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SPDY Best Practices

Application level


Use a single connection - it’s better for SPDY performance and for the internet to use as few connections as possible. For SPDY, this will result in better packing of data into packets, better header compression, less connection state, fewer handshakes, etc. It improves TCP behavior across the internet and reduces bufferbloat. Interacts better with NATs as well as it requires less state.

Don’t shard hostnames - This is a hack used by web apps to work around browser parallelism limits. For all the reasons that we suggest using a single connection, hostname sharding is suboptimal when you can use SPDY instead. Furthermore, hostname sharding requires extra DNS queries and complicates web apps due to using multiple origins.

Use server push instead of inlining - This is a hack used by web apps to reduce RTTs. Inlining reduces the cacheability of web pages and may increase web page sizes due to base64 encoding. Instead, the server can just push the content.

Use request prioritization - Clients can advise the server on relative priorities for resources. Some basic heuristics which clients should all do generally is to make sure that html > js, css > *.

Use reasonable SPDY frame sizes - Although the spec allows for large frames, it is often desirable to use smaller frames, because this allows for better interleaving of frames from different streams.

Don't use CSS sprites with external stylesheets - Resources in external stylesheets are obviously only discovered after the external stylesheet has been downloaded, and only once the rule matches an element. The advantage they provide of reducing HTTP requests is unnecessary with SPDY due to its multiplexing. Therefore, CSS sprites just make it slower.

SSL

Use smaller, but complete, certificate chains - The size of certificate chains can substantially affect connection startup performance due to increased serialization latency and using up precious bytes in the initcwnd, costing extra RTTs. Also, when the server does not present the full certificate chain, then clients have to fetch intermediate certificates, adding yet more roundtrips before the connection becomes usable by the application.

Use wildcard certs - SPDY allows for connection sharing, which helps reduce the number of connections, which is beneficial for all the reasons already stated.

Don’t use overly large SSL write buffers - Large TLS application records will span multiple packets. Since applications must process complete TLS application records, this may add latency. Google servers use 2k buffers.

TCP

Servers should ensure initcwnd is at least 10 - initcwnd is a major bottleneck that impacts initial page load times. HTTP works around this by opening multiple connections simultaneously, essentially achieving n * initcwnd initial congestion window sizes.

Turn off tcp_slow_start_after_idle - Linux by default will have tcp_slow_start_after_idle set to 1. This drops cwnd back down to initcwnd and goes back to slow start after the connection has been “idle” (defined as the RTO). This kills persistent single connection performance and should be turned off.

Persist cwnd - SPDY provides a SETTINGS frame to allow for persisting settings across connections. Clients can use this information to remember their previous stable cwnd level and replay that to the server. Servers can use kernel modifications to set the cwnd for the connection.
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