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7.2 HTTP 1.1 Response Headers and Their Meaning

147

no store

: Document should never be cached and should not 

even be stored in a temporary location on disk. This header is 

intended to prevent inadvertent copies of sensitive information.

must revalidate

: Client must revalidate document with 

original server (not just intermediate proxies) each time it is used.

proxy revalidate

: This is the same as 

must revalidate

, 

except that it applies only to shared caches.

max age=xxx

: Document should be considered stale after xxx 

seconds. This is a convenient alternative to the 

Expires

 header, 

but only works with HTTP 1.1 clients. If both 

max age

 and 

Expires

 are present in the response, the 

max age

 value takes 

precedence.

s max age=xxx

: Shared caches should consider the document 

stale after xxx seconds.

The 

Cache Control

 header is new in HTTP 1.1. 

Connection

A value of 

close

 for this response header instructs the browser not to 

use persistent HTTP connections. Technically, persistent connections 

are the default when the client supports HTTP 1.1 and does not specify 

a  

Connection: close

  request header (or when an HTTP 1.0 client 

specifies  

Connection: keep alive

 ). However, since persistent con 

nections require a 

Content Length

 response header, there is no reason 

for a servlet to explicitly use the 

Connection

 header. Just omit the 

Con 

tent Length

 header if you aren't using persistent connections. See 

Section 7.4 (Using Persistent HTTP Connections) for an example of the 

use of persistent HTTP connections from servlets.

Content Encoding

This header indicates the way in which the page was encoded during 

transmission. The browser should reverse the encoding before deciding 

what to do with the document. Compressing the document with gzip 

can result in huge savings in transmission time; for an example, see Sec 

tion 4.4 (Sending Compressed Web Pages).

Content Language

The 

Content Language

 header signifies the language in which the 

document is written. The value of the header should be one of the stan 

dard language codes such as 

en

, 

en us

, 

da

, etc. See RFC 1766 for 

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