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Enterprise QoS Reality Check

Enterprise QoS Reality Check. Terry Gray Director, Networks & Distributed Computing University of Washington. Context. UW network: 45,000 machines 500 GB/day 50 remote locations ATM-free zone Experts in breaking networks. Network Manager Goals.

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Enterprise QoS Reality Check

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  1. Enterprise QoS Reality Check Terry Gray Director, Networks & Distributed Computing University of Washington

  2. Context • UW network: • 45,000 machines • 500 GB/day • 50 remote locations • ATM-free zone • Experts in breaking networks

  3. Network Manager Goals • Meet client expectations by matching capacity with demand… via: • increasing capacity • optimizing use of available capacity • reducing demand • Avoid becoming a victim of QoS myths... • Re: Admission Control, Reservations, and Bandwidth Guarantees

  4. Admission Control • Does not create capacity… • How many fast-busy signals = success? • Makes sense IFF • BW cost > BW Management cost • Denial (busy signals) better than degradation • Cost model precludes adding capacity • A tactical solution to a strategic problem: • namely, matching capacity with demand.

  5. Reservations • Require sequestered bandwidth • Require end-times • Are useless for “small chunk” allocations • Are generally not beloved by users • Require manual coordination for “large chunk” allocations

  6. Bandwidth Guarantees • In a shared medium, there are no guarantees, only probabilities: • P(denial) or P(degradation) • No free lunch: • If you don’t invest in capacity, you will need to invest in more technology, but also: people to develop and manage bandwidth policies

  7. Bandwidth Management • Different strategies needed for different congestion zones and timescales... • Congestion zones: • Subnet • Backbone • WAN • Timescales • Per packet (Traffic Shaping, DiffServ) • Per flow or session (Admission Control) • Persistent (MPLS, DWDM)

  8. Three Kinds of Traffic • Preferred (usually = Interactive) • Best Effort • Sacrificial

  9. Provisioning • Claim: • You must adequately provision for Preferred and Best-Effort traffic, or you will die at the hands of the few or the many… • Conclusion: • QoS is a labelling and feedback problem, not a signalling and admission control problem

  10. Feedback Alternatives • Admission controls (fast-busy signals): • explicit short term feedback; • doesn't solve real problem. • Usage pricing: • implicit long term feedback; • revenue stream for adding capacity • Social pressure: • e.g. top user lists

  11. One Approach • Combo of provisioning & simple diffserv • Try laissez faire tagging by end-system • Premium-port strategy if that fails • Encourage resilient application design

  12. QoS Worries • Increasing network complexity • Impact on network reliability • Effects of “Policy Jitter” • Beware the prophets of DEN • WAN QoS accounting • Building upgrades (cat 3 wireplant) • Wireless

  13. For more info… • http://staff.washington.edu/gray/papers • gray@washington.edu

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