CSISCSIS

TAC-TOI-01

Quality of Service & Traffic Engineering

(QoS & TE)Khaled Mohamed

Credit: some of the sides are from Cisco Systems

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Agenda

• QOS and TE in IP Network

• The QoS and TE Architectures

• QOS and TE Service Types

• The Technical Scenarios Reasons

• The Applications and Their Needs

• Q&A

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QoS in IP NetworksThus far: “making the best of best effort”

Future: next-generation Internet with QoS guarantees– Differentiated Services: differential guarantees– Integrated Services: firm guarantees

• Example: guarantees an audio application 1Mbps; the remaining to 0.5Mbps to Web transfer

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Principles for QoS Guarantees

packet classification: router can distinguish between different classes

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Principles for QoS Guarantees

• prevents applications from misbehaving (e.g., multimedia app. sends higher than declared rate)

scheduling and policing: provide protection (isolation)

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Principles for QoS Guarantees• Allocating fixed (non-sharable) bandwidth to flow: inefficient

use of bandwidth if flows doesn’t use its allocation

high utilization: while providing isolation, it is desirable to use resources as efficiently as possible

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Principles for QoS Guarantees• Basic fact of life: cannot support traffic demands beyond

link capacity

call admission: flow declares its needs, network may block call (e.g., busy signal) if it cannot meet needs

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Summary of QoS Principles

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Traffic SpecificationThree common-used criteria: • (Long term) Average Rate: how many pkts can be

sent per unit time (in the long run)– crucial question: what is the interval length: 100 packets

per sec or 6000 packets per min have same average!

• Peak Rate: e.g., 6000 pkts per min. (ppm) avg.; 1500 ppm peak rate

• (Max.) Burst Size: max. number of pkts sent consecutively (with no intervening idle)

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Traffic SpecificationToken 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).

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Scheduling Mechanisms• scheduling: choose next packet to send on link• FIFO (first in first out) scheduling: send in order of arrival

to queue– discard policy: if packet arrives to full queue: who to discard?

• tail drop: drop arriving packet• priority: drop/remove on priority basis• random: drop/remove randomly

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Scheduling Policies: morePriority scheduling: transmit highest priority

queued packet • multiple classes, with different priorities

– class may depend on marking or other header info, e.g. IP source/dest, port numbers, etc..

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Scheduling Policies: still moreround robin scheduling:• multiple classes• cyclically scan class queues, serving one from each class (if available)

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Scheduling Policies: still moreWeighted Fair Queuing:

• generalized Round Robin

• each class gets weighted amount of service in each cycle

.

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Delay Guarantees• token bucket, WFQ combine to guarantee

upper bound on delay, i.e., QoS guarantee!

WFQ

token rate, r

bucket size, b

assigned flow rate, R

D = b/Rmax

arrivingtraffic

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QOS Type of Services

Integrated services

• Differentiated services

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IETF IntServ Services• Architecture for providing QoS guarantees in IP

networks for individual application sessions

• Assumptions– use a common infrastructure for both real-time

and non-real-time communications– resource must be explicitly managed in order to

meet the requirements of real-time applications• resource reservation: routers maintain state info (a la VC) of

allocated resources, QoS req’s

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Intserv QoS: Service Models [rfc2211, rfc 2212]

Guaranteed service:• worst case traffic arrival: leaky-

bucket-policed source • simple (mathematically

provable) bound on delay [Parekh 1992, Cruz 1988]

Controlled load service:• "a quality of service closely

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

WFQ

token rate, r

bucket size, b

per-flowrate, R

D = b/Rmax

arrivingtraffic Controlled link sharing:

• Sharing link among different classes

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Reference Architecture

Admission Control

Data InData Out

Con

trol P

lan

eD

ata

Pla

ne

Scheduler

Routing Routing

MessagesRSVP

messages

Classifier

RSVP

Route Lookup

Forwarding Table Per Flow QoS Table

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Classifier

Buffermanagement

Scheduler

flow 1

flow 2

flow n

…

Per-flow State

A Closer Look at the Data Path

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Intserv: QoS Guarantee Scenario• Resource reservation

– call setup, signaling (RSVP)

– traffic, QoS declaration

– per-element admission control

– QoS-sensitive scheduling (e.g., WFQ)

request/reply

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RSVP ProtocolA flow needs performance guarantee must :

• declare its QoS requirement

– R-spec: defines the QoS being requested

• characterize traffic it will send into network

– T-spec: defines traffic characteristics

• signaling protocol: needed to carry R-spec and T-spec to routers (where reservation is required)

– RSVP

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RSVP: Soft-state Receiver-initiatedEnd-to-End Reservation

• Sender periodically sends PATH messages to receiver R, each router updates the PATH message by increasing hop count and adding its propagation delay

• When receiver R gets the PATH message, it knows– Traffic characteristics (tspec): (r,b,R)– Number of hops, propagation delay introduced by the routers

• Receiver R sends back this information + required worst-case delay in RESV• Each router along path provides a per-hop delay guarantee and forwards

RESV with updated info – In the simplest case, the routers can just split the delay– State timed out if not refreshed

R1R1

R2R2

R3R3

SS

RR(b,r,R,0,0)

(b,r,R,2,D-D3 )

(b,r,R,1,D-D3-D2)

(b,r,R,0,0) (b,r,R,3,D

)

delay budget PATHRESV

(b,r,R,1,d 1

) (b,r,R,2,d1 +d

2 ) (b,r,

R,3,d 1+d 2

+d 3)

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Implementing IntServ

• Use WFQ to implement controlled link sharing among different organizations

• WFQ provides guaranteed service

• Controlled-load and best-effort flows are separated by priority

WFQ

guaranteedflow n

priority

10%30%

controlledflows

best effortflows

WFQ

guaranteedflow 1

CS department gets 50%

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IETF Differentiated ServicesConcerns with Intserv:• Scalability: signaling, maintaining per-flow router state difficult with large number of

flows • Flexible Service Models: Intserv has only two classes. Also want “qualitative” service

classes– “behaves like a wire”– relative service distinction: Platinum, Gold, Silver

Diffserv approach: • simple functions in network core, relatively complex functions at edge routers (or hosts)• Don’t define service classes, provide functional components to build service classes

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The DiffServ Traffic Conditioner Block (TCB)

• Classifier: Identifies packets for assignment to Classes

• Meter: Checks compliance to traffic parameters (Token Bucket) and passes result to Marker and Shaper/Dropper to trigger particular action for in/out-of-profile packets

• Marker: Writes/rewrites the DSCP value

• Shaper: Delays some packets for them to be compliant with the profile

• Dropper: Drops packets that exceed the profile (Bc or Be)

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DiffServ ArchitectureEdge router:- per-flow traffic management

- marks packets as in-profile and out-profile

Core router:

- per class traffic management

- buffering and scheduling

based on marking at edge

- preference given to in-profile packets- Assured Forwarding

scheduling

...

r

b

marking

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Edge-router Packet Marking

• class-based marking: packets of different classes marked differently

• intra-class marking: conforming portion of flow marked differently than non-conforming one

• profile: pre-negotiated rate A, bucket size B

• packet marking at edge based on per-flow profile

Possible usage of marking:

User packets

Rate A

B

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Classification and Conditioning

• Packet is marked in the Type of Service (TOS) in IPv4, and Traffic Class in IPv6

• 6 bits used for Differentiated Service Code Point (DSCP) and determine PHB that the packet will receive

• 2 bits are currently unused

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Classification and Conditioning may be desirable to limit traffic injection rate of

some class:

• user declares traffic profile (eg, rate, burst size)

• traffic metered, shaped if non-conforming

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Forwarding (PHB)• PHB result in a different observable (measurable)

forwarding performance behavior• PHB does not specify what mechanisms to use to

ensure required PHB performance behavior• Examples:

– Class A gets x% of outgoing link bandwidth over time intervals of a specified length

– Class A packets leave first before packets from class B

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Forwarding (PHB)

PHBs being developed:• Expedited Forwarding: pkt departure rate of a

class equals or exceeds specified rate – logical link with a minimum guaranteed rate

• Assured Forwarding: 4 classes of traffic– each guaranteed minimum amount of bandwidth– each with three drop preference partitions

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Direction of Data-FlowDirection of Data-Flow

100Mbps100Mbps

WANWAN2Mbps2Mbps

Why QoS?Congestion Scenario #1—Speed Mismatch

• The #1 Reason for Congestion!• Possibly Persistent when going from LAN to WAN• Usually Transient when going from LAN to LAN!

1000Mbps1000Mbps 100Mbps100Mbps

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Direction of Data-FlowDirection of Data-Flow

Choke Point

Choke Points

Why QoS?Congestion Scenario #2—Aggregation

• Transient Congestion fairly typical!

HQ Hubi

Remote

2Mbps2Mbps 512Kbps512Kbps

N*56KbpsN*56Kbps

FR/ATMFR/ATM

1000Mbps1000Mbps1000Mbps1000Mbps

S1 S2

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STM-64/OC-192cSTM-64/OC-192c

Core1Core1 Core2Core2

STM-16/OC-48cSTM-16/OC-48c

Net-1Net-1

Net-2Net-2

Net-nNet-n

Why QoS?? Congestion Scenario #3—Confluence

• Always need mechanisms to provide guarantees!• Transient Congestion occurs!

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VoiceVoice FTPFTP ERP andMission-Critical

ERP andMission-Critical

Bandwidth Low toModerate

Low toModerate

Moderateto High

Moderateto High

LowLow

Random Drop Sensitive LowLow HighHigh ModerateTo High

ModerateTo High

Delay Sensitive HighHigh LowLow Low toModerate

Low toModerate

Jitter Sensitive HighHigh LowLow ModerateModerate

Typical ApplicationQoS Requirements

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Q & A

Thank You