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QoS in NetworksQoS in NetworksCS680CS680

Prof. A. SahooProf. A. Sahoo

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Topics to be CoveredTopics to be Covered• Introduction• Integrated Services

- Architecture- Service models

- Guaranteed service- Controlled load service

- RSVP- Salient parts of RFC 2205

• Packet scheduling (various scheduling mechanisms e.g. max-min fair scheduling, GPS, WFQ, WRR etc.)

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Topics to be CoveredTopics to be Covered• Differentiated Services

– Basic approach– PHBs– Services– Diffserv fields– Traffic classification and conditioning– AF PHB– EF PHB– Traffic Policing– IntServ over DiffServ– Congestion control

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Topics to be CoveredTopics to be Covered• Multiprotocol Label Switching

– Motivation– Label switching proposals (tag switching, IP

switching etc.)– MPLS architecture

• Salient parts of RFC 3031

– Label Distribution Protocols

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Topics to be CoveredTopics to be Covered• Traffic description methods and Delay Analysis• Internet Traffic Engineering

– The Fish Problem– Building blocks of TE– Constraint based routing– Multipath load sharing

• QoS based routing• QoS in Wireless Network• Virtual Private Network• Papers in those topics

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Books and ReferencesBooks and References• Internet QoS – Architecture and Mechanisms for

Quality of Service - Zheng Wang, Publisher: Morgan Kauffman.• MPLS Technology and Application – B. Davie, Y.

Rekhter, Publisher: Morgan Kauffman.• An Engineering Approach to Computer

Networking- S. Keshav, publisher : Addison-Wesley.

• Various RFCs and papers.

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PrerequisitePrerequisite• UG or PG course in Computer Networks

– Understanding of MAC, routing, TCP/IP protocols

• Basics in Algorithms• Basics in Queueing Theory

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Class EvaluationClass Evaluation• Surprise Quizzes – 10%• Mid sem – 30%• End sem – 40%• Course Project – 20%

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Grading PolicyGrading Policy• Based on clustering

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Pre-RequisitePre-Requisite• Undergraduate Computer Networks

Knowledge– Understanding of TCP, IP routing protocols,

various MAC protocols

• Basic knowledge in probability theory• Basic understanding of queuing theory

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Other Academic PoliciesOther Academic Policies• Academic dishonesty

– Will not be tolerated at all• Will get fail grade for doing so

• Attendance Policy– As per Institute rule

• Falling Short will get you XX grade

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Introduction (cont’d)Introduction (cont’d)• For Many Years Internet was primarily

used for networking research. File transfer, email were the most popular application: They do not need any performance guarantee from underlying network.

• New applications such as VoIP, video conferencing, e-commerce apps are sensitive to network performance.

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Introduction (cont’d)Introduction (cont’d)• Internet cannot provide any resource

guarantees : the service is best effort• Internet does not provide service differentiation :

all packets are treated equal.• But applications such as VoIP require low delay,

jitter and packet loss; whereas file transfer app can tolerate fair amount of delay and loss. Thus there is a need to differentiate between packets of such applications.

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Current State of InternetCurrent State of Internet• Uses best-effort service model• No guarantee of timeliness of delivery• No service discrimination• Bandwidth and network congestion

problems• Unpredictable network response time

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What is QoSWhat is QoS• The capability to provide resource

assurance and service differentiation so that delay, jitter or loss sensitive applications can perform satisfactorily is often referred to as quality of service (QoS).

• can be provided through relative prioritization of resource allocation to different flows/packets in the network.

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QoSQoS• QoS parameters

– At bit level• Bit error rate

– At Packet level• Delay• Delay Jitter• Packet loss• Throughput

– At call level• Blocking probability

– At Application level• Throughput• Response time

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QoSQoS• QoS in circuit-switched network

– Quite Easy

• QoS in packet-switched network– Difficult (why?)

• Wired network• Wireless network• Inherent limitation of protocols• Queuing

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Resource AllocationResource Allocation• Many problems in the Internet come down to

issue of resource allocation.• Packets get delayed or dropped because network

resource cannot meet the traffic demands.• A network consists of shared resources :

bandwidth, buffer, serving traffic from competing users.

• To support QoS network must allocate resources and decide who should get how much resources.

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Resource Allocation (cont’d)Resource Allocation (cont’d)• Current Internet does not support active

resource allocation.• Network treats all packets equally and

then serves FCFS.• Hence current Internet offers best effort

service.• Adequate for some apps (e.g. file

transfer), but not suitable for realtime apps.

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Integrated ServicesIntegrated Services• Based on per flow resource reservation.• Apps must make a reservation before

transmitting traffic.• App characterize its traffic and resource

requirement.• Network uses routing protocol to find a

path.• A reservation protocol is used to install the

reservation state along that path.

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Integrated Services (cont’d)Integrated Services (cont’d)• At each hop, admission control checks whether

sufficient resources are available to accept the new reservation.

• Resource reservation enforced by packet classification and scheduling mechanisms.

• Two new service models were introduced : guaranteed service and controlled load.

• Guaranteed service provides deterministic worst case delay

• Controlled load provides less firm guarantee : it’s close to a lightly loaded best-effort network.

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Integrated Services (cont’d)Integrated Services (cont’d)• Overhead of setting up reservation is high.• Scalability problem : Backbone will have a

large number of flows and keeping flow information is not feasible.

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Differentiated ServiceDifferentiated Service• User traffic is divided into a small number

of forwarding classes.• For each forwarding class, the amount of

traffic that users can inject is limited at the edge of the network.

• Edge of a differentiated Services network responsible for mapping packets to their appropriate forwarding classes.

• Packet classification is usually based on service level agreement.

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Differentiated Service Differentiated Service (Cont’d)(Cont’d)

• Nodes at the edge of the network perform traffic policing to ensure conformance.

• Non-conforming traffic may be dropped, delayed or marked with a different forwarding class.

• Forwarding class is directly encoded into the packet header.

• Interior nodes use this info. to differentiate the treatment of packets.

• Does not require resource reservation.

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Differentiated Service Differentiated Service (Cont’d)(Cont’d)

• Forwarding classes apply to traffic aggregates rather than individual flows.

• No scalability problem.

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Multiprotocol Label Multiprotocol Label Switching (MPLS)Switching (MPLS)

• A short fixed length label is encoded into the packet header and is used for packet forwarding.

• When a label switch router (LSR) receives a MPLS packet, it uses incoming label to find the next hop and the corresponding outgoing label.

• In MPLS, the path a packet traverses is called label switched path (LSP).

• Network protocol independent

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Multiprotocol Label Multiprotocol Label Switching (MPLS)Switching (MPLS)

• Works alongside existing routing technologies and provide a mechanism for explicit control over routing paths.

• Used for traffic engineering, guaranteeing QoS and VPN.

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Traffic EngineeringTraffic Engineering• The basic problem : Given a network and traffic

demands, how can traffic flows in the network be organized so that an optimization objective can be achieved.

• Typically optimal operating point is reached when traffic is evenly distributed : leads to min queuing delay and packet loss.

• This cannot be achieved through destination based IP routing

• Advanced routing techniques constraint-based routing are used.

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Traffic Engineering (cont’d)Traffic Engineering (cont’d)• Routes are computed with multiple

constraints and aims for efficient resource utilization.

• Appropriate path selection with uniform traffic distribution and the congestion avoidance techniques improve the network performance

• MPLS can be used to achieve this goal.

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QoS based RoutingQoS based Routing• Basic routing protocols do not consider

any QoS parameters e.g. delay, packet loss.

• They are dynamic in terms of topology changes due to link or node failure

• But static in terms of performance of the network w.r.t. delay, packet loss etc

• QoS based routing provides a mechanism to get QoS support from the routing protocol.

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QoS based Routing (cont’d)QoS based Routing (cont’d)• Study few mechanisms proposed in

literature– Shortest Path First with Emergency Exits– OSPF Extension with QoS routing (RFC 2676)– OSPF based Load Sensitive Routing

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QoS in Wireless NetworkQoS in Wireless Network• Mostly study IEEE 802.11 based network• 802.11 MAC is a contention based

(CSMA/CA) MAC• Hence there is no guarantee/assurance for

an application to have bounded delay/packet loss

• Look at various proposals in literature that addresses the QoS issues

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QoS in Wireless Network QoS in Wireless Network (cont’d)(cont’d)

• DCF• Some enhancement to DCF for QoS• EDCA• Distributed Fair Scheduling in MAC• Wireless Token Ring Protocol• Blackburst