chapter 17 integrated and differentiated services 1 integrated and differentiated services comp5416...

Chapter 17 Integrated and Differentiated Services

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Integrated and Differentiated Services

COMP5416Chapter 17


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Review Demands on IP-based internets are rising IP-based internets were designed for elastic

applications that tolerate variations in throughput and loss

Now, they are used to support high volumes and various traffic mix including real-time and non real-time applications– These are sensitive to delay and throughput variations

and requires high quality of service (QoS) Thus, they now need to provide service

differentiations for different applications like ATM network!


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Introduction New additions to Internet increasing traffic

– High volume client/server application– Web– Real time voice and video

Need to manage traffic and control congestion Two complementary IETF standards:

– Integrated services (IntServ)• Provides collective service to set of traffic demands

placed in a domain– Limit demand per capacity & reserve resources to

meet QoS– Differentiated services (DiffServ)

• Classify traffic in groups• Different group traffic handled differently


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Internet Traffic Elastic

– Can adjust to changes in delay and throughput– E.g. common TCP and UDP application like email,

FTP, web

Inelastic– Does not easily adapt to changes in delay

and throughput – real time traffic such as web streaming,

voice over IP (VoIP)– Requires minimum throughput, bounded

delay and jitter (i.e. variation of delay)


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IntServ Architecture

IPv4 header fields for precedence/priority and type of service usually ignored

ATM is only network designed to support TCP, UDP and real-time traffic from inception– However, need new installation & costly

Need to support Quality of Service (QoS) within TCP/IP architecture– Requires adding functionality to routers– Means of requesting QoS


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IntServ Approach

Enable provision of QoS over IP (RFC2211,2212)

Enables sharing available capacity when congested

Currently, routers have these mechanisms:– Dynamic Routing Algorithms

• Select to minimise delay to balance load

– Active Queue Management (AQM)• Causes TCP sender to back off and reduce load

These are not sufficient, and are enhanced by IntServ


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IntServ Functions Admission control

– For specific QoS, reservation required for new flow– Resource reSerVation Protocol (RSVP) used

Routing algorithm– Base decision on QoS parameters, not shortest

path only Queuing discipline

– Take account of different flow requirements– Meet QoS

AQM policy– Manage congestion


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IntServ Implementation in Router

Background Functions

Forwarding functions


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IntServ Components – Background Functions Reservation Protocol

– Reserve resources for new flows Admission control

– Determines whether current resources enough to support new request

Management agent– Can use agent to modify traffic control database

and direct admission control Routing protocol

– Directs next hop for each address and flow


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IntServ Components – Forwarding Classifier and route selection

– Incoming packets mapped to classes• Single flow or set of flows with same QoS

– E.g. all video flows

• Based on IP header fields

– Determines next hop Packet scheduler

– Manages one or more queues for each output– Order in which queued packets sent

• Based on class, traffic control database, current and past activity on outgoing port

– Policing• Determine whether flow exceed its requested capacity


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IntServ Services Service defined on two levels

– General categories of service:• Guaranteed (~ CBR)• Controlled load (~ VBR)• Best effort (default) (~ UBR)

– Particular flow within category

Service for a flow is specified by certain parameters known as traffic specification (TSpec)

TSpec is part of the traffic contract


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IntServ Services – Guaranteed Service

Most demanding service Provides assured data rate Has specific upper bound on queuing delay

through network– Must be added to propagation delay to get total

delay– May be wise to set high to accommodate rare long

queue delays Has no queuing losses

– i.e. no buffer overflow


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IntServ Services – Controlled Load

Tightly approximates to best efforts under unloaded conditions

No upper bound on queuing delay– High percentage of packets do not experience delay over

minimum transit delay• Propagation delay plus router processing with no

queuing delay Very high percentage delivered

– Almost no queuing loss Useful for adaptive (or soft) real time applications

To provide these service categories, routers adopt suitable queuing discipline


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Queuing Discipline Traditionally FIFO or FCFS at each router

port No special treatment to high priority packets

(flows) Small packets held up by large packets

ahead of them in queue– Larger average delay for smaller packets– Flows of larger packets get better service

Greedy TCP connection can crowd out altruistic (i.e. unselfish) connections– If one connection does not back off, others may

back off more


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Fair Queuing (FQ) Multiple queues for each port

– One for each source or flow Queues serviced in round robin

– Each busy queue gets exactly one packet per cycle

Achieves load balancing among flows– No advantage to being greedy

• Your queue gets longer, increasing your delay

Drawback: Short packets penalized as each queue sends one packet per cycle


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FIFO and FQ


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Processor Sharing (PS)

Not practical but same principle adopted in another scheme

Multiple queues as in FQ Send one bit from each queue per round

– Longer packets no longer get an advantage Work out virtual start and finish time for a

given packet (of queue )iii PSF ],max[ 1

iii AFS

However, we wish to send packets, not bits in reality


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Bit-Round Fair Queuing (BRFQ)

Based on PS Each flow gets 1/nth of bandwidth (n flows) Compute virtual start and finish time as in PS When a packet finished, the next packet sent

is the one with the earliest virtual finish time Good approximation to performance of PS

– Throughput and delay of queues converge as time increases


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Comparisonof FIFO, FQ and BRFQ


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Generalised Processor Sharing (GPS)

BRFQ can’t provide different capacities to different flows

Enhancement called weighted fair queuing (WFQ), based on generalised PS

From PS, allocate weighting to each flow that determines how many bits are sent during each round– If weighted 5, then 5 bits are sent per round

w

PSF iii ],max[ 1

iii AFS

Gives means of responding to different service levels => The concept of service differentiation!

Can provide guarantees that delays do not exceed certain bounds


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Weighted Fair Queuing

Emulates GPS Same strategy as BRFQ Enables a router to assign weight to

each flow and guarantee bound on delay

Max buffer size needed proportional to defined max delay


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Example: BFRQ vs WFQ Received these packets all at about

the same time and same output link Service with BRFQ:

Packet Size Flow

1 100 1

2 100 1

3 60 2

4 120 2

5 60 2

– pkt tx sequence: 3,1,4,2,5

Service with WFQ– Assume flow 2 gets 1.5 of flow 1

– So, weight ratio 2 : 3

Packet Size Flow Fi

1 100 1 100

2 100 1 200

3 60 2 60

4 120 2 180

5 60 2 240

iii PSF

Packet Size Flow Fi

1 100 1 50

2 100 1 100

3 60 2 20

4 120 2 60

5 60 2 80

w

PSF iii

– Sequence: 3,1,4,5,2


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Active Queue Management (AQM)

A congestion control function! In face of congestion, informed discard

policy is needed Congestion management by proactive

packet discard is used:– Before buffer becomes full– Used on single FIFO queue or multiple

queues– E.g. Random Early Detection (RED)


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RED Motivation Surges may 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– And may entail Global Synchronisation in drop-tail policy

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

Just bigger buffers do not help Try to anticipate onset of congestion and tell one

connection to slow down


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RED Design Goals

Congestion avoidance Global synchronisation avoidance Avoidance of bias to bursty traffic

– Discard only arriving packets will do this Bound on average queue length

– Hence control on average delay– Average filters out transient congestion


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RED Buffer


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RED Algorithm

Calculate average queue size avg

if avg < THmin

queue packet

else if THmin avg Thmax

calculate probability Pa

with probability Pa

discard packet

else with probability 1-Pa

queue packet

else if avg THmax

discard packet


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Differentiated Services (DiffServ)

IntServ is complex to deploy! May not scale well for large volumes of traffic

– Amount of control signals ( overhead)– Maintenance of state information at routers

Intserv has only two classes DiffServ (RFC2475) designed to provide

simple, easy to implement, low overhead tool– simple functions in network core, relatively

complex functions at edge routers (or hosts)– Doesn’t define service classes, provide functional

components to build service classes


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Characteristics of DiffServ Use IPv4 header Type of Service or IPv6

Traffic Class field (called DS field)– So, no change to IP!

Service level agreement (SLA) established between provider and customer prior to use of DiffServ

All traffic with same DS field treated same– E.g. multiple voice connections


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DiffServ Architecture

Edge/Boundary router:- per-flow traffic management

- marks packets as in-profile and out-profile

Core/Interior router:

- per class traffic management

- buffering and scheduling

based on marking at edge

- preference given to in-profile packets

scheduling

...marking

©J.F Kurose and K.W. Ross


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Edge-router Packet Marking

profile: pre-negotiated rate packet marking at edge based on per-flow profile

class-based marking: packets of different classes marked differently

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

Possible usage of marking:



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Classification and Conditioning Packet is marked in the DS field 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

Traffic conditioning to provide desired service Classifier

– Separate packets into classes

Meter/Police– Measure traffic for conformance to profile

Marker– Policing by remarking codepoints if required

Shaper Dropper


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Classification and Conditioning

may be desirable to limit traffic injection rate of some class:

user declares traffic profile (e.g., rate, burst size) traffic metered, shaped or dropped if non-conforming



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Forwarding (PHB) I

Per Hop Behaviour results in a different observable (measurable) forwarding performance behaviour

PHB does not specify what mechanisms to use to ensure required behavior

Examples: – Class A gets x% of outgoing link bandwidth over

time intervals– Class A packets leave first before packets from

class B



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Forwarding (PHB) II

Defined PHBs: Expedited Forwarding: pkt departure rate of a

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

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



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Summary IntServ and DiffServ are QoS frameworks for

IP internets IntServ requires additional signalling protocol

to reserve resource and need to keep state per flow => not scalable

DiffServ works based on aggregate classes and has minimal impact on the end-systems => scalable & a more popular alternative

Next: Protocols for QoS Support

chapter 17 integrated and differentiated services 1 integrated and differentiated services comp5416...

Documents

integrated services

qos slide

intserv slide

services diffserv

realtime traffic

traffic demands

congestion slide

variation of delay slide