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It is then critical to ensure that this resolution process be as fast as possible, by applying<br />
the following best practices:<br />
1. Limit the number of unique domains/hostnames. Of course this is not always in<br />
your control, however as we’ll see in the sharding section, the relative performance<br />
impact of the number of unique hostnames will only grow when moving<br />
to <strong>HTTP</strong>/2<br />
2. Ensure low resolution latencies. Understand the topology of your DNS serving<br />
infrastructure and perform regular resolution time measurements from all the<br />
locations where your end users are (you can achieve this by using synthetic/real<br />
user monitoring). If you decide to rely on a third party provider, select one best<br />
suited to your needs, as they can offer wide differences of service quality.<br />
3. Leverage DNS prefetch from your initial html or response. This will start DNS<br />
resolution of all the hostnames on the page while the initial html is being downloaded<br />
and processed. For instance, see below the effect of DNS prefetching a<br />
google api hostname in a webpagetest waterfall. Note the dark blue piece representing<br />
the DNS resolution happening before the actual request is initiated:<br />
Figure 4.2 - DNS prefetch<br />
Figure 4.3 - Timeline object<br />
Optimize TCP connections<br />
Opening a TCP connection between a client and a serving origin is an expensive process.<br />
At the minimum, a non-secure (<strong>HTTP</strong>) connection involves a request-response<br />
round trip and some dedicated resource (memory + CPU) at both ends. A secure<br />
connection (<strong>HTTP</strong>S) generally incurs additional latency caused by 2 more round<br />
trips between the client and the origin server. If the client and servers are on opposite<br />
ends of the US, a total of 3 round-trips will take place before the connection is established,<br />
taking up to 250 ms, so you ought to carefully manage your connections. Recommended<br />
mitigations include:<br />
1. Leverage Preconnect, as it will remove connection times from the waterfall critical<br />
path<br />
2. Use a CDN. CDNs will terminate the http/s connection at the edge, located close<br />
to the requesting client, and therefore can greatly minimize the round trip latencies<br />
incurred by establishing a new connection.<br />
3. Implement the latest TLS best practices. (<strong>HTTP</strong>S only)<br />
Best Practices for Web Performance | 15
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