PageSpeed: Leverage proxy caching (deprecated)
Enabling public caching in the HTTP headers for static resources allows the browser to download resources from a nearby proxy server rather than from a remoter origin server.
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In addition to browser caching, HTTP provides for proxy caching, which enables static resources to be cached on public web proxy servers, most notably those used by ISPs. This means that even first-time users to your site can benefit from caching: once a static resource has been requested by one user through the proxy, that resource is available for all other users whose requests go through that same proxy. Since those locations are likely to be in closer network proximity to your users than your servers, proxy caching can result in a significant reduction in network latency. Also, if enabled proxy caching effectively gives you free web site hosting, since responses served from proxy caches don't draw on your servers' bandwidth at all.
You use the
Cache-control: public header to indicate that a resource can be cached by public web proxies in addition to the browser that issued the request. With some exceptions (described in the sidebar), you should configure your web server to set this header to public for cacheable resources.