AJAX - The Complete Reference

590 Part IV: Appendixes

General Headers

General headers are, as their name indicates, very general. These headers provide the most

basic information about a request or response. All of the general headers are detailed in

Table B-3; while some like Date are quite common, others like Upgrade or Trailer will

rarely be seen.

Entity Headers

Another class of headers, entity headers, are related to the actual message payload of an

HTTP request or response. Since most requests, except POST or PUT method requests, do

not send data to a server, these headers are often seen more in responses. Table B-4 presents

a brief overview of this class of headers.

Header Description Examples

Cache-

Control

Passes cache control information to

server or client. There are many options

for the header, and this header may need

to be accompanied by other headers

such as date/time indications, a unique

payload header like ETag or Pragma

headers in order to completely control

caching.

Cache-control: max-age=6666989

Cache-control: private

Connection Used by the user-agent or server to

specify how the connection should be

handled. Typically, the value of close

is used to tell a server to close the

connection after the response. The

server may also respond with the

header, and it will often set it to a value

of keep-alive to indicate to the client

that the connection should be persistent.

However, by default without a close value

a persistent connection is generally

implied by HTTP/1.1.

Connection: close

Connection: keep-alive

Date Indicates the date and time that a

request or response was made in

Greenwich Mean Time (GMT). GMT time

is mandatory for time consistency, given

the worldwide nature of the Web.

Date: Sun, 11 Aug 2001

13:24:52 GMT

Pragma Passes control information. For example,

this header can be used to inform a

caching proxy server to fetch a fresh

copy of a page. It is also used in

responses for cache control.

Pragma: no-cache

TABLE B-3 General HTTP 1.1 Headers