W3C

Architecture of the World Wide Web

Editor's Draft 6 February 2003

This version:
http://www.w3.org/2001/tag/2002/webarch-20030206
Latest editor's draft
http://www.w3.org/2001/tag/webarch/
Previous version:
http://www.w3.org/2001/tag/2002/webarch-20021206
Latest version
http://www.w3.org/TR/webarch/
Editor:
Ian Jacobs, W3C
Authors:
See acknowledgments.

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Abstract

The World Wide Web is a networked information system. Web Architecture consists of the requirements, constraints, principles, and design choices that influence the design of the system and the behavior of agents within the system. When followed, the large-scale effect is that of a shared information space. This document organizes the technical discussion of the system in three parts: identification, representation, and interaction. This document also addresses some non-technical (social) issues that contribute to the shared information space.

This document strives to establish a reference set of requirements, constraints, principles, and design choices for Web architecture.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. The latest status of this document series is maintained at the W3C.

This Editor's draft includes some changes incorporated by Chris Lilley, notably in section 3 and section 4.

This draft is intended for review by the TAG. It does not represent consensus within the TAG. This document has been developed by W3C's Technical Architecture Group (TAG) (charter). A list of changes in this document is available.

This draft remains incomplete; sections 1 and 2 are the most developed, 3 and 4 the least. The TAG has published a number of findings that address specific architecture issues. Parts of those findings may appear in subsequent drafts. Please also consult the list of issues under consideration by the TAG.

This draft includes some editorial notes and also references to open TAG issues. These do not represent all open issues in the document. They are expected to disappear from future drafts.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than "work in progress."

The latest information regarding patent disclosures related to this document is available on the Web. As of this publication, there are no disclosures.

Please send comments on this document to the public W3C TAG mailing list www-tag@w3.org (archive).

A list of current W3C Recommendations and other technical documents can be found at the W3C Web site.

Table of Contents

1. Introduction

The World Wide Web (or, Web) is a networked information system consisting of agents (programs acting on behalf of another person, entity, or process) that exchange information.

This document organizes Web architecture into:

  1. Identification. Agents identify objects in the system (called "resources") with Uniform Resource Identifiers (URIs), defined in [RFC2396].
  2. Representation. Agents represent resources using a nonexclusive set of data formats, separately or in combination (e.g., XHTML, CSS, PNG, XLink, RDF/XML, SMIL animation). This section also discusses technologies for building new data formats (XML, XML Namespaces).
  3. Interaction. Agents exchange representations via protocols, including HTTP [RFC2616], FTP, and SMTP1. Several of these protocols share a reliance on the Multipurpose Internet Mail Extensions (MIME) standards for the format of message bodies [RFC2045] and for Internet Media Types [RFC2046].

The terms MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY are used in accordance with RFC 2119 [RFC2119].

1.1. Audience of this document

The intended audience for this document includes:

  1. Participants in W3C groups,
  2. Other groups and individuals developing technologies to be integrated into the Web.

The authors have made every effort to keep this document terse, with the expectation that additional documents will elaborate on the required properties, constraints, and principles, rationale, and examples.

Readers will benefit from familiarity with the Requests for Comments (RFC) series from the IETF, some of which define pieces of the architecture discussed in this document.

1.2. Scope of this document

This document focuses on the architecture of the Web. For instance, the principles enumerated in this document are those closely related to the Web. Some general design principles are discussed in this document, but many are not: minimal constraint (fewer rules makes the system more flexible), modularity, minimum redundancy, extensibility, simplicity, and robustness.

Other groups within W3C are addressing architectural design goals in the following areas:

  1. Internationalization; see W3C's Internationalization Activity.
  2. Accessibility; see W3C's Web Accessibility Initiative.
  3. Device independence; see W3C's Device Independence Activity.

For information about architectural principles of the Internet, refer to [RFC1958].

1.3. Summary of required properties, constraints, principles, and good practice notes

The terms used in the following list are elaborated on in the document, as are the categories of principle, constraint etc.

Use URIs: [constraint]
Use URIs: All important resources SHOULD be identified by a URI.
URI case: [practice]
URI case: It SHOULD NOT be assumed that URIs which differ only in character case can be used interchangeably.
Resource descriptions: [practice]
Resource descriptions: Owners of important resources SHOULD make available representations that describe the nature and purpose of those resources.
Safe retrieval: [principle]
Safe retrieval: Agents do not incur obligations by retrieving a representation.
Consistent representations: [practice]
Consistent representations: It is confusing and costly when, for a given URI, representations vary in unpredictable ways.
New URI schemes: [practice]
New URI schemes: Authors of specifications SHOULD avoid introducing new URI schemes when existing schemes can be used to meet the goals of the specifications.
Coneg with fragments: [practice]
Coneg with fragments: Authors SHOULD NOT use HTTP content negotiation for different media types that do not share the same fragment identifier semantics.

Some of the items in the above list may conflict with current practice, and so education and outreach will be required to improve on that practice. Other items may fill in gaps in published specifications or may call attention to known weaknesses in those specifications.

The architecture described in this document is the result of experience. There has been some theoretical and modeling work in the area of Web Architecture, notably Roy Fielding's work on "Representational State Transfer" [REST].

2. Identification and resources

The Web is a universe of resources. A resource is defined by [RFC2396] to be anything that has identity. Examples include documents, files, menu items, machines, and services, as well as people, organizations, and concepts. Web architecture starts with a uniform syntax for resource identifiers, so that we can refer to resources, access them, describe them, and share them. The Uniform Resource Identifier (URI) syntax employs an extensible set of URI schemes. Several URI schemes incorporate identification mechanisms that pre-date the Web into this (generic URI) syntax:

Other URI schemes have been introduced since the advent of the Web, including those introduced as a consequence of new protocols. Examples of URIs for these schemes include:

One can append a fragment identifier to a URI to yield an identifier for part of, or a view of, a resource2. The following URIs include fragment identifiers:

Note that while this composition is syntactically fully general, it is meaningless in some URI schemes. The URI mailto:nobody@example.org#abc is meaningless in practice.

A generic syntax for URIs is defined by [RFC2396]. The current document uses the term "URI" to mean, in RFC2396 terms, an absolute URI reference3 optionally followed by a fragment identifier. The TAG is working actively to convince the IETF to revise RFC2396 so that the definition of "URI" aligns with the current document.

2.1. Resources, URIs, and the shared information space

When one resource refers to another via a URI, a link is formed. When many resources are linked this way, the large-scale effect is a shared information space, where resources are identifiable by URI. The value of the Web increases with the number of resources identified by URI; this is due to the "network effect." In turn, resources are more valuable when they are identifiable on the Web. Hence:

Constraint

Use URIs: All important resources SHOULD be identified by a URI.4

There are many benefits to making resources identifiable by URI. Some are by design (e.g., linking and bookmarking), while others have arisen naturally (e.g., global search services). See the TAG finding URIs, Addressability, and the use of HTTP GET for some details about the interaction of this principle in HTTP application design.

2.1.1. Identity questions

In general, it is not possible to inspect a URI and determine what resource it identifies. For example, in general, one cannot look at http://www.example.com/lj45sr and know that it refers to "my old car" or "the weather forecast for Oaxaca."

If two parties use the same URI (see comparison of identifiers), the parties are referring to the same resource. This does not mean that two different URIs necessarily refer to different resources. Web architecture does not constrain resources to be uniquely named. The problem of determining whether two different URIs refer to the same resource or not is, in the general case, arbitrarily hard.

Fortunately, the problem does not need a complete nor ubiquitously deployed solution in order for the Web to operate usefully. Approaches to the problem include avoiding it, formal approaches, and heuristic approaches.

2.2. Uses of URIs

The two primary uses of URIs are:

  1. To compare identifiers
  2. To dereference a URI

2.2.1. Comparing identifiers

There may be applications (e.g., XML namespace names [XMLNS]) where comparison is expected to be the sole or primary use of a URI. Certain URI schemes provide rules for determining the syntactic equivalence of URIs, i.e., whether two URIs are different spellings of the same identifier. These rules vary from scheme to scheme.

For example, URNs begin with two colon-delimited fields, the first of which is the string urn and the second is the "namespace identifier" (NID). In URNs, these two fields are to be compared in a case-insensitive fashion. The remainder of the URN following the second colon is subject to rules dependent on the content of the second field (following the first colon) - thus the equivalence rules may vary within URN namespace identifiers.

Section 3.2.3 of the HTTP specification [RFC2616] states that, when comparing two HTTP URIs, the host name part must be considered case-insensitive, so http://WWW.EXAMPLE/ and http://www.example/ identify the same resource.

Good practice

URI case: It SHOULD NOT be assumed that URIs which differ only in character case can be used interchangeably.

In other words: if you want to refer to a resource and you know of a URI that refers to it, you should spell the URI the same way. Producers of URIs should be conservative by maximizing the consistency of identifiers used to refer to any given resource, and by ensuring sufficient difference between identifiers used for different resources.

Consumers of URIs, on the other hand, should be liberal in allowing URI producers maximum freedom in choosing URIs. Even though producers should not use http://example.com/MyStuff and http://example.com/myStuff to identify different resources, they may, and clients that assume that these URIs refer to the same resource do so at their own risk.

Note: Equivalence of URIs is not the same as consistent representations of a resource.

Issue: URIEquivalence-15: When are two URI variants considered equivalent? See also issue IRIEverywhere-27 - Should W3C specifications start promoting IRIs?

2.2.2. Dereferencing a URI

To dereference a URI is to apply in succession a finite set of relevant specifications, beginning with the specification that governs the scheme of the URI.

A "representation" is a data object that represents or describes a resource state, and is the vehicle for conveying the meaning of a resource. A resource is an abstraction for which there is a conceptual mapping to a (possibly empty) set of representations.

As an example of the application of specifications in succession, suppose that http://weather.yahoo.com/forecast/MXOA0069 is used within an a element of an SVG document. The sequence of specifications applied is:

  1. The URI specification [RFC2396]. This specification says (in section 3.1) that the scheme "define the semantics for the remainder of the URI string." In this case, the URI scheme is HTTP.
  2. The HTTP/1.1 protocol. Section 3.2.2 of RFC2616 [RFC2616] explains the semantics of HTTP URIs.
  3. The SVG 1.0 Recommendation [SVG10], which imports the link semantics defined by XLink 1.0 [XLink10]. Section 17.1 of the SVG specification suggests that interaction with an a link involves retrieving a representation of a resource, identified by the XLink href attribute: "By activating these links (by clicking with the mouse, through keyboard input, and voice commands), users may visit these resources." This means that the GET method defined in HTTP/1.1 is used to retrieve the representation of the resource.
  4. Once the representation has been retrieved, the media type of the representation governs its interpretation (here, for rendering).

Representations, when transferred by a Web protocol, are often accompanied by metadata in the message (for example, HTTP headers). In particular, the value of the media type in the set of metadata is key to the correct interpretation of a resource representation, and governs the handling of fragment identifiers. Note that, in general, one cannot determine the media type(s) of representation(s) of a resource by inspecting a URI for that resource. For example, do not assume that a URI that ends with the string ".html" refers to a resource that has an HTML representation.

See section 2 for more information about formats used to encode representations.

2.2.3. Retrieving a representation

Depending on the protocol used, there may be several ways to dereference a URI. One of the most important actions on the Web is to retrieve a representation of a resource (such as with HTTP GET), which means to retrieve a representation of the state of the resource. There are other ways to interact with a resource (such as with HTTP POST). Dereference mechanisms vary by URI scheme. For instance, the URN scheme [RFC 2141] does not specify a dereference procedure.

Good practice

Resource descriptions: Owners of important resources SHOULD make available representations that describe the nature and purpose of those resources.

Principle

Safe retrieval: Agents do not incur obligations by retrieving a representation.

For instance, a user does not incur an obligation by following an HTML link that causes the user agent to retrieve a representation. Tools such as proxies and search engines can retrieve representations without user interaction; it would be harmful to the Web if such operations incurred obligations. See the TAG finding "URIs, Addressability, and the use of HTTP GET" for more information about safe retrieval.

Issue: deepLinking-25: What to say in defense of principle that deep linking is not an illegal act?

2.2.4. Consistent representations

URIs represent a worldwide contract for who can create names and how the resources they designate take on meaning. In the case of HTTP URIs, for example, the agreement is that the authoritative meaning of the resource designated by the URI is established by retrieving a representation of the resource (per the HTTP specification [RFC2616]) and then interpreting the representation according to the relevant specifications. The authoritative meaning of a resource is established by following specifications.

Representations of a resource may vary as a function of factors including time, the identity of the agent accessing the resource, data submitted to the resource when interacting with it, and changes external to the resource. Consider the previous URI http://weather.yahoo.com/forecast/MXOA0069: representations for the designed resource (the weather in Oaxaca) depend on (at least) time, the expressed preference of the user for Fahrenheit or Celsius, the identity of the user-agent software receiving the representation, and, presumably, the weather in Oaxaca.

Good practice

Consistent representations: It is confusing and costly when, for a given URI, representations vary in unpredictable ways.

For example, serving two images as equivalents through HTTP content negotiation, where one image represents a square and the other a circle, will undermine confidence in the URI used to retrieve those images.

A description of what a URI identifies should be unambiguous. For instance, saying that the URI http://www.example.com/moby identifies "Moby Dick" can lead to confusion because this might be interpreted as any one of the following very distinct resources: a particular printing of this work (say, by ISBN), or the work itself in an abstract sense (for example, using RDF), or the fictional white whale, or a particular copy of the book on the shelves of a library (via the Web interface of the library's online catalog), or the record in the library's electronic catalog which contains the metadata about the work, or the Gutenberg project's online version. Similarly, one should not use the same URI to refer to a person and to that person's mailbox.

Ambiguous descriptions of what a URI identifies increase the likelihood that two parties will think the same URI identifies different resources, and thus that the parties will use the URI inconsistently. This can be costly, as in the case of two databases in which the same URI is used inconsistently; merging the two databases might lead to confusion or errors. In this document, we do not talk about "the meaning of a URI," only the meaning of the resource identified by a URI. Although people commonly ascribe meaning to resources based on their experience with those resources, that "meaning through use" is not the authoritative meaning. As stated above, the authoritative meaning of a resource is established by following specifications.

2.2.5. Persistence

There are thus strong social expectations that once a URI identifies a particular resource, it should continue indefinitely to refer to that resource; this is called the persistence of the URI. Persistence is always a matter of policy and commitment on the part of authorities assigning URIs rather than a constraint imposed by technological means.

For example, each W3C technical report (e.g., "the SVG specification") is in fact a series of documents that mature over time (from Working Drafts, Candidate Recommendations, Proposed Recommendations, to Recommendation). W3C assigns a URI to the "latest version" in the series (e.g., http://www.w3.org/TR/SVG). W3C also assigns a URI for each specification in the series (called the "this version URI", e.g., http://www.w3.org/TR/2001/PR-SVG-20010719/). W3C policy is that representations of the "latest version" resource will change over time (with each new publication of an SVG specification). W3C policy is also that representations of a specification designated by a "this version" identifier will not change over time, to the best of W3C's ability to maintain its archives intact.

HTTP [RFC2616] has been designed to promote consistency. For example, HTTP redirection (via some of the 3xx response codes) permits servers to tell a client that further action needs to be taken by the client in order to fulfill the request (e.g., the resource has been assigned a new URI). In addition, content negotiation also promotes consistency, as a site manager would not be required to define new URIs for each new format that is supported, as would be the case with protocols that don't support content negotiation, such as FTP.

For more discussion about persistence, refer to [Cool].6

2.3. URI Schemes

One important characteristic of a URI is its scheme (the string that precedes the first colon in a URI). For example the scheme of the URI http://www.example.com/ is "http", and for ftp://ftp.example.com/ it is "ftp". It is common to classify URIs by scheme, calling the two preceding examples respectively an "HTTP URI" and an "FTP URI".

Since many aspects of URI processing are scheme-dependent, and since a huge range of software is expected to be able to process URIs, the cost of introduction of new URI schemes is very high.

Good practice

New URI schemes: Authors of specifications SHOULD avoid introducing new URI schemes when existing schemes can be used to meet the goals of the specifications.

While "myscheme:blort" is a URI that satisfies the syntactic constraints of [RFC2396], if "myscheme" is not registered, you are not guaranteed that somebody else isn't already using it for something else.

The IANA registry [IANASchemes] lists registered URI schemes and the specifications that define them. For instance, the IANA registry indicates that the "http" scheme is defined by [RFC2616]. Refer to RFC2717 for information about registering a new URI scheme.

The deployment and use of different URI schemes may require varying degrees of central coordination and administration. For example, MAILTO, FTP, and HTTP URIs depend (in practice at least) on the use of the DNS infrastructure. Also, there is a central registry of URN namespace identifiers.

2.4. Fragment identifiers

In some URI schemes it is meaningful for a URI to end with a fragment identifier. The fragment identifier is interpreted only after the retrieval of a representation. Section 4.1 of [RFC2396] states that "the format and interpretation of fragment identifiers is dependent on the media type [RFC2046] of the retrieval result," that is, the representation.

For instance, if the representation is an HTML document, the fragment identifies a hypertext anchor. In the case of a graphics format, the fragment might identify a circle or spline. In the Resource Description Framework [RDF10], fragments can be used to identify anything, be it abstract (e.g., a dream) or concrete (e.g., an automobile).

Good practice

Coneg with fragments: Authors SHOULD NOT use HTTP content negotiation for different media types that do not share the same fragment identifier semantics.

Editor's note: There has been some discussion but no agreement that new access protocols should provide a means to convert fragment identifiers according to media type.

3. Representations

Data on the Web manifests itself through resource representations. A resource representation consists of:

  1. An Internet Media Type, which may include optional or mandatory parameters
  2. The actual dats "a sequence of bits"

A format specification describes the semantics and structure of the format - either directly as a bit sequence or with some indirection (as when multiple character encodings are permitted).

Refer to other W3C format guidelines: Charmod, XAG, etc.

3.1. Scope

What is a format, and how does it relate to the concept of a document. Do all documents have a format? Is a document a collection of resources of different formats organized into a whole? Is a document the same as a resource? the same as a message body? as a non-multipart message body? What is the distinction between documents and data, if any. Does 'document' imply human readable and if so, does it imply presentation? Does it imply a hierarchically structured, report-like document with headings and subheadings? Is a catalog a document? Is a rave flyer a document?

Negotiation (stuff above might go here also) by network request, by listed alternatives in content any preference? Resource variants, foo.css and foo.html unlikely to be equivalent.

3.2. Processing model

On the interpretation and processing of formats (see namespaceDocument-8 and mixedNamespaceMeaning-13):

3.3. Format specification design guidelines

Editor's note: This section is in its early stages.

3.3.1. When to use XML

For hierarchicaly structured information which does not contain a large proportion of binary information, XML is a natural choice. It is widely but not universally applicable for format specifications. For example, an audio or video format is unlikely to be well suited to representation in XML. Advantages of using XML include

  1. Explicit representation of the hierarchical structure
  2. Persistence; there is lots of redundancy
  3. Facilitates internationalization
  4. Clean error-handling; early detection of errors
  5. Mix of structure and text or data content
  6. Composability of multiple namespaces

Refer also to "Guidelines For The Use of XML in IETF Protocols" [IETFXML] for information about the use of XML within IETF standards-track protocols and "XML Accessibility Guidelines" [XAG] for help designing XML formats that lower barriers to Web accessibility for people with disabilities

3.3.2. XML Namespaces and Namespace Documents

XML vocabularies use URIs, per "Namespaces in XML" [XMLNS], as "namespace names" to create globally unique element and attribute names. These URIs are identified in an XML document with a namespace declaration.

Editor's note: Possible practice note: Designers SHOULD use XML Namespaces when they use XML.

Editor's note: The following text is all well and good, but mainly discusses namespace documents. Need some prior discussion on why using XML namespaces is good (composability, modularity) and what the drawbacks are (DTD hacks, etc) particularly to back up the suggested practice note.

Although "Namespaces in XML" makes it clear that it is not necessary for the namespace name to be a retrievable resource, the "resource description" principle suggests that it SHOULD be a retrievable resource.

Presented with a namespace name that it does not recognize, the URI is the only key that a person or application has to find out more about the namespace. The natural way to find out more about a resource identified by a URI is to dereference it.

There are many reasons why a person or agent might want more information about the namespace. A person might want to:

An agent might undertake to retreive that information for a user, or it might be searching for other kinds of information, such as:

It follows that there is, in general, no single type of resource that can be returned in response to a request for the namespace name that will always be the most appropriate.

Consequently, it often makes sense to use some sort of hybrid document that indirectly provides access to a variety of resources as the document available from the namespace name. One example of such a hybrid document is RDDL [@@Reference?@@].

Note, however, that RDDL or a document like it, is no more universally correct than any other type of resource. For any particular namespace, there might be a single best answer (schema, ontology, HTML documentation, etc.), as determined by the developers of that namespace.

Issue: namespaceDocument-8: What should a "namespace document" look like?

Editor's note: Where should we put a section on mixing namespaces; is the section on processing model more appropriate? See issue mixedNamespaceMeaning-13.

Editor's note: Mixing namespaces can however be done without agreeing on a processing model so although any processing model would affect mixed namespaces, it is not the same issue.

3.4. Separating Content and Presentation

Issue: contentPresentation-26: Separation of semantic and presentational markup, to the extent possible, is architecturally sound.

Separating the concepts content, presentation, and interaction allows more easily composable specifications. For example, a markup language can be specified independently of a style sheet language. The separation facilitates alternate presentations of the same content, which is seen to have an accessibility advantage and to be more suited to the multiple modalities of Web access.

There is no hard and fast division between what is 'purely semantic content' and what is 'just presentation'. The term "semantics" is often used or misued in this context, however any structured format is likely to have some semantics; and some semantics refer to precise details of presentation. Thus, 'semantics' is not used in this section. Instead, a given format may be seen to reside somewhere on a continuum from highly abstract to highly concrete. The more concrete, the less presentational flexibility remains. Highly abstract formats often require extensive transformation before being presented. Less abstract formats can often be presented directly just by decorating the source tree with formatting properties, for example using CSS. Highly concrete formats may still retain some presentational flexibility, for example restyling a pie diagram to fit into a different presentation.

In general, moving from more abstract to more concrete can be done (with loss of some abstractions) and is frequently done (for presentation); moving from a more concrete to a more abstract representation (eg, HTML to RDF) is sometimes possible in specific cases but may require extra information and is not possible in the general case.

It is sometimes asserted that XML is content and styling is presentation. However, presentational information may itself be complex and structured, and thus a good candidate for encoding in XML. Examples of such highly concrete XML formats are XSL Formatting Objects, SVG, the presentational part of MathML, and Voice XML.

As an example, an abstract dataset may contain relationships between sales areas, individual sales people, individual products, and time periods. A more concrete representation might be an HTML table comparing total sales per area over a three year period by quarter. Abstractions conerning individuals, dates of sales, and popularities of products have been lost, and the decision made to display the information in a two dimensional tabular format. However the table may be styled in different fonts and colors, columns or rows may be added, the table may be transposed, and the table may be serialized to a voice browser or screen reader. An even more concrete representation might be a pie chart in SVG of total sales over the whole period by sales area. Seasonal patterns have been lost, but there is still a limited degree of presentational flexibility in terms of color and size and access to the descriptions. Creation of a report on the sales performance of individuals or the growth in popularity of different products would require access to the most abstract form of the data.

3.4.1. Content, Presentation, and Interaction

This section attempts to organize some areas of future discussion.

3.4.1.1. Content

Composability (ns-meaning). Use of XML for tree structured content. Linking in general v. idref in one document. Human readable v. machine data. Served or not (hidden behind server - semantic firewall, accessibility. Linking into parts of the content, transclusion of parts. Compound documents, components from multiple servers - scalability, deep linking. Processing models, error handling.

3.4.1.2. Presentation

Presentation by decoration (application of CSS to XML as presentation), and by derivation (creation of html/svg/etc as presentation). Linking (bidirectionally) between content and presentations. Inheritance of properties across namespaces. Consistency of property names. Subsets. 'Applies to' as opposed to 'set on'. Specificity of properties as attributes, chaining styling, restyling. Time-lines, linking to portions of a time-line.

3.4.1.3. Interactivity

Animation, scripting, events, client/server interaction. Declarative v. script based - accessibility, power; formalization of common functionality (loop animation, rollovers) in declarative form. DOM - making additional methods, add to rather than replacing XML DOM. Effect of script/programming language limitations on choice of element and attribute names. Linking to active components - XForms example with model and abstract form control, can be extended to presentational instantiation of form control.

3.5. Ideas and issues

  1. For new format specifications, use XML family of specifications unless there's a good reason not to. Which XML specifications? Which particular family members?
  2. Format designers should use URIs without constraining content providers to particular URI schemes.
  3. Allow for Web-wide linking, not just internal document linking.
  4. Qnames: Issues rdfmsQnameUriMapping-6, qnameAsId-18 and finding "Using QNames as Identifiers in Content"
  5. Formatting properties: Issue formattingProperties-19, contentPresentation-26
  6. Error handling: Issue errorHandling-20
  7. Media type registration: RFC3023Charset-21, finding Internet Media Type registration, consistency of use. Also, makes sure to define fragment identifier semantics.
  8. Effect of Mobile on architecture - size, complexity, memory constraints. Binary infosets, storage efficiency. Composable subsets.
  9. What is the scope of using XLink? xlinkScope-23
  10. Can a specification include rules for overriding HTTP content type parameters? contentTypeOverride-24
  11. Create formats that allow authors to hide URIs from view (e.g., behind link text). For authors: at times it is useful or necessary to reveal a URI (e.g., in an advertisement on the side of a bus), in which case, good social behavior requires that the URI be easy to use.

4. Interaction

As mentioned in the introduction, the Web is designed to create the large-scale effect of a shared information space that scales well and behaves predictably. It is also not static - it is primarily use by people to get information, and that process is not one of passive consumption. Besides selecting which documents to read, people also interact with them - scrolling, zooming, filling in forms, following hyperlinks, and viewing and interacting with animations and scripting. A document is thus not merely a piece of XML markup and associated stylesheet but also descriptions of hyperlinking, scripting through the Document Object Model, declarative animation, associated audio and visual media, forms, etc.

4.1. Device Independence and Multimodal Interaction

Although much interaction to date has taken place through fairly similar desktop and laptop computers - with a keyboard, mouse, sctreen - increasingly interaction also uses other devices - PDSa, cellphones, spoken interaction - and also using accessibility helpers ranging from screen magnifiers through to Braille devices. The increasing necessity for such multimodal interaction informs the architectural principles and best practices relating to Web interaction.

4.2. HTTP and REST

4.3. Ideas and issues

  1. Consistency of media types and message contents (from "TAG Finding: Internet Media Type registration, consistency of use"
  2. Consistency of communicating character encoding (same source).
  3. HTTP as a substrate protocol [TAG issue HTTPSubstrate-16]

5. General design principles

Editor's note: There may be some general principles that hold across all three previous chapters. Should we put them in this appendix and refer to them from each section? Tim Bray has expressed the opinion that we should not have a separate section on general design principles.

5.1. Information hiding

When designing specifications that address independent functions of a system, avoidable references between the specifications are in general harmful. They are harmful because they impede the independent evolution of the specifications.

For example, it is a strength of XML that XPath cannot query the HTTP header. It is a strength of HTTP that it does not refer to details of the underlying TCP to the extent that it cannot be run over a different transport service. Similarly, the RDF data graph has a significance that is independent of the actual serialization. However, there is a flaw: the embedded XML parsetype="Literal" data type.

Sometimes it is necessary (and good for given application) to break layers. For example, it is good for an HTTP client to be aware of TCP speeds and round trip times to different mirror servers in order to optimize the choice of server. When designing specification, identify the functionalities that break layers so it is clear when they are being used.

6. Glossary

This section is non-normative.

6.1. Principles, constraints, etc.

Editor's note: The TAG is still experimenting with the categorization of points in this document. This list is likely to change.

The important points of this document are categorized as follows:

Constraint
An architectural constraint is a restriction in behavior or interaction within the system. Constraints may be imposed for technical, policy, or other reasons.
Design Choice
In the design of the Web, some design choices, like the names of the <p> and <li> elements in HTML, or the choice of the colon character in URIs, are somewhat arbitrary; if <par>, <elt>, or * had been chosen instead, the large-scale result would, most likely, have been the same. Other design choices are more fundamental; these are the focus of this document.
Good practice
Good practice -- by software developers, content authors, site managers, users, and specification writers -- increases the value of the Web.
Principle
An architectural principle is a fundamental law 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 determined by its slowest component."
Property
Architectural properties include both the functional properties achieved by the system, such as accessibility and global scope, and non-functional properties, such as relative ease of evolution, reusability of components, efficiency, and dynamic extensibility.

6.2. Technical terms

Agents
programs acting on behalf of another person, entity, or process
Dereference
To dereference a URI is to apply in succession a finite set of relevant specifications, beginning with the specification that governs the scheme of the URI.
Link
When one resource refers to another via a URI, a link is formed.
MIME
standards for the format of message bodies [RFC2045] and for Internet Media Types [RFC2046].
Persistence
There are thus strong social expectations that once a URI identifies a particular resource, it should continue indefinitely to refer to that resource; this is called the persistence of the URI.
Resource
A resource is defined by [RFC2396] to be anything that has identity.
Retrieve a representation
to retrieve a representation of the state of the resource.
URI Scheme
One important characteristic of a URI is its scheme (the string that precedes the first colon in a URI).

7. References

7.1. Normative References

IANASchemes
IANA's online registry of URI Schemes is available at http://www.iana.org/assignments/uri-schemes.
Dan Connolly's list of URI schemes is a useful resource for finding out which references define various URI schemes.
RFC2045
IETF "RFC 2045: Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", N. Freed, N. Borenstein, November 1996. Available at http://www.ietf.org/rfc/rfc2045.txt.
RFC2046
IETF "RFC 2046: Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", N. Freed, N. Borenstein, November 1996. Available at http://www.ietf.org/rfc/rfc2046.txt.
RFC2119
IETF "RFC 2119: Key words for use in RFCs to Indicate Requirement Levels", S. Bradner, March 1997. Available at http://www.ietf.org/rfc/rfc2119.txt.
RFC2396
IETF "RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax", T. Berners-Lee, R. Fielding, L. Masinter, August 1998. Available at http://www.ietf.org/rfc/rfc2396.txt.
RFC2616
IETF "RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1", J. Gettys, J. Mogul, H. Frystyk, L. Masinter, P. Leach, T. Berners-Lee, June 1999. Available at http://www.ietf.org/rfc/rfc2616.txt.
RFC2717
IETF "Registration Procedures for URL Scheme Names", R. Petke, I. King, November 1999. Available at http://www.ietf.org/rfc/rfc2717.txt.

7.2. Non-Normative References

Axioms
"Universal Resource Identifiers - Axioms of Web Architecture", T. Berners-Lee, living document dated December 1996. Available at http://www.w3.org/DesignIssues/Axioms.
Cool
"Cool URIs don't change" T. Berners-Lee, W3C, 1998 Available at http://www.w3.org/Provider/Style/URI.
CSS2
"Cascading Style Sheets, level 2", B. Bos, H. Lie, C. Lilley, I. Jacobs, 12 May 1998. This W3C Recommendation is available at http://www.w3.org/TR/1998/REC-CSS2-19980512/.
DAMLOIL
"DAML+OIL (March 2001) Reference Description", D. Connolly, F. van Harmelen, I. Horrocks, D. L. McGuinness, P. F. Patel-Schneider, 18 Dec 2001. This W3C Note is available at http://www.w3.org/TR/2001/NOTE-daml+oil-reference-20011218.
Eng90
"Knowledge-Domain Interoperability and an Open Hyperdocument System", D. C. Engelbart, June 1990.
Fielding
"Principled Design of the Modern Web Architecture", R.T. Fielding and R.N. Taylor, UC Irvine. In Proceedings of the 2000 International Conference on Software Engineering (ICSE 2000), Limerick, Ireland, June 2000, pp. 407-416. This document is available at http://www.ics.uci.edu/~fielding/pubs/webarch_icse2000.pdf.
Fragments
"Fragment Identifiers on URIs", T. Berners-Lee, living document dated April 1997. Available at http://www.w3.org/DesignIssues/Fragment.
HTML40
"HTML 4.01 Specification", D. Raggett, A. Le Hors, I. Jacobs, 24 December 1999. This W3C Recommendation is available at http://www.w3.org/TR/1999/REC-html401-19991224/.
IETFXML
IETF "Guidelines For The Use of XML in IETF Protocols," S. Hollenbeck, M. Rose, L. Masinter, eds., 2 November 2002. This IETF Internet Draft is available at http://www.imc.org/ietf-xml-use/xml-guidelines-07.txt. If this document is no longer available, refer to the ietf-xml-use mailing list.
OWL10
"Web Ontology Language (OWL) Reference Version 1.0", M. Dean, D. Connolly, F. van Harmelen, J. Hendler, I. Horrocks, D. L. McGuinness, P. F. Patel-Schneider, L. A. Stein, eds., 12 Nov 2002. This W3C Working Draft is available at http://www.w3.org/TR/2002/WD-owl-ref-20021112/.
P3P10
"The Platform for Privacy Preferences 1.0 (P3P1.0) Specification", M. Marchiori, ed., 16 April 2002. This W3C Recommendation is available at http://www.w3.org/TR/2002/REC-P3P-20020416/.
RDF10
"Resource Description Framework (RDF) Model and Syntax Specification", O. Lassila, R. R. Swick, eds., 22 February 1999. This W3C Recommendation is available at http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/.
REST
" Representational State Transfer (REST)", Chapter 5 of "Architectural Styles and the Design of Network-based Software Architectures", Doctoral Thesis of R. T. Fielding, 2000. Available at http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm.
RFC1958
IETF "RFC 1958: Architectural Principles of the Internet", B. Carpenter, June 1996. Available at http://www.ietf.org/rfc/rfc1958.txt.
RFC2141
IETF "RFC 2141: URN Syntax", R. Moats, May 1997. Available at http://www.ietf.org/rfc/rfc2141.txt.
RFC2718
"Guidelines for new URL Schemes", L. Masinter, H. Alvestrand, D. Zigmond, R. Petke, November 1999. Available at: http://www.ietf.org/rfc/rfc2718.txt.
RFC3236
IETF "RFC 3236: The 'application/xhtml+xml' Media Type", M. Baker, P. Stark, January 2002. Available at: http://www.rfc-editor.org/rfc/rfc3236.txt
SVG10
"Scalable Vector Graphics (SVG) 1.0 Specification", J. Ferraiolo, ed., 4 Sep 2001. This W3C Recommendation is available at http://www.w3.org/TR/2001/REC-SVG-20010904/.
UniqueDNS
" IAB Technical Comment on the Unique DNS Root", B. Carpenter, 27 Sep 1999. Available at http://www.icann.org/correspondence/iab-tech-comment-27sept99.htm.
XAG
"XML Accessibility Guidelines", Daniel Dardailler et al., 3 October 2002. Available at http://www.w3.org/TR/xag
XHTML10
"XHTML 1.0: The Extensible HyperText Markup Language: A Reformulation of HTML 4 in XML 1.0", S. Pemberton et al., 26 January 2000, revised 1 August 2002. Available at http://www.w3.org/TR/2002/REC-xhtml1-20020801/.
XLink10
"XML Linking Language (XLink) Version 1.0", S. DeRose, E. Maler, D. Orchard, 27 June 2001. This W3C Recommendation is available at http://www.w3.org/TR/2001/REC-xlink-20010627/.
XML10
"Extensible Markup Language (XML) 1.0 (Second Edition)", T. Bray, J. Paoli, C.M. Sperberg-McQueen, E. Maler, 6 October 2000. This W3C Recommendation is available at http://www.w3.org/TR/2000/REC-xml-20001006.
XMLNS
"Namespaces in XML", T. Bray, D. Hollander, A. Layman, 14 Jan 1999. This W3C Recommendation is available at http://www.w3.org/TR/1999/REC-xml-names-19990114/.
W3CPROCESS
"W3C Process Document", 19 July 2001 Version. Available at http://www.w3.org/Consortium/Process-20010719/.

8. End notes

  1. @@Text here on why SMTP part of Web@@ (Note 1 context.)
  2. When comparison is expected to be the sole or primary operation on a URI, it does not matter whether one has chosen a URI with our without a fragment identifier. However, when one expects to interact with a resource, there are some advantages to using a URI without a fragment identifier: only URIs work with intermediaries in the Web architecture (e.g., proxies) or with redirection (in HTTP, for example). (Note 2 context.)
  3. [RFC2396] defines a URI reference to be either an absolute URI reference or a relative URI reference. The syntax for a relative URI reference is a shortened form of that for an absolute URI reference, where some prefix of the URI is missing and certain path components ("." and "..") have a special meaning when, and only when, interpreting a relative path. For example, in a document whose base URI is http://example/dir1/dir2/file1, the relative URI reference ../file2 is a shortened form of http://example/dir1/file2 and the relative URI reference #abc is a shortened form for http://example/dir1/dir2/file1#abc. (Note 3 context.)
  4. 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]. (Note 4 context.)
  5. See example from Dan Connolly for details. (Note 5 context.)
  6. The title is somewhat misleading. It's not the URIs that change, it's what they identify. (Note 6 context.)

9. Acknowledgments

The authors of this document are the participants of W3C's Technical Architecture Group: Tim Berners-Lee (Chair, W3C), Tim Bray (Antarctica Systems), Dan Connolly (W3C), Paul Cotton (Microsoft), Roy Fielding (Day Software), Chris Lilley (W3C), David Orchard (BEA Systems), Norman Walsh (Sun), and Stuart Williams (Hewlett-Packard).

The TAG thanks people for their thoughtful contributions on the TAG's public mailing list, www-tag (archive).