RFC 7232 defines a conditional request mechanism to prevent
response objects being unnecessarily transferred over the network.
If that mechanism is used by the client and a cache MISS occurs
it can prevent new cache entries being created.
This option determines whether Squid on cache MISS will pass the
client revalidation request to the server or tries to fetch new
content for caching. It can be useful while the cache is mostly
empty to more quickly have the cache populated by generating
non-conditional GETs.
When set to 'on' (default), Squid will pass all client If-* headers
to the server. This permits server responses without a cacheable
payload to be delivered and on MISS no new cache entry is created.
When set to 'off' and if the request is cacheable, Squid will
remove the clients If-Modified-Since and If-None-Match headers from
the request sent to the server. This requests a 200 status response
from the server to create a new cache entry with.