[LINK] RFI: How Does the Do-Not-Cache Instruction Work?

[Roger Clarke]

At 21:18 +1000 10/8/08, Avi Miller wrote:
>There are no-cache directives that are used by web applications to 
>tell proxy servers and browsers to always re-request content. I 
>found this page that has a fairly non-technical explanation of the 
>various caching directives that are available in HTTP/1.1:
>http://palisade.plynt.com/issues/2008Jul/cache-control-attributes/

Excellent, thanks Avi!

http://www.mnot.net/cache_docs/#META explains the 'Expires' and 
'Cache-Control' settings in the HTTP header.  (It's so much easier to 
read documentation when you know the relevant keywords!!).

As Jan mentioned, my need is in relation to Google's StreetView.

The underlying question I'm trying to dig down into is whether 
StreetView scenes are likely to be cached anywhere, whether on users' 
machines or on intervening proxy-servers run by ISPs.

Knowing the answer would make it easier to understand what happens 
when Google amends an image (e.g. by manually blurring a face) or 
takes it down.

Looking at the traffic, the values in the headers from Google are variously:
Cache-Control: private
Cache-Control: private, max-age=3600

(Boy, there's a vast amount of data, isn't there?!).

p. 108-109 of RFC 2616 on HTTP/1.1 (June 1999)
http://www.ietf.org/rfc/rfc2616.txt
"private
"Indicates that all or part of the response message is intended for a 
single user and MUST NOT be cached by a shared cache. This allows an 
origin server to state that the specified parts of the response are 
intended for only one user and are not a valid response for requests 
by other users. A private (non-shared) cache MAY cache the response."


Ergo:  well-behaved intermediate proxy-servers will not cache 
StreetView images.  But local cache may contain an image for an hour.


ANY CORRECTION TO THAT UNDERSTANDING WOULD BE GRATEFULLY ACCEPTED!!


BTW, by using Control-Shift, I now see that the StreetView window 
uses Adobe Flash.  (Do they say that anywhere?  Does that mean that 
people who *don't* have it installed in their browser can't see 
StreetView scenes?).

Is Adobe Flash content just as capable of being cached as other 
formats?  I can't immediately see why not.  So I assume it's cached 
client-side too.


-- 
Roger Clarke                  http://www.anu.edu.au/people/Roger.Clarke/
			            
Xamax Consultancy Pty Ltd      78 Sidaway St, Chapman ACT 2611 AUSTRALIA
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Visiting Professor in Info Science & Eng  Australian National University
Visiting Professor in the eCommerce Program      University of Hong Kong
Visiting Professor in the Cyberspace Law & Policy Centre      Uni of NSW