HTTP/1.0
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... headers unless they know for certain (by out of band means) that
there are no HTTP/1.0 proxies in the response chain. HTTP/1.1
...
... Caching and HTTP/1.0 ...
... cache documents (following Expires:
<a date in the past> or Pragma: no-cache (HTTP/1.0), or Cache-
control: no-cache ...