Internet Quality of Service

Quality of Service (QoS)

• The best-effort model , in which the network tries to deliver data from source to destination but makes no promises about end-to-end delay, is not sufficient for real-time applications.

• The primary goal of Quality of Service (QoS) is to support different levels of services by provide priorities including dedicated bandwidth, controlled jitter and latency (required by some real-time and interactive traffic), and improved loss characteristics.

Real-time Applications

• Require “deliver on time” assurances

• Example application (audio)– sample voice once every 125us– each sample has a playback time– packets experience variable delay in network– add constant factor to playback time: playback point

Microphone

Speaker

Sampler,A Dconverter

Buffer,D A

Playback BufferS

eque

nce

num

ber

Packetgeneration

Networkdelay

Buffer

Playback

Time

Packetarrival

Application Requirements

Application

file transfere-mail

Web documentsreal-time audio/video

stored audio/videointeractive games

financial apps

Data loss

no lossno lossloss-tolerantloss-tolerant

loss-tolerantloss-tolerantno loss

Bandwidth

elasticelasticelasticaudio: 5Kb-1Mbvideo:10Kb-5Mbsame as above few Kbps upelastic

Time Sensitive

nononoyes, 100’s msec

yes, few secsyes, 100’s msecyes and no

Taxonomy

Applications

Real time

Tolerant

Adaptive Nonadaptive

Delay-adaptive

Rate-adaptive

Intolerant

Rate-adaptive Nonadaptive

Interactive Interactivebulk

Asynchronous

Elastic

QoS Approaches

• IPv4 Type of Service (TOS)– IPv6 Traffic class + Flow label

• Integrated Services (Intserv)

• Differentiated Services (Deffserv)

• ATM service classes– CBR, VBR-rt, VBR-nrt, ABR, UBR

IPv4

• RFC1349• Precedence: the importance or priority of the datagram• TOS:

– 1000 -- minimize delay– 0100 -- maximize throughput– 0010 -- maximize reliability– 0001 -- minimize monetary cost– 0000 -- normal service

• MBZ: Must Be Zero (Unused)– Suggested to be used as ECN field in RFC3168

Precedence TOS MBZ

1 2 3 4 5 6 7 8

Integrated Services

• Intserv provides individualized quality-of-service guarantees to individual application sessions by per-flow resource reservation.

• IETF intserv working group (concluded)• Resource ReSerVation Protocol (RSVP, RFC 22

05)• Subnet Bandwidth Manager (SBM, RFC 2814)

Service Classes

• Guaranteed service (RFC2212)– Provides firm bounds on the queueing delays

that a packet will experience in a network element

• Controlled-load service (RFC 2211)– provides the flow with a quality of service clos

ely approximating the QoS that same flow would receive from an unloaded network element

Mechanisms

• Traffic characterization– Tspec (RFC2210)– Rspec (RFC2215)

• Admission control

• Reservation protocol– RSVP

• Packet processing– Weighted Fair Queuing (WFQ)

Flowspecs

• Rspec: describes service requested from network– controlled-load: none– guaranteed: delay target

• Tspec: describes flow’s traffic characteristics– average bandwidth + burstiness: token bucket filter– token rate r– bucket depth B– must have a token to send a byte– must have n tokens to send n bytes– start with no tokens– accumulate tokens at rate of r per second– can accumulate no more than B tokens

Token Bucket

• If the service rate at a network element is R, then the queuing delay is bounded by b/R.

Admission Control

• A router decides whether to admit a flow based on the R-spec and T-spec of the flow and currently available resources on the router.

Reservation Protocol

• RSVP is employed in Intserv to setup path and reserve resources.

RSVP

• Receiver-oriented reservation• Receiver heterogeneity• Designed to support multicast• Merge requirements in case of multicast

RSVP

• Support multiple senders

• Use soft state (refresh periodically)– Source transmits PATH messages every 30 s

econds– Destination responds with RESV message

• Separate from route establishment

• QoS can change dynamically

RSVP

Sender 1

Sender 2

PATH

PATH

RESV(merged)

RESV

RESV

Receiver B

Receiver A

R

R

R

R

R

RSVP

• RSVP does not specify how the network provides the reserved bandwidth to the data flows.

• RSVP is not a routing protocol.

• RSVP is a signaling protocol that allows host to establish and tear down reservations for data flows.

Packet Processing

• Packet classification associates each packet with the appropriate reservation class.– IPv4: SA, DA, SP, DP, Protocol– IPv6: Flow label

• Packet scheduling manages queues so that each packet receives the requested service.– Guaranteed: calculate end-to-end delay– Controlled load: assign the aggregate flow with a

weight based on the amount of traffic admitted

WFQ

• WFQ provides different amount of service among queues according to their weights.

Challenges

• Scalability– Per-flow states

• Overhead– Signaling messages– Refresh messages– Packet processing

• Security– RSVP deny of service

Differentiated Services

• Diffserv provides scalable and flexible service differentiation to handle different classes of traffic in different ways within the Internet.

• IETF diffserv working group• DS field (RFC2474, 2475)• Per-Hop Behaviors (PHB, RFC2474, 2597,

2598)

Architecture

• “Complex at edge, Simple at core.”– At edge:

• Classifying• Metering• Marking• Conditioning

– At core• BA classifying• Queuing and scheduling

Mechanisms

• Diffserv Code Point (DSCP)

• Behavior Aggregate (BA)

• Per-Hop Behaviors (PHB)– Expedited Forwarding (EF, RFC3246)– Assured Forwarding (AF, RFC2597)

DSCP

• Edge routers mark packets of different classes with different DSCP.

• Core routers treat packets with different level of services according to its DSCP.

• DS field: TOS(IPv4), Traffic class(IPv6)

• CU: currently unused

Edge Functions

• Meter monitors whether the incoming packet flow conforms to the negotiated traffic profile.

• Shaper spaces the incoming packets to the negotiated traffic rate.

PHB

• Per-Hop Behavior is a description of the externally observable forwarding treatment applied at a differentiated services-compliant node to a behavior aggregate.

• EF is intended to provide a building block for low delay, low jitter and low loss services by ensuring that the EF aggregate is served at a certain configured rate.

• The AF PHB group provides delivery of IP packets in four independently forwarded AF classes. Within each AF class, an IP packet can be assigned one of three different levels of drop precedence.

backw ard com patib ility

TO S (R FC 791)

IP precedence (R FC 1349)

0 1 2 3 4 5 6 7

TO S

111 N etw ork contro l

Precedence 0

110 In ternetw ork contro l

101 C ritical

100 Flash override

011 Flash

010 Im m ediate

001 Priority

000 R outine

B est-Effo rt traffic

0 1 2 3 4 5 6 7

D SCP C U

XPool 1 X X X X 0

1 1 1 0 0 0

1 1 0 0 0 0

1 0 1 0 0 0

1 0 0 0 0 0

0 1 1 0 0 0

0 1 0 0 0 0

0 0 1 0 0 0

0 0 0 0 0 0B est-Effo rtD efau lt PH B

C lass Selector C odepo int

1 0 1 1 0 0

Low Prio rity

H igh Prio rity

0 1 0 0 0 0

0 1 0 0 1 0

0 1 0 1 0 0

0 1 1 0 0 0

0 1 1 0 1 0

0 1 1 1 0 0

1 0 0 0 0 0

1 0 0 0 1 0

1 0 0 1 0 0

1 0 1 0 0 0

1 0 1 0 1 0

1 0 1 1 0 0

C lass 1 C lass 2

C lass 3 C lass 4

Low Drop

M edium D rop

H igh D rop

Low Drop

M edium D rop

H igh D rop

A ssuredForw ard ing

PH B

ExpeditedForw ard ing

PH B

IP precedence

PHB Implementations

• Priority queue

• WFQ

• RED with In and Out (RIO)

• Weighted RED (WRED)P(drop)

1.0

MaxP

Minin MaxinMaxoutMinout

AvgLen

E2E DiffServ

• DS Domain

• Bandwidth Broker (BB)

• Service Level Agreement (SLA)

BB

Adjacent BB Adjacent BB

User/AppInterface

ApplicationServer

User/Host

NetworkOperator

Inter-DomainInterface

Intra-DomainInterface

EdgeRouters

EdgeRouters

DataRepository

RoutingInformation

Policy ManagerInterface

Network ManagementInterface

ATM QoS

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

Service Classes

ABR

• ABR Resource Management (RM) cells

• Virtual Source/Virtual Destination

S3S2S1H1 H2RM cells

Source Virtualdestination

RM cells

Virtualsource

Destination

RSVP v.s. Q.2931

• RSVP– receiver generates res

ervation– soft state (refresh/time

out)– separate from route es

tablishment– QoS can change dyna

mically– receiver heterogeneity

• ATM Q.2931– sender generates con

nection request– hard state (explicit del

ete)– concurrent with route e

stablishment– QoS is static for life of

connection– uniform QoS to all rec

eivers