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Re: Feedback for draft-nottingham-http-link-header-03

From: Mark Nottingham <mnot@mnot.net>
Date: Thu, 11 Dec 2008 10:54:51 +1100
Cc: Yves Lafon <ylafon@w3.org>, Julian Reschke <julian.reschke@gmx.de>, HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <F2C33909-37F7-43FE-8F82-6C736AC96631@mnot.net>
To: Roy T. Fielding <fielding@gbiv.com>

Right, OK; that's the subtle bit. Julian, make sense?

On 11/12/2008, at 8:19 AM, Roy T. Fielding wrote:

> On Dec 10, 2008, at 3:01 AM, Yves Lafon wrote:
>> On Wed, 10 Dec 2008, Julian Reschke wrote:
>>> Mark Nottingham wrote:
>>>> ...
>>>>      <t>HTML4 also has a "rev" parameter for links that allows a  
>>>> link's relation to be reversed. The Link header
>>>>        has a "rev" parameter to allow the expression of these  
>>>> links in HTTP headers, but its use is not encouraged,
>>>>        due to the confusion this mechanism causes as well as  
>>>> conflicting interpretations among HTML versions.</t>
>>>> ...
>>> OK, it seems I'm missing something here. Could somebody explain to  
>>> me, what the conflicting interpretations are, and which we prefer  
>>> (and why?)?
>>> HTML2 (<http://tools.ietf.org/html/rfc1866#section-5.7.3>):
>>>   REL
>>>           The REL attribute gives the relationship(s) described by
>>>           the hyperlink. The value is a whitespace separated list
>>>           of relationship names. The semantics of link
>>>           relationships are not specified in this document.
>>>   REV
>>>           same as the REL attribute, but the semantics of the
>>>           relationship are in the reverse direction. A link from A
>>>           to B with REL="X" expresses the same relationship as a
>>>           link from B to A with REV="X". An anchor may have both
>>>           REL and REV attributes.
>> In that case, it's not a reversed link, but still a normal forward  
>> link, but it expressed a reversed relationship.
>> A: link rel="bigger than" B
>> is the same relation as
>> B: link rev="smaller than" A
>> Same relation, but different links (and different authority  
>> claiming the relationship between both A and B).
>> In the HTML4 example, it's reverse and forward links, so the links  
>> are supposed equivalent, so not only the relation is equivalent,  
>> but also the link.
>> I prefer far more the "relationship equivalence" as defined in HTML2.
> Yep.  The HTML2 definition is correct but easily misunderstood because
> "expresses the same relationship" does not mean "is the same link".
> What it should have said is that rev reverses the relationship  
> semantics,
> e.g., rev=made is equivalent to rel=creator.  Both define a link  
> from A to B.
> ....Roy

Mark Nottingham     http://www.mnot.net/
Received on Wednesday, 10 December 2008 23:55:34 UTC

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