< draft-fielding-uri-rfc2396bis-04.txt   draft-fielding-uri-rfc2396bis-05.txt >
Network Working Group T. Berners-Lee Network Working Group T. Berners-Lee
Internet-Draft MIT/LCS Internet-Draft W3C/MIT
Updates: 1738 (if approved) R. Fielding Updates: 1738 (if approved) R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved) Day Software Obsoletes: 2732, 2396, 1808 (if approved) Day Software
Expires: August 16, 2004 L. Masinter L. Masinter
Adobe Expires: October 15, 2004 Adobe
February 16, 2004 April 16, 2004
Uniform Resource Identifier (URI): Generic Syntax Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-04 draft-fielding-uri-rfc2396bis-05
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with By submitting this Internet-Draft, I certify that any applicable
all provisions of Section 10 of RFC2026. patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that other Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts. groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on August 16, 2004.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
A Uniform Resource Identifier (URI) is a compact string of characters A Uniform Resource Identifier (URI) is a compact sequence of
for identifying an abstract or physical resource. This specification characters for identifying an abstract or physical resource. This
defines the generic URI syntax and a process for resolving URI specification defines the generic URI syntax and a process for
references that might be in relative form, along with guidelines and resolving URI references that might be in relative form, along with
security considerations for the use of URIs on the Internet. guidelines and security considerations for the use of URIs on the
Internet.
The URI syntax defines a grammar that is a superset of all valid The URI syntax defines a grammar that is a superset of all valid
URIs, such that an implementation can parse the common components of URIs, such that an implementation can parse the common components of
a URI reference without knowing the scheme-specific requirements of a URI reference without knowing the scheme-specific requirements of
every possible identifier. This specification does not define a every possible identifier. This specification does not define a
generative grammar for URIs; that task is performed by the individual generative grammar for URIs; that task is performed by the individual
specifications of each URI scheme. specifications of each URI scheme.
Editorial Note Editorial Note
Discussion of this draft and comments to the editors should be sent Discussion of this draft and comments to the editors should be sent
to the uri@w3.org mailing list. An issues list and version history to the uri@w3.org mailing list. An issues list and version history
is available at <http://gbiv.com/protocols/uri/rev-2002/issues.html>. 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 . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . 6
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . . . . 6 1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . 6
1.2 Design Considerations . . . . . . . . . . . . . . . . . . . 6 1.2 Design Considerations . . . . . . . . . . . . . . . . . . 7
1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . 7
1.2.2 Separating Identification from Interaction . . . . . . . . . 7 1.2.2 Separating Identification from Interaction . . . . . . 8
1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . . . . 9 1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . 9
1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . . 10 1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . 10
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . 11 2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1 Percent Encoding . . . . . . . . . . . . . . . . . . . . . . 11 2.1 Percent-Encoding . . . . . . . . . . . . . . . . . . . . . 11
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . . 12 2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . 11
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . . 12 2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . 12
2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . . 13 2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . 13
3. Syntax Components . . . . . . . . . . . . . . . . . . . . . 15 2.5 Identifying Data . . . . . . . . . . . . . . . . . . . . . 13
3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.1 User Information . . . . . . . . . . . . . . . . . . . . . . 16 3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2.1 User Information . . . . . . . . . . . . . . . . . . . 17
3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . . 24 4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . . 24 4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . 24
4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . . 25 4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 Same-document Reference . . . . . . . . . . . . . . . . . . 25 4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . 25
4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . . 25 4.4 Same-document Reference . . . . . . . . . . . . . . . . . 25
5. Reference Resolution . . . . . . . . . . . . . . . . . . . . 27 4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . 26
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . . 27
5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 27
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 28
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . . . . 28
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 28
5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . . 28
5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . . . . 29
5.2.2 Transform References . . . . . . . . . . . . . . . . . . . . 29
5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . . . . 30
5.3 Component Recomposition . . . . . . . . . . . . . . . . . . 32
5.4 Reference Resolution Examples . . . . . . . . . . . . . . . 33
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . . . . 33
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . . . . 33
6. Normalization and Comparison . . . . . . . . . . . . . . . . 35
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . . 36
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 36
6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 37
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 38
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 39
6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 39
7. Security Considerations . . . . . . . . . . . . . . . . . . 41
7.1 Reliability and Consistency . . . . . . . . . . . . . . . . 41
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . . 41
7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . . 42
7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . . 42
7.5 Sensitive Information . . . . . . . . . . . . . . . . . . . 43
7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 43
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 45
Normative References . . . . . . . . . . . . . . . . . . . . 46
Informative References . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 50
B. Parsing a URI Reference with a Regular Expression . . . . . 52
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . 53
D. Summary of Non-editorial Changes . . . . . . . . . . . . . . 55
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 55
D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 55
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . 62
1. Introduction 5. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 27
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . 27
5.1.1 Base URI Embedded in Content . . . . . . . . . . . . . 27
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . 28
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . 28
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . 28
5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . 29
5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . 29
5.2.2 Transform References . . . . . . . . . . . . . . . . . 29
5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . 30
5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . 31
5.3 Component Recomposition . . . . . . . . . . . . . . . . . 33
5.4 Reference Resolution Examples . . . . . . . . . . . . . . 34
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . 34
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . 34
6. Normalization and Comparison . . . . . . . . . . . . . . . . . 36
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . 36
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . 37
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . 37
6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . 37
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . 38
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . 39
6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . 40
7. Security Considerations . . . . . . . . . . . . . . . . . . . 40
7.1 Reliability and Consistency . . . . . . . . . . . . . . . 40
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . 41
7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . 41
7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . 42
7.5 Sensitive Information . . . . . . . . . . . . . . . . . . 43
7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . 43
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 45
9.2 Informative References . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 47
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 48
B. Parsing a URI Reference with a Regular Expression . . . . . . 50
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 51
D. Summary of Non-editorial Changes . . . . . . . . . . . . . . . 52
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . 52
D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . 53
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Intellectual Property and Copyright Statements . . . . . . . . 58
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 such 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], and is designed to meet the recommendations laid
out in "Functional Recommendations for Internet Resource Locators" out in "Functional Recommendations for Internet Resource Locators"
[RFC1736] and "Functional Requirements for Uniform Resource Names" [RFC1736] and "Functional Requirements for Uniform Resource Names"
[RFC1737]. [RFC1737].
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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 excludes those portions of RFC 1738 that defined the specific It excludes those portions of RFC 1738 that defined the specific
syntax of individual URI schemes; those portions will be updated as syntax of individual URI schemes; those portions will be updated as
separate documents. The process for registration of new URI schemes separate documents. The process for registration of new URI schemes
is defined separately by [RFC2717]. Advice for designers of new URI is defined separately by [RFC2717]. 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 "character This specification uses the terms "character" and "coded character
encoding" in accordance with the definitions provided in [RFC2978]. set" in accordance with the definitions provided in [RFC2978], and
"character encoding" in place of what [RFC2978] refers to as a
"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 of
resource identifiers to be used in the same context, even when the resource identifiers to be used in the same context, even when the
mechanisms used to access those resources may differ; it allows mechanisms used to access those resources may differ; it allows
uniform semantic interpretation of common syntactic conventions uniform semantic interpretation of common syntactic conventions
across different types of resource identifiers; it allows across different types of resource identifiers; it allows
introduction of new types of resource identifiers without 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; and,
it allows the identifiers to be reused in many different contexts, it 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-existing, large, and widely-used set of resource identifiers. pre-existing, large, and widely-used set of resource identifiers.
Resource Resource
Anything that has been named or described can be a resource.
Anything that can be named or described can be a resource.
Familiar examples include an electronic document, an image, a Familiar examples include an electronic document, an image, a
service (e.g., "today's weather report for Los Angeles"), and a service (e.g., "today's weather report for Los Angeles"), and a
collection of other resources. 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, abstract bound books in a library can also be resources. Likewise, abstract
concepts can be resources, such as the operators and operands of a concepts can be resources, such as the operators and operands of a
mathematical equation or the types of a relationship (e.g., mathematical equation, the types of a relationship (e.g., "parent"
"parent" or "employee"). or "employee"), or numeric values (e.g., zero, one, and infinity).
These things are called resources because they each can be
considered a source of supply or support, or an available means,
for some system, where such systems may be as diverse as the World
Wide Web, a filesystem, an ontological graph, a theorem prover, or
some other form of system for the direct or indirect observation
and/or manipulation of resources. Note that "supply" is not
necessary for a thing to be considered a resource: the ability to
simply refer to that thing is often sufficient to support the
operation of a given system.
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. identification. Our use of the terms "identify" and "identifying"
refer to this process of distinguishing from many to one; they
should not be mistaken as an assumption that the identifier
defines the identity of what is referenced, though that may be the
case for some identifiers.
A URI is an identifier that consists of a sequence of characters A URI is an identifier that consists of a sequence of characters
matching the syntax defined by the syntax rule named "URI" in Section matching the syntax rule named <URI> in Section 3. A URI can be used
3. A URI can be used to refer to a resource. This specification does to refer to a resource. This specification does not place any limits
not place any limits on the nature of a resource or the reasons why on the nature of a resource, the reasons why an application might
an application might wish to refer to a resource. URIs have a global wish to refer to a resource, or the kinds of system that might use
scope and should be interpreted consistently regardless of context, URIs for the sake of identifying resources.
but that interpretation may be defined in relation to the user's
context (e.g., "http://localhost/" refers to a resource that is
relative to the user's network interface and yet not specific to any
one user).
1.1.1 Generic Syntax URIs have a global scope and must be interpreted consistently
regardless of context, though the result of that interpretation may
be in relation to the end-user's context. For example, "http://
localhost/" has the same interpretation for every user of that
reference, even though the network interface corresponding to
"localhost" may be different for each end-user: interpretation is
independent of access. 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 action intended to refer to a single, globally
unique thing must use a URI that distinguishes that resource from all
other things. URIs that identify in relation to the end-user's local
context should only be used when the context itself is a defining
aspect of the resource, such as when an on-line Linux manual refers
to a file on the end-user's filesystem (e.g., "file:///etc/hosts").
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 that are needed to implement a
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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 defining the range of syntax allowed for all URIs,
including those schemes that have yet to be defined. including those schemes that have yet to be defined.
A parser of the generic URI syntax is capable of parsing any URI A parser of the generic URI syntax is capable of parsing any URI
reference into its major components; once the scheme is determined, reference into its major components; once the scheme is determined,
further scheme-specific parsing can be performed on the components. further scheme-specific parsing can be performed on the components.
In other words, the URI generic syntax is a superset of the syntax of In other words, the URI generic syntax is a superset of the syntax of
all URI schemes. all URI schemes.
1.1.2 Examples 1.1.2 Examples
The following examples illustrate URIs that are in common use. The following examples illustrate URIs that are in common use.
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
mailto:John.Doe@example.com mailto:John.Doe@example.com
news:comp.infosystems.www.servers.unix news:comp.infosystems.www.servers.unix
telnet://melvyl.ucop.edu/ telnet://melvyl.ucop.edu/
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 need 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 any quality of the
scheme. Future specifications and related documentation should use scheme. Future specifications and related documentation should use
the general term "URI", rather than the more restrictive terms URL the general term "URI", rather than the more restrictive terms URL
and URN [RFC3305]. 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
integers from a coded character set. The interpretation of a URI character encoding octets. The interpretation of a URI depends only
depends only on the characters used and not how those characters are on the characters used and not how those characters are represented
represented in a network protocol. 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:
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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 in 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 such
representation is defined 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 will specify the which the URI is referenced; such a definition should specify the
character encoding scheme used to map those characters to octets character encoding used to map those characters to octets prior to
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. In fact, the URI alone 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, an operation (if any)
associated with a URI reference is defined by the protocol element, associated with a URI reference is defined by the protocol element,
data format attribute, or natural language text in which it appears. data format 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 such words as "access",
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locally cached representation, resolution of the URI to determine an locally cached representation, resolution of the URI to determine an
appropriate access mechanism (if any), and dereference of the URI for appropriate access mechanism (if any), and dereference of the URI for
the sake of applying a retrieval operation. Depending on the the sake of applying a retrieval operation. Depending on the
protocols used to perform the retrieval, additional information might protocols used to perform the retrieval, additional information might
be supplied about the resource (resource metadata) and its relation be supplied about the resource (resource metadata) and its relation
to other resources. to other resources.
URI references in information systems are designed to be URI references in information 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 at the
time it is accessed and may vary over time or due to other aspects of time it is accessed and may vary over time or due to other aspects of
the interaction. When an author creates a reference to such a the interaction. Such references are created in order to be be used
resource, they do so with the intention that the reference be used in in the future: what is being identified is not some specific result
the future; what is being identified is not some specific result that that was obtained in the past, but rather some characteristic that is
was obtained in the past, but rather some characteristic that is
expected to be true for future results. In such cases, the resource expected to be true for future results. In such cases, the resource
referred to by the URI is actually a sameness of characteristics as referred to by the URI is actually a sameness of characteristics as
observed over time, perhaps elucidated by additional comments or observed over time, perhaps elucidated by additional comments or
assertions made by the resource provider. assertions 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 such a URI 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, and 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 for the purpose of delimiting components
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the reference context and the target URI. The reference resolution the reference context and the target URI. The reference resolution
algorithm, presented in Section 5, defines how such a reference is algorithm, presented in Section 5, defines how such a reference is
transformed to the target URI. Since relative references can only be transformed to the target URI. Since relative references can only be
used within the context of a hierarchical URI, designers of new URI used within the context of a hierarchical URI, designers of new URI
schemes should use a syntax consistent with the generic syntax's schemes should use a syntax consistent with the generic syntax's
hierarchical components unless there are compelling reasons to forbid hierarchical components unless there are compelling reasons to forbid
relative referencing within that scheme. relative referencing within that scheme.
All URIs are parsed by generic syntax parsers when used. A URI scheme All URIs are parsed by generic syntax parsers when used. A URI scheme
that wishes to remain opaque to hierarchical processing must disallow that wishes to remain opaque to hierarchical processing must disallow
the use of slash and question mark characters. However, since a the use of slash and question mark characters. However, since a URI
non-relative URI reference is only modified by the generic parser if reference is only modified by the generic parser if it contains a
it contains complete path segments of "." or ".." (see Section 3.3), dot-segment (a complete path segment of "." or "..", as described in
URIs may safely use "/" for other purposes if they do not allow Section 3.3), URI schemes may safely use "/" for other purposes if
dot-segments. they do not allow dot-segments.
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),
CTL (control characters), DIGIT (decimal digits), DQUOTE (double DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal
quote), HEXDIG (hexadecimal digits), LF (line feed), and SP (space). digits), LF (line feed), and SP (space). The complete URI syntax is
The complete URI syntax is collected in Appendix A. collected in Appendix A.
2. Characters
Although ABNF notation defines its terminal values to be non-negative 2. Characters
integers (codepoints) based on the US-ASCII coded character set
[ASCII], we must invert that relation in order to understand the URI
syntax, since URIs are defined as strings of characters independent
of any particular encoding. Therefore, the integer values must be
mapped back to their corresponding characters via US-ASCII in order
to complete the syntax rules.
This specification does not mandate the use of any particular The URI syntax provides a method of encoding data, presumably for the
character encoding scheme for mapping between URI characters and the sake of identifying a resource, as a sequence of characters. The URI
octets used to store or transmit those characters. When a URI appears characters are, in turn, frequently encoded as octets for transport
in a protocol element, the character encoding is defined by that or presentation. This specification does not mandate any particular
protocol; absent such a definition, a URI is assumed to use the same character encoding for mapping between URI characters and the octets
used to store or transmit those characters. When a URI appears in a
protocol element, the character encoding is defined by that protocol;
absent 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
integers (codepoints) based on the US-ASCII coded character set
[ASCII]. Since a URI is a sequence of characters, we must invert
that relation in order to understand the URI syntax. Therefore, the
integer values used by the ABNF must be mapped back to their
corresponding characters via US-ASCII in order to complete the syntax
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 (Section 2.2) digits, letters, and a few graphic symbols. A reserved subset of
subset of those characters may be used to delimit syntax components those characters may be used to delimit syntax components within a
within a URI, while the remaining characters, including both the URI, while the remaining characters, including both the unreserved
unreserved (Section 2.3) set and those reserved characters not acting set and those reserved characters not acting as delimiters, define
as delimiters, define each component's data. each 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 triplet, component. A percent-encoded octet is encoded as a character triplet,
consisting of the percent character "%" followed by the two 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). character (SP). Section 2.4 describes when percent-encoding and
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. Two URIs that
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 are equivalent. For consistency, URI producers and
normalizers should use uppercase hexadecimal digits for all normalizers should use uppercase hexadecimal digits for all
percent-encodings. percent-encodings.
2.2 Reserved Characters 2.2 Reserved Characters
URIs include components and sub-components 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 forming the URI.
reserved = gen-delims / sub-delims reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@" gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")" sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "=" / "*" / "+" / "," / ";" / "="
A subset of the reserved characters (gen-delims) are used as The purpose of reserved characters is to provide a set of delimiting
delimiters of the generic URI components described in Section 3. A characters that are distinguishable from other data within a URI.
component's ABNF syntax rule will not use the reserved or gen-delims
rule names directly; instead, each syntax rule lists those reserved
characters that are allowed within that component (i.e., not
delimiting it). The allowed reserved characters, including those in
the sub-delims set and any of the gen-delims that are not a delimiter
of that component, are reserved for use as sub-component delimiters
within the component. Only the most common sub-components are
defined by this specification; other sub-components may be defined by
a URI scheme's specification, or by the implementation-specific
syntax of a URI's dereferencing algorithm, provided that such
sub-components are delimited by characters in that component's
reserved set. If no such delimiting role has been assigned, then a
reserved character appearing in a component represents the data octet
corresponding to its encoding in US-ASCII.
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-encoding a reserved character, or decoding a percent-encoded Percent-encoding a reserved character, or decoding a percent-encoded
octet that corresponds to a reserved character, will change how the octet that corresponds to a reserved character, will change how the
URI is interpreted by most applications. URI is interpreted by most applications. Thus, characters in the
reserved set are protected from normalization and are therefore safe
to be used by scheme-specific and producer-specific algorithms for
delimiting data subcomponents within a URI.
2.3 Unreserved Characters A subset of the reserved characters (gen-delims) are used as
delimiters of the generic URI components described in Section 3. A
component's ABNF syntax rule will not use the reserved or gen-delims
rule names directly; instead, each syntax rule lists the characters
allowed within that component (i.e., not delimiting it) and any of
those characters that are also in the reserved set are "reserved" for
use as subcomponent delimiters within the component. Only the most
common subcomponents are defined by this specification; other
subcomponents may be defined by a URI scheme's specification, or by
the implementation-specific syntax of a URI's dereferencing
algorithm, provided that such subcomponents are delimited by
characters in the reserved set allowed within that component.
URI producing applications should percent-encode data octets that
correspond to characters in the reserved set. However, if a reserved
character is found in a URI component and no delimiting role is known
for that character, then it should be interpreted as representing the
data octet corresponding to that character's encoding in US-ASCII.
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 octet are equivalent: they identify its corresponding percent-encoded octet are equivalent: they identify
the same resource. However, percent-encoded unreserved characters the same resource. However, percent-encoded unreserved characters
may change the result of some URI comparisons (Section 6), may change the result of some URI comparisons (Section 6),
potentially leading to incorrect or inefficient behavior. For potentially leading to incorrect or inefficient behavior. For
consistency, percent-encoded octets in the ranges of ALPHA (%41-%5A consistency, percent-encoded octets in the ranges of ALPHA (%41-%5A
and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), underscore and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), underscore
(%5F), or tilde (%7E) should not be created by URI producers and, (%5F), or tilde (%7E) should not be created by URI producers and,
when found in a URI, should be decoded to their corresponding when found in a URI, should be decoded to their corresponding
unreserved character by URI normalizers. unreserved character 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 that 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. It is during that process that an
implementation determines which of the reserved characters are to be implementation determines which of the reserved characters are to be
used as sub-component delimiters and which can be safely used as used as subcomponent delimiters and which can be safely used as data.
data. Once produced, a URI is always in its percent-encoded form. Once produced, a URI is always in its percent-encoded form.
When a URI is dereferenced, the components and sub-components 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, since otherwise the data may
be mistaken for component delimiters. The only exception is for be 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 software; the "%7E" can be replaced by "~" by older URI processing software; 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" in order
for that octet to be used as data within a URI. Implementations must for that octet to be used as data within a URI. Implementations must
not percent-encode or decode the same string more than once, since not percent-encode or decode the same string more than once, since
decoding an already decoded string might lead to misinterpreting a decoding an already decoded string might lead to misinterpreting a
percent data octet as the beginning of a percent-encoding, or vice percent data octet as the beginning of a percent-encoding, or vice
versa in the case of percent-encoding an already percent-encoded versa in the case of percent-encoding an already percent-encoded
string. string.
URI characters serve as an external interface for identification 2.5 Identifying Data
between systems. A system that internally provides identifiers in
the form of a different character encoding, such as EBCDIC, will
generally perform character translation of textual identifiers to
UTF-8 [RFC3629] (or some other superset of the US-ASCII character
encoding) at an internal interface, since that results in more
meaningful identifiers than simply percent-encoding the original
octets. When interpreting an incoming URI on such an interface,
percent-encoded octets must be decoded before the reverse transcoding
can be applied.
In some cases, the interface between a URI component and the URI characters provide identifying data for each of the URI
identifying data it has been crafted to represent is much less direct components, serving as an external interface for identification
than a character encoding translation. For example, portions of a between systems. Although the presence and nature of the URI
URI might reflect a query on non-ASCII data, numeric coordinates on a production interface is hidden from clients that use its URIs, and
map, etc. Likewise, a URI scheme may define components with thus beyond the scope of the interoperability requirements defined by
additional encoding requirements, such as base64, that are applied this specification, it is a frequent source of confusion and errors
prior to forming the component and producing the URI. in the interpretation of URI character issues. Implementers need to
be aware that there are multiple character encodings involved in the
production and transmission of URIs: local name and data encoding,
public interface encoding, URI character encoding, data format
encoding, and protocol encoding.
When a URI scheme defines a component that represents textual data The first encoding of identifying data is the one in which the local
consisting of characters from the Unicode (ISO/IEC 10646-1) character names or data are stored. URI producing applications (a.k.a., origin
set, the data should be encoded first as octets according to the servers) will typically use the local encoding as the basis for
UTF-8 character encoding [RFC3629], and then only those octets that producing meaningful names. The URI producer will transform the
do not correspond to characters in the unreserved set should be local encoding to one that is suitable for a public interface, and
then transform the public interface encoding into the restricted set
of URI characters (reserved, unreserved, and percent-encodings).
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
data formats are often subsequently encoded for transmission over
Internet protocols.
For most systems, an unreserved character appearing within a URI
component is interpreted as representing the data octet corresponding
to that character's encoding in US-ASCII. Consumers of URIs assume
that the letter "X" corresponds to the octet "01011000", and there is
no harm in making that assumption even when it is incorrect. A
system that internally provides identifiers in the form of a
different character encoding, such as EBCDIC, will generally perform
character translation of textual identifiers to UTF-8 [RFC3629] (or
some other superset of the US-ASCII character encoding) at an
internal interface, thereby providing more meaningful identifiers
than simply percent-encoding the original octets.
For example, consider an information service that provides data,
stored locally using an EBCDIC-based filesystem, to clients on the
Internet through an HTTP server. When an author creates a file on
that filesystem with the name "Laguna Beach", their expectation is
that the "http" URI corresponding to that resource would also contain
the meaningful string "Laguna%20Beach". If, however, that server
produces URIs using an overly-simplistic raw octet mapping, then the
result would be a URI containing
"%D3%81%87%A4%95%81@%C2%85%81%83%88". An internal transcoding
interface fixes that problem by transcoding the local name to a
superset of US-ASCII prior to producing the URI. Naturally, proper
interpretation of an incoming URI on such an interface requires that
percent-encoded octets be decoded (e.g., "%20" to SP) before the
reverse transcoding is applied to obtain the local name.
In some cases, the internal interface between a URI component and the
identifying data that it has been crafted to represent is much less
direct than a character encoding translation. For example, portions
of a URI might reflect a query on non-ASCII data, numeric coordinates
on a map, etc. Likewise, a URI scheme may define components with
additional encoding requirements that are applied prior to forming
the component and producing the URI.
When a new URI scheme defines a component that represents textual
data consisting of characters from the Unicode (ISO/IEC 10646-1)
character set, the data should be encoded first as octets according
to the UTF-8 character encoding [RFC3629], and then only those octets
that do not correspond to characters in the unreserved set should be
percent-encoded. For example, the character A would be represented percent-encoded. For example, the character A would be represented
as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be
represented as "%C3%80", and the character KATAKANA LETTER A would be represented as "%C3%80", and the character KATAKANA LETTER A would be
represented as "%E3%82%A2". represented 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 ":" ["//" authority] path ["?" query] ["#" fragment] URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part = "//" authority path-abempty
/ path-abs
/ path-rootless
/ path-empty
The scheme and path components are required, though path may be empty The scheme and path components are required, though path may be empty
(no characters). An ABNF-driven parser will find that the border (no characters). When authority is present, the path must either be
between authority and path is ambiguous; they are disambiguated by empty or begin with a slash ("/") character. When authority is not
the "first-match-wins" (a.k.a. "greedy") algorithm. In other words, present, the path cannot begin with two slash characters ("//").
if authority is present then the first segment of the path must be These restrictions result in five different ABNF rules for a path
empty. (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 scheme is
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specify schemes must do so using lowercase letters. An specify schemes must do so using lowercase letters. An
implementation should accept uppercase letters as equivalent to implementation should accept uppercase letters as equivalent to
lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for
the sake of robustness, but should only produce lowercase scheme the sake of robustness, but should only produce lowercase scheme
names, for consistency. names, for 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 for registration of new URI schemes is defined separately by
[RFC2717]. The scheme registry maintains the mapping between scheme [RFC2717]. The scheme registry maintains the mapping between scheme
names and their specifications. Advice for designers of new URI names and their specifications. Advice for designers of new URI
schemes can be found in [RFC2718]. schemes can be found in [RFC2718].
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 that might indicate the URI has been constructed to
mislead the user (Section 7.6). 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, such 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 userinfo from host if the userinfo component is empty (zero separates host from port if the port component is empty. Some schemes
length) and should omit the ":" delimiter that separates host from do not allow the userinfo and/or port subcomponents.
port if the port component is empty. Some schemes do not allow the
userinfo and/or port sub-components.
3.2.1 User Information If a URI contains an authority component, then the path component
must either be empty or begin with a slash ("/") character.
Non-validating parsers (those that merely separate a URI reference
into its major components) will often ignore the subcomponent
structure of authority, treating it as an opaque string from the
double-slash to the first terminating delimiter, until such time as
the URI is dereferenced.
The userinfo sub-component may consist of a user name and, 3.2.1 User Information
optionally, scheme-specific information about how to gain
authorization to access the resource. The user information, if The userinfo subcomponent may consist of a user name and, optionally,
present, is followed by a commercial at-sign ("@") that delimits it scheme-specific information about how to gain authorization to access
from the host. the resource. The user information, if present, is followed by a
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
sub-component unless such data is the empty string (indicating no subcomponent unless the data after the colon is the empty string
password) or "anonymous". Applications may choose to ignore or reject (indicating no password). Applications may choose to ignore or reject
such data when received as part of a reference, and should reject the such data when received as part of a reference, and should reject the
storage of such data in unencrypted form. The passing of storage of such data in unencrypted form. The passing of
authentication information in clear text has proven to be a security authentication information in clear text has proven to be a security
risk in almost every case where it has been used. 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 (Section misleadingly crafted to look like a trusted domain name (Section
7.6). 7.6).
3.2.2 Host 3.2.2 Host
The host sub-component 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-decimal form, or a host name. dotted-decimal form, or a registered name. The host subcomponent is
case-insensitive. The presence of a host subcomponent within a URI
does not imply that the scheme requires access to the given host on
the Internet. In many cases, the host syntax is used only for the
sake of reusing the existing registration process created and
deployed for DNS, thus obtaining a globally unique name without the
cost of deploying another registry. However, such use comes with its
own costs: domain name ownership may change over time for reasons not
anticipated by the URI producer. In other cases, the data within the
host component identifies a registered name that has nothing to do
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
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. Again, the distinguish between an IPv4address and a reg-name. In order to
"first-match-wins" algorithm applies: If host matches the rule for disambiguate, the syntax, we apply the "first-match-wins" algorithm:
IPv4address, then it should be considered an IPv4 address literal and If host matches the rule for IPv4address, then it should be
not a reg-name. Although host is case-insensitive, producers and considered an IPv4 address literal and not a reg-name. Although host
normalizers should use lowercase for host names and hexadecimal is case-insensitive, producers and normalizers should use lowercase
addresses for the sake of uniformity, while only using uppercase for registered names and hexadecimal addresses for the sake of
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 optional version flag may be used to indicate such a format
explicitly rather than relying on heuristic determination. explicitly rather than relying on heuristic determination.
IP-literal = "[" ( IPv6address / IPvFuture ) "]" IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" 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 existing IPv4
and IPv6 literal addresses. If a URI containing an IP-literal that and IPv6 literal addresses. If a URI containing an IP-literal that
starts with "v" (case-insensitive), indicating that the version flag starts with "v" (case-insensitive), indicating that the version flag
is present, is dereferenced by an application that does not know the is present, is dereferenced by an application that does not know the
meaning of that version flag, then the application should return an meaning of that version flag, then the application should return an
appropriate error for "address mechanism not supported". appropriate error for "address mechanism not supported".
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grammar. grammar.
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 string of characters that A host identified by a registered name is a sequence of characters
is intended for lookup within a locally-defined host or service name that is usually intended for lookup within a locally-defined host or
registry. The most common of such registry mechanisms is the Domain service name registry, though the URI's scheme-specific semantics may
Name System (DNS), as defined by Section 3 of [RFC1034] and Section require that a specific registry (or fixed name table) be used
2.1 of [RFC1123]. A DNS name consists of a sequence of domain labels instead. The most common name registry mechanism is the Domain Name
System (DNS). A registered name intended for lookup in the DNS uses
the syntax defined in Section 3.5 of [RFC1034] and Section 2.1 of
[RFC1123]. Such a name consists of a sequence of domain labels
separated by ".", each domain label starting and ending with an separated by ".", each domain label starting and ending with an
alphanumeric character and possibly also containing "-" characters. alphanumeric character and possibly also containing "-" characters.
The rightmost domain label of a fully qualified domain name in DNS The rightmost domain label of a fully qualified domain name in DNS
may be followed by a single "." and should be followed by one if it may be followed by a single "." and should be followed by one if it
is necessary to distinguish between the complete domain name and some is necessary to distinguish between the complete domain name and some
local domain. local domain.
reg-name = 0*255( unreserved / pct-encoded / sub-delims ) reg-name = 0*255( unreserved / pct-encoded / sub-delims )
If the host component is defined and the registered name is empty If the URI scheme defines a default for host, then that default
(zero length), then the name defaults to "localhost" (Section 6.2.3 applies when the host subcomponent is undefined or when the
discusses how this should be normalized). If "localhost" is not registered name is empty (zero length). For example, the "file" URI
determined by a host name lookup, then it should be interpreted to scheme is defined such that no authority, an empty host, and
mean the machine on which the URI is being resolved. "localhost" all mean the end-user's machine, whereas the "http"
scheme considers a missing authority or empty host to be 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-name beyond that necessary for interoperability. Instead, it reg-name beyond that necessary for interoperability. Instead, it
delegates the issue of host name syntax conformance to the operating delegates the issue of registered name syntax conformance to the
system of each application performing URI resolution, and that operating system of each application performing URI resolution, and
operating system decides what it will allow for the purpose of host that operating system decides what it will allow for the purpose of
identification. A URI resolution implementation might use DNS, host host identification. A URI resolution implementation might use DNS,
tables, yellow pages, NetInfo, WINS, or any other system for lookup host tables, yellow pages, NetInfo, WINS, or any other system for
of host and service names. However, a globally-scoped naming system, lookup of registered names. However, a globally-scoped naming system,
such as DNS fully-qualified domain names, is necessary for URIs that such as DNS fully-qualified domain names, is necessary for URIs that
are intended to have global scope. URI producers should use host are intended to have global scope. URI producers should use names
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. immediately apparent.
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 host or service 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 octets independent of the underlying name resolution technology; such
must represent characters encoded in the UTF-8 character encoding non-ASCII characters must first be encoded according to UTF-8
[RFC3629] prior to being percent-encoded. When a non-ASCII host name [RFC3629] and then each octet of the corresponding UTF-8 sequence
represents an internationalized domain name intended for resolution must be percent-encoded to be represented as URI characters. URI
via DNS, the name must be transformed to the IDNA encoding [RFC3490] producing applications must not use percent-encoding in host unless
prior to name lookup. URI producers should provide such host names in it is used to represent a UTF-8 character sequence. When a non-ASCII
the IDNA encoding, rather than a percent-encoding, if they wish to registered name represents an internationalized domain name intended
maximize interoperability with legacy URI resolvers. for resolution via the DNS, the name must be transformed to the IDNA
encoding [RFC3490] prior to name lookup. URI producers should
The presence of host within a URI does not imply that the scheme provide such registered names in the IDNA encoding, rather than a
requires access to the given host on the Internet. In many cases, percent-encoding, if they wish to maximize interoperability with
the host syntax is used only for the sake of reusing the existing legacy URI resolvers.
registration process created and deployed for DNS, thus obtaining a
globally unique name without the cost of deploying another registry.
However, such use comes with its own costs: domain name ownership may
change over time for reasons not anticipated by the URI producer.
3.2.3 Port 3.2.3 Port
The port sub-component 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, etc.) is defined by the URI scheme. URI producers
and normalizers should omit the port component and its ":" delimiter and normalizers should omit the port component and its ":" delimiter
if port is empty or its value would be the same as the scheme's if port is empty or its value would be the same as the scheme's
default. 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). If a URI contains an URI's scheme and naming authority (if any). The path is terminated by
authority component, then the initial path segment must be empty the first question mark ("?") or number sign ("#") character, or by
(i.e., the path must begin with a slash ("/") character or be the end of the URI.
entirely empty). The path is terminated by the first question mark
("?") or number sign ("#") character, or by the end of the URI. If a URI contains an authority component, then the path component
must either be empty or begin with a slash ("/") character. If a URI
does not contain an authority component, then the path cannot begin
with two slash characters ("//"). In addition, a URI reference
(Section 4.1) may begin with a relative path, in which case the first
path segment cannot contain a colon (":") character. The ABNF
requires five separate rules to disambiguate these cases, only one of
which will match a given URI reference. We use the generic term
"path component" to describe the URI substring that is matched by the
parser to one of these rules.
path = path-abempty ; begins with "/" or is empty
/ path-abs ; begins with "/" but not "//"
/ path-noscheme ; begins with a non-colon segment
/ path-rootless ; begins with a segment
/ path-empty ; zero characters
path-abempty = *( "/" segment )
path-abs = "/" [ segment-nz *( "/" segment ) ]
path-noscheme = segment-nzc *( "/" segment )
path-rootless = segment-nz *( "/" segment )
path-empty = 0<pchar>
path = segment *( "/" segment )
segment = *pchar segment = *pchar
segment-nz = 1*pchar
segment-nzc = 1*( unreserved / pct-encoded / sub-delims / "@" )
pchar = unreserved / pct-encoded / sub-delims / ":" / "@" pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
A path consists of a sequence of path segments separated by a slash A path consists of a sequence of path segments separated by a slash
("/") 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:fred@example.com> has a path of "fred@example.com", whereas <mailto:fred@example.com> has a path of "fred@example.com", 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 ".." are defined for relative reference The path segments "." and "..", also known as dot-segments, are
within the path name hierarchy. They are intended for use at the defined for relative reference within the path name hierarchy. They
beginning of a relative path reference (Section 4.2) for indicating are intended for use at the beginning of a relative path reference
relative position within the hierarchical tree of names. This is (Section 4.2) for indicating relative position within the
similar to their role within some operating systems' file directory hierarchical tree of names. This is similar to their role within
structure to indicate the current directory and parent directory, some operating systems' file directory structure to indicate the
respectively. However, unlike a file system, these dot-segments are current directory and parent directory, respectively. However, unlike
only interpreted within the URI path hierarchy and are removed as a file system, these dot-segments are only interpreted within the URI
part of the resolution process (Section 5.2). path hierarchy and are removed as part of the resolution process
(Section 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 for the
purpose of delimiting scheme-specific or dereference-handler-specific purpose of delimiting scheme-specific or dereference-handler-specific
sub-components. For example, the semicolon (";") and equals ("=") subcomponents. For example, the semicolon (";") and equals ("=")
reserved characters are often used for delimiting parameters and reserved characters are often used for delimiting parameters and
parameter values applicable to that segment. The comma (",") parameter values applicable to that segment. The comma (",")
reserved character is often used for similar purposes. For example, reserved character is often used for similar purposes. For example,
one URI producer might use a segment like "name;v=1.1" to indicate a one URI producer might use a segment like "name;v=1.1" to indicate a
reference to version 1.1 of "name", whereas another might use a reference to version 1.1 of "name", whereas another might use a
segment like "name,1.1" to indicate the same. Parameter types may be segment like "name,1.1" to indicate the same. Parameter types may be
defined by scheme-specific semantics, but in most cases the syntax of defined by scheme-specific semantics, but in most cases the syntax of
a parameter is specific to the implementation of the URI's a parameter is specific to 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 mark (if any). The query component is indicated by the first question mark
("?") character and terminated by a number sign ("#") character or by ("?") character and terminated by a number sign ("#") character or by
the end of the URI. 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, but should not be used as such within a within the query component. Beware that some older, erroneous
URI that is expected to be the base for relative references (Section implementations do not handle such URIs correctly when they are used
5.1). Incorrect implementations of reference resolution often fail as the base for relative references (Section 5.1), apparently because
to distinguish query data from path data when looking for they fail to to distinguish query data from path data when looking
hierarchical separators, thus resulting in non-interoperable results. for hierarchical separators. However, since query components are
However, since query components are often used to carry identifying often used to carry identifying information in the form of
information in the form of "key=value" pairs, and one frequently used "key=value" pairs, and one frequently used value is a reference to
value is a reference to another URI, it is sometimes better for another URI, it is sometimes better for usability to avoid
usability to avoid percent-encoding those characters. percent-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. Individual media types may define their own URI is dereferenced. If no such representation exists, then the
restrictions on, or structure within, the fragment identifier syntax semantics of the fragment are considered unknown and, effectively,
for specifying different types of subsets, views, or external unconstrained. Fragment identifier semantics are independent of the
references that are identifiable as secondary resources by that media URI scheme and thus cannot be redefined by scheme specifications.
type. If the primary resource has multiple representations, as is
often the case for resources whose representation is selected based Individual media types may define their own restrictions on, or
on attributes of the retrieval request (a.k.a., content negotiation), structure within, the fragment identifier syntax for specifying
then whatever is identified by the fragment should be consistent different types of subsets, views, or external references that are
across all of those representations: each representation should identifiable as secondary resources by that media type. If the
either define the fragment such that it corresponds to the same primary resource has multiple representations, as is often the case
secondary resource, regardless of how it is represented, or the for resources whose representation is selected based on attributes of
fragment should be left undefined by the representation (i.e., not the retrieval request (a.k.a., content negotiation), then whatever is
found). identified by the fragment should be consistent across all of those
representations: each representation should either define the
fragment such that it corresponds to the same secondary resource,
regardless of how it is represented, or the fragment should be left
undefined by the representation (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 systems as Fragment identifiers have a special role in information systems as
the primary form of client-side indirect referencing, allowing an the primary form of client-side indirect referencing, allowing an
author to specifically identify those aspects of an existing resource author to specifically identify those aspects of an existing resource
that are only indirectly provided by the resource owner. As such, that are only indirectly provided by the resource owner. As such, the
interpretation of the fragment identifier during a retrieval action fragment identifier is not used in the scheme-specific processing of
is performed solely by the user agent; the fragment identifier is not a URI; instead, the fragment identifier is separated from the rest of
passed to other systems during the process of retrieval. Although the URI prior to a dereference, and thus the identifying information
this is often perceived to be a loss of information, particularly in within the fragment itself is dereferenced solely by the user agent
regards to accurate redirection of references as content moves over and regardless of the URI scheme. Although this separate handling is
time, it also serves to prevent information providers from denying often perceived to be a loss of information, particularly in regards
reference authors the right to selectively refer to information to accurate redirection of references as resources move over time, it
within a resource. also serves to prevent information providers from denying reference
authors the right to selectively refer to information within a
resource. Indirect referencing also provides additional flexibility
and extensibility to systems that use URIs, since new media types are
easier to define and deploy than new 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, but should not be used represent data within the fragment identifier. Beware that some
as such within a URI that is expected to be the base for relative older, erroneous implementations do not handle such URIs correctly
references (Section 5.1) for the same reasons as described above for when they are used as the base for relative references (Section 5.1).
query.
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. In order to
save space and take advantage of hierarchical locality, many Internet save 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, while 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-URI URI-reference = URI / relative-URI
A URI-reference may be relative: if the reference's prefix matches A URI-reference may be relative: if the reference's prefix matches
the syntax of a scheme followed by its colon separator, then the the syntax of a scheme followed by its colon separator, then the
reference is a URI rather than a relative-URI. reference is a URI rather than a relative-URI.
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, and then each component is whether or not the reference is relative, after which each component
parsed for its subparts and their validation. The ABNF of is parsed for its subparts and their validation. The ABNF of
URI-reference, along with the "first-match-wins" disambiguation rule, URI-reference, along with the "first-match-wins" disambiguation rule,
is sufficient to define a validating parser for the generic syntax. is sufficient to define a validating parser for the generic syntax.
Readers familiar with regular expressions should see Appendix B for Readers familiar with regular expressions should see Appendix B for
an example of a non-validating URI-reference parser that will take an example of a non-validating URI-reference parser that will take
any given string and extract the URI components. any given string and extract the URI components.
4.2 Relative URI 4.2 Relative URI
A relative URI reference takes advantage of the hierarchical syntax A relative URI reference takes advantage of the hierarchical syntax
(Section 1.2.3) in order to express a reference that is relative to (Section 1.2.3) in order to express a reference that is relative to
the name space of another hierarchical URI. the name space of another hierarchical URI.
relative-URI = ["//" authority] path ["?" query] ["#" fragment] relative-URI = relative-part [ "?" query ] [ "#" fragment ]
relative-part = "//" authority path-abempty
/ path-abs
/ path-noscheme
/ 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 relative a network-path reference; such references are rarely used. A relative
reference that begins with a single slash character is termed an reference that begins with a single slash character is termed an
absolute-path reference. A relative reference that does not begin absolute-path reference. A relative reference that does not begin
with a slash character is termed a relative-path reference. 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
because it would be mistaken for a scheme name. Such a segment must because it would be mistaken for a scheme name. Such a segment must
be preceded by a dot-segment (e.g., "./this:that") to make a be preceded by a dot-segment (e.g., "./this:that") to make a
relative-path reference. relative-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 ":" ["//" authority] path ["?" query] absolute-URI = scheme ":" hier-part [ "?" query ]
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 the purpose of a
retrieval action, the target of that reference is defined to be retrieval action, the target of that reference is defined to be
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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 Section 6.2.2 and Section 6.2.3, is allowed but
rarely performed in practice. Normalization may increase the set of rarely performed in practice. Normalization may increase the set of
same-document references, which may be of benefit to some caching same-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, with the assumption that context-based heuristics are machines, with the assumption that context-based heuristics are
sufficient to complete the URI (e.g., most host names beginning with sufficient to complete the URI (e.g., most registered names beginning
"www" are likely to have a URI prefix of "http://"). Although there with "www" are likely to have a URI prefix of "http://"). Although
is no standard set of heuristics for disambiguating a URI suffix, there is no standard set of heuristics for disambiguating a URI
many client implementations allow them to be entered by the user and suffix, many client implementations allow them to be entered by the
heuristically resolved. user and heuristically resolved.
While this practice of using suffix references is common, it should While this practice of using suffix references is common, it should
be avoided whenever possible and never used in situations where be avoided whenever possible and never used in situations where
long-term references are expected. The heuristics noted above will long-term references are expected. The heuristics noted above will
change over time, particularly when a new URI scheme becomes popular, change over time, particularly when a new URI scheme becomes popular,
and are often incorrect when used out of context. Furthermore, they and are often incorrect when used out of context. Furthermore, they
can lead to security issues along the lines of those described in can lead to security issues along the lines of those described in
[RFC1535]. [RFC1535].
Since a URI suffix has the same syntax as a relative path reference, Since a URI suffix has the same syntax as a relative path reference,
a suffix reference cannot be used in contexts where a relative a 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 those places where there is no defined base URI, such as dialog boxes
and off-line advertisements. and 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, such 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 there exists a "base URI" against
which the relative reference is applied. Aside from fragment-only which 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. prior to parsing URI references that might be relative.
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
skipping to change at page 27, line 42 skipping to change at page 27, line 42
| | | | (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 within Document 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 such that it can be readily
obtained by a parser. This can be useful for descriptive documents, obtained by a parser. This can be useful for descriptive documents,
such as tables of content, which may be transmitted to others through such as tables of content, 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., E-Mail 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 each media type's specification. available, is described by the data format specification associated
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
[RFC2110]. 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 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. Since this definition is
necessarily application-dependent, failing to define a base URI using necessarily application-dependent, failing to define a base URI using
one of the other methods may result in the same content being 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 application.
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 references established. Aside from fragment-only references, relative references
can only be used reliably in situations where the base URI is can only be used reliably in situations where the base URI is
well-defined. 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 componets 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 using some other algorithm, provided that the results
match what would be given by this algorithm. match what would be given by this algorithm.
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 Section 6.2.2 and
Section 6.2.3, is optional. A URI reference must be transformed to Section 6.2.3, is optional. A URI reference must be transformed to
its target URI before it can be normalized. its 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 30, line 25 skipping to change at page 30, 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 a 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. Replace any prefix of "./" or "../" at the beginning of the input 2. While the input buffer is not empty, loop:
buffer with "/".
3. While the input buffer is not empty, loop: a. If the input buffer begins with a prefix of "../" or "./",
then remove that prefix from the input buffer; otherwise,
1. 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 "/"; otherwise prefix with "/" in the input buffer; otherwise,
2. 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 "/" and remove the last segment and its preceding prefix with "/" in the input buffer and remove the last
"/" (if any) from the output buffer; otherwise segment and its preceding "/" (if any) from the output
buffer; otherwise,
3. Remove the first segment and its preceding "/" (if any) from d. If the input buffer consists only of "." or "..", then remove
the input buffer and append them to the output buffer. that from the input buffer; otherwise,
4. Finally, the output buffer is returned as the result of e. Move the first path segment in the input buffer to the end of
the output buffer, including the initial "/" character (if
any) and any subsequent characters up to, but not including,
the next "/" character or the end of the input buffer.
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
express an identifier relative to the hierarchy of names in the base
URI. The remove_dot_segments algorithm respects that hierarchy by
removing extra dot-segments rather than treating them as an error or
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 example merged paths, showing the state of the two buffers after each
step. step.
STEP OUTPUT BUFFER INPUT BUFFER STEP OUTPUT BUFFER INPUT BUFFER
1 : /a/b/c/./../../g 1 : /a/b/c/./../../g
3c: /a /b/c/./../../g 2e: /a /b/c/./../../g
3c: /a/b /c/./../../g 2e: /a/b /c/./../../g
3c: /a/b/c /./../../g 2e: /a/b/c /./../../g
3a: /a/b/c /../../g 2b: /a/b/c /../../g
3b: /a/b /../g 2c: /a/b /../g
3b: /a /g 2c: /a /g
3c: /a/g 2e: /a/g
STEP OUTPUT BUFFER INPUT BUFFER STEP OUTPUT BUFFER INPUT BUFFER
1 : mid/content=5/../6 1 : mid/content=5/../6
3c: mid /content=5/../6 2e: mid /content=5/../6
3c: mid/content=5 /../6 2e: mid/content=5 /../6
3b: mid /6 2c: mid /6
3c: 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 using two segment stacks rather than
strings. strings.
Note: Some client applications will fail to separate a reference's Note: Beware that some older, erroneous implementations will fail
query component from its path component before merging the base to separate a reference's query component from its path component
and reference paths. This may result in loss of information if prior to merging the base and reference paths, resulting in an
the query component contains the strings "/../" or "/./". interoperability failure if the query component contains the
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 33, line 5 skipping to change at page 34, line 5
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 URI reference is transformed to its target URI as follows. a relative URI 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 33, line 39 skipping to change at page 34, line 39
"" = "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 above.
Parsers must be careful in handling cases where there are more Parsers must be careful in handling cases where there are more
relative path ".." segments than there are hierarchical levels in the relative path ".." segments than there are hierarchical levels in the
base URI's path. Note that the ".." syntax cannot be used to change base URI's path. Note that the ".." syntax cannot be used to change
the authority component of a URI. the authority component of a URI.
skipping to change at page 35, line 5 skipping to change at page 36, line 5
Some parsers allow the scheme name to be present in a relative URI Some parsers allow the scheme name to be present in a relative URI
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 if 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 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 in respect to some particular purpose,
and software with differing purposes will often be subject to and software with differing purposes will often be subject to
differing design trade-offs in regards to how much effort should be differing design trade-offs in regards to how much effort should be
spent in reducing duplicate identifiers. This section describes a spent in reducing duplicate identifiers. This section describes a
variety of methods that may be used to compare URIs, the trade-offs variety of methods that may be used to compare URIs, the trade-offs
between them, and the types of applications that might use them. between them, and the types of applications that might use them. A
canonical form for URI references is defined to reduce the occurrence
of false negative comparisons.
6.1 Equivalence 6.1 Equivalence
Since URIs exist to identify resources, presumably they should be Since 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 such a definition of equivalence is not of much practical use, since
there is no way for software to compare two resources without there is no way for software to compare two resources without
knowledge of the implementation-specific syntax of each URI's knowledge of the implementation-specific syntax of each URI's
dereferencing algorithm. For this reason, determination of dereferencing algorithm. For this reason, determination of
equivalence or difference of URIs is based on string comparison, equivalence or difference of URIs is based on string comparison,
perhaps augmented by reference to additional rules provided by URI perhaps augmented by reference to additional rules provided by URI
scheme definitions. We use the terms "different" and "equivalent" to scheme definitions. We use the terms "different" and "equivalent" to
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different URIs. Therefore, comparison methods are designed to different URIs. Therefore, comparison methods are designed to
minimize false negatives while strictly avoiding false positives. 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 URI references; the references should be converted to their relative URI references; the references should be converted to their
target URI forms before comparison. When URIs are being compared for target URI forms before comparison. When URIs are being compared for
the purpose of selecting (or avoiding) a network action, such as the purpose of selecting (or avoiding) a network action, such as
retrieval of a representation, the fragment components (if any) retrieval of a representation, the fragment components (if any)
should be excluded from the comparison. should be excluded from 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 in negatives is reduced. As noted above, false negatives cannot in
principle be eliminated. In practice, their probability can be principle be eliminated. In practice, their probability can be
reduced, but this reduction requires more processing and is not reduced, but this reduction requires more processing and is not
cost-effective for all applications. cost-effective for all 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 those
practices that are cheap but have a relatively higher chance of practices that are cheap but have a relatively higher chance of
producing false negatives, and proceeding to those that have higher producing false negatives, and proceeding to those that have higher
computational cost and lower risk of false negatives. computational 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, considered as character strings, are identical, then it
is safe to conclude that they are equivalent. This type of 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. This Testing strings for equivalence requires some basic precautions. This
procedure is often referred to as "bit-for-bit" or "byte-for-byte" procedure is often referred to as "bit-for-bit" or "byte-for-byte"
comparison, which is potentially misleading. Testing of strings for comparison, which is potentially misleading. Testing of strings for
equality is normally based on pairwise comparison of the characters equality is normally based on pairwise comparison of the characters
that make up the strings, starting from the first and proceeding that make up the strings, starting from the first and proceeding
until both strings are exhausted and all characters found to be until both strings are exhausted and all characters found to be
equal, a pair of characters compares unequal, or one of the strings equal, a pair of characters compares unequal, or one of the strings
is exhausted before the other. is exhausted before the other.
Such character comparisons require that each pair of characters be Such character comparisons require 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 be in a Java String
object (UTF-16), bit-for-bit comparisons applied naively will produce object (UTF-16), bit-for-bit comparisons applied naively will produce
both false-positive and false-negative errors. It is better to speak errors. It is better to speak of equality on a
of equality on a character-for-character rather than byte-for-byte or character-for-character rather than byte-for-byte or bit-for-bit
bit-for-bit basis. In practical terms, character-by-character basis. In practical terms, character-by-character comparisons should
comparisons should be done codepoint-by-codepoint after conversion to be done codepoint-by-codepoint after conversion to a common character
a common character encoding. encoding.
6.2.2 Syntax-based Normalization 6.2.2 Syntax-based Normalization
Software may use logic based on the definitions provided by this Software may use logic based on the definitions provided by this
specification to reduce the probability of false negatives. Such specification to reduce the probability of false negatives. Such
processing is moderately higher in cost than character-for-character processing is moderately higher in cost than character-for-character
string comparison. For example, an application using this approach string comparison. For example, an application using this approach
could reasonably consider the following two URIs equivalent: could reasonably consider the 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, encoding normalization, empty-component case normalization, percent-encoding normalization, and removal of
normalization, and removal of dot-segments. dot-segments.
6.2.2.1 Case Normalization 6.2.2.1 Case Normalization
When a URI scheme uses components of the generic syntax, it will also When a URI scheme uses components of the generic syntax, it will also
use the common syntax equivalence rules, namely that the scheme and use the common syntax equivalence rules, 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/>. Applications should not equivalent to <http://www.example.com/>. Applications should not
assume anything about the case sensitivity of other URI components, assume anything about the case sensitivity of other URI components,
since that is dependent on the implementation used to handle a since that is dependent on the implementation used to handle a
dereference. dereference.
The hexadecimal digits within a percent-encoding triplet (e.g., "%3a" The hexadecimal digits within a percent-encoding triplet (e.g., "%3a"
versus "%3A") are case-insensitive and therefore should be normalized versus "%3A") are case-insensitive and therefore should be normalized
to use uppercase letters for the digits A-F. to use uppercase letters for the digits A-F.
6.2.2.2 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 variance among otherwise identical URIs. In addition to the
case-insensitivity issue noted above, some URI producers case-insensitivity issue noted above, some URI producers
percent-encode octets that do not require percent-encoding, resulting percent-encode octets that do not require percent-encoding, resulting
in URIs that are equivalent to their non-encoded counterparts. Such in URIs that are equivalent to their non-encoded counterparts. Such
URIs should be normalized by decoding any percent-encoded octet that URIs should be normalized by decoding any percent-encoded octet that
corresponds to an unreserved character, as described in Section 2.3. corresponds to an unreserved character, as described in Section 2.3.
6.2.2.3 Empty-component Normalization 6.2.2.3 Path Segment Normalization
Components of the generic URI syntax are delimited from other
components by optional separators. For example, a query component is
separated from the path by a question mark ("?") and a port
sub-component is separated from host by a colon (":"). A URI in
which a delimiter is present and the (sub-)component it delimits is
empty is equivalent to the same URI without that delimiter. For
example, the following are all equivalent:
ftp://example.com/
ftp://example.com:/
ftp://@example.com:/
ftp://@example.com:/?
ftp://@example.com:/?#
URI producers and normalizers should omit a delimiter if the
component it delimits is empty, as exemplified by the first URI
above, with one exception: a double-slash delimiter indicating an
authority component should not be removed, even when the authority is
empty, since doing so can lead to misinterpreting the path.
6.2.2.4 Path Segment Normalization
The complete path segments "." and ".." have a special meaning within The complete path segments "." and ".." have a special meaning within
hierarchical URI schemes. As such, they should not appear in hierarchical URI schemes. As such, they should not appear in
absolute paths; if they are found, they can be removed by applying absolute paths; if they are found, they can be removed by applying
the remove_dot_segments algorithm to the path, as described in the remove_dot_segments algorithm to the path, as described in
Section 5.2. Section 5.2.
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. Software described by the defining specification for each scheme. Software
may use scheme-specific rules, at further processing cost, to reduce may use scheme-specific rules, at further processing cost, to reduce
the probability of false negatives. For example, since the "http" the probability of false negatives. For example, since the "http"
scheme makes use of an authority component, has a default port of scheme makes use of an authority component, has a default port of
"80", and defines an empty path to be equivalent to "/", the "80", and defines an empty path to be equivalent to "/", the
following four URIs are equivalent: following four URIs are equivalent:
http://example.com http://example.com
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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", where 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. In other words, the second of the above URI examples is the elided. In other words, the second of the above URI examples is the
normal form for the "http" scheme. normal form for the "http" 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. For many scheme specifications, an of an empty authority component or empty host subcomponent. For many
empty authority is considered an error; for others, it is considered scheme specifications, an empty authority or host is considered an
equivalent to "localhost". For the sake of uniformity, future scheme error; for others, it is considered equivalent to "localhost" or the
specifications should define an empty authority as being equivalent end-user's host. When a scheme defines a default for authority and a
to "localhost", and URI producers and normalizers should use URI reference to that default is desired, the reference should have
"localhost" instead of an empty authority. an empty authority for the sake of uniformity, brevity, and
internationalization. If, however, either the userinfo or port
subcomponent is non-empty, then the host should be given explicitly
even if it matches the default.
6.2.4 Protocol-based Normalization 6.2.4 Protocol-based Normalization
Web spiders, for which substantial effort to reduce the incidence of Web spiders, for which substantial effort to reduce the incidence of
false negatives is often cost-effective, are observed to implement false negatives is often cost-effective, are observed to implement
even more aggressive techniques in URI comparison. For example, if even more aggressive techniques in URI comparison. For example, if
they observe that a URI such as they 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).
6.3 Canonical Form 6.3 Canonical Form
It is in the best interests of everyone to avoid false-negatives in It is in the best interests of everyone concerned to avoid
comparing URIs and to minimize the amount of software processing for false-negatives in comparing URIs and to minimize the amount of
such comparisons. Those who produce and make reference to URIs can software processing for such comparisons. Those who produce and make
reduce the cost of processing and the risk of false negatives by reference to URIs can reduce the cost of processing and the risk of
consistently providing them in a form that is reasonably canonical false negatives by consistently providing them in a form that is
with respect to their scheme. Specifically: reasonably canonical with respect to their scheme. Specifically:
o Always provide the URI scheme in lowercase characters. o Always provide the URI scheme in lowercase characters.
o Always provide the host, if any, in lowercase characters. o Always provide the host, if any, in lowercase characters.
o Only perform percent-encoding where it is essential. o Only perform percent-encoding where it is essential.
o Always use uppercase A-through-F characters when percent-encoding. o Always use uppercase A-through-F characters when percent-encoding.
o Prevent /./ and /../ from appearing in non-relative URI paths. o Prevent dot-segments appearing in non-relative URI paths.
o Omit delimiters when their associated (sub-)component is empty.
o For schemes that define an empty authority to be equivalent to o For schemes that define a default authority, use an empty
"localhost", use "localhost". authority if the default is desired.
o For schemes that define an empty path to be equivalent to a path o For schemes that define an empty path to be equivalent to a path
of "/", use "/". of "/", use "/".
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, since 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, having once used a given URI to retrieve
some information, the same information will be retrievable by that some information, the same information will be retrievable by that
URI in the future. Nor is there any guarantee that the information URI in 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 name space or maintains it
over time. Such a guarantee can only be obtained from the person(s) over time. Such a guarantee can only be obtained from the person(s)
controlling that name space and the resource in question. A specific controlling that name space 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 such 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 to occur. The unsafe URI is typically constructed
by specifying a port number other than that reserved for the network by 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 that is
running a different protocol service and data within the URI contains running 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
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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), such percent-encoded octets
must not be decoded before transmission across that protocol. must not be decoded before transmission across that protocol.
Transfer of the percent-encoding, which might violate the protocol, Transfer of the percent-encoding, which might violate the protocol,
is less harmful than allowing decoded octets to be interpreted as is less harmful than allowing decoded octets to be interpreted as
additional operations or parameters, perhaps triggering an unexpected additional operations or parameters, perhaps triggering an unexpected
and possibly harmful remote operation. and 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 sub-components prior to decoding the octets, since components and subcomponents prior to decoding the octets, since
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 filesystem or related system
functions. Filesystems typically assign an operational meaning to functions. Filesystems typically assign an operational meaning to
special characters, such as the "/", "\", ":", "[", and "]" special characters, such as the "/", "\", ":", "[", and "]"
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"lpt", etc. In some cases, merely testing for the existence of such a "lpt", etc. In some cases, merely testing for the existence of such a
name will cause the operating system to pause or invoke unrelated 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 application 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
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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 such 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, rather than 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 which 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 sub-component 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 that is intended to mislead a human user by appearing to identify
one (trusted) naming authority while actually identifying a different one (trusted) naming authority while actually identifying a different
authority hidden behind the noise. For example authority hidden behind the noise. For example
ftp://ftp.example.com&story=breaking_news@10.0.0.1/top_story.htm ftp://cnn.example.com&story=breaking_news@10.0.0.1/top_story.htm
might lead a human user to assume that the host is might lead a human user to assume that the host is 'cnn.example.com',
'trusted.example.com', whereas it is actually '10.0.0.1'. Note that whereas it is actually '10.0.0.1'. Note that a misleading userinfo
a misleading userinfo sub-component could be much longer than the subcomponent could be much longer than the example above.
example above.
A misleading URI, such as the one above, is an attack on the user's A misleading URI, such as the one 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 on the software itself. User agents may be able to reduce the attack on the software itself. User agents may be able to reduce the
impact of such attacks by distinguishing the various components of impact of such attacks by distinguishing the various components of
the URI when rendered, such as by using a different color or tone to the URI when 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 general
panacea. More information on URI-based semantic attacks can be found panacea. More information on URI-based semantic attacks can be found
in [Siedzik]. in [Siedzik].
8. Acknowledgments 8. Acknowledgments
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 acknowledgments 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, Tony Hammond, Pat Hayes, Duerst, Stefan Eissing, Clive D.W. Feather, Tony Hammond, Pat Hayes,
Henry Holtzman, Ian B. Jacobs, Michael Kay, John C. Klensin, Graham Henry Holtzman, Ian B. Jacobs, Michael Kay, John C. Klensin, Graham
Klyne, Dan Kohn, Bruce Lilly, Andrew Main, Ira McDonald, Michael Klyne, Dan Kohn, Bruce Lilly, Andrew Main, Ira McDonald, Michael
Mealling, Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Mealling, Ray Merkert, Stephen Pollei, Julian Reschke, Tomas Rokicki,
Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne, Stuart Miles Sabin, Kai Schaetzl, Mark Thomson, Ronald Tschalaer, Norm
Williams, and Henry Zongaro are gratefully acknowledged. Walsh, Marc Warne, Stuart Williams, and Henry Zongaro are gratefully
acknowledged.
Normative References 9. References
9.1 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.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003. 10646", STD 63, RFC 3629, November 2003.
Informative References 9.2 Informative References
[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.
skipping to change at page 47, line 41 skipping to change at page 46, line 5
[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", RFC
1808, June 1995. 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.
[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC 2110,
March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998. Languages", BCP 18, RFC 2277, January 1998.
[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,
"MIME Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, March 1999.
[RFC2717] Petke, R. and I. King, "Registration Procedures for URL [RFC2717] 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.
[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.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
skipping to change at page 49, line 9 skipping to change at page 47, line 9
(IPv6) Addressing Architecture", RFC 3513, April 2003. (IPv6) Addressing Architecture", RFC 3513, April 2003.
[Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April [Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April
2001, <http://www.giac.org/practical/gsec/ 2001, <http://www.giac.org/practical/gsec/
Richard_Siedzik_GSEC.pdf>. Richard_Siedzik_GSEC.pdf>.
Authors' Addresses Authors' Addresses
Tim Berners-Lee Tim Berners-Lee
World Wide Web Consortium World Wide Web Consortium
MIT/LCS, Room NE43-356 Massachusetts Institute of Technology
200 Technology Square 77 Massachusetts Avenue
Cambridge, MA 02139 Cambridge, MA 02139
USA USA
Phone: +1-617-253-5702 Phone: +1-617-253-5702
Fax: +1-617-258-5999 Fax: +1-617-258-5999
EMail: timbl@w3.org EMail: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/ URI: http://www.w3.org/People/Berners-Lee/
Roy T. Fielding Roy T. Fielding
Day Software Day Software
skipping to change at page 50, line 5 skipping to change at page 48, line 5
Larry Masinter Larry Masinter
Adobe Systems Incorporated Adobe Systems Incorporated
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA USA
Phone: +1-408-536-3024 Phone: +1-408-536-3024
EMail: LMM@acm.org EMail: LMM@acm.org
URI: http://larry.masinter.net/ URI: http://larry.masinter.net/
Appendix A. Collected ABNF for URI Appendix A. Collected ABNF for URI
URI = scheme ":" ["//" authority] path ["?" query] ["#" fragment] URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part = "//" authority path-abempty
/ path-abs
/ path-rootless
/ path-empty
URI-reference = URI / relative-URI URI-reference = URI / relative-URI
relative-URI = ["//" authority] path ["?" query] ["#" fragment] absolute-URI = scheme ":" hier-part [ "?" query ]
absolute-URI = scheme ":" ["//" authority] path ["?" query] relative-URI = relative-part [ "?" query ] [ "#" fragment ]
relative-part = "//" authority path-abempty
/ path-abs
/ path-noscheme
/ 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" 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 ] "::"
skipping to change at page 50, line 40 skipping to change at page 49, line 4
/ [ *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 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
reg-name = 0*255( unreserved / pct-encoded / sub-delims ) reg-name = 0*255( unreserved / pct-encoded / sub-delims )
path = segment *( "/" segment ) path = path-abempty ; begins with "/" or is empty
/ path-abs ; begins with "/" but not "//"
/ path-noscheme ; begins with a non-colon segment
/ path-rootless ; begins with a segment
/ path-empty ; zero characters
path-abempty = *( "/" segment )
path-abs = "/" [ segment-nz *( "/" segment ) ]
path-noscheme = segment-nzc *( "/" segment )
path-rootless = segment-nz *( "/" segment )
path-empty = 0<pchar>
segment = *pchar segment = *pchar
segment-nz = 1*pchar
segment-nzc = 1*( unreserved / pct-encoded / sub-delims / "@" )
pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
query = *( pchar / "/" / "?" ) query = *( pchar / "/" / "?" )
fragment = *( pchar / "/" / "?" ) fragment = *( pchar / "/" / "?" )
pct-encoded = "%" HEXDIG HEXDIG pct-encoded = "%" HEXDIG HEXDIG
pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
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" Since 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.
^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))? ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
skipping to change at page 53, line 5 skipping to change at page 51, line 5
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 and, going in the opposite direction, we can recreate a URI reference
from its components using the algorithm of Section 5.3. from its components 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 electronic mail, USENET news messages, and, most importantly,
printed on paper. In such cases, it is important to be able to printed on paper. In such cases, it is important to be able to
delimit the URI from the rest of the text, and in particular from delimit the URI from the rest of the text, and in particular from
punctuation marks that might be mistaken for part of the URI. punctuation marks that might be mistaken for 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
skipping to change at page 54, line 4 skipping to change at page 52, line 13
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. Summary of Non-editorial Changes Appendix D. Summary of Non-editorial Changes
D.1 Additions D.1 Additions
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 a authority component, as identifiers for the host portion of a 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 not allowed outside their use as delimiters
for an IP literal within host. In order to make this change without for an IP literal within host. In order to make this change without
changing the technical definition of the path, query, and fragment changing the technical definition of the path, query, and fragment
components, those rules were redefined to directly specify the components, those rules were redefined to directly specify the
skipping to change at page 55, line 24 skipping to change at page 52, line 37
authority component and not allowed outside their use as delimiters authority component and not allowed outside their use as delimiters
for an IP literal within host. In order to make this change without for an IP literal within host. In order to make this change without
changing the technical definition of the path, query, and fragment changing the technical definition of the path, query, and fragment
components, those rules were redefined to directly specify the components, those rules were redefined to directly specify the
characters allowed rather than be defined in terms of uric. characters allowed rather than be defined in terms of uric.
Since [RFC2732] defers to [RFC3513] for definition of an IPv6 literal Since [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 (Section 6) on URI normalization and comparison has been
completely rewritten and extended using input from Tim Bray and completely rewritten and extended using input from Tim Bray and
discussion within the W3C Technical Architecture Group. discussion within the W3C Technical Architecture Group.
An ABNF rule for URI has been introduced to correspond to the common An ABNF rule for URI has been introduced to correspond to the common
usage of the term: an absolute URI with optional fragment. usage of the term: an absolute URI with optional fragment.
D.2 Modifications from RFC 2396 D.2 Modifications from RFC 2396
The ad-hoc BNF syntax has been replaced with the ABNF of [RFC2234]. The ad-hoc BNF syntax has been replaced with the ABNF of [RFC2234].
This change required all rule names that formerly included underscore This change required all rule names that formerly included underscore
characters to be renamed with a dash instead. characters to be renamed with a dash instead.
Section 2 on characters has been rewritten to explain what characters Section 2 on characters has been rewritten to explain what characters
are reserved, when they are reserved, and why they are reserved even are reserved, when they are reserved, and why they are reserved even
when not used as delimiters by the generic syntax. The mark when not used as delimiters by the generic syntax. The mark
characters that are typically unsafe to decode, including the characters that are typically unsafe to decode, including the
exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open
skipping to change at page 56, line 7 skipping to change at page 53, line 27
set in order to clarify the distinction between reserved and set in order to clarify the distinction between reserved and
unreserved and hopefully answer the most common question of scheme unreserved and hopefully answer the most common question of scheme
designers. Likewise, the section on percent-encoded characters has designers. Likewise, the section on percent-encoded characters has
been rewritten, and URI normalizers are now given license to decode been rewritten, and URI normalizers are now given license to decode
any percent-encoded octets corresponding to unreserved characters. any percent-encoded octets corresponding to unreserved characters.
In general, the terms "escaped" and "unescaped" have been replaced In general, the terms "escaped" and "unescaped" have been replaced
with "percent-encoded" and "decoded", respectively, to reduce with "percent-encoded" and "decoded", respectively, to reduce
confusion with other forms of escape mechanisms. 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 significantly reduce complexity. As more friendly to LALR parsers and reduce complexity. As a result, the
a result, the layout form of syntax description has been removed, layout form of syntax description has been removed, along with the
along with the uric, uric_no_slash, hier_part, opaque_part, net_path, uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,
abs_path, rel_path, path_segments, rel_segment, and mark rules. All path_segments, rel_segment, and mark rules. All references to
references to "opaque" URIs have been replaced with a better "opaque" URIs have been replaced with a better description of how the
description of how the path component may be opaque to hierarchy. The path component may be opaque to hierarchy. The ambiguity regarding
ambiguity regarding the parsing of URI-reference as a URI or a the parsing of URI-reference as a URI or a relative-URI with a colon
relative-URI with a colon in the first segment is now explained and in the first segment has been eliminated through the use of five
disambiguated in the section defining relative-URI. 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-URI rules, generic syntax components and within the URI and relative-URI 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 a relative path to be empty. The ABNF has been corrected to allow a relative path 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 the "about:" scheme used internally by many WWW browser
implementations. The ambiguity regarding the boundary between implementations. The ambiguity regarding the boundary between
authority and path is now explained and disambiguated in the same authority and path has been eliminated through the use of five
section. 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 and limited to 255 path characters. This defined within the host rule and limited to 255 path characters. This
change allows current implementations, where whatever name provided change allows current implementations, where whatever name provided
is simply fed to the local name resolution mechanism, to be is simply fed to the local name resolution mechanism, to be
consistent with the specification and removes the need to re-specify consistent with the specification and removes the need to re-specify
DNS name formats here. It also allows the host component to contain DNS name formats here. It also allows the host component to contain
percent-encoded octets, which is necessary to enable percent-encoded octets, which is necessary to enable
internationalized domain names to be provided in URIs, processed in internationalized domain names to be provided in URIs, processed in
their native character encodings at the application layers above URI their native character encodings at the application layers above URI
skipping to change at page 58, line 10 skipping to change at page 54, line 10
paths in order to match common implementations and improve the paths in order to match common implementations and 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.
Index Index
A A
ABNF 10 ABNF 10
absolute 25 absolute 25
absolute-path 24 absolute-path 25
absolute-URI 25 absolute-URI 25
access 7 access 8
authority 15, 16 authority 15, 16
B B
base URI 27 base URI 27
C C
characters 11 character encoding 4
character 4
characters 10
coded character set 4
D D
dec-octet 18 dec-octet 19
dereference 7 dereference 8
dot-segments 20 dot-segments 21
F F
fragment 22 fragment 15, 23
G G
gen-delims 12 gen-delims 11
generic syntax 5 generic syntax 6
H H
h16 17 h16 18
hier-part 15
hierarchical 9 hierarchical 9
host 17 host 17
I I
identifier 5 identifier 5
IP-literal 17 IP-literal 18
IPv4 18 IPv4 19
IPv4address 18 IPv4address 19
IPv6 17 IPv6 18
IPv6address 17 IPv6address 18
IPvFuture 17 IPvFuture 18
L L
locator 6 locator 6
ls32 17 ls32 18
M M
merge 30 merge 30
N N
name 6 name 6
network-path 24 network-path 25
P P
path 15, 20 path 15, 21
pchar 20 path-abempty 21
path-abs 21
path-empty 21
path-noscheme 21
path-rootless 21
path-abempty 15
path-abs 15
path-empty 15
path-rootless 15
pchar 21
pct-encoded 11 pct-encoded 11
percent-encoding 11 percent-encoding 11
port 20 port 20
Q Q
query 21 query 15, 22
R R
reg-name 19 reg-name 19
registered name 19 registered name 19
relative 9, 27 relative 9, 27
relative-path 24 relative-path 25
relative-URI 24 relative-URI 25
remove_dot_segments 30 remove_dot_segments 30, 31
representation 8 representation 8
reserved 12 reserved 11
resolution 7, 27 resolution 8, 27
resource 4 resource 4
retrieval 8 retrieval 8
S S
same-document 25 same-document 25
sameness 8 sameness 8
scheme 15 scheme 15, 15
segment 20 segment 21
sub-delims 12 segment-nz 21
suffix 25 segment-nzc 21
sub-delims 11
suffix 26
T T
transcription 6 transcription 7
U U
uniform 4 uniform 4
unreserved 12 unreserved 12
URI grammar URI grammar
absolute-URI 25 absolute-URI 25
ALPHA 10 ALPHA 10
authority 15, 16 authority 15, 16
CR 10 CR 10
CTL 10 dec-octet 19
dec-octet 18
DIGIT 10 DIGIT 10
DQUOTE 10 DQUOTE 10
fragment 15, 22, 24 fragment 15, 23, 25
gen-delims 12 gen-delims 11
h16 18 h16 18
HEXDIG 10 HEXDIG 10
hier-part 15
host 16, 17 host 16, 17
IP-literal 17 IP-literal 18
IPv4address 18 IPv4address 19
IPv6address 17, 18 IPv6address 18
IPvFuture 17 IPvFuture 18
LF 10 LF 10
ls32 18 ls32 18
mark 12 mark 12
OCTET 10 OCTET 10
path 15 path 21
path-segments 20 path-abempty 15, 21
pchar 20, 21, 22 path-abs 15, 21
path-empty 15, 21
path-noscheme 21
path-rootless 15, 21
pchar 21, 22, 23
pct-encoded 11 pct-encoded 11
port 16, 20 port 16, 20
query 15, 21, 24, 25 query 15, 22, 25
reg-name 19 reg-name 19
relative-URI 24, 24 relative-URI 24, 25
reserved 12 reserved 11
scheme 15, 15, 25 scheme 15, 16, 25
segment 20 segment 21
segment-nz 21
segment-nzc 21
SP 10 SP 10
sub-delims 12 sub-delims 11
unreserved 12 unreserved 12
URI 15, 24 URI 15, 24
URI-reference 24 URI-reference 24
userinfo 16, 16 userinfo 16, 17
URI 15 URI 15
URI-reference 24 URI-reference 24
URL 6 URL 6
URN 6 URN 6
userinfo 16 userinfo 17
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
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Internet Society. Internet Society.
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