AJAX - The Complete Reference

592 Part IV: Appendixes

Header Description Examples

Allow Indicates the HTTP methods supported

by the resource. Typically seen in a 405

(Method Not Allowed) response.

Allow: GET, HEAD

Content-Encoding Indicates the encoding the data is in. Content-encoding: gzip

Content-Language Indicates the language used for the

request or response.

Content-Language: es

Content-Length Gives the length in bytes of the message

being sent to or from the server, if any.

Remember that the browser can upload or

pass data using the PUT or POST method.

In the case of a HEAD request, this value

indicates what would have been passed

back if a GET had been used.

Content-Length: 1968

Content-Location The actual location (URL) of a resource.

Not always used as it is considered a

security risk by some because it may

expose translated addresses. Its use

in requests is not as well understood,

despite it being an entity header.

Content-Location:

intranet.ajaxref.com/

files/httpcodes.txt

Content-MD5 Contains an MD5 checksum of a

message body; useful for verifying the

integrity of a passed message.

Content-MD5: 00587edd8c0f

dcfcb6bda6856dfb304b

Content-Range Used with a partial body to specify where

this piece fits in the entire body. It must

specify the start and end byte positions

of the partial body. In addition, it should

include the total length of the entire

body if possible.

Content-range: bytes

500-999/7220

Content-Type Indicates the MIME type of a message

being sent to or from a server. Typically

all file responses would be stamped

with a Content-Type to indicate

to the browser what kind of data it is

receiving. In this role, Content-Type is

the most important of the headers in a

response. However, it can also be used

from a request point of view. The value in

this case is for data submission and is

particularly important in the case of form

post or file upload.

Content-Type: image/gif

Content-Type:

application/x-www-form-

urlencoded

Expires Gives the date/time after which the data

should be considered stale and should

not be returned from a cache. Times far

in the past indicate immediate cache

invalidation.

Expires: Thu, 04 Dec 1997

16:00:00 GMT

Last-Modified Indicates the last modification of a data

entity. Integral for cache control, Found

most often in responses.

Last-Modified: Mon, 30

Apr 2007 22:37:16 GMT

TABLE B-4 Entity Headers in HTTP 1.1