浏览器用于估算合理过期时间的算法是什么？13.2.2 启发式过期
由于源服务器并非总是提供明确的过期时间，HTTP缓存通常会分配启发式过期时间，采用使用其他标头值（如Last-Modified时间）来估计合理的过期时间的算法。 HTTP / 1.1规范没有提供具体的算法，但对其结果施加了最坏情况的限制。由于启发式过期时间可能会损害语义透明性，因此应谨慎使用，并鼓励源服务器尽可能提供明确的过期时间。 HTTP/1.1 RFC 2616
Cache-Control HTTP header。Only use HTTP headers to control browser caching. If you decide to go against this be aware that IE only recognizes two cache control directives that are set inside HTML:
<META HTTP-EQUIV="Pragma" CONTENT="no-cache">
<META HTTP-EQUIV="Expires" CONTENT="-1">
and seemingly only the former is useful in the HTTPS scenario. Further, there can be problems when trying to use Pragma in IE. Finally, Chrome ignores cache directives in meta tags reducing their usefulness even further.
Don't use the Expires header. In modern browsers Expires is superseded by Cache-Control. Expires: 0 and Pragma: no-cache are technically invalid response headers. Yes, they have existed since the beginning but not all modern browsers (e.g. Chrome) use them and they have been superseded by Cache-Control.
The Vary header is a minefield. How Vary behaves in older IEs. How Vary behaves with XHR. Finding the details out is left as an exercise to the reader - and leaves the impression it is preferable to use different URLs for different content...
Allow the browser to make conditional requests by setting ETags.
Etags allow a browser to do a lightweight check to see if the content has changed and it can avoid making a full request if it hasn't.
Be aware some browsers are just broken and need hacks. IE 8 can have issues downloading files which it has been told not to cache.
HttpResponseHeaders::GetFreshnessLifetimes()。nsHttpResponseHead::ComputeFreshnessLifetime()。computeFreshnessLifetimeForHTTPFamily()。来自Chromium源代码：https://code.google.com/p/chromium/codesearch#chromium/src/net/http/http_response_headers.cc&l=1082&rcl=1421094684
if ((response_code_ == 200 || response_code_ == 203 ||
response_code_ == 206) && !must_revalidate) {
// TODO(darin): Implement a smarter heuristic.
Time last_modified_value;
if (GetLastModifiedValue(&last_modified_value)) {
// The last-modified value can be a date in the future!
if (last_modified_value <= date_value) {
lifetimes.freshness = (date_value - last_modified_value) / 10;
return lifetimes;
}
}
}
date_value是Date字段的值吗？它看起来每次我发出请求时都会改变，是这样吗？而且它的值始终是源服务器上的日期。这是否意味着lifetimes.freshness也一直在增加（因为date_value增加但last_modified_value应该保持不变）？ - yaobinreturn (creationTime - lastModifiedValue) * 0.1;
max-age = (DownloadTime - LastModified) * 0.1来进行缓存控制： http://blogs.msdn.com/b/ie/archive/2010/07/14/caching-improvements-in-internet-explorer-9.aspx
这与Mozilla的做法非常相似（这篇文章已经有些旧了，我不知道它是否有改变）: https://developer.mozilla.org/en-US/docs/HTTP_Caching_FAQ
据我最近查看的情况，Gecko 估计过期时间为 现在时间 + （现在时间 - 上次修改时间）/10。