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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 1 Modelling and controlling the future? Peter Key Microsoft Research Cambridge peterkey@microsoft.com http://www.research.microsoft.com/~peterkey/

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 2 A 5 th Century world view (Democritus) All matter is composed of atoma Differ in shape size and weight Too small to be seen Appearances caused by higher-layer effects (post-modern interpretation?) Indestructible

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 3 21 st C Networking Everything composed of packets Packet level simulations Packet level QoS (none!) –Packets seem to get lost … Science studies macro level (eg thermodynamics etc) as well as micro-level But network Modelling & control still largely atomic!

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 4 Missing the point … “More generally, there are almost no interesting problems in networking for which an M/M/1 model is appropriate unless the result is negative (i.e., if you can show that even with an M/M/1 queue, the proposed algorithm or traffic management scheme fails woefully then the algorithm/scheme is clearly worthless).” (Name withheld to protect the …)

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 5 Moving up the stack Application performance matters to users Too much emphasis on Transport layer Can have application level reactions MAC matters for wireless Process Host to host Internet Network I/f Session Transport Application Presentation Network Data Link PHY

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 6 Modelling applications Need stochastic demand models What are the right timescales? –eg traffic may be LRD –but locally Poisson (infinitely divisible limit ) –Interacting feedback control loops Right level of detail? Want Network level models & not just particle (queuing) models

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 7 Modelling large systems Flows N r (t), stochastic arrivals rate r Fixed expected duration r variable b/w x r Packet level measures, p(N r (t) x r (t),…) Scale system by C r C C r & n r (t) N r (t) /C then have a FLLN for network equilibrium behaviour

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 8 Modelling large systems -II Diffusion limit (OU) for second-order properties Theory vs simulation

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 9 Distributed admission control

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 10 System Capacity C Active connections, N a probing connections, N p Acks Probability marking (loss) 0 1 Load Mark active probing Delay Virtual Capacity cC

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 11 Example: Admission Control Q: How Long to probe for & stability? –Critical to ensure mean holding time / rtt Large Q: What rate? –Zero rate (“free probing”) or –“optimal” non-zero rate =half rate at which packets marked / lost Q: Feedback signal? –Tail marking (drop tail) cannot protect system –Early warning marking (apparently sacrificing5%) with probing gives a controllable system Q: Dynamic Rate Adjustment? –If “correct” at entry, can be “Infrequent”

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 12 Performance with Loss based feedback

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 13 Performance with Marking Give up a small amount of efficiency (eg 5-10%) to gain control

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 14 Small networks? Analysis breaks down for small networks –Ad-hoc / home networks (small diameter) –or where ratio of peak b/w /capacity large (small capacity) Variance not a good predictor of behaviour Timescale separation can break down If rtts are small, then end-system effects are no longer negligible

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 15 Mixing traffic? If scaling applied to fixed-volume traffic (eg file transfers), limit distribution unstable or has mean zero BUT, mixing with fixed duration traffic stabilizes the system –See forthcoming ITC paper!

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 16 User preferences? How should different types of traffic be treated? –Need to have real incentives somewhere (incentive compatibility implies real money) –or mandated behaviour Current TCP-friendliness definition inhibits evolution Distributing resources: Zero-sum game at network level, not at user level Sneaker-net currently cheaper than real-net, contention at access points, difference increasing

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 17 Example: Scheduling file transfers Capacity C TCP allocates equal weights (or bias against small) –forces concave user utility function on rate But Shortest remaining processing time –“optimal”, implementation, stability (& fairness?) questions We can design a distributed solution which gives greater weight to mice – everyone better off!

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 18 Modelling application level recap Have given some examples of application level issues –Flow level models / demand –User level preferences

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 19 Overlays – an alternative method of sharing? Overlays can provide both a addressing/routing substrate and a “community” Increase availability of certain resources (eg information, processing) not others (eg b/w) Can performance guarantees be given? –use of FEC can just be an arms race.. Asymmetric resources imply benefits for the “rich”! Eg Splitstream can save on sources costs

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Key Ne X tworking’03 June 23-25,2003, Chania, Crete, Greece The First COST-IST(EU)-NSF(USA) Workshop on EXCHANGES & TRENDS IN N ETWORKING 20 Some concluding remarks Modelling needs to guide design not follow … Current Internet provides connectivity but what else? –With better signals (eg early-warning, ECN) could give (soft) QoS guarantees –But why should users cooperate? –Learn from game theory & economics? –Incentives for content, connectivity, routing are interconnected

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