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Problems with ..-state-mgmt-05.txt

From: Koen Holtman <koen@win.tue.nl>
Date: Tue, 31 Dec 1996 15:46:10 +0100 (MET)
Message-Id: <199612311446.PAA08472@wsooti08.win.tue.nl>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Cc: Koen Holtman <koen@win.tue.nl>

draft-ietf-http-state-mgmt-05.txt (HTTP State Management Mechanism)
has been a Proposed Standard for about a month now.  This message
discusses two problems with draft-ietf-http-state-mgmt-05.txt I have
seen so far.  I feel that these problems need to be resolved in some
way as soon as possible.

These problems may or may not have an impact on the standards track
progress of the draft.  I will not discuss that issue in this message,
as others know more about such procedural matters.

Problem 1. Advice about sending Cache-Control: must-revalidate
==========

(Problem first identified by Anselm Baird-Smith)

Section 4.2.3 (Controlling Caching) says:

|The origin server should send the following additional HTTP/1.1 response
|headers, depending on circumstances:
[...]
|   * To allow caching of a document and require that it be validated
|     before returning it to the client: Cache-control: must-revalidate.
                                                        ^^^^^^^^^^^^^^^^ 

We recently found out that `must-revalidate' in HTTP/1.1 means `must
revalidate if stale', not `must always revalidate'.  This  means that
the advice above is wrong, it should be:

   * To allow caching of a document and require that it be validated
     before returning it to the client: 
     Cache-control: max-age=0, must-revalidate.
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Essentially the same problem occurs in the next item of the spec text:

|   * To allow caching of a document, but to require that proxy caches
|     (not user agent caches) validate it before returning it to the
|     client: Cache-control: proxy-revalidate.

and the same fix should be applied.


Problem 2. Downwards compatibility with existing cookie implementations
==========

(problem first identified by Martijn Koster)

If the following header (without the line breaks) is sent to Microsoft
Internet Explorer V3:

Set-cookie: xx="1=2&3-4";
    Comment="blah";
    Version=1; Max-Age=15552000; Path=/;
    Expires=Sun, 27 Apr 1997 01:16:23 GMT

then MSIE v3 will return the cookie

Cookie: Max-Age=15552000

and not xx="1=2&3-4", which would be the most reasonable thing to do
according to Netscape's cookie specification.  (By the way, Microsoft
will probably fix this problem in MSIE v4.)

The downwards compatibility scheme in the state management draft,
however, _relies_ on all existing browsers sending back xx="1=2&3-4"
when getting the Set-Cookie header above.  It therefore seems that
downwards compatibility, and smooth deployment, is impossible for the
current specification.

The state management draft assumes that existing clients are tolerant
when parsing Set-Cookie headers.  As is shown by MSIE v3, this
assumption is wrong.  A downwards compatibility scheme, which in my
opinion is crucially important if this specification is to be deployed
at all, will therefore have to rely on some other assumption.  

I propose using the assumption that unknown response headers are
ignored by all clients.  This leads to the following fix: the state
management specification defines a _new_ header for setting cookies,
for example "Cookie-Control", together with a rule that if a
Cookie-Control header is present in the response message, a compliant
client must ignore all Set-Cookie headers.  This would allow downwards
compatibility by sending two headers:

Cookie-Control: xx="1=2&3-4";
    Comment="blah";
    Version=1; Max-Age=15552000; Path=/;
Set-Cookie: xx="1=2&3-4"; Path=/; Expires=Sun, 27 Apr 1997 01:16:23 GMT

The "Version=1" directive in the Cookie-Control header above could
possibly also be removed.


Koen.
Received on Tuesday, 31 December 1996 07:01:48 EST

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