RFC 2068

9.3 GET

   The GET method means retrieve whatever information (in the form of an
   entity) is identified by the Request-URI. If the Request-URI refers
   to a data-producing process, it is the produced data which shall be
   returned as the entity in the response and not the source text of the
   process, unless that text happens to be the output of the process.

   The semantics of the GET method change to a "conditional GET" if the
   request message includes an If-Modified-Since, If-Unmodified-Since,
   If-Match, If-None-Match, or If-Range header field. A conditional GET
   method requests that the entity be transferred only under the
   circumstances described by the conditional header field(s). The
   conditional GET method is intended to reduce unnecessary network
   usage by allowing cached entities to be refreshed without requiring
   multiple requests or transferring data already held by the client.

   The semantics of the GET method change to a "partial GET" if the
   request message includes a Range header field. A partial GET requests
   that only part of the entity be transferred, as described in section
   14.36. The partial GET method is intended to reduce unnecessary
   network usage by allowing partially-retrieved entities to be
   completed without transferring data already held by the client.

   The response to a GET request is cachable if and only if it meets the
   requirements for HTTP caching described in section 13.