< draft-fielding-uri-rfc2396bis   rfc3986.txt 
Network Working Group T. Berners-Lee Network Working Group T. Berners-Lee
Internet-Draft W3C/MIT Request for Comments: 3986 W3C/MIT
Updates: 1738 (if approved) R. Fielding STD: 66 R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved) Day Software Updates: 1738 Day Software
L. Masinter Obsoletes: 2732, 2396, 1808 L. Masinter
Expires: March 26, 2005 Adobe Category: Standards Track Adobe Systems
September 25, 2004 January 2005
Uniform Resource Identifier (URI): Generic Syntax Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-07
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Status of This Memo
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<http://www.ietf.org/shadow.html>. Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). Copyright (C) The Internet Society (2005).
Abstract Abstract
A Uniform Resource Identifier (URI) is a compact sequence of A Uniform Resource Identifier (URI) is a compact sequence of
characters for identifying an abstract or physical resource. This characters that identifies an abstract or physical resource. This
specification defines the generic URI syntax and a process for specification defines the generic URI syntax and a process for
resolving URI references that might be in relative form, along with resolving URI references that might be in relative form, along with
guidelines and security considerations for the use of URIs on the guidelines and security considerations for the use of URIs on the
Internet. The URI syntax defines a grammar that is a superset of all Internet. The URI syntax defines a grammar that is a superset of all
valid URIs, such that an implementation can parse the common valid URIs, allowing an implementation to parse the common components
components of a URI reference without knowing the scheme-specific of a URI reference without knowing the scheme-specific requirements
requirements of every possible identifier. This specification does of every possible identifier. This specification does not define a
not define a generative grammar for URIs; that task is performed by generative grammar for URIs; that task is performed by the individual
the individual specifications of each URI scheme. specifications of each URI scheme.
Editorial Note
Discussion of this draft and comments to the editors should be sent
to the [email protected] mailing list. An issues list and version history
is available at <http://gbiv.com/protocols/uri/rev-2002/issues.html>.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . 4 1.1. Overview of URIs . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . 6 1.1.1. Generic Syntax . . . . . . . . . . . . . . . . . 6
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . 7 1.1.2. Examples . . . . . . . . . . . . . . . . . . . . 7
1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . 7 1.1.3. URI, URL, and URN . . . . . . . . . . . . . . . 7
1.2 Design Considerations . . . . . . . . . . . . . . . . . . 7 1.2. Design Considerations . . . . . . . . . . . . . . . . . 8
1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . 7 1.2.1. Transcription . . . . . . . . . . . . . . . . . 8
1.2.2 Separating Identification from Interaction . . . . . . 9 1.2.2. Separating Identification from Interaction . . . 9
1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . 10 1.2.3. Hierarchical Identifiers . . . . . . . . . . . . 10
1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . 11 1.3. Syntax Notation . . . . . . . . . . . . . . . . . . . . 11
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Percent-Encoding . . . . . . . . . . . . . . . . . . . . . 12 2.1. Percent-Encoding . . . . . . . . . . . . . . . . . . . . 12
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . 12 2.2. Reserved Characters . . . . . . . . . . . . . . . . . . 12
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . 13 2.3. Unreserved Characters . . . . . . . . . . . . . . . . . 13
2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . 13 2.4. When to Encode or Decode . . . . . . . . . . . . . . . . 14
2.5 Identifying Data . . . . . . . . . . . . . . . . . . . . . 14 2.5. Identifying Data . . . . . . . . . . . . . . . . . . . . 14
3. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 16 3. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 16
3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1. Scheme . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2. Authority . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.1 User Information . . . . . . . . . . . . . . . . . . . 18 3.2.1. User Information . . . . . . . . . . . . . . . . 18
3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.2.2. Host . . . . . . . . . . . . . . . . . . . . . . 18
3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2.3. Port . . . . . . . . . . . . . . . . . . . . . . 22
3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.3. Path . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.4. Query . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.5. Fragment . . . . . . . . . . . . . . . . . . . . . . . . 24
4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . 25 4.1. URI Reference . . . . . . . . . . . . . . . . . . . . . 25
4.2 Relative Reference . . . . . . . . . . . . . . . . . . . . 26 4.2. Relative Reference . . . . . . . . . . . . . . . . . . . 26
4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . 26 4.3. Absolute URI . . . . . . . . . . . . . . . . . . . . . . 27
4.4 Same-document Reference . . . . . . . . . . . . . . . . . 27 4.4. Same-Document Reference . . . . . . . . . . . . . . . . 27
4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . 27 4.5. Suffix Reference . . . . . . . . . . . . . . . . . . . . 27
5. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 28 5. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 28
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . 28 5.1. Establishing a Base URI . . . . . . . . . . . . . . . . 28
5.1.1 Base URI Embedded in Content . . . . . . . . . . . . . 29 5.1.1. Base URI Embedded in Content . . . . . . . . . . 29
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . 29 5.1.2. Base URI from the Encapsulating Entity . . . . . 29
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . 30 5.1.3. Base URI from the Retrieval URI . . . . . . . . 30
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . 30 5.1.4. Default Base URI . . . . . . . . . . . . . . . . 30
5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . 30 5.2. Relative Resolution . . . . . . . . . . . . . . . . . . 30
5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . 30 5.2.1. Pre-parse the Base URI . . . . . . . . . . . . . 31
5.2.2 Transform References . . . . . . . . . . . . . . . . . 31 5.2.2. Transform References . . . . . . . . . . . . . . 31
5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . 32 5.2.3. Merge Paths . . . . . . . . . . . . . . . . . . 32
5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . 32 5.2.4. Remove Dot Segments . . . . . . . . . . . . . . 33
5.3 Component Recomposition . . . . . . . . . . . . . . . . . 34 5.3. Component Recomposition . . . . . . . . . . . . . . . . 35
5.4 Reference Resolution Examples . . . . . . . . . . . . . . 34 5.4. Reference Resolution Examples . . . . . . . . . . . . . 35
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . 35 5.4.1. Normal Examples . . . . . . . . . . . . . . . . 36
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . 35 5.4.2. Abnormal Examples . . . . . . . . . . . . . . . 36
6. Normalization and Comparison . . . . . . . . . . . . . . . . . 36
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . 37 6. Normalization and Comparison . . . . . . . . . . . . . . . . . 38
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . 37 6.1. Equivalence . . . . . . . . . . . . . . . . . . . . . . 38
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . 38 6.2. Comparison Ladder . . . . . . . . . . . . . . . . . . . 39
6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . 39 6.2.1. Simple String Comparison . . . . . . . . . . . . 39
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . 40 6.2.2. Syntax-Based Normalization . . . . . . . . . . . 40
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . 41 6.2.3. Scheme-Based Normalization . . . . . . . . . . . 41
7. Security Considerations . . . . . . . . . . . . . . . . . . . 41 6.2.4. Protocol-Based Normalization . . . . . . . . . . 42
7.1 Reliability and Consistency . . . . . . . . . . . . . . . 41 7. Security Considerations . . . . . . . . . . . . . . . . . . . 43
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . 42 7.1. Reliability and Consistency . . . . . . . . . . . . . . 43
7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . 42 7.2. Malicious Construction . . . . . . . . . . . . . . . . . 43
7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . 43 7.3. Back-End Transcoding . . . . . . . . . . . . . . . . . . 44
7.5 Sensitive Information . . . . . . . . . . . . . . . . . . 44 7.4. Rare IP Address Formats . . . . . . . . . . . . . . . . 45
7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . 44 7.5. Sensitive Information . . . . . . . . . . . . . . . . . 45
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45 7.6. Semantic Attacks . . . . . . . . . . . . . . . . . . . . 45
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 46 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 46 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
10.2 Informative References . . . . . . . . . . . . . . . . . . . 46 10.1. Normative References . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 48 10.2. Informative References . . . . . . . . . . . . . . . . . 47
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 49 A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 49
B. Parsing a URI Reference with a Regular Expression . . . . . . 51 B. Parsing a URI Reference with a Regular Expression . . . . . . 50
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 52 C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 51
D. Changes from RFC 2396 . . . . . . . . . . . . . . . . . . . . 53 D. Changes from RFC 2396 . . . . . . . . . . . . . . . . . . . . 53
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . 53 D.1. Additions . . . . . . . . . . . . . . . . . . . . . . . 53
D.2 Modifications . . . . . . . . . . . . . . . . . . . . . . 54 D.2. Modifications . . . . . . . . . . . . . . . . . . . . . 53
E. Instructions to RFC Editor . . . . . . . . . . . . . . . . . . 56 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60
Intellectual Property and Copyright Statements . . . . . . . . 61 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 61
1. Introduction 1. Introduction
A Uniform Resource Identifier (URI) provides a simple and extensible A Uniform Resource Identifier (URI) provides a simple and extensible
means for identifying a resource. This specification of URI syntax means for identifying a resource. This specification of URI syntax
and semantics is derived from concepts introduced by the World Wide and semantics is derived from concepts introduced by the World Wide
Web global information initiative, whose use of such identifiers Web global information initiative, whose use of these identifiers
dates from 1990 and is described in "Universal Resource Identifiers dates from 1990 and is described in "Universal Resource Identifiers
in WWW" [RFC1630], and is designed to meet the recommendations laid in WWW" [RFC1630]. The syntax is designed to meet the
out in "Functional Recommendations for Internet Resource Locators" recommendations laid out in "Functional Recommendations for Internet
[RFC1736] and "Functional Requirements for Uniform Resource Names" Resource Locators" [RFC1736] and "Functional Requirements for Uniform
[RFC1737]. Resource Names" [RFC1737].
This document obsoletes [RFC2396], which merged "Uniform Resource This document obsoletes [RFC2396], which merged "Uniform Resource
Locators" [RFC1738] and "Relative Uniform Resource Locators" Locators" [RFC1738] and "Relative Uniform Resource Locators"
[RFC1808] in order to define a single, generic syntax for all URIs. [RFC1808] in order to define a single, generic syntax for all URIs.
It contains the updates from, and obsoletes, [RFC2732], which It obsoletes [RFC2732], which introduced syntax for an IPv6 address.
introduced syntax for IPv6 addresses. It excludes those portions of It excludes portions of RFC 1738 that defined the specific syntax of
RFC 1738 that defined the specific syntax of individual URI schemes; individual URI schemes; those portions will be updated as separate
those portions will be updated as separate documents. The process documents. The process for registration of new URI schemes is
for registration of new URI schemes is defined separately by [BCP35]. defined separately by [BCP35]. Advice for designers of new URI
Advice for designers of new URI schemes can be found in [RFC2718]. schemes can be found in [RFC2718]. All significant changes from RFC
2396 are noted in Appendix D.
All significant changes from RFC 2396 are noted in Appendix D.
This specification uses the terms "character" and "coded character This specification uses the terms "character" and "coded character
set" in accordance with the definitions provided in [BCP19], and set" in accordance with the definitions provided in [BCP19], and
"character encoding" in place of what [BCP19] refers to as a "character encoding" in place of what [BCP19] refers to as a
"charset". "charset".
1.1 Overview of URIs 1.1. Overview of URIs
URIs are characterized as follows: URIs are characterized as follows:
Uniform Uniform
Uniformity provides several benefits: it allows different types of Uniformity provides several benefits. It allows different types
resource identifiers to be used in the same context, even when the of resource identifiers to be used in the same context, even when
mechanisms used to access those resources may differ; it allows the mechanisms used to access those resources may differ. It
uniform semantic interpretation of common syntactic conventions allows uniform semantic interpretation of common syntactic
across different types of resource identifiers; it allows conventions across different types of resource identifiers. It
introduction of new types of resource identifiers without allows introduction of new types of resource identifiers without
interfering with the way that existing identifiers are used; and, interfering with the way that existing identifiers are used. It
it allows the identifiers to be reused in many different contexts, allows the identifiers to be reused in many different contexts,
thus permitting new applications or protocols to leverage a thus permitting new applications or protocols to leverage a pre-
pre-existing, large, and widely-used set of resource identifiers. existing, large, and widely used set of resource identifiers.
Resource Resource
This specification does not limit the scope of what might be a This specification does not limit the scope of what might be a
resource; rather, the term "resource" is used in a general sense resource; rather, the term "resource" is used in a general sense
for whatever might be identified by a URI. Familiar examples for whatever might be identified by a URI. Familiar examples
include an electronic document, an image, a source of information include an electronic document, an image, a source of information
with consistent purpose (e.g., "today's weather report for Los with a consistent purpose (e.g., "today's weather report for Los
Angeles"), a service (e.g., an HTTP to SMS gateway), a collection Angeles"), a service (e.g., an HTTP-to-SMS gateway), and a
of other resources, and so on. A resource is not necessarily collection of other resources. A resource is not necessarily
accessible via the Internet; e.g., human beings, corporations, and accessible via the Internet; e.g., human beings, corporations, and
bound books in a library can also be resources. Likewise, bound books in a library can also be resources. Likewise,
abstract concepts can be resources, such as the operators and abstract concepts can be resources, such as the operators and
operands of a mathematical equation, the types of a relationship operands of a mathematical equation, the types of a relationship
(e.g., "parent" or "employee"), or numeric values (e.g., zero, (e.g., "parent" or "employee"), or numeric values (e.g., zero,
one, and infinity). one, and infinity).
Identifier Identifier
An identifier embodies the information required to distinguish An identifier embodies the information required to distinguish
what is being identified from all other things within its scope of what is being identified from all other things within its scope of
identification. Our use of the terms "identify" and "identifying" identification. Our use of the terms "identify" and "identifying"
refer to this purpose of distinguishing one resource from all refer to this purpose of distinguishing one resource from all
other resources, regardless of how that purpose is accomplished other resources, regardless of how that purpose is accomplished
(e.g., by name, address, context, etc.). These terms should not (e.g., by name, address, or context). These terms should not be
be mistaken as an assumption that an identifier defines or mistaken as an assumption that an identifier defines or embodies
embodies the identity of what is referenced, though that may be the identity of what is referenced, though that may be the case
the case for some identifiers. Nor should it be assumed that a for some identifiers. Nor should it be assumed that a system
system using URIs will access the resource identified: in many using URIs will access the resource identified: in many cases,
cases, URIs are used to denote resources without any intention URIs are used to denote resources without any intention that they
that they be accessed. Likewise, the "one" resource identified be accessed. Likewise, the "one" resource identified might not be
might not be singular in nature (e.g., a resource might be a named singular in nature (e.g., a resource might be a named set or a
set or a mapping that varies over time). mapping that varies over time).
A URI is an identifier, consisting of a sequence of characters A URI is an identifier consisting of a sequence of characters
matching the syntax rule named <URI> in Section 3, that enables matching the syntax rule named <URI> in Section 3. It enables
uniform identification of resources via a separately defined, uniform identification of resources via a separately defined
extensible set of naming schemes (Section 3.1). How that extensible set of naming schemes (Section 3.1). How that
identification is accomplished, assigned, or enabled is delegated to identification is accomplished, assigned, or enabled is delegated to
each scheme specification. each scheme specification.
This specification does not place any limits on the nature of a This specification does not place any limits on the nature of a
resource, the reasons why an application might wish to refer to a resource, the reasons why an application might seek to refer to a
resource, or the kinds of system that might use URIs for the sake of resource, or the kinds of systems that might use URIs for the sake of
identifying resources. This specification does not require that a identifying resources. This specification does not require that a
URI persists in identifying the same resource over all time, though URI persists in identifying the same resource over time, though that
that is a common goal of all URI schemes. Nevertheless, nothing in is a common goal of all URI schemes. Nevertheless, nothing in this
this specification prevents an application from limiting itself to specification prevents an application from limiting itself to
particular types of resources, or to a subset of URIs that maintains particular types of resources, or to a subset of URIs that maintains
characteristics desired by that application. characteristics desired by that application.
URIs have a global scope and are interpreted consistently regardless URIs have a global scope and are interpreted consistently regardless
of context, though the result of that interpretation may be in of context, though the result of that interpretation may be in
relation to the end-user's context. For example, "http://localhost/" relation to the end-user's context. For example, "http://localhost/"
has the same interpretation for every user of that reference, even has the same interpretation for every user of that reference, even
though the network interface corresponding to "localhost" may be though the network interface corresponding to "localhost" may be
different for each end-user: interpretation is independent of access. different for each end-user: interpretation is independent of access.
However, an action made on the basis of that reference will take However, an action made on the basis of that reference will take
place in relation to the end-user's context, which implies that an place in relation to the end-user's context, which implies that an
action intended to refer to a single, globally unique thing must use action intended to refer to a globally unique thing must use a URI
a URI that distinguishes that resource from all other things. URIs that distinguishes that resource from all other things. URIs that
that identify in relation to the end-user's local context should only identify in relation to the end-user's local context should only be
be used when the context itself is a defining aspect of the resource, used when the context itself is a defining aspect of the resource,
such as when an on-line help manual refers to a file on the such as when an on-line help manual refers to a file on the end-
end-user's filesystem (e.g., "file:///etc/hosts"). user's file system (e.g., "file:///etc/hosts").
1.1.1 Generic Syntax 1.1.1. Generic Syntax
Each URI begins with a scheme name, as defined in Section 3.1, that Each URI begins with a scheme name, as defined in Section 3.1, that
refers to a specification for assigning identifiers within that refers to a specification for assigning identifiers within that
scheme. As such, the URI syntax is a federated and extensible naming scheme. As such, the URI syntax is a federated and extensible naming
system wherein each scheme's specification may further restrict the system wherein each scheme's specification may further restrict the
syntax and semantics of identifiers using that scheme. syntax and semantics of identifiers using that scheme.
This specification defines those elements of the URI syntax that are This specification defines those elements of the URI syntax that are
required of all URI schemes or are common to many URI schemes. It required of all URI schemes or are common to many URI schemes. It
thus defines the syntax and semantics that are needed to implement a thus defines the syntax and semantics needed to implement a scheme-
scheme-independent parsing mechanism for URI references, such that independent parsing mechanism for URI references, by which the
the scheme-dependent handling of a URI can be postponed until the scheme-dependent handling of a URI can be postponed until the
scheme-dependent semantics are needed. Likewise, protocols and data scheme-dependent semantics are needed. Likewise, protocols and data
formats that make use of URI references can refer to this formats that make use of URI references can refer to this
specification as defining the range of syntax allowed for all URIs, specification as a definition for the range of syntax allowed for all
including those schemes that have yet to be defined, thus decoupling URIs, including those schemes that have yet to be defined. This
the evolution of identification schemes from the evolution of decouples the evolution of identification schemes from the evolution
protocols, data formats, and implementations that make use of URIs. of protocols, data formats, and implementations that make use of
URIs.
A parser of the generic URI syntax is capable of parsing any URI A parser of the generic URI syntax can parse any URI reference into
reference into its major components; once the scheme is determined, its major components. Once the scheme is determined, further
further scheme-specific parsing can be performed on the components. scheme-specific parsing can be performed on the components. In other
In other words, the URI generic syntax is a superset of the syntax of words, the URI generic syntax is a superset of the syntax of all URI
all URI schemes. schemes.
1.1.2 Examples 1.1.2. Examples
The following example URIs illustrate several URI schemes and The following example URIs illustrate several URI schemes and
variations in their common syntax components: variations in their common syntax components:
ftp://ftp.is.co.za/rfc/rfc1808.txt ftp://ftp.is.co.za/rfc/rfc1808.txt
http://www.ietf.org/rfc/rfc2396.txt http://www.ietf.org/rfc/rfc2396.txt
ldap://[2001:db8::7]/c=GB?objectClass?one ldap://[2001:db8::7]/c=GB?objectClass?one
mailto:[email protected] mailto:[email protected]
news:comp.infosystems.www.servers.unix news:comp.infosystems.www.servers.unix
tel:+1-816-555-1212 tel:+1-816-555-1212
telnet://192.0.2.16:80/ telnet://192.0.2.16:80/
urn:oasis:names:specification:docbook:dtd:xml:4.1.2 urn:oasis:names:specification:docbook:dtd:xml:4.1.2
1.1.3 URI, URL, and URN 1.1.3. URI, URL, and URN
A URI can be further classified as a locator, a name, or both. The A URI can be further classified as a locator, a name, or both. The
term "Uniform Resource Locator" (URL) refers to the subset of URIs term "Uniform Resource Locator" (URL) refers to the subset of URIs
that, in addition to identifying a resource, provide a means of that, in addition to identifying a resource, provide a means of
locating the resource by describing its primary access mechanism locating the resource by describing its primary access mechanism
(e.g., its network "location"). The term "Uniform Resource Name" (e.g., its network "location"). The term "Uniform Resource Name"
(URN) has been used historically to refer to both URIs under the (URN) has been used historically to refer to both URIs under the
"urn" scheme [RFC2141], which are required to remain globally unique "urn" scheme [RFC2141], which are required to remain globally unique
and persistent even when the resource ceases to exist or becomes and persistent even when the resource ceases to exist or becomes
unavailable, and to any other URI with the properties of a name. unavailable, and to any other URI with the properties of a name.
An individual scheme does not need to be classified as being just one An individual scheme does not have to be classified as being just one
of "name" or "locator". Instances of URIs from any given scheme may of "name" or "locator". Instances of URIs from any given scheme may
have the characteristics of names or locators or both, often have the characteristics of names or locators or both, often
depending on the persistence and care in the assignment of depending on the persistence and care in the assignment of
identifiers by the naming authority, rather than any quality of the identifiers by the naming authority, rather than on any quality of
scheme. Future specifications and related documentation should use the scheme. Future specifications and related documentation should
the general term "URI", rather than the more restrictive terms URL use the general term "URI" rather than the more restrictive terms
and URN [RFC3305]. "URL" and "URN" [RFC3305].
1.2 Design Considerations 1.2. Design Considerations
1.2.1 Transcription 1.2.1. Transcription
The URI syntax has been designed with global transcription as one of The URI syntax has been designed with global transcription as one of
its main considerations. A URI is a sequence of characters from a its main considerations. A URI is a sequence of characters from a
very limited set: the letters of the basic Latin alphabet, digits, very limited set: the letters of the basic Latin alphabet, digits,
and a few special characters. A URI may be represented in a variety and a few special characters. A URI may be represented in a variety
of ways: e.g., ink on paper, pixels on a screen, or a sequence of of ways; e.g., ink on paper, pixels on a screen, or a sequence of
character encoding octets. The interpretation of a URI depends only character encoding octets. The interpretation of a URI depends only
on the characters used and not how those characters are represented on the characters used and not on how those characters are
in a network protocol. represented in a network protocol.
The goal of transcription can be described by a simple scenario. The goal of transcription can be described by a simple scenario.
Imagine two colleagues, Sam and Kim, sitting in a pub at an Imagine two colleagues, Sam and Kim, sitting in a pub at an
international conference and exchanging research ideas. Sam asks Kim international conference and exchanging research ideas. Sam asks Kim
for a location to get more information, so Kim writes the URI for the for a location to get more information, so Kim writes the URI for the
research site on a napkin. Upon returning home, Sam takes out the research site on a napkin. Upon returning home, Sam takes out the
napkin and types the URI into a computer, which then retrieves the napkin and types the URI into a computer, which then retrieves the
information to which Kim referred. information to which Kim referred.
There are several design considerations revealed by the scenario: There are several design considerations revealed by the scenario:
o A URI is a sequence of characters that is not always represented o A URI is a sequence of characters that is not always represented
as a sequence of octets. as a sequence of octets.
o A URI might be transcribed from a non-network source, and thus o A URI might be transcribed from a non-network source and thus
should consist of characters that are most likely to be able to be should consist of characters that are most likely able to be
entered into a computer, within the constraints imposed by entered into a computer, within the constraints imposed by
keyboards (and related input devices) across languages and keyboards (and related input devices) across languages and
locales. locales.
o A URI often needs to be remembered by people, and it is easier for o A URI often has to be remembered by people, and it is easier for
people to remember a URI when it consists of meaningful or people to remember a URI when it consists of meaningful or
familiar components. familiar components.
These design considerations are not always in alignment. For These design considerations are not always in alignment. For
example, it is often the case that the most meaningful name for a URI example, it is often the case that the most meaningful name for a URI
component would require characters that cannot be typed into some component would require characters that cannot be typed into some
systems. The ability to transcribe a resource identifier from one systems. The ability to transcribe a resource identifier from one
medium to another has been considered more important than having a medium to another has been considered more important than having a
URI consist of the most meaningful of components. URI consist of the most meaningful of components.
In local or regional contexts and with improving technology, users In local or regional contexts and with improving technology, users
might benefit from being able to use a wider range of characters; might benefit from being able to use a wider range of characters;
such use is not defined by this specification. Percent-encoded such use is not defined by this specification. Percent-encoded
octets (Section 2.1) may be used within a URI to represent characters octets (Section 2.1) may be used within a URI to represent characters
outside the range of the US-ASCII coded character set if such outside the range of the US-ASCII coded character set if this
representation is allowed by the scheme or by the protocol element in representation is allowed by the scheme or by the protocol element in
which the URI is referenced; such a definition should specify the which the URI is referenced. Such a definition should specify the
character encoding used to map those characters to octets prior to character encoding used to map those characters to octets prior to
being percent-encoded for the URI. being percent-encoded for the URI.
1.2.2 Separating Identification from Interaction 1.2.2. Separating Identification from Interaction
A common misunderstanding of URIs is that they are only used to refer A common misunderstanding of URIs is that they are only used to refer
to accessible resources. In fact, the URI alone only provides to accessible resources. The URI itself only provides
identification; access to the resource is neither guaranteed nor identification; access to the resource is neither guaranteed nor
implied by the presence of a URI. Instead, an operation (if any) implied by the presence of a URI. Instead, any operation associated
associated with a URI reference is defined by the protocol element, with a URI reference is defined by the protocol element, data format
data format attribute, or natural language text in which it appears. attribute, or natural language text in which it appears.
Given a URI, a system may attempt to perform a variety of operations Given a URI, a system may attempt to perform a variety of operations
on the resource, as might be characterized by such words as "access", on the resource, as might be characterized by words such as "access",
"update", "replace", or "find attributes". Such operations are "update", "replace", or "find attributes". Such operations are
defined by the protocols that make use of URIs, not by this defined by the protocols that make use of URIs, not by this
specification. However, we do use a few general terms for describing specification. However, we do use a few general terms for describing
common operations on URIs. URI "resolution" is the process of common operations on URIs. URI "resolution" is the process of
determining an access mechanism and the appropriate parameters determining an access mechanism and the appropriate parameters
necessary to dereference a URI; such resolution may require several necessary to dereference a URI; this resolution may require several
iterations. To use that access mechanism to perform an action on the iterations. To use that access mechanism to perform an action on the
URI's resource is to "dereference" the URI. URI's resource is to "dereference" the URI.
When URIs are used within information retrieval systems to identify When URIs are used within information retrieval systems to identify
sources of information, the most common form of URI dereference is sources of information, the most common form of URI dereference is
"retrieval": making use of a URI in order to retrieve a "retrieval": making use of a URI in order to retrieve a
representation of its associated resource. A "representation" is a representation of its associated resource. A "representation" is a
sequence of octets, along with representation metadata describing sequence of octets, along with representation metadata describing
those octets, that constitutes a record of the state of the resource those octets, that constitutes a record of the state of the resource
at the time that the representation is generated. Retrieval is at the time when the representation is generated. Retrieval is
achieved by a process that might include using the URI as a cache key achieved by a process that might include using the URI as a cache key
to check for a locally cached representation, resolution of the URI to check for a locally cached representation, resolution of the URI
to determine an appropriate access mechanism (if any), and to determine an appropriate access mechanism (if any), and
dereference of the URI for the sake of applying a retrieval dereference of the URI for the sake of applying a retrieval
operation. Depending on the protocols used to perform the retrieval, operation. Depending on the protocols used to perform the retrieval,
additional information might be supplied about the resource (resource additional information might be supplied about the resource (resource
metadata) and its relation to other resources. metadata) and its relation to other resources.
URI references in information retrieval systems are designed to be URI references in information retrieval systems are designed to be
late-binding: the result of an access is generally determined at the late-binding: the result of an access is generally determined when it
time it is accessed and may vary over time or due to other aspects of is accessed and may vary over time or due to other aspects of the
the interaction. Such references are created in order to be used in interaction. These references are created in order to be used in the
the future: what is being identified is not some specific result that future: what is being identified is not some specific result that was
was obtained in the past, but rather some characteristic that is obtained in the past, but rather some characteristic that is expected
expected to be true for future results. In such cases, the resource to be true for future results. In such cases, the resource referred
referred to by the URI is actually a sameness of characteristics as to by the URI is actually a sameness of characteristics as observed
observed over time, perhaps elucidated by additional comments or over time, perhaps elucidated by additional comments or assertions
assertions made by the resource provider. made by the resource provider.
Although many URI schemes are named after protocols, this does not Although many URI schemes are named after protocols, this does not
imply that use of such a URI will result in access to the resource imply that use of these URIs will result in access to the resource
via the named protocol. URIs are often used simply for the sake of via the named protocol. URIs are often used simply for the sake of
identification. Even when a URI is used to retrieve a representation identification. Even when a URI is used to retrieve a representation
of a resource, that access might be through gateways, proxies, of a resource, that access might be through gateways, proxies,
caches, and name resolution services that are independent of the caches, and name resolution services that are independent of the
protocol associated with the scheme name, and the resolution of some protocol associated with the scheme name. The resolution of some
URIs may require the use of more than one protocol (e.g., both DNS URIs may require the use of more than one protocol (e.g., both DNS
and HTTP are typically used to access an "http" URI's origin server and HTTP are typically used to access an "http" URI's origin server
when a representation isn't found in a local cache). when a representation isn't found in a local cache).
1.2.3 Hierarchical Identifiers 1.2.3. Hierarchical Identifiers
The URI syntax is organized hierarchically, with components listed in The URI syntax is organized hierarchically, with components listed in
order of decreasing significance from left to right. For some URI order of decreasing significance from left to right. For some URI
schemes, the visible hierarchy is limited to the scheme itself: schemes, the visible hierarchy is limited to the scheme itself:
everything after the scheme component delimiter (":") is considered everything after the scheme component delimiter (":") is considered
opaque to URI processing. Other URI schemes make the hierarchy opaque to URI processing. Other URI schemes make the hierarchy
explicit and visible to generic parsing algorithms. explicit and visible to generic parsing algorithms.
The generic syntax uses the slash ("/"), question mark ("?"), and The generic syntax uses the slash ("/"), question mark ("?"), and
number sign ("#") characters for the purpose of delimiting components number sign ("#") characters to delimit components that are
that are significant to the generic parser's hierarchical significant to the generic parser's hierarchical interpretation of an
interpretation of an identifier. In addition to aiding the identifier. In addition to aiding the readability of such
readability of such identifiers through the consistent use of identifiers through the consistent use of familiar syntax, this
familiar syntax, this uniform representation of hierarchy across uniform representation of hierarchy across naming schemes allows
naming schemes allows scheme-independent references to be made scheme-independent references to be made relative to that hierarchy.
relative to that hierarchy.
It is often the case that a group or "tree" of documents has been It is often the case that a group or "tree" of documents has been
constructed to serve a common purpose, wherein the vast majority of constructed to serve a common purpose, wherein the vast majority of
URI references in these documents point to resources within the tree URI references in these documents point to resources within the tree
rather than outside of it. Similarly, documents located at a rather than outside it. Similarly, documents located at a particular
particular site are much more likely to refer to other resources at site are much more likely to refer to other resources at that site
that site than to resources at remote sites. Relative referencing of than to resources at remote sites. Relative referencing of URIs
URIs allows document trees to be partially independent of their allows document trees to be partially independent of their location
location and access scheme. For instance, it is possible for a and access scheme. For instance, it is possible for a single set of
single set of hypertext documents to be simultaneously accessible and hypertext documents to be simultaneously accessible and traversable
traversable via each of the "file", "http", and "ftp" schemes if the via each of the "file", "http", and "ftp" schemes if the documents
documents refer to each other using relative references. refer to each other with relative references. Furthermore, such
Furthermore, such document trees can be moved, as a whole, without document trees can be moved, as a whole, without changing any of the
changing any of the relative references. relative references.
A relative reference (Section 4.2) refers to a resource by describing A relative reference (Section 4.2) refers to a resource by describing
the difference within a hierarchical name space between the reference the difference within a hierarchical name space between the reference
context and the target URI. The reference resolution algorithm, context and the target URI. The reference resolution algorithm,
presented in Section 5, defines how such a reference is transformed presented in Section 5, defines how such a reference is transformed
to the target URI. Since relative references can only be used within to the target URI. As relative references can only be used within
the context of a hierarchical URI, designers of new URI schemes the context of a hierarchical URI, designers of new URI schemes
should use a syntax consistent with the generic syntax's hierarchical should use a syntax consistent with the generic syntax's hierarchical
components unless there are compelling reasons to forbid relative components unless there are compelling reasons to forbid relative
referencing within that scheme. referencing within that scheme.
NOTE: Previous specifications used the terms "partial URI" and NOTE: Previous specifications used the terms "partial URI" and
"relative URI" to denote a relative reference to a URI. Since "relative URI" to denote a relative reference to a URI. As some
some readers misunderstood those terms to mean that relative URIs readers misunderstood those terms to mean that relative URIs are a
are a subset of URIs, rather than a method of referencing URIs, subset of URIs rather than a method of referencing URIs, this
this specification simply refers to them as relative references. specification simply refers to them as relative references.
All URI references are parsed by generic syntax parsers when used. All URI references are parsed by generic syntax parsers when used.
However, since hierarchical processing has no effect on an absolute However, because hierarchical processing has no effect on an absolute
URI used in a reference unless it contains one or more dot-segments URI used in a reference unless it contains one or more dot-segments
(complete path segments of "." or "..", as described in Section 3.3), (complete path segments of "." or "..", as described in Section 3.3),
URI scheme specifications can define opaque identifiers by URI scheme specifications can define opaque identifiers by
disallowing use of slash characters, question mark characters, and disallowing use of slash characters, question mark characters, and
the URIs "scheme:." and "scheme:..". the URIs "scheme:." and "scheme:..".
1.3 Syntax Notation 1.3. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC2234], including the following core ABNF syntax rules notation of [RFC2234], including the following core ABNF syntax rules
defined by that specification: ALPHA (letters), CR (carriage return), defined by that specification: ALPHA (letters), CR (carriage return),
DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal
digits), LF (line feed), and SP (space). The complete URI syntax is digits), LF (line feed), and SP (space). The complete URI syntax is
collected in Appendix A. collected in Appendix A.
2. Characters 2. Characters
The URI syntax provides a method of encoding data, presumably for the The URI syntax provides a method of encoding data, presumably for the
sake of identifying a resource, as a sequence of characters. The URI sake of identifying a resource, as a sequence of characters. The URI
characters are, in turn, frequently encoded as octets for transport characters are, in turn, frequently encoded as octets for transport
or presentation. This specification does not mandate any particular or presentation. This specification does not mandate any particular
character encoding for mapping between URI characters and the octets character encoding for mapping between URI characters and the octets
used to store or transmit those characters. When a URI appears in a used to store or transmit those characters. When a URI appears in a
protocol element, the character encoding is defined by that protocol; protocol element, the character encoding is defined by that protocol;
absent such a definition, a URI is assumed to be in the same without such a definition, a URI is assumed to be in the same
character encoding as the surrounding text. character encoding as the surrounding text.
The ABNF notation defines its terminal values to be non-negative The ABNF notation defines its terminal values to be non-negative
integers (codepoints) based on the US-ASCII coded character set integers (codepoints) based on the US-ASCII coded character set
[ASCII]. Since a URI is a sequence of characters, we must invert [ASCII]. Because a URI is a sequence of characters, we must invert
that relation in order to understand the URI syntax. Therefore, the that relation in order to understand the URI syntax. Therefore, the
integer values used by the ABNF must be mapped back to their integer values used by the ABNF must be mapped back to their
corresponding characters via US-ASCII in order to complete the syntax corresponding characters via US-ASCII in order to complete the syntax
rules. rules.
A URI is composed from a limited set of characters consisting of A URI is composed from a limited set of characters consisting of
digits, letters, and a few graphic symbols. A reserved subset of digits, letters, and a few graphic symbols. A reserved subset of
those characters may be used to delimit syntax components within a those characters may be used to delimit syntax components within a
URI, while the remaining characters, including both the unreserved URI while the remaining characters, including both the unreserved set
set and those reserved characters not acting as delimiters, define and those reserved characters not acting as delimiters, define each
each component's identifying data. component's identifying data.
2.1 Percent-Encoding 2.1. Percent-Encoding
A percent-encoding mechanism is used to represent a data octet in a A percent-encoding mechanism is used to represent a data octet in a
component when that octet's corresponding character is outside the component when that octet's corresponding character is outside the
allowed set or is being used as a delimiter of, or within, the allowed set or is being used as a delimiter of, or within, the
component. A percent-encoded octet is encoded as a character component. A percent-encoded octet is encoded as a character
triplet, consisting of the percent character "%" followed by the two triplet, consisting of the percent character "%" followed by the two
hexadecimal digits representing that octet's numeric value. For hexadecimal digits representing that octet's numeric value. For
example, "%20" is the percent-encoding for the binary octet example, "%20" is the percent-encoding for the binary octet
"00100000" (ABNF: %x20), which in US-ASCII corresponds to the space "00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
character (SP). Section 2.4 describes when percent-encoding and character (SP). Section 2.4 describes when percent-encoding and
decoding is applied. decoding is applied.
pct-encoded = "%" HEXDIG HEXDIG pct-encoded = "%" HEXDIG HEXDIG
The uppercase hexadecimal digits 'A' through 'F' are equivalent to The uppercase hexadecimal digits 'A' through 'F' are equivalent to
the lowercase digits 'a' through 'f', respectively. Two URIs that the lowercase digits 'a' through 'f', respectively. If two URIs
differ only in the case of hexadecimal digits used in percent-encoded differ only in the case of hexadecimal digits used in percent-encoded
octets are equivalent. For consistency, URI producers and octets, they are equivalent. For consistency, URI producers and
normalizers should use uppercase hexadecimal digits for all normalizers should use uppercase hexadecimal digits for all percent-
percent-encodings. encodings.
2.2 Reserved Characters 2.2. Reserved Characters
URIs include components and subcomponents that are delimited by URIs include components and subcomponents that are delimited by
characters in the "reserved" set. These characters are called characters in the "reserved" set. These characters are called
"reserved" because they may (or may not) be defined as delimiters by "reserved" because they may (or may not) be defined as delimiters by
the generic syntax, by each scheme-specific syntax, or by the the generic syntax, by each scheme-specific syntax, or by the
implementation-specific syntax of a URI's dereferencing algorithm. implementation-specific syntax of a URI's dereferencing algorithm.
If data for a URI component would conflict with a reserved If data for a URI component would conflict with a reserved
character's purpose as a delimiter, then the conflicting data must be character's purpose as a delimiter, then the conflicting data must be
percent-encoded before forming the URI. percent-encoded before the URI is formed.
reserved = gen-delims / sub-delims reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@" gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")" sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "=" / "*" / "+" / "," / ";" / "="
The purpose of reserved characters is to provide a set of delimiting The purpose of reserved characters is to provide a set of delimiting
characters that are distinguishable from other data within a URI. characters that are distinguishable from other data within a URI.
URIs that differ in the replacement of a reserved character with its URIs that differ in the replacement of a reserved character with its
corresponding percent-encoded octet are not equivalent. corresponding percent-encoded octet are not equivalent. Percent-
Percent-encoding a reserved character, or decoding a percent-encoded encoding a reserved character, or decoding a percent-encoded octet
octet that corresponds to a reserved character, will change how the that corresponds to a reserved character, will change how the URI is
URI is interpreted by most applications. Thus, characters in the interpreted by most applications. Thus, characters in the reserved
reserved set are protected from normalization and are therefore safe set are protected from normalization and are therefore safe to be
to be used by scheme-specific and producer-specific algorithms for used by scheme-specific and producer-specific algorithms for
delimiting data subcomponents within a URI. delimiting data subcomponents within a URI.
A subset of the reserved characters (gen-delims) are used as A subset of the reserved characters (gen-delims) is used as
delimiters of the generic URI components described in Section 3. A delimiters of the generic URI components described in Section 3. A
component's ABNF syntax rule will not use the reserved or gen-delims component's ABNF syntax rule will not use the reserved or gen-delims
rule names directly; instead, each syntax rule lists the characters rule names directly; instead, each syntax rule lists the characters
allowed within that component (i.e., not delimiting it) and any of allowed within that component (i.e., not delimiting it), and any of
those characters that are also in the reserved set are "reserved" for those characters that are also in the reserved set are "reserved" for
use as subcomponent delimiters within the component. Only the most use as subcomponent delimiters within the component. Only the most
common subcomponents are defined by this specification; other common subcomponents are defined by this specification; other
subcomponents may be defined by a URI scheme's specification, or by subcomponents may be defined by a URI scheme's specification, or by
the implementation-specific syntax of a URI's dereferencing the implementation-specific syntax of a URI's dereferencing
algorithm, provided that such subcomponents are delimited by algorithm, provided that such subcomponents are delimited by
characters in the reserved set allowed within that component. characters in the reserved set allowed within that component.
URI producing applications should percent-encode data octets that URI producing applications should percent-encode data octets that
correspond to characters in the reserved set. However, if a reserved correspond to characters in the reserved set unless these characters
character is found in a URI component and no delimiting role is known are specifically allowed by the URI scheme to represent data in that
for that character, then it should be interpreted as representing the component. If a reserved character is found in a URI component and
data octet corresponding to that character's encoding in US-ASCII. no delimiting role is known for that character, then it must be
interpreted as representing the data octet corresponding to that
character's encoding in US-ASCII.
2.3 Unreserved Characters 2.3. Unreserved Characters
Characters that are allowed in a URI but do not have a reserved Characters that are allowed in a URI but do not have a reserved
purpose are called unreserved. These include uppercase and lowercase purpose are called unreserved. These include uppercase and lowercase
letters, decimal digits, hyphen, period, underscore, and tilde. letters, decimal digits, hyphen, period, underscore, and tilde.
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
URIs that differ in the replacement of an unreserved character with URIs that differ in the replacement of an unreserved character with
its corresponding percent-encoded US-ASCII octet are equivalent: they its corresponding percent-encoded US-ASCII octet are equivalent: they
identify the same resource. However, URI comparison implementations identify the same resource. However, URI comparison implementations
do not always perform normalization prior to comparison Section 6. do not always perform normalization prior to comparison (see Section
For consistency, percent-encoded octets in the ranges of ALPHA 6). For consistency, percent-encoded octets in the ranges of ALPHA
(%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E),
underscore (%5F), or tilde (%7E) should not be created by URI underscore (%5F), or tilde (%7E) should not be created by URI
producers and, when found in a URI, should be decoded to their producers and, when found in a URI, should be decoded to their
corresponding unreserved character by URI normalizers. corresponding unreserved characters by URI normalizers.
2.4 When to Encode or Decode 2.4. When to Encode or Decode
Under normal circumstances, the only time that octets within a URI Under normal circumstances, the only time when octets within a URI
are percent-encoded is during the process of producing the URI from are percent-encoded is during the process of producing the URI from
its component parts. It is during that process that an its component parts. This is when an implementation determines which
implementation determines which of the reserved characters are to be of the reserved characters are to be used as subcomponent delimiters
used as subcomponent delimiters and which can be safely used as data. and which can be safely used as data. Once produced, a URI is always
Once produced, a URI is always in its percent-encoded form. in its percent-encoded form.
When a URI is dereferenced, the components and subcomponents When a URI is dereferenced, the components and subcomponents
significant to the scheme-specific dereferencing process (if any) significant to the scheme-specific dereferencing process (if any)
must be parsed and separated before the percent-encoded octets within must be parsed and separated before the percent-encoded octets within
those components can be safely decoded, since otherwise the data may those components can be safely decoded, as otherwise the data may be
be mistaken for component delimiters. The only exception is for mistaken for component delimiters. The only exception is for
percent-encoded octets corresponding to characters in the unreserved percent-encoded octets corresponding to characters in the unreserved
set, which can be decoded at any time. For example, the octet set, which can be decoded at any time. For example, the octet
corresponding to the tilde ("~") character is often encoded as "%7E" corresponding to the tilde ("~") character is often encoded as "%7E"
by older URI processing implementations; the "%7E" can be replaced by by older URI processing implementations; the "%7E" can be replaced by
"~" without changing its interpretation. "~" without changing its interpretation.
Because the percent ("%") character serves as the indicator for Because the percent ("%") character serves as the indicator for
percent-encoded octets, it must be percent-encoded as "%25" in order percent-encoded octets, it must be percent-encoded as "%25" for that
for that octet to be used as data within a URI. Implementations must octet to be used as data within a URI. Implementations must not
not percent-encode or decode the same string more than once, since percent-encode or decode the same string more than once, as decoding
decoding an already decoded string might lead to misinterpreting a an already decoded string might lead to misinterpreting a percent
percent data octet as the beginning of a percent-encoding, or vice data octet as the beginning of a percent-encoding, or vice versa in
versa in the case of percent-encoding an already percent-encoded the case of percent-encoding an already percent-encoded string.
string.
2.5 Identifying Data 2.5. Identifying Data
URI characters provide identifying data for each of the URI URI characters provide identifying data for each of the URI
components, serving as an external interface for identification components, serving as an external interface for identification
between systems. Although the presence and nature of the URI between systems. Although the presence and nature of the URI
production interface is hidden from clients that use its URIs, and production interface is hidden from clients that use its URIs (and is
thus beyond the scope of the interoperability requirements defined by thus beyond the scope of the interoperability requirements defined by
this specification, it is a frequent source of confusion and errors this specification), it is a frequent source of confusion and errors
in the interpretation of URI character issues. Implementers need to in the interpretation of URI character issues. Implementers have to
be aware that there are multiple character encodings involved in the be aware that there are multiple character encodings involved in the
production and transmission of URIs: local name and data encoding, production and transmission of URIs: local name and data encoding,
public interface encoding, URI character encoding, data format public interface encoding, URI character encoding, data format
encoding, and protocol encoding. encoding, and protocol encoding.
The first encoding of identifying data is the one in which the local Local names, such as file system names, are stored with a local
names or data are stored. URI producing applications (a.k.a., origin character encoding. URI producing applications (e.g., origin
servers) will typically use the local encoding as the basis for servers) will typically use the local encoding as the basis for
producing meaningful names. The URI producer will transform the producing meaningful names. The URI producer will transform the
local encoding to one that is suitable for a public interface, and local encoding to one that is suitable for a public interface and
then transform the public interface encoding into the restricted set then transform the public interface encoding into the restricted set
of URI characters (reserved, unreserved, and percent-encodings). of URI characters (reserved, unreserved, and percent-encodings).
Those characters are, in turn, encoded as octets to be used as a Those characters are, in turn, encoded as octets to be used as a
reference within a data format (e.g., a document charset), and such reference within a data format (e.g., a document charset), and such
data formats are often subsequently encoded for transmission over data formats are often subsequently encoded for transmission over
Internet protocols. Internet protocols.
For most systems, an unreserved character appearing within a URI For most systems, an unreserved character appearing within a URI
component is interpreted as representing the data octet corresponding component is interpreted as representing the data octet corresponding
to that character's encoding in US-ASCII. Consumers of URIs assume to that character's encoding in US-ASCII. Consumers of URIs assume
that the letter "X" corresponds to the octet "01011000", and there is that the letter "X" corresponds to the octet "01011000", and even
no harm in making that assumption even when it is incorrect. A when that assumption is incorrect, there is no harm in making it. A
system that internally provides identifiers in the form of a system that internally provides identifiers in the form of a
different character encoding, such as EBCDIC, will generally perform different character encoding, such as EBCDIC, will generally perform
character translation of textual identifiers to UTF-8 [STD63] (or character translation of textual identifiers to UTF-8 [STD63] (or
some other superset of the US-ASCII character encoding) at an some other superset of the US-ASCII character encoding) at an
internal interface, thereby providing more meaningful identifiers internal interface, thereby providing more meaningful identifiers
than simply percent-encoding the original octets. than those resulting from simply percent-encoding the original
octets.
For example, consider an information service that provides data, For example, consider an information service that provides data,
stored locally using an EBCDIC-based filesystem, to clients on the stored locally using an EBCDIC-based file system, to clients on the
Internet through an HTTP server. When an author creates a file on Internet through an HTTP server. When an author creates a file with
that filesystem with the name "Laguna Beach", their expectation is the name "Laguna Beach" on that file system, the "http" URI
that the "http" URI corresponding to that resource would also contain corresponding to that resource is expected to contain the meaningful
the meaningful string "Laguna%20Beach". If, however, that server string "Laguna%20Beach". If, however, that server produces URIs by
produces URIs using an overly-simplistic raw octet mapping, then the using an overly simplistic raw octet mapping, then the result would
result would be a URI containing be a URI containing "%D3%81%87%A4%95%81@%C2%85%81%83%88". An
"%D3%81%87%A4%95%81@%C2%85%81%83%88". An internal transcoding internal transcoding interface fixes this problem by transcoding the
interface fixes that problem by transcoding the local name to a local name to a superset of US-ASCII prior to producing the URI.
superset of US-ASCII prior to producing the URI. Naturally, proper Naturally, proper interpretation of an incoming URI on such an
interpretation of an incoming URI on such an interface requires that interface requires that percent-encoded octets be decoded (e.g.,
percent-encoded octets be decoded (e.g., "%20" to SP) before the "%20" to SP) before the reverse transcoding is applied to obtain the
reverse transcoding is applied to obtain the local name. local name.
In some cases, the internal interface between a URI component and the In some cases, the internal interface between a URI component and the
identifying data that it has been crafted to represent is much less identifying data that it has been crafted to represent is much less
direct than a character encoding translation. For example, portions direct than a character encoding translation. For example, portions
of a URI might reflect a query on non-ASCII data, numeric coordinates of a URI might reflect a query on non-ASCII data, or numeric
on a map, etc. Likewise, a URI scheme may define components with coordinates on a map. Likewise, a URI scheme may define components
additional encoding requirements that are applied prior to forming with additional encoding requirements that are applied prior to
the component and producing the URI. forming the component and producing the URI.
When a new URI scheme defines a component that represents textual When a new URI scheme defines a component that represents textual
data consisting of characters from the Unicode character set [UCS], data consisting of characters from the Universal Character Set [UCS],
the data should be encoded first as octets according to the UTF-8 the data should first be encoded as octets according to the UTF-8
character encoding [STD63], and then only those octets that do not character encoding [STD63]; then only those octets that do not
correspond to characters in the unreserved set should be correspond to characters in the unreserved set should be percent-
percent-encoded. For example, the character A would be represented encoded. For example, the character A would be represented as "A",
as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be the character LATIN CAPITAL LETTER A WITH GRAVE would be represented
represented as "%C3%80", and the character KATAKANA LETTER A would be as "%C3%80", and the character KATAKANA LETTER A would be represented
represented as "%E3%82%A2". as "%E3%82%A2".
3. Syntax Components 3. Syntax Components
The generic URI syntax consists of a hierarchical sequence of The generic URI syntax consists of a hierarchical sequence of
components referred to as the scheme, authority, path, query, and components referred to as the scheme, authority, path, query, and
fragment. fragment.
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ] URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part = "//" authority path-abempty hier-part = "//" authority path-abempty
/ path-absolute / path-absolute
/ path-rootless / path-rootless
/ path-empty / path-empty
The scheme and path components are required, though path may be empty The scheme and path components are required, though the path may be
(no characters). When authority is present, the path must either be empty (no characters). When authority is present, the path must
empty or begin with a slash ("/") character. When authority is not either be empty or begin with a slash ("/") character. When
present, the path cannot begin with two slash characters ("//"). authority is not present, the path cannot begin with two slash
These restrictions result in five different ABNF rules for a path characters ("//"). These restrictions result in five different ABNF
(Section 3.3), only one of which will match any given URI reference. rules for a path (Section 3.3), only one of which will match any
given URI reference.
The following are two example URIs and their component parts: The following are two example URIs and their component parts:
foo://example.com:8042/over/there?name=ferret#nose foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/ \_/ \______________/\_________/ \_________/ \__/
| | | | | | | | | |
scheme authority path query fragment scheme authority path query fragment
| _____________________|__ | _____________________|__
/ \ / \ / \ / \
urn:example:animal:ferret:nose urn:example:animal:ferret:nose
3.1 Scheme 3.1. Scheme
Each URI begins with a scheme name that refers to a specification for Each URI begins with a scheme name that refers to a specification for
assigning identifiers within that scheme. As such, the URI syntax is assigning identifiers within that scheme. As such, the URI syntax is
a federated and extensible naming system wherein each scheme's a federated and extensible naming system wherein each scheme's
specification may further restrict the syntax and semantics of specification may further restrict the syntax and semantics of
identifiers using that scheme. identifiers using that scheme.
Scheme names consist of a sequence of characters beginning with a Scheme names consist of a sequence of characters beginning with a
letter and followed by any combination of letters, digits, plus letter and followed by any combination of letters, digits, plus
("+"), period ("."), or hyphen ("-"). Although scheme is ("+"), period ("."), or hyphen ("-"). Although schemes are case-
case-insensitive, the canonical form is lowercase and documents that insensitive, the canonical form is lowercase and documents that
specify schemes must do so using lowercase letters. An specify schemes must do so with lowercase letters. An implementation
implementation should accept uppercase letters as equivalent to should accept uppercase letters as equivalent to lowercase in scheme
lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for names (e.g., allow "HTTP" as well as "http") for the sake of
the sake of robustness, but should only produce lowercase scheme robustness but should only produce lowercase scheme names for
names, for consistency. consistency.
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." ) scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
Individual schemes are not specified by this document. The process Individual schemes are not specified by this document. The process
for registration of new URI schemes is defined separately by [BCP35]. for registration of new URI schemes is defined separately by [BCP35].
The scheme registry maintains the mapping between scheme names and The scheme registry maintains the mapping between scheme names and
their specifications. Advice for designers of new URI schemes can be their specifications. Advice for designers of new URI schemes can be
found in [RFC2718]. URI scheme specifications must define their own found in [RFC2718]. URI scheme specifications must define their own
syntax such that all strings matching their scheme-specific syntax syntax so that all strings matching their scheme-specific syntax will
will also match the <absolute-URI> grammar, as described in also match the <absolute-URI> grammar, as described in Section 4.3.
Section 4.3.
When presented with a URI that violates one or more scheme-specific When presented with a URI that violates one or more scheme-specific
restrictions, the scheme-specific resolution process should flag the restrictions, the scheme-specific resolution process should flag the
reference as an error rather than ignore the unused parts; doing so reference as an error rather than ignore the unused parts; doing so
reduces the number of equivalent URIs and helps detect abuses of the reduces the number of equivalent URIs and helps detect abuses of the
generic syntax that might indicate the URI has been constructed to generic syntax, which might indicate that the URI has been
mislead the user (Section 7.6). constructed to mislead the user (Section 7.6).
3.2 Authority 3.2. Authority
Many URI schemes include a hierarchical element for a naming Many URI schemes include a hierarchical element for a naming
authority, such that governance of the name space defined by the authority so that governance of the name space defined by the
remainder of the URI is delegated to that authority (which may, in remainder of the URI is delegated to that authority (which may, in
turn, delegate it further). The generic syntax provides a common turn, delegate it further). The generic syntax provides a common
means for distinguishing an authority based on a registered name or means for distinguishing an authority based on a registered name or
server address, along with optional port and user information. server address, along with optional port and user information.
The authority component is preceded by a double slash ("//") and is The authority component is preceded by a double slash ("//") and is
terminated by the next slash ("/"), question mark ("?"), or number terminated by the next slash ("/"), question mark ("?"), or number
sign ("#") character, or by the end of the URI. sign ("#") character, or by the end of the URI.
authority = [ userinfo "@" ] host [ ":" port ] authority = [ userinfo "@" ] host [ ":" port ]
URI producers and normalizers should omit the ":" delimiter that URI producers and normalizers should omit the ":" delimiter that
separates host from port if the port component is empty. Some separates host from port if the port component is empty. Some
schemes do not allow the userinfo and/or port subcomponents. schemes do not allow the userinfo and/or port subcomponents.
If a URI contains an authority component, then the path component If a URI contains an authority component, then the path component
must either be empty or begin with a slash ("/") character. must either be empty or begin with a slash ("/") character. Non-
Non-validating parsers (those that merely separate a URI reference validating parsers (those that merely separate a URI reference into
into its major components) will often ignore the subcomponent its major components) will often ignore the subcomponent structure of
structure of authority, treating it as an opaque string from the authority, treating it as an opaque string from the double-slash to
double-slash to the first terminating delimiter, until such time as the first terminating delimiter, until such time as the URI is
the URI is dereferenced. dereferenced.
3.2.1 User Information 3.2.1. User Information
The userinfo subcomponent may consist of a user name and, optionally, The userinfo subcomponent may consist of a user name and, optionally,
scheme-specific information about how to gain authorization to access scheme-specific information about how to gain authorization to access
the resource. The user information, if present, is followed by a the resource. The user information, if present, is followed by a
commercial at-sign ("@") that delimits it from the host. commercial at-sign ("@") that delimits it from the host.
userinfo = *( unreserved / pct-encoded / sub-delims / ":" ) userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
Use of the format "user:password" in the userinfo field is Use of the format "user:password" in the userinfo field is
deprecated. Applications should not render as clear text any data deprecated. Applications should not render as clear text any data
after the first colon (":") character found within a userinfo after the first colon (":") character found within a userinfo
subcomponent unless the data after the colon is the empty string subcomponent unless the data after the colon is the empty string
(indicating no password). Applications may choose to ignore or (indicating no password). Applications may choose to ignore or
reject such data when received as part of a reference, and should reject such data when it is received as part of a reference and
reject the storage of such data in unencrypted form. The passing of should reject the storage of such data in unencrypted form. The
authentication information in clear text has proven to be a security passing of authentication information in clear text has proven to be
risk in almost every case where it has been used. a security risk in almost every case where it has been used.
Applications that render a URI for the sake of user feedback, such as Applications that render a URI for the sake of user feedback, such as
in graphical hypertext browsing, should render userinfo in a way that in graphical hypertext browsing, should render userinfo in a way that
is distinguished from the rest of a URI, when feasible. Such is distinguished from the rest of a URI, when feasible. Such
rendering will assist the user in cases where the userinfo has been rendering will assist the user in cases where the userinfo has been
misleadingly crafted to look like a trusted domain name misleadingly crafted to look like a trusted domain name
(Section 7.6). (Section 7.6).
3.2.2 Host 3.2.2. Host
The host subcomponent of authority is identified by an IP literal The host subcomponent of authority is identified by an IP literal
encapsulated within square brackets, an IPv4 address in encapsulated within square brackets, an IPv4 address in dotted-
dotted-decimal form, or a registered name. The host subcomponent is decimal form, or a registered name. The host subcomponent is case-
case-insensitive. The presence of a host subcomponent within a URI insensitive. The presence of a host subcomponent within a URI does
does not imply that the scheme requires access to the given host on not imply that the scheme requires access to the given host on the
the Internet. In many cases, the host syntax is used only for the Internet. In many cases, the host syntax is used only for the sake
sake of reusing the existing registration process created and of reusing the existing registration process created and deployed for
deployed for DNS, thus obtaining a globally unique name without the DNS, thus obtaining a globally unique name without the cost of
cost of deploying another registry. However, such use comes with its deploying another registry. However, such use comes with its own
own costs: domain name ownership may change over time for reasons not costs: domain name ownership may change over time for reasons not
anticipated by the URI producer. In other cases, the data within the anticipated by the URI producer. In other cases, the data within the
host component identifies a registered name that has nothing to do host component identifies a registered name that has nothing to do
with an Internet host. We use the name "host" for the ABNF rule with an Internet host. We use the name "host" for the ABNF rule
because that is its most common purpose, not its only purpose, and because that is its most common purpose, not its only purpose.
thus should not be considered as semantically limiting the data
within it.
host = IP-literal / IPv4address / reg-name host = IP-literal / IPv4address / reg-name
The syntax rule for host is ambiguous because it does not completely The syntax rule for host is ambiguous because it does not completely
distinguish between an IPv4address and a reg-name. In order to distinguish between an IPv4address and a reg-name. In order to
disambiguate the syntax, we apply the "first-match-wins" algorithm: disambiguate the syntax, we apply the "first-match-wins" algorithm:
If host matches the rule for IPv4address, then it should be If host matches the rule for IPv4address, then it should be
considered an IPv4 address literal and not a reg-name. Although host considered an IPv4 address literal and not a reg-name. Although host
is case-insensitive, producers and normalizers should use lowercase is case-insensitive, producers and normalizers should use lowercase
for registered names and hexadecimal addresses for the sake of for registered names and hexadecimal addresses for the sake of
uniformity, while only using uppercase letters for percent-encodings. uniformity, while only using uppercase letters for percent-encodings.
A host identified by an Internet Protocol literal address, version 6 A host identified by an Internet Protocol literal address, version 6
[RFC3513] or later, is distinguished by enclosing the IP literal [RFC3513] or later, is distinguished by enclosing the IP literal
within square brackets ("[" and "]"). This is the only place where within square brackets ("[" and "]"). This is the only place where
square bracket characters are allowed in the URI syntax. In square bracket characters are allowed in the URI syntax. In
anticipation of future, as-yet-undefined IP literal address formats, anticipation of future, as-yet-undefined IP literal address formats,
an optional version flag may be used to indicate such a format an implementation may use an optional version flag to indicate such a
explicitly rather than relying on heuristic determination. format explicitly rather than rely on heuristic determination.
IP-literal = "[" ( IPv6address / IPvFuture ) "]" IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" ) IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
The version flag does not indicate the IP version; rather, it The version flag does not indicate the IP version; rather, it
indicates future versions of the literal format. As such, indicates future versions of the literal format. As such,
implementations must not provide the version flag for existing IPv4 implementations must not provide the version flag for the existing
and IPv6 literal addresses. If a URI containing an IP-literal that IPv4 and IPv6 literal address forms described below. If a URI
starts with "v" (case-insensitive), indicating that the version flag containing an IP-literal that starts with "v" (case-insensitive),
is present, is dereferenced by an application that does not know the indicating that the version flag is present, is dereferenced by an
meaning of that version flag, then the application should return an application that does not know the meaning of that version flag, then
appropriate error for "address mechanism not supported". the application should return an appropriate error for "address
mechanism not supported".
A host identified by an IPv6 literal address is represented inside A host identified by an IPv6 literal address is represented inside
the square brackets without a preceding version flag. The ABNF the square brackets without a preceding version flag. The ABNF
provided here is a translation of the text definition of an IPv6 provided here is a translation of the text definition of an IPv6
literal address provided in [RFC3513]. A 128-bit IPv6 address is literal address provided in [RFC3513]. This syntax does not support
divided into eight 16-bit pieces. Each piece is represented IPv6 scoped addressing zone identifiers.
numerically in case-insensitive hexadecimal, using one to four
hexadecimal digits (leading zeroes are permitted). The eight encoded A 128-bit IPv6 address is divided into eight 16-bit pieces. Each
pieces are given most-significant first, separated by colon piece is represented numerically in case-insensitive hexadecimal,
characters. Optionally, the least-significant two pieces may instead using one to four hexadecimal digits (leading zeroes are permitted).
be represented in IPv4 address textual format. A sequence of one or The eight encoded pieces are given most-significant first, separated
more consecutive zero-valued 16-bit pieces within the address may be by colon characters. Optionally, the least-significant two pieces
elided, omitting all their digits and leaving exactly two consecutive may instead be represented in IPv4 address textual format. A
colons in their place to mark the elision. sequence of one or more consecutive zero-valued 16-bit pieces within
the address may be elided, omitting all their digits and leaving
exactly two consecutive colons in their place to mark the elision.
IPv6address = 6( h16 ":" ) ls32 IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32 / "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32 / [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32 / [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16 / [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::" / [ *6( h16 ":" ) h16 ] "::"
skipping to change at page 20, line 38 skipping to change at page 20, line 48
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9 dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99 / %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199 / "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249 / "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255 / "25" %x30-35 ; 250-255
A host identified by a registered name is a sequence of characters A host identified by a registered name is a sequence of characters
that is usually intended for lookup within a locally-defined host or usually intended for lookup within a locally defined host or service
service name registry, though the URI's scheme-specific semantics may name registry, though the URI's scheme-specific semantics may require
require that a specific registry (or fixed name table) be used that a specific registry (or fixed name table) be used instead. The
instead. The most common name registry mechanism is the Domain Name most common name registry mechanism is the Domain Name System (DNS).
System (DNS). A registered name intended for lookup in the DNS uses A registered name intended for lookup in the DNS uses the syntax
the syntax defined in Section 3.5 of [RFC1034] and Section 2.1 of defined in Section 3.5 of [RFC1034] and Section 2.1 of [RFC1123].
[RFC1123]. Such a name consists of a sequence of domain labels Such a name consists of a sequence of domain labels separated by ".",
separated by ".", each domain label starting and ending with an each domain label starting and ending with an alphanumeric character
alphanumeric character and possibly also containing "-" characters. and possibly also containing "-" characters. The rightmost domain
The rightmost domain label of a fully qualified domain name in DNS label of a fully qualified domain name in DNS may be followed by a
may be followed by a single "." and should be followed by one if it single "." and should be if it is necessary to distinguish between
is necessary to distinguish between the complete domain name and some the complete domain name and some local domain.
local domain.
reg-name = *( unreserved / pct-encoded / sub-delims ) reg-name = *( unreserved / pct-encoded / sub-delims )
If the URI scheme defines a default for host, then that default If the URI scheme defines a default for host, then that default
applies when the host subcomponent is undefined or when the applies when the host subcomponent is undefined or when the
registered name is empty (zero length). For example, the "file" URI registered name is empty (zero length). For example, the "file" URI
scheme is defined such that no authority, an empty host, and scheme is defined so that no authority, an empty host, and
"localhost" all mean the end-user's machine, whereas the "http" "localhost" all mean the end-user's machine, whereas the "http"
scheme considers a missing authority or empty host to be invalid. scheme considers a missing authority or empty host invalid.
This specification does not mandate a particular registered name This specification does not mandate a particular registered name
lookup technology and therefore does not restrict the syntax of lookup technology and therefore does not restrict the syntax of reg-
reg-name beyond that necessary for interoperability. Instead, it name beyond what is necessary for interoperability. Instead, it
delegates the issue of registered name syntax conformance to the delegates the issue of registered name syntax conformance to the
operating system of each application performing URI resolution, and operating system of each application performing URI resolution, and
that operating system decides what it will allow for the purpose of that operating system decides what it will allow for the purpose of
host identification. A URI resolution implementation might use DNS, host identification. A URI resolution implementation might use DNS,
host tables, yellow pages, NetInfo, WINS, or any other system for host tables, yellow pages, NetInfo, WINS, or any other system for
lookup of registered names. However, a globally-scoped naming lookup of registered names. However, a globally scoped naming
system, such as DNS fully-qualified domain names, is necessary for system, such as DNS fully qualified domain names, is necessary for
URIs that are intended to have global scope. URI producers should URIs intended to have global scope. URI producers should use names
use names that conform to the DNS syntax, even when use of DNS is not that conform to the DNS syntax, even when use of DNS is not
immediately apparent, and should limit such names to no more than 255 immediately apparent, and should limit these names to no more than
characters in length. 255 characters in length.
The reg-name syntax allows percent-encoded octets in order to The reg-name syntax allows percent-encoded octets in order to
represent non-ASCII registered names in a uniform way that is represent non-ASCII registered names in a uniform way that is
independent of the underlying name resolution technology; such independent of the underlying name resolution technology. Non-ASCII
non-ASCII characters must first be encoded according to UTF-8 [STD63] characters must first be encoded according to UTF-8 [STD63], and then
and then each octet of the corresponding UTF-8 sequence must be each octet of the corresponding UTF-8 sequence must be percent-
percent-encoded to be represented as URI characters. URI producing encoded to be represented as URI characters. URI producing
applications must not use percent-encoding in host unless it is used applications must not use percent-encoding in host unless it is used
to represent a UTF-8 character sequence. When a non-ASCII registered to represent a UTF-8 character sequence. When a non-ASCII registered
name represents an internationalized domain name intended for name represents an internationalized domain name intended for
resolution via the DNS, the name must be transformed to the IDNA resolution via the DNS, the name must be transformed to the IDNA
encoding [RFC3490] prior to name lookup. URI producers should encoding [RFC3490] prior to name lookup. URI producers should
provide such registered names in the IDNA encoding, rather than a provide these registered names in the IDNA encoding, rather than a
percent-encoding, if they wish to maximize interoperability with percent-encoding, if they wish to maximize interoperability with
legacy URI resolvers. legacy URI resolvers.
3.2.3 Port 3.2.3. Port
The port subcomponent of authority is designated by an optional port The port subcomponent of authority is designated by an optional port
number in decimal following the host and delimited from it by a number in decimal following the host and delimited from it by a
single colon (":") character. single colon (":") character.
port = *DIGIT port = *DIGIT
A scheme may define a default port. For example, the "http" scheme A scheme may define a default port. For example, the "http" scheme
defines a default port of "80", corresponding to its reserved TCP defines a default port of "80", corresponding to its reserved TCP
port number. The type of port designated by the port number (e.g., port number. The type of port designated by the port number (e.g.,
TCP, UDP, SCTP, etc.) is defined by the URI scheme. URI producers TCP, UDP, SCTP) is defined by the URI scheme. URI producers and
and normalizers should omit the port component and its ":" delimiter normalizers should omit the port component and its ":" delimiter if
if port is empty or its value would be the same as the scheme's port is empty or if its value would be the same as that of the
default. scheme's default.
3.3 Path 3.3. Path
The path component contains data, usually organized in hierarchical The path component contains data, usually organized in hierarchical
form, that, along with data in the non-hierarchical query component form, that, along with data in the non-hierarchical query component
(Section 3.4), serves to identify a resource within the scope of the (Section 3.4), serves to identify a resource within the scope of the
URI's scheme and naming authority (if any). The path is terminated URI's scheme and naming authority (if any). The path is terminated
by the first question mark ("?") or number sign ("#") character, or by the first question mark ("?") or number sign ("#") character, or
by the end of the URI. by the end of the URI.
If a URI contains an authority component, then the path component If a URI contains an authority component, then the path component
must either be empty or begin with a slash ("/") character. If a URI must either be empty or begin with a slash ("/") character. If a URI
skipping to change at page 23, line 9 skipping to change at page 23, line 22
("/") character. A path is always defined for a URI, though the ("/") character. A path is always defined for a URI, though the
defined path may be empty (zero length). Use of the slash character defined path may be empty (zero length). Use of the slash character
to indicate hierarchy is only required when a URI will be used as the to indicate hierarchy is only required when a URI will be used as the
context for relative references. For example, the URI context for relative references. For example, the URI
<mailto:[email protected]> has a path of "[email protected]", whereas <mailto:[email protected]> has a path of "[email protected]", whereas
the URI <foo://info.example.com?fred> has an empty path. the URI <foo://info.example.com?fred> has an empty path.
The path segments "." and "..", also known as dot-segments, are The path segments "." and "..", also known as dot-segments, are
defined for relative reference within the path name hierarchy. They defined for relative reference within the path name hierarchy. They
are intended for use at the beginning of a relative-path reference are intended for use at the beginning of a relative-path reference
(Section 4.2) for indicating relative position within the (Section 4.2) to indicate relative position within the hierarchical
hierarchical tree of names. This is similar to their role within tree of names. This is similar to their role within some operating
some operating systems' file directory structure to indicate the systems' file directory structures to indicate the current directory
current directory and parent directory, respectively. However, and parent directory, respectively. However, unlike in a file
unlike a file system, these dot-segments are only interpreted within system, these dot-segments are only interpreted within the URI path
the URI path hierarchy and are removed as part of the resolution hierarchy and are removed as part of the resolution process (Section
process (Section 5.2). 5.2).
Aside from dot-segments in hierarchical paths, a path segment is Aside from dot-segments in hierarchical paths, a path segment is
considered opaque by the generic syntax. URI-producing applications considered opaque by the generic syntax. URI producing applications
often use the reserved characters allowed in a segment for the often use the reserved characters allowed in a segment to delimit
purpose of delimiting scheme-specific or dereference-handler-specific scheme-specific or dereference-handler-specific subcomponents. For
subcomponents. For example, the semicolon (";") and equals ("=") example, the semicolon (";") and equals ("=") reserved characters are
reserved characters are often used for delimiting parameters and often used to delimit parameters and parameter values applicable to
parameter values applicable to that segment. The comma (",") that segment. The comma (",") reserved character is often used for
reserved character is often used for similar purposes. For example, similar purposes. For example, one URI producer might use a segment
one URI producer might use a segment like "name;v=1.1" to indicate a such as "name;v=1.1" to indicate a reference to version 1.1 of
reference to version 1.1 of "name", whereas another might use a "name", whereas another might use a segment such as "name,1.1" to
segment like "name,1.1" to indicate the same. Parameter types may be indicate the same. Parameter types may be defined by scheme-specific
defined by scheme-specific semantics, but in most cases the syntax of semantics, but in most cases the syntax of a parameter is specific to
a parameter is specific to the implementation of the URI's the implementation of the URI's dereferencing algorithm.
dereferencing algorithm.
3.4 Query 3.4. Query
The query component contains non-hierarchical data that, along with The query component contains non-hierarchical data that, along with
data in the path component (Section 3.3), serves to identify a data in the path component (Section 3.3), serves to identify a
resource within the scope of the URI's scheme and naming authority resource within the scope of the URI's scheme and naming authority
(if any). The query component is indicated by the first question (if any). The query component is indicated by the first question
mark ("?") character and terminated by a number sign ("#") character mark ("?") character and terminated by a number sign ("#") character
or by the end of the URI. or by the end of the URI.
query = *( pchar / "/" / "?" ) query = *( pchar / "/" / "?" )
The characters slash ("/") and question mark ("?") may represent data The characters slash ("/") and question mark ("?") may represent data
within the query component. Beware that some older, erroneous within the query component. Beware that some older, erroneous
implementations may not handle such data correctly when used as the implementations may not handle such data correctly when it is used as
base URI for relative references (Section 5.1), apparently because the base URI for relative references (Section 5.1), apparently
they fail to to distinguish query data from path data when looking because they fail to distinguish query data from path data when
for hierarchical separators. However, since query components are looking for hierarchical separators. However, as query components
often used to carry identifying information in the form of are often used to carry identifying information in the form of
"key=value" pairs, and one frequently used value is a reference to "key=value" pairs and one frequently used value is a reference to
another URI, it is sometimes better for usability to avoid another URI, it is sometimes better for usability to avoid percent-
percent-encoding those characters. encoding those characters.
3.5 Fragment 3.5. Fragment
The fragment identifier component of a URI allows indirect The fragment identifier component of a URI allows indirect
identification of a secondary resource by reference to a primary identification of a secondary resource by reference to a primary
resource and additional identifying information. The identified resource and additional identifying information. The identified
secondary resource may be some portion or subset of the primary secondary resource may be some portion or subset of the primary
resource, some view on representations of the primary resource, or resource, some view on representations of the primary resource, or
some other resource defined or described by those representations. A some other resource defined or described by those representations. A
fragment identifier component is indicated by the presence of a fragment identifier component is indicated by the presence of a
number sign ("#") character and terminated by the end of the URI. number sign ("#") character and terminated by the end of the URI.
fragment = *( pchar / "/" / "?" ) fragment = *( pchar / "/" / "?" )
The semantics of a fragment identifier are defined by the set of The semantics of a fragment identifier are defined by the set of
representations that might result from a retrieval action on the representations that might result from a retrieval action on the
primary resource. The fragment's format and resolution is therefore primary resource. The fragment's format and resolution is therefore
dependent on the media type [RFC2046] of a potentially retrieved dependent on the media type [RFC2046] of a potentially retrieved
representation, even though such a retrieval is only performed if the representation, even though such a retrieval is only performed if the
URI is dereferenced. If no such representation exists, then the URI is dereferenced. If no such representation exists, then the
semantics of the fragment are considered unknown and, effectively, semantics of the fragment are considered unknown and are effectively
unconstrained. Fragment identifier semantics are independent of the unconstrained. Fragment identifier semantics are independent of the
URI scheme and thus cannot be redefined by scheme specifications. URI scheme and thus cannot be redefined by scheme specifications.
Individual media types may define their own restrictions on, or Individual media types may define their own restrictions on or
structure within, the fragment identifier syntax for specifying structures within the fragment identifier syntax for specifying
different types of subsets, views, or external references that are different types of subsets, views, or external references that are
identifiable as secondary resources by that media type. If the identifiable as secondary resources by that media type. If the
primary resource has multiple representations, as is often the case primary resource has multiple representations, as is often the case
for resources whose representation is selected based on attributes of for resources whose representation is selected based on attributes of
the retrieval request (a.k.a., content negotiation), then whatever is the retrieval request (a.k.a., content negotiation), then whatever is
identified by the fragment should be consistent across all of those identified by the fragment should be consistent across all of those
representations: each representation should either define the representations. Each representation should either define the
fragment such that it corresponds to the same secondary resource, fragment so that it corresponds to the same secondary resource,
regardless of how it is represented, or the fragment should be left regardless of how it is represented, or should leave the fragment
undefined by the representation (i.e., not found). undefined (i.e., not found).
As with any URI, use of a fragment identifier component does not As with any URI, use of a fragment identifier component does not
imply that a retrieval action will take place. A URI with a fragment imply that a retrieval action will take place. A URI with a fragment
identifier may be used to refer to the secondary resource without any identifier may be used to refer to the secondary resource without any
implication that the primary resource is accessible or will ever be implication that the primary resource is accessible or will ever be
accessed. accessed.
Fragment identifiers have a special role in information retrieval Fragment identifiers have a special role in information retrieval
systems as the primary form of client-side indirect referencing, systems as the primary form of client-side indirect referencing,
allowing an author to specifically identify those aspects of an allowing an author to specifically identify aspects of an existing
existing resource that are only indirectly provided by the resource resource that are only indirectly provided by the resource owner. As
owner. As such, the fragment identifier is not used in the such, the fragment identifier is not used in the scheme-specific
scheme-specific processing of a URI; instead, the fragment identifier processing of a URI; instead, the fragment identifier is separated
is separated from the rest of the URI prior to a dereference, and from the rest of the URI prior to a dereference, and thus the
thus the identifying information within the fragment itself is identifying information within the fragment itself is dereferenced
dereferenced solely by the user agent and regardless of the URI solely by the user agent, regardless of the URI scheme. Although
scheme. Although this separate handling is often perceived to be a this separate handling is often perceived to be a loss of
loss of information, particularly in regards to accurate redirection information, particularly for accurate redirection of references as
of references as resources move over time, it also serves to prevent resources move over time, it also serves to prevent information
information providers from denying reference authors the right to providers from denying reference authors the right to refer to
selectively refer to information within a resource. Indirect information within a resource selectively. Indirect referencing also
referencing also provides additional flexibility and extensibility to provides additional flexibility and extensibility to systems that use
systems that use URIs, since new media types are easier to define and URIs, as new media types are easier to define and deploy than new
deploy than new schemes of identification. schemes of identification.
The characters slash ("/") and question mark ("?") are allowed to The characters slash ("/") and question mark ("?") are allowed to
represent data within the fragment identifier. Beware that some represent data within the fragment identifier. Beware that some
older, erroneous implementations may not handle such data correctly older, erroneous implementations may not handle this data correctly
when used as the base URI for relative references (Section 5.1). when it is used as the base URI for relative references (Section
5.1).
4. Usage 4. Usage
When applications make reference to a URI, they do not always use the When applications make reference to a URI, they do not always use the
full form of reference defined by the "URI" syntax rule. In order to full form of reference defined by the "URI" syntax rule. To save
save space and take advantage of hierarchical locality, many Internet space and take advantage of hierarchical locality, many Internet
protocol elements and media type formats allow an abbreviation of a protocol elements and media type formats allow an abbreviation of a
URI, while others restrict the syntax to a particular form of URI. URI, whereas others restrict the syntax to a particular form of URI.
We define the most common forms of reference syntax in this We define the most common forms of reference syntax in this
specification because they impact and depend upon the design of the specification because they impact and depend upon the design of the
generic syntax, requiring a uniform parsing algorithm in order to be generic syntax, requiring a uniform parsing algorithm in order to be
interpreted consistently. interpreted consistently.
4.1 URI Reference 4.1. URI Reference
URI-reference is used to denote the most common usage of a resource URI-reference is used to denote the most common usage of a resource
identifier. identifier.
URI-reference = URI / relative-ref URI-reference = URI / relative-ref
A URI-reference is either a URI or a relative reference. If the A URI-reference is either a URI or a relative reference. If the
URI-reference's prefix does not match the syntax of a scheme followed URI-reference's prefix does not match the syntax of a scheme followed
by its colon separator, then the URI-reference is a relative by its colon separator, then the URI-reference is a relative
reference. reference.
A URI-reference is typically parsed first into the five URI A URI-reference is typically parsed first into the five URI
components, in order to determine what components are present and components, in order to determine what components are present and
whether or not the reference is relative, after which each component whether the reference is relative. Then, each component is parsed
is parsed for its subparts and their validation. The ABNF of for its subparts and their validation. The ABNF of URI-reference,
URI-reference, along with the "first-match-wins" disambiguation rule, along with the "first-match-wins" disambiguation rule, is sufficient
is sufficient to define a validating parser for the generic syntax. to define a validating parser for the generic syntax. Readers
Readers familiar with regular expressions should see Appendix B for familiar with regular expressions should see Appendix B for an
an example of a non-validating URI-reference parser that will take example of a non-validating URI-reference parser that will take any
any given string and extract the URI components. given string and extract the URI components.
4.2 Relative Reference 4.2. Relative Reference
A relative reference takes advantage of the hierarchical syntax A relative reference takes advantage of the hierarchical syntax
(Section 1.2.3) in order to express a URI reference relative to the (Section 1.2.3) to express a URI reference relative to the name space
name space of another hierarchical URI. of another hierarchical URI.
relative-ref = relative-part [ "?" query ] [ "#" fragment ] relative-ref = relative-part [ "?" query ] [ "#" fragment ]
relative-part = "//" authority path-abempty relative-part = "//" authority path-abempty
/ path-absolute / path-absolute
/ path-noscheme / path-noscheme
/ path-empty / path-empty
The URI referred to by a relative reference, also known as the target The URI referred to by a relative reference, also known as the target
URI, is obtained by applying the reference resolution algorithm of URI, is obtained by applying the reference resolution algorithm of
Section 5. Section 5.
A relative reference that begins with two slash characters is termed A relative reference that begins with two slash characters is termed
a network-path reference; such references are rarely used. A a network-path reference; such references are rarely used. A
relative reference that begins with a single slash character is relative reference that begins with a single slash character is
termed an absolute-path reference. A relative reference that does termed an absolute-path reference. A relative reference that does
not begin with a slash character is termed a relative-path reference. not begin with a slash character is termed a relative-path reference.
A path segment that contains a colon character (e.g., "this:that") A path segment that contains a colon character (e.g., "this:that")
cannot be used as the first segment of a relative-path reference cannot be used as the first segment of a relative-path reference, as
because it would be mistaken for a scheme name. Such a segment must it would be mistaken for a scheme name. Such a segment must be
be preceded by a dot-segment (e.g., "./this:that") to make a preceded by a dot-segment (e.g., "./this:that") to make a relative-
relative-path reference. path reference.
4.3 Absolute URI 4.3. Absolute URI
Some protocol elements allow only the absolute form of a URI without Some protocol elements allow only the absolute form of a URI without
a fragment identifier. For example, defining a base URI for later a fragment identifier. For example, defining a base URI for later
use by relative references calls for an absolute-URI syntax rule that use by relative references calls for an absolute-URI syntax rule that
does not allow a fragment. does not allow a fragment.
absolute-URI = scheme ":" hier-part [ "?" query ] absolute-URI = scheme ":" hier-part [ "?" query ]
URI scheme specifications must define their own syntax such that all URI scheme specifications must define their own syntax so that all
strings matching their scheme-specific syntax will also match the strings matching their scheme-specific syntax will also match the
<absolute-URI> grammar. Scheme specifications are not responsible <absolute-URI> grammar. Scheme specifications will not define
for defining fragment identifier syntax or usage, regardless of its fragment identifier syntax or usage, regardless of its applicability
applicability to resources identifiable via that scheme, since to resources identifiable via that scheme, as fragment identification
fragment identification is orthogonal to scheme definition. However, is orthogonal to scheme definition. However, scheme specifications
scheme specifications are encouraged to include a wide range of are encouraged to include a wide range of examples, including
examples, including examples that show use of the scheme's URIs with examples that show use of the scheme's URIs with fragment identifiers
fragment identifiers when such usage is appropriate. when such usage is appropriate.
4.4 Same-document Reference 4.4. Same-Document Reference
When a URI reference refers to a URI that is, aside from its fragment When a URI reference refers to a URI that is, aside from its fragment
component (if any), identical to the base URI (Section 5.1), that component (if any), identical to the base URI (Section 5.1), that
reference is called a "same-document" reference. The most frequent reference is called a "same-document" reference. The most frequent
examples of same-document references are relative references that are examples of same-document references are relative references that are
empty or include only the number sign ("#") separator followed by a empty or include only the number sign ("#") separator followed by a
fragment identifier. fragment identifier.
When a same-document reference is dereferenced for the purpose of a When a same-document reference is dereferenced for a retrieval
retrieval action, the target of that reference is defined to be action, the target of that reference is defined to be within the same
within the same entity (representation, document, or message) as the entity (representation, document, or message) as the reference;
reference; therefore, a dereference should not result in a new therefore, a dereference should not result in a new retrieval action.
retrieval action.
Normalization of the base and target URIs prior to their comparison, Normalization of the base and target URIs prior to their comparison,
as described in Section 6.2.2 and Section 6.2.3, is allowed but as described in Sections 6.2.2 and 6.2.3, is allowed but rarely
rarely performed in practice. Normalization may increase the set of performed in practice. Normalization may increase the set of same-
same-document references, which may be of benefit to some caching document references, which may be of benefit to some caching
applications. As such, reference authors should not assume that a applications. As such, reference authors should not assume that a
slightly different, though equivalent, reference URI will (or will slightly different, though equivalent, reference URI will (or will
not) be interpreted as a same-document reference by any given not) be interpreted as a same-document reference by any given
application. application.
4.5 Suffix Reference 4.5. Suffix Reference
The URI syntax is designed for unambiguous reference to resources and The URI syntax is designed for unambiguous reference to resources and
extensibility via the URI scheme. However, as URI identification and extensibility via the URI scheme. However, as URI identification and
usage have become commonplace, traditional media (television, radio, usage have become commonplace, traditional media (television, radio,
newspapers, billboards, etc.) have increasingly used a suffix of the newspapers, billboards, etc.) have increasingly used a suffix of the
URI as a reference, consisting of only the authority and path URI as a reference, consisting of only the authority and path
portions of the URI, such as portions of the URI, such as
www.w3.org/Addressing/ www.w3.org/Addressing/
or simply a DNS registered name on its own. Such references are or simply a DNS registered name on its own. Such references are
primarily intended for human interpretation, rather than for primarily intended for human interpretation rather than for machines,
machines, with the assumption that context-based heuristics are with the assumption that context-based heuristics are sufficient to
sufficient to complete the URI (e.g., most registered names beginning complete the URI (e.g., most registered names beginning with "www"
with "www" are likely to have a URI prefix of "http://"). Although are likely to have a URI prefix of "http://"). Although there is no
there is no standard set of heuristics for disambiguating a URI standard set of heuristics for disambiguating a URI suffix, many
suffix, many client implementations allow them to be entered by the client implementations allow them to be entered by the user and
user and heuristically resolved. heuristically resolved.
While this practice of using suffix references is common, it should Although this practice of using suffix references is common, it
be avoided whenever possible and never used in situations where should be avoided whenever possible and should never be used in
long-term references are expected. The heuristics noted above will situations where long-term references are expected. The heuristics
change over time, particularly when a new URI scheme becomes popular, noted above will change over time, particularly when a new URI scheme
and are often incorrect when used out of context. Furthermore, they becomes popular, and are often incorrect when used out of context.
can lead to security issues along the lines of those described in Furthermore, they can lead to security issues along the lines of
[RFC1535]. those described in [RFC1535].
Since a URI suffix has the same syntax as a relative-path reference, As a URI suffix has the same syntax as a relative-path reference, a
a suffix reference cannot be used in contexts where a relative suffix reference cannot be used in contexts where a relative
reference is expected. As a result, suffix references are limited to reference is expected. As a result, suffix references are limited to
those places where there is no defined base URI, such as dialog boxes places where there is no defined base URI, such as dialog boxes and
and off-line advertisements. off-line advertisements.
5. Reference Resolution 5. Reference Resolution
This section defines the process of resolving a URI reference within This section defines the process of resolving a URI reference within
a context that allows relative references, such that the result is a a context that allows relative references so that the result is a
string matching the <URI> syntax rule of Section 3. string matching the <URI> syntax rule of Section 3.
5.1 Establishing a Base URI 5.1. Establishing a Base URI
The term "relative" implies that there exists a "base URI" against The term "relative" implies that a "base URI" exists against which
which the relative reference is applied. Aside from fragment-only the relative reference is applied. Aside from fragment-only
references (Section 4.4), relative references are only usable when a references (Section 4.4), relative references are only usable when a
base URI is known. A base URI must be established by the parser base URI is known. A base URI must be established by the parser
prior to parsing URI references that might be relative. A base URI prior to parsing URI references that might be relative. A base URI
must conform to the <absolute-URI> syntax rule (Section 4.3): if the must conform to the <absolute-URI> syntax rule (Section 4.3). If the
base URI is obtained from a URI reference, then that reference must base URI is obtained from a URI reference, then that reference must
be converted to absolute form and stripped of any fragment component be converted to absolute form and stripped of any fragment component
prior to use as a base URI. prior to its use as a base URI.
The base URI of a reference can be established in one of four ways, The base URI of a reference can be established in one of four ways,
discussed below in order of precedence. The order of precedence can discussed below in order of precedence. The order of precedence can
be thought of in terms of layers, where the innermost defined base be thought of in terms of layers, where the innermost defined base
URI has the highest precedence. This can be visualized graphically URI has the highest precedence. This can be visualized graphically
as: as follows:
.----------------------------------------------------------. .----------------------------------------------------------.
| .----------------------------------------------------. | | .----------------------------------------------------. |
| | .----------------------------------------------. | | | | .----------------------------------------------. | |
| | | .----------------------------------------. | | | | | | .----------------------------------------. | | |
| | | | .----------------------------------. | | | | | | | | .----------------------------------. | | | |
| | | | | <relative-reference> | | | | | | | | | | <relative-reference> | | | | |
| | | | `----------------------------------' | | | | | | | | `----------------------------------' | | | |
| | | | (5.1.1) Base URI embedded in content | | | | | | | | (5.1.1) Base URI embedded in content | | | |
| | | `----------------------------------------' | | | | | | `----------------------------------------' | | |
| | | (5.1.2) Base URI of the encapsulating entity | | | | | | (5.1.2) Base URI of the encapsulating entity | | |
| | | (message, representation, or none) | | | | | | (message, representation, or none) | | |
| | `----------------------------------------------' | | | | `----------------------------------------------' | |
| | (5.1.3) URI used to retrieve the entity | | | | (5.1.3) URI used to retrieve the entity | |
| `----------------------------------------------------' | | `----------------------------------------------------' |
| (5.1.4) Default Base URI (application-dependent) | | (5.1.4) Default Base URI (application-dependent) |
`----------------------------------------------------------' `----------------------------------------------------------'
5.1.1 Base URI Embedded in Content 5.1.1. Base URI Embedded in Content
Within certain media types, a base URI for relative references can be Within certain media types, a base URI for relative references can be
embedded within the content itself such that it can be readily embedded within the content itself so that it can be readily obtained
obtained by a parser. This can be useful for descriptive documents, by a parser. This can be useful for descriptive documents, such as
such as tables of content, which may be transmitted to others through tables of contents, which may be transmitted to others through
protocols other than their usual retrieval context (e.g., E-Mail or protocols other than their usual retrieval context (e.g., email or
USENET news). USENET news).
It is beyond the scope of this specification to specify how, for each It is beyond the scope of this specification to specify how, for each
media type, a base URI can be embedded. The appropriate syntax, when media type, a base URI can be embedded. The appropriate syntax, when
available, is described by the data format specification associated available, is described by the data format specification associated
with each media type. with each media type.
5.1.2 Base URI from the Encapsulating Entity 5.1.2. Base URI from the Encapsulating Entity
If no base URI is embedded, the base URI is defined by the If no base URI is embedded, the base URI is defined by the
representation's retrieval context. For a document that is enclosed representation's retrieval context. For a document that is enclosed
within another entity, such as a message or archive, the retrieval within another entity, such as a message or archive, the retrieval
context is that entity; thus, the default base URI of a context is that entity. Thus, the default base URI of a
representation is the base URI of the entity in which the representation is the base URI of the entity in which the
representation is encapsulated. representation is encapsulated.
A mechanism for embedding a base URI within MIME container types A mechanism for embedding a base URI within MIME container types
(e.g., the message and multipart types) is defined by MHTML (e.g., the message and multipart types) is defined by MHTML
[RFC2557]. Protocols that do not use the MIME message header syntax, [RFC2557]. Protocols that do not use the MIME message header syntax,
but do allow some form of tagged metadata to be included within but that do allow some form of tagged metadata to be included within
messages, may define their own syntax for defining a base URI as part messages, may define their own syntax for defining a base URI as part
of a message. of a message.
5.1.3 Base URI from the Retrieval URI 5.1.3. Base URI from the Retrieval URI
If no base URI is embedded and the representation is not encapsulated If no base URI is embedded and the representation is not encapsulated
within some other entity, then, if a URI was used to retrieve the within some other entity, then, if a URI was used to retrieve the
representation, that URI shall be considered the base URI. Note that representation, that URI shall be considered the base URI. Note that
if the retrieval was the result of a redirected request, the last URI if the retrieval was the result of a redirected request, the last URI
used (i.e., the URI that resulted in the actual retrieval of the used (i.e., the URI that resulted in the actual retrieval of the
representation) is the base URI. representation) is the base URI.
5.1.4 Default Base URI 5.1.4. Default Base URI
If none of the conditions described above apply, then the base URI is If none of the conditions described above apply, then the base URI is
defined by the context of the application. Since this definition is defined by the context of the application. As this definition is
necessarily application-dependent, failing to define a base URI using necessarily application-dependent, failing to define a base URI by
one of the other methods may result in the same content being using one of the other methods may result in the same content being
interpreted differently by different types of application. interpreted differently by different types of applications.
A sender of a representation containing relative references is A sender of a representation containing relative references is
responsible for ensuring that a base URI for those references can be responsible for ensuring that a base URI for those references can be
established. Aside from fragment-only references, relative established. Aside from fragment-only references, relative
references can only be used reliably in situations where the base URI references can only be used reliably in situations where the base URI
is well-defined. is well defined.
5.2 Relative Resolution 5.2. Relative Resolution
This section describes an algorithm for converting a URI reference This section describes an algorithm for converting a URI reference
that might be relative to a given base URI into the parsed components that might be relative to a given base URI into the parsed components
of the reference's target. The components can then be recomposed, as of the reference's target. The components can then be recomposed, as
described in Section 5.3, to form the target URI. This algorithm described in Section 5.3, to form the target URI. This algorithm
provides definitive results that can be used to test the output of provides definitive results that can be used to test the output of
other implementations. Applications may implement relative reference other implementations. Applications may implement relative reference
resolution using some other algorithm, provided that the results resolution by using some other algorithm, provided that the results
match what would be given by this algorithm. match what would be given by this one.
5.2.1 Pre-parse the Base URI 5.2.1. Pre-parse the Base URI
The base URI (Base) is established according to the procedure of The base URI (Base) is established according to the procedure of
Section 5.1 and parsed into the five main components described in Section 5.1 and parsed into the five main components described in
Section 3. Note that only the scheme component is required to be Section 3. Note that only the scheme component is required to be
present in a base URI; the other components may be empty or present in a base URI; the other components may be empty or
undefined. A component is undefined if its associated delimiter does undefined. A component is undefined if its associated delimiter does
not appear in the URI reference; the path component is never not appear in the URI reference; the path component is never
undefined, though it may be empty. undefined, though it may be empty.
Normalization of the base URI, as described in Section 6.2.2 and Normalization of the base URI, as described in Sections 6.2.2 and
Section 6.2.3, is optional. A URI reference must be transformed to 6.2.3, is optional. A URI reference must be transformed to its
its target URI before it can be normalized. target URI before it can be normalized.
5.2.2 Transform References 5.2.2. Transform References
For each URI reference (R), the following pseudocode describes an For each URI reference (R), the following pseudocode describes an
algorithm for transforming R into its target URI (T): algorithm for transforming R into its target URI (T):
-- The URI reference is parsed into the five URI components -- The URI reference is parsed into the five URI components
-- --
(R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R); (R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
-- A non-strict parser may ignore a scheme in the reference -- A non-strict parser may ignore a scheme in the reference
-- if it is identical to the base URI's scheme. -- if it is identical to the base URI's scheme.
skipping to change at page 32, line 5 skipping to change at page 32, line 38
endif; endif;
T.query = R.query; T.query = R.query;
endif; endif;
T.authority = Base.authority; T.authority = Base.authority;
endif; endif;
T.scheme = Base.scheme; T.scheme = Base.scheme;
endif; endif;
T.fragment = R.fragment; T.fragment = R.fragment;
5.2.3 Merge Paths 5.2.3. Merge Paths
The pseudocode above refers to a "merge" routine for merging a The pseudocode above refers to a "merge" routine for merging a
relative-path reference with the path of the base URI. This is relative-path reference with the path of the base URI. This is
accomplished as follows: accomplished as follows:
o If the base URI has a defined authority component and an empty o If the base URI has a defined authority component and an empty
path, then return a string consisting of "/" concatenated with the path, then return a string consisting of "/" concatenated with the
reference's path; otherwise, reference's path; otherwise,
o Return a string consisting of the reference's path component o return a string consisting of the reference's path component
appended to all but the last segment of the base URI's path (i.e., appended to all but the last segment of the base URI's path (i.e.,
excluding any characters after the right-most "/" in the base URI excluding any characters after the right-most "/" in the base URI
path, or excluding the entire base URI path if it does not contain path, or excluding the entire base URI path if it does not contain
any "/" characters). any "/" characters).
5.2.4 Remove Dot Segments 5.2.4. Remove Dot Segments
The pseudocode also refers to a "remove_dot_segments" routine for The pseudocode also refers to a "remove_dot_segments" routine for
interpreting and removing the special "." and ".." complete path interpreting and removing the special "." and ".." complete path
segments from a referenced path. This is done after the path is segments from a referenced path. This is done after the path is
extracted from a reference, whether or not the path was relative, in extracted from a reference, whether or not the path was relative, in
order to remove any invalid or extraneous dot-segments prior to order to remove any invalid or extraneous dot-segments prior to
forming the target URI. Although there are many ways to accomplish forming the target URI. Although there are many ways to accomplish
this removal process, we describe a simple method using two string this removal process, we describe a simple method using two string
buffers. buffers.
1. The input buffer is initialized with the now-appended path 1. The input buffer is initialized with the now-appended path
components and the output buffer is initialized to the empty components and the output buffer is initialized to the empty
string. string.
2. While the input buffer is not empty, loop: 2. While the input buffer is not empty, loop as follows:
A. If the input buffer begins with a prefix of "../" or "./", A. If the input buffer begins with a prefix of "../" or "./",
then remove that prefix from the input buffer; otherwise, then remove that prefix from the input buffer; otherwise,
B. If the input buffer begins with a prefix of "/./" or "/.", B. if the input buffer begins with a prefix of "/./" or "/.",
where "." is a complete path segment, then replace that where "." is a complete path segment, then replace that
prefix with "/" in the input buffer; otherwise, prefix with "/" in the input buffer; otherwise,
C. If the input buffer begins with a prefix of "/../" or "/..", C. if the input buffer begins with a prefix of "/../" or "/..",
where ".." is a complete path segment, then replace that where ".." is a complete path segment, then replace that
prefix with "/" in the input buffer and remove the last prefix with "/" in the input buffer and remove the last
segment and its preceding "/" (if any) from the output segment and its preceding "/" (if any) from the output
buffer; otherwise, buffer; otherwise,
D. If the input buffer consists only of "." or "..", then remove D. if the input buffer consists only of "." or "..", then remove
that from the input buffer; otherwise, that from the input buffer; otherwise,
E. Move the first path segment in the input buffer to the end of E. move the first path segment in the input buffer to the end of
the output buffer, including the initial "/" character (if the output buffer, including the initial "/" character (if
any) and any subsequent characters up to, but not including, any) and any subsequent characters up to, but not including,
the next "/" character or the end of the input buffer. the next "/" character or the end of the input buffer.
3. Finally, the output buffer is returned as the result of 3. Finally, the output buffer is returned as the result of
remove_dot_segments. remove_dot_segments.
Note that dot-segments are intended for use in URI references to Note that dot-segments are intended for use in URI references to
express an identifier relative to the hierarchy of names in the base express an identifier relative to the hierarchy of names in the base
URI. The remove_dot_segments algorithm respects that hierarchy by URI. The remove_dot_segments algorithm respects that hierarchy by
removing extra dot-segments rather than treating them as an error or removing extra dot-segments rather than treat them as an error or
leaving them to be misinterpreted by dereference implementations. leaving them to be misinterpreted by dereference implementations.
The following illustrates how the above steps are applied for two The following illustrates how the above steps are applied for two
example merged paths, showing the state of the two buffers after each examples of merged paths, showing the state of the two buffers after
step. each step.
STEP OUTPUT BUFFER INPUT BUFFER STEP OUTPUT BUFFER INPUT BUFFER
1 : /a/b/c/./../../g 1 : /a/b/c/./../../g
2E: /a /b/c/./../../g 2E: /a /b/c/./../../g
2E: /a/b /c/./../../g 2E: /a/b /c/./../../g
2E: /a/b/c /./../../g 2E: /a/b/c /./../../g
2B: /a/b/c /../../g 2B: /a/b/c /../../g
2C: /a/b /../g 2C: /a/b /../g
2C: /a /g 2C: /a /g
skipping to change at page 33, line 46 skipping to change at page 34, line 35
STEP OUTPUT BUFFER INPUT BUFFER STEP OUTPUT BUFFER INPUT BUFFER
1 : mid/content=5/../6 1 : mid/content=5/../6
2E: mid /content=5/../6 2E: mid /content=5/../6
2E: mid/content=5 /../6 2E: mid/content=5 /../6
2C: mid /6 2C: mid /6
2E: mid/6 2E: mid/6
Some applications may find it more efficient to implement the Some applications may find it more efficient to implement the
remove_dot_segments algorithm using two segment stacks rather than remove_dot_segments algorithm by using two segment stacks rather than
strings. strings.
Note: Beware that some older, erroneous implementations will fail Note: Beware that some older, erroneous implementations will fail
to separate a reference's query component from its path component to separate a reference's query component from its path component
prior to merging the base and reference paths, resulting in an prior to merging the base and reference paths, resulting in an
interoperability failure if the query component contains the interoperability failure if the query component contains the
strings "/../" or "/./". strings "/../" or "/./".
5.3 Component Recomposition 5.3. Component Recomposition
Parsed URI components can be recomposed to obtain the corresponding Parsed URI components can be recomposed to obtain the corresponding
URI reference string. Using pseudocode, this would be: URI reference string. Using pseudocode, this would be:
result = "" result = ""
if defined(scheme) then if defined(scheme) then
append scheme to result; append scheme to result;
append ":" to result; append ":" to result;
endif; endif;
skipping to change at page 34, line 42 skipping to change at page 35, line 42
endif; endif;
return result; return result;
Note that we are careful to preserve the distinction between a Note that we are careful to preserve the distinction between a
component that is undefined, meaning that its separator was not component that is undefined, meaning that its separator was not
present in the reference, and a component that is empty, meaning that present in the reference, and a component that is empty, meaning that
the separator was present and was immediately followed by the next the separator was present and was immediately followed by the next
component separator or the end of the reference. component separator or the end of the reference.
5.4 Reference Resolution Examples 5.4. Reference Resolution Examples
Within a representation with a well-defined base URI of Within a representation with a well defined base URI of
http://a/b/c/d;p?q http://a/b/c/d;p?q
a relative reference is transformed to its target URI as follows. a relative reference is transformed to its target URI as follows.
5.4.1 Normal Examples 5.4.1. Normal Examples
"g:h" = "g:h" "g:h" = "g:h"
"g" = "http://a/b/c/g" "g" = "http://a/b/c/g"
"./g" = "http://a/b/c/g" "./g" = "http://a/b/c/g"
"g/" = "http://a/b/c/g/" "g/" = "http://a/b/c/g/"
"/g" = "http://a/g" "/g" = "http://a/g"
"//g" = "http://g" "//g" = "http://g"
"?y" = "http://a/b/c/d;p?y" "?y" = "http://a/b/c/d;p?y"
"g?y" = "http://a/b/c/g?y" "g?y" = "http://a/b/c/g?y"
"#s" = "http://a/b/c/d;p?q#s" "#s" = "http://a/b/c/d;p?q#s"
skipping to change at page 35, line 31 skipping to change at page 36, line 31
"" = "http://a/b/c/d;p?q" "" = "http://a/b/c/d;p?q"
"." = "http://a/b/c/" "." = "http://a/b/c/"
"./" = "http://a/b/c/" "./" = "http://a/b/c/"
".." = "http://a/b/" ".." = "http://a/b/"
"../" = "http://a/b/" "../" = "http://a/b/"
"../g" = "http://a/b/g" "../g" = "http://a/b/g"
"../.." = "http://a/" "../.." = "http://a/"
"../../" = "http://a/" "../../" = "http://a/"
"../../g" = "http://a/g" "../../g" = "http://a/g"
5.4.2 Abnormal Examples 5.4.2. Abnormal Examples
Although the following abnormal examples are unlikely to occur in Although the following abnormal examples are unlikely to occur in
normal practice, all URI parsers should be capable of resolving them normal practice, all URI parsers should be capable of resolving them
consistently. Each example uses the same base as above. consistently. Each example uses the same base as that above.
Parsers must be careful in handling cases where there are more ".." Parsers must be careful in handling cases where there are more ".."
segments in a relative-path reference than there are hierarchical segments in a relative-path reference than there are hierarchical
levels in the base URI's path. Note that the ".." syntax cannot be levels in the base URI's path. Note that the ".." syntax cannot be
used to change the authority component of a URI. used to change the authority component of a URI.
"../../../g" = "http://a/g" "../../../g" = "http://a/g"
"../../../../g" = "http://a/g" "../../../../g" = "http://a/g"
Similarly, parsers must remove the dot-segments "." and ".." when Similarly, parsers must remove the dot-segments "." and ".." when
skipping to change at page 36, line 25 skipping to change at page 37, line 29
"./../g" = "http://a/b/g" "./../g" = "http://a/b/g"
"./g/." = "http://a/b/c/g/" "./g/." = "http://a/b/c/g/"
"g/./h" = "http://a/b/c/g/h" "g/./h" = "http://a/b/c/g/h"
"g/../h" = "http://a/b/c/h" "g/../h" = "http://a/b/c/h"
"g;x=1/./y" = "http://a/b/c/g;x=1/y" "g;x=1/./y" = "http://a/b/c/g;x=1/y"
"g;x=1/../y" = "http://a/b/c/y" "g;x=1/../y" = "http://a/b/c/y"
Some applications fail to separate the reference's query and/or Some applications fail to separate the reference's query and/or
fragment components from the path component before merging it with fragment components from the path component before merging it with
the base path and removing dot-segments. This error is rarely the base path and removing dot-segments. This error is rarely
noticed, since typical usage of a fragment never includes the noticed, as typical usage of a fragment never includes the hierarchy
hierarchy ("/") character, and the query component is not normally ("/") character and the query component is not normally used within
used within relative references. relative references.
"g?y/./x" = "http://a/b/c/g?y/./x" "g?y/./x" = "http://a/b/c/g?y/./x"
"g?y/../x" = "http://a/b/c/g?y/../x" "g?y/../x" = "http://a/b/c/g?y/../x"
"g#s/./x" = "http://a/b/c/g#s/./x" "g#s/./x" = "http://a/b/c/g#s/./x"
"g#s/../x" = "http://a/b/c/g#s/../x" "g#s/../x" = "http://a/b/c/g#s/../x"
Some parsers allow the scheme name to be present in a relative Some parsers allow the scheme name to be present in a relative
reference if it is the same as the base URI scheme. This is reference if it is the same as the base URI scheme. This is
considered to be a loophole in prior specifications of partial URI considered to be a loophole in prior specifications of partial URI
[RFC1630]. Its use should be avoided, but is allowed for backward [RFC1630]. Its use should be avoided but is allowed for backward
compatibility. compatibility.
"http:g" = "http:g" ; for strict parsers "http:g" = "http:g" ; for strict parsers
/ "http://a/b/c/g" ; for backward compatibility / "http://a/b/c/g" ; for backward compatibility
6. Normalization and Comparison 6. Normalization and Comparison
One of the most common operations on URIs is simple comparison: One of the most common operations on URIs is simple comparison:
determining if two URIs are equivalent without using the URIs to determining whether two URIs are equivalent without using the URIs to
access their respective resource(s). A comparison is performed every access their respective resource(s). A comparison is performed every
time a response cache is accessed, a browser checks its history to time a response cache is accessed, a browser checks its history to
color a link, or an XML parser processes tags within a namespace. color a link, or an XML parser processes tags within a namespace.
Extensive normalization prior to comparison of URIs is often used by Extensive normalization prior to comparison of URIs is often used by
spiders and indexing engines to prune a search space or reduce spiders and indexing engines to prune a search space or to reduce
duplication of request actions and response storage. duplication of request actions and response storage.
URI comparison is performed in respect to some particular purpose, URI comparison is performed for some particular purpose. Protocols
and implementations with differing purposes will often be subject to or implementations that compare URIs for different purposes will
differing design trade-offs in regards to how much effort should be often be subject to differing design trade-offs in regards to how
spent in reducing aliased identifiers. This section describes a much effort should be spent in reducing aliased identifiers. This
variety of methods that may be used to compare URIs, the trade-offs section describes various methods that may be used to compare URIs,
between them, and the types of applications that might use them. the trade-offs between them, and the types of applications that might
use them.
6.1 Equivalence 6.1. Equivalence
Since URIs exist to identify resources, presumably they should be Because URIs exist to identify resources, presumably they should be
considered equivalent when they identify the same resource. However, considered equivalent when they identify the same resource. However,
such a definition of equivalence is not of much practical use, since this definition of equivalence is not of much practical use, as there
there is no way for an implementation to compare two resources that is no way for an implementation to compare two resources unless it
are not under its own control. For this reason, determination of has full knowledge or control of them. For this reason,
equivalence or difference of URIs is based on string comparison, determination of equivalence or difference of URIs is based on string
perhaps augmented by reference to additional rules provided by URI comparison, perhaps augmented by reference to additional rules
scheme definitions. We use the terms "different" and "equivalent" to provided by URI scheme definitions. We use the terms "different" and
describe the possible outcomes of such comparisons, but there are "equivalent" to describe the possible outcomes of such comparisons,
many application-dependent versions of equivalence. but there are many application-dependent versions of equivalence.
Even though it is possible to determine that two URIs are equivalent, Even though it is possible to determine that two URIs are equivalent,
URI comparison is not sufficient to determine if two URIs identify URI comparison is not sufficient to determine whether two URIs
different resources. For example, an owner of two different domain identify different resources. For example, an owner of two different
names could decide to serve the same resource from both, resulting in domain names could decide to serve the same resource from both,
two different URIs. Therefore, comparison methods are designed to resulting in two different URIs. Therefore, comparison methods are
minimize false negatives while strictly avoiding false positives. designed to minimize false negatives while strictly avoiding false
positives.
In testing for equivalence, applications should not directly compare In testing for equivalence, applications should not directly compare
relative references; the references should be converted to their relative references; the references should be converted to their
respective target URIs before comparison. When URIs are being respective target URIs before comparison. When URIs are compared to
compared for the purpose of selecting (or avoiding) a network action, select (or avoid) a network action, such as retrieval of a
such as retrieval of a representation, fragment components (if any) representation, fragment components (if any) should be excluded from
should be excluded from the comparison. the comparison.
6.2 Comparison Ladder 6.2. Comparison Ladder
A variety of methods are used in practice to test URI equivalence. A variety of methods are used in practice to test URI equivalence.
These methods fall into a range, distinguished by the amount of These methods fall into a range, distinguished by the amount of
processing required and the degree to which the probability of false processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot be negatives is reduced. As noted above, false negatives cannot be
eliminated. In practice, their probability can be reduced, but this eliminated. In practice, their probability can be reduced, but this
reduction requires more processing and is not cost-effective for all reduction requires more processing and is not cost-effective for all
applications. applications.
If this range of comparison practices is considered as a ladder, the If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with those following discussion will climb the ladder, starting with practices
practices that are cheap but have a relatively higher chance of that are cheap but have a relatively higher chance of producing false
producing false negatives, and proceeding to those that have higher negatives, and proceeding to those that have higher computational
computational cost and lower risk of false negatives. cost and lower risk of false negatives.
6.2.1 Simple String Comparison 6.2.1. Simple String Comparison
If two URIs, considered as character strings, are identical, then it If two URIs, when considered as character strings, are identical,
is safe to conclude that they are equivalent. This type of then it is safe to conclude that they are equivalent. This type of
equivalence test has very low computational cost and is in wide use equivalence test has very low computational cost and is in wide use
in a variety of applications, particularly in the domain of parsing. in a variety of applications, particularly in the domain of parsing.
Testing strings for equivalence requires some basic precautions. Testing strings for equivalence requires some basic precautions.
This procedure is often referred to as "bit-for-bit" or This procedure is often referred to as "bit-for-bit" or
"byte-for-byte" comparison, which is potentially misleading. Testing "byte-for-byte" comparison, which is potentially misleading. Testing
of strings for equality is normally based on pairwise comparison of strings for equality is normally based on pair comparison of the
the characters that make up the strings, starting from the first and characters that make up the strings, starting from the first and
proceeding until both strings are exhausted and all characters found proceeding until both strings are exhausted and all characters are
to be equal, a pair of characters compares unequal, or one of the found to be equal, until a pair of characters compares unequal, or
strings is exhausted before the other. until one of the strings is exhausted before the other.
Such character comparisons require that each pair of characters be This character comparison requires that each pair of characters be
put in comparable form. For example, should one URI be stored in a put in comparable form. For example, should one URI be stored in a
byte array in EBCDIC encoding, and the second be in a Java String byte array in EBCDIC encoding and the second in a Java String object
object (UTF-16), bit-for-bit comparisons applied naively will produce (UTF-16), bit-for-bit comparisons applied naively will produce
errors. It is better to speak of equality on a errors. It is better to speak of equality on a character-for-
character-for-character rather than byte-for-byte or bit-for-bit character basis rather than on a byte-for-byte or bit-for-bit basis.
basis. In practical terms, character-by-character comparisons should In practical terms, character-by-character comparisons should be done
be done codepoint-by-codepoint after conversion to a common character codepoint-by-codepoint after conversion to a common character
encoding. encoding.
False negatives are caused by the production and use of URI aliases. False negatives are caused by the production and use of URI aliases.
Unnecessary aliases can be reduced, regardless of the comparison Unnecessary aliases can be reduced, regardless of the comparison
method, by consistently providing URI references in an method, by consistently providing URI references in an already-
already-normalized form (i.e., a form identical to what would be normalized form (i.e., a form identical to what would be produced
produced after normalization is applied, as described below). after normalization is applied, as described below).
Protocols and data formats often choose to limit some URI comparisons
to simple string comparison, based on the theory that people and Protocols and data formats often limit some URI comparisons to simple
string comparison, based on the theory that people and
implementations will, in their own best interest, be consistent in implementations will, in their own best interest, be consistent in
providing URI references, or at least consistent enough to negate any providing URI references, or at least consistent enough to negate any
efficiency that might be obtained from further normalization. efficiency that might be obtained from further normalization.
6.2.2 Syntax-based Normalization 6.2.2. Syntax-Based Normalization
Implementations may use logic based on the definitions provided by Implementations may use logic based on the definitions provided by
this specification to reduce the probability of false negatives. this specification to reduce the probability of false negatives.
Such processing is moderately higher in cost than This processing is moderately higher in cost than character-for-
character-for-character string comparison. For example, an character string comparison. For example, an application using this
application using this approach could reasonably consider the approach could reasonably consider the following two URIs equivalent:
following two URIs equivalent:
example://a/b/c/%7Bfoo%7D example://a/b/c/%7Bfoo%7D
eXAMPLE://a/./b/../b/%63/%7bfoo%7d eXAMPLE://a/./b/../b/%63/%7bfoo%7d
Web user agents, such as browsers, typically apply this type of URI Web user agents, such as browsers, typically apply this type of URI
normalization when determining whether a cached response is normalization when determining whether a cached response is
available. Syntax-based normalization includes such techniques as available. Syntax-based normalization includes such techniques as
case normalization, percent-encoding normalization, and removal of case normalization, percent-encoding normalization, and removal of
dot-segments. dot-segments.
6.2.2.1 Case Normalization 6.2.2.1. Case Normalization
For all URIs, the hexadecimal digits within a percent-encoding For all URIs, the hexadecimal digits within a percent-encoding
triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
should be normalized to use uppercase letters for the digits A-F. should be normalized to use uppercase letters for the digits A-F.
When a URI uses components of the generic syntax, the component When a URI uses components of the generic syntax, the component
syntax equivalence rules always apply; namely, that the scheme and syntax equivalence rules always apply; namely, that the scheme and
host are case-insensitive and therefore should be normalized to host are case-insensitive and therefore should be normalized to
lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
equivalent to <http://www.example.com/>. The other generic syntax equivalent to <http://www.example.com/>. The other generic syntax
components are assumed to be case-sensitive unless specifically components are assumed to be case-sensitive unless specifically
defined otherwise by the scheme (see Section 6.2.3). defined otherwise by the scheme (see Section 6.2.3).
6.2.2.2 Percent-Encoding Normalization 6.2.2.2. Percent-Encoding Normalization
The percent-encoding mechanism (Section 2.1) is a frequent source of The percent-encoding mechanism (Section 2.1) is a frequent source of
variance among otherwise identical URIs. In addition to the case variance among otherwise identical URIs. In addition to the case
normalization issue noted above, some URI producers percent-encode normalization issue noted above, some URI producers percent-encode
octets that do not require percent-encoding, resulting in URIs that octets that do not require percent-encoding, resulting in URIs that
are equivalent to their non-encoded counterparts. Such URIs should are equivalent to their non-encoded counterparts. These URIs should
be normalized by decoding any percent-encoded octet that corresponds be normalized by decoding any percent-encoded octet that corresponds
to an unreserved character, as described in Section 2.3. to an unreserved character, as described in Section 2.3.
6.2.2.3 Path Segment Normalization 6.2.2.3. Path Segment Normalization
The complete path segments "." and ".." are intended only for use The complete path segments "." and ".." are intended only for use
within relative references (Section 4.1) and are removed as part of within relative references (Section 4.1) and are removed as part of
the reference resolution process (Section 5.2). However, some the reference resolution process (Section 5.2). However, some
deployed implementations incorrectly assume that reference resolution deployed implementations incorrectly assume that reference resolution
is not necessary when the reference is already a URI, and thus fail is not necessary when the reference is already a URI and thus fail to
to remove dot-segments when they occur in non-relative paths. URI remove dot-segments when they occur in non-relative paths. URI
normalizers should remove dot-segments by applying the normalizers should remove dot-segments by applying the
remove_dot_segments algorithm to the path, as described in remove_dot_segments algorithm to the path, as described in
Section 5.2.4. Section 5.2.4.
6.2.3 Scheme-based Normalization 6.2.3. Scheme-Based Normalization
The syntax and semantics of URIs vary from scheme to scheme, as The syntax and semantics of URIs vary from scheme to scheme, as
described by the defining specification for each scheme. described by the defining specification for each scheme.
Implementations may use scheme-specific rules, at further processing Implementations may use scheme-specific rules, at further processing
cost, to reduce the probability of false negatives. For example, cost, to reduce the probability of false negatives. For example,
since the "http" scheme makes use of an authority component, has a because the "http" scheme makes use of an authority component, has a
default port of "80", and defines an empty path to be equivalent to default port of "80", and defines an empty path to be equivalent to
"/", the following four URIs are equivalent: "/", the following four URIs are equivalent:
http://example.com http://example.com
http://example.com/ http://example.com/
http://example.com:/ http://example.com:/
http://example.com:80/ http://example.com:80/
In general, a URI that uses the generic syntax for authority with an In general, a URI that uses the generic syntax for authority with an
empty path should be normalized to a path of "/"; likewise, an empty path should be normalized to a path of "/". Likewise, an
explicit ":port", where the port is empty or the default for the explicit ":port", for which the port is empty or the default for the
scheme, is equivalent to one where the port and its ":" delimiter are scheme, is equivalent to one where the port and its ":" delimiter are
elided, and thus should be removed by scheme-based normalization. elided and thus should be removed by scheme-based normalization. For
For example, the second URI above is the normal form for the "http" example, the second URI above is the normal form for the "http"
scheme. scheme.
Another case where normalization varies by scheme is in the handling Another case where normalization varies by scheme is in the handling
of an empty authority component or empty host subcomponent. For many of an empty authority component or empty host subcomponent. For many
scheme specifications, an empty authority or host is considered an scheme specifications, an empty authority or host is considered an
error; for others, it is considered equivalent to "localhost" or the error; for others, it is considered equivalent to "localhost" or the
end-user's host. When a scheme defines a default for authority and a end-user's host. When a scheme defines a default for authority and a
URI reference to that default is desired, the reference should be URI reference to that default is desired, the reference should be
normalized to an empty authority for the sake of uniformity, brevity, normalized to an empty authority for the sake of uniformity, brevity,
and internationalization. If, however, either the userinfo or port and internationalization. If, however, either the userinfo or port
subcomponent is non-empty, then the host should be given explicitly subcomponents are non-empty, then the host should be given explicitly
even if it matches the default. even if it matches the default.
Normalization should not remove delimiters when their associated Normalization should not remove delimiters when their associated
component is empty unless licensed to do so by the scheme component is empty unless licensed to do so by the scheme
specification. For example, the URI "http://example.com/?" cannot be specification. For example, the URI "http://example.com/?" cannot be
assumed to be equivalent to any of the examples above. Likewise, the assumed to be equivalent to any of the examples above. Likewise, the
presence or absence of delimiters within a userinfo subcomponent is presence or absence of delimiters within a userinfo subcomponent is
usually significant to its interpretation. The fragment component is usually significant to its interpretation. The fragment component is
not subject to any scheme-based normalization; thus, two URIs that not subject to any scheme-based normalization; thus, two URIs that
differ only by the suffix "#" are considered different regardless of differ only by the suffix "#" are considered different regardless of
the scheme. the scheme.
Some schemes define additional subcomponents that consist of Some schemes define additional subcomponents that consist of case-
case-insensitive data, giving an implicit license to normalizers to insensitive data, giving an implicit license to normalizers to
convert such data to a common case (e.g., all lowercase). For convert this data to a common case (e.g., all lowercase). For
example, URI schemes that define a subcomponent of path to contain an example, URI schemes that define a subcomponent of path to contain an
Internet hostname, such as the "mailto" URI scheme, cause that Internet hostname, such as the "mailto" URI scheme, cause that
subcomponent to be case-insensitive and thus subject to case subcomponent to be case-insensitive and thus subject to case
normalization (e.g., "mailto:[email protected]" is equivalent to normalization (e.g., "mailto:[email protected]" is equivalent to
"mailto:[email protected]" even though the generic syntax considers the "mailto:[email protected]", even though the generic syntax considers
path component to be case-sensitive). the path component to be case-sensitive).
Other scheme-specific normalizations are possible. Other scheme-specific normalizations are possible.
6.2.4 Protocol-based Normalization 6.2.4. Protocol-Based Normalization
Web spiders, for which substantial effort to reduce the incidence of Substantial effort to reduce the incidence of false negatives is
false negatives is often cost-effective, are observed to implement often cost-effective for web spiders. Therefore, they implement even
even more aggressive techniques in URI comparison. For example, if more aggressive techniques in URI comparison. For example, if they
they observe that a URI such as observe that a URI such as
http://example.com/data http://example.com/data
redirects to a URI differing only in the trailing slash redirects to a URI differing only in the trailing slash
http://example.com/data/ http://example.com/data/
they will likely regard the two as equivalent in the future. This they will likely regard the two as equivalent in the future. This
kind of technique is only appropriate when equivalence is clearly kind of technique is only appropriate when equivalence is clearly
indicated by both the result of accessing the resources and the indicated by both the result of accessing the resources and the
common conventions of their scheme's dereference algorithm (in this common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems case, use of redirection by HTTP origin servers to avoid problems
with relative references). with relative references).
7. Security Considerations 7. Security Considerations
A URI does not in itself pose a security threat. However, since URIs A URI does not in itself pose a security threat. However, as URIs
are often used to provide a compact set of instructions for access to are often used to provide a compact set of instructions for access to
network resources, care must be taken to properly interpret the data network resources, care must be taken to properly interpret the data
within a URI, to prevent that data from causing unintended access, within a URI, to prevent that data from causing unintended access,
and to avoid including data that should not be revealed in plain and to avoid including data that should not be revealed in plain
text. text.
7.1 Reliability and Consistency 7.1. Reliability and Consistency
There is no guarantee that, having once used a given URI to retrieve There is no guarantee that once a URI has been used to retrieve
some information, the same information will be retrievable by that information, the same information will be retrievable by that URI in
URI in the future. Nor is there any guarantee that the information the future. Nor is there any guarantee that the information
retrievable via that URI in the future will be observably similar to retrievable via that URI in the future will be observably similar to
that retrieved in the past. The URI syntax does not constrain how a that retrieved in the past. The URI syntax does not constrain how a
given scheme or authority apportions its name space or maintains it given scheme or authority apportions its namespace or maintains it
over time. Such a guarantee can only be obtained from the person(s) over time. Such guarantees can only be obtained from the person(s)
controlling that name space and the resource in question. A specific controlling that namespace and the resource in question. A specific
URI scheme may define additional semantics, such as name persistence, URI scheme may define additional semantics, such as name persistence,
if those semantics are required of all naming authorities for that if those semantics are required of all naming authorities for that
scheme. scheme.
7.2 Malicious Construction 7.2. Malicious Construction
It is sometimes possible to construct a URI such that an attempt to It is sometimes possible to construct a URI so that an attempt to
perform a seemingly harmless, idempotent operation, such as the perform a seemingly harmless, idempotent operation, such as the
retrieval of a representation, will in fact cause a possibly damaging retrieval of a representation, will in fact cause a possibly damaging
remote operation to occur. The unsafe URI is typically constructed remote operation. The unsafe URI is typically constructed by
by specifying a port number other than that reserved for the network specifying a port number other than that reserved for the network
protocol in question. The client unwittingly contacts a site that is protocol in question. The client unwittingly contacts a site running
running a different protocol service and data within the URI contains a different protocol service, and data within the URI contains
instructions that, when interpreted according to this other protocol, instructions that, when interpreted according to this other protocol,
cause an unexpected operation. A frequent example of such abuse has cause an unexpected operation. A frequent example of such abuse has
been the use of a protocol-based scheme with a port component of been the use of a protocol-based scheme with a port component of
"25", thereby fooling user agent software into sending an unintended "25", thereby fooling user agent software into sending an unintended
or impersonating message via an SMTP server. or impersonating message via an SMTP server.
Applications should prevent dereference of a URI that specifies a TCP Applications should prevent dereference of a URI that specifies a TCP
port number within the "well-known port" range (0 - 1023) unless the port number within the "well-known port" range (0 - 1023) unless the
protocol being used to dereference that URI is compatible with the protocol being used to dereference that URI is compatible with the
protocol expected on that well-known port. Although IANA maintains a protocol expected on that well-known port. Although IANA maintains a
registry of well-known ports, applications should make such registry of well-known ports, applications should make such
restrictions user-configurable to avoid preventing the deployment of restrictions user-configurable to avoid preventing the deployment of
new services. new services.
When a URI contains percent-encoded octets that match the delimiters When a URI contains percent-encoded octets that match the delimiters
for a given resolution or dereference protocol (for example, CR and for a given resolution or dereference protocol (for example, CR and
LF characters for the TELNET protocol), such percent-encoded octets LF characters for the TELNET protocol), these percent-encodings must
must not be decoded before transmission across that protocol. not be decoded before transmission across that protocol. Transfer of
Transfer of the percent-encoding, which might violate the protocol, the percent-encoding, which might violate the protocol, is less
is less harmful than allowing decoded octets to be interpreted as harmful than allowing decoded octets to be interpreted as additional
additional operations or parameters, perhaps triggering an unexpected operations or parameters, perhaps triggering an unexpected and
and possibly harmful remote operation. possibly harmful remote operation.
7.3 Back-end Transcoding 7.3. Back-End Transcoding
When a URI is dereferenced, the data within it is often parsed by When a URI is dereferenced, the data within it is often parsed by
both the user agent and one or more servers. In HTTP, for example, a both the user agent and one or more servers. In HTTP, for example, a
typical user agent will parse a URI into its five major components, typical user agent will parse a URI into its five major components,
access the authority's server, and send it the data within the access the authority's server, and send it the data within the
authority, path, and query components. A typical server will take authority, path, and query components. A typical server will take
that information, parse the path into segments and the query into that information, parse the path into segments and the query into
key/value pairs, and then invoke implementation-specific handlers to key/value pairs, and then invoke implementation-specific handlers to
respond to the request. As a result, a common security concern for respond to the request. As a result, a common security concern for
server implementations that handle a URI, either as a whole or split server implementations that handle a URI, either as a whole or split
into separate components, is proper interpretation of the octet data into separate components, is proper interpretation of the octet data
represented by the characters and percent-encodings within that URI. represented by the characters and percent-encodings within that URI.
Percent-encoded octets must be decoded at some point during the Percent-encoded octets must be decoded at some point during the
dereference process. Applications must split the URI into its dereference process. Applications must split the URI into its
components and subcomponents prior to decoding the octets, since components and subcomponents prior to decoding the octets, as
otherwise the decoded octets might be mistaken for delimiters. otherwise the decoded octets might be mistaken for delimiters.
Security checks of the data within a URI should be applied after Security checks of the data within a URI should be applied after
decoding the octets. Note, however, that the "%00" percent-encoding decoding the octets. Note, however, that the "%00" percent-encoding
(NUL) may require special handling and should be rejected if the (NUL) may require special handling and should be rejected if the
application is not expecting to receive raw data within a component. application is not expecting to receive raw data within a component.
Special care should be taken when the URI path interpretation process Special care should be taken when the URI path interpretation process
involves the use of a back-end filesystem or related system involves the use of a back-end file system or related system
functions. Filesystems typically assign an operational meaning to functions. File systems typically assign an operational meaning to
special characters, such as the "/", "\", ":", "[", and "]" special characters, such as the "/", "\", ":", "[", and "]"
characters, and special device names like ".", "..", "...", "aux", characters, and to special device names like ".", "..", "...", "aux",
"lpt", etc. In some cases, merely testing for the existence of such "lpt", etc. In some cases, merely testing for the existence of such
a name will cause the operating system to pause or invoke unrelated a name will cause the operating system to pause or invoke unrelated
system calls, leading to significant security concerns regarding system calls, leading to significant security concerns regarding
denial of service and unintended data transfer. It would be denial of service and unintended data transfer. It would be
impossible for this specification to list all such significant impossible for this specification to list all such significant
characters and device names; implementers should research the characters and device names. Implementers should research the
reserved names and characters for the types of storage device that reserved names and characters for the types of storage device that
may be attached to their application and restrict the use of data may be attached to their applications and restrict the use of data
obtained from URI components accordingly. obtained from URI components accordingly.
7.4 Rare IP Address Formats 7.4. Rare IP Address Formats
Although the URI syntax for IPv4address only allows the common, Although the URI syntax for IPv4address only allows the common
dotted-decimal form of IPv4 address literal, many implementations dotted-decimal form of IPv4 address literal, many implementations
that process URIs make use of platform-dependent system routines, that process URIs make use of platform-dependent system routines,
such as gethostbyname() and inet_aton(), to translate the string such as gethostbyname() and inet_aton(), to translate the string
literal to an actual IP address. Unfortunately, such system routines literal to an actual IP address. Unfortunately, such system routines
often allow and process a much larger set of formats than those often allow and process a much larger set of formats than those
described in Section 3.2.2. described in Section 3.2.2.
For example, many implementations allow dotted forms of three For example, many implementations allow dotted forms of three
numbers, wherein the last part is interpreted as a 16-bit quantity numbers, wherein the last part is interpreted as a 16-bit quantity
and placed in the right-most two bytes of the network address (e.g., and placed in the right-most two bytes of the network address (e.g.,
a Class B network). Likewise, a dotted form of two numbers means the a Class B network). Likewise, a dotted form of two numbers means
last part is interpreted as a 24-bit quantity and placed in the right that the last part is interpreted as a 24-bit quantity and placed in
most three bytes of the network address (Class A), and a single the right-most three bytes of the network address (Class A), and a
number (without dots) is interpreted as a 32-bit quantity and stored single number (without dots) is interpreted as a 32-bit quantity and
directly in the network address. Adding further to the confusion, stored directly in the network address. Adding further to the
some implementations allow each dotted part to be interpreted as confusion, some implementations allow each dotted part to be
decimal, octal, or hexadecimal, as specified in the C language (i.e., interpreted as decimal, octal, or hexadecimal, as specified in the C
a leading 0x or 0X implies hexadecimal; otherwise, a leading 0 language (i.e., a leading 0x or 0X implies hexadecimal; a leading 0
implies octal; otherwise, the number is interpreted as decimal). implies octal; otherwise, the number is interpreted as decimal).
These additional IP address formats are not allowed in the URI syntax These additional IP address formats are not allowed in the URI syntax
due to differences between platform implementations. However, they due to differences between platform implementations. However, they
can become a security concern if an application attempts to filter can become a security concern if an application attempts to filter
access to resources based on the IP address in string literal format. access to resources based on the IP address in string literal format.
If such filtering is performed, literals should be converted to If this filtering is performed, literals should be converted to
numeric form and filtered based on the numeric value, rather than a numeric form and filtered based on the numeric value, and not on a
prefix or suffix of the string form. prefix or suffix of the string form.
7.5 Sensitive Information 7.5. Sensitive Information
URI producers should not provide a URI that contains a username or URI producers should not provide a URI that contains a username or
password which is intended to be secret: URIs are frequently password that is intended to be secret. URIs are frequently
displayed by browsers, stored in clear text bookmarks, and logged by displayed by browsers, stored in clear text bookmarks, and logged by
user agent history and intermediary applications (proxies). A user agent history and intermediary applications (proxies). A
password appearing within the userinfo component is deprecated and password appearing within the userinfo component is deprecated and
should be considered an error (or simply ignored) except in those should be considered an error (or simply ignored) except in those
rare cases where the 'password' parameter is intended to be public. rare cases where the 'password' parameter is intended to be public.
7.6 Semantic Attacks 7.6. Semantic Attacks
Because the userinfo subcomponent is rarely used and appears before Because the userinfo subcomponent is rarely used and appears before
the host in the authority component, it can be used to construct a the host in the authority component, it can be used to construct a
URI that is intended to mislead a human user by appearing to identify URI intended to mislead a human user by appearing to identify one
one (trusted) naming authority while actually identifying a different (trusted) naming authority while actually identifying a different
authority hidden behind the noise. For example authority hidden behind the noise. For example
ftp://cnn.example.com&[email protected]/top_story.htm ftp://cnn.example.com&[email protected]/top_story.htm
might lead a human user to assume that the host is 'cnn.example.com', might lead a human user to assume that the host is 'cnn.example.com',
whereas it is actually '10.0.0.1'. Note that a misleading userinfo whereas it is actually '10.0.0.1'. Note that a misleading userinfo
subcomponent could be much longer than the example above. subcomponent could be much longer than the example above.
A misleading URI, such as the one above, is an attack on the user's A misleading URI, such as that above, is an attack on the user's
preconceived notions about the meaning of a URI, rather than an preconceived notions about the meaning of a URI rather than an attack
attack on the software itself. User agents may be able to reduce the on the software itself. User agents may be able to reduce the impact
impact of such attacks by distinguishing the various components of of such attacks by distinguishing the various components of the URI
the URI when rendered, such as by using a different color or tone to when they are rendered, such as by using a different color or tone to
render userinfo if any is present, though there is no general render userinfo if any is present, though there is no panacea. More
panacea. More information on URI-based semantic attacks can be found information on URI-based semantic attacks can be found in [Siedzik].
in [Siedzik].
8. IANA Considerations 8. IANA Considerations
URI scheme names, as defined by <scheme> in Section 3.1, form a URI scheme names, as defined by <scheme> in Section 3.1, form a
registered name space that is managed by IANA according to the registered namespace that is managed by IANA according to the
procedures defined in [BCP35]. No IANA actions are required by this procedures defined in [BCP35]. No IANA actions are required by this
document. document.
9. Acknowledgments 9. Acknowledgements
This specification is derived from RFC 2396 [RFC2396], RFC 1808 This specification is derived from RFC 2396 [RFC2396], RFC 1808
[RFC1808], and RFC 1738 [RFC1738]; the acknowledgments in those [RFC1808], and RFC 1738 [RFC1738]; the acknowledgements in those
documents still apply. It also incorporates the update (with documents still apply. It also incorporates the update (with
corrections) for IPv6 literals in the host syntax, as defined by corrections) for IPv6 literals in the host syntax, as defined by
Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in
[RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz, [RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz,
Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll, Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
Duerst, Stefan Eissing, Clive D.W. Feather, Al Gilman, Tony Hammond, Duerst, Stefan Eissing, Clive D.W. Feather, Al Gilman, Tony Hammond,
Elliotte Harold, Pat Hayes, Henry Holtzman, Ian B. Jacobs, Michael Elliotte Harold, Pat Hayes, Henry Holtzman, Ian B. Jacobs, Michael
Kay, John C. Klensin, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew Kay, John C. Klensin, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew
Main, Dave McAlpin, Ira McDonald, Michael Mealling, Ray Merkert, Main, Dave McAlpin, Ira McDonald, Michael Mealling, Ray Merkert,
Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Kai Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Kai
Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne, Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne,
Stuart Williams, and Henry Zongaro are gratefully acknowledged. Stuart Williams, and Henry Zongaro are gratefully acknowledged.
10. References 10. References
10.1 Normative References 10.1. Normative References
[ASCII] American National Standards Institute, "Coded Character [ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986. Interchange", ANSI X3.4, 1986.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997. Specifications: ABNF", RFC 2234, November 1997.
[STD63] Yergeau, F., "UTF-8, a transformation format of ISO [STD63] Yergeau, F., "UTF-8, a transformation format of
10646", STD 63, RFC 3629, November 2003. ISO 10646", STD 63, RFC 3629, November 2003.
[UCS] International Organization for Standardization, [UCS] International Organization for Standardization,
"Information Technology - Universal Multiple-Octet Coded "Information Technology - Universal Multiple-Octet Coded
Character Set (UCS)", ISO/IEC 10646:2003, December 2003. Character Set (UCS)", ISO/IEC 10646:2003, December 2003.
10.2 Informative References 10.2. Informative References
[BCP19] Freed, N. and J. Postel, "IANA Charset Registration [BCP19] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000. Procedures", BCP 19, RFC 2978, October 2000.
[BCP35] Petke, R. and I. King, "Registration Procedures for URL [BCP35] Petke, R. and I. King, "Registration Procedures for URL
Scheme Names", BCP 35, RFC 2717, November 1999. Scheme Names", BCP 35, RFC 2717, November 1999.
[RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985. host table specification", RFC 952, October 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989. and Support", STD 3, RFC 1123, October 1989.
[RFC1535] Gavron, E., "A Security Problem and Proposed Correction [RFC1535] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535, October With Widely Deployed DNS Software", RFC 1535,
1993. October 1993.
[RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A [RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses Unifying Syntax for the Expression of Names and Addresses
of Objects on the Network as used in the World-Wide Web", of Objects on the Network as used in the World-Wide Web",
RFC 1630, June 1994. RFC 1630, June 1994.
[RFC1736] Kunze, J., "Functional Recommendations for Internet [RFC1736] Kunze, J., "Functional Recommendations for Internet
Resource Locators", RFC 1736, February 1995. Resource Locators", RFC 1736, February 1995.
[RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for [RFC1737] Sollins, K. and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994. Uniform Resource Names", RFC 1737, December 1994.
[RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform [RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994. Resource Locators (URL)", RFC 1738, December 1994.
[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC [RFC1808] Fielding, R., "Relative Uniform Resource Locators",
1808, June 1995. RFC 1808, June 1995.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996. November 1996.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform [RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998. August 1998.
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D. [RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
Jensen, "HTTP Extensions for Distributed Authoring -- Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999. WEBDAV", RFC 2518, February 1999.
[RFC2557] Palme, F., Hopmann, A., Shelness, N. and E. Stefferud, [RFC2557] Palme, J., Hopmann, A., and N. Shelness, "MIME
"MIME Encapsulation of Aggregate Documents, such as HTML Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, March 1999. (MHTML)", RFC 2557, March 1999.
[RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke, [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,
"Guidelines for new URL Schemes", RFC 2718, November 1999. "Guidelines for new URL Schemes", RFC 2718, November 1999.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for [RFC2732] Hinden, R., Carpenter, B., and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999. Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
[RFC3305] Mealling, M. and R. Denenberg, "Report from the Joint W3C/ [RFC3305] Mealling, M. and R. Denenberg, "Report from the Joint
IETF URI Planning Interest Group: Uniform Resource W3C/IETF URI Planning Interest Group: Uniform Resource
Identifiers (URIs), URLs, and Uniform Resource Names Identifiers (URIs), URLs, and Uniform Resource Names
(URNs): Clarifications and Recommendations", RFC 3305, (URNs): Clarifications and Recommendations", RFC 3305,
August 2002. August 2002.
[RFC3490] Faltstrom, P., Hoffman, P. and A. Costello, [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", "Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003. RFC 3490, March 2003.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6 [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003. (IPv6) Addressing Architecture", RFC 3513, April 2003.
[Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", [Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?",
April 2001, <http://www.giac.org/practical/gsec/ April 2001, <http://www.giac.org/practical/gsec/
Richard_Siedzik_GSEC.pdf>. Richard_Siedzik_GSEC.pdf>.
Authors' Addresses
Tim Berners-Lee
World Wide Web Consortium
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
USA
Phone: +1-617-253-5702
Fax: +1-617-258-5999
EMail: [email protected]
URI: http://www.w3.org/People/Berners-Lee/
Roy T. Fielding
Day Software
5251 California Ave., Suite 110
Irvine, CA 92617
USA
Phone: +1-949-679-2960
Fax: +1-949-679-2972
EMail: [email protected]
URI: http://roy.gbiv.com/
Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Phone: +1-408-536-3024
EMail: [email protected]
URI: http://larry.masinter.net/
Appendix A. Collected ABNF for URI Appendix A. Collected ABNF for URI
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ] URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part = "//" authority path-abempty hier-part = "//" authority path-abempty
/ path-absolute / path-absolute
/ path-rootless / path-rootless
/ path-empty / path-empty
URI-reference = URI / relative-ref URI-reference = URI / relative-ref
absolute-URI = scheme ":" hier-part [ "?" query ] absolute-URI = scheme ":" hier-part [ "?" query ]
relative-ref = relative-part [ "?" query ] [ "#" fragment ] relative-ref = relative-part [ "?" query ] [ "#" fragment ]
relative-part = "//" authority path-abempty relative-part = "//" authority path-abempty
/ path-absolute / path-absolute
/ path-noscheme / path-noscheme
/ path-empty / path-empty
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." ) scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
authority = [ userinfo "@" ] host [ ":" port ] authority = [ userinfo "@" ] host [ ":" port ]
userinfo = *( unreserved / pct-encoded / sub-delims / ":" ) userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
host = IP-literal / IPv4address / reg-name host = IP-literal / IPv4address / reg-name
port = *DIGIT port = *DIGIT
IP-literal = "[" ( IPv6address / IPvFuture ) "]" IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" ) IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
IPv6address = 6( h16 ":" ) ls32 IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32 / "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32 / [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32 / [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16 / [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::" / [ *6( h16 ":" ) h16 ] "::"
h16 = 1*4HEXDIG h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
dec-octet = DIGIT ; 0-9 reg-name = *( unreserved / pct-encoded / sub-delims )
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
reg-name = *( unreserved / pct-encoded / sub-delims ) path = path-abempty ; begins with "/" or is empty
/ path-absolute ; begins with "/" but not "//"
/ path-noscheme ; begins with a non-colon segment
/ path-rootless ; begins with a segment
/ path-empty ; zero characters
path = path-abempty ; begins with "/" or is empty path-abempty = *( "/" segment )
/ path-absolute ; begins with "/" but not "//" path-absolute = "/" [ segment-nz *( "/" segment ) ]
/ path-noscheme ; begins with a non-colon segment path-noscheme = segment-nz-nc *( "/" segment )
/ path-rootless ; begins with a segment path-rootless = segment-nz *( "/" segment )
/ path-empty ; zero characters path-empty = 0<pchar>
path-abempty = *( "/" segment ) segment = *pchar
path-absolute = "/" [ segment-nz *( "/" segment ) ] segment-nz = 1*pchar
path-noscheme = segment-nz-nc *( "/" segment ) segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
path-rootless = segment-nz *( "/" segment ) ; non-zero-length segment without any colon ":"
path-empty = 0<pchar>
segment = *pchar pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
segment-nz = 1*pchar
segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
; non-zero-length segment without any colon ":"
pchar = unreserved / pct-encoded / sub-delims / ":" / "@" query = *( pchar / "/" / "?" )
query = *( pchar / "/" / "?" ) fragment = *( pchar / "/" / "?" )
fragment = *( pchar / "/" / "?" )
pct-encoded = "%" HEXDIG HEXDIG pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@" gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")" sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "=" / "*" / "+" / "," / ";" / "="
Appendix B. Parsing a URI Reference with a Regular Expression Appendix B. Parsing a URI Reference with a Regular Expression
Since the "first-match-wins" algorithm is identical to the "greedy" As the "first-match-wins" algorithm is identical to the "greedy"
disambiguation method used by POSIX regular expressions, it is disambiguation method used by POSIX regular expressions, it is
natural and commonplace to use a regular expression for parsing the natural and commonplace to use a regular expression for parsing the
potential five components of a URI reference. potential five components of a URI reference.
The following line is the regular expression for breaking-down a The following line is the regular expression for breaking-down a
well-formed URI reference into its components. well-formed URI reference into its components.
^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))? ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
12 3 4 5 6 7 8 9 12 3 4 5 6 7 8 9
skipping to change at page 51, line 39 skipping to change at page 51, line 29
$3 = //www.ics.uci.edu $3 = //www.ics.uci.edu
$4 = www.ics.uci.edu $4 = www.ics.uci.edu
$5 = /pub/ietf/uri/ $5 = /pub/ietf/uri/
$6 = <undefined> $6 = <undefined>
$7 = <undefined> $7 = <undefined>
$8 = #Related $8 = #Related
$9 = Related $9 = Related
where <undefined> indicates that the component is not present, as is where <undefined> indicates that the component is not present, as is
the case for the query component in the above example. Therefore, we the case for the query component in the above example. Therefore, we
can determine the value of the four components and fragment as can determine the value of the five components as
scheme = $2 scheme = $2
authority = $4 authority = $4
path = $5 path = $5
query = $7 query = $7
fragment = $9 fragment = $9
and, going in the opposite direction, we can recreate a URI reference Going in the opposite direction, we can recreate a URI reference from
from its components using the algorithm of Section 5.3. its components by using the algorithm of Section 5.3.
Appendix C. Delimiting a URI in Context Appendix C. Delimiting a URI in Context
URIs are often transmitted through formats that do not provide a URIs are often transmitted through formats that do not provide a
clear context for their interpretation. For example, there are many clear context for their interpretation. For example, there are many
occasions when a URI is included in plain text; examples include text occasions when a URI is included in plain text; examples include text
sent in electronic mail, USENET news messages, and, most importantly, sent in email, USENET news, and on printed paper. In such cases, it
printed on paper. In such cases, it is important to be able to is important to be able to delimit the URI from the rest of the text,
delimit the URI from the rest of the text, and in particular from and in particular from punctuation marks that might be mistaken for
punctuation marks that might be mistaken for part of the URI. part of the URI.
In practice, URIs are delimited in a variety of ways, but usually In practice, URIs are delimited in a variety of ways, but usually
within double-quotes "http://example.com/", angle brackets within double-quotes "http://example.com/", angle brackets
<http://example.com/>, or just using whitespace <http://example.com/>, or just by using whitespace:
http://example.com/ http://example.com/
These wrappers do not form part of the URI. These wrappers do not form part of the URI.
In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may
need to be added to break a long URI across lines. The whitespace have to be added to break a long URI across lines. The whitespace
should be ignored when extracting the URI. should be ignored when the URI is extracted.
No whitespace should be introduced after a hyphen ("-") character. No whitespace should be introduced after a hyphen ("-") character.
Because some typesetters and printers may (erroneously) introduce a Because some typesetters and printers may (erroneously) introduce a
hyphen at the end of line when breaking a line, the interpreter of a hyphen at the end of line when breaking it, the interpreter of a URI
URI containing a line break immediately after a hyphen should ignore containing a line break immediately after a hyphen should ignore all
all whitespace around the line break, and should be aware that the whitespace around the line break and should be aware that the hyphen
hyphen may or may not actually be part of the URI. may or may not actually be part of the URI.
Using <> angle brackets around each URI is especially recommended as Using <> angle brackets around each URI is especially recommended as
a delimiting style for a reference that contains embedded whitespace. a delimiting style for a reference that contains embedded whitespace.
The prefix "URL:" (with or without a trailing space) was formerly The prefix "URL:" (with or without a trailing space) was formerly
recommended as a way to help distinguish a URI from other bracketed recommended as a way to help distinguish a URI from other bracketed
designators, though it is not commonly used in practice and is no designators, though it is not commonly used in practice and is no
longer recommended. longer recommended.
For robustness, software that accepts user-typed URI should attempt For robustness, software that accepts user-typed URI should attempt
to recognize and strip both delimiters and embedded whitespace. to recognize and strip both delimiters and embedded whitespace.
For example, the text: For example, the text
Yes, Jim, I found it under "http://www.w3.org/Addressing/", Yes, Jim, I found it under "http://www.w3.org/Addressing/",
but you can probably pick it up from <ftp://foo.example. but you can probably pick it up from <ftp://foo.example.
com/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/ com/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/
ietf/uri/historical.html#WARNING>. ietf/uri/historical.html#WARNING>.
contains the URI references contains the URI references
http://www.w3.org/Addressing/ http://www.w3.org/Addressing/
ftp://foo.example.com/rfc/ ftp://foo.example.com/rfc/
http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING
Appendix D. Changes from RFC 2396 Appendix D. Changes from RFC 2396
D.1 Additions D.1. Additions
An ABNF rule for URI has been introduced to correspond to one common An ABNF rule for URI has been introduced to correspond to one common
usage of the term: an absolute URI with optional fragment. usage of the term: an absolute URI with optional fragment.
IPv6 (and later) literals have been added to the list of possible IPv6 (and later) literals have been added to the list of possible
identifiers for the host portion of an authority component, as identifiers for the host portion of an authority component, as
described by [RFC2732], with the addition of "[" and "]" to the described by [RFC2732], with the addition of "[" and "]" to the
reserved set and a version flag to anticipate future versions of IP reserved set and a version flag to anticipate future versions of IP
literals. Square brackets are now specified as reserved within the literals. Square brackets are now specified as reserved within the
authority component and not allowed outside their use as delimiters authority component and are not allowed outside their use as
for an IP literal within host. In order to make this change without delimiters for an IP literal within host. In order to make this
changing the technical definition of the path, query, and fragment change without changing the technical definition of the path, query,
components, those rules were redefined to directly specify the and fragment components, those rules were redefined to directly
characters allowed. specify the characters allowed.
Since [RFC2732] defers to [RFC3513] for definition of an IPv6 literal As [RFC2732] defers to [RFC3513] for definition of an IPv6 literal
address, which unfortunately lacks an ABNF description of address, which, unfortunately, lacks an ABNF description of
IPv6address, we created a new ABNF rule for IPv6address that matches IPv6address, we created a new ABNF rule for IPv6address that matches
the text representations defined by Section 2.2 of [RFC3513]. the text representations defined by Section 2.2 of [RFC3513].
Likewise, the definition of IPv4address has been improved in order to Likewise, the definition of IPv4address has been improved in order to
limit each decimal octet to the range 0-255. limit each decimal octet to the range 0-255.
Section 6 (Section 6) on URI normalization and comparison has been Section 6, on URI normalization and comparison, has been completely
completely rewritten and extended using input from Tim Bray and rewritten and extended by using input from Tim Bray and discussion
discussion within the W3C Technical Architecture Group. within the W3C Technical Architecture Group.
D.2 Modifications D.2. Modifications
The ad-hoc BNF syntax of RFC 2396 has been replaced with the ABNF of The ad-hoc BNF syntax of RFC 2396 has been replaced with the ABNF of
[RFC2234]. This change required all rule names that formerly [RFC2234]. This change required all rule names that formerly
included underscore characters to be renamed with a dash instead. In included underscore characters to be renamed with a dash instead. In
addition, a number of syntax rules have been eliminated or simplified addition, a number of syntax rules have been eliminated or simplified
to make the overall grammar more comprehensible. Specifications that to make the overall grammar more comprehensible. Specifications that
refer to the obsolete grammar rules may be understood by replacing refer to the obsolete grammar rules may be understood by replacing
those rules according to the following table: those rules according to the following table:
+----------------+--------------------------------------------------+ +----------------+--------------------------------------------------+
| obsolete rule | translation | | obsolete rule | translation |
+----------------+--------------------------------------------------+ +----------------+--------------------------------------------------+
| absoluteURI | absolute-URI | | absoluteURI | absolute-URI |
| relativeURI | relative-part [ "?" query ] | | relativeURI | relative-part [ "?" query ] |
| hier_part | ( "//" authority path-abempty / | | hier_part | ( "//" authority path-abempty / |
| | path-absolute ) [ "?" query ] | | | path-absolute ) [ "?" query ] |
| | | | | |
| opaque_part | path-rootless [ "?" query ] | | opaque_part | path-rootless [ "?" query ] |
| net_path | "//" authority path-abempty | | net_path | "//" authority path-abempty |
| abs_path | path-absolute | | abs_path | path-absolute |
| rel_path | path-rootless | | rel_path | path-rootless |
| rel_segment | segment-nz-nc | | rel_segment | segment-nz-nc |
| reg_name | reg-name | | reg_name | reg-name |
| server | authority | | server | authority |
| hostport | host [ ":" port ] | | hostport | host [ ":" port ] |
| hostname | reg-name | | hostname | reg-name |
skipping to change at page 55, line 5 skipping to change at page 54, line 42
| | / "(" / ")" | | | / "(" / ")" |
| | | | | |
| escaped | pct-encoded | | escaped | pct-encoded |
| hex | HEXDIG | | hex | HEXDIG |
| alphanum | ALPHA / DIGIT | | alphanum | ALPHA / DIGIT |
+----------------+--------------------------------------------------+ +----------------+--------------------------------------------------+
Use of the above obsolete rules for the definition of scheme-specific Use of the above obsolete rules for the definition of scheme-specific
syntax is deprecated. syntax is deprecated.
Section 2 on characters has been rewritten to explain what characters Section 2, on characters, has been rewritten to explain what
are reserved, when they are reserved, and why they are reserved even characters are reserved, when they are reserved, and why they are
when not used as delimiters by the generic syntax. The mark reserved, even when they are not used as delimiters by the generic
characters that are typically unsafe to decode, including the syntax. The mark characters that are typically unsafe to decode,
exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open including the exclamation mark ("!"), asterisk ("*"), single-quote
and close parentheses ("(" and ")"), have been moved to the reserved ("'"), and open and close parentheses ("(" and ")"), have been moved
set in order to clarify the distinction between reserved and to the reserved set in order to clarify the distinction between
unreserved and hopefully answer the most common question of scheme reserved and unreserved and, hopefully, to answer the most common
designers. Likewise, the section on percent-encoded characters has question of scheme designers. Likewise, the section on
been rewritten, and URI normalizers are now given license to decode percent-encoded characters has been rewritten, and URI normalizers
any percent-encoded octets corresponding to unreserved characters. are now given license to decode any percent-encoded octets
In general, the terms "escaped" and "unescaped" have been replaced corresponding to unreserved characters. In general, the terms
with "percent-encoded" and "decoded", respectively, to reduce "escaped" and "unescaped" have been replaced with "percent-encoded"
confusion with other forms of escape mechanisms. and "decoded", respectively, to reduce confusion with other forms of
escape mechanisms.
The ABNF for URI and URI-reference has been redesigned to make them The ABNF for URI and URI-reference has been redesigned to make them
more friendly to LALR parsers and reduce complexity. As a result, more friendly to LALR parsers and to reduce complexity. As a result,
the layout form of syntax description has been removed, along with the layout form of syntax description has been removed, along with
the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path, the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,
path_segments, rel_segment, and mark rules. All references to path_segments, rel_segment, and mark rules. All references to
"opaque" URIs have been replaced with a better description of how the "opaque" URIs have been replaced with a better description of how the
path component may be opaque to hierarchy. The relativeURI rule has path component may be opaque to hierarchy. The relativeURI rule has
been replaced with relative-ref to avoid unnecessary confusion over been replaced with relative-ref to avoid unnecessary confusion over
whether or not they are a subset of URI. The ambiguity regarding the whether they are a subset of URI. The ambiguity regarding the
parsing of URI-reference as a URI or a relative-ref with a colon in parsing of URI-reference as a URI or a relative-ref with a colon in
the first segment has been eliminated through the use of five the first segment has been eliminated through the use of five
separate path matching rules. separate path matching rules.
The fragment identifier has been moved back into the section on The fragment identifier has been moved back into the section on
generic syntax components and within the URI and relative-ref rules, generic syntax components and within the URI and relative-ref rules,
though it remains excluded from absolute-URI. The number sign ("#") though it remains excluded from absolute-URI. The number sign ("#")
character has been moved back to the reserved set as a result of character has been moved back to the reserved set as a result of
reintegrating the fragment syntax. reintegrating the fragment syntax.
The ABNF has been corrected to allow the path component to be empty. The ABNF has been corrected to allow the path component to be empty.
This also allows an absolute-URI to consist of nothing after the This also allows an absolute-URI to consist of nothing after the
"scheme:", as is present in practice with the "dav:" namespace "scheme:", as is present in practice with the "dav:" namespace
[RFC2518] and the "about:" scheme used internally by many WWW browser [RFC2518] and with the "about:" scheme used internally by many WWW
implementations. The ambiguity regarding the boundary between browser implementations. The ambiguity regarding the boundary
authority and path has been eliminated through the use of five between authority and path has been eliminated through the use of
separate path matching rules. five separate path matching rules.
Registry-based naming authorities that use the generic syntax are now Registry-based naming authorities that use the generic syntax are now
defined within the host rule. This change allows current defined within the host rule. This change allows current
implementations, where whatever name provided is simply fed to the implementations, where whatever name provided is simply fed to the
local name resolution mechanism, to be consistent with the local name resolution mechanism, to be consistent with the
specification and removes the need to re-specify DNS name formats specification. It also removes the need to re-specify DNS name
here. It also allows the host component to contain percent-encoded formats here. Furthermore, it allows the host component to contain
octets, which is necessary to enable internationalized domain names percent-encoded octets, which is necessary to enable
to be provided in URIs, processed in their native character encodings internationalized domain names to be provided in URIs, processed in
at the application layers above URI processing, and passed to an IDNA their native character encodings at the application layers above URI
library as a registered name in the UTF-8 character encoding. The processing, and passed to an IDNA library as a registered name in the
server, hostport, hostname, domainlabel, toplabel, and alphanum rules UTF-8 character encoding. The server, hostport, hostname,
have been removed. domainlabel, toplabel, and alphanum rules have been removed.
The resolving relative references algorithm of [RFC2396] has been The resolving relative references algorithm of [RFC2396] has been
rewritten using pseudocode for this revision to improve clarity and rewritten with pseudocode for this revision to improve clarity and
fix the following issues: fix the following issues:
o [RFC2396] section 5.2, step 6a, failed to account for a base URI o [RFC2396] section 5.2, step 6a, failed to account for a base URI
with no path. with no path.
o Restored the behavior of [RFC1808] where, if the reference o Restored the behavior of [RFC1808] where, if the reference
contains an empty path and a defined query component, then the contains an empty path and a defined query component, the target
target URI inherits the base URI's path component. URI inherits the base URI's path component.
o The determination of whether a URI reference is a same-document o The determination of whether a URI reference is a same-document
reference has been decoupled from the URI parser, simplifying the reference has been decoupled from the URI parser, simplifying the
URI processing interface within applications in a way consistent URI processing interface within applications in a way consistent
with the internal architecture of deployed URI processing with the internal architecture of deployed URI processing
implementations. The determination is now based on comparison to implementations. The determination is now based on comparison to
the base URI after transforming a reference to absolute form, the base URI after transforming a reference to absolute form,
rather than on the format of the reference itself. This change rather than on the format of the reference itself. This change
may result in more references being considered "same-document" may result in more references being considered "same-document"
under this specification than would be under the rules given in under this specification than there would be under the rules given
RFC 2396, especially when normalization is used to reduce aliases. in RFC 2396, especially when normalization is used to reduce
However, it does not change the status of existing same-document aliases. However, it does not change the status of existing
references. same-document references.
o Separated the path merge routine into two routines: merge, for o Separated the path merge routine into two routines: merge, for
describing combination of the base URI path with a relative-path describing combination of the base URI path with a relative-path
reference, and remove_dot_segments, for describing how to remove reference, and remove_dot_segments, for describing how to remove
the special "." and ".." segments from a composed path. The the special "." and ".." segments from a composed path. The
remove_dot_segments algorithm is now applied to all URI reference remove_dot_segments algorithm is now applied to all URI reference
paths in order to match common implementations and improve the paths in order to match common implementations and to improve the
normalization of URIs in practice. This change only impacts the normalization of URIs in practice. This change only impacts the
parsing of abnormal references and same-scheme references wherein parsing of abnormal references and same-scheme references wherein
the base URI has a non-hierarchical path. the base URI has a non-hierarchical path.
Appendix E. Instructions to RFC Editor
Prior to publication as an RFC, please remove this section and the
"Editorial Note" that appears after the Abstract. If [BCP35] or any
of the normative references are updated prior to publication, the
associated reference in this document can be safely updated as well.
This document has been produced using the xml2rfc tool set; the XML
version can be obtained via the URI listed in the editorial note.
Index Index
A A
ABNF 11 ABNF 11
absolute 26 absolute 27
absolute-path 26 absolute-path 26
absolute-URI 26 absolute-URI 27
access 9 access 9
authority 16, 17 authority 17, 18
B B
base URI 28 base URI 28
C C
character encoding 4 character encoding 4
character 4 character 4
characters 11 characters 8, 11
coded character set 4 coded character set 4
D D
dec-octet 20 dec-octet 20
dereference 9 dereference 9
dot-segments 22 dot-segments 23
F F
fragment 16, 24 fragment 16, 24
G G
gen-delims 12 gen-delims 13
generic syntax 6 generic syntax 6
H H
h16 19 h16 20
hier-part 16 hier-part 16
hierarchical 10 hierarchical 10
host 18 host 18
I I
identifier 5 identifier 5
IP-literal 19 IP-literal 19
IPv4 20 IPv4 20
IPv4address 20 IPv4address 19, 20
IPv6 19 IPv6 19
IPv6address 19, 20 IPv6address 19, 20
IPvFuture 19 IPvFuture 19
L L
locator 7 locator 7
ls32 19 ls32 20
M M
merge 32 merge 32
N N
name 7 name 7
network-path 26 network-path 26
P P
path 16, 22 path 16, 22, 26
path-abempty 22 path-abempty 22
path-absolute 22 path-absolute 22
path-empty 22 path-empty 22
path-noscheme 22 path-noscheme 22
path-rootless 22 path-rootless 22
path-abempty 16 path-abempty 16, 22, 26
path-absolute 16 path-absolute 16, 22, 26
path-empty 16 path-empty 16, 22, 26
path-rootless 16 path-rootless 16, 22
pchar 22 pchar 23
pct-encoded 12 pct-encoded 12
percent-encoding 12 percent-encoding 12
port 21 port 22
Q Q
query 16, 23 query 16, 23
R R
reg-name 20 reg-name 21
registered name 20 registered name 20
relative 10, 28 relative 10, 28
relative-path 26 relative-path 26
relative-ref 26 relative-ref 26
remove_dot_segments 32 remove_dot_segments 33
representation 9 representation 9
reserved 12 reserved 12
resolution 9, 28 resolution 9, 28
resource 5 resource 5
retrieval 9 retrieval 9
S S
same-document 27 same-document 27
sameness 9 sameness 9
scheme 16, 16 scheme 16, 17
segment 22 segment 22, 23
segment-nz 22 segment-nz 23
segment-nz-nc 22 segment-nz-nc 23
sub-delims 12 sub-delims 13
suffix 27 suffix 27
T T
transcription 7 transcription 8
U U
uniform 4 uniform 4
unreserved 13
URI grammar
absolute-URI 26
ALPHA 11
authority 16, 17
CR 11
dec-octet 20
DIGIT 11
DQUOTE 11
fragment 16, 24, 26
gen-delims 12
h16 19
HEXDIG 11
hier-part 16
host 17, 18
IP-literal 19
IPv4address 20
IPv6address 19, 20
IPvFuture 19
LF 11
ls32 19
mark 13
OCTET 11
path 22
path-abempty 16, 22
path-absolute 16, 22
path-empty 16, 22
path-noscheme 22
path-rootless 16, 22
pchar 22, 23, 24
pct-encoded 12
port 17, 21
query 16, 23, 26, 26
reg-name 20
relative-ref 25, 26
reserved 12
scheme 16, 16, 26
segment 22
segment-nz 22
segment-nz-nc 22
SP 11
sub-delims 12
unreserved 13 unreserved 13
URI 16, 25 URI grammar
absolute-URI 27
ALPHA 11
authority 18
CR 11
dec-octet 20
DIGIT 11
DQUOTE 11
fragment 24
gen-delims 13
h16 20
HEXDIG 11
hier-part 16
host 19
IP-literal 19
IPv4address 20
IPv6address 20
IPvFuture 19
LF 11
ls32 20
OCTET 11
path 22
path-abempty 22
path-absolute 22
path-empty 22
path-noscheme 22
path-rootless 22
pchar 23
pct-encoded 12
port 22
query 24
reg-name 21
relative-ref 26
reserved 13
scheme 17
segment 23
segment-nz 23
segment-nz-nc 23
SP 11
sub-delims 13
unreserved 13
URI 16
URI-reference 25
userinfo 18
URI 16
URI-reference 25 URI-reference 25
userinfo 17, 18 URL 7
URI 16 URN 7
URI-reference 25 userinfo 18
URL 7
URN 7
userinfo 17, 18
Intellectual Property Statement Authors' Addresses
Tim Berners-Lee
World Wide Web Consortium
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
USA
Phone: +1-617-253-5702
Fax: +1-617-258-5999
EMail: [email protected]
URI: http://www.w3.org/People/Berners-Lee/
Roy T. Fielding
Day Software
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Irvine, CA 92617
USA
Phone: +1-949-679-2960
Fax: +1-949-679-2972
EMail: [email protected]
URI: http://roy.gbiv.com/
Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Phone: +1-408-536-3024
EMail: [email protected]
URI: http://larry.masinter.net/
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