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15-441: Computer Networking

15-441: Computer Networking. Lecture 18: QoS. Thanks to David Anderson and Srini Seshan. Overview. Why QOS? Integrated services RSVP Differentiated services. Motivation. Internet currently provides one single class of “best-effort” service No assurances about delivery

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15-441: Computer Networking

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  1. 15-441: Computer Networking Lecture 18: QoS Thanks to David Anderson and Srini Seshan

  2. Overview • Why QOS? • Integrated services • RSVP • Differentiated services Lecture 21: 2006-11-16

  3. Motivation • 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 inelastic Lecture 21: 2006-11-16

  4. Why 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 application Lecture 21: 2006-11-16

  5. U U Elastic Hard real-time BW BW Delay-adaptive U BW Utility Curve Shapes Stay to the right and you are fine for all curves Lecture 21: 2006-11-16

  6. Utility curve – Elastic traffic U Elastic Bandwidth Does equal allocation of bandwidth maximize total utility? Lecture 21: 2006-11-16

  7. U Elastic BW Admission 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! Lecture 21: 2006-11-16

  8. U Hard real-time BW Delay-adaptive U BW Utility Curves – Inelastic traffic Does equal allocation of bandwidth maximize total utility? Lecture 21: 2006-11-16

  9. Inelastic 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 • Sometimes called “tolerant real-time” since they can adapt to the performance of the network • Hard real-time applications • Require hard limits on performance • E.g. control applications Lecture 21: 2006-11-16

  10. Delay-adaptive U BW Admission 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 adding more people would reduce overall utility • Basically avoids overload Lecture 21: 2006-11-16

  11. Overview • Why QOS? • Integrated services • RSVP • Differentiated services Lecture 21: 2006-11-16

  12. Components of Integrated Services • Type of commitment What does the network promise? • Packet scheduling How does the network meet promises? • Service interface How does the application describe what it wants? • Establishing the guarantee How is the promise communicated to/from the network How is admission of new applications controlled? Lecture 21: 2006-11-16

  13. Type of Commitments • Guaranteed service • For hard real-time applications • Fixed guarantee, network meets commitment if clients send at agreed-upon rate • Predicted service • For 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) • Implicit assumption – network does not change much over time • Datagram/best effort service Lecture 21: 2006-11-16

  14. Scheduling for Guaranteed Traffic • Use token bucket filter to characterize traffic • Described by rate r and bucket depth b • Use Weighted Fair-Queueing at the routers • Parekh’s bound for worst case queuing delay = b/r Lecture 21: 2006-11-16

  15. Token Bucket Filter Tokens enter bucket at rate r Operation: • If bucket fills, tokens are discarded • Sending a packet of size P uses P tokens • If bucket has P tokens, packet sent at max rate, else must wait for tokens to accumulate Bucket depth b: capacity of bucket Lecture 21: 2006-11-16

  16. Token Bucket Operation Tokens Tokens Tokens Overflow Packet Packet Not enough tokens  wait for tokens to accumulate Enough tokens  packet goes through, tokens removed Lecture 21: 2006-11-16

  17. Token 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*T • Information useful to admission algorithm Lecture 21: 2006-11-16

  18. Token Bucket Specs BW Flow B 2 Flow A: r = 1 MBps, B=1 byte 1 Flow A Flow B: r = 1 MBps, B=1MB 1 2 3 Time Lecture 21: 2006-11-16

  19. Guarantee 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/r, (where r may be much larger than average rate) • Assumes that r < link speed at any router • All sources limiting themselves to r will result in no network queuing Lecture 21: 2006-11-16

  20. Sharing versus Isolation • Isolation • Isolates well-behaved from misbehaving sources • Sharing • Mixing of different sources in a way beneficial to all • FIFO: sharing • each traffic source impacts other connections directly • e.g. malicious user can grab extra bandwidth • the simplest and most common queueing discipline • averages out the delay across all flows • Priority queues: one-way sharing • high-priority traffic sources have impact on lower priority traffic only • has to be combined with admission control and traffic enforcement to avoid starvation of low-priority traffic • WFQ: two-way isolation • provides a guaranteed minimum throughput (and maximum delay) Lecture 21: 2006-11-16

  21. Putting 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 queue • Predictive traffic classes • Worst case delay for classes separated by order of magnitude • When high priority needs extra bandwidth – steals it from lower class • Best effort traffic acts as lowest priority class Lecture 21: 2006-11-16

  22. Service Interfaces • Guaranteed Traffic • Host specifies rate to network • Why not bucket size b? • If delay not good, ask for higher rate • Predicted Traffic • Specifies (r, b) token bucket parameters • Specifies delay D and loss rate L • Network assigns priority class • Policing at edges to drop or tag packets • Needed to provide isolation – why is this not done for guaranteed traffic? • WFQ provides this for guaranteed traffic Lecture 21: 2006-11-16

  23. Overview • Why QOS? • Integrated services • RSVP • Differentiated services Lecture 21: 2006-11-16

  24. Components of Integrated Services • Type of commitment What does the network promise? • Packet scheduling How does the network meet promises? • Service interface How does the application describe what it wants? • Establishing the guarantee How is the promise communicated How is admission of new applications controlled? Lecture 21: 2006-11-16

  25. Service Interfaces • Guaranteed Traffic • Host specifies rate to network • Why not bucket size b? • If delay not good, ask for higher rate • Predicted Traffic • Specifies (r, b) token bucket parameters • Specifies delay D and loss rate L • Network assigns priority class • Policing at edges to drop or tag packets • Needed to provide isolation – why is this not done for guaranteed traffic? • WFQ provides this for guaranteed traffic Lecture 21: 2006-11-16

  26. C A D B Resource 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 Lecture 21: 2006-11-16

  27. RSVP Motivation • Resource reservation mechanism for multi-point applications • E.g., video or voice conference • Heterogeneous receivers • Changing membership • Use network efficiently • Minimize reserved bandwidth • Share reservations between receivers • Limit control overhead (scaling). • Adapt to routing changes C D B A I J H E G F Lecture 21: 2006-11-16

  28. PATH Messages • PATH messages carry sender’s Tspec • Token bucket parameters • 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 error Lecture 21: 2006-11-16

  29. RESV 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 message Lecture 21: 2006-11-16

  30. Path and Reservation Messages Sender 1 PATH R Sender 2 RESV (merged) PATH RESV R R Receiver 1 R RESV Reserved bandwidth is maximum of what downstream receivers can use Receiver 2 Lecture 21: 2006-11-16

  31. Soft State • Periodic PATH and RESV msgs refresh established reservation state • Path messages may follow new routes • Old information times out • Properties • Adapts to changes routes and sources • Recovers from failures • Cleans up state after receivers drop out Lecture 21: 2006-11-16

  32. Overview • Why QOS? • Integrated services • RSVP • Differentiated services Lecture 21: 2006-11-16

  33. Edge routers do fine grain enforcement Typically slower links at edge E.g. mail sorting in post offices Label packets with a type field Uses IP TOS bits E.g. a priority stamp Core routers process packets based on packet marking and defined per hop behavior More scalable than IntServ No per flow state or signaling Differentiated Services:Motivation and Design Classification and conditioning Lecture 21: 2006-11-16

  34. Expedited Forwarding PHB User sends within profile & network commits to delivery with requested profile • Strong guarantee • Possible service: providing a virtual wire • Admitted based on peak rate • Rate limiting of EF packets at edges only, using token bucket to shape transmission • Simple forwarding: classify packet in one of two queues, use priority • EF packets are forwarded with minimal delay and loss (up to the capacity of the router) Lecture 21: 2006-11-16

  35. Expedited Forwarding Traffic Flow Company A Packets in premium flows have bit set Premium packet flow restricted to R bytes/sec internal router ISP host edge router first hop router edge router Unmarked packet flow Lecture 21: 2006-11-16

  36. Assured Forwarding PHB • AF defines 4 classes • Strong assurance for traffic within profile & allow source to exceed profile • Implement services that differ relative to each other (e.g., gold service, silver service…) • Admission based on expected capacity usage profiles • Within each class, there are three drop priorities • Traffic unlikely to be dropped if user maintains profile • User and network agree to some traffic profile • Edges mark packets up to allowed rate as “in-profile” or high priority • Other packets are marked with one of 2 lower “out-of-profile” priorities • A congested router drops lower priority packets first • Implemented using clever queue management (RED with In/Out bit) Lecture 21: 2006-11-16

  37. Edge Router Input Functionality Traffic Conditioner 1 Flow 1 Traffic Conditioner N Flow N Arriving packet Forwarding engine Packet classifier Best effort classify packets based on packet header Lecture 21: 2006-11-16

  38. Traffic Conditioning Drop on overflow Packet output Wait for token Set EF bit Packet input No token token Packet output Packet input Test if token Set AF “in” bit Lecture 21: 2006-11-16

  39. Router Output Processing EF What type? High-priority Q Packets out AF Low-priority Q with priority dropAQM (RIO) Lecture 21: 2006-11-16

  40. Edge Router Policing no Token available? Clear “in” bit AF “in” set Forwarding engine Arriving packet Not marked Is packet marked? EF set no Token available? Drop packet Lecture 21: 2006-11-16

  41. Comparison Best-Effort Diffserv Intserv Service • Connectivity • No isolation • No guarantees • Per aggregation isolation • Per aggregation guarantee • Per flow isolation • Per flow guarantee Service Scope • End-to-end • Domain • End-to-end Complexity • No set-up • Long term setup • Per flow setup Scalability • Highly scalable • (nodes maintain only routing state) • Scalable (edge routers maintains per aggregate state; core routers per class state) • Not scalable (each router maintains per flow state) Lecture 21: 2006-11-16

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