15-744: Computer NetworkingQoSOverviewMotivationInelastic ApplicationsWhy a New Service Model?Utility Curve ShapesUtility curve – Elastic trafficAdmission ControlUtility Curves – Inelastic trafficSlide 11Slide 12Components of Integrated Services1. Type of commitmentPlayback ApplicationsCharacteristics of Playback ApplicationsApplications VariationsSlide 18Type of CommitmentsSlide 20Scheduling for Guaranteed TrafficToken Bucket FilterToken Bucket OperationToken Bucket CharacteristicsToken Bucket SpecsPredicted ServiceSlide 27Unified SchedulingService InterfacesSlide 30Internet Video TodayClient-Server Streaming: Adaptation Quality to LinkProblems Adapting to Network StateCongestion Manager ArchitectureTransmission APIFeedback about Network StateSlide 38DiffServBasic ArchitecturePer-hop Behaviors (PHBs)Slide 42Expedited Forwarding PHBExpedited Forwarding Traffic FlowAssured Forwarding PHBRed with In or Out (RIO)RIO Drop ProbabilitiesEdge Router Input FunctionalityTraffic ConditioningRouter Output ProcessingEdge Router PolicingComparisonPossible Token Bucket UsesGuarantee Proven by Parekh15-744: Computer NetworkingL-7 QoSQoS•IntServ•DiffServ•Assigned reading•[She95] Fundamental Design Issues for the Future Internet•Optional•[CSZ92] Supporting Real-Time Applications in an Integrated Services Packet Network: Architecture and Mechanisms•[CF98] Explicit Allocation of Best-Effort Packet Delivery Service23Overview•Why QOS?•Integrated services•Internet video•Differentiated services4Motivation•Internet currently provides one single class of “best-effort” service•No assurances about delivery•Existing applications are elastic•Tolerate delays and losses•Can adapt to congestion•Future “real-time” applications may be inelastic5Inelastic Applications•Continuous media applications•Lower and upper limit on acceptable performance.•BW below which video and audio are not intelligible•Internet telephones, teleconferencing with high delay (200 - 300ms) impair human interaction•Hard real-time applications•Require hard limits on performance•E.g. control applications6Why a New Service Model?•What is the basic objective of network design?•Maximize total bandwidth? Minimize latency?•Maximize user satisfaction – the total utility given to users•What does utility vs. bandwidth look like?•Must be non-decreasing function •Shape depends on application7Utility Curve ShapesStay to the right and youare fine for all curvesBWUElasticBWUHard real-timeBWUDelay-adaptive8Utility curve – Elastic trafficBandwidthUElasticDoes equal allocation of bandwidth maximize total utility?9Admission Control•If U(bandwidth) is concave elastic applications•Incremental utility is decreasing with increasing bandwidth•Is always advantageous to have more flows with lower bandwidth•No need of admission control; This is why the Internet works!BWUElastic10Utility Curves – Inelastic trafficBWUHard real-timeBWUDelay-adaptiveDoes equal allocation of bandwidth maximize total utility?11Admission Control•If U is convex inelastic applications•U(number of flows) is no longer monotonically increasing•Need admission control to maximize total utility•Admission control deciding when the addition of new people would result in reduction of utility•Basically avoids overloadBWUDelay-adaptive12Overview•Why QOS?•Integrated services•Internet video•Differentiated services13Components of Integrated Services1. Type of commitment What does the network promise?2. Packet scheduling How does the network meet promises?3. Service interface How does the application describe what it wants?4. Establishing the guarantee How is the promise communicated to/from the network How is admission of new applications controlled?141. Type of commitment What kind of promises/services should network offer? Depends on the characteristics of the applications that will use the network ….15Playback Applications•Sample signal packetize transmit buffer playback•Fits most multimedia applications•Performance concern:•Jitter – variation in end-to-end delay•Delay = fixed + variable = (propagation + packetization) + queuing•Solution: •Playback point – delay introduced by buffer to hide network jitterCharacteristics of Playback Applications•In general lower delay is preferable.•Doesn’t matter when packet arrives as long as it is before playback point•Network guarantees (e.g. bound on jitter) would make it easier to set playback point•Applications can tolerate some loss1617Applications Variations•Rigid & adaptive applications •Rigid – set fixed playback point •Adaptive – adapt playback point•Gamble that network conditions will be the same as in the past•Are prepared to deal with errors in their estimate•Will have an earlier playback point than rigid applications•Tolerant & intolerant applications•Tolerance to brief interruptions in service•4 combinations18Applications VariationsReally only two classes of applications1) Intolerant and rigid2) Tolerant and adaptiveOther combinations make little sense3) Intolerant and adaptive - Cannot adapt without interruption4) Tolerant and rigid - Missed opportunity to improve delay So what service classes should the network offer?19 Type of Commitments•Guaranteed service•For intolerant and rigid applications•Fixed guarantee, network meets commitment as long as clients send at match traffic agreement•Predicted service•For tolerant and adaptive applications•Two components•If conditions do not change, commit to current service•If conditions change, take steps to deliver consistent performance (help apps minimize playback delay)•Implicit assumption – network does not change much over time•Datagram/best effort service20Components of Integrated Services1. Type of commitment What does the network promise?2. Packet scheduling How does the network meet promises?3. Service interface How does the application describe what it wants?4. Establishing the guarantee How is the promise communicated to/from the network How is admission of new applications controlled?21Scheduling for Guaranteed Traffic•Use token bucket filter to characterize traffic•Described by rate r and bucket depth b•Use WFQ at the routers•Parekh’s bound for worst case queuing delay = b/r22Token Bucket FilterOperation:•If bucket fills, tokens are discarded•Sending a
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