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 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

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.

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

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

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)

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

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

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

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

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

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