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Web app developers spend most of our time not thinking about how data is actually transmitted through the bowels of the network stack. Abstractions at the application layer let us pretend that networks read and write whole messages as smooth streams of bytes. Generally this is a good thing. But knowing what's going underneath is crucial to performance tuning and application design. The character of our users' internet connections is changing and some of the rules of thumb we rely on may need to be revised.