CS640: Introduction to Computer NetworksWhy a New Service Model?Utility curve – Elastic trafficAdmission ControlUtility Curves – Inelastic trafficQoS and Admission ControlQoS Instantiation #1: Integrated ServicesType of CommitmentsScheduling for Guaranteed TrafficToken Bucket FilterToken Bucket CharacteristicsToken Bucket SpecsGuarantee Proven by ParekhPutting It All TogetherResource Reservation Protocol (RSVP)PATH MessagesRESV MessagesDifferentiated Services: Motivation and DesignDiffServ ExampleExpedited ForwardingAssured ForwardingTraffic Conditioning: At Customer EdgeEdge Router Policing: At ISP EdgeRouter Output ProcessingCS640: Introduction to Computer NetworksAditya AkellaLecture 20 – QoS2Why a New Service Model?•Best effort clearly insufficient–Some applications need more assurances from the network•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 application3Utility curve – Elastic trafficBandwidthUElasticDoes equal allocation of bandwidth maximize total utility?4Admission 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 and explicit QoS mechanisms BWUElastic5Utility Curves – Inelastic trafficBWUHard real-timeBWUDelay-adaptiveDoes equal allocation of bandwidth maximize total utility?6QoS and Admission Control•If U is convex inelastic applications–U(number of flows) is no longer monotonically increasing•Need admission control and special QoS mechanisms–Admission control deciding when the addition of new people would result in reduction of utilityBWUDelay-adaptive7QoS Instantiation #1:Integrated ServicesKey components:1. 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?8 Type of Commitments•Guaranteed service–For hard real-time applications–Fixed guarantee, network meets commitment as long as rates clients send at match traffic agreement•Predicted service–For tolerant (e.g. delay-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). Ensure that such apps continue to see a lightly loaded network.•Datagram/best effort service9Scheduling for Guaranteed Traffic•Use token bucket filter to characterize traffic–Described by rate r and bucket depth b–FlowSpec or flow specification•Use Weighted Fair-Queueing at the routers•Parekh’s bound for worst case queuing delay = b/r10Token Bucket FilterTokens enter bucket at rate rBucket depth b: capacity of bucketOverflowTokensTokensPacketEnough tokens packet goes through,tokens removedTokensPacketNot enough tokens wait for tokens to accumulate11Token Bucket Characteristics•On the long run, rate is limited to r•On the short run, a burst of size b can be sent•Amount of traffic entering at interval T is bounded by:–Traffic = b + r*T12Token Bucket SpecsBWTime121 2 3Flow AFlow BFlow A: r = 1 MBps, B=1 byteFlow B: r = 1 MBps, B=1MB13Guarantee Proven by Parekh•Given:–Flow i shaped with token bucket and leaky bucket rate control (depth b and rate r)–Network nodes do WFQ•Cumulative queuing delay Di suffered by flow i has upper bound–Di < b/r14Putting It All Together•Assume 3 types of traffic: guaranteed, predictive, best-effort•Scheduling: use WFQ in routers•Each guaranteed flow gets its own queue•All predicted service flows and best effort aggregates in single separate priority queue–Predictive traffic classes•Worst case delay for classes separated by order of magnitude•Strict priority queueing – coupled with admission control into each priority level•Higher priority steals scheduling cycles from lower priority - One way isolation–Best effort traffic acts as lowest priority class15Resource Reservation Protocol(RSVP)•Carries resource requests all the way through the network•Main goal: establish “state” in each of the routers so they “know” how they should treat flows.–State = packet classifier parameters, bandwidth reservation, ..•At each hop consults admission control and sets up reservation. Informs requester if failure•Key properties–Receiver driven–Soft state•Periodically refresh reservationsABCD16PATH Messages•PATH messages carry sender’s flow properties•Routers note the direction PATH messages arrived and set up reverse path to sender•Receivers send RESV messages that follow reverse path and setup reservations•If reservation cannot be made, user gets an error17RESV Messages•Forwarded via reverse path of PATH•Queuing delay and bandwidth requirements•Source traffic characteristics (from PATH)•Filter specification–Which transmissions can use the reserved resources•Router performs admission control and reserves resources–If request rejected, send error message18Differentiated Services:Motivation and Design•Edge routers do coarse grain enforcement–Label packets with a type field •Uses IP TOS bits•E.g. a priority stamp•Core routers process packets based on packet marking•More scalable than IntServ–No signaling–No per-flow state in the core–More useful between a pair of neighboring networks, while IntServ was end-to-end–Typically used by multi-campus enterprises with all campuses connected to the same ISPClassification and conditioning19DiffServ Examplefirst hoprouterinternalrouteredgerouterhostedgerouterISPCompany AUnmarkedpacket flowPackets in premiumflows have bit setPremium packet flowrestricted to R bytes/secSet bitsappropriatelyCheck if bitsconformSign a service level agreementwith ISP. (SLA)20Expedited ForwardingUser sends within agreed profile & network commits to delivery with requested profile–Strong guarantee–User cannot exceed profile packets will get dropped•Core router Simple forwarding: if packet marked as EF, put in priority queue–EF packets are forwarded with minimal delay and loss (up to the capacity of the router)21Assured Forwarding •AF defines 4 classes –Strong assurance for traffic within
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