PART IV
Appendix B: HTTP 1.1 Reference 595
Request Header Description Examples
If-Match Makes a request conditionally only
if the items match some selector
value passed in; typically related to
an encountered ETag header value.
If-Match: 4135cda4
If-Modified-
Since
Indicates file freshness to
determine if a response is
required. For example, when used
in conjunction with a GET request
for a particular file, the requested
file is checked to see if it has been
modified since the time specified
in the field. If the file has not been
modified, a "not modified" code
(304) is sent to the client so a
cached version of the document
can be used; otherwise, the file is
returned normally.
If-Modified-Since: Wed, 01
Sep 2004 13:24:52 GMT
If-None-Match Does the opposite of If-Match.
The request method is conditional
only if the selector does not match
anything.
If-None-Match: 4135cda4
If-Range If a client has a partial copy of
an object in its cache and wishes
to have an up-to-date copy of the
entire object there, it could use the
Range request header with this
header specifying a particular range
value to decide to update the file.
Modification selection can take
place on time as well.
If-Range: Wed, 01 Sep 2004
13:24:52 GMT
If-Unmodified-
Since
If the requested file has not been
modified since the specified time,
the server should perform the
requested method; otherwise, the
method should fail.
If-Unmodified-Since: Wed, 01
Sep 2004 13:24:52 GMT
Max-Forwards Indicates the limit of the number
of proxies or gateways that can
forward the request. Often ignored
in practice and used only with the
TRACE method.
Max-Forwards: 6
Proxy-
Authorization
Allows the client to identify itself
or the user to a proxy that requires
authentication.
Proxy-Authorization: Basic,
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TABLE B-5 HTTP 1.1 Request Headers (continued)