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Fast TCP Doesn’t Matter to ISPs (What Am I Missing?) Albert Greenberg Microsoft 1 Traffic and Cost • Consumer Broadband >> Business • Consumer broadband growth trends are stable • Individual consumer broadband flows have little impact on ISP backbones – Transmission rates per broadband subscriber are tiered and rate limited • High speed access costs are high – TCP is likely not the factor stopping a Disney from last minute distribution of content to theatres world wide 2 Fast TCP A Win for ISPs? • What if I’m wrong? • Biggest deal = file transfer size – Downloading (stored content) >> Streaming on the Internet – Fast TCP might increase consumer bandwidth demand • Economic wins – Bandwidth is the killer app, so this brings more revenue for ISPs – Fast, robust download protocols have potential to help video-ondemand services in IPTV framework • What would the impact be on network engineering? 3 Backbone • Sources of router stress – PPS: high rates of little packets (e.g., VoIP), yet Fast TCP won’t promote this – Protocols: stresses CPU, yet this is in the control plane not the data plane – Congestion: yet backbone physical pipes >> individual flow • Caveat: stat mux trouble could arise if using a set of parallel links for higher capacity • Core link load is low under normal conditions – Backbones dimensioned to handle rare failure conditions • Tremendous stat mux gains on backbones – 40Gbps links rolling in now – A 1Gbps flow is still 1/40th of this Backbone Traffic sea level Backbone Could be this: Traffic sea level (after high speed TCP) Backbone Not this: Traffic sea level (after high speed TCP) 4 Access • Access is where the cost is, and where the potential for problems is – Few 100 broadband subs per 1st aggregation router – Few QoS mechanisms deployed between the first IP aggregation router and broadband subscriber router – Higher speed access attracts subs who are early-adopters/power users, and who increase overall burstiness • E.g., Kensuke Fukada, Kenjiro Cho, Hiroshi Esaki, The Impact Of Residential Broadband Traffic On Japanese ISP Backbones, ACM SIGCOMM Computer Communication Review, Vol. 35, Issue 1, 2005, pp. 15-22. • Yet TCP dynamics are constrained by the access links – Access is rate limited, per sub – Fast TCP still backs off – QoS mechanisms in the last mile are available (to date generally not needed) – Capacity planning practice, which is largely empirical, can be adapted to accommodate increased burstiness; • E.g., Matthew Roughan, Charles R. Kalmanek, Pragmatic Modeling of Broadband Access Traffic, Computer Communications, Volume/Number: 26(8), May, 2003, pp.804-816. 5 Takeaways • Impact on Internet health – Don’t see potential downside • Impact on Internet economics – See potential upside • Impact on network engineering? – Access >> Backbone – Attendant increase in sluggishness in TCP feedback loop (more bytes shipped per RTT) risks potential for: • TCP synchronization and oscillation • QoS impact (jitter, loss) on legacy TCP flows • QoS impact of jumbo frames (if fast TCP becomes common, jumbos may as well) 6