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fragment identifiers

From: Roy T. Fielding <fielding@apache.org>
Date: Mon, 22 Jul 2002 19:27:47 -0700
To: www-tag@w3.org
Message-Id: <C7210802-9DE3-11D6-9635-000393753936@apache.org>

The TAG gave me an action item to describe some of the design history
and rationale for fragment identifiers.  This is my attempt to write
it down in a "few" paragraphs.
  = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

Fragment identifiers have had a long, bawdy relationship to URI
that goes back to the very first implementations of WWW addresses.
Theoretically speaking, a fragment identifier is a client-side
indirect reference via the URI to which it is attached. In other
words, a URI reference of the form

    URI#fragment

tells the client that it can access the "thing" identified by the
fragment by first performing a GET action (or its equivalent) on
the URI to retrieve a representation of that resource, and then
pass the fragment portion of the reference to the media-type specific
renderer for that representation in order for that viewer to complete
the reference.  How it completes the reference is dependent on the
media-type, but the most common action is to center or focus the
renderer's view of the representation on some fragment of the whole
representation (hence, the name fragment identifier).

Originally, fragment identifiers were defined as part of the URI
syntax [WWW Addressing, 1992]:

     The format of a hypertext name consists of the name of the naming
     subscheme to be used, then a name in a format particular to that
     subscheme, then an optional anchor identifier within the document.
     For example, the format is for all internet-based access methods:

         scheme : // host.domain:port / path / path # anchor

     A suffix # anchor id allows one to refer to a particular anchor
     within a document.

     A suffix ? followed by words separated by + signs allows one to
     search an index (see details ).

     References from one document to another with a similar name may
     be abbreviated to a relative name. This imposes certain
     restrictions on the way that the "path" is represented.

When WWW addresses were proposed to the IETF for standardization
as URI, a vocal subset of the Internet community believed that the
anchor syntax was specific to Web-like retrieval actions, and
since their software wanted to use identifiers with no option for
retrieval, they felt it necessary that anchors not be considered
part of the URI syntax at all.  TimBL changed the name from anchor
to fragment, and from address to identifier, to be more inclusive
of the wider community's goals, but the result remained a deadlock
within the URI working group of the IETF.  TimBL published RFC 1630
as a way of saying "well, we can't wait any longer, so here's what
we did and you may choose to adopt it or not."

    [RFC 1630]        fragmentaddress        uri [ # fragmentid ]

Note that this separated the definition of URI from that of
fragmentaddress, which was formerly called documentaddress and
later called URI-reference in RFC 2396.

Unfortunately, that still left the URI standardization process
in the hands of the IETF URI working group, which at the time was
convinced that the Web was only one way to do information retrieval
and therefore the Internet should not be "limited" to standardization
of URI the way they had already been implemented in the Web.  After
a year of no progress and facing dissolution of the working group,
the editors put together a document representing rough consensus on
the URL (and only URL), which became RFC 1738.  It skipped the whole
idea of fragments, except to exclude "#" from URL:

    The character "#" is unsafe and should always be encoded
    because it is used in World Wide Web and in other systems
    to delimit a URL from a fragment/anchor identifier that might
    follow it.

Needless to say, this sort of compromise simply didn't work.  None
of the deployed Web software implemented RFC 1738.  It didn't
correspond to how they parsed URI, how they processed URI, or
how they interpreted the result of retrieving representations
using URI.  This was rather frustrating to me, since I was both
a believer in the IETF process and an implementer of WWW technology.
So, when I wrote the spec for relative URL (RFC 1808), I bypassed
the issue and defined the syntax as simply

    URL = ( absoluteURL | relativeURL ) [ "#" fragment ]

Including fragment in the reference syntax is necessary because
the protocol elements that made use of this standard, such as
HTML "a" href, needed a consistent syntax.  Furthermore, as
individuals defined new URI schemes or updated the ones in 1738,
we found that they either forgot about the fragment issue
(leading to interoperability problems), or were doing strange
contortions in the BNF to account for both the 1738 grammar plus
fragments, or were simply ignoring 1738 and making normative
references to 1630.  However, people rightly pointed out that
making absoluteURL exclude fragment while URL includes them
results in some painful thinking whenever URL is used in a
sentence.

Time passed, and in 1997 we needed to update the URL specifications
in accordance with deployed practice.  Finally, there was a chance
to distinguish between people's opinions about what is desired to
be a URI and what it actually meant to be a URI in practice, and
produce a specification that defined URI in terms of how they were
used within deployed implementations of Internet protocols.  The
result was RFC 2396, which introduced the notion of URI-reference,
which was actually equivalent to TimBL's original definition of
documentaddress that was later renamed fragmentaddress, but in a
form that was slightly more politically correct.  *phew*

Right. So, now we have RFC 2396's definition of fragment identifiers,
which is actually the least restrictive and most open to media-type
specific interpretation of any formal definition that has ever been
specified for fragments, with the single exception of TimBL's
design notes regarding RDF that were written after 2396 was sent
to the RFC editor.

The reason we call it a URI that identifies a resource, rather
than a UDI that identifies a document, is because we want a URI to
reference things in the future -- to point to a source of future
useful things.  That's what resource means.  It is therefore
impossible to "retrieve" a resource, since the fact that it is
available "over there" is an essential part of it being a resource;
the resource remains over there, so the only thing that is
retrieved is an instantaneous representation of the resource
at the point in time at which it was generated by the origin.

Document means a lot of different things to different people,
one of which is a bag of bits representing the framework for a
renderable page.  All uses of the term "document" in RFC 2396
refer to the virtual document described by the retrieved
representation of a resource, where the virtual document may
consist of multiple individual representations within a single
rendering framework (e.g., a web page may consist of HTML,
stylesheets, in-line images, etc.).  In HTML, a fragment
identifies a portion of the complete virtual document, not
just the bits within the HTML framework.

I'll restate my most recent thinking on the subject from a prior
message to www-tag:

Fragment identifiers are client-side indirect references, similar
to how server-driven negotiation in HTTP allows a server-side
indirect reference.  The fragment identifier will, if the resource
provider has done it right, identify the same thing across multiple
representations.  Even a resource mapping to static content will
have multiple representations over time -- they will all be
byte-equivalent, but not age-equivalent.  Thus, if the resource
provider has done it right, a fragment identifier can be used to
consistently define a "thing" similar to a resource.  We do not,
however, call that "thing" a resource because it simply is not
available on the WWW interface as a resource -- the WWW does not
and never has treated the fragment identifier under the same rules
of processing as the resource identifier, since doing so would
interfere with the intent and result of client-side indirection.

Retrieval and name-equivalence are the only two "actions" allowed
on a fragment because the interface does not allow a fragment
identifier to traverse the architectural boundary between client
and server.  Fragments are not first-class resources; not even when
they consistently identify semantics across multiple representations.

The aspect of fragments that is media-type-specific is the mechanism
of the indirect reference when it is dereferenced.  The mechanism is
not known (and cannot be known) until a representation is in hand.
That is, either the fragment identifier is used in a same-document
reference or an action equivalent to GET is performed on the URI
preceding the fragment in order to obtain that representation.
The representation, once in hand, determines what needs to be done
to complete the retrieval action.

Finally, there is the issue of same-document references: URI
references that do not contain a URI, and therefore are interpreted
as referring to the representation in hand.  They do not refer to
the resource from which that representation was obtained, regardless
of the media type: such an interpretation simply cannot be supported
in any information system that allows resource content to change
over time.  Allowing a media type to assert that a same-document
reference is applicable to the resource identified by the URI used
to retrieve that representation is equivalent to saying that the
media type cannot be delivered through gateways, cannot be archived,
and cannot be versioned.  There are no such media types on the Web.

BTW, none of this is specific to HTTP.


Cheers,

Roy T. Fielding, Chief Scientist, Day Software
                  (roy.fielding@day.com) <http://www.day.com/>

                  Chairman, The Apache Software Foundation
                  (fielding@apache.org)  <http://www.apache.org/>
Received on Monday, 22 July 2002 22:27:56 GMT

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