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Re: Conventions for Sharing User Agent Profiles

From: <hallam@etna.ai.mit.edu>
Date: Mon, 12 Aug 96 17:39:46 -0400
Message-Id: <9608122139.AA02447@Etna.ai.mit.edu>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Cc: hallam@etna.ai.mit.edu
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/1321

>Anyway, I would guess that if we can come up with a standard
>encoding for the u-a-p resource pointed to by a u-a-p URL (and
>which would be a prerequisite for any such scheme) then with
>a little more effort we might be able to come up with compressed
>encoding that could be transmitted in the request headers without
>many more bytes that it would take to transmit the u-a-p URL.  And
>transmitting it in-band does solve the problems that might arise
>from indirection.

Pulling together Jeff's points of security and downloading in 
an intranet I see the specific weakness of Jim's scheme being
that it uses URLs when in this particular case we could use
something that has the properties of a URN.

Let us start by defining for the sake of argument an Object
Identifier URI which has a prefix OID:. We will describe a
specific subcategory of these OIDs, specifically the set
of identifiers which are a deterministic function of their
designata, in this case via an unbiased one way function
such as SHA-1.

So our "URN"-ish identifier which for the sake of avoiding
confusion with the URI requirements doc I will call a URS
(Universal Resource Sign) is something like :-


Which refers to the headers h, where h = 

"Accept: text/plain, text/html, image/gif, image/jpeg
User-Agent: Linemode/64.5

and  BASE64(SHA(h)) =  "khgq234o8t6quirgq2872135"

The typical usage of this technique would be as follows :-

GET http://foom.com/index.html http/1.2
Date: <whatever>
Include: OID:SHA-khgq234o8t6quirgq2872135

Which would be equivalent to :-

GET http://foom.com/index.html http/1.2
Date: <whatever>
Accept: text/plain, text/html, image/gif, image/jpeg
User-Agent: Linemode/64.5

Now say that the server does not have the mapping for 
OID:SHA-khgq234o8t6quirgq2872135 avaliable. In that case it might

1) Return a default value anyway 
	- especially usefull if the content is invariate wrt the
	headers included by reference.

2) Ask for the expansion of the headers.
3) Say that it doesn't have time to play this game and that the
	client should respond in traditional manner.

In the later case we might see :-

3xx Don't Follow You
Server: frob/54.3

And the Client would then send some message to provide the
appropriate binding or linkage, eg :-

POST OID:SHA-khgq234o8t6quirgq2872135 http/1.2
Date: <whatever>
Content-Type: application/http

Accept: text/plain, text/html, image/gif, image/jpeg
User-Agent: Linemode/64.5

Perhaps the method in this case needs to be adjusted 
somewhat... Its not quite a POST, more a "RETRY" type
of affair. 

Note that in all this we still need to have some way of avoiding the
initial loosing trip problem. Unless we know that we are dealing with 
a 1.2 server we have to start talking 1.0 and tentatively upgrade.

Who is interested in a DNS modification to provide for server profiles?

I see that we could do some really interesting stuff with a very
simple scheme. Here I would suggest that we go just a little bit
further than the current DNS hacks suggested and provide a WebX
record analogous to the MX record which contains a mapping table
from URIs to URIs :-

[Attached to record www.w3.org]

WEBX http://www.w3.org/* 1 version=1.2
WEBX http://www.w3.org/* 2 interval=30sec version=1.2
WEBX http://www.w3.org/* 2 interval=1day version=1.2
WEBX http://www.w3.org/* 3 version=1.1

This means that there are four sites serving http:// type URIs.
Server has firstness quality and uses protocol
http/1.2, Servers and have secondness
and also serve 1.2, has thirdness and is version 1.1.

The firstness, secondness and thirdness are inspired by the
trichotemy of Pierce. Basically this divides the relationship of
a sign to its designatum into three categories. In the first
category there is a direct connection between the sign and the 
thing designated by the sign. In the second category there is 
a conventional relationship between the sign and the designata,
in the third category the sign relates to another sign. Loosely
interperting these abstract categories in terms of HTTP we arrive
at three different qualities of cache scheme :-

Firstness:	The server is either the authoratative source
	for the resources or it is guaranteed to behave in a
	smantically equivalent manner (e.g. two servers operating
	off a mirrored filestore with exact semantics)
	[Alternatively the relationship of the resource to the
	URL is either static or otheriwse known in advance]

Secondness:	The server is guaranteed to behave as the 
	authoratative source would have done within a specified
	time interval. This is equivalent to a server which recieves
	change notifications from the originating host or some
	other host with quality firstness.

Thirdness:	There was at some time in the past a relationship
	between the two. This is CERN proxy style caching.

The advantage of this division is that it maps very directly to
three classes of server implementation with a range of implementation
costs and functionality. It would be futile to try transaction
control on a server with thirdness quality. On the other hand data
from a server with secondness quality and a time constant of 30sec
is likely to be as good as a direct connection from most people's
point of view.

I know that this is not the HTTP protocol per se but it is a method
of addressing certain problems that are otherwise hard or impossible
to avoid. I think we should think of the Web as a whole and not just
towards small pieces that happen to fit in particular pockets.


[P.S., Yes you could call this a URN scheme if you really
wanted but note that the scheme still defines a proceedure for
retreiving a resource and hence is in fact a URL. The main
concern of those asking for a naming scheme is addressed - the
referent is insensitive to the actual server used [This could be
improved upon by use of an "index" server offering redirections
in response to requests".] On the other hand certain requirements
proposed for URNs which were cited as essential are not met. 
Indeed I suspect that no mechanism which involves a resolution
mechanism can ever meet those requirements. A name is merely a
sign that has a conventional assosiation with its designatum,
the binding occurs from the fact of common usage within a 
community ]
Received on Monday, 12 August 1996 14:36:32 UTC

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