CS 268: Computer NetworkingL-7 QoSQoS• IntServ• DiffServ• Assigned reading• [ [She95] Fundamental Design Issues forthe Future Internet• [CSZ92] Supporting Real-Time Applicationsin an Integrated Services Packet Network:Architecture and Mechanisms23Overview• Why QOS?• Integrated services• RSVP• Differentiated services4Motivation• Internet currently provides one single classof “best-effort” service• No assurances about delivery• Existing applications are elastic• Tolerate delays and losses• Can adapt to congestion• Future “real-time” applications may beinelastic5Inelastic 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 networkdesign?• Maximize total bandwidth? Minimize latency?• Maximize user satisfaction – the total utilitygiven 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 ofbandwidth maximize total utility?9Admission Control• If U(bandwidth) is concave elastic applications• Incremental utility is decreasingwith increasing bandwidth• Is always advantageous tohave more flows with lowerbandwidth• No need of admission control; This is why the Internet works!BWUElastic10Utility Curves – Inelastic trafficBWUHard real-timeBWUDelay-adaptiveDoes equal allocation ofbandwidth maximize total utility?11Admission Control• If U is convex inelasticapplications• U(number of flows) is no longermonotonically increasing• Need admission control tomaximize total utility• Admission control deciding when the addition ofnew people would result inreduction of utility• Basically avoids overloadBWUDelay-adaptive12Overview• Why QOS?• Integrated services• RSVP• 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 shouldnetwork offer? Depends on the characteristics of theapplications 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 hidenetwork jitterCharacteristics of Playback Applications• In general lower delay is preferable.• Doesn’t matter when packet arrives as longas 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 and adaptive applications• Rigid: set fixed playback point• Adaptive: adapt playback point• Gamble that network conditions will be the same asin the past• Are prepared to deal with errors in their estimate• Will have an earlier playback point than rigidapplications• Tolerant and intolerant applications• Tolerance to brief interruptions in service• Four 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 networkoffer?19 Type of Commitments• Guaranteed service• For intolerant and rigid applications• Fixed guarantee, network meets commitment as longas 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 consistentperformance (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/r• b = bucket depth• r = rate of arrival22Token Bucket FilterOperation:• If bucket fills, tokens arediscarded• Sending a packet of size Puses P tokens• If bucket has P tokens,packet sent at max rate, elsemust wait for tokens toaccumulateTokens enter bucket at rate rBucket depth b:capacity of bucket23Token Bucket OperationTokensPacketOverflowTokens TokensPacketEnough tokens packet goes through,tokens removedNot enough tokens wait for tokens toaccumulate24Token Bucket Characteristics• In long run, rate is limited to r• In short run, a burst of size b can be sent• Amount of traffic entering at interval T isbounded by:• Traffic = b + r*T• Information useful to admission algorithm25Token Bucket SpecsBWTime121 2 3Flow AFlow BFlow A: r = 1 MBps, B=1 byteFlow B: r = 1 MBps, B=1MB26Predicted ServiceGoals:• Isolation• Isolates well-behaved from misbehaving sources• Sharing• Mixing of different sources in a way beneficial to allMechanisms:• WFQ• Great isolation but no sharing• FIFO• Great sharing but no isolation27Predicted Service• FIFO jitter increases with the number of hops• Use opportunity for sharing across hops• FIFO+• At each hop: measure
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