Download Fast TCP Doesn`t Matter to ISPs (What Am I

Fast TCP Doesn’t Matter to ISPs
(What Am I Missing?)
Albert Greenberg
Microsoft
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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
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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?
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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)
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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.
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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)
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