Jump to first page DiffServ Overview University of British Columbia Cpsc 527 Advanced Computer...

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University of British Columbia

Cpsc 527Advanced Computer Communications

Lecture 7b DiffServ OverviewDiffServ Overview

Instructor: Dr. Son T. VuongEmail: [email protected]

The World Connected

Spring 2010 – Dr. Son Vuong Cpsc 527 2

Outline

Overview of QoS and DiffServ Our research on DiffServ

A Class of Protocols Dynamic (Fair) Distribution DiffServ

Conclusions


What is the Service on Internet

A "Service" defines some significant characteristics of packet transmission in one direction across a set of one or more paths within a network.

These characteristics may be specified in quantitative or statistical terms of: throughput, delay, jitter, loss.


Current Problems with the Internet

Best-effort service has strained its current infrastructure capabilities.

A challenge is to design routers that can provide service differentiation for various types of traffic.

Two solutions proposed in IETF: Integrated service (IntServ): individual flows with

specific QoS requirements, e.g. RSVP flows Differentiated services (DiffServ): aggregate of

flows and per-hop behavior.


What is QoS and why we need it?

QoS has many different measures: Delay, delay variance (jitter), rate Fairness, flow isolation Reliability, e.g. drop rate

QoS is needed in packet networks to support multimedia applications: IP Telephony, mobile web-access, Video teleconferencing, net-PC applications, Grid services, etc.


Application QoS requirements

Different applications=different QoS requirements.

Delay-sensitive real-time applications: Voice, Interactive Video.

Delay-tolerant real-time applications: Streaming Video.

Elastic applications that tolerate a wide range of delay variations: FTP, Telnet and e-mail.


How to Provide QoS in a Best-Effort Internet?

More bandwidth does not solve the problem it only moves it to the core of the network: IP router technology has a limit to its computational

capabilities. Also, fairness is not solved when bandwidth is

increased. Solution: packet scheduling and resource reservation.


Improving QOS in IP Networks IETF groups are working on proposals to provide better QOS control in IP

networks, i.e., going beyond best effort to provide some assurance for QOS Work in Progress includes RSVP, Differentiated Services, and Integrated

Services Simple model

for sharing and congestion studies:


Principles for QOS Guarantees Consider a phone application at 1Mbps and an FTP application sharing a 1.5 Mbps

link. bursts of FTP can congest the router and cause audio packets to be dropped. want to give priority to audio over FTP

PRINCIPLE 1: Marking of packets is needed for router to distinguish between different classes; and new router policy to treat packets accordingly


Principles for QOS Guarantees (more) Applications misbehave (audio sends packets at a rate higher than 1Mbps

assumed above); PRINCIPLE 2: provide protection (isolation) for one class from other classes Require Policing Mechanisms to ensure sources adhere to bandwidth

requirements; Marking and Policing need to be done at the edges:


Principles for QOS Guarantees (more) Alternative to Marking and Policing: allocate a set portion of bandwidth

to each application flow; can lead to inefficient use of bandwidth if one of the flows does not use its allocation

PRINCIPLE 3: While providing isolation, it is desirable to use resources as efficiently as possible


Principles for QOS Guarantees (more) Cannot support traffic beyond link capacity PRINCIPLE 4: Need a Call Admission Process; application flow declares

its needs, network may block call if it cannot satisfy the needs


Summary


Scheduling And Policing Mechanisms

Scheduling: choosing the next packet for transmission on a link can be done following a number of policies;

FIFO: in order of arrival to the queue; packets that arrive to a full buffer are either discarded, or a discard policy is used to determine which packet to discard among the arrival and those already queued


Scheduling Policies Priority Queuing: classes have different priorities; class may

depend on explicit marking or other header info, eg IP source or destination, TCP Port numbers, etc.

Transmit a packet from the highest priority class with a non-empty queue

Preemptive and non-preemptive versions


Scheduling Policies (more) Round Robin: scan class queues serving one from each class that

has a non-empty queue


Scheduling Policies (more) Weighted Fair Queuing: is a generalized

Round Robin in which an attempt is made to provide a class with a differentiated amount of service over a given period of time


Policing Mechanisms Three criteria:

(Long term) Average Rate (100 packets per sec or 6000 packets per min??), crucial aspect is the interval length

Peak Rate: e.g., 6000 p p minute Avg and 1500 p p sec Peak (Max.) Burst Size: Max. number of packets sent

consecutively, ie over a short period of time


Policing Mechanisms

Token Bucket: limit input to specified Burst Size and Average Rate.

bucket can hold b tokens tokens generated at rate r token/sec unless bucket

full over interval of length t: number of packets

admitted less than or equal to (r t + b).


Policing Mechanisms (more)

token bucket, WFQ combine to provide guaranteed upper bound on delay, i.e., QoS guarantee!

WFQ

token rate, r

bucket size, b

per-flowrate, R

D = b/Rmax

arrivingtraffic


What Is Scheduling and Why Do We Need It?

In real networks, a large number of flows may require different guarantees from the network.

Scheduling determines the next eligible packet, among all the arriving packets, to be sent out in order to satisfy the required guarantees.

Attributes of a good scheduler: Rate and delay guarantees. Fairness. Utilization. Simplicity.


Scheduling (revisited) Scheduling: per-flow or aggregated flows. Per-flow (IntServ):

Provides exact delay and fairness guarantees. Requires maintaining state information for all

active sessions on a link. Aggregation (DiffServ):

Delay and fairness are not as easy to guarantee. No (or minimal amount of) states need to be

maintained.


Types of Schedulers

Rate-controlled: Delay / jitter Earliest Due Date (D-EDD and J-EDD).

Non-rate-controlled: Generalized Processor Sharing (GPS) and Weighted Fair Queueing (WFQ).


How GPS Works

Every session has its own queue and share. Weighted Round Robin (WRR) service order with an

infinitesimal amount serviced from each session. Empty queues are skipped when their turn arrives.


End-to-End Network DelayEnd-to-End Network Delay

Delay=Host Delay+Access Delay+Network Delay Even if the network provided the required QoS

without the host applications or access networks providing their guarantees then the end-to-end guarantees will not be met.

HostApplication

AccessNetwork Core Network


WFQ vs. GPS Departures

Arrivals

1

.2 .

.3

. . .

N-1

. . .N

GPSscheduler

WFQscheduler


Scheduling Policies (more) Weighted Fair Queuing: is a generalized

Round Robin in which an attempt is made to provide a class with a differentiated amount of service over a given period of time


Proactive Packet Discard

Congestion management by proactive packet discard Before buffer full Used on single FIFO queue or multiple

queues for elastic traffic E.g. Random Early Detection (RED)


Random Early Detection (RED)Motivation

Surges fill buffers and cause discards On TCP this is a signal to enter slow start phase,

reducing load Lost packets need to be resent

Adds to load and delay Global synchronization

Traffic burst fills queues so packets lost Many TCP connections enter slow start Traffic drops so network under utilized Connections leave slow start at same time

causing burst Bigger buffers do not help Try to anticipate onset of congestion and tell one

connection to slow down


RED Design Goals

Congestion avoidance Global synchronization avoidance

Current systems inform connections to back off implicitly by dropping packets

Avoidance of bias to bursty traffic Discard arriving packets will do this

Bound on average queue length Hence control on average delay


RED Algorithm – Overview

Calculate average queue size avgif avg < THmin

queue packetelse if THmin avg Thmax

calculate probability Pa

with probability Pa

discard packetelse with probability 1-Pa

queue packetelse if avg THmax

discard packet


RED Buffer


RED Algorithm Detail


Architecture of DiffServ

DiffServ Domain A Set of per-hop (forwarding) behaviors (PHB). Packet classification functions. Traffic conditioning functions including:

metering marking shaping policing


DS Nodes

DS boundary Nodes Classify the ingress packets. Perform traffic conditioning according to a

traffic conditioning agreement (TCA) DS Interior Nodes

Perform limited traffic conditioning functions such as DS codepoint re-marking.


DS Definition

000000 : Best - effort behavior Pool Codepoint space Policy 1 xxxxx0 Standards Action 2 xxxx11 EXP/LU 3 xxxx01 EXP/LU (*)

(*) May be used for future Standards Action allocations as necessary

37 Cpsc 527 Spring 2010 – Dr. Son Vuong

Example of DiffServ Domains

RRR

R

H

H

H

HH

H

BB


Per-Hop Behaviors (PHB)

PHB is a description of forwarding behavior of a DS node for a particular DS behavior aggregate.

Via PHBs, a node allocates its resources (buffer , bandwidth , delay, loss) to behavior aggregates.

PHBs are implemented via buffer management and packet scheduling mechanisms.

A PHB is selected at a node by a mapping of the DS codepoint in a received packet.


Traffic Conditioning Specification

The TCS specifies detailed service parameters for each service level :

Service performance parameters such as expected throughput, drop probability, latency.

Traffic profiles which must be adhered to for the requested service to be provided, such as token bucket parameters.

Disposition of traffic submitted in excess of the specified profile.

Marking services provided. Shaping services provided.


Packet Classifier and Traffic Conditioner

Classifier : select packets in a traffic stream based on packet header.

Meter: measures the temporal properties of the stream of packets selected by a traffic profile.

Marker: sets the DS field of a packet to a particular codepoint, add marked packet to a DS behavior aggregate.

Shaper: shapes the traffic stream to bring it into compliance with a traffic profile.

Dropper: discards some or all packets in a traffic stream to bring it into compliance with a traffic profile.


Service Level Specification (SLS)

In addition to TCS, the SLS specifies more general service :

Availability/Reliability Encryption services. Routing constraints. Authentication mechanisms. Mechanisms for service monitoring and auditing. Responsibilities such as location of the equipment

and functionality, action if the contract is broken, support capabilities.

Pricing and billing mechanisms.


Profile in Router

Profile ----> behavior of the router There may be many profiles in the router at any

time. Profile contains :

Name Attribute (in/out) Policy Scope Other information (Address , port, permission,

month mask, time of day, direction …)


Example of Profile

Name Entry2

PolicyScope DataTraffic

TimeOfDayRange 090000 to 170000

IncomingTOS 111

SourceAddressRange 139.24.2.12

to 139.24.2.255

Direction Outgoing

MaxeRate 5000

OutgoingTOS 101


Examples of Services

Better than Best-Effort (BBE) Service : This is a qualitative service which promises

to carry specific web server traffic at a higher priority than best-effort traffic.

Premium Service : Customer purchases a peak rate Packets below that rate carried with no delay Packets above that rate dropped


Our research on DiffServ

We focus on differentiated service since this aggregate service scales better than the per-flow integrated service.

We focus on developing a class of protocols including the Meter, Marker, Shaper at the Boundary nodes and the per-hop behavior of all interior nodes in a DS Domain.

For performance evaluation, the network simulator NS2 was used.


DiffServ and BestEffort Queues

DiffServ queue

Best-effort queue

Spring 2010 – Dr. Son Vuong Cpsc 527 47The simulation of DiffServ and BestEffort for TCP/IP traffic


The Dynamic Distribution DiffServ

The scheme unites DiffServ flows with Dynamic Distribution to the aggregate before sending them to the network.

Dynamic Distribution prevents any aggregate from taking all the bandwidth in a period of time, thus allows every aggregate a chance of passing through the router at all times.

Dynamic Distribution also guarantees Multimedia applications an upper bound on packet delay inside the DiffServ domain without the per-flow requirement.


Multiple Queues in DiffServ Router

Figure 1. Multiple-Queues in a Router of the DiffServ Domain

DiffServ queues

Backlogged packets


The Scheme of Dynamic Distribution Differentiated Service

This scheme simplifies processing at routers in DiffServ domain (cf. the IntServ scheme) while maintaining QoS guarantee for every flow.

At Boundary Nodes, packets are classified by their flows, which may number in thousands at one node or tens of thousand in the whole domain

At Inside Nodes, packets classified by their aggregates, numbered in hundreds at a node.


The Scheme at Boundary Node

Classifier: selects packets in a traffic stream based on DiffServ byte in header.

Meter: The leaky-bucket meter scheme for every flow with its subscribed bandwidth.

Dropper: drops all out-profile packets. Shaper: Fair-Queue scheme for every DiffServ

flow depending on the subscribing bandwidth. Marker: an aggregate marking scheme PHB: exact Fair-Queue forwarding scheme.


The Scheme for Interior Nodes

Classifier: selects packets in a traffic stream based on DiffServ byte and places them in appropriate queues.

Shaper: time divided into equal slots, each further divided mini-slots. Each aggregate receives a number of mini slots based on calculation at the beginning of the slot.

PHB: Frame-based forwarding scheme with Dynamic Distribution of resources.


The proposed Dynamic Distribution DiffServ scheme is flexible, simple and efficient

Guarantees delay bound without the per-flow complexity, suitable for multimedia traffic.

Future work: Various types of multimedia application traffic such

as video teleconferencing Various performance measures, including delay,

throughput and jitter Multicasting and security DS services

Summary

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