limits on HTTP Prefer Request Header in [ISSUE-89] resolution

All --

Tied to [ISSUE-89]...

Starting with the HTTP Prefer Request Header draft RFC [1], 
I find a requirement to register any token we want to use, and 
provide a list of its possible values (and their effects).  You 
can see the current list of such at [2].

Values and parameters of these tokens are restricted by [3],
which allows for quoted-string handling of (I think) every
character that might occur in a URI.


That said, reading the draft RFC more deeply, it looks like we 
might be better off starting with the existing "return=minimal" 
Preference [4], and extending it with previously undefined and 
thus optional "ldp" parameter --

   Prefer: return=minimal; ldp="ldp-none"

   Prefer: return=minimal; ldp="ldp-containment"

   Prefer: return=minimal; ldp="ldp-membership"

   Prefer: return=minimal; ldp="ldp-membership,ldp-containment"

   Prefer: return=minimal; ldp="ldp-all"

The first would theoretically say "leave out all containment 
and membership triples."  The last might be considered/defined 
as equivalent to --

   Prefer: return=representation

-- and to include any currently undefined parameters, which
John's draft [5] left as possible for the future.

All of these "ldp-*" strings could (and probably should) be
replaced with URIs which return their meanings.


Notwithstanding any of the above, I think it important to note 
that client use of this Request Header forces two Response Headers 
from the server -- Preference-Applied and Vary.

The mandated content of these headers is discussed in the HTTP 
Prefer Request Header draft RFC.


As always, a small question quickly gets larger.

Ted




[1] http://tools.ietf.org/html/draft-snell-http-prefer-18
[2] http://www.iana.org/assignments/http-parameters/http-parameters.xhtml#preferences
[3] http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#section-3.2.4
[4] http://tools.ietf.org/html/draft-snell-http-prefer-18#section-4.2
[5] http://lists.w3.org/Archives/Public/public-ldp-wg/2014Jan/0124.html



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