This section describes the status
of this document at the time of its publication. Other documents
may supersede this document. A list of current W3C publications and
the latest revision of this technical report can be found in the
W3C technical reports
index at http://www.w3.org/TR/.
This is the 15 December 2004
Recommendation of “Architecture of the World Wide Web, Volume One.”
This document has been reviewed by W3C Members, by software
developers, and by other W3C groups and interested parties, and is
endorsed by the Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited from
another document. W3C's role in making the Recommendation is to
draw attention to the specification and to promote its widespread
deployment. This enhances the functionality and interoperability of
the Web.
This document was developed by W3C's
Technical
Architecture Group (TAG), which, by charter
maintains a list of architectural issues. The scope of this document
is a useful subset of those issues; it is not intended to address
all of them. The TAG intends to address the remaining (and future)
issues now that Volume One is published as a W3C Recommendation. A
complete history of changes so this document is available.
Please send comments on this document to
public-webarch-comments@w3.org (public archive of public-webarch-comments). TAG
technical discussion takes place on www-tag@w3.org (public archive of www-tag).
This document was produced under the
W3C IPR policy of the July 2001 Process Document. The
TAG maintains a public list
of patent disclosures relevant to this document; that page also
includes instructions for disclosing a patent. An individual who
has actual knowledge of a patent which the individual believes
contains Essential Claim(s) with respect to this specification
should disclose the information in accordance with section 6 of the W3C Patent Policy.
The World Wide Web (WWW, or
simply Web) is an information
space in which the items of interest, referred to as resources, are
identified by global identifiers called Uniform Resource
Identifiers (URI).
Examples such as the following
travel scenario
are used throughout this document to illustrate typical behavior of
Web
agents—people or software acting on this information
space. A user
agent acts on behalf of a user. Software agents include
servers, proxies, spiders, browsers, and multimedia players.
Story
While planning a trip to Mexico,
Nadia reads “Oaxaca weather information:
'http://weather.example.com/oaxaca'” in a glossy travel magazine.
Nadia has enough experience with the Web to recognize that
"http://weather.example.com/oaxaca" is a URI and that she is likely
to be able to retrieve associated information with her Web browser.
When Nadia enters the URI into her browser:
- The browser recognizes that what Nadia typed is a URI.
- The browser performs an information retrieval action in
accordance with its configured behavior for resources identified
via the "http" URI scheme.
- The authority responsible for "weather.example.com" provides
information in a response to the retrieval request.
- The browser interprets the response, identified as XHTML by the
server, and performs additional retrieval actions for inline
graphics and other content as necessary.
- The browser displays the retrieved information, which includes
hypertext links to other information. Nadia can follow these
hypertext links to retrieve additional information.
This scenario illustrates the three
architectural bases of the Web that are discussed in this
document:
-
Identification (§2). URIs are used to identify
resources. In this travel scenario, the resource is a periodically
updated report on the weather in Oaxaca, and the URI is
“http://weather.example.com/oaxaca”.
-
Interaction (§3). Web agents communicate using
standardized protocols that enable interaction through the exchange
of messages which adhere to a defined syntax and semantics. By
entering a URI into a retrieval dialog or selecting a hypertext
link, Nadia tells her browser to perform a retrieval action for the
resource identified by the URI. In this example, the browser sends
an HTTP GET request (part of the HTTP protocol) to the server at
"weather.example.com", via TCP/IP port 80, and the server sends
back a message containing what it determines to be a representation
of the resource as of the time that representation was generated.
Note that this example is specific to hypertext browsing of
information—other kinds of interaction are possible, both within
browsers and through the use of other types of Web agent; our
example is intended to illustrate one common interaction, not
define the range of possible interactions or limit the ways in
which agents might use the Web.
-
Formats (§4). Most protocols used for representation
retrieval and/or submission make use of a sequence of one or more
messages, which taken together contain a payload of representation
data and metadata, to transfer the representation between agents.
The choice of interaction protocol places limits on the formats of
representation data and metadata that can be transmitted. HTTP, for
example, typically transmits a single octet stream plus metadata,
and uses the "Content-Type" and "Content-Encoding" header fields to
further identify the format of the representation. In this
scenario, the representation transferred is in XHTML, as identified
by the "Content-type" HTTP header field containing the registered
Internet media type name, "application/xhtml+xml". That Internet
media type name indicates that the representation data can be
processed according to the XHTML specification.
Nadia's browser is configured and
programmed to interpret the receipt of an "application/xhtml+xml"
typed representation as an instruction to render the content of
that representation according to the XHTML rendering model,
including any subsidiary interactions (such as requests for
external style sheets or in-line images) called for by the
representation. In the scenario, the XHTML representation data
received from the initial request instructs Nadia's browser to also
retrieve and render in-line the weather maps, each identified by a
URI and thus causing an additional retrieval action, resulting in
additional representations that are processed by the browser
according to their own data formats (e.g., "application/svg+xml"
indicates the SVG data format), and this process continues until
all of the data formats have been rendered. The result of all of
this processing, once the browser has reached an application
steady-state that completes Nadia's initial requested action, is
commonly referred to as a "Web page".
The following illustration shows the
relationship between identifier, resource, and representation.
In the remainder of this document, we
highlight important architectural points regarding Web identifiers,
protocols, and formats. We also discuss some important general architectural principles (§5)
and how they apply to the Web.
This document describes the
properties we desire of the Web and the design choices that have
been made to achieve them. It promotes the reuse of existing
standards when suitable, and gives guidance on how to innovate in a
manner consistent with Web architecture.
The terms MUST, MUST NOT, SHOULD,
SHOULD NOT, and MAY are used in the principles, constraints, and
good practice notes in accordance with RFC 2119 [RFC2119].
This document does not include
conformance provisions for these reasons:
- Conforming software is expected to be so diverse that it would
not be useful to be able to refer to the class of conforming
software agents.
- Some of the good practice notes concern people; specifications
generally define conformance for software, not people.
- We do not believe that the addition of a conformance section is
likely to increase the utility of the document.
This document is intended to inform
discussions about issues of Web architecture. The intended audience
for this document includes:
- Participants in W3C Activities
- Other groups and individuals designing technologies to be
integrated into the Web
- Implementers of W3C specifications
- Web content authors and publishers
Note: This document
does not distinguish in any formal way the terms "language" and
"format." Context determines which term is used. The phrase
"specification designer" encompasses language, format, and protocol
designers.
This document presents the general
architecture of the Web. Other groups inside and outside W3C also
address specialized aspects of Web architecture, including
accessibility, quality assurance, internationalization, device
independence, and Web Services. The section on Architectural Specifications (§7.1) includes
references to these related specifications.
This document strives for a balance
between brevity and precision while including illustrative
examples. TAG findings are informational documents that complement
the current document by providing more detail about selected
topics. This document includes some excerpts from the findings.
Since the findings evolve independently, this document includes
references to approved TAG findings. For other TAG issues covered
by this document but without an approved finding, references are to
entries in the TAG issues list.
Many of the examples in this document
that involve human activity suppose the familiar Web interaction
model (illustrated at the beginning of the Introduction) where a
person follows a link via a user agent, the user agent retrieves
and presents data, the user follows another link, etc. This
document does not discuss in any detail other interaction models
such as voice browsing (see, for example, [VOICEXML2]). The choice of interaction model may
have an impact on expected agent behavior. For instance, when a
graphical user agent running on a laptop computer or hand-held
device encounters an error, the user agent can report errors
directly to the user through visual and audio cues, and present the
user with options for resolving the errors. On the other hand, when
someone is browsing the Web through voice input and audio-only
output, stopping the dialog to wait for user input may reduce
usability since it is so easy to "lose one's place" when browsing
with only audio-output. This document does not discuss how the
principles, constraints, and good practices identified here apply
in all interaction contexts.
The important points of this document
are categorized as follows:
- Principle
- An architectural principle is a fundamental rule that applies
to a large number of situations and variables. Architectural
principles include "separation of concerns", "generic interface",
"self-descriptive syntax," "visible semantics," "network effect"
(Metcalfe's Law), and Amdahl's Law: "The speed of a system is
limited by its slowest component."
- Constraint
- In the design of the Web, some choices, like the names of the
p and li elements in HTML, the choice of
the colon (:) character in URIs, or grouping bits into eight-bit
units (octets), are somewhat arbitrary; if paragraph
had been chosen instead of p or asterisk (*) instead
of colon, the large-scale result would, most likely, have been the
same. This document focuses on more fundamental design choices:
design choices that lead to constraints, i.e., restrictions in
behavior or interaction within the system. Constraints may be
imposed for technical, policy, or other reasons to achieve
desirable properties in the system, such as accessibility, global
scope, relative ease of evolution, efficiency, and dynamic
extensibility.
- Good
practice
- Good practice—by software developers, content authors, site
managers, users, and specification designers—increases the value of
the Web.
In order to communicate internally, a
community agrees (to a reasonable extent) on a set of terms and
their meanings. One goal of the Web, since its inception, has been
to build a global community in which any party can share
information with any other party. To achieve this goal, the Web
makes use of a single global identification system: the URI. URIs
are a cornerstone of Web architecture, providing identification
that is common across the Web. The global scope of URIs promotes
large-scale "network effects": the value of an identifier increases
the more it is used consistently (for example, the more it is used
in hypertext
links (§4.4)).
Principle: Global Identifiers
Global naming
leads to global network effects.
This principle dates back at least as
far as Douglas Engelbart's seminal work on open hypertext systems;
see section Every Object Addressable in [Eng90].
The choice of syntax for global
identifiers is somewhat arbitrary; it is their global scope that is
important. The Uniform Resource
Identifier, [URI], has
been successfully deployed since the creation of the Web. There are
substantial benefits to participating in the existing network of
URIs, including linking, bookmarking, caching, and indexing by
search engines, and there are substantial costs to creating a new
identification system that has the same properties as URIs.
Good
practice: Identify with URIs
To benefit from and
increase the value of the World Wide Web, agents should provide
URIs as identifiers for resources.
A resource should have an associated
URI if another party might reasonably want to create a hypertext
link to it, make or refute assertions about it, retrieve or cache a
representation of it, include all or part of it by reference into
another representation, annotate it, or perform other operations on
it. Software developers should expect that sharing URIs across
applications will be useful, even if that utility is not initially
evident. The TAG finding "URIs, Addressability, and the use of HTTP GET and
POST" discusses additional benefits and considerations
of URI addressability.
Note: Some URI
schemes (such as the "ftp" URI scheme specification) use the term
"designate" where this document uses "identify."
By design a URI identifies one
resource. We do not limit the scope of what might be a resource. The term
"resource" is used in a general sense for whatever might be
identified by a URI. It is conventional on the hypertext Web to
describe Web pages, images, product catalogs, etc. as “resources”.
The distinguishing characteristic of these resources is that all of
their essential characteristics can be conveyed in a message. We
identify this set as “information
resources.”
This document is an example of an
information resource. It consists of words and punctuation symbols
and graphics and other artifacts that can be encoded, with varying
degrees of fidelity, into a sequence of bits. There is nothing
about the essential information content of this document that
cannot in principle be transfered in a message. In the case of this
document, the message payload is the representation of this document.
However, our use of the term resource
is intentionally more broad. Other things, such as cars and dogs
(and, if you've printed this document on physical sheets of paper,
the artifact that you are holding in your hand), are resources too.
They are not information resources, however, because their essence
is not information. Although it is possible to describe a great
many things about a car or a dog in a sequence of bits, the sum of
those things will invariably be an approximation of the essential
character of the resource.
We define the term “information
resource” because we observe that it is useful in discussions of
Web technology and may be useful in constructing specifications for
facilities built for use on the Web.
Constraint: URIs Identify a Single
Resource
Assign distinct
URIs to distinct resources.
Since the scope of a URI is global,
the resource identified by a URI does not depend on the context in
which the URI appears (see also the section about indirect
identification (§2.2.3)).
[URI]
is an agreement about how the Internet community allocates names
and associates them with the resources they identify. URIs are
divided into schemes (§2.4)
that define, via their scheme specification, the mechanism by which
scheme-specific identifiers are associated with resources. For
example, the "http" URI scheme ([RFC2616]) uses DNS and TCP-based HTTP servers for the
purpose of identifier allocation and resolution. As a result,
identifiers such as "http://example.com/somepath#someFrag" often
take on meaning through the community experience of performing an
HTTP GET request on the identifier and, if given a successful
response, interpreting the response as a representation of the
identified resource. (See also Fragment Identifiers (§2.6).) Of course, a retrieval
action like GET is not the only way to obtain information about a
resource. One might also publish a document that purports to define
the meaning of a particular URI. These other sources of information
may suggest meanings for such identifiers, but it's a local policy
decision whether those suggestions should be heeded.
Just as one might wish to refer to a
person by different names (by full name, first name only, sports
nickname, romantic nickname, and so forth), Web architecture allows
the association of more than one URI with a resource. URIs that
identify the same resource are called URI aliases. The section on
URI aliases (§2.3.1)
discusses some of the potential costs of creating multiple URIs for
the same resource.
Several sections of this document
address questions about the relationship between URIs and
resources, including:
By design, a URI identifies one
resource. Using the same URI to directly identify different
resources produces a URI collision. Collision often
imposes a cost in communication due to the effort required to
resolve ambiguities.
Suppose, for example, that one
organization makes use of a URI to refer to the movie The
Sting, and another organization uses the same URI to refer
to a discussion forum about The Sting. To a third
party, aware of both organizations, this collision creates
confusion about what the URI identifies, undermining the value of
the URI. If one wanted to talk about the creation date of the
resource identified by the URI, for instance, it would not be clear
whether this meant "when the movie was created" or "when the
discussion forum about the movie was created."
Social and technical solutions have
been devised to help avoid URI collision. However, the success or
failure of these different approaches depends on the extent to
which there is consensus in the Internet community on abiding by
the defining specifications.
The section on URI allocation (§2.2.2)
examines approaches for establishing the authoritative source of
information about what resource a URI identifies.
URIs are sometimes used for indirect identification
(§2.2.3). This does not necessarily lead to collisions.
URI allocation is the process of
associating a URI with a resource. Allocation can be performed both
by resource owners and by other parties. It is important to avoid
URI collision
(§2.2.1).
URI ownership is a relation
between a URI and a social entity, such as a person, organization,
or specification. URI ownership gives the relevant social entity
certain rights, including:
- to pass on ownership of some or all owned URIs to another
owner—delegation; and
- to associate a resource with an owned URI—URI allocation.
By social convention, URI ownership
is delegated from the IANA URI scheme registry [IANASchemes], itself a social
entity, to IANA-registered URI scheme specifications. Some URI
scheme specifications further delegate ownership to subordinate
registries or to other nominated owners, who may further delegate
ownership. In the case of a specification, ownership ultimately
lies with the community that maintains the specification.
The approach taken for the "http" URI
scheme, for example, follows the pattern whereby the Internet
community delegates authority, via the IANA URI scheme registry and
the DNS, over a set of URIs with a common prefix to one particular
owner. One consequence of this approach is the Web's heavy reliance
on the central DNS registry. A different approach is taken by the
URN Syntax scheme [RFC2141]
which delegates ownership of portions of URN space to URN Namespace
specifications which themselves are registered in an
IANA-maintained registry of URN Namespace Identifiers.
URI owners are responsible for
avoiding the assignment of equivalent URIs to multiple resources.
Thus, if a URI scheme specification does provide for the delegation
of individual or organized sets of URIs, it should take pains to
ensure that ownership ultimately resides in the hands of a single
social entity. Allowing multiple owners increases the likelihood of
URI collisions.
URI owners may organize or deploy
infrastruture to ensure that representations of associated
resources are available and, where appropriate, interaction with
the resource is possible through the exchange of representations.
There are social expectations for responsible representation
management (§3.5) by URI owners. Additional social implications
of URI ownership are not discussed here.
See TAG issue siteData-36, which concerns the expropriation of naming
authority.
Some schemes use techniques other
than delegated ownership to avoid collision. For example, the
specification for the data URL (sic) scheme [RFC2397] specifies that the resource identified by
a data scheme URI has only one possible representation. The
representation data makes up the URI that identifies that resource.
Thus, the specification itself determines how data URIs are
allocated; no delegation is possible.
Other schemes (such as
"news:comp.text.xml") rely on a social process.
To say that the URI
"mailto:nadia@example.com" identifies both an Internet mailbox and
Nadia, the person, introduces a URI collision. However, we can use
the URI to indirectly identify Nadia. Identifiers are commonly used
in this way.
Listening to a news broadcast, one
might hear a report on Britain that begins, "Today, 10 Downing
Street announced a series of new economic measures." Generally, "10
Downing Street" identifies the official residence of Britain's
Prime Minister. In this context, the news reporter is using it (as
English rhetoric allows) to indirectly identify the British
government. Similarly, URIs identify resources, but they can also
be used in many constructs to indirectly identify other resources.
Globally adopted assignment policies make some URIs appealing as
general-purpose identifiers. Local policy establishes what they
indirectly identify.
Suppose that
nadia@example.com is Nadia's email address. The
organizers of a conference Nadia attends might use
"mailto:nadia@example.com" to refer indirectly to her (e.g., by
using the URI as a database key in their database of conference
participants). This does not introduce a URI collision.
URIs that are identical,
character-by-character, refer to the same resource. Since Web
Architecture allows the association of multiple URIs with a given
resource, two URIs that are not character-by-character identical
may still refer to the same resource. Different URIs do not
necessarily refer to different resources but there is generally a
higher computational cost to determine that different URIs refer to
the same resource.
To reduce the risk of a false
negative (i.e., an incorrect conclusion that two URIs do not refer
to the same resource) or a false positive (i.e., an incorrect
conclusion that two URIs do refer to the same resource), some
specifications describe equivalence tests in addition to
character-by-character comparison. Agents that reach conclusions
based on comparisons that are not licensed by the relevant
specifications take responsibility for any problems that result;
see the section on error handling (§5.3) for more information about
responsible behavior when reaching unlicensed conclusions. Section
6 of [URI] provides more
information about comparing URIs and reducing the risk of false
negatives and positives.
See also the assertion that two URIs identify
the same resource (§2.7.2).
Although there are benefits (such as
naming flexibility) to URI aliases, there are also costs. URI
aliases are harmful when they divide the Web of related resources.
A corollary of Metcalfe's Principle (the "network effect") is that
the value of a given resource can be measured by the number and
value of other resources in its network neighborhood, that is, the
resources that link to it.
The problem with aliases is that if
half of the neighborhood points to one URI for a given resource,
and the other half points to a second, different URI for that same
resource, the neighborhood is divided. Not only is the aliased
resource undervalued because of this split, the entire neighborhood
of resources loses value because of the missing second-order
relationships that should have existed among the referring
resources by virtue of their references to the aliased
resource.
Good
practice: Avoiding URI aliases
A URI owner SHOULD
NOT associate arbitrarily different URIs with the same
resource.
URI consumers also have a role in
ensuring URI consistency. For instance, when transcribing a URI,
agents should not gratuitously percent-encode characters. The term
"character" refers to URI characters as defined in section 2 of
[URI]; percent-encoding is
discussed in section 2.1 of that specification.
Good
practice: Consistent URI usage
An agent that
receives a URI SHOULD refer to the associated resource using the
same URI, character-by-character.
When a URI alias does become common
currency, the URI owner
should use protocol techniques such as server-side redirects to
relate the two resources. The community benefits when the URI owner
supports redirection of an aliased URI to the corresponding
"official" URI. For more information on redirection, see section
10.3, Redirection, in [RFC2616]. See also [CHIPS] for a discussion of some best practices for
server administrators.
URI aliasing only occurs when more
than one URI is used to identify the same resource. The fact that
different resources sometimes have the same representation does not
make the URIs for those resources aliases.
Story
Dirk would like to add a link from
his Web site to the Oaxaca weather site. He uses the URI
http://weather.example.com/oaxaca and labels his link “report on
weather in Oaxaca on 1 August 2004”. Nadia points out to
Dirk that he is setting misleading expectations for the URI he has
used. The Oaxaca weather site policy is that the URI in question
identifies a report on the current weather in Oaxaca—on any given
day—and not the weather on 1 August. Of course, on the first of
August in 2004, Dirk's link will be correct, but the rest of the
time he will be misleading readers. Nadia points out to Dirk that
the managers of the Oaxaca weather site do make available a
different URI permanently assigned to a resource reporting on the
weather on 1 August 2004.
In this story, there are two
resources: “a report on the current weather in Oaxaca” and “a
report on the weather in Oaxaca on 1 August 2004”. The
managers of the Oaxaca weather site assign two URIs to these two
different resources. On 1 August 2004, the
representations for these resources are identical. That fact that
dereferencing two different URIs produces identical representations
does not imply that the two URIs are aliases.
In the URI
"http://weather.example.com/", the "http" that appears before the
colon (":") names a URI scheme. Each URI scheme has a specification
that explains the scheme-specific details of how scheme identifiers
are allocated and become associated with a resource. The URI syntax
is thus a federated and extensible naming system wherein each
scheme's specification may further restrict the syntax and
semantics of identifiers within that scheme.
Examples of URIs from various schemes
include:
- mailto:joe@example.org
- ftp://example.org/aDirectory/aFile
- news:comp.infosystems.www
- tel:+1-816-555-1212
- ldap://ldap.example.org/c=GB?objectClass?one
- urn:oasis:names:tc:entity:xmlns:xml:catalog
While Web architecture allows the
definition of new schemes, introducing a new scheme is costly. Many
aspects of URI processing are scheme-dependent, and a large amount
of deployed software already processes URIs of well-known schemes.
Introducing a new URI scheme requires the development and
deployment not only of client software to handle the scheme, but
also of ancillary agents such as gateways, proxies, and caches. See
[RFC2718] for other
considerations and costs related to URI scheme design.
Because of these costs, if a URI
scheme exists that meets the needs of an application, designers
should use it rather than invent one.
Good
practice: Reuse URI schemes
A specification
SHOULD reuse an existing URI scheme (rather than create a new one)
when it provides the desired properties of identifiers and their
relation to resources.
Consider our travel scenario: should the agent providing
information about the weather in Oaxaca register a new URI scheme
"weather" for the identification of resources related to the
weather? They might then publish URIs such as
"weather://travel.example.com/oaxaca". When a software agent
dereferences such a URI, if what really happens is that HTTP GET is
invoked to retrieve a representation of the resource, then an
"http" URI would have sufficed.
The Internet Assigned Numbers
Authority (IANA) maintains a registry [IANASchemes] of mappings between
URI scheme names and scheme specifications. For instance, the IANA
registry indicates that the "http" scheme is defined in [RFC2616]. The process for registering a
new URI scheme is defined in [RFC2717].
Unregistered URI schemes SHOULD NOT
be used for a number of reasons:
- There is no generally accepted way to locate the scheme
specification.
- Someone else may be using the scheme for other purposes.
- One should not expect that general-purpose software will do
anything useful with URIs of this scheme beyond URI
comparison.
One misguided motivation for
registering a new URI scheme is to allow a software agent to launch
a particular application when retrieving a representation. The same
thing can be accomplished at lower expense by dispatching instead
on the type of the representation, thereby allowing use of existing
transfer protocols and implementations.
Even if an agent cannot process
representation data in an unknown format, it can at least retrieve
it. The data may contain enough information to allow a user or user
agent to make some use of it. When an agent does not handle a new
URI scheme, it cannot retrieve a representation.
When designing a new data format, the
preferred mechanism to promote its deployment on the Web is the
Internet media type (see Representation Types and Internet Media Types (§3.2)).
Media types also provide a means for building new information
applications, as described in future directions for data formats (§4.6).
It is tempting to guess the nature of
a resource by inspection of a URI that identifies it. However, the
Web is designed so that agents communicate resource information
state through representations, not identifiers. In general, one cannot
determine the type of a resource representation by inspecting a URI
for that resource. For example, the ".html" at the end of
"http://example.com/page.html" provides no guarantee that
representations of the identified resource will be served with the
Internet media type "text/html". The publisher is free to allocate
identifiers and define how they are served. The HTTP protocol does
not constrain the Internet media type based on the path component
of the URI; the URI owner is free to configure the server to return
a representation using PNG or any other data format.
Resource state may evolve over time.
Requiring a URI owner to publish a new URI for each change in
resource state would lead to a significant number of broken
references. For robustness, Web architecture promotes independence
between an identifier and the state of the identified resource.
Good
practice: URI opacity
Agents making use
of URIs SHOULD NOT attempt to infer properties of the referenced
resource.
In practice, a small number of
inferences can be made because they are explicitly licensed by the
relevant specifications. Some of these inferences are discussed in
the details of
retrieving a representation (§3.1.1).
The example URI used in the
travel scenario
("http://weather.example.com/oaxaca") suggests to a human reader
that the identified resource has something to do with the weather
in Oaxaca. A site reporting the weather in Oaxaca could just as
easily be identified by the URI "http://vjc.example.com/315". And
the URI "http://weather.example.com/vancouver" might identify the
resource "my photo album."
On the other hand, the URI
"mailto:joe@example.com" indicates that the URI refers to a
mailbox. The "mailto" URI scheme specification authorizes agents to
infer that URIs of this form identify Internet mailboxes.
Some URI assignment authorities
document and publish their URI assignment policies. For more
information about URI opacity, see TAG issues metaDataInURI-31 and siteData-36.
Story
When browsing the XHTML document
that Nadia receives as a representation of the resource identified
by "http://weather.example.com/oaxaca", she finds that the URI
"http://weather.example.com/oaxaca#weekend" refers to the part of
the representation that conveys information about the weekend
outlook. This URI includes the fragment identifier "weekend" (the
string after the "#").
The fragment identifier component of a URI
allows indirect identification of a secondary
resource by reference to a primary resource and
additional identifying information. The secondary resource may be
some portion or subset of the primary resource, some view on
representations of the primary resource, or some other resource
defined or described by those representations. The terms "primary
resource" and "secondary resource" are defined in section 3.5 of
[URI].
The terms “primary” and “secondary”
in this context do not limit the nature of the resource—they are
not classes. In this context, primary and secondary simply indicate
that there is a relationship between the resources for the purposes
of one URI: the URI with a fragment identifier. Any resource can be
identified as a secondary resource. It might also be identified
using a URI without a fragment identifier, and a resource may be
identified as a secondary resource via multiple URIs. The purpose
of these terms is to enable discussion of the relationship between
such resources, not to limit the nature of a resource.
The interpretation of fragment
identifiers is discussed in the section on media types and fragment
identifier semantics (§3.2.1).
See TAG issue abstractComponentRefs-37, which concerns the use
of fragment identifiers with namespace names to identify abstract
components.
There remain open questions
regarding identifiers on the Web.
The integration of
internationalized identifiers (i.e., composed of characters beyond
those allowed by [URI]) into the
Web architecture is an important and open issue. See TAG issue
IRIEverywhere-27 for discussion about work going
on in this area.
Emerging Semantic Web technologies,
including the "Web Ontology Language (OWL)" [OWL10], define RDF properties such as
sameAs to assert that two URIs identify the same
resource or inverseFunctionalProperty to imply it.
Communication between agents over a
network about resources involves URIs, messages, and data. The
Web's protocols (including HTTP, FTP, SOAP, NNTP, and SMTP) are
based on the exchange of messages. A message may include data as well as
metadata about a resource (such as the "Alternates" and "Vary" HTTP
headers), the message data, and the message itself (such as the
"Transfer-encoding" HTTP header). A message may even include
metadata about the message metadata (for message-integrity checks,
for instance).
Story
Nadia follows a hypertext link
labeled "satellite image" expecting to retrieve a satellite photo
of the Oaxaca region. The link to the satellite image is an XHTML
link encoded as <a
href="http://example.com/satimage/oaxaca">satellite
image</a>. Nadia's browser analyzes the URI and
determines that its scheme
is "http". The browser configuration determines how it locates the
identified information, which might be via a cache of prior
retrieval actions, by contacting an intermediary (such as a proxy
server), or by direct access to the server identified by a portion
of the URI. In this example, the browser opens a network connection
to port 80 on the server at "example.com" and sends a "GET" message
as specified by the HTTP protocol, requesting a representation of
the resource.
The server sends a response message
to the browser, once again according to the HTTP protocol. The
message consists of several headers and a JPEG image. The browser
reads the headers, learns from the "Content-Type" field that the
Internet media type of the representation is "image/jpeg", reads
the sequence of octets that make up the representation data, and
renders the image.
This section describes the
architectural principles and constraints regarding interactions
between agents, including such topics as network protocols and
interaction styles, along with interactions between the Web as a
system and the people that make use of it. The fact that the Web is
a highly distributed system affects architectural constraints and
assumptions about interactions.
Agents may use a URI to access the
referenced resource; this is called dereferencing the URI. Access may
take many forms, including retrieving a representation of the
resource (for instance, by using HTTP GET or HEAD), adding or
modifying a representation of the resource (for instance, by using
HTTP POST or PUT, which in some cases may change the actual state
of the resource if the submitted representations are interpreted as
instructions to that end), and deleting some or all representations
of the resource (for instance, by using HTTP DELETE, which in some
cases may result in the deletion of the resource itself).
There may be more than one way to
access a resource for a given URI; application context determines
which access method an agent uses. For instance, a browser might
use HTTP GET to retrieve a representation of a resource, whereas a
hypertext link checker might use HTTP HEAD on the same URI simply
to establish whether a representation is available. Some URI
schemes set expectations about available access methods, others
(such as the URN scheme [RFC
2141]) do not. Section 1.2.2 of [URI] discusses the separation of identification and
interaction in more detail. For more information about
relationships between multiple access methods and URI
addressability, see the TAG finding "URIs, Addressability, and the use of HTTP GET and
POST".
Although many URI schemes (§2.4) are named after protocols, this
does not imply that use of such a URI will necessarily result in
access to the resource via the named protocol. Even when an agent
uses a URI to retrieve a representation, that access might be
through gateways, proxies, caches, and name resolution services
that are independent of the protocol associated with the scheme
name.
Many URI schemes define a default
interaction protocol for attempting access to the identified
resource. That interaction protocol is often the basis for
allocating identifiers within that scheme, just as "http" URIs are
defined in terms of TCP-based HTTP servers. However, this does not
imply that all interaction with such resources is limited to the
default interaction protocol. For example, information retrieval
systems often make use of proxies to interact with a multitude of
URI schemes, such as HTTP proxies being used to access "ftp" and
"wais" resources. Proxies can also to provide enhanced services,
such as annotation proxies that combine normal information
retrieval with additional metadata retrieval to provide a seamless,
multidimensional view of resources using the same protocols and
user agents as the non-annotated Web. Likewise, future protocols
may be defined that encompass our current systems, using entirely
different interaction mechanisms, without changing the existing
identifier schemes. See also, principle of orthogonal specifications (§5.1).
Dereferencing a URI generally
involves a succession of steps as described in multiple
specifications and implemented by the agent. The following example
illustrates the series of specifications that governs the process
when a user agent is instructed to follow a hypertext link (§4.4) that is part of an SVG
document. In this example, the URI is
"http://weather.example.com/oaxaca" and the application context
calls for the user agent to retrieve and render a representation of
the identified resource.
- Since the URI is part of a hypertext link in an SVG document,
the first relevant specification is the SVG 1.1 Recommendation
[SVG11]. Section 17.1 of
this specification imports the link semantics defined in XLink 1.0
[XLink10]: "The remote resource
(the destination for the link) is defined by a URI specified by the
XLink
href attribute on the 'a' element."
The SVG specification goes on to state that interpretation of an
a element involves retrieving a representation of a
resource, identified by the href attribute in the
XLink namespace: "By activating these links (by clicking with the
mouse, through keyboard input, voice commands, etc.), users may
visit these resources."
- The XLink 1.0 [XLink10]
specification, which defines the
href attribute in
section 5.4, states that "The value of the href attribute must be a
URI reference as defined in [IETF RFC 2396], or must result in a
URI reference after the escaping procedure described below is
applied."
- The URI specification [URI]
states that "Each URI begins with a scheme name that refers to a
specification for assigning identifiers within that scheme." The
URI scheme name in this example is "http".
- [IANASchemes] states
that the "http" scheme is defined by the HTTP/1.1 specification
(RFC 2616 [RFC2616], section
3.2.2).
- In this SVG context, the agent constructs an HTTP GET request
(per section 9.3 of [RFC2616])
to retrieve the representation.
- Section 6 of [RFC2616]
defines how the server constructs a corresponding response message,
including the 'Content-Type' field.
- Section 1.4 of [RFC2616]
states "HTTP communication usually takes place over TCP/IP
connections." This example addresses neither that step in the
process nor other steps such as Domain Name System
(DNS) resolution.
- The agent interprets the returned representation according to
the data format specification that corresponds to the
representation's Internet Media Type (§3.2) (the value of the HTTP
'Content-Type') in the relevant IANA registry [MEDIATYPEREG].
Precisely which representation(s)
are retrieved depends on a number of factors, including:
- Whether the URI owner makes available any representations at
all;
- Whether the agent making the request has access privileges for
those representations (see the section on linking and access control (§3.5.2));
- If the URI owner has provided more than one representation (in
different formats such as HTML, PNG, or RDF; in different languages
such as English and Spanish; or transformed dynamically according
to the hardware or software capabilities of the recipient), the
resulting representation may depend on negotiation between the user
agent and server.
- The time of the request; the world changes over time, so
representations of resources are also likely to change over
time.
Assuming that a representation has
been successfully retrieved, the expressive power of the
representation's format will affect how precisely the
representation provider communicates resource state. If the
representation communicates the state of the resource inaccurately,
this inaccuracy or ambiguity may lead to confusion among users
about what the resource is. If different users reach different
conclusions about what the resource is, they may interpret this as
a URI collision (§2.2.1).
Some communities, such as the ones developing the Semantic Web,
seek to provide a framework for accurately communicating the
semantics of a resource in a machine readable way. Machine readable
semantics may alleviate some of the ambiguity associated with
natural language descriptions of resources.
A representation is data that
encodes information about resource state. Representations do not
necessarily describe the resource, or portray a likeness of the
resource, or represent the resource in other senses of the word
"represent".
Representations of a resource may
be sent or received using interaction protocols. These protocols in
turn determine the form in which representations are conveyed on
the Web. HTTP, for example, provides for transmission of
representations as octet streams typed using Internet media types
[RFC2046].
Just as it is important to reuse
existing URI schemes whenever possible, there are significant
benefits to using media typed octet streams for representations
even in the unusual case where a new URI scheme and associated
protocol is to be defined. For example, if the Oaxaca weather were
conveyed to Nadia's browser using a protocol other than HTTP, then
software to render formats such as text/xhmtl+xml and image/png
would still be usable if the new protocol supported transmission of
those types. This is an example of the principle of orthogonal specifications (§5.1).
Good
practice: Reuse representation formats
New protocols
created for the Web SHOULD transmit representations as octet
streams typed by Internet media types.
The Internet media type mechanism
does have some limitations. For instance, media type strings do not
support versioning (§4.2.1)
or other parameters. See TAG issues uriMediaType-9 and mediaTypeManagement-45 which concern aspects of
the media type mechanism.
The Internet Media Type defines the
syntax and semantics of the fragment identifier (introduced in
Fragment Identifiers (§2.6)), if
any, that may be used in conjunction with a representation.
Story
In one of his XHTML pages, Dirk
creates a hypertext link to an image that Nadia has published on
the Web. He creates a hypertext link with <a
href="http://www.example.com/images/nadia#hat">Nadia's
hat</a>. Emma views Dirk's XHTML page in her Web
browser and follows the link. The HTML implementation in her
browser removes the fragment from the URI and requests the image
"http://www.example.com/images/nadia". Nadia serves an SVG
representation of the image (with Internet media type
"image/svg+xml"). Emma's Web browser starts up an SVG
implementation to view the image. It passes it the original URI
including the fragment, "http://www.example.com/images/nadia#hat"
to this implementation, causing a view of the hat to be displayed
rather than the complete image.
Note that the HTML implementation
in Emma's browser did not need to understand the syntax or
semantics of the SVG fragment (nor does the SVG implementation
have to understand HTML, WebCGM, RDF ... fragment syntax or
semantics; it merely had to recognize the # delimiter from the URI
syntax [URI] and remove the fragment when accessing the resource).
This orthogonality
(§5.1) is an important feature of Web architecture; it is what
enabled Emma's browser to provide a useful service without
requiring an upgrade.
The semantics of a fragment
identifier are defined by the set of representations that might
result from a retrieval action on the primary resource. The
fragment's format and resolution are therefore dependent on the
type of a potentially retrieved representation, even though such a
retrieval is only performed if the URI is dereferenced. If no such
representation exists, then the semantics of the fragment are
considered unknown and, effectively, unconstrained. Fragment
identifier semantics are orthogonal to URI schemes and thus cannot
be redefined by URI scheme specifications.
Interpretation of the fragment
identifier is performed solely by the agent that dereferences a
URI; the fragment identifier is not passed to other systems during
the process of retrieval. This means that some intermediaries in
Web architecture (such as proxies) have no interaction with
fragment identifiers and that redirection (in HTTP [RFC2616], for example) does not account
for fragments.
Content negotiation refers to the
practice of making available multiple representations via the same
URI. Negotiation between the requesting agent and the server
determines which representation is served (usually with the goal of
serving the "best" representation a receiving agent can process).
HTTP is an example of a protocol that enables representation
providers to use content negotiation.
Individual data formats may define
their own rules for use of the fragment identifier syntax for
specifying different types of subsets, views, or external
references that are identifiable as secondary resources by that
media type. Therefore, representation providers must manage content
negotiation carefully when used with a URI that contains a fragment
identifier. Consider an example where the owner of the URI
"http://weather.example.com/oaxaca/map#zicatela" uses content
negotiation to serve two representations of the identified
resource. Three situations can arise:
- The interpretation of "zicatela" is defined consistently by
both data format specifications. The representation provider
decides when definitions of fragment identifier semantics are are
sufficiently consistent.
- The interpretation of "zicatela" is defined inconsistently by
the data format specifications.
- The interpretation of "zicatela" is defined in one data format
specification but not the other.
The first situation—consistent
semantics—poses no problem.
The second case is a server
management error: representation providers must not use content
negotiation to serve representation formats that have inconsistent
fragment identifier semantics. This situation also leads to
URI collision
(§2.2.1).
The third case is not a server
management error. It is a means by which the Web can grow. Because
the Web is a distributed system in which formats and agents are
deployed in a non-uniform manner, Web architecture does not
constrain authors to only use "lowest common denominator" formats.
Content authors may take advantage of new data formats while still
ensuring reasonable backward-compatibility for agents that do not
yet implement them.
In case three, behavior by the
receiving agent should vary depending on whether the negotiated
format defines fragment identifier semantics. When a received data
format does not define fragment identifier semantics, the agent
should not perform silent error recovery unless the user has given consent;
see [CUAP] for additional
suggested agent behavior in this case.
See related TAG issue RDFinXHTML-35.
Successful communication between
two parties depends on a reasonably shared understanding of the
semantics of exchanged messages, both data and metadata. At times,
there may be inconsistencies between a message sender's data and
metadata. Examples, observed in practice, of inconsistencies
between representation data and metadata include:
- The actual character encoding of a representation (e.g.,
"iso-8859-1", specified by the
encoding attribute in
an XML declaration) is inconsistent with the charset parameter in
the representation metadata (e.g., "utf-8", specified by the
'Content-Type' field in an HTTP header).
- The namespace
(§4.5.3) of the root element of XML representation data (e.g.,
as specified by the "xmlns" attribute) is inconsistent with the
value of the 'Content-Type' field in an HTTP header.
On the other hand, there is no
inconsistency in serving HTML content with the media type
"text/plain", for example, as this combination is licensed by
specifications.
Receiving agents should detect
protocol inconsistencies and perform proper error recovery.
Constraint: Data-metadata
inconsistency
Agents MUST NOT
ignore message metadata without the consent of the user.
Thus, for example, if the parties
responsible for "weather.example.com" mistakenly label the
satellite photo of Oaxaca as "image/gif" instead of "image/jpeg",
and if Nadia's browser detects a problem, Nadia's browser must not
ignore the problem (e.g., by simply rendering the JPEG image)
without Nadia's consent. Nadia's browser can notify Nadia of the
problem or notify Nadia and take corrective action.
Furthermore, representation
providers can help reduce the risk of inconsistencies through
careful assignment of representation metadata (especially that
which applies across representations). The section on media types for XML (§4.5.7)
presents an example of reducing the risk of error by providing no
metadata about character encoding when serving XML.
The accuracy of metadata relies on
the server administrators, the authors of representations, and the
software that they use. Practically, the capabilities of the tools
and the social relationships may be the limiting factors.
The accuracy of these and other
metadata fields is just as important for dynamic Web resources,
where a little bit of thought and programming can often ensure
correct metadata for a huge number of resources.
Often there is a separation of
control between the users who create representations of resources
and the server managers who maintain the Web site software. Given
that it is generally the Web site software that provides the
metadata associated with a resource, it follows that coordination
between the server managers and content creators is required.
Good
practice: Metadata
association
Server managers
SHOULD allow representation creators to control the metadata
associated with their representations.
In particular, content creators
need to be able to control the content type (for extensibility) and
the character encoding (for proper internationalization).
The TAG finding "Authoritative Metadata" discusses in more detail
how to handle data/metadata inconsistency and how server
configuration can be used to avoid it.
Nadia's retrieval of weather
information (an example of a read-only query or lookup) qualifies
as a "safe" interaction; a safe interaction is one where
the agent does not incur any obligation beyond the interaction. An
agent may incur an obligation through other means (such as by
signing a contract). If an agent does not have an obligation before
a safe interaction, it does not have that obligation
afterwards.
Other Web interactions resemble
orders more than queries. These unsafe interactions may
cause a change to the state of a resource and the user may be held
responsible for the consequences of these interactions. Unsafe
interactions include subscribing to a newsletter, posting to a
list, or modifying a database. Note: In this
context, the word "unsafe" does not necessarily mean "dangerous";
the term "safe" is used in section 9.1.1 of [RFC2616] and "unsafe" is the natural opposite.
Story
Nadia decides to book a vacation to
Oaxaca at "booking.example.com." She enters data into a series of
online forms and is ultimately asked for credit card information to
purchase the airline tickets. She provides this information in
another form. When she presses the "Purchase" button, her browser
opens another network connection to the server at
"booking.example.com" and sends a message composed of form data
using the POST method. This is an unsafe interaction; Nadia wishes to change the
state of the system by exchanging money for airline tickets.
The server reads the POST request,
and after performing the booking transaction returns a message to
Nadia's browser that contains a representation of the results of
Nadia's request. The representation data is in XHTML so that it can
be saved or printed out for Nadia's records.
Note that neither the data
transmitted with the POST nor the data received in the response
necessarily correspond to any resource identified by a URI.
Safe interactions are important
because these are interactions where users can browse with
confidence and where agents (including search engines and browsers
that pre-cache data for the user) can follow hypertext links
safely. Users (or agents acting on their behalf) do not commit
themselves to anything by querying a resource or following a
hypertext link.
Principle: Safe retrieval
Agents do not
incur obligations by retrieving a representation.
For instance, it is incorrect to
publish a URI that, when followed as part of a hypertext link,
subscribes a user to a mailing list. Remember that search engines
may follow such hypertext links.
The fact that HTTP GET, the access
method most often used when following a hypertext link, is safe
does not imply that all safe interactions must be done through HTTP
GET. At times, there may be good reasons (such as confidentiality
requirements or practical limits on URI length) to conduct an
otherwise safe operation using a mechanism generally reserved for
unsafe operations (e.g., HTTP POST).
For more information about safe and
unsafe operations using HTTP GET and POST, and handling security
concerns around the use of HTTP GET, see the TAG finding
"URIs, Addressability, and the use of HTTP GET and
POST".
Story
Nadia pays for her airline tickets
online (through a POST interaction as described above). She
receives a Web page with confirmation information and wishes to
bookmark it so that she can refer to it when she calculates her
expenses. Although Nadia can print out the results, or save them to
a file, she would also like to bookmark them.
Transaction requests and results
are valuable resources, and like all valuable resources, it is
useful to be able to refer to them with a persistent URI (§3.5.1).
However, in practice, Nadia cannot bookmark her commitment to pay
(expressed via the POST request) or the airline company's
acknowledgment and commitment to provide her with a flight
(expressed via the response to the POST).
There are ways to provide
persistent URIs for transaction requests and their results. For
transaction requests, user agents can provide an interface for
managing transactions where the user agent has incurred an
obligation on behalf of the user. For transaction results, HTTP
allows representation providers to associate a URI with the results
of an HTTP POST request using the "Content-Location" header
(described in section 14.14 of [RFC2616]).
Story
Since Nadia finds the Oaxaca
weather site useful, she emails a review to her friend Dirk
recommending that he check out 'http://weather.example.com/oaxaca'.
Dirk clicks on the resulting hypertext link in the email he
receives and is frustrated by a 404 (not found). Dirk tries again
the next day and receives a representation with "news" that is
two-weeks old. He tries one more time the next day only to receive
a representation that claims that the weather in Oaxaca is sunny,
even though his friends in Oaxaca tell him by phone that in fact it
is raining. Dirk and Nadia conclude that the URI owners are
unreliable or unpredictable. Although the URI owner has chosen the
Web as a communication medium, the owner has lost two customers due
to ineffective representation management.
A URI owner may supply zero or more
authoritative representations of the resource identified by that
URI. There is a benefit to the community in providing
representations.
Good
practice: Available representation
A URI owner
SHOULD provide representations of the resource it identifies
For example, owners of XML
namespace URIs should use them to identify a namespace document
(§4.5.4).
Just because representations are
available does not mean that it is always desirable to retrieve
them. In fact, in some cases the opposite is true.
Principle: Reference does not imply
dereference
An application
developer or specification author SHOULD NOT require networked
retrieval of representations each time they are referenced.
Dereferencing a URI has a
(potentially significant) cost in computing and bandwidth
resources, may have security implications, and may impose
significant latency on the dereferencing application. Dereferencing
URIs should be avoided except when necessary.
The following sections discuss some
aspects of representation management, including promoting URI persistence (§3.5.1),
managing access to resources
(§3.5.2), and supporting navigation (§3.5.3).
As is the case with many human
interactions, confidence in interactions via the Web depends on
stability and predictability. For an information resource,
persistence depends on the consistency of representations. The
representation provider decides when representations are
sufficiently consistent (although that determination generally
takes user expectations into account).
Although persistence in this case
is observable as a result of representation retrieval, the term
URI
persistence is used to describe the desirable property
that, once associated with a resource, a URI should continue
indefinitely to refer to that resource.
Good
practice: Consistent representation
A URI owner
SHOULD provide representations of the identified resource
consistently and predictably.
URI persistence is a matter of
policy and commitment on the part of the URI owner. The choice of a particular URI scheme
provides no guarantee that those URIs will be persistent or that
they will not be persistent.
HTTP [RFC2616] has been designed to help manage URI
persistence. For example, HTTP redirection (using the 3xx response
codes) permits servers to tell an agent that further action needs
to be taken by the agent in order to fulfill the request (for
example, a new URI is associated with the resource).
In addition, content negotiation also promotes consistency, as
a site manager is not required to define new URIs when adding
support for a new format specification. Protocols that do not
support content negotiation (such as FTP) require a new identifier
when a new data format is introduced. Improper use of content
negotiation can lead to inconsistent representations.
For more discussion about URI
persistence, see [Cool].
It is reasonable to limit access to
a resource (for commercial or security reasons, for example), but
merely identifying the resource is like referring to a book by
title. In exceptional circumstances, people may have agreed to keep
titles or URIs confidential (for example, a book author and a
publisher may agree to keep the URI of page containing additional
material secret until after the book is published), otherwise they
are free to exchange them.
As an analogy: The owners of a
building might have a policy that the public may only enter the
building via the main front door, and only during business hours.
People who work in the building and who make deliveries to it might
use other doors as appropriate. Such a policy would be enforced by
a combination of security personnel and mechanical devices such as
locks and pass-cards. One would not enforce this policy by hiding
some of the building entrances, nor by requesting legislation
requiring the use of the front door and forbidding anyone to reveal
the fact that there are other doors to the building.
Story
Nadia sends to Dirk the URI of the
current article she is reading. With his browser, Dirk follows the
hypertext link and is asked to enter his subscriber username and
password. Since Dirk is also a subscriber to services provided by
"weather.example.com," he can access the same information as Nadia.
Thus, the authority for "weather.example.com" can limit access to
authorized parties and still provide the benefits of URIs.
The Web provides several mechanisms
to control access to resources; these mechanisms do not rely on
hiding or suppressing URIs for those resources. For more
information, see the TAG finding "'Deep Linking' in the World Wide Web".
It is a strength of Web
Architecture that links can be made and shared; a user who has
found an interesting part of the Web can share this experience just
by republishing a URI.
Story
Nadia and Dirk want to visit the
Museum of Weather Forecasting in Oaxaca. Nadia goes to
"http://maps.example.com", locates the museum, and mails the URI
"http://maps.example.com/oaxaca?lat=17.065;lon=-96.716;scale=6" to
Dirk. Dirk goes to "http://mymaps.example.com", locates the museum,
and mails the URI
"http://mymaps.example.com/geo?sessionID=765345;userID=Dirk" to
Nadia. Dirk reads Nadia's email and is able to follow the link to
the map. Nadia reads Dirk's email, follows the link, and receives
an error message 'No such session/user'. Nadia has to start again
from "http://mymaps.example.com" and find the museum location once
more.
For resources that are generated on
demand, machine generation of URIs is common. For resources that
might usefully be bookmarked for later perusal, or shared with
others, server managers should avoid needlessly restricting the
reusability of such URIs. If the intention is to restrict
information to a particular user, as might be the case in a home
banking application for example, designers should use appropriate
access control (§3.5.2)
mechanisms.
Interactions conducted with HTTP
POST (where HTTP GET could have been used) also limit navigation
possibilities. The user cannot create a bookmark or share the URI
because HTTP POST transactions do not typically result in a
different URI as the user interacts with the site.
There remain open questions
regarding Web interactions. The TAG expects future versions of this
document to address in more detail the relationship between the
architecture described herein, Web Services, peer-to-peer systems, instant
messaging systems (such as [RFC3920]), streaming audio (such as RTSP [RFC2326]), and voice-over-IP (such as
SIP [RFC3261]).
A data format specification (for
example, for XHTML, RDF/XML, SMIL, XLink, CSS, and PNG) embodies an
agreement on the correct interpretation of representation data. The
first data format used on the Web was HTML. Since then, data
formats have grown in number. Web architecture does not constrain
which data formats content providers can use. This flexibility is
important because there is constant evolution in applications,
resulting in new data formats and refinements of existing formats.
Although Web architecture allows for the deployment of new data
formats, the creation and deployment of new formats (and agents
able to handle them) is expensive. Thus, before inventing a new
data format (or "meta" format such as XML), designers should
carefully consider re-using one that is already available.
For a data format to be usefully
interoperable between two parties, the parties must agree (to a
reasonable extent) about its syntax and semantics. Shared
understanding of a data format promotes interoperability but does
not imply constraints on usage; for instance, a sender of data
cannot count on being able to constrain the behavior of a data
receiver.
Below we describe some
characteristics of a data format that facilitate integration into
Web architecture. This document does not address generally
beneficial characteristics of a specification such as readability,
simplicity, attention to programmer goals, attention to user needs,
accessibility, nor internationalization. The section on
