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Re: Possible optimization to State-Info proposal

From: Koen Holtman <koen@win.tue.nl>
Date: Fri, 25 Aug 1995 23:21:13 +0200 (MET DST)
Message-Id: <199508252121.XAA02807@wswiop05.win.tue.nl>
To: Dave Kristol <dmk@allegra.att.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com, Koen Holtman <koen@win.tue.nl>
Dave Kristol:
>

 [dropping the requirement that proxy caches must always forward
 state-info headers from clients]

>Nice idea!  If the dust settles without major objections, I'll add this
>to a state-info-01 I-D.  That will be a couple of weeks, after some
>vacation.

Dropping the state-info forwarding requirement would make your
state-info-01 almost identical to the planned second version of my
non-persistent cookie proposal.  I guess this means that I don't have
to write my second version anymore.

Let me try to summarize the state-info/caching requirements we seem to
have converged at, in the form of modifications to
draft-kristol-http-state-info-00.txt.  Dave, feel free to cut and
paste from the text below if you can use it.

[Note to Daniel W. Connolly, added later: I just realized that a lot
of the material in Section 4.3 below could go straight into a "HTTP
Caching: rules and Heuristics" document.  Feel free to cut and paste.]

----------snip----------

1.  ABSTRACT

[Delete the discussion of library browsing, this is no longer directly
supported if state-info is not always passed though.  (However, there
are still ways to implement library browsing, see 5.3 below)]

[...]

4.  PROPOSAL OUTLINE

[...]

4.1  Origin Server Role

[...]

An origin server may only include State-Info headers in responses to
non-idempotent requests.  (Non-idempotent requests are all request
that do not use the GET and HEAD methods)

[...]

4.2  User Agent Role

[...]

For reasons of privacy protection (see Section 6), a user agent should
ignore State-Info headers contained in responses to idempotent (GET
and HEAD) requests.

A user agent in the state ``have state-info'' should include a
State-info request header in all requests to the origin server, whether
these are idempotent or not.

Caches in user agents should be careful to implement the caching
semantics defined in the HTTP protocol, especially when handling
requests or responses containing State-Info headers.

If the user agent allows the user to configure the its cache to

  `check the for validity of document (i.e. issue conditional get if
   expired) only once per session',

this configuration option (which makes the user agent violate the HTTP
specification anyway) should not override HTTP cache semantics for
transactions where requests or responses containing State-Info headers
are involved, as this will make stateful dialogs impossible or, worse,
dangerously unreliable.

[...]

4.3  Caching Proxy Role

Caches that conform completely to the (draft) HTTP 1.0 or 1.1
specification need not be changed to support State-Info.

As per the requirements in the HTTP 1.0 and 1.1 drafts, caching
proxies

  + must never cache responses to non-idempotent requests
    (note that the drafts probably need to make this requirement
    more explicit)

  + must not cache a response to an idempotent request if the response
    contains a Pragma or Expires header with a value that disallows
    caching

Thus, origin servers can expect that proxies forward to them:

 - all non-idempotent requests (which may be carrying State-Info
   headers) issued by user agents

 - all idempotent requests (which may be carrying State-Info headers)
   for every `dynamic' URI D issued by user agents.  An URI D is
   `dynamic' if the server has consistently put Pragma or Expires
   headers disallowing caching in every response message to
   non-idempotent requests for URI D.  Proxies may `downgrade' normal
   GET requests to conditional GET request when doing the forwarding.

An example of a `dynamic', idempotent URI is a `shopping basket
contents URI' will typically be accessed with the GET method: the link
to the shopping basket page will be a normal HTML <A HREF=...> link,
it need not be a form submit button.

Origin servers can only change the session state (State-Info value
stored by a user agent) in response to non-idempotent request done by
user agents.  However, session-state is not identical to server state
(the mapping URI->entity is part of the server state). Proxy and user
agent (cache) authors should be aware that server may *at any time*
change the entity bound to any `dynamic' URI.  (An example would be a
`chat page' under a dynamic URI that changes because another user
writes on it, or a `stock quotes' page that is dynamically updated.)

Proxy caches that, for whatever reason, are unwilling or unable *not*
to cache a `dynamic' entity belonging to an URI D should, if
State-Info headers were present in the request or response for D,
return a HTTP error code 501 (Not Implemented) to the requesting
client.

[Note: Below, I distinguish between not conforming to the HTTP spec,
which is bad, and breaching internet etiquette, which is much worse.
For example, a user agent that sends out a badly constructed
User-Agent: header may not conform to the HTTP spec, but does not
commit a grave breach of internet etiquette.]

Serving an outdated (incorrectly cached) dynamic response instead of
giving an error code is completely unacceptable behavior, because it
may break the synchronization between the session state maintained by
the server and the user's view of that state.  Willingly breaking this
synchronization should be considered as grave a breach of internet
etiquette, as bad as willingly changing the contents of relayed IP
packets belonging to a telnet session.



5.  IMPLEMENTATION CONSIDERATIONS

[...]

5.3 Browsing History Tracking

The state-info facilities only allow origin servers to track user
access to non-idempotent and `dynamic' idempotent URI's.  A `library'
or `magazine browsing' server may want to track all URI's accessed by
the user, allowing it to show a list of articles looked at already.
This tracking could be accomplished by making all pages `dynamic', or
by including a small `dynamic' inline picture on every page.  However,
if neither the user agent nor any proxy close to the user agent has
conditional GET capability, this technique may cause an unacceptably
large amount of web traffic to be generated.


6. PRIVACY

[...]

The requirement on user agents to ignore all State-Info headers
contained in responses to idempotent requests (GET, HEAD) helps to
protect the privacy of the user.

A service provider wanting to abuse the State-Info facilities to track
the path of each user through the server first has to get the user to
click a form submit button or issue another browsing command resulting
in the sending of a non-idempotent request.

Thus, a user that does only `regular' browsing by clicking HTML <A
HREF> links (or having the user agent resolve inline pictures or
idempotent request redirections) never has to worry about getting
`tagged' by a malicious service providers, even if the user has set
the user agent to always engage in a stateful session without prior
notice.

[Note: the following addition addresses the privacy problems I
discussed in my previous message in this thread.  The non-idempotent
requests rule discussed above makes solving these problems relatively
straightforward.]

   + It is recommended that a user agent should, as a configuration
     option, be able to pop up a dialog box when receiving a
     State-Info response header, like this:

        ---------------------------------------------------------
          Start a session with server foo.bar? 
           [ Yes ]
           [ Yes, always ]
           [ No ]
          (Help information: Starting a session will allow foo.bar
          to gather accurate statistics of your actions)
        ----------------------------------------------------------

    If one of the `yes' buttons is pressed, the user agent should
    change to the ``have state-info'' state.  The `Yes, always'
    button will have the additional effect of having the user agent
    start a session with foo.bar without popping up a dialog box on
    future invocations.

    [`no' alternative 1:] If the `No' button is pressed, the user
      agent should stay in the ``no state-info'' state.  When getting
      a new State-Info response header from foo.bar, the user agent
      should pop up the dialog box again.  This allows users to
      reconsider the earlier `No' decision.

    [`no' alternative 2:] If the `No' button is pressed, the user
      agent should also go to the ``have state-info'' state, but start
      sending State-info request headers containing empty strings
      instead of sending headers with the opaque information received
      in the response header.  When getting a new State-Info response
      header from foo.bar, the user agent should pop up the dialog box
      again.  This `No' behavior allows servers to suppress the
      sending of more State-Info response headers and start a
      non-stateful dialog with users that do not want to engage in
      stateful dialogs for privacy reasons.  On the other hand, if a
      server receives an empty-string state-info header, it can also
      choose to just send a new state-info response header again,
      thereby asking the user to reconsider the earlier `No' decision.

[...]

----------snip----------

Koen.
Received on Friday, 25 August 1995 14:23:19 EDT

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