Re: Question on the boundaries of content negotiation in the context of the Web of Data

I started to turn this into a request for TAG telecon agendum, and got
stuck on the word "equivalent".

Just to make sure I understand you - by "equivalent" are you referring
to HTTP 2616 section 13.3.3:

   Entity tags are normally "strong validators," but the protocol
   provides a mechanism to tag an entity tag as "weak." One can think of
   a strong validator as one that changes whenever the bits of an entity
   changes, while a weak value changes whenever the meaning of an entity
   changes. Alternatively, one can think of a strong validator as part
   of an identifier for a specific entity, while a weak validator is
   part of an identifier for a set of semantically equivalent entities.

and are you specifically asking about the use of entity tags?  Or were
you really asking the broader question about the use of CN that people
like me were eager to answer? Because I think these are two different

If you're asking for advice on "good practice" around the use of
entity tags, the only example given in RFC 2616 is that of hit
counters, which seems quite a long way from "semantic equivalence" of
an image and some RDF. I'd be surprised if anyone would argue in favor
of allowing a cached PNG to be returned when RDF was available and
preferred. On the other hand, the question of under which
circumstances (if any) you are advised to use CN to choose between PNG
and RDF has a very different character. Perhaps some server software
has chosen to assume co-representations are equivalent for caching
purposes, but if this is allowed by RFC 2616 I'd be very interested to
hear the argument.

I would think that CN is used (and intended to be used) not just for
choosing between semantically equivalent entities, but also for
semantic subsetting, such as abbreviated representations for mobile
devices, low-resolution displays, audio vs. written, etc. Subsetting
is certainly *not* equivalence.

Obviously there is appeal to a slippery adjective "semantic", which
you're never going to pin down in a manner that is both rigorous and
general, but you could legitimately ask someone to list some positive
and negative examples and situations where differences between
representations might or might not matter to users and/or


On Thu, Feb 12, 2009 at 7:29 AM, Michael Hausenblas
<> wrote:
> Dear TAG members, dear subscribers,
> I would like to ask you about your opinion on the following scenario. Please
> note that (1) though I'm a member of the W3C Media Fragments WG I speak only
> for myself, and (2) that all URIs used in the following are dereferenceable
> and made out of 100% recycled electrons.
> Given three URIs, namely,
> <>
> <>
> <>
> is it 'allowed' (that is, does it break the Web architecture) if one does
> the following:
> $curl -I -H "Accept: image/png"
> HTTP/1.1 200 OK
> Date: Thu, 12 Feb 2009 12:12:39 GMT
> Server: Apache/2.2.3 (CentOS)
> Content-Location: house.png
> Vary: negotiate,accept
> TCN: choice
> Last-Modified: Thu, 12 Feb 2009 11:54:07 GMT
> ETag: "5c0fd-2deb-462b760a7f5c0;462b77ce8a040"
> Accept-Ranges: bytes
> Content-Length: 11755
> Connection: close
> Content-Type: image/png
> $ curl -I -H "Accept: text/turtle"
> HTTP/1.1 200 OK
> Date: Thu, 12 Feb 2009 12:13:01 GMT
> Server: Apache/2.2.3 (CentOS)
> Content-Location: house.ttl
> Vary: negotiate,accept
> TCN: choice
> Last-Modified: Thu, 12 Feb 2009 11:54:06 GMT
> ETag: "5c0fc-173-462b76098b380;462b77ce8a040"
> Accept-Ranges: bytes
> Content-Length: 371
> Connection: close
> Content-Type: text/turtle
> Please note that I don't ask if this works. It does. Obviously. The
> question, to put it in other words, is: is the PNG *representation* derived
> via conneg from the generic resource <>
> equivalent to the RDF in Turtle?
> If not, why not? If it is, can you please point me to a finding, note, a
> specification, etc. that 'normatively' defines what 'equivalency' really is?
> Cheers,
>      Michael
> --
> Dr. Michael Hausenblas
> DERI - Digital Enterprise Research Institute
> National University of Ireland, Lower Dangan,
> Galway, Ireland, Europe
> Tel. +353 91 495730

Received on Wednesday, 18 February 2009 14:33:02 UTC