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transmitting over long distances. For example, the theoretical fastest speed for sending data from New York to London is 56ms; to Syd‐ ney, it's 160ms. This means that to serve data to a user in Sydney from your servers in New York, 160ms will pass to establish a connection, and another 160ms will pass before the first byte of data is returned. That means that 320ms is the fastest possible time, even in optimal conditions, that a single byte of data could be returned. Of course, most requests will involve multiple round trips for data and multiple connections. However, data often doesn't travel by an optimal route. The BGP (Border Gateway Protocol) that manages most of the rout‐ ing on the Internet is designed to find optimal routes between any two points. Like all other protocols, though, it can be prone to mis‐ configuration, which sometimes results in the selection of less-than- optimal routes. More commonly, such suboptimal routes are chosen due to the peering arrangements of your network provider. Peering determines which other networks a network will agree to forward traffic to. You should not assume that the Internet is a non-partisan place where data moves freely from system to system; the reality is that peering is a commercial arrangement, and companies will choose their peers based on financial, competitive, and other less-idealistic reasons. The upshot for your system is that it is important to be aware that the peering arrangement that your hosting company (and the com‐ panies they have arrangements with) has in place can affect the per‐ formance of your system. When choosing a data center, you can get information about these arrangements; however, cloud providers are not so open. Therefore, it's important to monitor what's happening to determine the best cloud provider for your end users. Performance Risks The variability of connectivity across the backbone really boils down to a single performance risk, but it's a fundamental one that you need to be aware of when building any web-based system. Unreliable delivery of content If you cannot control how data is being sent to a user, you can‐ not control the speed at which it arrives. This makes it very dif‐ 2. Backbone Connectivity | 5

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