list 1 default 4


interface serial 2/1

ip unnumbered loopback 0

custom-queue-list 1


access-list 100 permit tcp host any eq http

CQ List Definition

CQ Attachedto Interface

ACL Definition

Bitwise round robin fair queuing
“Bitwise Round Robin” Fair Queuing

TDM Model

  • Keshav, Demers, Shenker, and Zhang

  • Simulates a TDM

  • One flow per channel

Time Division Multiplexer

Tdm message arrival sequence
TDM Message Arrival Sequence






Time Division Multiplexer


Tdm message delivery sequence
TDM Message Delivery Sequence






Time Division Multiplexer


Fair queuing algorithm
Fair Queuing Algorithm

Employs virtual bit-by-bit round robin model (BRR)

BRR dynamics are described by the equation:

i-th packet from flow a arriving at time t0 is services at time t :

Servicing of i-th packet from flow a will start at Sia and finish at Fia :

Additional parameter is added for priority assignment to inactive flows :

Packets are ordered for transmission according to Biavalues.

Fair queuing approach
Fair Queuing Approach

  • Enqueue traffic in the sequence the TDM would deliver it

  • As a result, be as fair as the TDM

Effects of fair queuing
Effects of Fair Queuing

  • Low-bandwidth flows get

    • As much bandwidth as they can use

    • Timely service

  • High-bandwidth flows

    • Interleave traffic

    • Cooperatively share bandwidth

    • Absorb latency

What weighting does
What Weighting Does

  • In TDM

    • Channel speed determines message “duration”

  • In WFQ

    • Multiplier on message length changes simulated message “duration”

  • Result:

    • Flow’s “fair” share predictably unfair

Weighted fair queuing wfq
Weighted Fair Queuing (WFQ)

Traffic Destined for Interface

Transmit Queue

Output Line


Weighted Fair Scheduling

Configurable Number of Queues

  • Flow-Based Classification by:

  • Source and destination address

  • Protocol

  • Session identifier (port/socket)

Interface Buffer Resources

  • Weight Determined by:

  • Requested QoS (IP Procedure, RSVP)

  • Frame Relay FECN, BECN, DE(For FR Traffic)

  • Flow throughput (weighted-fair)

Weighted fair queuing wfq1
Weighted Fair Queuing (WFQ)

  • Fair bandwidth per flow allocation

  • Low delay for interactive applications

  • Protection from ill-behaved sources

Weighted fair queuing wfq2
Weighted Fair Queuing (WFQ)

Flow classified by the following fields:

  • Source address

  • Source port

  • Destination address

  • Destination port

  • ToS

    Weight of each flow (queue) depends on ToS:

    weight = 1/(precedence+1)

    Bandwidth distributed in 1/weight proportions

Weighted fair queuing wfq3
Weighted Fair Queuing (WFQ)

  • Packets are ordered according to the expected virtual departure time of their last bit.

  • Low volume flows have preference over high volume transfers.

  • Low volume flow is identified as using less than its share of bandwidth.

  • The special queue length threshold value is established, after which only low volume flows can enqueue. All the packets, that belong to high volume flows are dropped.

Drawbacks of weighted fair queuing
Drawbacks of Weighted Fair Queuing

  • Requires more sorting than other approaches

Weighted fair queuing wfq4





Weighted Fair Queuing (WFQ)

Weighted fair queuing wfq5





Weighted Fair Queuing (WFQ)

Wfq configuration sample
WFQ Configuration Sample

interface serial 2/1

ip unnumbered loopback 0

fair-queue 32 128 0

Queue Threshold


Number of reservable queues

Maximal numberof queues

Rtp priority queuing
RTP Priority Queuing

  • Classifies only by UDP port range

  • Only even ports from the range are classified

  • Establishes upper limit via integrated policer

  • Excess traffic dropped during congestion periods

  • RTP PQ has priority over LLQ

Rtp pq configuration sample
RTP PQ Configuration Sample

interface serial 2/1

ip unnumbered loopback 0

ip rtp priority 16384 16383 256

Starting UDP port

Bandwidth Limit(kbps)

Range length

Low latency queuing llq
Low Latency Queuing (LLQ)

  • Implemented using MQI

  • Very rich classification criteria (class-map)

  • Establishes upper limit via integrated policer

  • Excess traffic dropped during congestion periods

Llq configuration sample
LLQ Configuration Sample

IOS 12.0(5)T

class-map match-all voice

match access-group name voip


policy-map llq

class voip

priority 30

class class-default

fair-queue 64


interface serial 2/1

ip unnumbered loopback 0

service-policy output llq


ip access-list extended voip

permit ip host any

Class definitions

LLQ policy definition

LLQ Policy attachedto interface

ACL definition

Class based wfq cbwfq
Class Based WFQ (CBWFQ)

  • Based on the same algorithm as WFQ

  • Weights can be manually configured

  • Allows to easily specify guaranteed bandwidth for a class

  • Configuration based on Cisco MQI

Cbwfq configuration sample
CBWFQ Configuration Sample

IOS 12.0(5)T

class-map match-all premium

match access-group name premium-cust

class-map match-all low-priority

match protocol napster


policy-map cbwfq-sample

class premium

bandwidth 512

class low-priority

shape average 128

shape peak 512

class class-default

fair-queue 64


interface serial 2/1

ip unnumbered loopback 0

max-reserved-bandwidth 85

service-policy output cbwfq-sample


ip access-list extended premium-cust

permit ip host any

Class definitions

Qos policy definition

QoS Policy attachedto interface

ACL definition

Cbwfq configuration sample1
CBWFQ Configuration Sample

IOS 12.1(5)T

Hierarchical Design

class-map match-all premium

match access-group name premium-cust

class-map match-all voice

match ip precedence flash


policy-map total-shaper

class class-default

shape average 1536

service-policy class-policy

policy-map class-policy

class premium

bandwidth 512

class voice

priority 64

class class-default

fair-queue 128

interface fastethernet 1/0

ip unnumbered loopback 0

max-reserved-bandwidth 85

service-policy output total-shaper


ip access-list extended premium-cust

permit ip host any

Hierarchical cbwfq limitations
Hierarchical CBWFQ Limitations

  • Only two levels of hierarchy are supported

  • set command not supported in child policy

  • Shaping allows only in parent policy

  • LLQ can be configured only either in child or parent policies but not in both

  • FQ allowed only in child policy

Global synchronization effect


Link Capacity

Avg. Throughput


Global Synchronization Effect

Tail drop and tcp flow control
Tail Drop and TCP Flow Control

  • Packet drops from all TCP sessions simultaneously

  • High probability of multiple drops from the same TCP session

  • Uniformly distributed drops from high volume and interactive flows

    Result: Low average throughput!

Random early detection red
Random Early Detection (RED)

Developed by Van Jacobson in 1993

  • Starts randomly dropping packets before actual congestion occurs

  • Keeps average queue depth low

  • Increases average throughput

Global synchronization removed


Link Capacity

Avg. Throughput


Global Synchronization Removed

Random early detection red1



Tail Drop







 min





Random Early Detection (RED)

Random early detection red2
Random Early Detection (RED)

  • min – Minimal threshold after which RED starts packet drops. Minimal recommended value is 5 packets.

  • max – Maximal threshold after which all packets are dropped. Recommended value is 2-3 times min.

  • - Mark probability denominator denotes packet drop probability at max average queue depth. Optimal value – 0.1 .

  •  - Exponential weighting factor determines the level of backward value-dependence in average queue depth calculation:qavg = (qold· (1 - 2-)) + (qcur · 2-)General recommendation  = 9.

RED Parameters:

Tcp rate control 1
TCP Rate Control - 1

  • In TCP, the spacing of ACKs and the window size in the ACKs controls the transmitter’s rate.

  • Rate Control manipulates the ACKs as they pass through the rate control device by:

    • Adjusting the size of TCP ACK window

    • Inserting new ACKs

    • Re-spacing existing ACKs

  • Rate Control works only with TCP; other methods, such as Token Bucket, must be used with UDP.

  • Rate Control violates the protocol layering design, as it allows network devices to manipulate a higher-layer protocol’s operation. Nevertheless, it usually functions well and provides fine-grained control.

Weighted random early detection wred
Weighted Random Early Detection (WRED)

  • Modified version of RED

  • Weights determine the set of parameters: min , maxand .

  • Weight depends on ToS field value

  • Interactive flows are preserved

Wred configuration sample
WRED Configuration Sample

Interface based

interface serial 2/1

ip unnumbered loopback 0


random-detect 0 32 64 20

random-detect 1 32 64 20

random-detect 2 32 64 20

random-detect 3 32 64 20



Wred configuration sample1
WRED Configuration Sample

MQI based

policy-map red

class class-default


random-detect 0 32 64 20

random-detect 1 32 64 20

random-detect 2 32 64 20

random-detect 3 32 64 20

interface Serial2/1

ip unnumbered loopback 0

service-policy output red



WRED is incompatible with LLQ feature!

Link fragmentation and interleaving lfi



64 kbps

1500 bytes  190ms

Link Fragmentation and Interleaving (LFI)

For links < 128kbps

Link fragmentation and interleaving lfi1

64 kbps

Link Fragmentation and Interleaving (LFI)

Supported interfaces:

  • Multilink PPP

  • Frame Relay DLCI

  • ATM VC

Lfi configuration sample
LFI Configuration Sample

MLP version

interface virtual-template 1

ip unnumbered loopback 0

ppp multilink

ppp multilink interleave

ppp multilink fragment-delay 30

ip rtp interleave 16384 1024 512

Resource reservation protocol rsvp
Resource Reservation Protocol (RSVP)

  • End-to-end QoS signaling protocol

  • Used to establish dynamic reservations over the network

  • Always establishes simplex reservation

  • Supports unicast and multicast traffic

  • Actually uses WFQ and WRED mechanisms

Resource reservation protocol rsvp3
Resource Reservation Protocol (RSVP)

Reservation Types:

  • Guaranteed Rate (uses WFQ and LLQ)

  • Controlled Load (uses WRED)

Qos policy propagation over bgp
QoS Policy Propagation over BGP

  • QoS policy can be shared inside single AS or among different ASs.

  • Community attribute is usually used for color assignments

  • Prevents manual policy changes in network devices

Qppb configuration sample
QPPB Configuration Sample

Router A

Router B

ip bgp-community new-format


router bgp 10

neighbor remote-as 20

neighbor send-community

neighbor route-map cout out


route-map cout permit 10

match ip address 20

set community 60:9


access-list 20 permit

ip bgp-community new-format


router bgp 20

neighbor remote-as 10

table-map mark-pol


route-map mark-pol permit 10

match community 1

set ip precedence flash


ip community-list 1 permit 60:9


interface Serial 0/1

ip unnumbered loopback 0

bgp-policy source ip-prec-map

Topics not covered
Topics not Covered

  • Multiprotocol Label Switching (MPLS)

  • Frame Relay QoS

  • ATM QoS

  • Distributed Queuing Algorithms

  • Multicast


  • QoS is not an exotic feature any more

  • QoS allows specific applications (VoIP, VC) to share network infrastructure with best-effort traffic

  • QoS in IP networks simplifies their functionality avoiding Frame Relay and ATM usage