qos management in ip networks: integrated and...

Lund University / Presentation 2012

QoS Management in IP Networks:Integrated and Differentiated Services


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!


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


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)


IntServ Architecture

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

• ATM only network designed to support TCP, UDP and real-time traffic from inception– not available everywhere, costly reconfiguration

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


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


IntServ Functionsn Admission control

– For specific QoS, reservation required for new flow

– Resource reSerVation Protocol (RSVP) usedn Routing algorithm

– Base decision on QoS parameters, not shortest path only

n Queuing discipline– Take account of different flow requirements– Meet QoS

n AQM policy– Manage congestion


IntServ Implementation in Router

n Background Functions

n Forwarding functions


IntServ Components – Background Functions

n Reservation Protocol– Reserve resources for new flows

n Admission control– Determines whether current resources enough to support new request

n Management agent– Can use agent to modify traffic control database and direct admission

controln Routing protocol

– Directs next hop for each address and flow


IntServ Components – Forwarding

n 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

n 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


IntServ Servicesn Service defined on two levels

– General categories of service:• Guaranteed• Controlled load• Best effort (default)

– Particular flow within categoryn Service for a flow is specified by certain

parameters known as traffic specification (TSpec)

n TSpec is part of the traffic contract


IntServ Services – Guaranteed Service

n Most demanding servicen Provides assured data raten 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 delaysn Has no queuing losses

– i.e. no buffer overflow


IntServ Services – Controlled Load

n Tightly approximates to best effort under unloaded conditions

n 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 delayn Very high percentage delivered

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

n To provide these service categories, routers adopt suitable queuing discipline


Queuing Disciplines

n Traditionally FIFO or FCFS at each router port

n No special treatment to high priority packets (flows)

n 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

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

back off more


FCFS

• Work Conserving (if packet waiting, serve)• Klienrock Conservation Law

If delay for one flow is lowered, the delay for one or more other flows must increase

ρ = occupancyq = mean scheduler delayC = a constant

NX

n=1

⇢nqn = C


Non Work Conserving

• Scheduler can be idle even if packets waiting• Switches packets to

– The right destination– At the right time

• Reduces jitter• Makes traffic predictable• Mechanism known as “shaping”


Fair Queuing (FQ)

n Also work conservingn Multiple queues for each port

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

– Each busy queue gets exactly one packet per cyclen Achieves load balancing among flows

– No advantage to being greedy• Your queue gets longer, increasing your delay

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


FIFO and FQ


Priority Queuing

• K queues– 1 ≤ k ≤ K– Queue k+1 higher prio. than queue k– Higher prio. served first

• Simple implementation• Low processing overhead • No fairness, low prio. queues can be starved

3 2 1 5 4 8 7 6

1 2 3 4 5 6 7 8


Processor Sharing (PS)

n Not practical but same principle adopted in another scheme, BRFQ

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

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

given packet (of queue α)

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


Bit-Round Fair Queuing (BRFQ)

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

earliest virtual finish timen Good approximation to performance of PS

– Throughput and delay of queues converge as time increases


Generalised Processor Sharing (GPS)n BRFQ can’t provide different capacities to different

flowsn Enhancement called weighted fair queuing (WFQ),

based on generalised PSn 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

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


Weighted Fair Queuing

n Emulates GPSn Same strategy as BRFQn Enables a router to assign weight to each flow and guarantee

bound on delayn Max buffer size needed proportional to defined max delay


Sequence: 3,1,4,5,2

Packet Size Flow Fi

1 100 1 50

2 100 1 100

3 60 2 20

4 120 2 60

5 60 2 80

Example: BRFQ vs WFQn Received these packets all at about

the same time and same output link è

n 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

n Service with WFQ– Assume flow 2 gets 1.5 of flow 1 – So, weight ratio à 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


CBQ

• Assigns fractions of BW to class nodes

• Values minimum• Nodes can borrow unused BW

• Priority to flows within a class

100 %

40 %

20 %

60 %

40 %

Root

X Y

RT NRT


Differentiated Services (DiffServ)

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

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

n Intserv has only two classesn 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


Characteristics of DiffServ

n Use IPv4 header Type of Service or IPv6 Traffic Class field (called DS field)– So, no change to IP!

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

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


DiffServ ArchitectureEdge/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


Edge-router Packet Marking

n profile: pre-negotiated raten packet marking at edge based on per-

flow profile

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

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

Possible usage of marking:


Classification and Conditioning

n Packet is marked in the DS fieldn 6 bits used for Differentiated Service Code Point (DSCP) and

determine PHB that the packet will receiven 2 bits are currently unused



Traffic conditioning to provide desired servicen Classifier

– Separate packets into classesn Meter/Police

– Measure traffic for conformance to profilen Marker

– Policing by remarking codepoints if requiredn Shapern Dropper



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

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

©J.F Kurose and K.W. Ross


Forwarding (PHB) I

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

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

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

time intervals– Class A packets leave first before packets from

class B


Forwarding (PHB) II

Defined PHBs:n Expedited Forwarding: pkt departure rate of a class equals or

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

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


Summaryn IntServ and DiffServ are QoS frameworks for IP internetsn IntServ requires additional signalling protocol to reserve resource

and need to keep state per flow => not scalablen DiffServ works based on aggregate classes and has minimal impact

on the end-systems => scalable & a more popular alternative

qos management in ip networks: integrated and...

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