AJAX - The Complete Reference

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)